structured growth of zinc oxide nanorods on plastic optical fiber and light side coupling towards
217
STRUCTURED GROWTH OF ZINC OXIDE NANORODS ON PLASTIC OPTICAL FIBER AND LIGHT SIDE COUPLING TOWARDS SENSING APPLICATIONS HAZLI RAFIS BIN ABDUL RAHIM FACULTY OF ENGINEERING UNIVERSITY OF MALAYA KUALA LUMPUR 2017
Transcript of structured growth of zinc oxide nanorods on plastic optical fiber and light side coupling towards
STRUCTURED GROWTH OF ZINC OXIDE NANORODS ON PLASTIC OPTICAL FIBER AND LIGHT SIDE
COUPLING TOWARDS SENSING APPLICATIONS
HAZLI RAFIS BIN ABDUL RAHIM
FACULTY OF ENGINEERING UNIVERSITY OF MALAYA
KUALA LUMPUR
2017
STRUCTURED GROWTH OF ZINC OXIDE
NANORODS ON PLASTIC OPTICAL FIBER AND
LIGHT SIDE COUPLING TOWARDS SENSING
APPLICATIONS
HAZLI RAFIS BIN ABDUL RAHIM
THESIS SUBMITTED IN FULFILMENT OF THE
REQUIREMENTS FOR THE DEGREE OF DOCTOR OF
PHILOSOPHY
FACULTY OF ENGINEERING
UNIVERSITY OF MALAYA
KUALA LUMPUR
2017
ii
UNIVERSITY OF MALAYA
ORIGINAL LITERARY WORK DECLARATION
Name of Candidate: Hazli Rafis Bin Abdul Rahim (I.C No.: )
Registration/Matric No: KHA140007
Name of Degree: Doctor of Philosophy
Title of Thesis: Structured Growth of Zinc Oxide Nanorods on Plastic Optical
Fiber and Light Side Coupling Towards Sensing Applications
Field of Study: Electronic (Engineering and Engineering Trades)
I do solemnly and sincerely declare that:
(1) I am the sole author/writer of this Work;
(2) This Work is original;
(3) Any use of any work in which copyright exists was done by way of fair dealing
and for permitted purposes and any excerpt or extract from, or reference to or
reproduction of any copyright work has been disclosed expressly and
sufficiently and the title of the Work and its authorship have been
acknowledged in this Work;
(4) I do not have any actual knowledge nor do I ought reasonably to know that the
making of this work constitutes an infringement of any copyright work;
(5) I hereby assign all and every rights in the copyright to this Work to the
University of Malaya (“UM”), who henceforth shall be owner of the copyright
in this Work and that any reproduction or use in any form or by any means
whatsoever is prohibited without the written consent of UM having been first
had and obtained;
(6) I am fully aware that if in the course of making this Work I have infringed any
copyright whether intentionally or otherwise, I may be subject to legal action
or any other action as may be determined by UM.
Candidate’s Signature Date:
Subscribed and solemnly declared before,
Witness’s Signature Date:
Name:
Designation:
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ABSTRACT
A simple and cost effective optical fiber sensor using side coupling of light into
the core modes of plastic optical fiber (POF) coated with zinc oxide (ZnO) nanorods is
reported here. Nanorods coating enhanced coupling inside the fiber by scattering light but
were also capable of causing leakage. Structuring the growth to specific regions allowed
scattering from different segments along the fiber to contribute to the total coupled power.
A uniform, densed and highly aligned spiral patterned ZnO nanorods were grown on the
POF using the hydrothermal method and its effect was investigated. ZnO nanorods
growth time of 12 h and temperature of 90 °C provided the best coupling voltage. Side
coupling was measured to be a factor of 2.2 times better for spiral patterned coatings as
opposed to unpatterned coatings. The formation of multiple segments was used for
multiple-wavelength channels excitation where different bands were side coupled from
different segments. It was found that visible white light source significantly coupled the
light into the POF compared with infrared laser sources. A first order theoretical model
was derived to simulate the impact of millimeter (mm) scale spiral patterns on coupling
efficiency by varying the width and spacing of the coated and uncoated regions. The width
of spiral patterned ZnO nanorod coatings on POF was optimized theoretically for light
side coupling and was found to be 5 mm. An experimental validation was performed to
complete the optimization and the experimental results showing a well correlation with
simulation. Optimized width of spiral patterned ZnO nanorods grown on large core POFs
was used for the purpose of temperature and multiple optical channel alcohol vapor
sensing. Spiral patterned ZnO nanorods coating exhibited a significant response to
temperature change from 20 ˚C to 100 ˚C based on extinction concept which is the
attenuation of light by scattering and absorption as it traverses the ZnO nanorods.
Sensitivity was measured to be a factor of 1.3 times better for spiral patterned coatings as
opposed to unpatterned coating. The multiple optical channel alcohol sensing mechanism
iv
utilized changes in the output signal due to adsorption of methanol, ethanol and
isopropanol vapors. Three spectral bands consisting of red (620-750 nm), green (495-
570 nm) and blue (450-495 nm) were applied in measurements. The range of relative
intensity modulation (RIM) was determined to be between 25 to 300 ppm. Methanol
presented the strongest response compared to ethanol and isopropanol in all three spectral
channels. With regard to alcohol detection RIM by spectral band, the green channel
demonstrated the highest RIM values followed by the blue and red channels respectively.
v
ABSTRAK
Satu penderia optik yang mudah dan kos efektif menggunakan gandingan sisi
cahaya ke dalam ragam-ragam teras gentian optik plastik (POF) disalut dengan zink
oksida (ZnO) nanorods dilaporkan di sini. Salutan nanorod-nanorod mempertingkatkan
gandingan dalam gentian oleh serakan cahaya tetapi juga boleh menyebabkan kebocoran.
Penstrukturkan pertumbuhan ke kawasan-kawasan tertentu membolehkan penyerakan
daripada ruas yang berbeza di sepanjang gentian yang menyumbang kepada jumlah kuasa
terganding. Satu pilin corak ZnO nanorod yang seragam, tumpat dan terjajar dengan
tinggi dan yang ditumbuhkan di atas teras POF menggunakan kaedah hidroterma dan
kesannya disiasat. ZnO nanorod yang mempunyai masa pertumbuhan 12 jam dan suhu
90 °C telah menyediakan gandingan voltan terbaik. Gandingan sisi diukur dengan faktor
sebanyak 2.2 kali lebih baik untuk lapisan pilin corak berbanding dengan lapisan tidak
tercorak. Pembentukan berbilang ruas telah juga digunakan untuk pengujaan saluran-
saluran pelbagai panjang gelombang di mana jalur-jalur digandingkan secara sisi daripada
ruas yang berbeza. Didapati sumber cahaya putih boleh nampak dengan ketara
menggandingkan cahaya ke dalam POF berbanding dengan sumber laser infra-merah.
Satu model teori tertib pertama diterbitkan untuk menyelakukan kesan corak-corak pilin
berskala milimeter (mm) terhadap kecekapan gandingan dengan mengubah lebar dan
jarak kawasan bersalut dan tidak bersalut. Lebar lapisan corak pilin ZnO nanorod pada
POF teras telah dioptimumkan secara teori untuk gandingan sebelah cahaya dan didapati
5 mm adalah lebar tersebut. Satu pengesahan ujikaji telah dilakukan untuk melengkapkan
pengoptimuman dan keputusan ujikaji menunjukkan satu hubungan sekaitan yang baik
dengan penyelakuan. Lebar corak pilin ZnO nanorod yang ditunbuhkan atas POF teras
besar telah digunakan untuk penderiaan suhu dan wap alkohol pelbagai saluran optik.
Lapiran pilin corak ZnO nanorod mempamerkan satu tindak balas yang ketara kepada
perubahan suhu dari 20 ˚C hingga 100 ˚C berdasarkan konsep pemupusan yang
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merupakan pengecilan cahaya oleh serakan dan penyerapan apabila ia merentasi ZnO
nanorod. Kepekaan diukur yang menunjukan faktor 1.3 kali lebih baik untuk lapiran corak
pilin yang bertentangan dengan lapiran tidak bercorak. Mekanisme penderiaan wap
alkohol pelbagai saluran optik telah menggunakan perubahan-perubahan di dalam isyarat
keluaran disebakan oleh penyerapan wap-wap methanol, etanol dan isopropil. Tiga jalur
spektrum terdiri daripada merah (620-750 nm), hijau (495-570 nm) dan biru (450-495
nm) telah digunakan dalam pengukuran ini. Julat nisbi pemodulatan keamatan ditentukan
antara 25 hingga 300 ppm. Metanol menunjukkan tindakbals yang kuat berbanding etanol
dan isopropil dalam ketiga-tiga saluran spektrum. Dengan mengambil kira nisbi
pemodulatan keamatan pengesanan alkohol oleh jalur spektrum, saluran hijau
menunjukkan nilai nisbi pemodulatan keamatan tertinggi diikuti dengan masing-masing
oleh saluran biru dan merah.
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ACKNOWLEDGEMENTS
It is with immense gratitude that I would like to acknowledge the support and help
of my supervisor, Prof. Dr. Sulaiman Wadi Bin Harun. He has been a source of inspiration
and has convincingly conveyed a spirit of adventure in regard to research. The initial ideas
he suggested and his vast knowledge on optical sensor were invaluable in helping me get
set on the right track. The successful completion of my task would not have been possible
without his constant encouragement and guidance throughout the whole period of my
research work.
I am also indebted to my co-supervisor Prof. Dr. Waleed S. Mohammed who
helped me in building up a positive work attitude. His mastery over the subject and the
freedom he provided in carrying out my research were instrumental in gaining a lot of
confidence. I owe my deepest gratitude to Prof. Dr. Louis Gabor Hornyak, Director of
Center of Excellence in Nanotechnology, Asian Institute of Technology (AIT), Bangkok,
Thailand and Prof. Dr. Joydeep Dutta, Chair of Functional Materials division, KTH Royal
Institute of Technology, Stockholm, Sweden for reading my reports, commenting on my
views and helping me understand and enrich my ideas.
I share the credit of my work with Mr. Manjunath, PhD student, Bangkok
University, Thailand and Siddharth Thokchom, master student, Assam Don Bosco
University, India for their constant involvement and inspiring advice.
Most importantly, I would like to express my heart-felt gratitude to my parents
Abdul Rahim and Hasnah, thank you for your encouragement and prayers. My deepest
appreciations from bottom of my heart go to my wife Siti Khatijah for all your love and
support, and to our children Rayyan and Rayqal for being such as wonderful sons.
Lastly, I would like to thank Universiti Teknikal Malaysia Melaka (UTeM) and
Ministry of Higher Education Malaysia (MOHE) for sponsoring my PhD program under
SLAB/ SLAI scholarship.
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TABLE OF CONTENTS
Abstract ............................................................................................................................ iii
Abstrak .............................................................................................................................. v
Acknowledgements ......................................................................................................... vii
Table of Contents ........................................................................................................... viii
List of Figures ................................................................................................................. xii
List of Tables................................................................................................................ xviii
List of Symbols and Abbreviations ................................................................................ xix
List of Appendices ........................................................................................................ xxii
CHAPTER 1: INTRODUCTION .................................................................................. 1
1.1 General ..................................................................................................................... 1
1.2 The Role of Nanotechnology in Optical Sensor ...................................................... 2
1.3 Problem Statement ................................................................................................... 4
1.4 Hypothesis ............................................................................................................... 5
1.5 Objectives of the Study ............................................................................................ 6
1.6 Limitation of the Study ............................................................................................ 6
1.7 Organization of the Thesis ....................................................................................... 7
CHAPTER 2: LITERATURE REVIEW ...................................................................... 9
2.1 Introduction.............................................................................................................. 9
2.2 Optical Fiber .......................................................................................................... 10
2.3 Plastic Optical Fiber (POF) ................................................................................... 13
2.3.1 Optical Properties of POF ........................................................................ 14
2.3.2 Mechanical Properties of POF.................................................................. 15
2.3.3 Thermal Properties of POF ....................................................................... 17
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2.3.4 Chemical Infiltration ................................................................................ 18
2.4 Optical Sensor Using Plastic Optical Fiber ........................................................... 19
2.4.1 Optical Loss .............................................................................................. 20
2.4.2 Interferometry-Based Sensors .................................................................. 22
2.4.3 OTDR, OFDR and Scattering................................................................... 24
2.4.4 Fiber Bragg Grating (FBG) ...................................................................... 25
2.5 Zinc Oxide Nanorod-Structure .............................................................................. 25
2.6 Hydrothermal Synthesis Method of Zinc Oxide Nanostructure ............................ 29
2.7 Zinc Oxide in Global Applications ........................................................................ 34
2.8 Light Scattering and Side Coupling ....................................................................... 37
2.9 Recent Research on Temperature and Gas Sensing Using Optical Fiber .............. 46
CHAPTER 3: OPTIMIZATION OF ZINC OXIDE NANOROD COATINGS ON
LARGE CORE PLASTIC OPTICAL FIBER THROUGH HYDROTHERMAL
GROWTH……… .......................................................................................................... 50
3.1 Introduction............................................................................................................ 50
3.2 Optimization parameters for the hydrothermal method......................................... 51
3.3 ZnO Nanorods through Hydrothermal Growth ..................................................... 52
3.3.1 Fiber Preparation ...................................................................................... 53
3.3.2 Seeding Process ........................................................................................ 54
3.3.3 ZnO Nanorod Growth Process ................................................................. 59
3.4 Optimization of ZnO Nanorod Growth on POF .................................................... 60
3.4.1 Spiral Patterned Growth of Zno Nanorods on POF Using the Optimized
Growth Duration. ...................................................................................... 69
3.5 Optimization of Seeding Methods to Improve the Growth of Zno Nanorods on
POF…. ................................................................................................................... 72
3.6 Summary ................................................................................................................ 76
x
CHAPTER 4: CHARACTERIZATION OF LIGHT SIDE COUPLING TOWARDS
MULTIPLE OPTICAL CHANNEL AND OPTIMIZATION OF SPIRAL
PATTERNED WIDTH OF ZINC OXIDE NANOROD COATING FOR OPTIMAL
SIDE COUPLING ......................................................................................................... 78
4.1 Introduction............................................................................................................ 78
4.2 Mechanism of Light Scattering by ZnO Nanorod ................................................. 80
4.3 Mechanism of Light Scattering For Unpatterned and Spiral Patterned ZnO Nanorod
Layers and For the Multi-Channel Optical Fiber .................................................. 81
4.4 Experimental Characterization of Multi-Channel Optical Fiber towards Light Side
Coupling ................................................................................................................ 84
4.5 Modeling of Coupling Efficiency for Spiral Patterned and Unpatterned Coating by
Varying the Width of the Coated Region towards Light Side Coupling ............... 86
4.6 Theoretical Optimization of Spiral Patterned Width for Optimal Side Coupling . 93
4.7 Experimental Optimization of Spiral Patterned Width for Optimal Side
Coupling….. .......................................................................................................... 99
4.8 Summary .............................................................................................................. 102
CHAPTER 5: APPLIED LIGHT SIDE COUPLING WITH OPTIMIZED SPIRAL
PATTERNED ZINC OXIDE NANOROD COATINGS FOR TEMPERATURE
AND MULTIPLE OPTICAL CHANNEL ALCOHOL VAPOR SENSING ........ 104
5.1 Introduction.......................................................................................................... 104
5.2 SEM images of Optimized Spiral Patterned Zinc Oxide Nanorod Coatings for
Sensing Applications ........................................................................................... 105
5.3 Applied Light Side Coupling With Optimized Spiral Patterned Zinc Oxide Nanorod
Coatings for Temperature Sensing ...................................................................... 107
5.3.1 Experiment of Temperature Sensing ...................................................... 109
5.3.2 Results and Discussions ......................................................................... 110
xi
5.4 Applied Light Side Coupling With Optimized Spiral Patterned Zinc Oxide Nanorod
Coatings for Multiple Optical Channel Alcohol Vapor Sensing. ........................ 113
5.4.1 Experiment of Multiple Optical Channel for Alcohol Vapor Sensing ... 114
5.4.2 Results and Discussions ......................................................................... 117
5.5 Summary .............................................................................................................. 125
CHAPTER 6: CONCLUSION AND FUTURE WORK ......................................... 127
6.1 Conclusion ........................................................................................................... 127
6.2 Future work .......................................................................................................... 129
References ..................................................................................................................... 130
List of Publications, Papers Presented And patents ...................................................... 147
Appendix ....................................................................................................................... 151
xii
LIST OF FIGURES
Figure 2.1 Some applications of optical fiber sensors in industry (Rajan, 2015) ........... 10
Figure 2.2 The parts of optical fiber................................................................................ 11
Figure 2.3 Phenomena of light refraction and reflection inside optical fiber. ............... 12
Figure 2.4 (a) Multimode and (b) single mode ............................................................... 13
Figure 2.5 Attenuation loss of common POF as a function of wavelength (Zubia & Arrue,
2001) ............................................................................................................................... 15
Figure 2.6 The measurement of true stress versus strain for single-mode PMMA-doped
core (Kiesel et al., 2007) ................................................................................................. 16
Figure 2.7 The results of Dynamic Young’s modulus for PMMA MPOF, step index POF
and silica SMF28 (Stefani, Andresen, Yuan, & Bang, 2012) ......................................... 17
Figure 2.8 The responses of two POF FBG sensors with RH varied from 80% to 70% at
a temperature of 25°C (W. Zhang, Webb, & Peng, 2012) .............................................. 19
Figure 2.9 Schematic of POF-based accelerometer. The inset shows a magnification of
the fiber gap region. (Antunes et al., 2013)..................................................................... 21
Figure 2.10 (a) Schematic of VCO interrogator used for time-of-flight measurements and
(b) Image of upper side of aircraft flap with POF adhered to surface and prototype
instrumentation. (Gomez et al., 2009) ............................................................................. 23
Figure 2.11 Wurtzite structure of ZnO ............................................................................ 26
Figure 2.12 3D ZnO structures (a) nanorods (Dedova et al., 2007), (b) nanowires (Shan
et al., 2008), (c) nanoflowers (Miles et al., 2015) and (d) snowflakes (Jing et al., 2012).
......................................................................................................................................... 28
Figure 2.13 Growth morphologies of ZnO 1D nanostructures. (Z. L. Wang, 2004) ...... 29
Figure 2.14 SEM images of ZnO nanostructures grown with different aqueous solutions
of pH value of (a) 1.8, (b) 4.6, (c) 6.6, (d) 9.1, (e) 10.8 and (f) 11.2. (Amin et al., 2011)
......................................................................................................................................... 30
Figure 2.15 SEM images of ZnO nanorods on Si substrate with different precursor
concentrations of the growth aqueous solution (a) at 25 mM, (b) 50 mM, (c) 100 mM, (d)
300 mM. (Amin et al., 2011)........................................................................................... 31
Figure 2.16 SEM image of ZnO nanorods grown using Zn(NO3)2 and HMT (Baruah &
Dutta, 2008) .................................................................................................................... 34
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Figure 2.17 Various applications of ZnO (Kołodziejczak-Radzimska & Jesionowski,
2014) ............................................................................................................................... 35
Figure 2.18 Illustration of light scattering from one ZnO nanorod................................. 38
Figure 2.19 Schematic representation of two possible configurations of side coupling to
cladding modes with guided and leakage intensity responses of light paths in the side
coupling configuration (H Fallah et al., 2013) ................................................................ 39
Figure 2.20 Optical characterization setup for the light side coupling (Hoorieh Fallah et
al., 2014).......................................................................................................................... 41
Figure 2.21 The coupling intensity of different concentration of zinc acetate for ZnO
nanorods grown on wet etched optical fiber (H Fallah et al., 2013) ............................... 42
Figure 2.22 The measurement of coupling intensity (y- left axis), the average scattering
coefficient (y- right axis) and versus the concentration of zinc acetate (H Fallah et al.,
2013) ............................................................................................................................... 43
Figure 2.23 The coupling intensity for cladding mode and core mode at different
excitation location on the optical fiber (Hoorieh Fallah et al., 2014) ............................. 44
Figure 2.24 Optical nephelometer setup for testing scattering properties of ZnO grown on
glass substrate (Hoorieh Fallah et al., 2014) ................................................................... 45
Figure 2.25 The measurement of (a) normalized angular power spectra and (b) density,
ρa respect to the concentrations of zinc acetate used for preparing the ZnO seed layer on
glass substrate (Bora et al., 2014) ................................................................................... 46
Figure 2.26 The setup of liquid temperature sensor. (S. Kumar & Swaminathan, 2016)
......................................................................................................................................... 47
Figure 2.27 Optical fiber sensor based on SPR for chemical sensing. (Michel et al., 2016)
......................................................................................................................................... 49
Figure 3.1 Optimization parameters for the ZnO nanorods growth on POF using
hydrothermal method ...................................................................................................... 51
Figure 3.2 General procedures of ZnO nanorods synthesis using hydrothermal ............ 52
Figure 3.3 The process of fiber preparation (a) POF with black jacket (b) POF is exposed
with length of 10 cm for ZnO coating (c) 3M water proof tape is used to create spiral
template (d) manually creating spiral pattern on POF and (e) POF with spiral template
before the synthesis process. ........................................................................................... 53
Figure 3.4 Procedures of seeding process on POF .......................................................... 54
xiv
Figure 3.5 Process of 1mM ZnO nanoparticle solution preparation ............................... 55
Figure 3.6 Preparation of the pH controlled solution using NaOH................................. 55
Figure 3.7 Alkaline process of ZnO nanoparticles solution by NaOH ........................... 56
Figure 3.8 (a) Tween 80 preparation and (b) POF surface treatment ............................. 57
Figure 3.9 Dip and Dry method in seeding process ........................................................ 57
Figure 3.10 Drop and Dry method in seeding process .................................................... 58
Figure 3.11 Slow stirring method in seeding process ..................................................... 59
Figure 3.12 The process of ZnO nanorod growth on POFs ............................................ 60
Figure 3.13 Flow of the optimization process of ZnO nanorod growth on POF through
hydrothermal ................................................................................................................... 61
Figure 3.14 Low magnification SEM images of the POF coated with ZnO nanorods with
(a) surface treatment (Tween 80) and (b) without surface treatment .............................. 62
Figure 3.15 Vpp characterisation setup to measure the side coupling of ZnO nanorods for
unpatterned and spiral patterned POFs ........................................................................... 62
Figure 3.16 The modulated LED red light source used in the optical characterization .. 63
Figure 3.17 The exposed regions on the unpatterned type of POF (a) interface, (b) middle
and (c) tip ........................................................................................................................ 64
Figure 3.18 Average Vpp for 15 and 20 hours growth time with backscattering effects. 65
Figure 3.19 ZnO nanorods grown on POF (a) 15 hours (b) 20 hours ............................. 65
Figure 3.20 Backscattering effect is eliminated at interface regions after reducing the
growth time to 8, 10, and 12 hours.................................................................................. 66
Figure 3.21 Average Vpp at interfacial area for all growth times .................................... 67
Figure 3.22 The SEM images for growth durations: 8 hours (top left), 10 hours (top right)
and 12 hours (bottom) ..................................................................................................... 68
Figure 3.23 The specified regions on the spiral patterned POF for optical characterization.
......................................................................................................................................... 69
Figure 3.24 Average Vpp for the spiral patterned growth for 12 h which has more than
one interface and ZnO regions. The inset shows the regions covered by the aperture when
characterisation the structured and unstructured ZnO growth on POF ........................... 70
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Figure 3.25 (a) 13 kX SEM image of ZnO spiral patterned growth after synthesis (b) 25.0
kX SEM image of the nanorods and Inset: The ZnO nanorods at 60.0 kx magnification
for 12 hours ..................................................................................................................... 71
Figure 3.26 EDX spectrum of ZnO nanorods showing zinc and oxygen peaks ............. 72
Figure 3.27 The growth of ZnO nanorods using the drop and dry method (a) 5 kX SEM
image of spiral patterned growth on POF and (b) the morphology of ZnO nanorods at a
high magnification .......................................................................................................... 73
Figure 3.28 Schematic diagram showing the possible agglomeration of ZnO nanoparticles
upon evaporation of the solvent (a) thin layer of ZnO nanoparticles (b) agglomerated
clumps of ZnO nanoparticles with various orientations and (c) ZnO nanorods grow from
the seed crystallites in the different directions ................................................................ 74
Figure 3.29 The continuous slow stirring process .......................................................... 75
Figure 3.30 The growth of ZnO nanorods using the continuous slow stirring method (a)
5 kX SEM image of spiral patterned growth on POF and (b) the morphology of ZnO
nanorods at 10.0 kX ........................................................................................................ 76
Figure 4.1 Mechanism of light scatters into POF by ZnO nanorods at angle larger than
critical angle, θc ............................................................................................................... 80
Figure 4.2 Schematic diagram of light scattering for (a) Unpatterned growth of ZnO
nanorods with the coupling light (b) Spiral patterned growth of ZnO nanorods with more
interface and ZnO regions with the coupling light (c) Spiral patterned growth of ZnO
nanorods for a multi-channel excitation .......................................................................... 82
Figure 4.3 Spectral analysis setup to determine wavelength coupling maxima ............. 84
Figure 4.4 Transmittance of the visible white light spectrum ......................................... 85
Figure 4.5 Spectrum for near infrared (850 and 980 nm) for spiral patterned and
unpatterned growth.......................................................................................................... 86
Figure 4.6 (a) Spirally patterned coating of ZnO nanorods on POF and (b) unpatterned
coating of ZnO nanorods on POF with a visible light source ......................................... 87
Figure 4.7 Definition of polar coordinate ....................................................................... 88
Figure 4.8 (a) Dividing the POF coated with ZnO nanorods into discrete sections of width
Δz for both coating schemes (b) Optical Intensity components around a segment h of the
ZnO coated POF .............................................................................................................. 90
Figure 4.9 The scheme of light propagation for unpatterned continuous and spiral
patterned coating where ZnO coating region was fixed to 3 segments (3 mm) .............. 92
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Figure 4.10 The normalized coupling output for unpatterned and spiral patterned coating
by varying the width of ZnO nanorod coating on POFs ................................................. 94
Figure 4.11 (a) Spirally patterned coating of ZnO nanorods on POF and (b) unpatterned
coating of ZnO nanorods on POF with a laser light source (Gaussian beam) ................ 96
Figure 4.12 The coupling efficiencies for spiral patterned and unpatterned coating excited
by a laser light source ...................................................................................................... 97
Figure 4.13 Spiral patterned coating of ZnO nanorods (b) unpatterned coating of ZnO
nanorods with varied uncoated spacing .......................................................................... 98
Figure 4.14 The effects on coupling efficiency by varying the uncoated region ............ 99
Figure 4.15 Coating schemes (a) unpatterned POFs (b) Spiral patterned POFs ........... 100
Figure 4.16 Optimization setup to measure the output voltage for unpatterned and spiral
patterned ZnO nanorods ................................................................................................ 101
Figure 4.17 The experimental result of spiral patterned and unpatterned coating for 3, 5,
7 and 100 mm ................................................................................................................ 102
Figure 5.1 (a) The optimized spiral patterned ZnO nanorod coatings, (b) the perpendicular
growth of ZnO nanorods on POF at low magnification (c) at high magnification (d) hight
and diameter of the ZnO nanorod and (e) ZnO continuous coating on unpatterned POF
....................................................................................................................................... 106
Figure 5.2 Experimental setup for the proposed temperature sensor towards light side
coupling ......................................................................................................................... 110
Figure 5.3 The response of spiral patterned coating and unpatterned coating in
temperature sensing. ...................................................................................................... 111
Figure 5.4 The temperature sensing mechanism (a) before light illumination (b) upon light
illumination and (c) aluminum rod in close proximity to ZnO nanorods coating layer.
....................................................................................................................................... 112
Figure 5.5 The sensitivity of spiral patterned and unpatterned coating in temperature
sensing ........................................................................................................................... 113
Figure 5.6 Experimental setup to validate the alcohol sensing activities of spiral patterned
POF as multiple optical channels .................................................................................. 115
Figure 5.7 Spectroscopy responses of multiple optical channels sensor in blue, green, and
red wavelengths for (a) methanol, (b) ethanol and (c) isopropanol .............................. 118
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Figure 5.8 Schematic diagram of the alcohol sensing mechanism activated using visible
white light illumination (a) in air at room temperature (b) with visible white light and (c)
with methanol exposure ................................................................................................ 120
Figure 5.9 The responses of multiple optical channels sensor in channel (a) blue (b) green
and (c) red ..................................................................................................................... 121
Figure 5.10 The relative intensity modulation (RIM) of multiple optical channels sensor
exposed to ethanol, methanol and isopropanol vapors.................................................. 123
Figure 5.11 The validation of the multiple optical channels sensor for (a) channel blue/
channel red (b) channel green/ channel red................................................................... 124
xviii
LIST OF TABLES
Table 2.1 Properties of Zinc Oxide ................................................................................. 27
Table 4.1 Differences of normalized coupling output, ΔI between spiral patterned and
unpatterned POFs for different widths of ZnO coating from 0 to 7 mm ........................ 95
xix
LIST OF SYMBOLS AND ABBREVIATIONS
˚C : Degree Celsius
µm : Micrometer
nm : Nanometer
cm : Centimeter
g : Gram
mM : Mili mole
C2H5OH : Ethanol
CH3OH : Methanol
C3H8O : Isopropanol
C6H12N4 : Hexamethylenetetramine
HCL : Hydrochloric acid
CO : Carbon monoxide
O2 : Oxygen
CO2 : Carbondioxide
H2O : Water
NaOH : Sodium hydroxide
ZAH : Zinc acetate hydrate
Zn(CH3COO)2 : Zinc acetate
Zn(NO2)3 : Zinc Nitrate hexahydrate
Zn(OH)2 : Zinc hydroxide
ZnCl2 : Zinc chloride
ZnO : Zinc oxide
Zn2+ : Zinc ions
O2- : Oxygen ions
xx
OH- : Hydroxyl ions
dB/km : Decibels/kilometer
Vpp : Peak-to-peak Voltage
σ : Beam waist
𝑟 : Distance from the center of the beam
Csc : Scattering cross section
a : Rods density
: Portion of Scattered Light
θinc : Incident angle
θc : Critical angle
Δz : Width of Segment
z. : Coupling coefficient
Ip : Coupling output of spiral pattern
Iup : Coupling output of unpatterned
n : Refractive index
ΔI : Normalized coupling output
: Azimuthal angle
HMT : Hexamethylenetetramine
MMF : Multimode Fiber
SOF : Silica Optical Fiber
OFSs : Optical Fiber Sensors
POF : Plastic Optical Fiber
AI : Artificial intelligence
PMMA : Polymehyl Methacrylate
xxi
SEM : Scanning Electron Microscope
LED : Light Emitting Diode
FBG : Fiber Bragg Grating
DI : Deionized
DC : Direct Current
1D : One Dimensional
2D : Two Dimensional
3D : Three Dimensional
ZAH : Zinc Acetate Hydrate
EDX : Energy-dispersive X-ray
SPR : Space Plasmon Resonance
RH : Relative Humidity
CTOP : Specialty Amorphous Fluorinated Polymer
DMA : Dynamic Mechanical Analysis
MPOF : Multimode Plastic Optical Fiber
OTDR : Optical Time-Domain Reflectometry
OFDR Optical Frequency-Domain Reflectometry
VCO : Voltage-Controlled Oscillator
RI : Refractive Index
IR : Infrared
VZn : Zinc Vacancies
ca. : Around, about or approximately
RIM : Relative Intensity Modulation
GOF : Glass Optical Fiber
xxii
LIST OF APPENDICES
Publications and Papers Presented
Patent Filing Reports
1
CHAPTER 1: INTRODUCTION
1.1 General
Historically, the early research on optical fiber sensors (OFSs) was started in the
70s and related to medical instrument that was such as a fiber-optic endoscope consisting
of a bundle of flexible glass fibres able to coherently transmit an image (Edmonson,
1991). Nowadays, various approaches and technologies have been developed to gain
attention in sensing applications. Optical sensors using fiber optics definitely provide
reliable solutions in many fields since optical fibers can measure physical properties such
as strain (Ohno, Naruse, Kihara, & Shimada, 2001), displacement (Rahman, Harun,
Yasin, & Ahmad, 2012), temperature (Tyler et al., 2009), pressure (W. Wang, Wu, Tian,
Niezrecki, & Wang, 2010), velocity (Weng et al., 2006) and magnetism (Lv, Zhao, Wang,
& Wang, 2014). Every year, exploring the potentials of OFSs keep receiving high interest
because optical fibers offer well known advantages such as immunity to electrical and
magnetic fields, low attenuation, wide transmission bandwidth, small physical size and
weight, increased flexibility, analog and digital transmission, electrical insulation,
immunity to electromagnetic interference and interception and receiver sensitivity.
Beside these properties, FOSs also hold enormous potential for the use in chemical
applications due to the high sensitivity and slightly invasive technique (Mescia &
Prudenzano, 2013) which is important in monitoring environmental pollution, mainly if
FOSs are applied in radiation zone.
This thesis is concerned with the development of a simple and cost effective
system based on light scattering from zinc oxide (ZnO) nanorods grown in spiral pattern
on plastic optical fiber (POF) for temperature and alcohol vapors sensing applications.
The performance of the system is investigated based on the simulation and experimental
2
results. Artificial intelligence (AI) is suggested as an efficient technique for improving
the capability of the sensor system.
1.2 The Role of Nanotechnology in Optical Sensor
Recently, interest in integrating OFSs with nanotechnology anonymously
received global demands to increase sensitivity, repeatability, selectivity and stability of
their performances. Nanotechnology literally means the ability to manipulate individual
atoms and molecules to produce nanostructured materials and submicron objects that has
applications in the real world. According to National Science Foundation and National
Nanotechnology Initiative (NNI) (H. Chen et al., 2013), nanotechnology involves the
production and application of physical, chemical and biological systems at scales ranging
from individual atoms or molecules to about 100 nm, as well as the integration of the
resulting nanostructures into larger systems with fundamentally new properties and
functions because of their small structure. Nano-systems include micro/ nano-
electromechanical systems (MEMS/NEMS), micro-mechatronics, optoelectronics,
micro-fluidics and systems integration. These systems can sense, control and activate on
the micro/nano-scale and can function individually or in arrays to generate effects on the
macro-scale. Nanotechnology plays an important role in fabrication of sensors. Its usage
leads to new findings for the mechanism of reactions as well as fabrication of new types
of sensors.
Zinc oxide has gained substantial interest in the research community in part
because of its versatile wide-bandgap (3.37eV) semiconductor material that has
contributed to the development of numerous applications over the past few years.
Depending on its doping condition, ZnO can be conductive (including n-type and p-type
conductivity), semi-conductive, insulating, transparent and show piezoelectric behavior,
room temperature ferromagnetism, and huge magneto-optic and chemical sensing
3
properties (Kołodziejczak-Radzimska & Jesionowski, 2014). The paramount importance
is the transparency of ZnO to visible light that is in part responsible for exploring this
material for optoelectronics applications (Janotti & Van de Walle, 2009; Pauporté &
Lincot, 2000; Shinde, Shinde, Bhosale, & Rajpure, 2008; Xiang et al., 2007), biosensors
(Chang et al., 2010; T. Kong et al., 2009; S. A. Kumar & Chen, 2008), resonators (Cao et
al., 1998), medical devices (Rasmussen, Martinez, Louka, & Wingett, 2010), imaging
(Zvyagin et al., 2008) and wireless communication (J. Chen, Zeng, Li, Niu, & Pan, 2005).
As a rule, the optical signal in gas sensor arises from the interaction of gas
molecules with an incident electromagnetic radiation, which can take place at all
frequency and wavelength ranges. Every gas has specific properties and therefore has
specific interaction with electromagnetic radiation. This means that the results of these
interactions can be used for gas molecule identification. It was found that various methods
can be used for gas analysis (Sberveglieri, 2012). However, absorption spectroscopy is
still one of the most commonly used methods in optical gas sensing (Kraft, 2006). It has
been established that for many applications and absorption spectroscopic detection is a
reliable method of detecting various gases. Recently, the method has been widely used
by coating optical fiber with ZnO for detection of gases such as oxygen, O2 (Vanheusden,
Seager, Warren, Tallant, & Voigt, 1996), carbon dioxide, CO2 (Samarasekara, Yapa,
Kumara, & Perera, 2007), ammonia, NH3 (Aslam et al., 1999) and methane, CH4
(Bhattacharyya, Basu, Saha, & Basu, 2007).
Generally, temperature measurement systems are very important for many
industries and according to World Health Organization (WHO), the impact of climate and
temperature on health has been receiving increased attention in recent years. Accurate
and continuous temperature monitoring is a critical task for a wide variety of industries.
Controlling temperature levels is needed to eliminate harmful bacteria in cooking
(McWilliams & Lamb, 1994), cooling (Schmidt & Notohardjono, 2002), storing
4
(Seaman, 1997), shipping, displaying and production (Sugaya et al., 2002). From food
processing to medical applications, even a single degree can affect the quality of products,
reaction rate of molecules and health. Due to these demands, research on temperature
sensors continues to grow rapidly in order to improve the quality of life.
1.3 Problem Statement
Over the years, a bulk of the work has been done on the synthesis of ZnO nanorods
on flat surface such as metal, plastic and glass substrates (X. Wang, Summers, & Wang,
2004). The growth of ZnO on these flat surfaces promises a high guarantee for easily
controlling the morphological parameters such as alignment, density and uniformity. In
addition, the synthesis of ZnO nanorods on these flat surfaces typically promotes
extensive growth using hydrothermal method which involves a simple aqueous solution
at temperature below the boiling point of water compare with gas phase synthesis
(Ayouchi, Martin, Leinen, & Ramos-Barrado, 2003). Various ZnO nanorod structures
have been synthesized on these flat surfaces such as 1D nanorods (S. Xu & Wang, 2011),
2D nanoplate (Giri et al., 2015) and 3D nanoflowers (Wahab, Kim, Mishra, Yun, & Shin,
2010). However, patterned growth of ZnO nanorods on cylindrical surfaces with small
diameter such as optical fiber still remains an issue for optical sensing applications.
Commonly, unpatterned growth is preferable to do due to less time consumption and
reducible complexity for fabrication. Hence, there are many research reports about the
unpatterned growth of ZnO nanorods on POF for sensing applications (Baruah, K Pal, &
Dutta, 2012; Batumalay et al., 2014) but the backscattering of light occurs due to the high
density of ZnO nanorods presents all over the exposed core. Although the unpatterned
growth of ZnO nanorods enhances optical guidance in optical fiber, it is also responsible
for light leakage due to the very same scattering property (Hoorieh Fallah, Harun,
Mohammed, & Dutta, 2014). Consequently, these two situations have resulted low
5
coupling power which is undesirable in sensing applications. This open up the
possibilities in exploring new approach to increase the coupling power and minimize
backscattering from ZnO nanorods coating.
The work behind this research was initiated from the need to develop a low-cost,
high sensitivity and an uncomplicated sensor system. Generally, most optical sensing
applications are operated with laser light source by launching light from one end of the
optical fiber and output signal is collected from other end (Aneesh & Khijwania, 2011).
This is surely expensive and needs other mechanical supports to align properly the laser
beam into the fiber. Applying laser source onto ZnO nanorods coating to utilize scattering
of light into POF provided also less sensitivity caused by the inequality of beam structure
which has different distribution of intensity along the ZnO coating (Dickey, Weichman,
& Shagam, 2000) and the laser beam only focuses on specific coating area instead of
entire coating area. With this background and abstraction, a research work is needed to
provide another alternative of light source towards light side coupling applications. This
research work certainly can bring contribution to many fields such as in optoelectronics,
telecommunication and material engineering.
1.4 Hypothesis
1. ZnO crystal is transparent in the visible wavelength range and acts as a
waveguide for light.
2. Higher coupling of optical power for the patterned coating on POF than
the unpatterned coating.
3. Spiral pattern of ZnO nanorods showing a uniform decay and providing
higher coupling power.
4. Simple and efficient device as an optical transducer for sensing
applications.
6
1.5 Objectives of the Study
The main objective of this research study is to investigate the possibility of the
use of a simple and inexpensive sensing device towards light side coupling using plastic
optical fiber. The following sub objectives have to be met:
1. To optimize the synthesis process of ZnO nanorods growth on POF using
hydrothermal method.
2. To fabricate spiral patterned ZnO nanorods coating on POF using
hydrothermal method.
3. To optically characterize and optimize the spiral patterned ZnO nanorods
coating on POF using light side coupling method.
4. To develop a new theoretical model in analysing the width of spiral
patterned ZnO nanorod coating on POF for achieving maximum coupling.
5. To validate experimentally the sensing of spiral patterned ZnO nanorods
coating on POF.
1.6 Limitation of the Study
As the growth of ZnO nanorods were performed on a cylindrical surface of an optical
fiber the alignment as well as the distribution of the nanorods was an issue and the fibers
needed to be handled carefully. Also the fragile nature of the fibers made it prone to breakage
during fiber preparation and structuring. Achieving a uniform structuring of the fibers using
the plastic tape was difficult and had to be done accurately. The hydrothermal process being
a low temperature process, the crystal growth rate was low and the duration of growth took
more than ten hours to obtain the optimized ZnO nanorod morphology. Due to low POF
thermal specification (< 100 ˚C), temperature effects on physical POF should be considered
in synthesis process and sensing applications.
7
1.7 Organization of the Thesis
The thesis is organized into six chapters, each of which is then subdivided into
sections and subsections. Chapter 1 presents an introduction of this work comprising the
background study, statement of the problem, hypothesis, objectives of the research and
limitations of the study. In Chapter 2, the theoretical review of related research including
a thorough study on the optical fiber technology, its types and designs and the
implementation of optical fiber as an optical sensor will be presented. The chapter will
also discuss the structures of ZnO nanorod and the hydrothermal growth process. The
global applications of ZnO nanorods in various fields is also given. A detailed study on
the previous work of light side coupling into an optical fiber and its potential application
as an optical sensor will be presented. The last part of this chapter gives an overview of
recent research on temperature and gas sensing using optical fiber.
Chapter 3 will explain in detail the two main procedures in chemically growing
ZnO nanorods on POF and the implementation of it is presented by describing the
materials and methods involved. The physical and optical characterization utilized for
analyzing the structure of the ZnO nanorods and for optimizing the growth duration and
seeding method for maximum side coupling is also presented in this chapter. Chapter 4 will
discuss the characterization results of light side coupling for spiral patterned and
unpatterned coatings using spectra analysis. Second part will present a new theoretical
model of light side coupling to analyze the width of spiral patterned ZnO nanorod coating
on POF for maximum side coupling. An experimental validation was performed using
light side coupling method for different width of spiral ZnO nanorods coating and the
results are compared to the modelling in order to optimize the width of spiral patterned
ZnO nanorod coating on POF.
8
Chapter 5 will discuss and analyze two sensing applications of spiral patterned ZnO
nanorods coating on POF using light side coupling. First, temperature sensing was carried
out by varying temperature from 20 ˚C to 100 ˚C. Second, the sensor probe was used as
multiple optical channel for alcohol vapors sensing in visible wavelength. The
performances of the optical sensor were analyzed in three particular channels: blue (450
– 495 nm), green (495 – 570 nm) and red (620 – 750 nm). The results are presented in
graphs to compare the relative intensity modulation (RIM) of the sensing. Finally, in
Chapter 6 the conclusions are drawn and the future works to improve the proposed
technique are suggested.
9
CHAPTER 2: LITERATURE REVIEW
2.1 Introduction
In the past several decades since the invention of the laser in 1960 (Maiman, 1960)
and the development of modern low-loss optical fibers in 1966, optical fiber technology
has made a transition from the experimental stage to practical applications (K. Grattan &
Sun, 2000) . The main focus of the development of optical fiber has always been on
telecommunications, but the early 1970s saw some of the first experiments on low-loss
optical fibers being used for sensing purposes (Sathitanon & Pullteap, 2007). The field of
optical fiber sensing has continued to progress and has developed enormously since that
time. Magnetic (Dandridge, Tveten, Sigel, West, & Giallorenzi, 1980), pressure
(Budiansky, Drucker, Kino, & Rice, 1979; Hocker, 1979; Lagakos & Bucaro, 1981),
temperature (Yariv & Winsor, 1980), acceleration (Arditty, Papuchon, & Puech, 1981),
displacement, fluid level, current (Dandridge, Tveten, & Giallorenzi, 1981; Tangonan,
Persechini, Morrison, & Wysocki, 1980), and strain optical fiber sensors (Giallorenzi et
al., 1982) were among the first few types extensively investigated and explored for
sensing and measurement. For instance, distributed fiber-optic sensors have now been
installed in bridges and dams as shown in Figure 2.1 to monitor the performance and
structural damage of these facilities. OFSs are used to monitor the conditions within oil
wells and pipelines, railways, wings of airplanes and wind turbines. Compared with other
types of sensors, optical fiber sensors exhibit a number of advantages, such as immunity
to electromagnetic interference, applicability in high-voltage or explosive environments,
a very wide operating temperature range, multiplexing capabilities, and chemical
passivity (B. Lee, 2003).
10
Figure 2.1 Some applications of optical fiber sensors in industry (Rajan, 2015)
2.2 Optical Fiber
Generally, an optical fiber consists of a core, cladding, buffer and jacket as
illustrated in Figure 2.2 (J.-R. Lee, Dhital, & Yoon, 2011). Light travels through a
material and it is called the core. The core is surrounded by a dark material called the
cladding which reflects back any light that escapes the core. Finally, a plastic coating
called the buffer around each cladding protects the fiber from wear and tear. Hundreds or
thousands of these fibers are placed together in one cable protected by an outside covering
called the jacket.
11
Figure 2.2 The parts of optical fiber
The core and cladding of the fiber is composed of highly pure solid glass. A protective
layer then surrounds the cladding. In most of the optical fibers this protective layer is made
up of multilayer composition. The protective layer can be comprised of two layers: a soft
inner layer that cushions the fiber and allows the coating to be stripped from the glass
mechanically and a harder outer layer that protects the fiber during handling and termination
process (Kao, 1983). From the context of optical waveguiding it is clear that there should be
a variation of refractive index inside the fiber between the core and the cladding. Therefore,
the core and cladding are made up of two slightly different materials which are transparent to
light over the operating wavelength. To achieve the phenomenon of total internal reflection
which is the driving principle behind the operation of optical fibers the core has a higher
refractive index than the cladding.
The application of both reflection and refraction is used in the operation of a fiber
optic cable. When light falls on a shiny or mirrored surface it bounces off while when it travels
from one medium to another having different thickness or density it bends. The bending
depends on the angle at which light strikes the surface. At certain angle whole of the light is
reflected in the original medium completely. This phenomenon is called total internal
reflection which is the principle of operation of optical fiber (Cherin, 1983; Hentschel, 1983;
Keiser, 2000). Figure 2.3 shows the critical angle which is the minimum angle required for
light to reflect in the first medium completely. For all those angles greater than the critical
12
angle light rays will be totally internally reflected in the first medium. Therefore, when a light
ray is sent into the fiber, it is sent at an angle towards the side of the fiber that will reflect.
Figure 2.3 Phenomena of light refraction and reflection inside optical fiber.
There are generally two types of optical fibers: single mode and multimode as
depicted in Figure 2.4 (Lacy, 1982). The multimode fibers have a larger core than the single
mode fibers and it allows hundreds of modes of light to propagate through it. The larger core
diameters of multimode fibers facilitate the use of lower cost optical transmitters such as light
emitting diode (LED) and connectors. The single mode fibers on the other hand has a much
smaller diameter than the multimode fibers and allows only one mode also called as the
fundamental mode to pass through it. Single mode fibers are designed to maintain spatial and
spectral integrity of each optical signal over longer distances, allowing more information to
be transmitted. Its tremendous information carrying capacity and low intrinsic loss have made
single mode fiber the ideal transmission medium for a multitude of applications (Jeunhomme,
1983). Single mode fiber is typically used for longer distance and higher bandwidth
applications. Multimode fiber is used primarily in systems with short transmission distances
(under 2km) such as premises communications, private data networks and parallel optic
applications.
13
Figure 2.4 (a) Multimode and (b) single mode
2.3 Plastic Optical Fiber (POF)
Commonly, OFSs are based on silica optical fibers (SOFs) due to their wide use
in telecommunication applications and the high availability of components, equipment
and optical fiber specifications. Nowadays, POFs have practically experienced a stream
in applications for short distance telecommunication systems. Many researchers have
noticed their unique properties for sensing of strain, temperature, humidity, gas etc.
Simultaneously, researchers are also developing POFs with new properties including
single-mode fibers (Woyessa, Fasano, et al., 2016) and micro-structured fibers (Cordeiro
et al., 2006). These unique properties have been utilized to expand the range of sensing
applications beyond those previously realized with SOF sensors. There are several great
advantages of POF over SOFs, including their large elastic and plastic strain deformation
capabilities (Kiesel, Peters, Hassan, & Kowalsky, 2007), negative thermal sensitivities
(Peters, 2010), high numerical aperture (Ishigure, Horibe, Nihei, & Koike, 1995) and
lower stiffness (Rothmaier, Luong, & Clemens, 2008). Typically, POF materials include
polymethyl methacrylate (PMMA), polystyrene (PS) and amorphous fluorinated polymer
(CYTOP) (Olaf Ziemann, Krauser, Zamzow, & Daum, 2008a). Commercially, POFs are
14
typically available in multimode at their operating wavelength due to the challenges in its
fabrication. As a result, these POFs are low cost and easier to cut and connects as
compared to single mode SOFs. In market, multimode POFs are available with different
cross-sectional index distributions, including both step index and gradient index
configurations. Moreover, a large variety of POF diameters are available which are larger
than conventional single-mode silica optical fibers. Due to the large core diameter
(0.25mm-1mm) connectivity issues also does not rise, reducing the total cost of the system.
2.3.1 Optical Properties of POF
Figure 2.5 presents the intrinsic attenuation loss of common POF as a function of
wavelength. In applications, multimode POF sensor are commonly operated in three
different wavelengths; the visible wavelength range (400–700 nm) where the intrinsic
attenuation is low, near 850 nm where common telecommunication and low-cost
components are available and the near-infrared range (above 1300 nm) where the
specialty amorphous fluorinated polymer (CTOP) has low attenuation. At all
wavelengths, the intrinsic attenuation of POF materials are significantly higher than that
of SOF.
15
Figure 2.5 Attenuation loss of common POF as a function of wavelength (Zubia &
Arrue, 2001)
2.3.2 Mechanical Properties of POF
The mechanical properties of POFs are highly influenced by the fiber
manufacturing process and dopant concentration that is used to increase the core index of
refraction (Bosc & Toinen, 1993; Jiang, Kuzyk, Ding, Johns, & Welker, 2002). Due to
the annealing process to remove internal residual stresses and anisotropy in polymer
during manufacturing process, the mechanical and thermal behavior of a specific POF is
critical to calibrate prior to its use as a sensor. The mechanical properties of POFs are also
dependent upon loading condition and affected by environmental conditions such as high
temperature and humidity (O Ziemann, Daum, Bräuer, Schlick, & Frank, 2000; Olaf
Ziemann, Krauser, Zamzow, & Daum, 2008b). Figure 2.6 shows the measurement of true
stress (MPa) versus strain (%) for a PMMA optical fiber. POF properties
characteristically fall in the ranges of initial elastic modulus of 1 to 5 GPa, yield strain of
16
1%–6% and ultimate strains of 6%–100% as compared to 1%–5% for SOFs (Hayashi,
Mizuno, & Nakamura, 2012).
Figure 2.6 The measurement of true stress versus strain for single-mode PMMA-doped
core (Kiesel et al., 2007)
It was reported that the yield strain and initial stiffness of the material are both a function
of the applied strain rate. Moreover, the material behavior is nonlinear beyond the yield
strain and therefore the loading history is also critical to predict hysteresis and cyclic
behavior of the material. When POFs were strained beyond their yield limit, plastic
deformation occured in the fiber, resulting in a residual deformation even when the fiber
was unloaded. For sensing applications, this residual deformation can be considered a
shape memory effect that can store the maximum strain information to be extracted later
(Hayashi et al., 2012). PMMA also has a lower density (1195 kg m−3) than silica (2200
kg m−3), reducing the weight of distributed optical fiber sensors (van Eijkelenborg et al.,
2001).
Second, dynamic mechanical analysis (DMA) was reported to reveal the
dependence of the material behavior on applied loading rates by applying cyclic loads to
17
the materials at different frequencies. Young’s modulus at different frequencies can be
obtained from DMA. Young’s modulus starting from 7 Hz was observed in different
frequency responses for a solid POF, a micro-structured POF and a SOF for comparison
as shown in Figure 2.7. As compared to the silica fiber, it was presented that the lower
Young’s modulus at low frequencies of the polymer materials and the start of a frequency-
dependent modulus at lower frequencies in the polymer materials.
Figure 2.7 The results of Dynamic Young’s modulus for PMMA MPOF, step index
POF and silica SMF28 (Stefani, Andresen, Yuan, & Bang, 2012)
2.3.3 Thermal Properties of POF
The temperature sensitivity of POF was defined to be the phase shift per unit
change in temperature per unit length of the POF. Typically, the thermal sensitivity of
bulk PMMA is -154.3 rad/m/k (Silva-López et al., 2005). As for SOF, the intrinsic
temperature response of a POF must be a known variable for temperature compensation
of strain measurements. For instance, strain temperature cross talk in a multimode POF
of 33 με/ ˚C (J. Huang et al., 2012). Recent research has also reported the coupling
18
between the response of the POF to the temperature and humidity. The coefficient of
thermal expansion (COT) of POF is approximately −1 × 10–4/ °C and SOF is 5 × 10–7/
°C. A report found that when relative humidity (RH) is accurately controlled as
temperature is varied, the COT is nonlinear with temperature (Z. F. Zhang & Tao, 2013).
2.3.4 Chemical Infiltration
The inherent performance of POFs to absorb moisture can also make them
sensitive to chemical infiltration. In a previous work, the effect of vinyl ester and epoxy
resins on the integrity were measured using signal transmission of perfluorinated POFs
embedded in these resin systems as sensing applications (Hamouda, Peters, & Seyam,
2012). The more investigation of the two resin systems, vinyl ester was penetrated into
the POF during curing of the resin, resulting a significant increase in backscattering level
in the POF and eventual signal transmission loss. Figure 2.8 shows the visible change in
the POF cross section before and after exposure to the vinyl ester resin.
19
Figure 2.8 The responses of two POF FBG sensors with RH varied from 80% to 70% at
a temperature of 25°C (W. Zhang, Webb, & Peng, 2012)
In contrast, the epoxy resin did not penetrate the POF during cure, giving no increase in
backscattering level. POF3 (135µm) shows a faster response more closely following the
humidity change.
2.4 Optical Sensor Using Plastic Optical Fiber
POFs provides a low cost and efficient medium to be used as sensors. The sensors
fabricated using POF has potential applications in the field of medicine, environment,
chemical and biological area. Conventional POFs are used to make sensors for measuring
distance, position, shape, color, brightness, opacity, density, turbidity etc. Sophisticated
sensors that are used for particle tracking are made possible with the development of
fluorescent POFs. The studies of POF based sensors were started in early 70s and they were
first implemented in medical and industrial field (Fischer, Haupt, Reinboth, & Just, 2015).
Wide area research has been going on already for the production of improved versions of
20
POF based sensors to be used in applications where the traditional systems cannot apply. Also
the entire operating principle is based on optical domain and there is absolute immunity to
electromagnetic and radio frequency interference. The sensors are grouped by the particular
sensing mechanisms that they use to convert the physical parameters into properties of
propagating lightwaves which includes optical loss, optical time-domain reflectometry
(OTDR) (Liehr, Lenke, et al., 2009; Liehr, Nöther, & Krebber, 2009; Liehr, Wendt, &
Krebber, 2010), optical frequency-domain reflectometers (OFDR), scattering and long period
grating (Banerjee et al., 2003; Peters, 2010). POF based sensors are frequently used in
structural health monitoring, medicine and in environmental applications (Witt, Schukar, &
Krebber, 2008; Yokota, Okada, & Yamaguchi, 2007).
2.4.1 Optical Loss
A simple and low cost POF-based sensor is typically based on the measurement
of optical power losses. Commercially, the cost of such sensor is low because multimode
POF is usually preferable and inexpensive light sources can be used compare to SOF.
Moreover, a simple photodetector is required to convert the optical fiber power
transmitted the optical fiber into a voltage output. Figure 2.9 shows an example of POF
accelerometer based on the transfer of lightwaves between two multimode POFs
(Antunes, Varum, & Andre, 2013).
21
Figure 2.9 Schematic of POF-based accelerometer. The inset shows a magnification of
the fiber gap region. (Antunes et al., 2013)
In the setup, one POF was fixed to the inertial frame and the other moved with the object
whose acceleration is to be measured. Acceleration of the object moved the POF mounted
on the cantilever beam in a direction parallel to the cross section, creating a coupling loss
into the sensor fiber. This accelerometer offers some advantages which are low cost, ease
of fabrication and small size. Therefore, the accelerometer provides the low resolution
inherent in the power coupling measurements.
The measurement of power losses can be implemented by creating bending losses
in the POF using the geometry of U-shaped POF sensor. In the experiment, the radius of
the curved portion is well controlled such that the bending losses are repeatable in the
POF. By changing the index of refraction of a fluid external to the POF, the fraction of
light coupled into the surrounding fluid is changed. Therefore, this low-cost sensors can
22
be applied as a liquid level sensor such as fuel level monitoring (Montero, Lallana, &
Vázquez, 2012). The advantage here is that the optical sensor does not create a spark
hazard near the fuel.
2.4.2 Interferometry-Based Sensors
The time of flight interferometer takes advantage of low cost and ease of use of
multimode POFs. Due to an incoherent interferometer configuration, it does not require
phase measurement. For some structural applications, it is sufficient to measure the
integrated strain along the POF. The time-of-flight measurement system provides
sufficient displacement resolution for a full-scale structure. In previous works, time-of-
flight measurements was applied to monitor the global displacement of a vibrating aircraft
wing flap (Durana et al., 2009; Gómez et al., 2008). Figure 2.10 shows a diagram of the
voltage-controlled oscillator (VCO)-driven interrogator along with a photograph of the
aircraft flap with the surface mounted sensor POF. The interrogator is entirely constructed
of low-cost telecommunication components. The system is also portable and durable and
has relatively low power requirements. The measurement displacement range is
determined by the oscillator modulation frequency and can be quite large compared to
other interrogation methods.
23
Figure 2.10 (a) Schematic of VCO interrogator used for time-of-flight measurements
and (b) Image of upper side of aircraft flap with POF adhered to surface and prototype
instrumentation. (Gomez et al., 2009)
24
2.4.3 OTDR, OFDR and Scattering
Recently, several researches have also demonstrated POF-based sensor by
exploiting the unique properties of POF. The nonlinear stress–strain behavior of POFs at
large strain values as shown in Figure 2.6 is encoded along the POF and can be measured
through the scattering or other loss profiles along the POF, for example, through OTDR
(Liehr, Lenke, et al., 2009). In the work, multimode POFs with large diameter is embedded
into geotextiles for the monitoring of geotechnical and masonry structures. The geotextiles were
embedded in a railway embankment for monitoring of soil displacements. The large core
diameter allowed easy connection to handling of the sensors at the construction site, while
the use of the standard POF itself as the sensor permitted monitoring of a large area at
low cost. In addition, the high ultimate strain of the PMMA allowed the POF to elongate
with the large soil deformations. The OTDR measurements were limited by the
attenuation and dispersion characteristics of the POF. Replacing the POF with a low loss,
graded-index perfluorinated POF (PF-GIPOF) significantly improved both the
measurement resolution and maximum fiber length, up to 500 m, as a result of the reduced
dispersion and attenuation in the fiber. The resolution and speed of such measurements
can also be enhanced by applying incoherent OFDR rather than OTDR (Liehr & Krebber,
2012). The authors demonstrated OFDR measurements along a POF at 2 kHz data
acquisition rates with high spatial resolution on the order of a few micrometers. The POF
signal was sensitive to large strain magnitudes; therefore, this technique was applied to
the measurement of seismic induced strains (up to 125%).
25
2.4.4 Fiber Bragg Grating (FBG)
FBG sensors are one of the most widely applied silica optical fiber sensors as they can
provide local sensing information and can be multiplexed in large numbers along a single optical
fiber. In addition, the fact that the sensing information is wavelength encoded means that it is
invariant to fluctuations in laser power and coupling losses. Based on this success, numerous
researchers have developed techniques to inscribe FBGs in POFs with the motivation to exploit
the large tuning range of POF FBGs as compared to those in silica optical fibers. One
demonstration that highlights the unique properties of FBGs in POFs is that of high-
sensitivity pressure measurements to detect small pressure changes. In 2013, FBG POF
was used as pressure sensor (Rajan, Liu, Luo, Ambikairajah, & Peng, 2013). The cladding
of the POF was etched to significantly reduce the cladding diameter. Combining the low
elastic modulus of the polymer and small cladding diameter produced an FBG with high
sensitivity to small axial loads on the POF. The authors then attached the POF to a vinyl
diaphragm to transfer the surrounding pressure to an axial force on the POF and
demonstrated a complete sensor with the extremely high pressure sensitivity of 1.32 pm
Pa−1 over the range of 0.1–5.0 kPa. Challenges still are present in the application of POF
FBG sensors. Some of these challenges come from the inherent properties of the POF,
including the low maximum temperature threshold and viscoelastic strain response
(Peters, 2010). In addition, thermal erasing of POF FBGs can occur when the grating is
exposed to thermal loads for extended periods of time.
2.5 Zinc Oxide Nanorod-Structure
ZnO possesses a wurtzite structure that lacks a center of symmetry and thus it
exhibits strong piezoelectric and pyroelectric properties (Baruah & Dutta, 2008). The
conductivity of ZnO can also be increased through doping. ZnO also finds its uses in the
fields of acoustic wave filters, photonic crystals, photodetectors, light emitting diodes,
26
photodiodes, gas sensors, optical modulator waveguides, solar cells and varistors (Yi,
Wang, & Park, 2005). The transparency of ZnO nanrods in the visible wavelength range
coupled with its wide bandgap (3.37eV) is suitable for optoelectronic applications. ZnO
crystal also possesses high exiton binding energy (60meV) which allows efficient
exitonic emission at room temperature. Because of its hardness and rigidity, ZnO plays a
very important role in ceramics industry while its less toxicity, biocompatibility and
biodegradability make ZnO a best material in biomedicine and pre-ecological systems
(Kołodziejczak-Radzimska & Jesionowski, 2014).The crystal of ZnO has a hexagonal
wurtzite structure with lattice parameters of a = 0.3296 and c = 0.52065 nm. The
piezoelectric and pyroelectric properties are inherent from the tetrahedral arrangement of
Zn2+ and O2- ions as shown in the Figure 2.11. The piezoelectric and pyroelectric
properties are the resultant of the tetrahedral coordination and the absence of inversion
symmetry respectively.
Figure 2.11 Wurtzite structure of ZnO
27
Crystallography is used to determine the plain of ZnO nanorod structure. ZnO
possesses polar and non-polar facets with the basal plane (0001) being the most common
polar facet. Oppositely charged ions of Zn2+ (0001) and O2- (0001̅) occupy the ends of the
plane forming a dipole moment resulting in the variance of energy. ZnO ± (0001) are
quite peculiar in the sense that they are atomically flat, stable and exhibit no
reconstruction. The non-polar facets include the (21̅1̅0) and (011̅0) which have lower
surface energy than (0001) (Z. L. Wang, 2004). Table 1 show different properties of bulk
ZnO.
Table 2.1 Properties of Zinc Oxide
Crystal structure Hexagonal wurtzite
Lattice constant a=3.264 A°, c=5.207 A°
Molecular weight 81.38
Density 5.67 g/cm3
Melting point 1975 °C (3587 F)
Heat of fusion 4,470 cal/ mole
Thermal Conductivity 25 W/mK at 20 °C
Thermal expansion coefficient 4.3x10-6/ K at 20 °C and 7.7 x10-6/ K at 600 °C
Cohesive energy 1.89 eV
Band gap at room temperature 3.37 eV
Refractive index 2.008
Electron and hole effective mass me*=0.28 m0 , mh*=0.59 m0
Debye temperature 370 K
Lattice energy 964 kcal/mole
Dielectric constant ε0 = 8.75, ε∞ = 3.75
Exciton binding energy 60 meV
Piezoelectric coefficient 12 pC/N
Pyroelectric constant 6.8 A/s/cm2/K x 1010
Solubility 1.6 mg/L (30oC)
Standard enthalpy of formation -348.0 kJ/mol
Standard molar entropy 43.9 J・K-1 mol-1
The growth structures of ZnO nanostructures are quite varied. ZnO can occur in
1D (one dimentional), 2D (two dimentional) and 3D (three dimentional) structures. 1D
structures consists of the largest group which includes nanorods, needles, helixes, springs
and rings (Banerjee et al., 2003). 2D structures include nanoplates/nanosheets and
28
nanopellets (Chiu et al., 2010). Examples of ZnO 3D structures include nanorods
(Dedova, Volobujeva, Klauson, Mere, & Krunks, 2007), nanowires (Shan, Liu, & Hark,
2008), nanoflowers (Miles, Cameron, & Mattia, 2015), snowflakes (Jing et al., 2012) as
shown in Figure 2.12.
Figure 2.12 3D ZnO structures (a) nanorods (Dedova et al., 2007), (b) nanowires (Shan
et al., 2008), (c) nanoflowers (Miles et al., 2015) and (d) snowflakes (Jing et al., 2012).
The growth rate of these variety of novel structures can be tuned along three fast growing
directions: < 21̅1̅0 > (± [1̅21̅0 ], ±[ 21̅1̅0 ], ±[1̅1̅20]); < 011̅0> ( ±[ 011̅0], ±[ 101̅0], and
±[0001]. The surface morphologies of these structures depends on the relative surface
activities of various growth facets and the kinetic parameters are controlled by the growth
conditions. Some of the typical growth morphologies of 1D structures are given in Figure
2.13.
29
Figure 2.13 Growth morphologies of ZnO 1D nanostructures. (Z. L. Wang, 2004)
2.6 Hydrothermal Synthesis Method of Zinc Oxide Nanostructure
Extensive research for synthesis of ZnO nanoparticles in alcoholic medium using
hydrothermal method has been reported and widely accepted (Baruah et al., 2012). The
alcoholic medium growth provides faster nucleation and growth as compared to water
(Koziol, Boskovic, & Yahya, 2011). The hydrothermal method does not require the use
of organic solvents or additional processing of the product (grinding and calcination)
which makes it a simple and environmentally friendly technique. This process has many
advantages including the possibility of carrying out the synthesis at low temperatures the
diverse shapes and dimensions of the resulting crystals depending on the composition of
the starting mixture and the process temperature and pressure, the high degree of
crystallinity of the product and the high purity of the material obtained (Polsongkram et
al., 2008).
The hydrothermal method of ZnO nanorod synthesis is a solution based method.
Several literatures exist where the aqueous synthesis of ZnO nanoparticles using Zinc
nitrate hexahydrate (Zn(NO2)3) is reported. (Amin et al., 2011). (Zn(NO2)3) acts as the
source of Zn2+ ions and the growth was carried out at temperature of about 100 to 150 ˚C.
30
pH also plays an important role in the growth of ZnO nanorods. In a previous work, with
pH< 11 zinc hydroxide precursors are dissolved partially and ZnO powder is nucleated
in a heterogeneous system while for pH>11 the Zinc hydroxide precursors are dissolved
and a clear solution is formed with the ZnO powder nucleated in a homogeneous system
(Amin et al., 2011). Figure 2.14 shows the effect of pH on the growth of ZnO
nanostructures. When pH was increased, the growth rate increased due to the increases of
hydroxyl ions (OH−) concentration which gives arise to ZnO particles in the solution.
Figure 2.14 SEM images of ZnO nanostructures grown with different aqueous solutions
of pH value of (a) 1.8, (b) 4.6, (c) 6.6, (d) 9.1, (e) 10.8 and (f) 11.2. (Amin et al., 2011)
It is well known that increasing or decreasing the concentration of the chemical
reactants will eventually influence the resultant products. The density, length and
diameter of the ZnO nanorods are varied with the concentration applied during the
synthesis. Figure 2.15 shows the effect of different precursor concentrations on ZnO
nanorods growth. A linear relation can be seen between the increase of the concentration
31
and the nanorods dimensions, interestingly the diameter of the nanorods increases
gradually.
Figure 2.15 SEM images of ZnO nanorods on Si substrate with different precursor
concentrations of the growth aqueous solution (a) at 25 mM, (b) 50 mM, (c) 100 mM,
(d) 300 mM. (Amin et al., 2011)
An example of hydrothermal reaction is the synthesis of Zinc oxide using the
reagents Zinc Choride (ZnCl2) and Sodium Hydroxide (NaOH) in a ratio of 1:2 in an
aqueous environment (D. Chen, Jiao, & Cheng, 1999). The chemical reaction is given as
below
ZnCl2 + 2NaOH Zn (OH)2 + Na2+ + 2Cl- (2.1)
32
The white precipitate zinc hydroxide Zn(OH)2 underwent filtration and washing and then
the pH was controlled at around 5-6 using hydrochloric acid (HCl). The hydrothermal
heating takes place at an autoclave with a set temperature followed by cooling. ZnO is
obtained as the end product according to the equation
Zn(OH)2 ZnO + H2O (2.2)
Another example of hydrothermal method was proposed by the following reaction (Ismail,
El-Midany, Abdel-Aal, & El-Shall, 2005).
Zn(CH3COO)2 + 2NaOH Zn(OH)2 + 2CH3COONa (2.3)
Zn(OH)2 ZnO + H2O (2.4)
The chemical reaction between Zn(CH3COO)2 and NaOH was carried out in the presence
of hexamethylenetetramine (HMT) at room temperature. The resulting precipitate of
Zn(OH)2 was washed with water several times and then underwent thermal treatment in
a Teflon-lined autoclave. Based on SEM images, the authors concluded that the HMT, as
a surfactant, plays an important role in the modification of the ZnO particles. The shape
of the particles is also affected by the time and temperature of the hydrothermal process.
With an increase in time, temperature and surfactant concentration, the size of the
particles increases. Hydrothermal processing of the precursor, followed by drying,
produced spherical particles of ZnO with sizes in the range 55–110 nm depending on the
conditions of synthesis.
In previous work, a thin layer of ZnO nanoparticles were grown on glass substrate
by thermal decomposition of Zinc nitrate and HMT using the hydrothermal method
(Ashfold, Doherty, Ndifor-Angwafor, Riley, & Sun, 2007; Vergés, Mifsud, & Serna,
1990). HMT is a non-ionic highly water soluble tertradentate cyclic tertiary amine. It
Temperature
33
releases OH- on thermal decomposition which reacts with Zn2+ ions to form ZnO. The
chemical equation involved in the process is summarized as
(CH2)6N4 + 6H2O 6HCHO + NH3 (2.5)
NH3 + H2O NH4+ + OH- (2.6)
2OH- + 2Zn2+ ZnO(S) + H2O (2.7)
The role of the HMT is manifold. It not only supplies hydroxyl ions to drive the
precipitation reaction but also acts as a buffer as the rate of its hydrolysis decreases with
increasing pH and vice versa. The role of HMT in the growth process of ZnO nanowire
was also discussed in a very different manner in other literature (Sugunan, Warad,
Boman, & Dutta, 2006). It was proposed that HMT will attach to the non-polar facets of
zincite crystal preferably being a long chain polymer and a nonpolar chelating agent. It
therefore cuts off the excess Zn2+ ions to them leaving only the polar (0001) face for
epitaxial growth. HMT therefore acts like a shape inducing polymer surfactant rather than
as a buffer. The morphology of ZnO nanostructures can be controlled by varying the
amount of a soft surfactant, ethylenendiamine and the pH of the reaction mixture of zinc
acetate, sodium hydroxide and the surfactant. Homogeneous growth was obtained at a pH
of 12 and inhomogeneity occurs as the pH decreases. However, HMT to zinc nitrate
relative concentrations and growth temperature were reported having a profound effect
over density, orientation and growth of the ZnO nanorods (Mahmood, Bora, & Dutta,
2013). In the work, the effect of molar ratio of zinc nitrate and hexamine, ZnO nanorods
were grown in precursors with 10 mM concentration of zinc nitrate hexahydrate and
varying the HMT concentration (0 mM, 2 mM, 5 mM, 10 mM, 12 mM and 15 mM). As
a result, the 1:1 molar ratio of zinc nitrate and HMT in the precursor solution exhibited
the maximum photocatalytic efficiency. For synthesis temperature effect on the growth
rate of ZnO nanorods, 10 mM precursor solution was prepared to grow ZnO nanorods for
15 hours at temperature 40 ˚C, 50 ˚C, 70 ˚C and 90 ˚C. It was found that ZnO nanorod
34
growth at 90 ˚C exhibited the highest photocatalytic efficiency. Figure 2.16 shows the
scanning electron microscope (SEM) image of the hydrothermally grown ZnO nanorods
using Zn(NO3)2 and HMT.
Figure 2.16 SEM image of ZnO nanorods grown using Zn(NO3)2 and HMT (Baruah &
Dutta, 2008)
2.7 Zinc Oxide in Global Applications
Zinc oxide (ZnO) is widely used in many areas because of its diverse properties
both physical and chemical. ZnO finds its uses in a variety of fields which include
industries, pharmaceuticals, chemicals and paint industry. Figure 2.17 summarizes the
various applications of zinc oxide and their uses. ZnO is widely used in rubber industry
to improve the thermal conductivity of pure silicon rubber while retaining its high
electrical resistance and are promising candidates as high performance engineering
materials. The disinfecting and antibacterial properties of ZnO make it a suitable material
for producing various kinds of medicines. ZnO can be applied locally in the form of cream
35
or ointments or can be administered orally. It also exhibits peeling effect at higher
concentrations.
Figure 2.17 Various applications of ZnO (Kołodziejczak-Radzimska & Jesionowski,
2014)
ZnO is also used in various types of nutritional products and diet supplements, where it
serves to provide essential dietary zinc. Due to their ability to absorb UVA and UVB
radiation ZnO products are used in sun screen creams. Its wide energy band (3.37 eV)
and high bond energy (60 meV) (Y. Kong, Yu, Zhang, Fang, & Feng, 2001; Venkatesh
& Jeganathan, 2013) at room temperature mean that zinc oxide can be used in
photoelectronic (Purica, Budianu, & Rusu, 2001) and electronic equipment (Aoki,
Hatanaka, & Look, 2000), in devices emitting a surface acoustic wave (Gorla et al., 1999),
in field emitters (Jo et al., 2003), in sensors (F.-C. Lin, Takao, Shimizu, & Egashira,
1995), in UV lasers (Tang et al., 1998) and in solar cells (Krebs, Thomann, Thomann, &
Andreasen, 2008; Repins et al., 2008). ZnO also exhibits the phenomenon of
luminescence (chiefly photoluminescence—emission of light under exposure to
electromagnetic radiation). Because of this property it is used in FED (field emission
display) equipment, such as televisions (Ihara et al., 2002). It is superior to the
36
conventional materials, sulfur and phosphorus (compounds exhibiting phosphorescence),
because it is more resistant to UV rays and also has higher electrical conductivity.
Zinc oxide is also used in gas sensors (L. Wang et al., 2012; J. Xu, Pan, & Tian,
2000). It is a stable material whose weak selectivity with respect to particular gases can
be improved by adding other elements. The working temperature of ZnO is relatively
high (400–500 °C), but when nanometric particles are used this can be reduced to around
300 °C. The sensitivity of such devices depends on the porosity and grain size of the
material; sensitivity increases as the size of zinc oxide particles decreases. It is most
commonly used to detect CO and CO2 (in mines and in alarm equipment), but can also be
used for the detection of other gases (H2, SF6, C4H10, C2H5OH). Apart from the
applications mentioned above, zinc oxide can also be used in other branches of industry,
including for example concrete production. The addition of zinc oxide improves the
process time and the resistance of concrete to the action of water. Also, the addition of
ZnO to Portland cement slows down hardening and quenching (it reduces the gradual
evolution of heat), and also improves the whiteness and final strength of the cement
(Olmo, Chacon, & Irabien, 2001). It is also added to many food products, including
breakfast cereals. ZnO is used as a source of zinc, which is an essential nutrient
(Whittaker, 1998). Thanks to their special chemical and antifungal properties, zinc oxide
and its derivatives are also used in the process of producing and packing meat products
(e.g., meat and fish) and vegetable products (e.g., sweetcorn and peas) (Espitia et al.,
2012).
37
2.8 Light Scattering and Side Coupling
Light scattering first captured the imagination of the ancients with observations
of variations of colour in nature, including the blue sky, the rainbow and the dramatic
colours seen at dusk and dawn (Lynch & Livingston, 2001; Minnaert, 2013). The first
recorded light scattering observations date back to the 11th century when Alhasen of
Basra attempted to explain the color of the blue sky (Singh & Riess, 2001). Many great
scientific minds that followed pursued light scattering experiments, including Leonardo
da Vinci (Hey, 1983) and Sir Isaac Newton (Lilienfeld, 2004). Lord Rayleigh was the
first to provide a quantitative treatment of light scattering in the 19th century and the
concept of Rayleigh scattering survives to this day (Rayleigh, 1917). While light
scattering analysis is used in many fields of study, it is only recently that light scattering
has become useful for biomedical applications (Mourant, Hielscher, Eick, Johnson, &
Freyer, 1998; Murdock, Braydich-Stolle, Schrand, Schlager, & Hussain, 2008),
optoelectronics (Müller et al., 2002) etc. Based on these great discoveries of light
scattering, the concept was applied in optical field using ZnO nanostructures (H Fallah et
al., 2013). Basically, this research work integrates photonics and nanotechnology to apply
the behaviour of light scattering towards light side coupling that occurs due to the incident
angle of the incoming light is greater than the critical angle between the surrounding and
cladding or core coated with ZnO nanorods grown on the multimode optical fiber.
Through this concept, light side coupling is practically impossible to be applied with the
light incident angle larger than critical angle between core or cladding and air media. The
only way to trap light inside fiber by applying another material on the optical fiber as
scattering element. In the case, ZnO nanorods can excite the core or cladding mode which
is very sensitive to surrounding environment and scatter light into the fiber core. The
whole idea of side coupling is governed by the law of refraction. ZnO nanorods grown
on has a higher refractive index (nZnO ̴ 2) than the core material (nSilica ̴ ̴ 1.5) and allows
38
light coupling into nanorod waveguides. As light is launched at the side of the fiber the
ZnO nanorods being a photonic crystal acts as a waveguide and scatters light in various
angles. The part of the light whose incident angle, θi is greater than the critical angle is
coupled inside the fiber as the condition for total internal reflection is achieved inside the
core modes as indicated in Figure 2.18. This allows light propagation inside the optical
fiber and light leakage from these modes, permitting two possible light coupling
collection schemes which are light exits from the side and is collected either from the
optical fiber end or through a side optical fiber probe. The nanorod arrays on the
multimode optical fiber also provide a suitable environment for sensing applications due
to their large surface to volume ratio. The optical scattering properties of the nanorods
depends on its shape, density and uniformity and these parameters are tuned by choosing
proper growth conditions.
Figure 2.18 Illustration of light scattering from one ZnO nanorod
39
Figure 2.18 depicts the excitation of cladding modes leak while propagating due
to the presence of nanorods. The total power inside the optical fiber decays exponentially
due to scattering effects. In the case, the absorption of visible light was neglected which
is minimal because ZnO is a wide band gap semiconductor. The total power is written as
below:
𝑃𝑔𝑢𝑖𝑑𝑒𝑑(𝑧) = 𝑃𝑜 exp{−2 𝛼𝑠 𝑋 (𝑧 − 𝑧𝑜)} + 𝑃∞ (2.1)
Where
𝑃𝑜 and 𝑃∞ = the coupled light power at (z = zo ) and background ( z ∞)
𝛼𝑠 = nanorods scattering coefficient.
Figure 2.19 Schematic representation of two possible configurations of side coupling to
cladding modes with guided and leakage intensity responses of light paths in the side
coupling configuration (H Fallah et al., 2013)
40
However, the excitation inside the fiber is only maximized at Z = ZO and then the intensity
of the guided light reduces exponentially to the ZnO nanorods interface (Z = 0). For the
location farthest away from the interface, any light reaching the detector is minimised. In
the approach, a fundamental work was practically performed to characterize the scattering
coefficient, αs and coupling intensity for different concentrations of zinc acetate used
during seeding process in hydrothermal process (3 mM, 4 mM, 5 mM & 6 mM). The
optical characterization was performed by measuring the output power to cladding mode
in wet etched optical fiber coated with ZnO nanorods using longitudinal scanning
approach as graphically illustrated in Figure 2.20. The output power was recorded by a
photodetector placed at the end of the optical fiber while the coupling intensity was
captured using a charge-coupled device (CCD camera) at incident angle along the optical
fiber. The output from camera and photodetector were synchronized using a computer to
record images of coupling intensity and output power while moving the stage parallel to
the optical fiber. Then, the scattering coefficient can be directly determined from the
measurement using Equation (2.1).
41
Figure 2.20 Optical characterization setup for the light side coupling (Hoorieh Fallah et al.,
2014)
As results, the coupling intensity versus displacement of light source was plotted as
shown in Figure 2.21. It was observed that the use of 4 mM of zinc acetate for seeding showed
the brightest intensity. The coupling intensity rapidly reduces due to the increase in distance
of the excitation location of light source from photodetector.
42
Figure 2.21 The coupling intensity of different concentration of zinc acetate for ZnO
nanorods grown on wet etched optical fiber (H Fallah et al., 2013)
Figure 2.22 shows the measured coupling output and scattering coefficient versus the
concentration of zinc acetate for ZnO nanorods growth on the wet etched optical fiber to
excite cladding mode. Relatively, 4 mM of zinc acetate was found to be the highest
scattering coefficient. Higher zinc acetate concentration could lead to the lack of pattern
in directionality of ZnO nanorods that might affect the scattering coefficient values as
indicated through the large standard deviation.
43
Figure 2.22 The measurement of coupling intensity (y- left axis), the average scattering
coefficient (y- right axis) and versus the concentration of zinc acetate (H Fallah et al.,
2013)
The investigation on coupling intensity was extended to excite core mode in wet etched
of optical fiber using the experiment setup as shown in Figure 2.20. A multimode optical
fiber was wet etched for 20 minutes and subsequently coated with ZnO nanorods. Figure
2.23 shows the comparison of coupling intensity between cladding mode and core mode.
It was found that the power coupled to the core modes is higher than the cladding modes.
In addition, power exponentially decays due to leakage of the core mode through the
presence of ZnO nanorods on the wet etched region when the light source was moved
away from the focused region.
44
Figure 2.23 The coupling intensity for cladding mode and core mode at different
excitation location on the optical fiber (Hoorieh Fallah et al., 2014)
However, the primary limitation of wet etching method was that only a small region of
the fiber could be used for signal collection. This situation is not preferable for sensing
applications where extended light sources are required. Another fundamental work of
light side coupling was also performed to characterize the scattering properties of ZnO
nanorods such as cross section and maximum power coupling using nephelometry
method. Figure 2.24 shows the homemade nephelometer using a collimated fiber coupled
with white light LED source and a photodetector.
45
Figure 2.24 Optical nephelometer setup for testing scattering properties of ZnO grown
on glass substrate (Hoorieh Fallah et al., 2014)
In the optical characterization, ZnO nanorods were grown on a flat glass substrate
using hydrothermal method in order to optimize the growth condition. This is to avoid
the effects on angular spectrum of the scattering process since a cylindrical lens was used
in the work. Moreover, the growth of ZnO nanorods around the optical fiber might
increase the interaction among the ZnO nanorods. The characterization was performed
for different concentrations of zinc acetate using in seeding process which are 1 mM, 2
mM, 4 mM and 6 mM. As a result, a polar plot as shown in Figure 2.25 presents the
measured normalized angular power spectra and density, ρa for different zinc acetate
46
concentrations. It was observed that the lowest concentration of zinc acetate demonstrates
highest coupling power at incident angle larger than critical angle, θc. Meanwhile, higher
concentrations presented lower coupling power compared to 1 mM. The ZnO nanorods
density was given by the number of rods in a selected area divided by the area, A (cm2). It
was seen that the number of ZnO nanorods per unit area is almost constant and increasing the
concentration did not have significant effects on length and diameter on ZnO nanorods. Base
on the analysis, the seeding solution molarity was fixed to 1 mM for growing ZnO
nanorods on optical fiber.
Figure 2.25 The measurement of (a) normalized angular power spectra and (b) density,
ρa respect to the concentrations of zinc acetate used for preparing the ZnO seed layer on
glass substrate (Bora et al., 2014)
2.9 Recent Research on Temperature and Gas Sensing Using Optical Fiber
Interest in monitoring temperature has constantly been on the increase in recent
years. The monitoring is very important because it is necessary in many different fields
such as medical (Takaki, 1998), food industry (Law, Bermak, & Luong, 2010), sport
(Byrne & Lim, 2007), living residences (Wood et al., 2008) etc. For instance, a sensor
system is required in operating room to prevent the buildup of humidity that can pose
serious risks to patient health. Up to date in 2016, researches still use the very common
47
optical fiber in temperature sensing which is fiber Bragg gratings (FBG) using silica
optical fiber (Warren-Smith, Nguyen, Lang, Ebendorff-Heidepriem, & Monro, 2016;
Woyessa, Nielsen, Stefani, Markos, & Bang, 2016). The fabrication process of FBG
involves a complicated process and expensive equipment such as high performance laser
light source with high pulse energy for open structures. In addition, the Bragg grating
region is difficult to determine using laser light source with 1550 nm in wavelength. An
integration of visible light source and the spliced single-mode fiber is required so that
scattering from the ablation spots could be observed.
In other method, Mach-Zehnder interferometer (MZI) has been commonly used
in diverse sensing applications because of their flexible configurations (B. H. Lee et al.,
2012). MZI is a device used to determine relative phase shift between two collimated
beams from a coherent light source either by changing length of one of the arms or by
placing a sample in path of one of the beams. In a recent research, temperature sensor
based on POF and electro-optic effect of MZI using a laser source of wavelength 635 nm
was proposed (S. Kumar & Swaminathan, 2016). Figure 2.26 shows the proposed setup
of liquid temperature sensor.
Figure 2.26 The setup of liquid temperature sensor. (S. Kumar & Swaminathan, 2016)
48
The laser light source is used to generate a beam of light and then this beam is guided into
the POF. The setup employed a differential amplifier in order to detect precisely a small
variation in liquid temperature.
Gas sensing plays important roles in many applications including safety
management of oil and gas industry (Vogler & Sigrist, 2006) and exhaust gas monitoring
for combustion engines (Docquier & Candel, 2002). There are numerous gas sensing
technologies available to sense various gases. Among them, optical fiber sensors using
infrared (IR) absorption spectroscopy (Hoo, Jin, Ho, Wang, & Windeler, 2002); (Chong
et al., 2015) stands out due to the high detection specificity. IR spectroscopy is based on
the optical absorption of molecular vibration bands, which represent the particular of
various gas molecules. Therefore, IR absorption spectroscopy is widely used due to
reliable technique for both detection and identification of hazardous and greenhouse
gases. In addition, IR sensors have minimal drift, fast response, long lifetime and can be
conducted in real time and in situ without disturbing the target system (Hodgkinson &
Tatam, 2012). An optical fiber sensor for gas detection has been developed based on IR
absorption spectroscopy (Chong et al., 2016). The proposed sensor demonstrated ultra-
sensitive to detect the level of carbon dioxide (CO2) at 1570 nm wavelength.
Surface plasmon resonance (SPR) for gas detection and biosensing was
demonstrated by Nylander and Liedberg (Liedberg, Nylander, & Lunström, 1983;
Nylander, Liedberg, & Lind, 1982). Since then SPR sensing has been receiving
continuously growing attention from scientific community. SPR is a powerful technique
for direct sensitive chemical detection (Abdelghani et al., 1997). A latest research using
SPR technique was developed for in situ detection of xanthan gum (Michel, Xiao,
Skillman, & Alameh, 2016) using 1550 nm laser light source. Figure 2.27 shows the
experiment setup using optical equipment such as optical power meter, optical circular,
mirror, prism and laser beam which is not cost effective and complicated in measurement.
49
Figure 2.27 Optical fiber sensor based on SPR for chemical sensing. (Michel et al.,
2016)
In all the systems explained above, laser light source was widely used in
temperature and gas sensing and light was launched from one end of the fiber while signal
was collected at the other end. In order to reduce cost and complexity in design and
increase the utilization of visible light in sensing applications with high sensitivity, this
work applies the behavior of light scattering from ZnO nanorods into POF towards light
side coupling.
50
CHAPTER 3: OPTIMIZATION OF ZINC OXIDE NANOROD COATINGS ON
LARGE CORE PLASTIC OPTICAL FIBER THROUGH HYDROTHERMAL
GROWTH
3.1 Introduction
Zinc oxide coating is a layer containing zinc (Zn) and oxygen (O) which can be
synthetically produced using various chemical methods such as mechanochemical
process (Ao, Li, Yang, Zeng, & Ma, 2006; Stanković, Veselinović, Škapin, Marković, &
Uskoković, 2011), sol-gel (Benhebal et al., 2013; Mahato et al., 2009), hydrothermal (D.
Chen et al., 1999; Ismail et al., 2005) and emulsion (Stanković et al., 2011). As explained
in the previous chapter, zinc oxide (ZnO) has attracted tremendous interest due to its
noticeable performances in electronics, optics and photonics. ZnO is preferable to use in
numerous applications because it is insoluble in water, high chemical stability, high
electrochemical coupling coefficient, broad range of radiation absorption and high photo-
stability. Due to this, ZnO can serve greatly as a coating layer in optical sensor technology
to improve and enhance the sensitivity in sensing various physical parameters such as gas
concentrations, humidity level, temperature, pressure, strain, etc. Absorption of
molecules on the ZnO coating layer can be sensed through variation of ZnO properties
such as photoluminescence, electrical conductivity, vibration frequency, mass etc. This
chapter explains the synthesis process of ZnO nanorods growth to coat large core plastic
optical fiber (POF) using hydrothermal method.
51
3.2 Optimization parameters for the hydrothermal method
Figure 3.1 shows the important parameters for growing ZnO nanorods using
hydrothermal method. In previous works, the hydrothermal synthesis was used to grow
ZnO nanorods on glass substrate and silica multimode optical fiber. From these works,
chemicals, solution concentration (molarity) and temperature had been optimized
successfully to maximize light side coupling by exploiting scattering properties of the
ZnO nanorods (Baruah & Dutta, 2008; Bora et al., 2014; H Fallah et al., 2013; Mahmood
et al., 2013). Light induced by scattering at angles larger than the critical angle is guided
inside the fiber. Although ZnO nanorods enhance optical guidance in this way, they are
also responsible for light leakage due to the very same scattering property. In the previous
work also, coupling of light to the core mode was accomplished by exposing the core to
wet chemical etching. Light was then allowed to couple from an intermediate region near
the beginning of the core exposure domain while leakage was minimised at un-etched
fiber domains downstream. The primary limitation of this method was that only a small
portion of the fiber could be used for signal collection. This situation is undesirable for
applications such as receivers in telecommunications and sensing where extended light
sources are required.
Figure 3.1 Optimization parameters for the ZnO nanorods growth on POF using
hydrothermal method
52
The extended light source leads to less guidance of light inside the fiber resulting in low
efficiency and sensitivity. To increase the magnitude of light collection, two approaches
that were executed simultaneously were proposed. First, a large-core plastic fiber optic is
required to increase the scattering area; and second, a structured scattering layer tightly
bound to the surface of the POF is required to harvest light from different segments of the
POF. The scattering layer consists of ZnO nanorods as a fiber coating. Thus, the synthesis
process of ZnO nanorods through hydrothermal method needed to be optimized again due
to the different specifications of POF. The POF consists of polymethyl methacrylate resin
that is surrounded by a fluorinated polymer jacket that have a storage and operating
temperature lower than 100 ˚C compare to multimode silica optical fiber. Hence, two
important parameters are controlled in order to optimize the growth of ZnO nanorods on
POF as shown in Figure 3.1. First, growth duration for a promising ZnO nanorods coating
and second, seeding method to control uniformity and density of ZnO nanorods grown
on POF.
3.3 ZnO Nanorods through Hydrothermal Growth
The growth of ZnO nanorods on POF using hydrothermal synthesis involves three
major steps; sample preparation, seeding and growth process as depicted in Figure 3.2.
These procedures are discussed in this section.
Figure 3.2 General procedures of ZnO nanorods synthesis using hydrothermal
53
3.3.1 Fiber Preparation
Figure 3.3 shows the process of fiber preparation to create ZnO structured growth
on POF. In this work, ZnO nanorods were spirally grown on POF to increase the intensity
of guided light for sensing application. Standard multimode SK-80 POF fiber (Mitsubishi
Rayon Co., LTD; Japan) was used in this study as shown in Figure 3.3 (a). The outer part
of the POF is a fluorinated polymer jacket with inner-outer diameter in range of 1880-
2120 µm, respectively. The jacket covers the POF that consists of polymethyl
methacrylate resin with diameter ranging from 1840 to 2080 μm. At first, the jacket of
the POF was mechanically stripped to expose the POF over a length of 10 cm as depicted
in Figure 3.3(b). Following cleansing with a dry tissue, 3M water proof plastic tape
(Figure 3.3(c)) was applied to create manually spiral template along the exposed POF as
shown in Figure 3.3 (d). The width of spiral pattern on POF can be varied and Figure
3.3(e) shows the width was 0.5 cm (uncovered area) to be coated with ZnO nanorods.
Figure 3.3 The process of fiber preparation (a) POF with black jacket (b) POF is
exposed with length of 10 cm for ZnO coating (c) 3M water proof tape is used to create
spiral template (d) manually creating spiral pattern on POF and (e) POF with spiral
template before the synthesis process.
54
The tape was removed after the ZnO nanorods synthesis process to expose the bare
templated POF surface before experimental characterisation and sensing. For unpatterned
coating, the tape was not required to apply on the POF but ZnO nanorods were grown
entirely along the POF.
3.3.2 Seeding Process
Seeding process plays a very important role in ZnO nanorod coatings on POF.
The diameter, length, uniformity and density of the ZnO nanorods are primarily
dependent on the seeding process. Figure 3.4 shows the procedures of seeding process
which involve 4 main steps; seeding solution preparation, POF surface treatment, forming
nucleation site on POF and annealing.
Figure 3.4 Procedures of seeding process on POF
Firstly, two different solutions were prepared in order to synthesise ZnO seed
particles which are ZnO nanoparticles solution and pH controlled solution. For ZnO
nanoparticles solution, ca. 0.0044 g zinc acetate dihydrate [Zn(O2CCH3)2(H2O)2] (Merck
KGaA, Germany) was dissolved in 20 ml of ethanol (Merck KGaA, Germany) under slow
55
stirring at temperature of 50 ̊ C for 30 minutes to form a 1 mM solution. Then, the solution
was cooled in the ambient for some time before adding another 20 ml ethanol. Figure 3.5
shows the process of ZnO solution preparation with the final amount of 40 ml.
Figure 3.5 Process of 1mM ZnO nanoparticle solution preparation
The pH of ZnO nanoparticles solution is one of the important factors that
influencing the ZnO properties thorough hydrothermal process. The pH can change the
number of ZnO nuclei and growth units (H. Zhang et al., 2004). Sagar et al. (2007)
claimed that the increase in pH (from acidic to alkaline) of the ZnO nanoparticles solution
improves the growth of a ZnO film. To control the aqueous pH, a pH control solution was
prepared by dissolving sodium hydroxide (NaOH) in 20 ml ethanol to form 1mM solution
with temperature of 50 ˚C under slow stirring as shown in Figure 3.6.
Figure 3.6 Preparation of the pH controlled solution using NaOH
56
After 10 minutes, the 20 ml pH control solution was added into ZnO nanoparticles
solution using 1 ml pipet as illustrated in Figure 3.7. This technique provides more
hydroxyl ions (OH-) in the seeding solution (Baruah & Dutta, 2008). Then, the ZnO
nanoparticles solution was slowly stirred for 1 minute for every single drop of 1 ml pH
control solution. This process was repeated for 20 times. Then, the seeding solution was
kept in a water bath at temperature 60 ˚C for 3 hours. As result, a noticeable change in
the colour of the solution from clear to milky could be observed and pH level changed
from ~ 4 to ~ 9.
Figure 3.7 Alkaline process of ZnO nanoparticles solution by NaOH
For optimum uniformity of ZnO growth on POF, surface treatment was performed
using polysorbate 80 (tween 80) which is non-ionic surfactant that contains hydrophilic
group. In the process, 1 ml tween 80 was completely dissolved in 10 ml deionized (DI)
water under moderate stirring with temperature of 45 ˚C as depicted in Figure 3.8(a).
Then, the POF were vertically dipped into the solution for 10 minutes as shown in Figure
3.8(b). The POF samples were dried in air for 2 hours.
57
(a) (b)
Figure 3.8 (a) Tween 80 preparation and (b) POF surface treatment
In the process of forming nucleation site on the POF, three seeding methods were
carried out in order to achieve a proper profile of ZnO nanorods growth on POF. The
method used are as follows:
(i) Dip and Dry (Figure 3.9): The samples of POF were dipped in the seeding
solution for 1 minute and dried on a hot plate at a temperature of 70 ˚C for 1
minute. This process was repeated for 10 times. Following the conclusion of
the dipping process, the POFs were annealed at 70 °C for 3 hrs.
Figure 3.9 Dip and Dry method in seeding process
58
(ii) Drop and Dry (Figure 3.10): The samples of POFs were placed on a hot plate
at a temperature of 70 ˚C. The seeding solution was dropped on the POFs with
amount of 100 µl using micro-pipet and wait for 1 minute to dry the POFs. The
process was repeated for 10 times. Then, the POFs were annealed at the same
temperature for 3 hours.
Figure 3.10 Drop and Dry method in seeding process
(iii) Slow stirring (Figure 3.11): In the method, the exposed POFs were immersed
in the seeding solution under slow stirring for 30 minutes. This method can
avoid the ZnO nanostructure to attach with redundancy on the POFs. The
seeding process was also concluded by annealing the POFs for 3 hours at 70
˚C.
59
Figure 3.11 Slow stirring method in seeding process
3.3.3 ZnO Nanorod Growth Process
ZnO nanorods were grown following the seeding process. 2.97 g zinc nitrate
hexahydrate [Zn(NO3)2·6H2O] (Ajax Finechem Pty Ltd) and 1.40 g of
hexamethyleneteramine or HMT [(CH2)6N4] (Sigma-Aldrich) were dissolved in 1000 ml
of deionised (DI) water to form 10 mM solutions of each compound. The seeded POFs
were then vertically placed in 200 ml of the synthesis solution and heated in an oven set
at 90 °C as shown in Figure 3.12. Following 5 hours of heating, the solution was discarded
and replaced with a new solution in order to maintain constant growth conditions (Baruah
& Dutta, 2009b). Growth time was varied from 8 to 20 h. Following synthesis, POFs were
removed and rinsed several times with DI water.
60
Figure 3.12 The process of ZnO nanorod growth on POFs
3.4 Optimization of ZnO Nanorod Growth on POF
The synthesis of ZnO nanorod growth on POF as explained in section 3.3 was
optically optimized to investigate light scattering into the POF towards light side
coupling. In seeding process, the dip and dry method was applied in order to deposit ZnO
nanoparticles on the POF. The profile of ZnO nanorods growth on the POF was as well
characterized using scanning electron microscopy (SEM) and Energy dispersive X-ray
(EDX). The entire process of the optimization is summarized accordingly as depicted in
Figure 3.13.
The hydrothermal growth was firstly performed to grow ZnO nanorods on POF
for 20 hours at a temperatures as low as 90 ˚C (J. H. Kim et al., 2007; T.-U. Kim, Kim,
Pawar, Moon, & Kim, 2010; Tam et al., 2006) with and without the surface treatment
process. The obtained ZnO nanorords coated POF were then characterized by SEM (model:
Hitachi, 3400N) and EDX was performed during SEM.
61
Optimization of ZnO Nanorod
Growth on POF core
Nanotechnology
Characterization Growth times
SEM/
FESEM EDX
15 Hours 20 Hours
Light Side Coupling
Experiment
Backscattering
Reduce growth times
8 Hours 10 Hours 12 Hours
Record &
analyse the dataYes
Found the optimized
growth times
Optimization of seeding
method using the
optimized growth time
of ZnO nanorod
Slow stirring
Dip & Dry
Drop & Dry
Dip & DryUniformity
Alignment
Density
Optical
Experiment
Nanotechnology
Synthesis
ProcessSpiral Patterned
Coating
Figure 3.13 Flow of the optimization process of ZnO nanorod growth on POF through
hydrothermal
Figure 3.14 shows the SEM images of ZnO nanorods grown on the POF for 20 hours
with surface treatment (Tween 80) and without surface treatment, respectively. These POF
samples were observed at low magnification in order to compare the distribution of ZnO
coating layer attached on the POF. It was obtained that the POF treated with Tween 80 has a
good coating of ZnO layer as shown in Figure 3.14 (a) compare to the untreated POF as
depicted in Figure 3.14 (b). The result clearly shows that the coating of ZnO layer was not
firmly attached on the untreated POF. It can affect the intensity of guided light inside the POF
due to less light scattering.
62
Figure 3.14 Low magnification SEM images of the POF coated with ZnO nanorods with
(a) surface treatment (Tween 80) and (b) without surface treatment
For onward ZnO synthesis process, the surface treatment becomes a compulsory
procedure before seeding the POF with ZnO nanoparticles. Then, the unpatterned POF
samples coated with ZnO nanorods were prepared for two different growth time; 15 hours
and 20 hours. The POF samples were optically characterized towards light side coupling.
The optical characterisation apparatus is schematically depicted in Figure 3.15 below.
Figure 3.15 Vpp characterisation setup to measure the side coupling of ZnO nanorods
for unpatterned and spiral patterned POFs
63
The magnitude of the side coupling was measured in terms of ‘peak-to-peak’ voltage
(Vpp) following excitation by a modulated light-emitting diode (LED) red light source –
e.g. the extended light source as depicted in Figure 3.16. Light from the extended source
was restricted by an aperture onto specific sites on the POF in order to optimise the growth
conditions for maximum side coupling. The egress end of the optical fiber is linked to a
digital oscilloscope and subsequently to a computer for data recording and analysis.
Figure 3.16 The modulated LED red light source used in the optical characterization
POFs were illuminated by a 3 cm diameter broad band LED extended light source
placed 10 cm from the fiber surface. A rectangular aperture 1 × 3 cm was placed
perpendicularly to and directly on top of the fiber during signal acquisition. Three regions
were inspected for the unpatterned type of fiber: (i) the interfacial area between the ZnO
coating and the uncoated fiber near the detector end; (ii) the middle ZnO region; and (iii)
the tip that consisted of the terminal ZnO-air interface as illustrated in Figure 3.17. The
fiber tip was covered in all cases except for readings taken for the tip of the POF samples
and the tip reading is not considered for any purpose in this work. In addition, the tip
reading is changeable for each sample due to the edge of the tips.
64
Figure 3.17 The exposed regions on the unpatterned type of POF (a) interface, (b)
middle and (c) tip
The plots in Figure 3.18 shows the average Vpp on bare and unpatterned POFs for
15 hours and 20 hours. However, the average Vpp at the interface region (for both these
growth times) was greatly reduced due to backscattering that limits light side coupling to
the core modes. This backscattering problem also contributed to increase the average Vpp
at the ZnO region for 15 and 20 hours due to the presence of ZnO nanorods not inducing
light leakage. The bare POFs did not show any backscattering effects and very low
forward light scattering. The backscattering problem occurred due to longer growth times,
resulting in higher ZnO nanorods density on POFs as shown in Figure 3.19 for 15 hours
and 20 hours, respectively. Hence, the coating provided a greater barrier to light side
coupling due to backscattering. The SEM images of ZnO nanorods are captured at a
magnification of 25.00 kX clearly showed that the ZnO nanorods were not vertically
grown on POFs. This profile as well contributes to produce high backscattering and low
intensity of guided light.
65
Figure 3.18 Average Vpp for 15 and 20 hours growth time with backscattering effects
Figure 3.19 ZnO nanorods grown on POF (a) 15 hours (b) 20 hours
The problem was solved by reducing the ZnO nanorods growth duration to 8, 10,
or 12 hours by applying the same synthesis process of ZnO nanorods as explained in
section 3.3. Figure 3.20 shows the improvement in the average Vpp at interface regions
for the above mentioned growth durations: 8, 10, and 12 hours. As the extended light
source was shined at middle regions, the average Vpp significantly reduced due to ZnO
66
nanorods induced light leakage through the core modes. At interface region, the average
Vpp greatly increased due to more light coupled inside the core. However, it was
determined that the growth duration of 12 hours was optimal in limiting backscattering.
Tip readings (uncovered) are high due to ingress of light through the fiber optic in addition
to potential side coupling.
Figure 3.20 Backscattering effect is eliminated at interface regions after reducing the
growth time to 8, 10, and 12 hours
The optimized condition was concluded based on the highest average Vpp at the
interface region. The results of optimisation are summarised in Figure 3.21 showing only
Vpp against interface data. At 12 h growth time, Vpp is maximized, thereby demonstrating
high light side coupling with reduced leakage due to backscattering. This optimized
process of growing ZnO nanorods on POF was then applied to fabricate the spiral
patterned growth on POF as shown in Figure 3.3.
67
Figure 3.21 Average Vpp at interfacial area for all growth times
Meanwhile, the growth of ZnO nanorods on POFs for the growth durations; 8, 10
and 12 hours were also characterized and analysed through SEM images as shown in
Figure 3.22. For ZnO nanorods grown on POF for 8 hours, it was observed that the
thickness of ZnO coating layer was very thin and detached. Some parts of the coating
layer were not properly grown with ZnO nanorods. The growth of ZnO nanorods was
seen like a wave because the length was not proper uniform and short due to a short
growth duration. The growth of ZnO nanorods for 10 hours also showing an improper
coating layer but the thickness has a slight improvement due to the elongation of ZnO
nanorods. However, the orientation of ZnO nanorods is not homogenous as desired. A
better improvement in ZnO coating layer can be clearly seen for 12 hours, only a few
small parts were not coated and a strong attachment with the POF was achieved.
68
Therefore, the ZnO nanorods were not distributed appropriately on the POF and few
patches of ZnO flowers were presented among the nanorods.
Figure 3.22 The SEM images for growth durations: 8 hours (top left), 10 hours (top
right) and 12 hours (bottom)
69
3.4.1 Spiral Patterned Growth of Zno Nanorods on POF Using the Optimized
Growth Duration.
For spiral patterned POF, the POF samples were prepared following the steps as
shown in Figure 3.3. The synthesis process was performed as explained in section 3.3
with the optimized growth duration. For optical characterization, five ZnO regions were
analysed: (Interface 1) the interfacial area between the ZnO coating and the uncoated fiber
near the detector end; (ZnO 1) the adjacent pure ZnO region; (Interface 2) a second
interfacial domain between the ZnO and the uncoated fiber; (ZnO 2) a second pure ZnO
region; and (Tip/ Interface 3) the tip domain of ZnO and air as before (uncovered during
reading taken for the tip) as illustrated in Figure 3.23. In all cases, bare POFs devoid of
ZnO coating served as controls in the experiments. Five readings were acquired for each
measurement.
Figure 3.23 The specified regions on the spiral patterned POF for optical
characterization.
The same characterization setup as performed for the unpatterned POF was used
to investigate the side coupling in term of ‘peak to peak’ voltage (Vpp) as shown in Figure
3.15. The graph of average Vpp for the five regions on the spiral patterned growth is
depicted in Figure 3.24. Vpp was highest at Interface 1, the ZnO bare interface closest to
70
the detector. Vpp was significantly lower at ZnO 2, the pure ZnO region. A slight rise in
Vpp was observed at Interface 2, another interfacial region. ZnO 2, a pure ZnO region
located further from the detector showed similar values to ZnO 1. Interface 3 showed the
tip effect as before. Therefore, the spiral patterned on the POF has potential application
as multi-channel excitation and enhance the total coupling inside POF. It is worth
mentioning that Vpp was a factor of 2x lower than for the same region (interfacial region
closet to photodetector) on the unpatterned fiber. This is due to area reduction of the spiral
structure as shown by the inset in Figure 3.24. This is not the case when an extended light
source was used.
Figure 3.24 Average Vpp for the spiral patterned growth for 12 h which has more than
one interface and ZnO regions. The inset shows the regions covered by the aperture
when characterisation the structured and unstructured ZnO growth on POF
Figure 3.25 shows the SEM image of ZnO spiral patterned growth on POF for 12
hours. The SEM image in Figure 3.25 (a) with magnification set at 13.00 kX was used to
observe the ZnO spiral pattern on the POF. The ZnO coating layer strongly attached on
the POF with proper orientation. Figure 3.25 (b) depicts SEM images with magnification
71
of 25.00 kX which clearly shows vertical alignment, high density (63 nanorods/1.23 ×
10−12 m2 = 510 × 1011 nanorods/m2) and uniform distribution of ZnO nanorods on the
POF.
Figure 3.25 (a) 13 kX SEM image of ZnO spiral patterned growth after synthesis (b)
25.0 kX SEM image of the nanorods and Inset: The ZnO nanorods at 60.0 kx
magnification for 12 hours
The EDX spectra was performed during SEM to verify the nature of the species
attached on the POF and to allow a rough estimation of their relative amounts. The EDX
elemental analysis revealed that the topcoat layer consisted only of zinc and oxygen as
shown in Figure 3.26. The presence of Zinc indicates a high peak at about 1.0 keV and
oxygen peak appears at 0.35 keV.
72
Figure 3.26 EDX spectrum of ZnO nanorods showing zinc and oxygen peaks
3.5 Optimization of Seeding Methods to Improve the Growth of Zno Nanorods
on POF.
The SEM images in section 3.4 showing the morphology of ZnO nanorods grown
on POF was not proper as desired due to cylindrical surface of POF compare to flat
surfaces that promises a high guarantee for easily controlling the morphological
parameters such as alignment, density and uniformity. Deposition of ZnO nanoparticles
during seeding process plays an important role in the hydrothermal growth of ZnO
(Baruah & Dutta, 2009a). In an attempt to improve the morphology of ZnO, another two
seeding methods were carried out as explained clearly in section 3.3.2; drop-dry and
continuous slow stirring method. The nanorods were then grown following the
conventional growth process. The SEM images of ZnO spiral patterned growth on POF
using drop and dry method as depicted in Figure 3.27.
73
Figure 3.27 The growth of ZnO nanorods using the drop and dry method (a) 5 kX SEM
image of spiral patterned growth on POF and (b) the morphology of ZnO nanorods at a
high magnification
It was observed that the ZnO nanorods were successfully coated on the POF but
with low density. At the high magnification, it can be seen that the ZnO nanoparticles
agglomerate during the seeding process to form bigger clumps. This can be attributed to
occur due to the surface tension of the solvent (ethanol) which brings the particles
together during the drying process (Dutta & Hofmann, 2004). The gradual evaporation of
the solvent from the surface of the POF leads to cracks in the layer of the ZnO
nanoparticles grown on the POF. As the particles are brought together due to surface
tension of the solvent during evaporation, it is unlikely that the crystallites would be
preferentially oriented on the POF (M. Wang & Zhang, 2009). As a result of multifarious
orientations of the seed crystallites, the nanorods grow in various directions resulting an
agglomerated nanorods like growth as illustrated in Figure 3.28.
74
Figure 3.28 Schematic diagram showing the possible agglomeration of ZnO
nanoparticles upon evaporation of the solvent (a) thin layer of ZnO nanoparticles (b)
agglomerated clumps of ZnO nanoparticles with various orientations and (c) ZnO
nanorods grow from the seed crystallites in the different directions
To obtain highly oriented growth on POF, continuous slow stirring method
through direct hydrolysis proved to be a more promising technique than using pre-
synthesized ZnO nanoparticles seeds self-organised on the POF as discussed above.
Figure 3.29 shows the POFs were being dipped in the seeding solution for 30 minutes
under continuous slow stirring which is able to avoid the agglomeration of ZnO
nanoparticles. Orientation of the ZnO nanoparticles formed in the thin film grown on the
POF.
75
Figure 3.29 The continuous slow stirring process
Figure 3.30 shows the SEM image of the ZnO growth on the POF with the continuous
slow stirring method in seeding process. It was interesting to observe that not only did
this seeding process give uniform growth in the inner layers of the mesh, but it also
eliminated the formation of the loosely bound agglomerates. No microstructures could be
observed sitting on the nanorods.
76
Figure 3.30 The growth of ZnO nanorods using the continuous slow stirring method (a)
5 kX SEM image of spiral patterned growth on POF and (b) the morphology of ZnO
nanorods at 10.0 kX
3.6 Summary
ZnO nanorods were grown by the hydrothermal method directly onto POF. The
morphology of the ZnO nanorods could be varied through changes in growth duration
and seeding methods. The synthesis process to grow ZnO nanorods on POF was
optimised by maximising the side coupling to POF from an extended light source.
Backscattering occurred due to high density of ZnO nanorods growth for 15 and 20 hours,
resulting less coupling light inside the POF. This problem was solved by varying the
growth duration to 8, 10 and 12 hours. ZnO nanorods growth time of 12 hours and
temperature of 90 °C provided the best coupling voltage. This work also reports a novel
spiral patterned growth of ZnO nanorods on POF. Structuring the growth to specific
regions allows scattering from different segments along the POF to contribute to the total
77
coupled power. Seeding methods were as well optimized in this work because it is very
important to control the growth and orientation of ZnO nanorods on the POF. Vertically
aligned ZnO nanorods were obtained on the POF using a continuous slow stirring during
the seeding process.
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CHAPTER 4: CHARACTERIZATION OF LIGHT SIDE COUPLING
TOWARDS MULTIPLE OPTICAL CHANNEL AND OPTIMIZATION OF
SPIRAL PATTERNED WIDTH OF ZINC OXIDE NANOROD COATING FOR
OPTIMAL SIDE COUPLING
4.1 Introduction
The numerous breakthroughs in photonics that have taken place over the last 50
years gave rise to many applications using light scattering which often involves a
considerable amount of interdisciplinary knowledge. Chemists and physicists have
utilized light scattering such as small angle x-ray scattering (SAXS) to study the size and
shape of macromolecules in solution as well as a whole range of materials including
colloidal suspensions and solid polymers (Agbabiaka, Wiltfong, & Park, 2013; Lipfert,
Columbus, Chu, Lesley, & Doniach, 2007). A classical text with title “Light Scattering
by Small Particles” (Hulst & Van De Hulst, 1957) and the comprehensive book on "The
Scattering of Light and Other Electromagnetic Radiation" (Kerker, 1969) were widely
referred for a deeper understanding of the dynamical properties of systems often requires
theoretical and experimental examinations of the scattering phenomena.
Nowadays, ZnO has received tremendous interest as a scattering element
especially on various flat surfaces for many optical applications such as solar cells
(Berginski et al., 2007; Krč, Zeman, Kluth, Smole, & Topič, 2003), bio-imaging (Wu et
al., 2007) and etc. However, there are not many research attempts so far to develop a
patterned ZnO growth. The application of a spiral patterned ZnO nanorods with mm
dimensions on cylindrical surfaces with small diameter (e.g. ca. 2 mm) of a typical optical
fiber has still not been explored for optical applications. Practically, unpatterned growth
is preferred due to reduced complexity during fabrication and shorter treatment time. As
79
examples, some applications have been demonstrated with unpatterned growth of ZnO
coating on POF (Bora et al., 2014; H Fallah et al., 2013). However, it was found that
although unpatterned ZnO nanorod layers enhanced optical side coupling with the fiber,
significant levels of backscattering prevented the ingress of light into the fiber.
Furthermore, ZnO scattering centers provided a pathway for light leakage (Hoorieh Fallah
et al., 2014). Consequently, these two optical loss mechanisms resulted in low intensity
of side coupling of light, a condition that is undesirable in optical applications such as in
telecommunications, sensing and measurements. As explained in previous chapter, in
order to increase the intensity of side-coupled light, application of spiral patterned
coatings of ZnO nanorods on POF was proposed to mitigate the level of backscattering
and leakage.
This chapter will focus on two main objectives of this research work. First, the
spiral patterned coatings of ZnO nanorods on POF through continuity of the optimized
hydrothermal synthesis will be optically characterized towards light side coupling of
multiple optical channel. In this characterization, spectral analysis is performed for the
unpatterned and spiral patterned samples to identify the wavelength coupling maxima. A
broad spectrum white light source and two infrared laser sources were used (850 and 980
nm). The optical transmittance of patterned and unpatterned POFs is compared by
computing the coupling efficiency. Second, an optimization of the spiral spacing of ZnO
nanorod coated regions on the POF was carried out to produce maximal signal intensity.
Theoretically, high intensity light side coupling is expected between the scattering ZnO
layer and the fiber optic if the width of the ZnO spirally-patterned coating is optimized
for the purpose of experimental design.
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4.2 Mechanism of Light Scattering by ZnO Nanorod
It is worth mentioning here that across this thesis and in a previous publication (H
Fallah et al., 2013) , the term scattering is used to describe the main phenomenon
corresponding to side coupling as shown in Figure 4.1. It was reported that another
important factor has been observed to actually contribute into coupling light to the guided
modes inside POF particularly at large angles, θi, (near right angles). At angles close to
90°, light is guided inside the rods because ZnO nanorods have higher refractive index,
n3 compared to the polymer, n2 forming the POF, light at the outlet of the nanorods
diverges with wide field of view inside the fiber. Side coupling is obtained for the portion
of this diverging light which is at angles larger than the critical angle, θc between polymer
core and air, n1. Though, for simplicity and for the remaining of this thesis the term
scattering is used to describe the macroscopic effect of light side coupling.
Figure 4.1 Mechanism of light scatters into POF by ZnO nanorods at angle larger than
critical angle, θc
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4.3 Mechanism of Light Scattering For Unpatterned and Spiral Patterned ZnO
Nanorod Layers and For the Multi-Channel Optical Fiber
In conventional optical fiber systems, light is typically introduced from one end,
guided through the fiber and collected at the other end. This common method has been
widely used for sensing applications using plastic optic fiber (POF) coated with ZnO
nanostructures (Batumalay et al., 2014; Harith et al., 2015; Lokman et al., 2015). In
previous chapter, two approaches were proposed to increase the magnitude of light
collection and light side coupling was applied in order to optimize the growth of ZnO
nanorods on POF. This section explains the mechanism of light scattering for unpatterned
and spiral patterned ZnO nanorod coatings and for the multi-channel optical fiber case as
illustrated in Figure 4.2.
Light scattering is induced by the presence of ZnO nanorods on the surface
excitation locations along the POF. A portion of the scattered light is guided when
scattering angles are greater than the critical angle between the surrounding and the core
(Bora et al., 2014). The coupled light propagates through the POF to the terminal detector
(Iout). The presence of the nanorods as well causes light leakage through the side of the
fiber (Ileak) (Figure 4.2(a)). For example, if two point light sources, P(z1) and P(z2) along
a POF are illuminated simultaneously, then the excitation inside the fiber is maximised at
these points. However, due to the nanorods inducing light leakage, the intensity of the
guided light decreases exponentially to the ZnO nanorods interface. For the location
farthest away from the interface (e.g. z2), any light reaching the detector is minimised.
Hence, the power coupled from point z2 provides only minimal contribution to the total
guidance. Clearly, the way to increase the contribution originating from point z2 is to
reduce the amount of leakage.
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Figure 4.2 Schematic diagram of light scattering for (a) Unpatterned growth of ZnO
nanorods with the coupling light (b) Spiral patterned growth of ZnO nanorods with
more interface and ZnO regions with the coupling light (c) Spiral patterned growth of
ZnO nanorods for a multi-channel excitation
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Light leakage can be minimised by reducing the ZnO coverage through the
fabrication of a spiral patterned layer of ZnO nanorods as shown in Figure 4.2(b). The
reduction of the effective area of the scattering layer is expected to increase the
contribution from point z2. Considering an arbitrary point at the middle of the spiral
patterned ZnO layer (Figure 4.2(b)), the light coupled inside the fiber leaks exponentially
inside the coated region. The intensity remains steady in the uncoated region till the next
ZnO patterned region where the exponential decay occurs again. The intensity from point
z2 is increased due to a balance between the optimised side coupling from the ZnO patches
and the reduction of the leakage due to the reduction of the effective ZnO nanorods region.
On the basis of this hypothesis, one can predict possible enhancement of the total coupling
when an extended light source is used.
In another demonstration, the presence of patches of ZnO nanorods was used for
multi-channel excitation. Though, it is possible to achieve multi-wavelength excitation
with unpatterned growth, channels further from the ZnO edge suffers a sever loss. Higher
power is then required for channel equalisation. This effect is minimised here using the
spiral patterned POF as shown in Figure 4.2(c). Different wavelengths of light source,
P(z1), P(z2), and P(z3) are individually excited at different spiral patches of ZnO nanorods.
Due to the reduction of the effective scattering area, the peaks of the coupled light are
expected to be higher than multi-channel performed on unpatterned ZnO nanorods
growth. This gives rise to a possible application in wavelength division multiplexing. The
coupling efficiency of each channel depends on the spacing between the scattering
domains.
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4.4 Experimental Characterization of Multi-Channel Optical Fiber towards
Light Side Coupling
Spectral analysis was performed for the unpatterned and patterned samples to
identify the wavelength coupling maxima using the setup shown in Figure 4.3. A broad
spectrum white light source and two infrared laser sources were used (850 and 980 nm).
The one end of the POF fiber is linked to a spectrometer and subsequently to a computer
for data recording and analysis. The other one end of the POF fiber was covered with a
small aperture to avoid light entering from the end during spectral acquisition. The optical
transmittance of patterned and unpatterned POFs were compared. Transmittance was
calculated by the following expression.
𝑇𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑎𝑛𝑐𝑒 = 𝐶𝑜𝑢𝑝𝑙𝑒𝑑 𝑃𝑜𝑤𝑒𝑟
𝑆𝑜𝑢𝑟𝑐𝑒 𝑃𝑜𝑤𝑒𝑟
Figure 4.3 Spectral analysis setup to determine wavelength coupling maxima
Figure 4.4 represents the transmittance of visible white light for spiral patterned
and unpatterned POFs when an extended source was used. The result indicates that the
spiral patterned growth is able to increase coupling of the light source better than the
unpatterned growth due to the existence of more interfacial ZnO regions and reduction of
(4.1)
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active region on the POF. The plot in Figure 4.4 also shows that the spiral patterned
growth provides a higher light transmittance with an improvement factor of 2.2.
Figure 4.4 Transmittance of the visible white light spectrum
Figure 4.5 shows that the transmittance of light for the spiral patterned growth is
higher than the unpatterned growth when both infrared laser sources were tested.
However, the infrared laser source did not significantly couple at the particular
wavelength inside the POF due to the small waist of laser beam that only focuses on a
specific region. Consequently, less amount of light scatters into POF by ZnO nanorods is
coupled and guided inside the POF. Therefore, the coupling efficiency was too low for
useful applications.
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Figure 4.5 Spectrum for near infrared (850 and 980 nm) for spiral patterned and
unpatterned growth
4.5 Modeling of Coupling Efficiency for Spiral Patterned and Unpatterned
Coating by Varying the Width of the Coated Region towards Light Side
Coupling
In this section, a first order model is derived to simulate the impact of millimeter
(mm) scale spiral patterns on power leakage due to scattering by ZnO nanorods. In the
side coupling mechanism proposed here, ZnO nanorods allow light to couple inside the
guiding region (core of POF). ZnO nanorods as well guide the light outside the fiber core
with each bounce at the interface. These two counter-effects restrict the coupling to an
effective area around a region at the beginning of the ZnO coating. This limits the use of
this system in multiple channels as well as for application with extended sources. One
way to improve the system response is through spreading the effective coupling area of
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ZnO nanorods across the fiber. This is achieved by introducing patches of nanorods
coating. Optimizing the gaps and width of ZnO coating enhance the system response
depending on the light source used. More detailed analysis of the scheme was explained
in section 4.3. Two kinds of ZnO nanorod coating schemes on POF were analyzed: 1.
Spirally patterned ZnO nanorod coatings in which a light-blocking layer was applied, and
2. Unpatterned coatings in which ZnO nanorods coated the entire surface of the POF
uniformly. The two configurations are shown in Figure 4.6.
Figure 4.6 (a) Spirally patterned coating of ZnO nanorods on POF and (b) unpatterned
coating of ZnO nanorods on POF with a visible light source
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In the schemes illustrated in Figure 4.6, the visible-light source illuminates the upper
hemisphere of the coated POF when oriented normal to its surface. The ZnO nanorods
scatter light at different directions accordingly and the total light scattered inside the
optical fiber is expresses in the following equation
𝑃𝑜 = 𝑃𝑠𝑜𝑢𝑟𝑐𝑒 . 𝜌𝑎. 𝐶𝑠𝑐 (4.2)
In Equation (4.2), Psource, is the power of the source excitation. The constants Csc and a
are the scattering cross section of one rod (m) and rods density (number of nanorods per
unit area, Nrod/ µm2) respectively. However, not all light scattering is guided (coupled).
Only light scattered with angles larger than the critical angle contributes the coupling.
The Equation (4.2) can be written as
𝑃𝑜 = 𝑃𝑠𝑜𝑢𝑟𝑐𝑒 . 𝜌𝑎. 𝐶𝑠𝑐.ψ (4.3)
The constant ψ is the portion of the scattered light that couples into the guided modes of
the fiber. In order to derive an expression of ψ, a probability of distribution function of
the light scattering from one ZnO rod versus radial, θ and azimuthal, ϕ angles was
calculated and this is typically referred to as the phase function 𝑝(𝜃, 𝜙) and it is illustrated
in Figure 4.7.
Figure 4.7 Definition of polar coordinate
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As 𝑝(𝜃, 𝜙) is a probability distribution function, the integration over all angles should
be unity.
ψ = ∫ ∫ 𝑝(𝜃, 𝜙) sin 𝜃 𝑑𝜃𝑑𝜙 = 1𝜋
0
2𝜋
0 (4.4)
In Equations (4.4), the function p (θ, ϕ) is the phase function which is assumed to be
independent on the azimuthal angle, ϕ and hence the integration over ϕ is 2π. Hence in
Equation (4.4), the integration is over the radial angle, θ only. The expression in Equation
(4.4) is reduced to
ψ=2π∫ 𝑝(𝜃, 𝜙) sin 𝜃 𝑑𝜃 = 1𝜋
0 (4.5)
Only light scattered at angles large than critical angles contributes to the guidance inside
fiber. The fraction of scattered light that is guided can be written as:
ψ=2π∫ 𝑝(𝜃 − 𝜃𝑖𝑛𝑐) sin 𝜃 𝑑𝜃𝜋
𝜃𝑐 (4.6)
The function p (θ - θinc) is assumed to vary linearly with the incident angle, θinc. This
assumption can be justified here as small range of angles around normal incidence is
considered. At larger angles this model deviates from the actual system. The critical
angle, θc, is the one between the core POF and air. From Equation (4.3) and (4.6), the
maximum coupled power, 𝑃𝑜 to core or cladding mode is defined as
𝑃𝑜 = 𝑃𝑠𝑜𝑢𝑟𝑐𝑒 2𝜋𝐶𝑠𝑐𝜌𝑎𝜓 (4.7)
To study the coupling and source distribution effect, the POF surface was divided into
segments of width, Δz shown in Figure 4.8(a). The source excitation is assumed constant
over the width. At any segment h on the surface of the POF, exposed to a visible light
source, there is an arbitrary intensity profile Ps(z) causes a portion of Ps(z) to couple
to the guided modes. In addition to the excitation, a portion of the previously coupled
light (coming from segment Ph-1) adds to the amount of light coming out of segment h as
shown in Figure 4.8(b). Notice that, in the figure the coupling coefficient from segment
h is indicated as z. The power coupled out of segment h can be then written as
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𝑃ℎ = 𝜓𝜂𝑧,ℎ𝑃𝑠 + 𝑃ℎ−1 − (𝜂𝑧,ℎ𝑃ℎ−1) (4.8)
Figure 4.8 (a) Dividing the POF coated with ZnO nanorods into discrete sections of
width Δz for both coating schemes (b) Optical Intensity components around a segment h
of the ZnO coated POF
In simulations, the length of POF was selected to 100 segments of 1 mm each for
a total of 100 mm and Psource is the power of the source excitation that was fixed to 5 for
amplitude. Then, normalization of the outputs were applied, each value of the outputs
divided by Psource to have a maximum value equal to 1. Three coating regions of ZnO
nanorods were developed in order to create spiral patterned coating on the POF and the
widths of the ZnO nanorods coating were varied from 1 to 20 segments as shown in Figure
4.6(a). Meanwhile, the unpatterned POF was evaluated by varying the ZnO nanorods
coating from 1 to 100 segments which is fully coated as depicted in Figure 4.6(b). These
two scheme coatings were analysed using Equation (4.8) by applying finite difference
method. In this case, the widths of ZnO nanorods coating were fixed to 3 segments (3
mm) starting from segment 10 to 12 (1st ZnO region), 13 to 38 (uncoated region), 34 to
36 (2nd ZnO region), 37 to 62 (uncoated region) and 63-65 (3rd ZnO region). For ZnO
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unpatterned coating, there is only one ZnO region that is also fixed to 3 segments (10 –
12). The region omit is a coated with ZnO nanorods has the coupling coefficient, ηz higher
than zero and ηz for uncoated region is equal to zero. Thus, the power for the segment
before segment 10 (𝑃9) was equal to zero due to the ηz was zero. As the portion of light
from segment 9, P9 was substituted into Equation (4.8) to couple to the amount of light of
P10. The total light at segment 10 is
𝑃10 = 𝜓𝜂𝑧10𝑃𝑠 + 𝑃9 − (𝜂𝑧10𝑃9) = 𝜓𝜂𝑧10𝑃𝑠
The amplitude of P10 = 𝜓𝜂𝑧10𝑃𝑠 is coupled to the amount of light in segment 11. Thus,
P11 can be written as follow:
𝑃11 = 𝜓𝜂𝑧11𝑃𝑠+𝑃10 − (𝜂𝑧2𝑃10)
Then, the coupling light in segment 11 is coupled to the light presents inside segment 12,
the amplitude of P12 is given as
𝑃12 = 𝜓𝜂𝑧12𝑃𝑠+𝑃11 − (𝜂𝑧12𝑃11)
In this case, the coupled light from segment 10 to segment 12 is equal to P12 = ~ 0.7
because the width of ZnO nanorods coating was fixed to 3 segments. For the unpatterned
POF, the coupling light is consistently equal to P12 in uncoated region until reaching the
photodector. The consistency of the coupled light occurs due to coupling coefficients
from segment 13 to 100, ηz13 to ηz100 are equal to zero in uncoated region. Thus, the
coupling light reached the photodetector can be written as
𝑃13 = 𝜓𝜂𝑧13 𝑡𝑜 100 𝑃𝑠+𝑃12 − (𝜂𝑧13 𝑡𝑜 100 𝑃12) = 𝑃12
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In spiral patterned POF, this consistency of P12 remains steady in the uncoated
region (segment 13 to 38) till the second ZnO nanorods region (segment 34 to 36) that
has another three segments. The amount of P12 is coupled again in the first segment of
second ZnO nanorods region. The coupled light keeps increasing until the next uncoated
region. The effect of spiral patterned coating on POF leads to a significant improvement
of light intensity as depicted in Figure 4.9 achieving a level of coupling light of 0.98. In
the case, side coupling was obtained to be a factor of 1.4 times better for spiral patterned
coating as opposed to unpatterned continuous coatings.
Figure 4.9 The scheme of light propagation for unpatterned continuous and spiral
patterned coating where ZnO coating region was fixed to 3 segments (3 mm)
It is worth mentioning that the coupled power is normalized to the optical power incident
at each segment. Also in off-axis coupling azimuthal modes (or skew rays) are dominantly
coupled (Dwivedi, Sharma, & Gupta, 2007). These however might not be the only modes
to be excited in side coupling as radial modes can be excited as well. This is due to the
main fact that mode excitation happens due to matching the the momentum of the
scattered light to the propagation constant of guided mode. In general, the assumption of
specific power distribution among any set of modes (in any form) with the proposed first
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order model especially when large core fiber is used would not have a significant effect
on the driven results or the measurement as we estimate the leak due to surface scattering.
4.6 Theoretical Optimization of Spiral Patterned Width for Optimal Side
Coupling
Improved optical side coupling efficiency was demonstrated for spiral patterned
zinc oxide (ZnO) nanorods coated large core plastic optic fibers (POFs) as opposed to
unpatterned continuous coatings (Rahim et al., 2016). Nanorods enhanced coupling inside
the fiber by scattering light but were also capable of causing leakage. Structuring the
growth to specific regions allowed scattering from different segments along the fiber to
contribute to the total coupled power. In order to optimize the width of ZnO nanorods
coating on POF, an analysis was theoretically performed by analyzing the coupling
efficiency in three different considerations. First, as explained in section 4.5, the analysis
was continued by varying the width of spiral patterned coating from 1 to 20 mm segments
as shown in Figure 4.6(a). Meanwhile, the unpatterned was analyzed by varying the ZnO
nanorods coating from 1 to 20 segments which is fully coated as illustrated in Figure
4.6(b). A broad spectrum light source was applied for side coupling in the analysis.
Figure 4.10 illustrates the modeling results of normalized coupling output for unpatterned
and spiral patterned POF. The normalized coupling output increased greatly for spiral
patterned over that derived from unpatterned coatings as the width of ZnO nanorods
coating was varied from 0 to 20 mm.
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Figure 4.10 The normalized coupling output for unpatterned and spiral patterned
coating by varying the width of ZnO nanorod coating on POFs
Spiral patterned POFs coupled more light compared to unpatterned POF for nanorod
coating widths less than 5 mm as shown in Table 4.1. The greatest difference in coupling
output between patterned and unpatterned coatings was shown at ZnO width equal to 1
mm where ΔI(1 mm) = Ip – Iup = 0.369 due to spiral ZnO coating along the core POF
compared to unpatterned coating that had only one patch of ZnO region (1 mm) on the
POF. Although ΔI(1mm) was the highest, the coupling output for spiral pattern was not
considered because it was not the maximum value of light side coupling. The spiral
pattern coating achieved the maximum value of light side coupling at width equal to 5
mm (ΔI(5mm) =Ip – Iup = 0.135). Therefore, despite that ΔI(5 mm) was less than ΔI(1 mm) , the
use of maximal light side coupling was more dominant in applications. Meanwhile, the
unpatterned coating achieved the maximum value of light side coupling at ZnO coating
width longer than spiral patterns. Once the maximum coupling output was reached, the
coupling output remained consistent in POF’s with both types of coatings at the
normalized value equal to 1 even though the width of ZnO coating was varied.
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Table 4.1 Differences of normalized coupling output, ΔI between spiral patterned and
unpatterned POFs for different widths of ZnO coating from 0 to 7 mm
Widths of
ZnO coating
region (mm)
Normalized coupling output
Spiral pattern,
Ip
Unpatterned,
Iup
ΔI = Ip - Iup
0 0 0 0
1 0.699 0.330 0.369
2 0.910 0.551 0.359
3 0.973 0.699 0.274
4 0.992 0.798 0.194
5 1.000 0.865 0.135
6 1.000 0.909 0.091
7 1.000 0.939 0.061
Second, with the coating schemes used in (i), a laser light source (Gaussian beam)
was applied and the effects on the coupling efficiency was analyzed for spiral patterned
and unpatterned coatings. The source power, 𝑃𝑠 in Equation (4.8) was set as follows:
𝑃𝑠 = 1
𝜎√2𝜋exp (−
𝑟2
2𝜎2) (4.9)
Where,
σ = Beam waist
𝑟 = Distance from the center of the beam
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In the modeling, the center of the beam was aligned to be at a border between the exposed
POF and jacket as illustrated in Figure 4.11. The same procedures was applied for the
modeling with a broad spectrum light source was used in order to observe the coupling
efficiency with laser light source. ZnO nanorods coating on POF allows normal incident
light to scatter inside the POF at angle greater than the critical angle.
Figure 4.11 (a) Spirally patterned coating of ZnO nanorods on POF and (b)
unpatterned coating of ZnO nanorods on POF with a laser light source (Gaussian beam)
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Figure 4.12 shows the theoretical coupling efficiencies of spiral patterned and
unpatterned coatings. The same response as discussed in previous work (H Fallah et al.,
2013) occurred in the analysis which the total power inside the fiber reduced
exponentially for the both scheme coatings, due to scattering effects (neglecting visible
light absorption by the ZnO nanorods, which is minimal as ZnO is a wide band gap
semiconductor). As the width was increased, there was more light leakage through the
presence of ZnO coating on POF. However, spiral patterned POF coupled more light
compared to unpatterrned POF for all width of ZnO coating region. The peaks of coupling
efficiencies for spiral patterned and unpatterned coating were shown at ZnO width equal
to 2 mm.
Figure 4.12 The coupling efficiencies for spiral patterned and unpatterned coating
excited by a laser light source
Third, the effects on the coupling efficiencies for spiral patterned and unpatterned
coatings were evaluated by varying the spacing (uncoated region). Three widths of spiral
patterned coating on the POF were fixed at 5 segments and the spacing were varried from
1 to 20 segments and as shown in Figure 4.13(a). Meanwhile, the unpatterned POF was
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also analyzed by varying the uncoated region from 1 to 20 segments and a continuous
ZnO nanorods coating was created on POF with a width of 5 segments as depicted in
Figure 4.13(b). A broad spectrum light source was applied for side coupling in the
analysis due to high coupling efficiency as discussed in the first theoretical optimization.
Figure 4.13 Spiral patterned coating of ZnO nanorods (b) unpatterned coating of ZnO
nanorods with varied uncoated spacing
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The modeling result in Figure 4.14 clearly shows there is no light scattering into POF
when the width of uncoated region was varied. The light only coupled within the ZnO
coating region and remained steady in uncoated region due to the coupling coefficient, ηz
for uncoated region is equal to zero as explained in section 4.5. Although the width of
uncoated region was varied, the output remained unchanged for the both coating schemes.
The coupling output for spiral patterned coating is higher than unpatterned coating due to
more interfacial ZnO coating regions on POF to couple the light.
Figure 4.14 The effects on coupling efficiency by varying the uncoated region
4.7 Experimental Optimization of Spiral Patterned Width for Optimal Side
Coupling
POF fiber spiral patterning and ZnO nanorod seeding and synthesis procedures
were described in detail in the previous chapter. Standard polymethyl methacrylate (SK-
80 POF fibers from Mitsubishi Rayon Co., LTD; Japan) were utilized in the experiments
to serve as controls and the same fibers were modified to obtain spiral patterned POF with
a specified spiral pitch angle, spacing and width. The jacket of the POF were
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mechanically stripped to expose the core fiber over a length of 10 cm. The fiber length of
10 cm was chosen in this work in order to have a full illumination of light beam on the
stripped fiber from a light source with diameter of 3 cm that was placed in parallel at an
optimal distance of 3 cm from the POF surface. Figure 4.15 illustrates the ZnO coating
schemes; three widths were varied from 3, 5 to 7 mm for the spiral patterned and
unpatterened POF. These width coatings were selected from modelling result in Table 4.1
due to the significant output differences occurred at small width of ZnO coatings. A fully
coated POF (100 mm) was also fabricated to complete the validation. Tape-patterned and
unpatterned POFs were then placed in a ZnO seed solution and subsequently into the
growth solution to form ZnO nanorods. Percent surface coverage and nanorod orientation
were evaluated as described in the previous chapter by evaluation of scanning electron
micrographs recorded by a Hitachi, 3400N SEM system operating at 20 kV.
Figure 4.15 Coating schemes (a) unpatterned POFs (b) Spiral patterned POFs
Optimization of optical input through the POF waveguides was realized by
correlation with maximal values of the output voltage as depicted in Figure 4.16. A
function generator was used to modulate the light from a broadband LED light source
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(diameter = 3 cm). Sinusoidal intensity pattern was generated and transmitted through the
LED. At the receiver side, peak-to-peak voltage of photodetector output was recorded
(not the DC value). This scheme allows minimization of the ambient light effect and
external sources. The amplitude of output voltage changes according to the amount of
coupling inside the POF. The light source was placed in parallel and ~3 cm from the POF
surface. The diameter of the light source was oriented along the longitudinal axis of the
POF. The fiber tip was covered to avoid light entering from the end. The analysis was
performed on the spiral patterned POFs with three different widths of ZnO coatings (3
mm, 5 mm and 7 mm), the unpatterned POFs with the limited ZnO coating (3 mm, 5 mm,
7 mm) and full coated POF’s. Five readings were acquired for each measurement.
Figure 4.16 Optimization setup to measure the output voltage for unpatterned and spiral
patterned ZnO nanorods
Based on the simulation results, 3 mm, 5 mm, and 7 mm coating widths were
selected for experimental optimization and application. Figure 4.17 shows the
experimental results for spiral patterned and unpatterned POFs. Overall, it can be seen
that both coating schemes correlated well with simulations. The results clearly showed
that the unpatterned coatings of ZnO nanorods (3 mm, 5 mm and 7 mm) coupled less light
compared to spiral patterned POFs. In addition, the full ZnO coated POFs (100 mm)
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produced an output voltage that was less than spiral patterned POFs (3 mm, 5 mm or 7
mm) due to less illumination coverage of the visible light source in distance of 3 cm from
POF sample as shown in Figure 4.16.
Figure 4.17 The experimental result of spiral patterned and unpatterned coating for 3, 5,
7 and 100 mm
4.8 Summary
The spiral pattern on the POF also provides a higher light intensity multi-channel
compared with unpatterned ZnO nanorods POF. Spectral analysis was performed to
investigate light transmittance for different wavelength of light sources. It was found that
visible white light source significantly coupled the light into the POF compared with
infrared laser sources.
The present study also theoretically optimized the width of the ZnO nanorod spiral
coating with a visible light source. The effects on light side coupling were analyzed by
varying the width of ZnO coating region. A significant improvement was demonstrated
by spiral patterned coating at small widths of spiral coating region.
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The light side coupling also was theoretically proved with laser light source
(Gaussian beam) for spiral patterned and unpatterned coating. It is found that the light
exponentially decays when the width of the ZnO coating region was increased due to the
distribution of Gaussian beam. Consequently, the both scheme coatings contributed less
coupling efficiencies for applications.
The analysis on light side coupling were then performed by varying the uncoated
region for the both scheme coatings. There was no any change in amplitude of light when
the width of uncoated region was varied. Thus, the width of uncoated region can be
ignored in design for optimal efficiency.
Overall, spirally patterned coating theoretically proved an improvement in light
guiding compared to unpatterned coating. An optimized width of spiral patterned coating
was found to be 5 mm for efficient light coupling. There was reasonable correlation
between theory and experiment.
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CHAPTER 5: APPLIED LIGHT SIDE COUPLING WITH OPTIMIZED
SPIRAL PATTERNED ZINC OXIDE NANOROD COATINGS FOR
TEMPERATURE AND MULTIPLE OPTICAL CHANNEL ALCOHOL VAPOR
SENSING
5.1 Introduction
Optical sensors, another important application of optical fiber, have also
experienced fast development and attracted wide attention in fundamental scientific
research as well as in practical applications. Optical fiber can not only transport
information acquired by sensors at a high speed and in large volume, but it can also play
the role of a sensing element itself. In addition, compared with electric and other types of
sensors, optical fiber sensor technology has unique merits, such as immunity from
electromagnetic interference (B. Lee, 2003), being waterproof (Saito, Ichikawa, &
Oshima, 1987), and resistance to chemical corrosion (Giallorenzi, Bucaro, Dandridge, &
Cole, 1986). It has advantages over conventional bulky optical sensors, such as the
combination of sensing and signal transmission, smaller size (Cullum & Vo-Dinh, 2000),
and the possibility of building distributed systems (Mendez, Morse, & Mendez, 1990).
Optical fiber sensor technology has been used in various areas of industry, transportation
(Oehme & Wolfbeis, 1997), communication (Hill, Fujii, Johnson, & Kawasaki, 1978),
security (Szustakowski, Ciurapinski, Palka, & Zyczkowski, 2001), and defence (Cooper,
Elster, Jones, & Kelly, 2001), as well as in people’s daily life (Hocker, 1979). Its
importance has been growing with the advancement of the technology and the expansion
of the scope of its application.
However, these optical fiber sensors are usually associated with high cost, high
operational power requirements and complexity in operation. Laser light sources are
generally used in optical sensing applications but costs related to the laser and mechanical
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alignment apparatus can be relatively high (Aneesh & Khijwania, 2011). Application of
laser light sources onto coated fibers also poses several problems. Inequality of beam
distribution onto the fiber and small beam diameter can lead to fluctuations, non-
representation and low intensity (Dickey et al., 2000). Broadband visible light source
methods are simpler and less expensive to operate but suffer from low sensitivity.
However, specially coated optical fibers are able to improve the sensitivity of visible light
source methods. A new broadband visible light source sensor system that utilizes light
side coupling to ZnO nanorod coated POF is proposed. The ZnO rods act as scattering
elements that induce light transmission into the POF.
Here, in experiments using light side coupling method, molecules of ZnO on POF
which is composed of discrete electric charges illuminated by an electromagnetics wave
(visible light), electric charges in the ZnO coating layer are set into oscillatory motion by
the electric field of the incident light. Accelerated electric charges scatter light into POF.
In addition, the excited ZnO nanorods may transform part of incident light into other form
called absorption. This phenomena is called extinction that was applied to the spiral
patterned coating of ZnO nanorods on POF as a temperature sensor and multiple optical
channel waveguide sensor for detection of alcohols in the visible wavelength domain
were demonstrated based on a need to develop a low-cost, high sensitivity and
uncomplicated sensor system.
5.2 SEM images of Optimized Spiral Patterned Zinc Oxide Nanorod Coatings
for Sensing Applications
Figure 5.1 shows the SEM image of optimized spiral patterned coating of ZnO
nanorods for temperature and multiple optical channel alcohol vapor sensing. The SEM
image in Figure 5.1(a) with magnification set at 10.00 kX clearly shows the spiral
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patterned ZnO nanorods coating on POF. In the low-magnification image given below,
the width of ZnO coating is 5 mm and the uncoated spacing is 10 mm in width.
Figure 5.1 (a) The optimized spiral patterned ZnO nanorod coatings, (b) the
perpendicular growth of ZnO nanorods on POF at low magnification (c) at high
magnification (d) hight and diameter of the ZnO nanorod and (e) ZnO continuous
coating on unpatterned POF
(e)
2 mm
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In Figure 5.1(b), ZnO nanorods can be seen growing perpendicular to the surface of the
POF, an important geometry to enhance the light scattering mechanism for light side
coupling into the POF. Moreover, the growth of ZnO nanorods on POF surface in Figure
5.1(b) observed at magnification of 15.00 kX reveals high density (85 nanorods / 3.62 X
10-12 m2 = 23.50 X 106 nanorods/µm2) and uniform distribution. Figure 5.1(c) shows the
growth of ZnO nanorods with magnification of 30.00 kX. From the SEM images, the
obtained ZnO nanorods were about 3.41 µm ± 0.05 µm in length and 172.8 nm ± 20 nm
in diameter as shown in Figure 5.1(d).
5.3 Applied Light Side Coupling With Optimized Spiral Patterned Zinc Oxide
Nanorod Coatings for Temperature Sensing
Conventional temperature sensors have their limitations if large distances have to
be covered such as in many distributed measurements, electromagnetic interference leads
to the loss of signal to noise ratio, explosive environments does not allow safe use of
resistive devices and often in a plurality of applications they do not match when light-
weight structures are desired. The fiber optic sensors market is a multi-billion dollar
business which is prognosed to grow further and fiber optic based temperature sensors
are an important class therein as they are immune to electro-magnetic interference and
are thus robust and accurate in high-RF environments. Several measurement principles
have been described in the literature for measuring temperature sensors such as intensity
modulated fiber optic displacement sensor (FODS) (Rahman, Harun, Saidin, Yasin, &
Ahmad, 2012), lifetime measurements (Z. Zhang, Grattan, & Palmer, 1992), microfiber
loop resonator (MLR) (Harun, Lim, Damanhuri, & Ahmad, 2011) and stimulated
brillouin scattering (Kurashima, Horiguchi, & Tateda, 1990), interferometer (H-Romano
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et. al, 2015) and multicore fiber structure (A-Lopez et. al., 2014). Although, the
temperature sensing using polymer-coated microfiber interferometer reported by I. H.
Romano et al has a high sensitivity but it is not able to sense temperature changes at higher
range due to low melting point of the polymer. In order to be economically advantageous,
an optical fiber temperature sensor must be robust, easy-to-use, fast, accurate, stable over
a wide measurement range and suitable for a large variety of applications (Li et al., 2012).
In an application, many commercial electronic components can be damaged due to
exposure to high temperature (> 70˚C) and some can be damaged by exposure to low
temperatures (< 0˚C) (Mishra, Keimasi, & Das, 2004). Semiconductor devices and LCDs
(liquid crystal displays) are examples of commonly used components that are susceptible
to large temperature variations. In these cases, temperature sensing is indeed important
so that appropriate measures can be incorporated to prolong the life of these devices.
Optical fiber based temperature sensors are the only possibility in the presence of
electromagnetic fields such as in microwave fields, power plants or explosion-proof areas
and wherever measurement with electrical temperature sensors is not possible such as in
high tension cable lines, airplanes, spacecrafts, electrical motors etc (Ramakrishnan,
Rajan, Semenova, & Farrell, 2016).
In a previous report, temperature sensing was demonstrated using ZnO thin films
where spectral absorption changes in ZnO was monitored (Hvedstrup Jensen, 1974). In
this work we present optimized simple yet sensitive spiral patterned ZnO nanorod
coatings on POF based temperature sensor capable of utilizing ambient light coupled
through the nanorods into the fibers for sensing. Sensing performances of ZnO nanorod
coatings, spirally patterned on POF fibers are presented and the results are compared to
the sensing characteristics of the unpatterned fibers. Uncoated POF (bare) were not
considered for this application since it does not show any scattering effects due to side
coupling of light (Aneesh & Khijwania, 2011; Rahim et al., 2016).
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5.3.1 Experiment of Temperature Sensing
The proposed temperature sensor is schematically illustrated in Figure 5.2. For
maximal temperature detection, an aluminium rod with dimension of 0.3 and 10 cm in
length was used. The aluminium rod is placed vertically on a hot plate and in closed
contact with the physical POF coated with ZnO nanorods. For temperature monitoring, a
thermocouple (type J) was fixed in closed contact with the POF as well. The thermocouple
has a resolution of 1 ˚C and is able to measure the temperature within a range of 0 ˚C to
500 ˚C. A modulator circuit was used to minimize the noise in the measurement, the
white-light LED current driver was modulated with a periodical pattern signal generated
by a signal generator. The magnitude of light side coupling was measured by connecting
one of the POF to photodector and displayed in millivolt (mV) on oscilloscope under
illumination of the modulated visible white light source on the POF. The other one of the
POF tip was covered during the experiments to avoid light entering directly through the
tips. Then, temperature sensing measurement was carried out by varying temperature
from 20 ˚C to 100 ˚C. Five readings were recorded for each measurement. The sensitivity
(S) was obtained through the slope of sensing response for spirally patterned and
unpatterned ZnO nanorod coated POF devices.
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Figure 5.2 Experimental setup for the proposed temperature sensor towards light side
coupling
5.3.2 Results and Discussions
The real time responses of the ZnO nanorod coated optical fiber sensor to
temperature changes from 20 ˚C to 100 ˚C were recorded towards light side coupling.
The measurements were conducted by exposing the spirally patterned and unpatterned
ZnO nanorods coated POFs to temperature under visible light illumination. It was found
that the both coating schemes showed obvious output voltage changes upon exposure to
temperature as depicted in Figure 5.3. It is well known that the thermo-optic coefficient
of the POF is an order of magnitude higher than that of glass optical fiber (GOF), and the
refractive index (RI) of POF is affected by temperature variation. Therefore, the
temperature dependence must be taken into account for POF based RI sensors. Several
reports are available studying the temperature dependence of the RI sensors based on
GOF technology (Han, Lee, & Lee, 2004; P. Wang, Semenova, & Farrell, 2008; P. Wang,
Semenova, Wu, Zheng, & Farrell, 2010). As explained in section 4.4, the spirally
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patterned ZnO nanorod coating leads to an increase in coupling of the light source
compared to the unpatterned POF’s due to the higher interfacial ZnO regions on the POF.
Figure 5.3 The response of spiral patterned coating and unpatterned coating in
temperature sensing.
Earlier work showed that absorptivity of ZnO had a linear dependence on
temperature using reflection measurements (Hvedstrup Jensen, 1974) and refractive index
of ZnO coating layer changed at different temperature (Sanjeev & Kekuda, 2015). Figure
5.4 presents the sensing mechanism of the temperature sensor in this work using
extinction concept which is the attenuation of light by scattering and absorption as it
traverses the ZnO nanorods (Near, Hayden, & El-Sayed, 2012). Before visible light
illumination was applied onto ZnO nanorods, light does not scatter into the POF as shown
in Figure 5.4 (a). As visible light illuminates continuously onto the ZnO coating layer at
a temperature of 20 ˚C, light scattered into the POF and a high intensity of guided light
was detected due to low light absorption inside ZnO coating layer as illustrated in Figure
5.4 (b). When the POF coated with ZnO nanorods was closely touched to the heated
aluminium rod following the continuous light illumination, the absorption of light inside
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the ZnO nanorods coating increased and the amount of light scattering into the POF
relatively reduced with increasing temperature within the range as depicted in Figure
5.4(c). Consequently, the intensity of guided light inside the POF decreased due to less
light coupling. The excitation of oxygen molecules increased and obviously more energy
was required that contributed to a high absorption inside ZnO nanorods coating layer.
Figure 5.4 The temperature sensing mechanism (a) before light illumination (b) upon
light illumination and (c) aluminum rod in close proximity to ZnO nanorods coating
layer.
Figure 5.5 shows the sensitivities of the optical sensor coated with ZnO nanorods.
In the case of spirally patterned coating, the sensor response to temperature changes was
found to be 0.0623 mV/˚C. However, when the unpatterned coating was exposed to
temperature changes, the sensitivity decreased to 0.0484 mV/˚C due to less coupling light
inside POF. Moreover, spirally patterned coating consists of two exposed elements; ZnO
nanorods coating and uncoated regions (polymer). Thermal effect can be effectively
sensed by spiral patterned POF because the uncoated regions contributes also to optical
loss which is dependent on temperature. The loss is reported to increase with increasing
temperature (Husdi, Nakamura, & Ueha, 2004; Minakawa et al., 2014). In the temperature
sensing, sensitivity was measured to be a factor of 1.3 times better for spiral patterned
coatings as opposed to unpatterned coatings. Moreover, this temperature sensor
demonstrated a higher sensitivity compared to other optical fiber temperature sensors
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which usually use common sensing method by introducing light from one end and
collecting at the other end (Ju, Watekar, & Han, 2009; Rahman, Harun, Saidin, et al.,
2012). It was observed that the spiral patterned coating is able to monitor temperature to
0.1284 ˚C resolution and unpatterned coating has a resolution of 0.2273 ˚C. Based on this
performance, the optimized spiral patterned ZnO nanorod coating was further used for
multiple optical channel alcohol vapor sensing.
Figure 5.5 The sensitivity of spiral patterned and unpatterned coating in temperature
sensing
5.4 Applied Light Side Coupling With Optimized Spiral Patterned Zinc Oxide
Nanorod Coatings for Multiple Optical Channel Alcohol Vapor Sensing.
A chemical sensor is a device that transforms chemical information in the form of
concentration of a specific material into an analytically useful signal (Hulanicki, Glab, &
Ingman, 1991). A large number of commercially available chemical measurement
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systems are found in the market today and can be classified by the type of analytical signal
required for measurement. For optical-based instrumentation, these include, among
others, absorbance (Puyol et al., 2005), luminescence (Leiner, 1991), light scattering
(McFarland & Van Duyne, 2003), and fluorescence (Liebsch, Klimant, Krause, &
Wolfbeis, 2001). Due to the simplicity of directing light into a sensing platform, optical
fibers have found applications for the measurement of chemicals in food (McCorkle et
al., 2005; Morisawa & Muto, 2012; Narsaiah, Jha, Bhardwaj, Sharma, & Kumar, 2012;
Shenhav et al., 2013), security industry safes (Clevenson, Desjardins, Gan, & Englund,
2013) and clinical materials (Shenhav et al., 2013). Nowadays, optical fiber sensors have
been integrated with nanotechnology in utilizing visible light as light source for various
sensing applications.
In the experiment, the responses of alcohol vapors were observed in spectral of
visible wavelength towards light side coupling. The behavior of light scattering changed
when the spiral patterned ZnO nanorods coating on POF exposed to alcohol vapors. The
performances of the optical sensor were investigated as a multiple optical channel sensing
in visible wavelength.
5.4.1 Experiment of Multiple Optical Channel for Alcohol Vapor Sensing
Figure 5.6 shows the experimental setup used for optical sensing of alcohols. The
apparatus consisted of a spectrometer (USB4000, Ocean Optics) and a sensing chamber
(0.18 m x 0.2 m x 0.27 m). A visible white light source with wavelength 380 to 750 nm
was used to induce light side coupling. The intensity of the white light source was
modulated with a periodical pattern using the modulator in order to minimize the
background effect. It is worth mentioning that here a spectrometer was connected to the
end of the POF in order to record the spectrum of the coupled light. During the
investigation of sensor performance, ambient air was passed through the sensing chamber
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at room temperature (ca. 26 ºC) and relative humidity of 45% until a steady state condition
(0 ppm) was obtained. The white light source was placed 3 cm from the POF surface. A
known amount of alcohol was vaporized and introduced into the sensing chamber as the
target gas. Three kinds of alcohol were tested: 1. ethanol [CH3CH2OH] (Merck KGaA,
Germany, 99.8 %), 2. methanol [CH3OH] (J.T.Baker, USA, 99.8%) and 3. isopropanol
[C3H7OH] (Merck KGaA, Germany, 99.5%). The spectral response towards alcohol
vapor was recorded every 10 seconds from 0 ppm to 300 ppm. In the experiment, the
concentration (C) of target alcohol vapor in ppm was computed using the following
equation (Peng et al., 2010).
𝐶 = 𝑇×𝑉𝑡×𝐷𝑡
𝑉𝑐× 𝑀𝑡× 𝑅 (5.1)
where 𝑇 is the operating temperature in Kelvin (K), 𝑉𝑐 is the volume (ml) of the diluted
target gas which is equal to the volume of the sensing chamber. 𝑉𝑡, 𝐷𝑡 and 𝑀𝑡 represent
the volume (µl), density (g/ml) and molecular weight (g/mol) of the alcohol analyte,
respectively. R is the universal gas constant which is equal to 8.2 𝑥 104 JK-1mol-1.
Figure 5.6 Experimental setup to validate the alcohol sensing activities of spiral
patterned POF as multiple optical channels
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From the recorded visible spectrum (380 nm – 750 nm), the response from three
specific ranges (referred to here as channels) was studied - blue (450 – 495 nm), green
(495-570 nm) and red (620 – 750 nm). In the channels, the measured transmittance
average values and standard deviations were obtained for the all concentrations of the
alcohol vapors. The sensing performances in each channels were investigated by
analyzing the effects on light intensity towards the all alcohol vapor concentrations. The
sensing effects were presented in term of relative intensity modulation (RIM) in arbitrary
unit (a.u) (L. Grattan & Meggitt, 2013) that was calculated using the following equation.
𝑅𝐼𝑀 = 𝐼𝑓(𝑎𝑣) − 𝐼𝑖(𝑎𝑣)
𝐼𝑖(𝑎𝑣) (5.2)
where 𝐼𝑓 is the smallest average intensity after injection of alcohol vapor and 𝐼𝑖 is the
average initial intensity before injection of alcohol vapor under light illumination . The
RIMs were obtained for each alcohol vapor response in all three channels. To create an
inexpensive multichannel sensing system using red, green and blue LEDs and a simple
photodetector, a preliminary reference was developed through division of higher
responses from two of the three channels with respect to channel that produces lowest
response. It is worth noting here that the aim of this part of the experiment was to study
the efficiency of utilizing only three color channels (Red, Green, Blue) for different
vapors sensing. This can be possibly extended to the use of RGB LED with lower cost
photo-detector instead of a spectrometer.
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5.4.2 Results and Discussions
The improved side coupling by the spiral patterned coating of ZnO nanorods on
POF is exploited to demonstrate sensor performances in different wavelength domains of
visible light called channels to sense three different alcohol vapors (methanol, ethanol
and isopropanol) as shown in Figure 5.7. All sensing was accomplished with the
optimized spiral ZnO coating width of 5 mm. It was found that the sensor demonstrated
three different responses for methanol, ethanol and isopropanol vapors as a function of
molecular weights (methanol < ethanol < isopropanol), relative dielectric constants and
polarity. The relative dielectric constants of methanol (33), ethanol (24) and isopropanol
(20), for example, most likely influenced selective alcohol vapor molecule adsorption
onto different crystal faces of the ZnO nanorods (Wiederrecht, 2010). The amount of
vapor adsorption coupled with the region of adsorption on the ZnO therefore played the
major roles in the attenuation of the coupled light signal. Additionally, the refractive
indices of methanol (1.328), ethanol (1.361) and isopropanol (1.377) also affected the
interaction of light by varying the refractive index of ZnO nanorods coating (Yebo,
Lommens, Hens, & Baets, 2010). Interestingly, in the presence of methanol, the intensity
was seen to decrease significantly indicating lower side coupling of light into the POF.
This was due to the change in the refractive index of the ZnO coating layer caused by
methanol absorption. During sensor recovery, as methanol evaporated from the layer of
ZnO nanorods coating, the sensor output was observed to return closely to the initial
condition in ca. 7 minutes. For ethanol and isopropanol, the sensor also demonstrated
similar response patterns caused by rising adsorption onto ZnO nanorods.
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Figure 5.7 Spectroscopy responses of multiple optical channels sensor in blue, green,
and red wavelengths for (a) methanol, (b) ethanol and (c) isopropanol
However, the sensor demonstrated slight response to isopropanol vapor molecules
compared to ethanol. The recovery time for ethanol and isopropanol were ca. 5 minutes
and ca. 3 minutes, respectively. The decrease of light intensity when exposed to the
alcohol vapors and the recovery towards initial value is believed to be due to
chemisorption process, the interaction of hydrogen-bonding between –OH groups of
alcohol molecules with ZnO coating layer (Jaisai, Baruah, & Dutta, 2012). Due to small
size of methanol molecules compared to ethanol and the biggest molecule size,
isopropanol, the chemisorption process between methanol molecules and ZnO coating
layer is very high that took more time to recover relatively to initial value.
The change in the refractive index of the ZnO coating layer due to absorption
process that affects the amount of light scattering into POF can be explained clearly using
the sensing mechanism depicted in Figure 5.8. Initially, ZnO nanorods were exposed to
air at room temperature as shown in Figure 5.8 (a). In this case, ionized oxygen is
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chemisorbed onto the surface in its molecular form, O2−, as given in Equation (5.3)
(Alenezi et al., 2013).
O2 (gas) + e- ↔ O2- (5.3)
As the surface is illuminated continuously by visible light as shown in Figure
5.8(b), changes in the carrier density in the ZnO nanorods and the layer of depletion depth
occur. Once electron−hole pairs are generated by the visible light, holes migrate to the
surface and discharge the adsorbed oxygen molecules. This causes the depth of the
depletion layer to decrease, resulting in the desorption of surface oxygen. Over time,
unpaired electrons accumulate until the desorption and adsorption of oxygen reaches an
equilibrium state. The amount of adsorbed oxygen decreases compared to air conditions
as shown in Figure 5.8 (a). The presence of excitons under visible light irradiation leads
to the formation of atomic adsorbed oxygen, O−, which is substantially more chemically
active than O2− and creates favorable conditions for catalytic reactions (Barry & Stone,
1960; Fan, Srivastava, & Dravid, 2009). This phenomena contributed to the amount of
light scattering into optical core fiber by the ZnO nanorods. When gases (such as
methanol in this case) are introduced, the adsorbed oxygen on ZnO nanorods took part in
the oxidation of methanol in two possible ways (Equation (5.4) and (5.5)) (Patel, Patel,
& Vaishnav, 2003). The oxygen ions on the surface of ZnO reacted with the methanol
molecules and give up electrons to the conduction band and increase the carrier
concentration in the ZnO nanostructure as shown in Figure 5.8(c).
CH3OH + O−↔ HCHO + H2O + e− (5.4)
CH3OH + O2−↔ HCOOH + H2O + e− (5.5)
As a result, scattering attenuation was therefore a function of the type of molecular
species adsorbed onto the ZnO surface (e.g. its refractive index, n) and the amount of that
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material present, allowing these molecules to interact differently with the incoming light.
At the same time different organic molecules have different refractive indices, molecule
sizes and band-bending occurs at surface which will also affect the interaction of
incoming light by varying the n of the outer coating differently (Yebo et al., 2010).
Figure 5.8 Schematic diagram of the alcohol sensing mechanism activated using
visible white light illumination (a) in air at room temperature (b) with visible white light
and (c) with methanol exposure
As observed from the responses of the sensor in multiple optical channels as
shown in Figure 5.9, different wavelengths of light as well contributed to determining the
amount of chemical vapor absorption onto ZnO nanorods with attenuated light scattering
into the POF. In this result, the green channel presented the largest range of intensity
(0.11 – 0.87) followed by the blue channel (0.07 - 0.41) and then, with the lowest
intensity, the red channel (0.05 – 0.17). It can be concluded that the intensity of green
light in ZnO nanorods dramatically decreased with the increase in vapor concentration as
alcohol molecules are adsorbed onto the ZnO coating.
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Figure 5.9 The responses of multiple optical channels sensor in channel (a) blue (b)
green and (c) red
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Subsequently, there was a degree of attenuation of light scattering into POF. It was shown
in references (H. Lin et al., 2006; Vanheusden, Warren, Seager, Tallant, Caruso, et al.,
1996; Vanheusden, Warren, Seager, Tallant, Voigt, et al., 1996) that the luminance
characteristic of ZnO has a significant response in green spectral range due to the strong
influence by free-carrier depletion at the particle surface, particularly for small ZnO
particles. Moreover, upon exposure to visible light, ZnO nanorod coating will be
photoactivated leading to reduced inter - grain barrier height, thereby increasing the
density of free carriers in the material. Boiling points of primary alcohols in these
experiments are methanol: 65 ˚C; ethanol: 78 ˚C; isopropanol: 82 ˚C which are related
anyway to the bond dissociation energies. Hence, the order of sensitivity was in the
reverse order. This leads to the specific heats of vaporization which is the lowest for
isopropanol (0.471 kJ/g) compared to ethanol (0.925 kJ/g) and methanol (1.22 kJ/g).
Figure 5.10 shows the relative intensity modulations (RIMs) of the
multichannel optical sensor for alcohol vapors. The RIMs of the sensor in each channel
were calculated using Eq. (5.2). The absolute of the light intensity decreased rapidly with
increasing the alcohol vapors concentration from 0 – 300 ppm. The green channel
contributed the highest RIMs for all three alcohols. Measurements in the blue light
domain showed intermediate RIMs, and lastly, the red channel exhibited significantly
lower RIMs of all three light domains for all alcohols tested. For the green channel, the
sensor response for methanol was found to be 0.84 a.u. However, when ethanol and
isopropanol were tested, the RIM decreased to 0.55 a.u and 0.14 a.u, respectively. In the
case of the blue channel, sensor response to methanol also presented the highest RIM of
the three alcohols equal to 0.79 a.u. For ethanol and isopropanol, once again, decreased
but to lower levels than those observed for the green channel- 0.49 a.u and 0.13 a.u,
respectively. The same trends in alcohol RIM based on molecular weight was
demonstrated by the red channel but to the lowest levels of the three light domains.
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Figure 5.10 The relative intensity modulation (RIM) of multiple optical channels sensor
exposed to ethanol, methanol and isopropanol vapors
The optimum RIM for isopropanol is smaller than that of ethanol due to reduced
absorption onto the ZnO nanorods by the larger molecular weight and slightly less polar
molecule (Alenezi et al., 2013). Channel green demonstrated significant RIMs in sensing
the three alcohol vapors because ZnO nanorods have both polar and non-polar surfaces
as reported in (M. Huang et al., 2014) where zinc vacancies (VZn) at the nonpolar surfaces
are responsible for the green luminescence of ZnO nanostructures (Fabbri et al., 2014).
The presence of neutral VZn on ZnO coating generates a multiple effect: the absence of
Zn ions leaves out under coordinated O atoms, and the unpaired O electrons give rise to
empty states. As a consequence, this leads to a high absorption in green luminescence due
to strong O-H chemical bonds (Willander et al., 2010).
A preliminary reference was developed by performing cross validation on channel
blue and green with respect to channel red that produced the minimal response for the
alcohol vapors as shown in Figure 5.10 in order to create an inexpensive multichannel
sensing system using red, green and blue LEDs and a simple photodetector. To observe
the response of the sensor as a multichannel device, one of the three channels was set as
a reference to accommodate for source fluctuation and environment effect (heat and
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vibration for example). The other two readings are normalized to the reference and the
RIM of different gas vapors to these channels are examined.
Figure 5.11 The validation of the multiple optical channels sensor for (a) channel blue/
channel red (b) channel green/ channel red
In both validations, the three alcohol vapors illustrated a similar pattern response at two
different ranges which were ~1.3 – 2.5 for channel blue/channel red and ~ 2.0 – 5.2 for
channel green/ channel red. In these ranges, significant responses were seen clearly in
methanol and ethanol sensing compared to isopropanol that showed less response.
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5.5 Summary
The study implemented successfully light side coupling with optimized spiral
patterned zinc oxide nanorod coatings for temperature and multiple optical channel
alcohol vapor sensing. The inherent advantages of optical fibers paired with the
transparency of ZnO nanorods in the visible wavelength region were applied to design a
simple and cost effective optical sensor. With increasing temperature, the coupled light
was found to reduce, which was used for calibrating as a temperature sensor. The spirally
patterned ZnO nanorods coating demonstrated significant improvement (1.3 times better
sensitivity) in the coupled light power compared to unpatterned coatings, since scattering
of light is dependent on the refractive index of ZnO surface and highly sensitive to ZnO
absorption. These robust but simple temperature sensors can find wide ranging
applications for environmental monitoring (Schroeder, Yamate, & Udd, 1999),
biomedical purposes (Korolyov & Potapov, 2012) as well as in environments with
electromagnetic pollution and/or explosive conditions. For instance, it can be also used
as a visible light photosensitive indicator for those who have photosensitivity diseases
related to sunlight or interior lighting. This proposed sensor is able to indicate the intensity
level of light at that a particular area due to excitation of ZnO upon receiving photons to
scatter light into POF.
Practical application of the special POF system also showed promise as a multiple
optical channel sensor for alcohol vapors in the visible range of the spectrum. To explain
the mechanism of sensing for alcohols, it was proposed that the light scattering aspect of
the ZnO nanorods dependent on changes in the refractive index was affected by
adsorption of alcohol species. With regard to sensing performance in the three spectral
channels described previously, methanol showed the greatest RIM and range followed by
ethanol and isopropanol. In the investigation of a multiple optical channel sensor, the
green channel significantly produced higher RIMs in sensing methanol, ethanol and
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isopropanol vapors compared to the blue and red channels. Furthermore, a preliminary
reference was developed in order to propose a multiple optical channels sensor system
using inexpensive color LEDs (blue, green and red) as light sources and a simple
photodetector in applications.
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CHAPTER 6: CONCLUSION AND FUTURE WORK
This chapter presents the overall summary of the results and conclusions. The
research described in this thesis showed a possibility of applying structuring growth of
ZnO nanorods on POF for various sensing applications. Although some comprehensive
conclusions were able to be drawn, the whole spectrum of questions was not exhausted.
The summary of the author's conclusions and experiences is written below in hope that
they may be helpful to someone who will continue this research work.
6.1 Conclusion
Structured growth of ZnO nanorods on POF has been successfully performed
using hydrothermal method. The size and morphology of the ZnO nanorods could be
varied through the changes in the concentration of the reactants, temperature, growth
duration and seeding technique. The study also demonstrated controlled light scattering
and efficient light coupling into the core modes of the ZnO nanorod coated POF. The
hydrothermal growth duration of the ZnO nanorods were found to be important in the
light coupling efficiency. From the optical characterization it was seen that the spiral
structure growth of the ZnO nanorods on POF for a growth duration of 12 hours at 90 ˚C
provided the maximum coupling power. The spiral pattern structure also has an improved
transmittance factor of 2.2 compared to the unpatterned coating with an extended light
source.
A new theoretical model specifically for light scattering from ZnO nanorods
coated POF have been successfully analysed the effect on coupling power by varying the
width of spiral structure. From the analysis, the width of spiral patterned ZnO nanorod
coatings on POF was optimized theoretically for maximum light side coupling with a
constant amplitude of light source and found to be 5 mm. The theoretical model also
proved that Gaussian beam was not able to couple light efficiently towards light side
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coupling. The width of spiral patterned ZnO nanorods coating on POF was experimentally
optimized and the experimental results correlated well with the simulations.
A major highlight of this project was the use of POF as optical sensors using ZnO
nanorods. The inherent advantages of optical POF paired with the transparency of ZnO
nanorods in the visible wavelength region were incorporated to design a simple and cost
effective optical sensor for various applications. One-dimensional nanostructures with
very high surface to volume ratio can be attractive candidates for sensing purposes. The
possibility of light side coupling shown by the ZnO nanorods coated POF was exploited
as an optical temperature sensor. The experiment carried out on temperature sensing
showed a decrease in the coupled power as the temperature increases for the both coating
schemes. Spiral patterned coating demonstrated higher sensitivity compared to
unpatterned coating.
Further, the application was extended to the use of the fabricated POF as multiple
optical channel alcohol vapour sensor by utilizing the scattering properties of spiral
patterned ZnO nanorods grown on POF. The ZnO nanorod coated POF demonstrated
significant change in the coupled power in the presence of ethanol, methanol and
isopropanol vapours, since scattering of light is dependent on the nature of the surface
and highly sensitive to any changes on the surface. It was found that with increase in the
concentration of the alcohol vapours from 25 ppm to 300 ppm, the coupled light
decreases. This had a direct impact on the sensitivity. The spectral results were analysed
among which methanol gave a strongest response compared to ethanol and isopropanol
in three channels: red (620-750 nm), green (495-570 nm) and blue (450-495 nm). With
regard to alcohol detection sensitivity by spectral band, the green channel demonstrated
the highest RIM values followed by the blue and red channels respectively.
129
6.2 Future work
As future recommendations, the investigation can be further extended to increase
the coupling efficiency by using two or more POF coated with spiral patterned ZnO
nanorods coating. In addition, enhancement of the sensor system can be achieved through
an integration with an artificial intelligent (AI) system to sense even the smallest of
physical parameter changes. In application, the fabricated POF can be used as a probe in
optical wireless energy harvesting.
130
REFERENCES
Abdelghani, A., Chovelon, J., Jaffrezic-Renault, N., Ronot-Trioli, C., Veillas, C., &
Gagnaire, H. (1997). Surface plasmon resonance fibre-optic sensor for gas
detection. Sensors and Actuators B: Chemical, 39(1), 407-410.
Agbabiaka, A., Wiltfong, M., & Park, C. (2013). Small Angle X-Ray Scattering
Technique for the Particle Size Distribution of Nonporous Nanoparticles. Journal
of Nanoparticles, 2013.
Alenezi, M. R., Alshammari, A. S., Jayawardena, K., Beliatis, M. J., Henley, S. J., &
Silva, S. (2013). Role of the exposed polar facets in the performance of thermally
and UV activated ZnO nanostructured gas sensors. The Journal of Physical
Chemistry C, 117(34), 17850-17858.
Amin, G., Asif, M., Zainelabdin, A., Zaman, S., Nur, O., & Willander, M. (2011).
Influence of pH, precursor concentration, growth time, and temperature on the
morphology of ZnO nanostructures grown by the hydrothermal method. Journal
of Nanomaterials, 2011, 5.
Aneesh, R., & Khijwania, S. K. (2011). Zinc oxide nanoparticle based optical fiber
humidity sensor having linear response throughout a large dynamic range. Applied
optics, 50(27), 5310-5314.
Antonio-Lopez, J. E., Eznaveh, Z. S., LiKamWa, P., Schülzgen, A., & Amezcua-Correa,
R. (2014). Multicore fiber sensor for high-temperature applications up to 1000 C.
Opt Lett, 39(15), 4309-4312.
Antunes, P. F., Varum, H., & Andre, P. S. (2013). Intensity-encoded polymer optical fiber
accelerometer. Sensors Journal, IEEE, 13(5), 1716-1720.
Ao, W., Li, J., Yang, H., Zeng, X., & Ma, X. (2006). Mechanochemical synthesis of zinc
oxide nanocrystalline. Powder Technology, 168(3), 148-151.
Aoki, T., Hatanaka, Y., & Look, D. C. (2000). ZnO diode fabricated by excimer-laser
doping. Applied physics letters, 76(22), 3257-3258.
Arditty, H., Papuchon, M., & Puech, C. (1981). Fiber-optic rotation sensor: Toward an
integrated device; A review. Quantum Electronics, IEEE Journal of, 17(12),
2454-2454.
Ashfold, M. N., Doherty, R. P., Ndifor-Angwafor, N. G., Riley, D. J., & Sun, Y. (2007).
The kinetics of the hydrothermal growth of ZnO nanostructures. Thin Solid Films,
515(24), 8679-8683.
Aslam, M., Chaudhary, V., Mulla, I., Sainkar, S., Mandale, A., Belhekar, A., &
Vijayamohanan, K. (1999). A highly selective ammonia gas sensor using surface-
ruthenated zinc oxide. Sensors and Actuators A: Physical, 75(2), 162-167.
Ayouchi, R., Martin, F., Leinen, D., & Ramos-Barrado, J. (2003). Growth of pure ZnO
thin films prepared by chemical spray pyrolysis on silicon. Journal of Crystal
Growth, 247(3), 497-504.
131
Banerjee, D., Lao, J., Wang, D., Huang, J., Ren, Z., Steeves, D., . . . Sennett, M. (2003).
Large-quantity free-standing ZnO nanowires. Applied physics letters, 83(10),
2061-2063.
Barry, T., & Stone, F. (1960). The reactions of oxygen at dark and irradiated zinc oxide
surface. Paper presented at the Proceedings of the Royal Society of London A:
Mathematical, Physical and Engineering Sciences.
Baruah, S., & Dutta, J. (2008). Hydrothermal growth of ZnO nanostructures. SCIENCE
AND TECHNOLOGY OF ADVANCED MATERIALS.
Baruah, S., & Dutta, J. (2009a). Effect of seeded substrates on hydrothermally grown
ZnO nanorods. Journal of Sol-Gel Science and Technology, 50(3), 456-464. doi:
DOI 10.1007/s10971-009-1917-2
Baruah, S., & Dutta, J. (2009b). Hydrothermal growth of ZnO nanostructures. SCIENCE
AND TECHNOLOGY OF ADVANCED MATERIALS, 10(1), 18. Doi
10.1088/1468-6996/10/1/013001
Baruah, S., K Pal, S., & Dutta, J. (2012). Nanostructured zinc oxide for water treatment.
Nanoscience & Nanotechnology-Asia, 2(2), 90-102.
Batumalay, M., Harith, Z., Rafaie, H., Ahmad, F., Khasanah, M., Harun, S., . . . Ahmad,
H. (2014). Tapered plastic optical fiber coated with ZnO nanostructures for the
measurement of uric acid concentrations and changes in relative humidity.
Sensors and Actuators A: Physical, 210, 190-196.
Benhebal, H., Chaib, M., Salmon, T., Geens, J., Léonard, A., Lambert, S. D., . . .
Heinrichs, B. (2013). Photocatalytic degradation of phenol and benzoic acid using
zinc oxide powders prepared by the sol–gel process. Alexandria Engineering
Journal, 52(3), 517-523.
Berginski, M., Hüpkes, J., Schulte, M., Schöpe, G., Stiebig, H., Rech, B., & Wuttig, M.
(2007). The effect of front ZnO: Al surface texture and optical transparency on
efficient light trapping in silicon thin-film solar cells. Journal of applied physics,
101(7), 074903.
Bhattacharyya, P., Basu, P., Saha, H., & Basu, S. (2007). Fast response methane sensor
using nanocrystalline zinc oxide thin films derived by sol–gel method. Sensors
and Actuators B: Chemical, 124(1), 62-67.
Bora, T., Fallah, H., Chaudhari, M., Apiwattanadej, T., Harun, S. W., Mohammed, W. S.,
& Dutta, J. (2014). Controlled side coupling of light to cladding mode of ZnO
nanorod coated optical fibers and its implications for chemical vapor sensing.
Sensors and Actuators B: Chemical, 202, 543-550.
Bosc, D., & Toinen, C. (1993). Tensile mechanical properties and reduced internal
stresses of polymer optical fiber. Polymer composites, 14(5), 410-413.
Budiansky, B., Drucker, D., Kino, G., & Rice, J. (1979). Pressure sensitivity of a clad
optical fiber. Applied optics, 18(24), 4085-4088.
132
Byrne, C., & Lim, C. L. (2007). The ingestible telemetric body core temperature sensor:
a review of validity and exercise applications. British journal of sports medicine,
41(3), 126-133.
Cao, H., Zhao, Y., Ong, H., Ho, S., Dai, J. Y., Wu, J. Y., & Chang, R. (1998). Ultraviolet
lasing in resonators formed by scattering in semiconductor polycrystalline films.
Applied physics letters, 73(25), 3656-3658.
Chang, C.-C., Chiu, N.-F., Lin, D. S., Chu-Su, Y., Liang, Y.-H., & Lin, C.-W. (2010).
High-sensitivity detection of carbohydrate antigen 15-3 using a gold/zinc oxide
thin film surface plasmon resonance-based biosensor. analytical chemistry, 82(4),
1207-1212.
Chen, D., Jiao, X., & Cheng, G. (1999). Hydrothermal synthesis of zinc oxide powders
with different morphologies. Solid State Communications, 113(6), 363-366.
Chen, H., Roco, M. C., Son, J., Jiang, S., Larson, C. A., & Gao, Q. (2013). Global
nanotechnology development from 1991 to 2012: patents, scientific publications,
and effect of NSF funding. Journal of nanoparticle research, 15(9), 1-21.
Chen, J., Zeng, F., Li, D., Niu, J., & Pan, F. (2005). Deposition of high-quality zinc oxide
thin films on diamond substrates for high-frequency surface acoustic wave filter
applications. Thin Solid Films, 485(1), 257-261.
Cherin, A. H. (1983). An Introduction To Optical Fibers.
Chiu, W., Khiew, P., Cloke, M., Isa, D., Tan, T., Radiman, S., . . . Lim, H. (2010).
Photocatalytic study of two-dimensional ZnO nanopellets in the decomposition of
methylene blue. Chemical Engineering Journal, 158(2), 345-352.
Chong, X., Kim, K.-J., Li, E., Zhang, Y., Ohodnicki, P. R., Chang, C.-H., & Wang, A. X.
(2016). Near-infrared absorption gas sensing with metal-organic framework on
optical fibers. Sensors and Actuators B: Chemical, 232, 43-51.
Chong, X., Kim, K.-J., Ohodnicki, P. R., Li, E., Chang, C.-H., & Wang, A. X. (2015).
Ultrashort Near-Infrared Fiber-Optic Sensors for Carbon Dioxide Detection.
Sensors Journal, IEEE, 15(9), 5327-5332.
Clevenson, H., Desjardins, P., Gan, X., & Englund, D. (2013). Gas sensing using a
resilient polymer photonic crystal nanocavity with ultra-high quality factor. Paper
presented at the CLEO: Applications and Technology.
Cooper, K., Elster, J., Jones, M., & Kelly, R. (2001). Optical fiber-based corrosion sensor
systems for health monitoring of aging aircraft. Paper presented at the
AUTOTESTCON Proceedings, 2001. IEEE Systems Readiness Technology
Conference.
Cordeiro, C. M., Franco, M. A., Chesini, G., Barretto, E. C., Lwin, R., Cruz, C. B., &
Large, M. C. (2006). Microstructured-core optical fibre for evanescent sensing
applications. Opt Express, 14(26), 13056-13066.
133
Cullum, B. M., & Vo-Dinh, T. (2000). The development of optical nanosensors for
biological measurements. Trends in biotechnology, 18(9), 388-393.
Dandridge, A., Tveten, A., & Giallorenzi, T. (1981). Interferometric current sensors using
optical fibres. Electronics Letters, 17(15), 523-525.
Dandridge, A., Tveten, A., Sigel, G., West, E., & Giallorenzi, T. (1980). Optical fibre
magnetic field sensors. Electronics Letters, 16(11), 408-409.
Dedova, T., Volobujeva, O., Klauson, J., Mere, A., & Krunks, M. (2007). ZnO nanorods
via spray deposition of solutions containing zinc chloride and thiocarbamide.
Nanoscale Research Letters, 2(8), 391-396.
Dickey, F. M., Weichman, L. S., & Shagam, R. N. (2000). Laser beam shaping
techniques. Paper presented at the High-Power Laser Ablation.
Docquier, N., & Candel, S. (2002). Combustion control and sensors: a review. Progress
in energy and combustion science, 28(2), 107-150.
Durana, G., Kirchhof, M., Luber, M., Ocáriz, D., Sáez, I., Poisel, H., . . . Vázquez, C.
(2009). Use of a novel fiber optical strain sensor for monitoring the vertical
deflection of an aircraft flap. Sensors Journal, IEEE, 9(10), 1219-1225.
Dutta, J., & Hofmann, H. (2004). Self-organization of colloidal nanoparticles.
Encyclopedia of nanoscience and nanotechnology, 9, 617-640.
Dwivedi, Y. S., Sharma, A. K., & Gupta, B. D. (2007). Influence of skew rays on the
sensitivity and signal-to-noise ratio of a fiber-optic surface-plasmon-resonance
sensor: a theoretical study. Applied optics, 46(21), 4563-4569.
Edmonson, J. M. (1991). History of the instruments for gastrointestinal endoscopy.
Gastrointest Endosc, 37, S27-S56.
Espitia, P. J. P., Soares, N. d. F. F., dos Reis Coimbra, J. S., de Andrade, N. J., Cruz, R.
S., & Medeiros, E. A. A. (2012). Zinc oxide nanoparticles: synthesis,
antimicrobial activity and food packaging applications. Food and Bioprocess
Technology, 5(5), 1447-1464.
Fabbri, F., Villani, M., Catellani, A., Calzolari, A., Cicero, G., Calestani, D., . . .
Sekiguchi, T. (2014). Zn vacancy induced green luminescence on non-polar
surfaces in ZnO nanostructures. Scientific reports, 4.
Fallah, H., Chaudhari, M., Bora, T., Harun, S., Mohammed, W., & Dutta, J. (2013).
Demonstration of side coupling to cladding modes through zinc oxide nanorods
grown on multimode optical fiber. Opt Lett, 38(18), 3620-3622.
Fallah, H., Harun, S. W., Mohammed, W. S., & Dutta, J. (2014). Excitation of core modes
through side coupling to multimode optical fiber by hydrothermal growth of ZnO
nanorods for wide angle optical reception. JOSA B, 31(9), 2232-2238.
134
Fan, S.-W., Srivastava, A. K., & Dravid, V. P. (2009). UV-activated room-temperature
gas sensing mechanism of polycrystalline ZnO. Applied physics letters, 95(14),
142106.
Fischer, U. H., Haupt, M., Reinboth, C., & Just, J.-U. (2015). Applying the principles of
augmented learning to photonics laboratory work. Paper presented at the Tenth
International Topical Meeting on Education and Training in Optics and Photonics.
Giallorenzi, T. G., Bucaro, J. A., Dandridge, A., & Cole, J. H. (1986). Optical-fiber
sensors challenge the competition: Resistance to corrosion and immunity to
interference head the list of benefits in detecting stimuli ranging from pressure to
magnetism. Spectrum, IEEE, 23(9), 44-50.
Giallorenzi, T. G., Bucaro, J. A., Dandridge, A., Sigel Jr, G., Cole, J. H., Rashleigh, S.
C., & Priest, R. G. (1982). Optical fiber sensor technology. Microwave Theory
and Techniques, IEEE Transactions on, 30(4), 472-511.
Giri, A. K., Saha, A., Mondal, A., Ghosh, S. C., Kundu, S., & Panda, A. B. (2015).
Rectangular ZnO porous nano-plate assembly with excellent acetone sensing
performance and catalytic activity. RSC Advances, 5(124), 102134-102142.
Gomez, J., Zubia, J., Aranguren, G., Arrue, J., Poisel, H., & Saez, I. (2009). Comparing
polymer optical fiber, fiber Bragg grating, and traditional strain gauge for aircraft
structural health monitoring. Applied optics, 48(8), 1436-1443.
Gómez, J., Zubia, J., Aranguren, G., Durana, G., Illaro, J., Sáez, I., . . . Hartl, E. (2008).
Comparing polymer optical fiber (POF), fiber Bragg gratings and traditional
strain gauge for aircraft structural health monitoring. Paper presented at the 2008
IEEE Avionics, Fiber-Optics and Photonics Technology Conference.
Gorla, C., Emanetoglu, N., Liang, S., Mayo, W., Lu, Y., Wraback, M., & Shen, H. (1999).
Structural, optical, and surface acoustic wave properties of epitaxial ZnO films
grown on (0112) sapphire by metalorganic chemical vapor deposition. Journal of
applied physics, 85(5), 2595-2602.
Grattan, K., & Sun, T. (2000). Fiber optic sensor technology: an overview. Sensors and
Actuators A: Physical, 82(1), 40-61.
Grattan, L., & Meggitt, B. (2013). Optical fiber sensor technology: advanced
applications-Bragg gratings and distributed sensors: Springer Science &
Business Media.
Hamouda, T., Peters, K., & Seyam, A.-F. M. (2012). Effect of resin type on the signal
integrity of an embedded perfluorinated polymer optical fiber. Smart materials
and structures, 21(5), 055023.
Han, Y.-G., Lee, J., & Lee, S. (2004). Discrimination of bending and temperature
sensitivities with phase-shifted long-period fiber gratings depending on initial
coupling strength. Opt Express, 12(14), 3204-3208.
135
Harith, Z., Irawati, N., Rafaie, H. A., Batumalay, M., Harun, S. W., Nor, R. M., & Ahmad,
H. (2015). Tapered Plastic Optical Fiber Coated With Al-Doped ZnO
Nanostructures for Detecting Relative Humidity. Sensors Journal, IEEE, 15(2),
845-849.
Harun, S., Lim, K., Damanhuri, S., & Ahmad, H. (2011). Microfiber loop resonator based
temperature sensor. Journal of the European Optical Society-Rapid publications,
6.
Hayashi, N., Mizuno, Y., & Nakamura, K. (2012). Brillouin gain spectrum dependence
on large strain in perfluorinated graded-index polymer optical fiber. Opt Express,
20(19), 21101-21106.
Hentschel, C. (1983). Fiber optics handbook.
Hernández-Romano, I., Monzón-Hernández, D., Moreno-Hernández, C., Moreno-
Hernandez, D., & Villatoro, J. (2015). Highly sensitive temperature sensor based
on a polymer-coated microfiber interferometer. Ieee Photonics Technology
Letters, 27(24), 2591-2594.
Hey, J. (1983). From Leonardo to the Graser: light scattering in historical perspective. S
Afr J Sci, 79, 11-27.
Hill, K., Fujii, Y., Johnson, D. C., & Kawasaki, B. (1978). Photosensitivity in optical
fiber waveguides: Application to reflection filter fabrication. Applied physics
letters, 32(10), 647-649.
Hocker, G. (1979). Fiber-optic sensing of pressure and temperature. Applied optics, 18(9),
1445-1448.
Hodgkinson, J., & Tatam, R. P. (2012). Optical gas sensing: a review. Measurement
Science and Technology, 24(1), 012004.
Hoo, Y., Jin, W., Ho, H. L., Wang, D., & Windeler, R. S. (2002). Evanescent-wave gas
sensing using microstructure fiber. Optical Engineering, 41(1), 8-9.
Huang, J., Lan, X., Wang, H., Yuan, L., Wei, T., Gao, Z., & Xiao, H. (2012). Polymer
optical fiber for large strain measurement based on multimode interference. Opt
Lett, 37(20), 4308-4310.
Huang, M., Yan, Y., Feng, W., Weng, S., Zheng, Z., Fu, X., & Liu, P. (2014). Controllable
tuning various ratios of ZnO polar facets by crystal seed-assisted growth and their
photocatalytic activity. Crystal Growth & Design, 14(5), 2179-2186.
Hulanicki, A., Glab, S., & Ingman, F. (1991). Chemical sensors: definitions and
classification. Pure and Applied Chemistry, 63(9), 1247-1250.
Hulst, H. C., & Van De Hulst, H. (1957). Light scattering by small particles: Courier
Corporation.
136
Husdi, I. R., Nakamura, K., & Ueha, S. (2004). Sensing characteristics of plastic optical
fibres measured by optical time-domain reflectometry. Measurement Science and
Technology, 15(8), 1553.
Hvedstrup Jensen, G. (1974). Temperature Dependence of the Band Gap in ZnO from
Reflection Data. physica status solidi (b), 64(1), K51-K54.
Ihara, M., Takahiro, I., Kusunoki, T., Ohno, K., Senna, M., Isobe, T., & Konishi, M.
(2002). Method for producing light-emitting substance: Google Patents.
Ishigure, T., Horibe, A., Nihei, E., & Koike, Y. (1995). High-bandwidth, high-numerical
aperture graded-index polymer optical fiber. Lightwave Technology, Journal of,
13(8), 1686-1691.
Ismail, A. A., El-Midany, A., Abdel-Aal, E., & El-Shall, H. (2005). Application of
statistical design to optimize the preparation of ZnO nanoparticles via
hydrothermal technique. Materials Letters, 59(14), 1924-1928.
Jaisai, M., Baruah, S., & Dutta, J. (2012). Paper modified with ZnO nanorods–
antimicrobial studies. Beilstein journal of nanotechnology, 3(1), 684-691.
Janotti, A., & Van de Walle, C. G. (2009). Fundamentals of zinc oxide as a
semiconductor. Reports on Progress in Physics, 72(12), 126501.
Jeunhomme, L. B. (1983). Single-mode fiber optics. Principles and applications. Optical
Engineering, New York: Dekker, 1983, 1.
Jiang, C., Kuzyk, M. G., Ding, J.-L., Johns, W. E., & Welker, D. J. (2002). Fabrication
and mechanical behavior of dye-doped polymer optical fiber. Journal of applied
physics, 92(1), 4-12.
Jing, Q., Fu, W., Li, W., Yang, H., Li, M., Ma, J., . . . Zhang, Y. (2012). Synthesis of
snowflake-like multi-layered ZnO with controllable pore sizes and its
photocatalytic property. Applied surface science, 258(8), 3604-3610.
Jo, S., Lao, J., Ren, Z., Farrer, R., Baldacchini, T., & Fourkas, J. (2003). Field-emission
studies on thin films of zinc oxide nanowires. Applied physics letters, 83(23),
4821-4823.
Ju, S., Watekar, P. R., & Han, W.-T. (2009). Highly sensitive temperature sensor using
fiber Bragg grating on Pb/Ge-codoped fiber. Paper presented at the National Fiber
Optic Engineers Conference.
Kao, C. K. (1983). Optical Fiber Systems: Technology, Design, and Application.
Keiser, G. (2000). Optical fiber communication. NY: McGraw-Hill.
Kerker, M. (1969). The scattering of light: Academic Press, New York.
Kiesel, S., Peters, K., Hassan, T., & Kowalsky, M. (2007). Behaviour of intrinsic polymer
optical fibre sensor for large-strain applications. Measurement Science and
Technology, 18(10), 3144.
137
Kim, J. H., Kim, E. M., Andeen, D., Thomson, D., DenBaars, S. P., & Lange, F. F. (2007).
Growth of Heteroepitaxial ZnO Thin Films on GaN‐Buffered Al2O3 (0001)
Substrates by Low‐Temperature Hydrothermal Synthesis at 90° C. Advanced
Functional Materials, 17(3), 463-471.
Kim, T.-U., Kim, J.-A., Pawar, S., Moon, J.-H., & Kim, J. H. (2010). Creation of
nanoscale two-dimensional patterns of ZnO nanorods using laser interference
lithography followed by hydrothermal synthesis at 90 C. Crystal Growth &
Design, 10(10), 4256-4261.
Kołodziejczak-Radzimska, A., & Jesionowski, T. (2014). Zinc oxide—from synthesis to
application: a review. Materials, 7(4), 2833-2881.
Kong, T., Chen, Y., Ye, Y., Zhang, K., Wang, Z., & Wang, X. (2009). An amperometric
glucose biosensor based on the immobilization of glucose oxidase on the ZnO
nanotubes. Sensors and Actuators B: Chemical, 138(1), 344-350.
Kong, Y., Yu, D., Zhang, B., Fang, W., & Feng, S. (2001). Ultraviolet-emitting ZnO
nanowires synthesized by a physical vapor deposition approach. Applied physics
letters, 78(4), 407-409.
Korolyov, V., & Potapov, V. (2012). Biomedical Fiber-Optic Temperature and Pressure
Sensors. Biomedical Engineering, 1-4.
Koziol, K., Boskovic, B., & Yahya, N. (2011). Carbon and Oxide Nanostructures:
Synthesis, Characterisation and Applications. Columbia: Henry Dickens & Co.
Kraft, M. (2006). VIBRATIONAL SPECTROSCOPIC SENSORS Fundamentals,
Instrumentation and Applications Optical Chemical Sensors (pp. 117-155):
Springer.
Krč, J., Zeman, M., Kluth, O., Smole, F., & Topič, M. (2003). Effect of surface roughness
of ZnO: Al films on light scattering in hydrogenated amorphous silicon solar cells.
Thin Solid Films, 426(1), 296-304.
Krebs, F. C., Thomann, Y., Thomann, R., & Andreasen, J. W. (2008). A simple
nanostructured polymer/ZnO hybrid solar cell—preparation and operation in air.
Nanotechnology, 19(42), 424013.
Kumar, S., & Swaminathan, S. (2016). Design of liquid temperature sensor based on
bending loss phenomenon of plastic optic fiber and electro-optic effect of Mach-
Zehnder interferometer. Paper presented at the SPIE Photonics Europe.
Kumar, S. A., & Chen, S. M. (2008). Nanostructured zinc oxide particles in chemically
modified electrodes for biosensor applications. Analytical Letters, 41(2), 141-158.
Kurashima, T., Horiguchi, T., & Tateda, M. (1990). Distributed-temperature sensing
using stimulated Brillouin scattering in optical silica fibers. Opt Lett, 15(18),
1038-1040.
Lacy, E. (1982). Fiber Optics Prentice-Hall. Englewood Cliffs, New Jersey, 70.
138
Lagakos, N., & Bucaro, J. A. (1981). Pressure desensitization of optical fibers. Applied
optics, 20(15), 2716-2720.
Law, M. K., Bermak, A., & Luong, H. C. (2010). A sub-μmW embedded CMOS
temperature sensor for RFID food monitoring application. IEEE journal of solid-
state circuits, 45(6), 1246.
Lee, B. (2003). Review of the present status of optical fiber sensors. Optical fiber
technology, 9(2), 57-79.
Lee, B. H., Kim, Y. H., Park, K. S., Eom, J. B., Kim, M. J., Rho, B. S., & Choi, H. Y.
(2012). Interferometric fiber optic sensors. Sensors (Basel), 12(3), 2467-2486.
Lee, J.-R., Dhital, D., & Yoon, D.-J. (2011). Investigation of cladding and coating
stripping methods for specialty optical fibers. Optics and Lasers in Engineering,
49(3), 324-330.
Leiner, M. J. (1991). Luminescence chemical sensors for biomedical applications: scope
and limitations. Analytica chimica acta, 255(2), 209-222.
Li, X., Lin, S., Liang, J., Zhang, Y., Oigawa, H., & Ueda, T. (2012). Fiber-optic
temperature sensor based on difference of thermal expansion coefficient between
fused silica and metallic materials. IEEE Photonics Journal, 4(1), 155-162.
Liebsch, G., Klimant, I., Krause, C., & Wolfbeis, O. S. (2001). Fluorescent imaging of
pH with optical sensors using time domain dual lifetime referencing. analytical
chemistry, 73(17), 4354-4363.
Liedberg, B., Nylander, C., & Lunström, I. (1983). Surface plasmon resonance for gas
detection and biosensing. Sensors and Actuators, 4, 299-304.
Liehr, S., & Krebber, K. (2012). Application of quasi-distributed and dynamic length and
power change measurement using optical frequency domain reflectometry.
Sensors Journal, IEEE, 12(1), 237-245.
Liehr, S., Lenke, P., Wendt, M., Krebber, K., Seeger, M., Thiele, E., . . . Münich, J. C.
(2009). Polymer optical fiber sensors for distributed strain measurement and
application in structural health monitoring. Sensors Journal, IEEE, 9(11), 1330-
1338.
Liehr, S., Nöther, N., & Krebber, K. (2009). Incoherent optical frequency domain
reflectometry and distributed strain detection in polymer optical fibers.
Measurement Science and Technology, 21(1), 017001.
Liehr, S., Wendt, M., & Krebber, K. (2010). Distributed strain measurement in
perfluorinated polymer optical fibres using optical frequency domain
reflectometry. Measurement Science and Technology, 21(9), 094023.
Lilienfeld, P. (2004). A blue sky history. Optics and photonics news, 15(6), 32-39.
139
Lin, F.-C., Takao, Y., Shimizu, Y., & Egashira, M. (1995). Hydrogen-sensing mechanism
of zinc oxide varistor gas sensors. Sensors and Actuators B: Chemical, 25(1), 843-
850.
Lin, H., Cheng, C., Chou, Y., Huang, L., Chen, Y., & Tsen, K. (2006). Enhancement of
band gap emission stimulated by defect loss. Opt Express, 14(6), 2372-2379.
Lipfert, J., Columbus, L., Chu, V. B., Lesley, S. A., & Doniach, S. (2007). Size and shape
of detergent micelles determined by small-angle X-ray scattering. The Journal of
Physical Chemistry B, 111(43), 12427-12438.
Lokman, A., Harun, S., Harith, Z., Rafaie, H., Nor, R., & Arof, H. (2015). Inline Mach–
Zehnder interferometer with ZnO nanowires coating for the measurement of uric
acid concentrations. Sensors and Actuators A: Physical, 234, 206-211.
Lv, R.-Q., Zhao, Y., Wang, D., & Wang, Q. (2014). Magnetic fluid-filled optical fiber
Fabry–Pérot sensor for magnetic field measurement. Photonics Technology
Letters, IEEE, 26(3), 217-219.
Lynch, D. K., & Livingston, W. C. (2001). Color and light in nature: Cambridge
University Press.
Mahato, T., Prasad, G., Singh, B., Acharya, J., Srivastava, A., & Vijayaraghavan, R.
(2009). Nanocrystalline zinc oxide for the decontamination of sarin. Journal of
hazardous materials, 165(1), 928-932.
Mahmood, M. A., Bora, T., & Dutta, J. (2013). Studies on hydrothermally synthesised
zinc oxide nanorod arrays for their enhanced visible light photocatalysis.
International Journal of Environmental Technology and Management, 16(1-2),
146-159.
Maiman, T. H. (1960). Stimulated optical radiation in ruby.
McCorkle, D. L., Warmack, R. J., Patel, S. V., Mlsna, T., Hunter, S. R., & Ferrell, T. L.
(2005). Ethanol vapor detection in aqueous environments using micro-capacitors
and dielectric polymers. Sensors and Actuators B: Chemical, 107(2), 892-903.
McFarland, A. D., & Van Duyne, R. P. (2003). Single silver nanoparticles as real-time
optical sensors with zeptomole sensitivity. Nano Lett, 3(8), 1057-1062.
McWilliams, K. R., & Lamb, S. (1994). Device for controlling or limiting temperature in
an electric cooking appliance: Google Patents.
Mendez, A., Morse, T. F., & Mendez, F. (1990). Applications of embedded optical fiber
sensors in reinforced concrete buildings and structures. Paper presented at the
OE/FIBERS'89.
Mescia, L., & Prudenzano, F. (2013). Advances on optical fiber sensors. Fibers, 2(1), 1-
23.
140
Michel, D., Xiao, F., Skillman, L., & Alameh, K. (2016). Surface Plasmon Resonance
Sensor for In Situ Detection of Xanthan Gum. Selected Topics in Quantum
Electronics, IEEE Journal of, 22(3), 1-4.
Miles, D., Cameron, P., & Mattia, D. (2015). Hierarchical 3D ZnO nanowire structures
via fast anodization of zinc. Journal of Materials Chemistry A, 3(34), 17569-
17577.
Minakawa, K., Hayashi, N., Shinohara, Y., Tahara, M., Hosoda, H., Mizuno, Y., &
Nakamura, K. (2014). Wide-range temperature dependences of Brillouin
scattering properties in polymer optical fiber. Japanese Journal of Applied
Physics, 53(4), 042502.
Minnaert, M. (2013). The nature of light and colour in the open air: Courier Corporation.
Mishra, R., Keimasi, M., & Das, D. (2004). The temperature ratings of electronic parts.
Electronics Cooling, 10(1), 20.
Montero, D. S., Lallana, P. C., & Vázquez, C. (2012). A polymer optical fiber fuel level
sensor: Application to paramotoring and powered paragliding. Sensors (Basel),
12(5), 6186-6197.
Morisawa, M., & Muto, S. (2012). Plastic optical fiber sensing of alcohol concentration
in liquors. Journal of Sensors, 2012.
Mourant, J. R., Hielscher, A. H., Eick, A. A., Johnson, T. M., & Freyer, J. P. (1998).
Evidence of intrinsic differences in the light scattering properties of tumorigenic
and nontumorigenic cells. Cancer Cytopathology, 84(6), 366-374.
Müller, J., Sönnichsen, C., Von Poschinger, H., Von Plessen, G., Klar, T., & Feldmann,
J. (2002). Electrically controlled light scattering with single metal nanoparticles.
Applied physics letters, 81(1), 171-173.
Murdock, R. C., Braydich-Stolle, L., Schrand, A. M., Schlager, J. J., & Hussain, S. M.
(2008). Characterization of nanomaterial dispersion in solution prior to in vitro
exposure using dynamic light scattering technique. Toxicological Sciences,
101(2), 239-253.
Narsaiah, K., Jha, S. N., Bhardwaj, R., Sharma, R., & Kumar, R. (2012). Optical
biosensors for food quality and safety assurance—a review. Journal of food
science and technology, 49(4), 383-406.
Near, R., Hayden, S., & El-Sayed, M. (2012). Extinction vs absorption: which is the
indicator of plasmonic field strength for silver nanocubes? The Journal of
Physical Chemistry C, 116(43), 23019-23026.
Nylander, C., Liedberg, B., & Lind, T. (1982). Gas detection by means of surface plasmon
resonance. Sensors and Actuators, 3, 79-88.
Oehme, I., & Wolfbeis, O. S. (1997). Optical sensors for determination of heavy metal
ions. Microchimica Acta, 126(3-4), 177-192.
141
Ohno, H., Naruse, H., Kihara, M., & Shimada, A. (2001). Industrial applications of the
BOTDR optical fiber strain sensor. Optical fiber technology, 7(1), 45-64.
Olmo, I. F., Chacon, E., & Irabien, A. (2001). Influence of lead, zinc, iron (III) and
chromium (III) oxides on the setting time and strength development of Portland
cement. Cement and Concrete Research, 31(8), 1213-1219.
Patel, N., Patel, P., & Vaishnav, V. (2003). Indium tin oxide (ITO) thin film gas sensor
for detection of methanol at room temperature. Sensors and Actuators B:
Chemical, 96(1), 180-189.
Pauporté, T., & Lincot, D. (2000). Electrodeposition of semiconductors for optoelectronic
devices: results on zinc oxide. Electrochimica Acta, 45(20), 3345-3353.
Peng, L., Zhai, J., Wang, D., Zhang, Y., Wang, P., Zhao, Q., & Xie, T. (2010). Size-and
photoelectric characteristics-dependent formaldehyde sensitivity of ZnO
irradiated with UV light. Sensors and Actuators B: Chemical, 148(1), 66-73.
Peters, K. (2010). Polymer optical fiber sensors—a review. Smart materials and
structures, 20(1), 013002.
Polsongkram, D., Chamninok, P., Pukird, S., Chow, L., Lupan, O., Chai, G., . . . Schulte,
A. (2008). Effect of synthesis conditions on the growth of ZnO nanorods via
hydrothermal method. Physica B: Condensed Matter, 403(19), 3713-3717.
Purica, M., Budianu, E., & Rusu, E. (2001). ZnO thin films on semiconductor substrate
for large area photodetector applications. Thin Solid Films, 383(1), 284-286.
Puyol, M., Villuendas, F., Domínguez, C., Cadarso, V., Llobera, A., Salinas, I., . . .
Alonso, J. (2005). Absorbance-based integrated optical sensors Frontiers in
Chemical Sensors (pp. 1-44): Springer.
Rahim, H. R. B. A., Manjunath, S., Fallah, H., Thokchom, S., Harun, S. W., Mohammed,
W. S., . . . Dutta, J. (2016). Side coupling of multiple optical channels by spiral
patterned zinc oxide coatings on large core plastic optical fibers. IET Micro &
Nano Letters, 11(2), 122-126.
Rahman, H. A., Harun, S. W., Saidin, N., Yasin, M., & Ahmad, H. (2012). Fiber optic
displacement sensor for temperature measurement. Sensors Journal, IEEE, 12(5),
1361-1364.
Rahman, H. A., Harun, S. W., Yasin, M., & Ahmad, H. (2012). Fiber-optic salinity sensor
using fiber-optic displacement measurement with flat and concave mirror.
Selected Topics in Quantum Electronics, IEEE Journal of, 18(5), 1529-1533.
Rajan, G. (2015). Optical Fiber Sensors: Advanced Techniques and Applications (Vol.
36): CRC press.
Rajan, G., Liu, B., Luo, Y., Ambikairajah, E., & Peng, G.-D. (2013). High sensitivity
force and pressure measurements using etched singlemode polymer fiber Bragg
gratings. Sensors Journal, IEEE, 13(5), 1794-1800.
142
Ramakrishnan, M., Rajan, G., Semenova, Y., & Farrell, G. (2016). Overview of Fiber
Optic Sensor Technologies for Strain/Temperature Sensing Applications in
Composite Materials. Sensors (Basel), 16(1), 99.
Rasmussen, J. W., Martinez, E., Louka, P., & Wingett, D. G. (2010). Zinc oxide
nanoparticles for selective destruction of tumor cells and potential for drug
delivery applications. Expert opinion on drug delivery, 7(9), 1063-1077.
Rayleigh, L. (1917). On the reflection of light from a regularly stratified medium.
Proceedings of the Royal Society of London. Series A, Containing Papers of a
Mathematical and Physical Character, 93(655), 565-577.
Repins, I., Contreras, M. A., Egaas, B., DeHart, C., Scharf, J., Perkins, C. L., . . . Noufi,
R. (2008). 19· 9%‐efficient ZnO/CdS/CuInGaSe2 solar cell with 81· 2% fill
factor. Progress in Photovoltaics: Research and applications, 16(3), 235-239.
Rothmaier, M., Luong, M. P., & Clemens, F. (2008). Textile pressure sensor made of
flexible plastic optical fibers. Sensors (Basel), 8(7), 4318-4329.
Saito, Y., Ichikawa, O., & Oshima, T. (1987). Waterproof optical fiber cable and method
of the production thereof: Google Patents.
Samarasekara, P., Yapa, N., Kumara, N., & Perera, M. (2007). CO2 gas sensitivity of
sputtered zinc oxide thin films. Bulletin of Materials Science, 30(2), 113-116.
Sanjeev, S., & Kekuda, D. (2015). Effect of Annealing Temperature on the Structural and
Optical Properties of Zinc Oxide (ZnO) Thin Films Prepared by Spin Coating
Process. Paper presented at the IOP Conference Series: Materials Science and
Engineering.
Sathitanon, N., & Pullteap, S. (2007). A fiber optic interferometric sensor for dynamic
measurement. measurements, 7, 8.
Sberveglieri, G. (2012). Gas sensors: principles, operation and developments: Springer
Science & Business Media.
Schmidt, R. R., & Notohardjono, B. D. (2002). High-end server low-temperature cooling.
IBM Journal of Research and Development, 46(6), 739-751.
Schroeder, R. J., Yamate, T., & Udd, E. (1999). High pressure and temperature sensing
for the oil industry using fiber Bragg gratings written onto side hole single mode
fiber. Paper presented at the PROCEEDINGS-SPIE THE INTERNATIONAL
SOCIETY FOR OPTICAL ENGINEERING.
Seaman, W. E. (1997). Electronically controlled container for storing temperature
sensitive material: Google Patents.
Shan, C., Liu, Z., & Hark, S. (2008). Temperature dependent photoluminescence study
on phosphorus doped ZnO nanowires. Applied physics letters, 92(7), 73103-
73103.
143
Shenhav, A., Brodie, Z., Beiderman, Y., Garcia, J., Mico, V., & Zalevsky, Z. (2013).
Optical sensor for remote estimation of alcohol concentration in blood stream.
Optics communications, 289, 149-157.
Shinde, S., Shinde, P., Bhosale, C., & Rajpure, K. (2008). Optoelectronic properties of
sprayed transparent and conducting indium doped zinc oxide thin films. Journal
of Physics D: Applied Physics, 41(10), 105109.
Silva-López, M., Fender, A., MacPherson, W. N., Barton, J. S., Jones, J. D., Zhao, D., . .
. Bennion, I. (2005). Strain and temperature sensitivity of a single-mode polymer
optical fiber. Opt Lett, 30(23), 3129-3131.
Singh, R., & Riess, F. (2001). The 1930 Nobel prize for physics: a close decision? Notes
and Records of the Royal Society, 55(2), 267-283.
Stanković, A., Veselinović, L., Škapin, S., Marković, S., & Uskoković, D. (2011).
Controlled mechanochemically assisted synthesis of ZnO nanopowders in the
presence of oxalic acid. Journal of materials science, 46(11), 3716-3724.
Stefani, A., Andresen, S., Yuan, W., & Bang, O. (2012). Dynamic characterization of
polymer optical fibers. Sensors Journal, IEEE, 12(10), 3047-3053.
Sugaya, M., Murai, F., Kaneko, Y., Kanetomo, M., Hirasawa, S., Watanabe, T., . . .
Kuroda, K. (2002). Substrate temperature control system and method for
controlling temperature of substrate: Google Patents.
Sugunan, A., Warad, H. C., Boman, M., & Dutta, J. (2006). Zinc oxide nanowires in
chemical bath on seeded substrates: role of hexamine. Journal of Sol-Gel Science
and Technology, 39(1), 49-56.
Szustakowski, M., Ciurapinski, W., Palka, N., & Zyczkowski, M. (2001). Recent
development of fiber optic sensors for perimeter security. Paper presented at the
Security Technology, 2001 IEEE 35th International Carnahan Conference on.
Takaki, S. (1998). Temperature sensor for medical application: Google Patents.
Tam, K., Cheung, C., Leung, Y., Djurišic, A., Ling, C., Beling, C., . . . Phillips, D. (2006).
Defects in ZnO nanorods prepared by a hydrothermal method. The Journal of
Physical Chemistry B, 110(42), 20865-20871.
Tang, Z., Wong, G. K., Yu, P., Kawasaki, M., Ohtomo, A., Koinuma, H., & Segawa, Y.
(1998). Room-temperature ultraviolet laser emission from self-assembled ZnO
microcrystallite thin films. Applied physics letters, 72(25), 3270-3272.
Tangonan, G. L., Persechini, D., Morrison, R., & Wysocki, J. (1980). Current sensing
with metal-coated multimode optical fibres. Electronics Letters, 16(25), 958-959.
Tyler, S. W., Selker, J. S., Hausner, M. B., Hatch, C. E., Torgersen, T., Thodal, C. E., &
Schladow, S. G. (2009). Environmental temperature sensing using Raman spectra
DTS fiber‐optic methods. Water Resources Research, 45(4).
144
van Eijkelenborg, M. A., Large, M. C., Argyros, A., Zagari, J., Manos, S., Issa, N. A., . .
. de Sterke, C. M. (2001). Microstructured polymer optical fibre. Opt Express,
9(7), 319-327.
Vanheusden, K., Seager, C., Warren, W. t., Tallant, D., & Voigt, J. (1996). Correlation
between photoluminescence and oxygen vacancies in ZnO phosphors. Applied
physics letters, 68(3), 403-405.
Vanheusden, K., Warren, W., Seager, C., Tallant, D., Caruso, J., Hampden-Smith, M., &
Kodas, T. (1996). Nature of the green luminescent center in zinc oxide. Paper
presented at the MRS Proceedings.
Vanheusden, K., Warren, W., Seager, C., Tallant, D., Voigt, J., & Gnade, B. (1996).
Mechanisms behind green photoluminescence in ZnO phosphor powders. Journal
of applied physics, 79(10), 7983-7990.
Venkatesh, P. S., & Jeganathan, K. (2013). Investigations on the growth and
characterization of vertically aligned zinc oxide nanowires by radio frequency
magnetronsputtering. Journal of Solid State Chemistry, 200, 84-89.
Vergés, M. A., Mifsud, A., & Serna, C. (1990). Formation of rod-like zinc oxide
microcrystals in homogeneous solutions. Journal of the Chemical Society,
Faraday Transactions, 86(6), 959-963.
Vogler, D., & Sigrist, M. (2006). Near-infrared laser based cavity ringdown spectroscopy
for applications in petrochemical industry. Applied Physics B, 85(2-3), 349-354.
Wahab, R., Kim, Y.-S., Mishra, A., Yun, S.-I., & Shin, H.-S. (2010). Formation of ZnO
micro-flowers prepared via solution process and their antibacterial activity.
Nanoscale Research Letters, 5(10), 1675-1681.
Wang, L., Kang, Y., Liu, X., Zhang, S., Huang, W., & Wang, S. (2012). ZnO nanorod
gas sensor for ethanol detection. Sensors and Actuators B: Chemical, 162(1), 237-
243.
Wang, M., & Zhang, L. (2009). The influence of orientation on the photoluminescence
behavior of ZnO thin films obtained by chemical solution deposition. Materials
Letters, 63(2), 301-303.
Wang, P., Semenova, Y., & Farrell, G. (2008). Temperature dependence of macrobending
loss in all-fiber bend loss edge filter. Optics communications, 281(17), 4312-4316.
Wang, P., Semenova, Y., Wu, Q., Zheng, J., & Farrell, G. (2010). Temperature
performance of a macrobending single-mode fiber-based refractometer. Applied
optics, 49(10), 1744-1749.
Wang, W., Wu, N., Tian, Y., Niezrecki, C., & Wang, X. (2010). Miniature all-silica
optical fiber pressure sensor with an ultrathin uniform diaphragm. Opt Express,
18(9), 9006-9014.
145
Wang, X., Summers, C. J., & Wang, Z. L. (2004). Large-scale hexagonal-patterned
growth of aligned ZnO nanorods for nano-optoelectronics and nanosensor arrays.
Nano Lett, 4(3), 423-426.
Wang, Z. L. (2004). Zinc oxide nanostructures: growth, properties and applications.
Journal of Physics: Condensed Matter, 16(25), R829.
Warren-Smith, S. C., Nguyen, L. V., Lang, C., Ebendorff-Heidepriem, H., & Monro, T.
M. (2016). Temperature sensing up to 1300° C using suspended-core
microstructured optical fibers. Opt Express, 24(4), 3714-3719.
Weng, J., Tan, H., Wang, X., Ma, Y., Hu, S., & Wang, X. (2006). Optical-fiber
interferometer for velocity measurements with picosecond resolution. Applied
physics letters, 89(11), 111101.
Whittaker, P. (1998). Iron and zinc interactions in humans. The American journal of
clinical nutrition, 68(2), 442S-446S.
Wiederrecht, G. (2010). Handbook of nanoscale optics and electronics: Academic Press.
Willander, M., Nur, O., Sadaf, J. R., Qadir, M. I., Zaman, S., Zainelabdin, A., . . . Hussain,
I. (2010). Luminescence from zinc oxide nanostructures and polymers and their
hybrid devices. Materials, 3(4), 2643-2667.
Witt, J., Schukar, M., & Krebber, K. (2008). Medicinal textiles with integrated polymer-
optical fibers for respiration monitoring. Tm-Technisches Messen, 75(12), 670-
677.
Wood, A. D., Stankovic, J. A., Virone, G., Selavo, L., He, Z., Cao, Q., . . . Stoleru, R.
(2008). Context-aware wireless sensor networks for assisted living and residential
monitoring. Network, IEEE, 22(4), 26-33.
Woyessa, G., Fasano, A., Stefani, A., Markos, C., Nielsen, K., Rasmussen, H. K., & Bang,
O. (2016). Single mode step-index polymer optical fiber for humidity insensitive
high temperature fiber Bragg grating sensors. Opt Express, 24(2), 1253-1260.
Woyessa, G., Nielsen, K., Stefani, A., Markos, C., & Bang, O. (2016). Temperature
insensitive hysteresis free highly sensitive polymer optical fiber Bragg grating
humidity sensor. Opt Express, 24(2), 1206-1213.
Wu, Y., Lim, C., Fu, S., Tok, A., Lau, H., Boey, F., & Zeng, X. (2007). Surface
modifications of ZnO quantum dots for bio-imaging. Nanotechnology, 18(21),
215604.
Xiang, B., Wang, P., Zhang, X., Dayeh, S. A., Aplin, D. P., Soci, C., . . . Wang, D. (2007).
Rational synthesis of p-type zinc oxide nanowire arrays using simple chemical
vapor deposition. Nano Lett, 7(2), 323-328.
Xu, J., Pan, Q., & Tian, Z. (2000). Grain size control and gas sensing properties of ZnO
gas sensor. Sensors and Actuators B: Chemical, 66(1), 277-279.
146
Xu, S., & Wang, Z. L. (2011). One-dimensional ZnO nanostructures: solution growth and
functional properties. Nano Research, 4(11), 1013-1098.
Yariv, A., & Winsor, H. V. (1980). Proposal for detection of magnetic fields through
magnetostrictive perturbation of optical fibers. Opt. Lett, 5(3), 87-89.
Yebo, N. A., Lommens, P., Hens, Z., & Baets, R. (2010). An integrated optic ethanol
vapor sensor based on a silicon-on-insulator microring resonator coated with a
porous ZnO film. Opt Express, 18(11), 11859-11866.
Yi, G.-C., Wang, C., & Park, W. I. (2005). ZnO nanorods: synthesis, characterization and
applications. Semiconductor Science and Technology, 20(4), S22.
Yokota, M., Okada, T., & Yamaguchi, I. (2007). An optical sensor for analysis of soil
nutrients by using LED light sources. Measurement Science and Technology,
18(7), 2197.
Zhang, H., Yang, D., Ma, X., Ji, Y., Xu, J., & Que, D. (2004). Synthesis of flower-like
ZnO nanostructures by an organic-free hydrothermal process. Nanotechnology,
15(5), 622.
Zhang, W., Webb, D. J., & Peng, G.-D. (2012). Investigation into time response of
polymer fiber Bragg grating based humidity sensors. Journal of lightwave
technology, 30(8), 1090-1096.
Zhang, Z., Grattan, K., & Palmer, A. (1992). Fiber optic temperature sensor based on the
cross referencing between blackbody radiation and fluorescence lifetime. Review
of scientific Instruments, 63(5), 3177-3181.
Zhang, Z. F., & Tao, X. M. (2013). Intrinsic temperature sensitivity of fiber Bragg
gratings in PMMA-based optical fibers. Photonics Technology Letters, IEEE,
25(3), 310-312.
Ziemann, O., Daum, W., Bräuer, A., Schlick, J., & Frank, W. (2000). Results of a German
6.000 hours accelerated aging test of PMMA POF and consequences for the
practical use of POF. Paper presented at the Proceedings of the 9th International
Conference on Plastic Optical Fibres, Boston.
Ziemann, O., Krauser, J., Zamzow, P. E., & Daum, W. (2008a). POF handbook. Springer.
Ziemann, O., Krauser, J., Zamzow, P. E., & Daum, W. (2008b). POF handbook: optical
short range transmission systems: Springer Science & Business Media.
Zubia, J., & Arrue, J. (2001). Plastic optical fibers: An introduction to their technological
processes and applications. Optical fiber technology, 7(2), 101-140.
Zvyagin, A. V., Zhao, X., Gierden, A., Sanchez, W., Ross, J. A., & Roberts, M. S. (2008).
Imaging of zinc oxide nanoparticle penetration in human skin in vitro and in vivo.
Journal of biomedical optics, 13(6), 064031-064031-064039.
147
LIST OF PUBLICATIONS, PAPERS PRESENTED AND PATENTS
Journal Publications (ISI)
1) Rahim, H. R. B. A., Manjunath, S., Fallah, H., Thokchom, S., Harun, S. W.,
Hornyak. L. G., Mohammed, W. S., & Dutta, J. (2016). Side coupling of multiple
optical channels by spiral patterned zinc oxide coatings on large core plastic
optical fibers. IET Micro & Nano Letters, 11(2), 122-126.
2) Rahim, H. R. B. A, Lokman. M. Q, Harun. S. W., Hornyak. L. G., Mohammed,
W. S., & Dutta, J. (2016). Applied Light Side Coupling With Optimized Spiral
Patterned Zinc Oxide Nanorod Coatings for Multiple Optical Channel Alcohol
Vapor Sensing. Journal of Nanophotonics, SPIE.
3) Rahim, H. R. B. A, Lokman. M. Q, Harun. S. W., Mohammed, W. S., & Dutta,
J. (2017), Temperature Sensing By Side Coupling Of Light Through Zinc Oxide
Nanorods On Optical Fibers, Elsevier: Sensors and Actuators A: Physical 257
(2017) 15–19.
4) Rahim, H. R. B. A, H., Irawati, N., Rafaie, H. A., Ahmad, H., Harun, S. W., &
Nor, R. M. (2015). Detection Of Different Concentrations Of Uric Acid Using
Tapered Silica Optical Sensor Coated With Zinc Oxide (ZnO). Jurnal Teknologi,
74(8).
148
5) Lokman, M. Q., Rahim, H. R. B. A., Harun, S. W., Hornyak, G. L., &
Mohammed, W. S. (2016). Light backscattering (eg reflectance) by ZnO nanorods
on tips of plastic optical fibres with application for humidity and alcohol vapour
sensing. Micro & Nano Letters, 11(12), 832-836.
6) M. Q. Lokman, H. R. A. Rahim, M. Yasin, Z. Jusoh, S. W. Harun. (2016),
Evaluation of Light Backscattering by Zinc Oxide Nanorods on Fiber End of
Silica Optical Fiber, Journal of Optoelectronics and Advanced Materials – Rapid
Communications, 10 (11-12), 885 – 888.
7) A. R. Muhammad, M. T. Ahmad, R. Zakaria, H. R. A. Rahim, S. F. A. Z. Yusoff,
K. S. Hamdan, H. H. M. Yusof, H. Arof, S. W. Harun (2017), Q-Switching Pulse
Operation in 1.5-µm Region Using Copper Nanoparticles as Saturable Absorber,
Chinese Physical Letter, 34(3), 034205-1- 4.
Papers Presented at Conferences
1) Rafis, H., Irawati, N., Rafaie, H. A., Ahmad, H., Harun, S. W., & Nor, R. M.
“Detection of Different Concentrations of Uric Acid Using Tapered Silica Optical
Sensor Coated with Zinc Oxide (ZnO)”, Laser Technology and Optic Symposium
2014 (LATOS 2014), Universiti Teknologi Malaysia (UTM).
2) H. Rafis, S. Manjunath, Hoorieh Fallah, S.W.Harun, Waleed S. Mohammed, G.
Louis Hornyak, Joydeep Dutta "Spiral Structured Growth of ZnO on Plastic
Optical Fiber Towards Light Side Coupling ", International Symposium on
Modern Optics and Its Applications (ISMOA 2015) in Bandung, Indonesia
(2015).
149
3) Rafis, A. R. H., Sulaima, W. H., & Waleed, S. M. (2016, November). Improved
optical side coupling efficiency by spiral patterned zinc oxide nanorod coatings
on large core plastic optical fiber. In Second International Seminar on Photonics,
Optics, and Its Applications (ISPhOA 2016) (pp. 101500T-101500T).
International Society for Optics and Photonics, SPIE. (1st Poster Award)
4) Pandey, C. A., Rahim, R., Manjunath, S., Hornyak, G. L., & Mohammed, W. S.
(2015, July). Synthesis and characterization of hydrothermally grown zinc oxide
(ZnO) nanorods for optical waveguide application. In International Conference on
Photonics Solutions 2015 (pp. 96590X-96590X). International Society for Optics
and Photonics, SPIE.
5) H. Rafis, S.W.Harun, Waleed S. Mohammed “Multiple Optical Channel Alcohol
Vapor Sensing in Visible Wavelength” 2016 Research and Innovation
Competition Week, 15-17 Mac 2016. (Silver Medal).
6) S.Thokchom, D. Lourembam , R. Borgohain , H. R. B. A. Rahim, Waleed S. M, S.
Baruah, ZnS coated ZnO nano-rods on Optical fiber for Gas sensing, In
International Conference on Advances Nanotechnology 2017 (iCAN2017),
Sustainable Nanotechnology Organization.
150
Patents
1) Coated Plastic Optical Fiber (PI 2016701346), University of Malaya, Intellectual
Properties of Malaysia (2016).
2) Multiple Channel Optical Sensing towards Light Side Coupling, University of
Malaya, Intellectual Properties of Malaysia (2016).
3) A Backscattering Light in Optical Sensor System, University of Malaya,
Intellectual Properties of Malaysia (2016).
151
APPENDIX
A selection of published works are attached in this appendix.
Side coupling of multiple optical channels by spiral patterned zinc oxide coatings onlarge core plastic optical fibers
Hazli Rafis Bin Abdul Rahim1,2,3, Somarapalli Manjunath4, Hoorieh Fallah2, Siddharth Thokchom5,Sulaiman Wadi Harun1,2, Waleed Soliman Mohammed6, Louis Gabor Hornyak7, Joydeep Dutta8
1Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia2Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia3Faculty of Electronic and Computer Engineering, Universiti Teknikal Malaysia Melaka, 76100 Melaka, Malaysia4School of Electronic Engineering, Vellore Institute of Technology University, 632014 Vellore, India5School of Technology, Assam Don Bosco University, Airport Road, Azara, Guwahati 781017, Assam, India.6Center of Research in Optoelectronics, Communication and Control Systems (BU-CROCCS), School of Engineering, BangkokUniversity, 12120 Patumthani, Thailand7Center of Excellence in Nanotechnology, Asian Institute of Technology, 12120 Patumthani, Thailand8Chair of Functional Materials division, KTH Royal Institute of Technology, Stockholm, SwedenE-mail: [email protected]
Published in Micro & Nano Letters; Received on 27th October 2015; Revised on 7th December 2015; Accepted on 4th January 2016
Improved optical side coupling efficiency was demonstrated for spiral patterned zinc oxide (ZnO) nanorods coated large core plastic opticfibers (POFs) as opposed to unpatterned continuous coatings. ZnO nanorods were grown by the hydrothermal method directly onto POFsurfaces. Nanorods coating enhanced coupling inside the fiber by scattering light but were also capable of causing leakage. Structuring thegrowth to specific regions allows scattering from different segments along the fiber to contribute to the total coupled power. ZnOnanorods growth time of 12 h and temperature of 90 °C provided the best coupling voltage. Side coupling was measured to be a factor of2.2 times better for spiral patterned coatings as opposed to unpatterned coatings. The formation of multiple segments was as well used formultiple-wavelength channels excitation where different bands were side coupled from different segments.
1. Introduction: Zinc oxide (ZnO) is a versatile wide-bandgap(3.37 eV) semiconductor material that has contributed to thedevelopment of numerous applications over the past few years.Depending on its doping condition, ZnO can be conductive(including n-type and p-type conductivity), semi-conductive,insulating, transparent and show piezoelectric behaviour, roomtemperature ferromagnetism, and huge magneto-optic andchemical sensing properties [1]. This versatility makes ZnO asuitable material for a variety of integrated nanosystems thatinclude optoelectronics [2–5], biosensors [6–8], resonators [9],medical devices [10, 11], imaging [12, 13], and wirelesscommunication [14]. In optical fiber systems, light is typicallyintroduced from one end, guided through the fiber and collectedat the other end. This common method has been widely used forsensing applications using plastic optic fiber (POF) coated withZnO nanostructures [15–17]. In previous work, we havedemonstrated the feasibility of side coupling to optical fibers byexploiting the scattering properties of ZnO nanorods coated onsilica multimode optical fibers [18]. Light induced by scatteringat angles larger than the critical angle is guided inside the fiber.Although ZnO nanorods enhance optical guidance in this way,
they are also responsible for light leakage due to the very same scat-tering property. In our previous work, coupling of light to the coremode was accomplished by exposing the core to wet chemicaletching. Light was then allowed to couple from an intermediateregion near the beginning of the core exposure domain whileleakage was minimised at unetched fiber domains downstream.The primary limitation of this method was that only a smallportion of the fiber could be used for signal collection. This situ-ation is undesirable for applications such as receivers in telecommu-nications and sensing where extended light sources are required.The extended light source leads to less guidance of light insidethe fiber resulting in low efficiency and sensitivity.
To increase the magnitude of light collection, we proposed twoapproaches that were executed simultaneously. First, a large-coreplastic fiber optic is required to increase the scattering area; andsecond, a structured scattering layer tightly bound to the surfaceof the POF is required to harvest light from different segments ofthe POF. The scattering layer consists of ZnO nanorods as a fibercoating. Fig. 1 illustrates the mechanism of light scattering forunpatterned (Fig. 1a) and spiral patterned (Fig. 1b) ZnO nanorodslayers and for the multi-channel optical fiber case (Fig. 1c). Lightscattering is induced by the presence of ZnO nanorods on thesurface excitation locations along the POF. A portion of the scat-tered light is guided when scattering angles are greater than the crit-ical angle between the surrounding and the core [19]. The coupledlight propagates through the POF to the terminal detector (Iout). Thepresense of the nanorods as well causes light leakage through theside of the fiber (Ileak) (Fig. 1a). For example, if two point lightsources, P(z1) and P(z2) along a POF are illuminated simultaneous-ly, then the excitation inside the fiber is maximised at these points.However, due to the nanorods induced leakage, the intensity of theguided light decreases exponentially to the ZnO nanorods interface.For the location farthest away from the interface (e.g. z2), any lightreaching the detector is minimised. Hence, the power coupled frompoint z2 provides only minimal contribution to the total guidance.Clearly, the way to increase the contribution originating frompoint z2 is to reduce the amount of leakage.
Light leakage can be minimised by reducing the ZnO coveragethrough the application of a spiral patterned layer of ZnO nanorodsas shown in Fig. 1b. The reduction of the effective area of the scat-tering layer is expected to increase the contribution from point z2.Considering an arbitrary point at the middle of the spiral patternedZnO layer (Fig. 1b), the light coupled inside the fiber leaks expo-nentially inside the coated region. The intensity remains steady inthe uncoated region till the next ZnO patterned region where the
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exponential decay occurs again. The intensity from point z2 isincreased due to a balance between the optimised side couplingfrom the ZnO patches and the reduction of the leakage due to thereduction of the effective ZnO nanorods region. On the basis ofthis hypothesis, one can predict possible enhancement of the totalcoupling when an extended light source is used.
In another demonstration, the presence of patches of ZnO nanor-ods was used for multi-channel excitation. Though, it is possible toachieve multi-wavelength excitation with unpatterned growth,channels further from the ZnO edge suffers a sever loss. Higherpower is then required for channel equalisation. This effect is mini-mised here using the spiral patterned POF as shown in Fig. 1c.Different wavelengths of light source, P(z1), P(z2), and P(z3) are in-dividually excited at different spiral patches of ZnO nanorods. Dueto the reduction of the effective scattering area, the peaks of thecoupled light are expected to be higher than multi-channel per-formed on unpatterned ZnO nanorods growth. This gives rise to apossible application in wavelength division multiplexing. Thecoupling efficiency of each channel depends on the spacingbetween the scattering domains.
2. Materials, fabrication, and characterisation2.1. Fibre preparation: Standard multimode SK-80 POF fibers(Mitsubishi Rayon Co., LTD; Japan) were used in experiments.The fiber core consists of polymethyl methacrylate resin withdiameter ranging from 1840 to 2080 µm. The core is surroundedby a fluorinated polymer jacket with inner–outer diameter in therange of 1880–2120 µm, respectively. Fig. 3a depicts a graphicalrepresentation of the spiral structure. At first, the jacket of thePOF is mechanically stripped to expose the core fiber over alength of 7 cm. Following cleansing with a dry tissue, the spiraltemplate is applied with plastic tape (Fig. 3a). This work focuseson creating spiral pattern using the plastic tape (0.4 cm). Thewidth of spiral ZnO patterned on POF was 0.2 cm. The tape wasremoved before experimental characterisation to expose the baretemplated POF surface.
2.2. ZnO seeding procedure: The synthesis of ZnO nanorods wasaccomplished as previously reported [20]. First, ZnO seedparticles were synthesised by dissolving ca. 0.0044 g zinc acetatedihydrate [Zn(O2CCH3)2·2H2O] (Merck KGaA, Germany) in 20ml of ethanol (Merck KGaA, Germany) to form a 1 mM solution.The resulting ZnO seed particles served as nucleation centres fornanorods growth. Unpatterned and spiral patterned POFs werethen dipped into the ZnO seed solution for 30 s and placed on ahotplate set at 70 °C for 2 min. The solvent was then allowed to
Fig. 1 Schematic diagram of light scattering fora Unpatterned growth of ZnO nanorods with the coupling lightb Spiral patterned growth of ZnO nanorods with more interface and ZnOregions with the coupling lightc Spiral patterned growth of ZnO nanorods for a multi-channel excitation
Fig. 2 Optical characterisation apparatusa Vpp characterisation setup to measure the scattering effect of ZnO nanorodsfor unpatterned and spiral patterned ZnO nanorodsb Spectral analysis setup to determine wavelength coupling maxima
Fig. 3 Graphical representation and SEM image ofa Spiral patterned form using plastic tapeb 13 kX SEM image of ZnO spiral patterned growth after synthesisc 25.0 kX SEM image of the nanorods
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evaporate. The process was repeated ten times to ensure optimalseed distribution on the surface of the POF. It was followed bydrying the samples at 70 °C under ambient conditions therebyconcluding the seeding process.
2.3. ZnO nanorods synthesis: ZnO nanorods were grown followingthe seeding process. 2.97 g zinc nitrate hexahydrate [Zn(NO3)2·6H2O] (Ajax Finechem Pty Ltd) and 1.40 g ofhexamethyleneteramine or HMT [(CH2)6N4] (Sigma-Aldrich)were dissolved in 400 ml of deionised (DI) water to form 10 mMsolutions of each compound. The seeded POFs were thenvertically placed in 200 ml of the synthesis solution and heated inan oven set at 90 °C. Following 5 h of heating, the solution was
discarded and replaced with a new solution in order to maintainconstant growth conditions. Growth time was varied from 8 to20 h. Following synthesis, POFs were removed and rinsed severaltimes in DI water.
2.4. Characterisation: Scanning electron microscopy (SEM) wasperformed at the National Nanotechnology Center, ThailandScience Park (Hitachi, 3400N). Energy dispersive X-ray (EDX)was performed during SEM. The optical characterisationapparatus is schematically depicted in Fig. 2a below. Themagnitude of the side coupling was measured in terms of‘peak-to-peak’ voltage (Vpp) following excitation by a modulatedred light source – e.g. the extended light source. Light from theextended source was restricted by an aperture onto specific siteson the POF in order to optimise the growth conditions formaximum side coupling. The egress end of the optical fiber islinked to a digital oscilloscope and subsequently to a computerfor data recording and analysis.
2.5. Experimental: Vpp was analysed according to the schematicdepicted in Fig. 2a. POFs were illuminated by a 3 cm diameterbroad band light-emitting diode extended light source placed 10cm from the fiber surface. A rectangular aperture 1 × 3 cm wasplaced perpendicularly to and directly on top of the fiber duringsignal acquisition. Three domains were inspected for theunpatterned type of fiber: (i) the interfacial area between the ZnOcoating and the uncoated fiber near the detector end; (ii) themiddle ZnO domain; and (iii) the tip domain that consisted of theterminal ZnO-air interface. The fiber tip was covered in all casesexcept for readings taken for the ‘tip domain’ of the unpatternedand spiral patterned POF. In the figure, dark regions represent
Fig. 4 EDX spectrum of ZnO nanorods showing zinc and oxygen peaks
Fig. 5 Average Vpp on bare and unpatterned POFsa Average Vpp for 15 and 20 h growth timeb Back scattering effect is eliminated at interface regions after reducing thegrowth time to 8, 10, and 12 h
Fig. 6 Average Vpp at interfacial area for all growth times
Fig. 7 Average Vpp for the spiral patterned growth for 12 h which has morethan one interface and ZnO regions. The inset shows the regions covered bythe aperture when characterisation the structured and unstructured ZnOgrowth on POF
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patterned ZnO nanorods scattering domains. For unpatterned POFs,the extended light source was positioned in the centre of the ZnOcoated area. Vpp was measured on bare and unpatterned ZnOnanorods POFs (variable growth time) to determine side couplinglimits. Optimised growth time was applied to Vpp measurementson patterned surfaces.
For patterned POFs, five ZnO domains were analysed: (i) theinterfacial area between the ZnO coating and the uncoated fibernear the detector end; (ii) the adjacent pure ZnO domain; (iii) asecond interfacial domain between the ZnO and the uncoatedfiber; (iv) a second pure ZnO domain; and (v) the tip domain ofZnO and air as before (uncovered during tip domain measure-ments). In all cases, bare POFs devoid of ZnO coating served ascontrols in the experiments. Five readings were acquired for eachmeasurement.
Spectral analysis was performed for the unpatterned and pat-terned samples to identify the wavelength coupling maxima usingthe setup shown in Fig. 2b. A broad spectrum white light sourceand two infrared laser sources were used (850 and 980 nm). Theoptical transmittance of patterned and unpatterned POFs were com-pared. No aperture was used during spectral acquisition.Transmittance was calculated by the following expression
Transmittance = coupled power
source power
3. Result and discussion3.1. Physical characterisation: SEM image in Fig. 3b withmagnification set at 13.00 kX was used to observe the ZnO spiralpattern on the POF. Fig. 3c depicts SEM images withmagnification of 25.00 kX which clearly shows verticalalignment, high density (63 nanorods/1.23 × 10−12 m2 = 510 ×1011 nanorods/m2) and uniform distribution of ZnO nanorods onthe POF.
EDX elemental analysis revealed that the topcoat layer consistedonly of zinc and oxygen as shown in Fig. 4.
3.2. Optical characterisation: The plots in Figs. 5a and b show theaverage Vpp on bare and unpatterned POFs. Initially the growthduration was set at 15 or 20 h. However, the average Vpp at theinterface region (for both these growth times) was greatly reduceddue to backscattering that limits light side coupling to the coremodes (Fig. 5a). Longer growth times resulted in higher ZnOnanorods density on POFs and hence, the coating provided agreater barrier to light side coupling due to backscattering. The
bare POFs did not show any backscattering effects. The problemwas solved by reducing the ZnO nanorods growth duration to 8,10, or 12 h. Fig. 5b shows the improvement in the average Vpp
for the above mentioned growth durations: 8, 10, and 12 h. Fromthis characterisation, it was determined that the growth durationof 12 h was optimal in limiting backscattering. The conclusionwas based on the highest average Vpp at the interface region. Thisoptimised process of growing ZnO nanorods on POF was thenapplied to fabricate the spiral patterned growth as shown in Fig. 3a.
The results of optimisation are summarised in Fig. 6 showingonly Vpp against interface data. At 12 h growth time, Vpp is maxi-mised, thereby demonstrating high light side coupling withreduced leakage due to backscattering. Tip readings (uncovered)are high due to ingress of light through the fiber optic in additionto potential side coupling.
The graph of average Vpp for the five domains on the spiral pat-terned growth is depicted in Fig. 7. Vpp was highest at Domain 1,the ZnO bare interface closest to the detector. Vpp was significantlylower at Domain 2, the pure ZnO region. A slight rise in Vpp wasobserved at Domain 3, another interfacial region. Domain 4, apure ZnO region located further from the detector showed similarvalues to Domain 2. Domain 5 showed the tip effect as before.Therefore, the spiral patterned on the POF has potential applicationas multi-channel excitation and enhance the total coupling insidePOF. It is worth mentioning that Vpp was a factor of 2× lowerthan for the same domain on the unpatterned fiber. This is due toarea reduction of the spiral structure as shown by the inset inFig. 7. This is not the case when an extended source was used.
Fig. 8 represents the transmittance of visible white light for spiralpatterned and unpatterned POFs when an extended source wasused. The result indicates that the spiral patterned growth is ableto increase coupling of the light source better than the unpatternedgrowth due to the existence of more interfacial ZnO regions on thePOF. The plot in Fig. 7 also shows that the spiral patterned growthprovides a higher light transmittance with an improvement factorof 2.2.
Fig. 9 shows that the transmittance of light for the spiral pat-terned growth is higher than the unpatterned growth when both in-frared laser sources were tested. However, the infrared laser sourcedid not significantly couple at the particular wavelength inside thePOF. Therefore, the coupling efficiency was too low for usefulapplications.
4. Conclusion: The synthesis process to grow ZnO nanorods onPOF core is optimised by maximising the side coupling to POFfrom an extended source. This work also reports a novel spiralpatterned growth of ZnO nanorods on POF. The light sidecoupling improves considerably for spiral patterned growth
Fig. 8 Transmittance of the visible white light spectruma Spiral patternedb Unpatterned growth
Fig. 9 Spectrum for near infrared (850 and 980 nm) for spiral patternedand unpatterned growth
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compared with unpatterned growth due to the presence of moreZnO regions on the POF. The spiral pattern on the POF alsoprovides a higher light intensity multi-channel compared withunpatterned ZnO nanorods POF. Spectral analysis is performed toinvestigate light transmittance for different wavelength of lightsources. It is found that visible white light source significantlycoupled the light into the POF compared with infrared lasersources. In future, investigation of coupling efficiency can beperformed by varying the width and spacing of the coated anduncoated regions.
5. Acknowledgment: The authors would like to acknowledgeUniversity of Malaya for financial support under high impactresearch grant (grant no: D000009-16001).
6 References
[1] Kolodziejczak-Radzimska A., Jesionowski T.: ‘Zinc oxide – fromsynthesis to application: a review’, Materials, 2014, 7, (4), pp.2833–2881
[2] Pauporté T., Lincot D.: ‘Electrodeposition of semiconductors foroptoelectronic devices: results on zinc oxide’, Electrochim. Acta,2000, 45, (20), pp. 3345–3353
[3] Xiang B., Wang P., Zhang X., ET AL.: ‘Rational synthesis of p-typezinc oxide nanowire arrays using simple chemical vapor deposition’,Nano Lett., 2007, 7, (2), pp. 323–328
[4] Janotti A., Van de Walle C.G.: ‘Fundamentals of zinc oxide as asemiconductor’, Rep. Prog. Phys., 2009, 72, (12), pp. 126501.
[5] Shinde S.S., Shinde P.S., Bhosale C.H., ET AL.: ‘Optoelectronic prop-erties of sprayed transparent and conducting indium doped zinc oxidethin films’, J. Phys. D, Appl. Phys., 2008, 41, (10), pp. 105109
[6] Chang C.C., Chiu N.F., Lin D.S., ET AL.: ‘High-sensitivity detection ofcarbohydrate antigen 15-3 using a gold/zinc oxide thin film surfaceplasmon resonance-based biosensor’, Anal. Chem., 2010, 82, (4),pp. 1207–1212
[7] Kumar S.A., Chen S.M.: ‘Nanostructured zinc oxide particles inchemically modified electrodes for biosensor applications’, Anal.Lett., 2008, 41, (2), pp. 141–158
[8] Kong T., Chen Y., Ye Y., ET AL.: ‘An amperometric glucose biosensorbased on the immobilization of glucose oxidase on the ZnO nano-tubes’, Sens. Actuators B, Chem., 2009, 138, (1), pp. 344350
[9] Cao H., Zhao Y.G., Ong H.C., ET AL.: ‘Ultraviolet lasing in resonatorsformed by scattering in semiconductor polycrystalline films’, Appl.Phys. Lett., 1998, 73, (25), pp. 3656–3658
[10] Rasmussen J.W., Martinez E., Louka P., ET AL.: ‘Zinc oxide nanopar-ticles for selective destruction of tumor cells and potential for drugdelivery applications’, Expert Opin. Drug Deliv., 2010, 7, (9), pp.10631077
[11] Lenz A.G., Karg E., Lentner B., ET AL.: ‘A dose-controlled system forair-liquid interface cell exposure and application to zinc oxide nano-particles’, Part. FibreToxicol., 2009, 6, (32), pp. B16
[12] Johnson J.C., Yan H., Schaller R.D., ET AL.: ‘Near-field imaging ofnonlinear optical mixing in single zinc oxide nanowires’, NanoLett., 2002, 2, (4), pp. 279–283
[13] Zvyagin A.V., Zhao X., Gierden A., ET AL.: ‘Imaging of zinc oxidenanoparticle penetration in human skin in vitro and in vivo’,J. Biomed. Opt., 2008, 13, (6), 064031-9
[14] Fallah H., Harun S.W., Mohammed W.S., ET AL.: ‘Excitation of coremodes through side coupling to multimode optical fiber by hydrother-mal growth of ZnO nanorods for wide angle optical reception’, J. Opt.Soc. Am. B, 2014, 31, (9), pp. p2232-2238
[15] Batumalay M., Harith Z., Rafaie H.A., ET AL.: ‘Tapered plastic opticalfiber coated with ZnO nanostructures for the measurement of uric acidconcentrations and changes in relative humidity’, Sens. Actuators A,Phys., 2014, 210, pp. 190–196
[16] Lokman A., Harun S.W., Harith Z., ET AL.: ‘Inline Mach–Zehnderinterferometer with ZnO nanowires coating for the measurement ofuric acid concentrations’, Sens. Actuators A. Phys., 2015, 234, pp.206–211
[17] Harith Z., Irawati N., Rafaie H.A., ET AL.: ‘Tapered plastic optical fibercoated with Al-doped ZnO nanostructures for detecting relative hu-midity’, IEEE Sens. J., 2015, 15, (2), pp. 845–849
[18] Chen J.J., Zeng F., Li D.M., ET AL.: ‘Deposition of high-quality zincoxide thin films on diamond substrates for high-frequency surfaceacoustic wave filter applications’, Thin Solid Films, 2005, 485, (1),pp. 257–261
[19] Bora T., Fallah H., Chaudhari M., ET AL.: ‘Controlled side coupling oflight to cladding mode of ZnO nanorods coated optical fibers and itsimplications for chemical vapor sensing’, Sens. Actuators B, Chem.,2014, 202, pp. 543–550
[20] Fallah H., Chaudhari M., Bora T., ET AL.: ‘Demonstration of sidecoupling to cladding modes through zinc oxide nano-rods grownon multimode optical fiber’,Opt. Lett., 2013, 38, (18), pp. 3620–3622
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Applied light-side coupling withoptimized spiral-patterned zinc oxidenanorod coatings for multiple opticalchannel alcohol vapor sensing
Hazli Rafis Bin Abdul RahimMuhammad Quisar Bin LokmanSulaiman Wadi HarunGabor Louis HornyakKarel SterckxWaleed Soliman MohammedJoydeep Dutta
Hazli Rafis Bin Abdul Rahim, Muhammad Quisar Bin Lokman, Sulaiman Wadi Harun, GaborLouis Hornyak, Karel Sterckx, Waleed Soliman Mohammed, Joydeep Dutta, “Applied light-side couplingwith optimized spiral-patterned zinc oxide nanorod coatings for multiple optical channel alcoholvapor sensing,” J. Nanophoton. 10(3), 036009 (2016), doi: 10.1117/1.JNP.10.036009.
Applied light-side coupling with optimizedspiral-patterned zinc oxide nanorod coatings
for multiple optical channel alcohol vapor sensing
Hazli Rafis Bin Abdul Rahim,a,b,c Muhammad Quisar Bin Lokman,a
Sulaiman Wadi Harun,a,b Gabor Louis Hornyak,d Karel Sterckx,e
Waleed Soliman Mohammed,e,* and Joydeep DuttafaUniversity of Malaya, Department of Electrical Engineering, Faculty of Engineering,
50603 Kuala Lumpur, MalaysiabUniversity of Malaya, Photonics Research Centre, 50603 Kuala Lumpur, Malaysia
cUniversiti Teknikal Malaysia Melaka, Faculty of Electronic and Computer Engineering,76100 Melaka, Malaysia
dAsian Institute of Technology, Center of Excellence in Nanotechnology,12120 Patumthani, Thailand
eBangkok University, Center of Research in Optoelectronics, Communication and ControlSystems (BU-CROCCS), School of Engineering, 12120 Patumthani, ThailandfKTH Royal Institute of Technology, Chair of Functional Materials Division,
Stockholm SE-100 44, Sweden
Abstract. The width of spiral-patterned zinc oxide (ZnO) nanorod coatings on plastic opticalfiber (POF) was optimized theoretically for light-side coupling and found to be 5 mm. StructuredZnO nanorods were grown on large core POFs for the purpose of alcohol vapor sensing. The aimof the spiral patterns was to enhance signal transmission by reduction of the effective ZnOgrowth area, thereby minimizing light leakage due to backscattering. The sensing mechanismutilized changes in the output signal due to adsorption of methanol, ethanol, and isopropanolvapors. Three spectral bands consisting of red (620 to 750 nm), green (495 to 570 nm), and blue(450 to 495 nm) were applied in measurements. The range of relative intensity modulation (RIM)was determined to be for concentrations between 25 to 300 ppm. Methanol presented the strong-est response compared to ethanol and isopropanol in all three spectral channels. With regard toalcohol detection RIM by spectral band, the green channel demonstrated the highest RIM valuesfollowed by the blue and red channels, respectively. © 2016 Society of Photo-Optical InstrumentationEngineers (SPIE) [DOI: 10.1117/1.JNP.10.036009]
Keywords: zinc oxide; spiral pattern; visible-light wavelength; optical fiber; light-side coupling;nanorods.
Paper 16049 received Mar. 22, 2016; accepted for publication Jul. 18, 2016; published onlineAug. 8, 2016.
1 Introduction
A chemical sensor is a device that transforms chemical information in the form of concentrationof a specific material into an analytically useful signal.1 A large number of commercially avail-able chemical measurement systems are found in the market today and can be classified by thetype of analytical signal required for measurement. For optical-based instrumentation, theseinclude, among others, absorbance,2 luminescence,3 light scattering,4 and fluorescence.5 Dueto the simplicity of directing light into a sensing platform, optical fibers have found applicationsfor the measurement of chemicals in food,6–9 security industry safes,10 and clinical materials.11
However, these chemical vapor sensors are usually associated with high-cost, high-operational
*Address all correspondence to: Waleed Soliman Mohammed, E-mail: [email protected]
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power requirements,12 and complexity in operation. Laser light sources are generally used inoptical sensing applications, but costs related to the laser and the mechanical alignment appa-ratus can be relatively high.13 Application of laser light sources onto coated fibers also posesseveral problems. Inequality of beam distribution onto the fiber and small beam diameter canlead to fluctuations, nonrepresentation, and low intensity.14 In addition, the most commonmethod adopted in optical sensing is based on the quenching of luminescence from a rangeof chemical species.15 Most of the luminescent indicators used for chemical sensing sufferfrom the disadvantages of having both shortwave excitation and small stokes shift, thus addingcomplexity to the required measurement instrumentation.16
In previous work, the authors proposed an approach to overcome these issues using light-sidecoupling through scattering of zinc oxide (ZnO) nanorods coated on silica multimode opticalfibers.17 ZnO nanorods coated on the fiber contributes into side coupling to the guided modes.However, ZnO nanorods as well causes light to leak outside the core. This can effectively reducethe device’s efficiency and sensitivity. One way to increase the magnitude of light intensity isthrough the use of a larger plastic optical fiber (POF) core that increases the scattering area.Structuring the growth of ZnO into separate patches allows coupling light from different seg-ments of the POF.18
Over the years, the bulk of work on ZnO focused on synthesis and surface modification,19
treatment,20 and protection.21 Avariety of structures including one-dimensional nanorods,22 two-dimensional nanoplates,23 and three-dimensional nanoflowers24 have been synthesized.However, patterned growth, the application of a helical pattern with mm dimensions, of ZnOnanorods on cylindrical surfaces with small diameter (e.g., ∼2 mm) of a typical optical fiber stillremains challenging for optical applications. Practically, unpatterned growth is preferred due toreduced complexity during fabrication and shorter treatment time. As a result for sensing appli-cations, our previous work was initially based on unpatterned growth of ZnO nanorods on thePOF core.25,26 However, we found that although unpatterned ZnO nanorod layers enhanced opti-cal side coupling with the fiber, significant levels of backscattering prevented the ingress of lightinto the fiber. Furthermore, ZnO scattering centers provided a pathway for light leakage.27
Consequently, these two optical loss mechanisms resulted in low intensity of side couplingof light, a condition that is undesirable in optical applications such as in telecommunications,sensing, and measurements. As reported previously, to increase the intensity of side-coupledlight, application of patterned coatings of ZnO nanorods on POF cores was proposed to mitigatethe level of backscattering and leakage.18 It is worth mentioning here that across this manuscriptand in our previous publications,17 the term scattering is used to describe the main phenomenoncorresponding to side coupling as shown in Fig. 1. Upon recent theoretical study by the authors,another important factor has been observed to actually contribute into coupling light to theguided modes inside POF particularly at large angles θi (near right angles). At angles closeto 90°, light is guided inside the rods and because ZnO nanorods have higher refractiveindex, n3 compared to the polymer, n2 forming the POF, light at the outlet of the nanorodsdiverges with wide field of view inside the fiber. Side coupling is obtained for the portionof this diverging light, which is at angles larger than the critical angle θc between the polymer
Fig. 1 Mechanism of light scatters into POF core by ZnO nanorods at angle larger than criticalangle.
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core and air n1. Although, for simplicity and for the remainder of this manuscript, the termscattering is used to describe the macroscopic effect of light-side coupling.
In this work, optimization of the spiral spacing of ZnO nanorod-coated regions on the fiberwas carried out to produce maximal signal intensity. Theoretically, high-intensity light-side cou-pling is expected between the scattering ZnO layer and the fiber optic if the width of the ZnOspirally patterned coating is optimized. Also, adaptation of the sensor as a multiple optical chan-nel waveguide sensor for detection of alcohols in the visible wavelength domain was studied.Finally, experimental demonstration of the patterned fiber optic as a multiple optical channelsensor for alcohol vapor is shown.
2 Modeling
Here, a first-order model is derived to simulate the impact of millimeter (mm) scale spiral pat-terns on power leakage due to scattering by ZnO nanorods. In the side coupling mechanismproposed here, ZnO nanorods allow light to couple inside the guiding region (core of POF).ZnO nanorods as well guide the light outside the fiber core with each bounce at the interface.These two counter effects restrict the coupling to an effective area around a region at the begin-ning of the ZnO coating. This limits the use of this system in multiple channels as well as forapplication with extended sources. One way to improve the system response is through spreadingthe effective coupling area of ZnO nanorods across the fiber. This is achieved by introducingpatches of nanorods coating. Optimizing the gaps and width of ZnO coating enhances the systemresponse depending on the light source used. More detailed analysis of the scheme was explainedin a previous publication.18 In the analysis, two cases of ZnO nanorod coating on POF core havealways been compared: spirally patterned ZnO nanorod coatings in which a light-blocking layerwas applied and unpatterned coating in which ZnO nanorods cover the entire surface of the POFuniformly. The two configurations are shown in Fig. 2. The objective was to optimize the widthof spiral-patterned ZnO nanorods coating for the purpose of experimental design.
Fig. 2 (a) Spiral-patterned coating of ZnO nanorods on POF core and (b) unpatterned coating ofZnO nanorods on POF core.
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In the schemes illustrated in Fig. 2, the visible-light source illuminates the upper hemisphereof the coated POF when oriented normal to its surface. The ZnO nanorods scatter light atdifferent directions accordingly and maximum coupled power Po to core or cladding mode isdefined as
EQ-TARGET;temp:intralink-;e001;116;687 Po ¼ Psource 2πCscρaψ ; (1)
where Psource is the power of the source excitation. The constants Csc and ρa are the scatteringcross section of one rod (m) and rods density (number of nanorods per unit area Nrod∕μm2,respectively. The constant, ψ is the portion of the scattered light that couples into the guidedmodes of the fiber as
EQ-TARGET;temp:intralink-;e002;116;609ψ ¼Zπ
θc
p ðθ − θincÞ sin θ dθ: (2)
The function pðθ − θincÞ is the phase function, which is the probability distribution function orthe scattered power as a function of the scattering angles θ. The function is assumed to varylinearly with the incident angle θinc. This assumption can be justified here as a small rangeof angles around normal incidence is considered. At larger angles, this model deviates fromthe actual system. The critical angle θc is the one between the core POF and air.
To study the coupling and source distribution effect, the POF surface was divided into seg-ments of width Δz shown in Fig. 3(a). The source excitation is assumed constant over the width.At any segment h on the surface of the POF, exposed to a visible-light source, there is an arbi-trary intensity profile PsðzÞ causes a portion of ψηPsðzÞ to couple to the guided modes. In addi-tion to the excitation, a portion of the previously coupled light (coming from segment Ph−1) addsto the amount of light coming out of segment h as shown in Fig. 3(b). Notice that, in the figure,the coupling coefficient from segment h is indicated as ηz. The power coupled out of segment hcan then be written as
EQ-TARGET;temp:intralink-;e003;116;394Ph ¼ ψηz;hPs þ Ph−1 − ðηz;hPh−1Þ: (3)
In simulations, the length of POF was selected to 100 segments of 1 mm each for a total of100 mm and Psource is the power of the source excitation that was fixed to 5 for amplitude. Threecoating regions of ZnO nanorods were developed to create spiral-patterned coating on the POFand the widths of the ZnO nanorods coating were varied from 1 to 20 segments as shown inFig. 2(a). Meanwhile, the unpatterned POF was evaluated by varying the ZnO nanorods coating
Fig. 3 (a) Dividing the POF coated with ZnO nanorods into discrete sections of width Δz for bothcoating schemes. (b) Optical intensity components around a segment h of the ZnO coated POF.
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from 1 to 100 segments, which is fully coated as depicted in Fig. 2(b). These two scheme coat-ings were analyzed using Eq. (3) by applying finite difference method. In this case, the widths ofZnO nanorods coating were fixed to three segments (3 mm) starting from segment 10 to 12 (firstZnO region), 13 to 38 (uncoated region), 34 to 36 (second ZnO region), 37 to 62 (uncoatedregion), and 63 to 65 (third ZnO region). For ZnO unpatterned coating, there is only oneZnO region that is also fixed to three segments (10 to 12). The region was coated with ZnOnanorods has the coupling coefficient, ηz higher than zero, and ηz for uncoated region isequal to zero. Thus, the power for the segment before segment 10 (P9) is equal to zero dueto the ηz is zero. As the portion of light from segment 9, P9 is substituted into Eq. (3) to coupleto the amount of light of P10. The total light at segment 10 is
EQ-TARGET;temp:intralink-;sec2;116;616P10 ¼ ψηz10Ps þ P9 − ðηz10P9Þ ¼ ψηz10Ps.
The amplitude of P10 ¼ ψηz10Ps is coupled to the amount of light in segment 11. Thus, P11 canbe written as follows:
EQ-TARGET;temp:intralink-;sec2;116;560P11 ¼ ψηz11Ps þ P10 − ðηz2P10Þ:
Then, the coupling light in segment 11 is coupled to the light presents inside segment 12, theamplitude of P12 is given as
EQ-TARGET;temp:intralink-;sec2;116;505P12 ¼ ψηz12Ps þ P11 − ðηz12P11Þ:
In this case, the coupled light from segment 10 to segment 12 is equal to P12 ¼ ∼0.7, because thewidth of ZnO nanorods coating has been fixed to three segments. For the unpatterned POF, thecoupling light is consistently equal to P12 in the uncoated region until reaching the photodector.The consistency of the coupled light occurs due to coupling coefficients from segment 13 to 100,ηz13 to ηz100 are equal to zero in the uncoated region. Thus, the coupling light reached the photo-detector can be written as
EQ-TARGET;temp:intralink-;sec2;116;402P13 ¼ ψηz13 to 100Ps þ P12 − ðηz13 to 100P12Þ ¼ P12:
In spiral-patterned POF, this consistency of P12 remains steady in the uncoated region (segment13 to 38) until the second ZnO nanorods region (segment 34 to 36) has another three segments.The amount of P12 is coupled again in the first segment of second ZnO nanorods region. Thecoupled light keeps increasing until the next uncoated region. The effect of spiral-patterned coat-ing on POF leads to a significant improvement of light intensity as depicted in Fig. 4 achieving alevel of coupling light of 0.98. In this case, side coupling was obtained to be a factor of 1.4 timesbetter for spiral-patterned coating as opposed to unpatterned continuous coatings.
It is worth mentioning that the coupled power is normalized to the optical power incident ateach segment. Also off-axis, coupling azimuthal modes (or skew rays) are dominantly coupled.28
These, however, might not be the only modes to be excited in side coupling as radial modes can
Fig. 4 The scheme of light propagation for unpatterned continuous and spiral-patterned coating,where ZnO coating region was fixed to three segments (3 mm).
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be excited as well. This is due to the main fact that mode excitation happens due to matching themomentum of the scattered light to the propagation constant of guided mode. In general, theassumption of specific power distribution among any set of modes (in any form) with the pro-posed first-order model especially when large core fiber is used would not have a significanteffect on the driven results or the measurement as we estimate the leak due to surface scattering.
3 Experimental
3.1 Fiber Preparation, Zinc Oxide Nanorod Synthesis, and Characterization
Following the previously optimized POF fiber spiral patterning and ZnO nanorod seeding andsynthesis procedures,18 ZnO nanorods were grown using hydrothermal method. Here, standardpolymethyl methacrylate (SK-80 POF fibers from Mitsubishi Rayon Co., Ltd, Japan) were uti-lized in experiments to serve as controls and the same fibers were modified to obtain spiral-patterned POF with a specified spiral pitch angle, spacing, and width. The jacket of thePOF were mechanically stripped to expose the core fiber over a length of 10 cm. Figure 5 illus-trates the ZnO coating schemes; three widths were varied from 3, 5, to 7 mm for the spiral-pat-terned and unpatterned POF. These width coatings were selected from modeling result in Fig. 8due to the significant output differences occurred at small width of ZnO coatings. A fully coatedPOF (100 mm) was also fabricated to complete the validation. Tape-patterned and unpatternedPOFs were then placed in a ZnO seed solution and subsequently into the growth solution to formZnO nanorods. Percent surface coverage and nanorod orientation were evaluated as described inprevious work by evaluation of scanning electron micrographs recorded by a Hitachi, 3400 Nsystem operating at 20 kV.18
3.2 Optimization
Optimization of optical input through the POF waveguides was realized by correlation withmaximal values of the output voltage (Fig. 6). A function generator was used to modulatethe light from a broadband LED light source (diameter ¼ 3 cm). Sinusoidal intensity patternwas generated and transmitted through the LED. At the receiver side, peak-to-peak voltageof photodetector output was recorded (not the direct current value). This scheme allows min-imization of the ambient light effect and external sources. The amplitude of output voltagechanges according to the amount of coupling inside the POF core. The light source was placedin parallel and at a distance of ∼3 cm from the POF surface. The diameter of the light source was
Fig. 5 Coating schemes (a) unpatterned POFs (3, 5, 7, and 100 mm). (b) Spiral-patterned widthsof ZnO nanorod-coated POFs (3, 5, and 7 mm).
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oriented along the longitudinal axis of the POF. The fiber tip was covered to avoid light enteringfrom the end. The analysis was performed on the spiral-patterned POFs with three differentwidths of ZnO coatings (3, 5, and 7 mm), the unpatterned POFs with the limited ZnO coating(3, 5, and 7 mm), and full coated POF’s. Five readings were acquired for each measurement.
3.3 Multiple Optical Channel Sensing of Alcohols
Figure 7 shows the experimental setup used for optical sensing of alcohols. The apparatusconsisted of a spectrometer (USB4000, Ocean Optics) and a sensing chamber (0.18 m ×0.2 m × 0.27 m). Avisible white light source with wavelength 380 to 750 nm was used to inducelight-side coupling. The intensity of the white light source was modulated with a periodicalpattern using the modulator to minimize the background effect as illustrated in Fig. 7. It isworth mentioning that here a spectrometer was connected to the end of the POF to recordthe spectrum of the coupled light. During the investigation of the sensor performance, ambientair was allowed through the sensing chamber at room temperature (∼26°C) and relative humidityof 45% until a steady-state condition (0 ppm) was obtained. The white light source was placed3 cm from the POF surface. A known amount of alcohol was vaporized and introduced into thesensing chamber as the target gas. Three kinds of alcohol were tested: (1) ethanol [CH3CH2OH](Merck KGaA, Germany, 99.8%), (2) methanol [CH3OH] (J. T. Baker, 99.8%), and (3) isopro-panol [C3H7OH] (Merck KGaA, Germany, 99.5%). The spectral response toward alcohol vaporwas recorded every 10 s from 0 to 300 ppm. In the experiment, the concentration (C) of targetalcohol vapor in ppm was computed using the following equation:29
EQ-TARGET;temp:intralink-;e004;116;299C ¼ T × Vt ×Dt
Vc ×Mt× R; (4)
Fig. 6 Optimization setup to measure the output voltage for unpatterned and spiral-patterned ZnOnanorods. The tip of the POF was covered by black tape in all measurements and a modulatedvisible-light source was placed at a distance of 3 cm from POF.
Fig. 7 Experimental setup to validate the alcohol sensing activities of spiral-patterned POF invisible wavelength as multiple optical channels (blue, green, and red).
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where T is the operating temperature in Kelvin (K) and Vc is the volume (ml) of the diluted targetgas, which is equal to the volume of the sensing chamber. Vt, Dt, and Mt represent the volume(μl), density (g∕ml), and molecular weight (g∕mol) of the alcohol analyte, respectively. R is theuniversal gas constant, which is equal to 8.2 × 104. From the recorded visible spectrum (380 to750 nm), the response from three specific ranges (referred to here as channels) was studied—blue (450 to 495 nm), green (495 to 570 nm), and red (620 to 750 nm). In the channels, themeasured transmittance average values and standard deviations were obtained for the all con-centrations of the alcohol vapors. The sensing performances in each channel were investigatedby analyzing the effects on light intensity toward all the alcohol vapor concentrations. The sens-ing effects were presented in term of relative intensity modulation (RIM) in an arbitrary unit (a.u.)30 that was calculated using the following equation:
EQ-TARGET;temp:intralink-;e005;116;604RIM ¼ IfðavÞ − IiðavÞIiðavÞ
; (5)
where If is the smallest average intensity after injection of alcohol vapor and Ii is the averageinitial intensity before injection of alcohol vapor under light illumination. The RIMs wereobtained for each alcohol vapor response in all three channels. To create an inexpensive multi-channel sensing system using red, green, blue LEDs, and a simple photodetector, a preliminaryreference is developed through division of higher responses from two of the three channels withrespect to the channel that produces lowest response. It is worth noting here that the aim of thispart of the experiment is to study the efficiency of utilizing only three color channels (red, green,and blue) for different vapors sensing. This can possibly be extended to the use of RGB LEDwith lower cost photodetector instead of a spectrometer.
4 Results and Discussion
Figure 8 illustrates the modeling results of normalized output for unpatterned and spiral-pat-terned POF. The normalized output increased greatly for spiral patterned POFs over that derivedfrom unpatterned coatings as the width of ZnO nanorods coating was varied from 0 to 20 mm.
Spiral-patterned POFs coupled more light compared to unpatterned POF for nanorodcoating widths <5 mm as shown in Table 1. The greatest difference in output between patternedand unpatterned coatings was shown at ZnO width equal to 1 mm where ΔIð1 mmÞ ¼Ip − Iup ¼ 0.369 due to spiral ZnO coating along the core POF compared to unpatterned coatingthat had only one patch of ZnO region (1 mm) on the POF. Although ΔIð1 mmÞ was the highest,the coupling output for spiral pattern was not considered because it was not the maximum valueof light-side coupling. The spiral pattern coating achieved the maximum value of light-side cou-pling at width equal to 5 mm [ΔIð5 mmÞ ¼ Ip − Iup ¼ 0.135]. Therefore, despite that ΔIð5 mmÞwas <ΔIð1 mmÞ, the use of maximal light-side coupling was more dominant in applications.
Fig. 8 The output voltage for unpatterned and spiral-patterned coating by varying the width of ZnOnanorods coating on POFs.
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Meanwhile, the unpatterned coating achieved the maximum value of light-side coupling at ZnOcoating width longer than spiral patterns. Once the maximum output was reached, the outputremained consistent in POF’s with both types of coatings at the normalized value equal to 1 eventhough the width of ZnO coating was varied.
Based on simulation results, 3, 5, and 7 mm coating widths were selected for experimentaloptimization and application. Figure 9 shows the experimental results for spiral-patterned andunpatterned POFs. Overall, it can be seen that both coating schemes correlated well with sim-ulations. The results clearly showed that the unpatterned coatings of ZnO nanorods (3, 5, and7 mm) coupled less light compared to spiral-patterned POFs. In addition, the full ZnO coatedPOFs (100 mm) produced an output voltage that was less than spiral-patterned POFs (3, 5, or7 mm) due to less illumination coverage of the visible-light source in the distance of 3 cm fromPOF sample as shown in Fig. 6.
4.1 Physical Characterization
The SEM image in Fig. 10(a) with magnification set at 10.00 kX clearly shows the spiral-pat-terned ZnO nanorods coating on POF. In the low-magnification image given in Fig. 10(a), thewidth of ZnO coating is 5 mm and the uncoated spacing is 10 mm in width. In Fig. 10(b), ZnOnanorods can be seen growing perpendicular to the surface of the POF, an important geometry toenhance the light scattering mechanism for light-side coupling into the POF. Moreover, the
Table 1 Differences of normalized coupling output, ΔI between spiral-patterned and unpatternedPOFs for different widths of ZnO coating from 0 to 7 mm.
Widths of ZnO coating region (mm)
Normalized coupling output
Spiral pattern Ip Unpatterned Iup ΔI ¼ Ip − Iup
0 0 0 0
1 0.699 0.330 0.369
2 0.910 0.551 0.359
3 0.973 0.699 0.274
4 0.992 0.798 0.194
5 1.000 0.865 0.135
6 1.000 0.909 0.091
7 1.000 0.939 0.061
Fig. 9 The experimental result of spiral-patterned and unpatterned coating for 3, 5, 7, and100 mm.
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growth of ZnO nanorods on POF surface in Fig. 10(b) observed at magnification of 15.00 kXreveals high density (85 nanorods∕3.62 × 10−12 m2 ¼ 23.50 × 106 nanorods∕μm2) and uni-form distribution. Figure 10(c) shows the growth of ZnO nanorods with magnification of30.00 kX. From the SEM images, the obtained ZnO nanorods were about 3.41� 0.05 μmin length and 172.8� 20 nm in diameter as shown in Fig. 10(d).
4.2 Multiple Optical Channels Sensing
The improved side coupling by the spiral-patterned coating of ZnO nanorods on POF isexploited to demonstrate sensor performances in different wavelength domains of visible lightcalled channels to sense three different alcohol vapors (methanol, ethanol, and isopropanol) asshown in Fig. 11. All sensing was accomplished with the optimized spiral ZnO coating width of5 mm. It was found that the sensor demonstrated three different responses for methanol, ethanol,and isopropanol vapors as a function of molecular weights (methanol < ethanol < isopropanol),relative dielectric constants, and polarity. The relative dielectric constants of methanol (33),
Fig. 10 (a) The optimized width (5 mm) of spiral-patterned ZnO nanorods coating on POF, (b) thedistribution of ZnO nanorods on POF at low magnification, (c) ZnO nanorods observed at highmagnification, and (d) the height and diameter of ZnO nanorod.
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ethanol (24), and isopropanol (20), e.g., most likely influenced selective alcohol vapor moleculeadsorption onto different crystal faces of the ZnO nanorods.31 The amount of vapor adsorptioncoupled with the region of adsorption on the ZnO, therefore, played major roles in the attenu-ation of the coupled light signal. Additionally, the refractive indices of methanol (1.328), ethanol(1.361), and isopropanol (1.377) also affected the interaction of light by varying the refractiveindex of ZnO nanorods coating.32 Interestingly, in the presence of methanol, the intensity wasseen to decrease significantly indicating lower side coupling of light into the POF core. This wasdue to the change in the refractive index of the ZnO coating layer caused by methanol absorption.During sensor recovery, as methanol evaporated from the layer of ZnO nanorods coating, thesensor output was observed to return closely to the initial condition in ∼7 min. For ethanol andisopropanol, the sensor also demonstrated similar response patterns caused by rising adsorptiononto ZnO nanorods. However, the sensor demonstrated slight response to isopropanol vapormolecules compared to ethanol. The recovery time for ethanol and isopropanol were ∼5 and∼3 min, respectively. The decrease of light intensity when exposed to the alcohol vaporsand the recovery toward initial value is believed to be due to chemisorption process, the inter-action of hydrogen bonding between –OH groups of alcohol molecules with ZnO coating layer.33
Due to the small size of methanol molecules compared to ethanol and the biggest molecule size,isopropanol, the chemisorption process between methanol molecules and ZnO coating layer isvery high, taking more time to recover relatively to initial value.
The change in the refractive index of the ZnO coating layer due to absorption process thataffects the amount of light scattering into POF can be explained clearly using the sensing mecha-nism depicted in Fig. 12. Initially, ZnO nanorods are exposed to air at room temperature asshown in Fig. 12(a). In this case, ionized oxygen is chemisorbed onto the surface in its molecularform O−
2 as given by34
EQ-TARGET;temp:intralink-;e006;116;135O2ðgasÞ þ e− ↔ O−2 : (6)
As the surface is illuminated continuously by visible light as shown in Fig. 12(b), changes in thecarrier density in the ZnO nanorods and the layer of depletion depth occur. Once electron–holepairs are generated by the visible light, holes migrate to the surface and discharge the adsorbed
Fig. 11 Spectroscopy responses of multiple optical channels sensor in channel blue, green, andred wavelengths for (a) methanol, (b) ethanol, and (c) isopropanol.
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oxygen molecules. This causes the depth of the depletion layer to decrease, resulting in thedesorption of surface oxygen. Over time, unpaired electrons accumulate until the desorption andadsorption of oxygen reaches an equilibrium state. The amount of adsorbed oxygen decreasescompared to air conditions as shown in Fig. 12(a). The presence of excitons under visible-lightirradiation leads to the formation of atomic adsorbed oxygen, O−, which is substantially morechemically active than O−
2 and creates favorable conditions for catalytic reactions.35,36 This phe-nomena contributed to the amount of light scattering into optical core fiber by the ZnO nanorods.When gases (such as methanol in this case) are introduced, the adsorbed oxygen on ZnO nano-rods took part in the oxidation of methanol in two possible ways [Eqs. (7) and (8)].37 The oxygenions on the surface of ZnO reacted with the methanol molecules and give up electrons to theconduction band and increase the carrier concentration in the ZnO nanostructure as shown inFig. 12(c),
EQ-TARGET;temp:intralink-;e007;116;331CH3OHþ O− ↔ HCHOþ H2Oþ e−; (7)
EQ-TARGET;temp:intralink-;e008;116;300CH3OH þ O−2 ↔ HCOOHþ H2Oþ e−: (8)
As a result, scattering attenuation was therefore a function of the type of molecular speciesadsorbed onto the ZnO surface (e.g., its refractive index, n) and the amount of that materialpresent, allowing these molecules to interact differently with the incoming light. At the sametime different organic molecules have different refractive indices, molecule sizes, and band bend-ing occurs at surface, which will also affect the interaction of incoming light by varying the n ofthe outer coating differently.38
As observed from the responses of the sensor in multiple optical channels as shown inFig. 13, different wavelengths of light contributed in determining the amount of chemicalvapor absorption onto ZnO nanorods with attenuated light scattering into the POF core. Inthis result, the green channel presented the largest range of intensity (0.11 to 0.87) followedby the blue channel (0.07 to 0.41) and then, with the lowest intensity, the red channel (0.05to 0.17). It can be concluded that the intensity of green light in ZnO nanorods dramaticallydecreased with the increase in vapor concentration as alcohol molecules are adsorbed onto theZnO coating. Subsequently, there was a degree of attenuation of light scattering into POF core. Itwas shown in Refs. 39–41 that the luminance characteristic of ZnO has a significant response ingreen spectral range due to the strong influence by free-carrier depletion at the particle surface,particularly for small ZnO particles.
Fig. 12 Schematic diagram of the alcohol sensing mechanism using POF coated with ZnO nano-rods activated using visible white light illumination (a) in air at room temperature, (b) with visiblewhite light, and (c) with methanol exposure.
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Moreover, upon exposure to visible light, ZnO nanorod coating will be photoactivated lead-ing to reduced intergrain barrier height, thereby increasing the density of free carriers in thematerial. Boiling points of primary alcohols in these experiments are methanol: 65°C; ethanol:78°C; isopropanol: 82°C, which are related anyway to the bond dissociation energies. Hence, theorder of RIM was in the reverse order. This leads to the specific heats of vaporization, which isthe lowest for isopropanol (0.471 kJ∕g) compared to ethanol (0.925 kJ∕g) and methanol(1.22 kJ∕g). At concentration <100 ppm, the sensor demonstrated a slight difference in responseto methanol and ethanol vapor. It means that the sensor is not able to differentiate significantlythe two alcohol vapors due to less amount of vaporization to extremely excite oxygen ions on thesurface of ZnO coating. A significant difference can be seen clearly when the sensor wasexposed to higher concentrations because the interaction between methanol molecules andthe oxygen ions of ZnO contributed to a higher carrier concentration compared to ethanol.
Fig. 13 The responses of multiple optical channels sensor as a function of concentration of meth-anol, ethanol, and isopropanol vapors (ppm) in channels (a) blue, (b) green, and (c) red.
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Figure 14 shows the RIMs of the multichannel optical sensor for alcohol vapors. The RIMs ofthe sensor in each channel were calculated using Eq. (5). The absolute value of the light intensitydecreased rapidly with increasing alcohol vapors concentration from 0 to 300 ppm. The greenchannel contributed the highest RIM for all three alcohols. Measurements in the blue lightdomain showed intermediate RIM, and lastly, the red channel exhibited significantly lowerRIM of all three light domains for all alcohols tested. For the green channel, the sensor responsefor methanol was found to be 0.84 a.u. However, when ethanol and isopropanol were tested, theRIM decreased to 0.55 and 0.14 a.u., respectively. In the case of the blue channel, sensor
Fig. 14 The RIM of multiple optical channels sensor exposed to ethanol, methanol, and isopro-panol vapors were analyzed in three visible wavelengths (blue, green, and red).
Fig. 15 The validation of the multiple optical channels sensor for (a) channel blue/channel red and(b) channel green/channel red.
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response to methanol also presented the highest RIM of the three alcohols equal to 0.79 a.u. Forethanol and isopropanol, once again, decreased but to lower levels than those observed for thegreen channel 0.49 and 0.13 a.u., respectively. The same trends in alcohol RIM based on molecu-lar weight was demonstrated by the red channel but to the lowest levels of the three lightdomains. The optimum RIM for isopropanol is smaller than that of ethanol due to reducedabsorption onto the ZnO nanorods by the larger molecular weight and slightly less polarmolecule.34 Channel green demonstrated significant RIM in sensing the three alcohol vaporsbecause ZnO nanorods have both polar and nonpolar surfaces as reported in Ref. 42, wherezinc vacancies (VZn) at the nonpolar surfaces are responsible for the green luminescence ofZnO nanostructures.43 The presence of neutral VZn on ZnO coating generates a multiple effect:the absence of Zn ions leaves out under coordinated O atoms, and the unpaired O electrons giverise to empty states. As a consequence, this leads to a high absorption in green luminescence dueto strong O–H chemical bonds.44
A preliminary reference was developed by performing cross validation on channel blue andgreen with respect to channel red that produced the minimal response for the alcohol vapors asshown in Fig. 15 to create an inexpensive multichannel sensing system using red, green, blueLEDs, and a simple photodetector. To observe the response of the sensor as a multichanneldevice, one of the three channels is set as a reference to accommodate for source fluctuationand environment effect (e.g., heat and vibration). The other two readings are normalized tothe reference and the RIM of different gas vapors to these channels are examined. In both val-idations, the three alcohol vapors illustrated a similar pattern response at two different ranges,which were ∼1.3 to 2.5 for channel blue/channel red and ∼2.0 to 5.2 for channel green/channelred. In these ranges, significant responses were seen clearly in methanol and ethanol sensingcompared to isopropanol that showed less response.
5 Conclusion
The present study optimized theoretically the width of the ZnO nanorod spiral coating and dem-onstrated experimentally with a visible-light source its utility as an alcohol vapor sensor. Thewidth was found to be 5 mm for efficient light-side coupling. There was reasonable correlationbetween theory and experiment. Practical application of the special POF system showed promiseas a multiple optical channel sensor for alcohol vapors in the visible range of the spectrum. Toexplain the mechanism of sensing for alcohols, we proposed that the light scattering aspect of theZnO nanorods dependent on changes in the refractive index was affected by adsorption of alco-hol species. With regard to sensing performance in the three spectral channels described pre-viously, methanol showed the greatest RIM and range followed by ethanol and isopropanol. Inthe investigation of a multiple optical channel sensor, the green channel significantly producedhigher RIMs in sensing methanol, ethanol, and isopropanol vapors compared to the blue and redchannels. Furthermore, a preliminary reference was developed to propose a multiple opticalchannels sensor system using inexpensive color LEDs (blue, green, and red) as light sourcesand a simple photodetector in applications.
Acknowledgments
The authors would like to acknowledge the University of Malaya for their financial supportunder a High Impact Research Grant under Grant No. D000009-16001.
References
1. A. Hulanicki, S. Glab, and F. O. L. K. E. Ingman, “Chemical sensors: definitions and clas-sification,” Pure Appl. Chem. 63(9), 1247–1250 (1991).
2. M. Puyol et al., “Absorbance-Based Integrated Optical Sensors in Frontiers in ChemicalSensors, pp. 1–44, Springer, Berlin, Heidelberg (2005).
3. M. J. Leiner, “Luminescence chemical sensors for biomedical applications: scope and lim-itations,” Anal. Chim. Acta 255(2), 209–222 (1991).
Bin Abdul Rahim et al.: Applied light-side coupling with optimized spiral-patterned zinc oxide nanorod. . .
Journal of Nanophotonics 036009-15 Jul–Sep 2016 • Vol. 10(3)
4. A. D. McFarland and R. P. Van Duyne, “Single silver nanoparticles as real-time opticalsensors with zeptomole sensitivity,” Nano Lett. 3(8), 1057–1062 (2003).
5. G. Liebsch et al., “Fluorescent imaging of pH with optical sensors using time domain duallifetime referencing,” Anal. Chem. 73(17), 4354–4363 (2001).
6. D. L. McCorkle et al., “Ethanol vapor detection in aqueous environments using micro-capacitors and dielectric polymers,” Sens. Actuators B 107(2), 892–903 (2005).
7. M. Morisawa and S. Muto, “Plastic optical fiber sensing of alcohol concentration inliquors,” J. Sens. 2012, 709849 (2012).
8. A. Shenhav et al., “Optical sensor for remote estimation of alcohol concentration in bloodstream,” Opt. Commun. 289, 149–157 (2013).
9. K. Narsaiah et al., “Optical biosensors for food quality and safety assurance–a review,”J. Food Sci. Technol. 49(4), 383–406 (2012).
10. H. Clevenson et al., “Gas sensing using a resilient polymer photonic crystal nanocavity withultra-high quality factor,” in CLEO: 2013, OSA Technical Digest (Online), Paper No.AF1J.2, Optical Society of America (2013).
11. Z. Zalevsky et al., “Optical remote sensor for alcohol concentration in blood,” in Frontiersin Optics 2012/Laser Science XXVIII, OSA Technical Digest (Online), Paper No. FTu4E.5,Optical Society of America (2012).
12. S. M. Barnard and D. R. Walt, “Fiber-optic organic vapor sensor,” Environ. Sci. Technol.25(7), 1301–1304 (1991).
13. R. Aneesh and S. K. Khijwania, “Zinc oxide nanoparticle based optical fiber humiditysensor having linear response throughout a large dynamic range,” Appl. Opt. 50(27),5310–5314 (2011).
14. F. M. Dickey, L. S. Weichman, and R. N. Shagam, “Laser beam shaping techniques,” inPaper presented at the High-Power Laser Ablation (2000).
15. B. D. MacCraith et al., “Fibre optic oxygen sensor based on fluorescence quenching ofevanescent-wave excited ruthenium complexes in sol-gel derived porous coatings,”Analyst 118(4), 385–388 (1993).
16. A. Sharma and O. S. Wolfbeis, “Fiber optic oxygen sensor based on fluorescence quenchingand energy transfer,” Appl. Spectrosc. 42(6), 1009–1011 (1988).
17. H. Fallah et al., “Demonstration of side coupling to cladding modes through zinc oxidenanorods grown on multimode optical fiber,” Opt. Lett. 38(18), 3620–3622 (2013).
18. H. R. B. A. Rahim et al., “Side coupling of multiple optical channels by spiral patternedzinc oxide coatings on large core plastic optical fibers,” Micro Nano Lett. 11(2), 122–126(2016).
19. R. Hong et al., “Synthesis and surface modification of ZnO nanoparticles,” Chem. Eng. J.119(2), 71–81 (2006).
20. M. Furuta et al., “Effect of surface treatment of gate-insulator on uniformity of bottom-gateZnO thin film transistors,” Electrochem. Solid-State Lett. 13(4), H101–H104 (2010).
21. S. H. Sonawane et al., “Improved active anticorrosion coatings using layer-by-layerassembled ZnO nanocontainers with benzotriazole,” Chem. Eng. J. 189, 464–472(2012).
22. S. Xu and Z. L. Wang, “One-dimensional ZnO nanostructures: solution growth and func-tional properties,” Nano Res. 4(11), 1013–1098 (2011).
23. A. K. Giri et al., “Rectangular ZnO porous nano-plate assembly with excellent acetonesensing performance and catalytic activity,” RSC Adv. 5, 102134–102142 (2015).
24. R. Wahab et al., “Formation of ZnO micro-flowers prepared via solution process and theirantibacterial activity,” Nanoscale Res. Lett. 5(10), 1675–1681 (2010).
25. S. Baruah, S. K. Pal, and J. Dutta, “Nanostructured zinc oxide for water treatment,”Nanosci. Nanotechnol. Asia 2(2), 90–102 (2012).
26. M. Batumalay et al., “Tapered plastic optical fiber coated with ZnO nanostructures for themeasurement of uric acid concentrations and changes in relative humidity,” Sens. ActuatorsA 210, 190–196 (2014).
27. H. Fallah et al., “Excitation of core modes through side coupling to multimode optical fiberby hydrothermal growth of ZnO micro-nanorods for wide angle optical reception,” J. Opt.Soc. Am. B 31(9), 2232–2238 (2014).
Bin Abdul Rahim et al.: Applied light-side coupling with optimized spiral-patterned zinc oxide nanorod. . .
Journal of Nanophotonics 036009-16 Jul–Sep 2016 • Vol. 10(3)
28. Y. S. Dwivedi, A. K. Sharma, and B. D. Gupta, “Influence of skew rays on the sensitivityand signal-to-noise ratio of a fiber-optic surface-plasmon-resonance sensor: a theoreticalstudy,” Appl. Opt. 46(21), 4563–4569 (2007).
29. L. Peng et al., “Size-and photoelectric characteristics-dependent formaldehyde sensitivity ofZnO Irradiated with UV light,” Sens. Actuators B 148(1), 66–73 (2010).
30. L. S. Grattan and B. T. Meggitt, Eds., Optical Fiber Sensor Technology: AdvancedApplications-Bragg Gratings And Distributed Sensors, Springer Science & BusinessMedia, Berlin, Germany (2013).
31. G. Wiederrecht, Handbook of Nanoscale Optics and Electronics, Academic Press,Cambridge, Massachusetts (2010).
32. N. A. Yebo et al., “An integrated optic ethanol vapor sensor based on a silicon-on-insulatormicroring resonator coated with a porous ZnO film,” Opt. Express 18(11), 11859–11866(2010).
33. M. Jaisai, S. Baruah, and J. Dutta, “Paper modified with ZnO nanorods-antimicrobial stud-ies,” Beilstein J. Nanotechnol. 3(1), 684–691 (2012).
34. M. R. Alenezi et al., “Role of the exposed polar facets in the performance of thermally andUV activated ZnO nanostructured gas sensors,” J. Phys. Chem. C 117(34), 17850–17858(2013).
35. T. Barry and F. Stone, “The reactions of oxygen at dark and irradiated zinc oxide surface,”Proc. R. Soc. London, Ser. A 255, 124–144 (1960).
36. S. Fan, A. K. Srivastava, and P. V. Dravid, “UV-activated room temperature gas sensingmechanism of polycrystalline ZnO,” Appl. Phys. Lett. 95(1–3), 142106 (2009).
37. N. G. Patel, P. D. Patel, and V. S. Vaishnav, “Indium tin oxide (ITO) thin film gas sensor fordetection of methanol at room temperature,” Sens. Actuators B 96(1–2), 180–189 (2003).
38. N. A. Yebo et al., “An integrated optic ethanol vapor sensor based on a silicon-on-insulatormicroring resonator coated with a porous ZnO film,”Opt. Express 18, 11859–11866 (2010).
39. K. Vanheusden et al., “Mechanisms behind green photoluminescence in ZnO phosphorpowders,” J. Appl. Phys. 79(10), 7983–7990 (1996).
40. K. Vanheusden et al., “Nature of the green luminescent center in zinc oxide,” inMRS Proc.,Vol. 424, p. 433, Cambridge University Press, Cambridge, United Kingdom (1996).
41. H. Y. Lin et al., “Enhancement of band gap emission stimulated by defect loss,” Opt.Express 14(6), 2372–2379 (2006).
42. M. Huang et al., “Controllable tuning various ratios of ZnO polar facets by crystal seed-assisted growth and their photocatalytic activity,” Cryst. Growth Des. 14(5), 2179–2186(2014).
43. F. Fabbri et al., “Zn vacancy induced green luminescence on non-polar surfaces in ZnOnanostructures,” Sci. Rep. 4 (2014).
44. M. Willander et al., “Luminescence from zinc oxide nanostructures and polymers and theirhybrid devices,” Materials 3(4), 2643–2667 (2010).
Biographies for the authors are not available.
Bin Abdul Rahim et al.: Applied light-side coupling with optimized spiral-patterned zinc oxide nanorod. . .
Journal of Nanophotonics 036009-17 Jul–Sep 2016 • Vol. 10(3)
Sensors and Actuators A 257 (2017) 15–19
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical
j ourna l h o mepage: www.elsev ier .com/ locate /sna
TEMPERATURE SENSING BY SIDE COUPLING OF LIGHT THROUGHZINC OXIDE NANORODS ON OPTICAL FIBERS
Hazli Rafis Bin Abdul Rahim a,c, Muhammad Quisar Bin Lokman a,Sulaiman Wadi Harun a,b, Joydeep Dutta d, Waleed Soliman Mohammed e,∗
a Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Photonics Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysiac Faculty of Electronic and Computer Engineering, Universiti Teknikal Malaysia Melaka, 76100 Melaka, Malaysiad Functional Materials division, Materials and Nano-Physics Department, ICT School, KTH Royal Institute of Technology, SE-164 40 Kista, Stockholm, Swedene Center of Research in Optoelectronics, Communication and Control Systems (BU-CROCCS), School of Engineering, Bangkok University, 12120 Patumthani,Thailand
a r t i c l e i n f o
Article history:Received 7 September 2016Received in revised form10 December 2016Accepted 6 February 2017Available online 7 February 2017
Keywords:zinc oxidespiral patternoptical fiberlight side couplingnanorodstemperature
a b s t r a c t
A temperature sensor fabricated by light side coupling through spirally patterned zinc oxide (ZnO)nanorods coated directly on plastic optical fiber (POF) is reported. A significant response to temperaturechanges from 20 ◦C to 100 ◦C based on extinction concept due to the attenuation of light by scatteringand absorption was used. Sensitivity increases by a factor of 1.3 in spirally patterned coatings comparedto optical fibers with continuous coating. The simplicity and economical sensor fabrication process forthe swift and sensitive detection of temperature changes using visible light source show potential inenvironmental and biomedical applications.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
Light side coupling in optical fibers through the growth of sin-gle crystalline zinc oxide (ZnO) nanorods directly on plastic opticalfibers was introduced in our previous work [1]. In this concept, thelight scattering due to the incident angle of incident light greaterthan the critical angle posed by the surrounding and optical fibercore is applied. In this configuration, usually, light propagationinside optical fiber occur through the ZnO nanorods but the inten-sity of guided light is often low due to the leakage of light throughthe core mode [2]. This problem was suitably addressed by usinglarge core plastic optical fiber (POF) to increase scattering area [3].Further improvement was achieved by reducing the net area of ZnOnanorods coating through a structured growth on the POF’s [4].POF’s are physically robust and suitable for operation in the visiblelight regime as compared to glass optical fibers (GOF’s). Generally,most optical sensing applications operate with laser light source
∗ Corresponding author.E-mail address: [email protected] (W.S. Mohammed).
by launching light from one end of the optical fiber and output sig-nal is collected from other end [5]. This is more complex and oftenexpensive due to the needs of coupling the light in the optical fiberto align the laser beam. Coherent sources with small beam sizeswhen coupled through ZnO nanorods coating would provide lowersensitivity caused by the inequality of beam structure as it willhave different distribution of intensity along the ZnO coating [6]and more importantly the laser beam can only focuses on specificcoating area instead of entire coating area.
Conventional temperature sensors have their limitations if largedistances have to be covered such as in many distributed mea-surements, electromagnetic interference leads to the loss of signalto noise ratio, explosive environments does not allow safe use ofresistive devices and often in a plurality of applications they donot match when light-weight structures are desired. The fiber opticsensors market is a multi billion dollar business which is prognosedto grow further and fiber optic based temperature sensors are animportant class therein as they are immune to electro-magneticinterference and are thus robust and accurate in high-RF environ-ments. Several measurement principles have been described inthe literature for measuring temperature sensors such as inten-
http://dx.doi.org/10.1016/j.sna.2017.02.0080924-4247/© 2017 Elsevier B.V. All rights reserved.
16 H.R.B.A. Rahim et al. / Sensors and Actuators A 257 (2017) 15–19
sity modulated fiber optic displacement sensor (FODS) [7], lifetimemeasurements [8], microfiber loop resonator (MLR) [9], stimulatedbrillouin scattering [10], interferometer [11] and multicore fiberstructure [12]. Although, the temperature sensing using polymer-coated microfiber interferometer reported by Romano et al. has ahigh sensitivity but it is not able to sense temperature changes athigher range due to low melting point of the polymer. In order tobe economically advantageous, an optical fiber temperature sen-sor must be robust, easy-to-use, fast, accurate, stable over a widemeasurement range and suitable for a large variety of applications[13]. In an application, many commercial electronic componentscan be damaged due to exposure to high temperature (>70 ◦C) andsome can be damaged by exposure to low temperatures (<0 ◦C)[14]. Semiconductor devices and LCDs (liquid crystal displays) areexamples of commonly used components that are susceptible tolarge temperature variations. In these cases, temperature sensing isindeed important so that appropriate measures can be incorporatedto prolong the life of these devices. Optical fiber based temperaturesensors are the only possibility in the presence of electromagneticfields such as in microwave fields, power plants or explosion-proofareas and wherever measurement with electrical temperature sen-sors is not possible such as in high tension cable lines, airplanes,spacecrafts, electrical motors etc [15].
In a previous report, temperature sensing was demonstratedusing ZnO thin films where spectral absorption changes in ZnOwas monitored [16]. In this work we present optimized simple yetsensitive spiral patterned ZnO nanorod coatings on POF based tem-perature sensor capable of utilizing ambient light coupled throughthe nanorods into the fibers for sensing. Sensing performances ofZnO nanorod coatings, spirally patterned on POF fibers are pre-sented and the results are compared to the sensing characteristicsof the unpatterned fibers. Uncoated POF (bare) were not consideredfor this application since it does not show any scattering effects dueto side coupling of light [3,4].
2. Fiber Preparation
POF fiber spiral patterning and ZnO nanorod seeding and synthe-sis procedures were described in detail in previous works [4,17,18].Standard polymethyl methacrylate (SK-80 POF fibers from Mit-subishi Rayon Co., LTD; Japan) were utilized in these experiments toserve as control. The jacket of the POF were mechanically strippedoff to expose the fiber over a length of 100 mm. Fig. 1 shows howself-adhesive plastic tape was wrapped to create spiral pattern onPOF. Fully coated POF (100 mm) were also fabricated to completethe validation.
The fiber length of 100 mm was chosen in this work in order tohave a full illumination of light beam on the stripped fiber from alight source with diameter of 30 mm that was placed in parallel atan optimal distance of 30 mm from the POF surface. Tape-patternedand unpatterned POFs were then placed in a ZnO seed solution andsubsequently into the growth solution to form ZnO nanorods. Scan-ning Electron Microscopy (SEM) was performed by a Hitachi, 3400NSEM system operating at 20 kV.
3. Experiment
The proposed temperature sensor is schematically illustrated inFig. 2. For maximal temperature detection, an aluminum rod withdimension of 0.3 and 10 cm in length was used. The aluminum rodis placed vertically on a hot plate and in closed contact with thephysical POF coated with ZnO nanorods. For temperature monitor-ing, a thermocouple (type J) was fixed in closed contact with thePOF. The thermocouple has a resolution of 1 ◦C and is able to mea-sure the temperature within a range of 0 ◦C to 500 ◦C. A modulator
Fig. 1. Spiral Structured on POF using self-adhesive plastic tape for ZnO nanorodscoating.
circuit was used to minimize the noise in the measurement, thewhite-light LED current driver was modulated with a periodicalpattern signal generated by a signal generator.
The magnitude of light side coupling was measured by connect-ing one of the POF to photodector and displayed in millivolt (mV)on oscilloscope under illumination of the modulated visible whitelight source. The other one of the POF tip was covered during theexperiments to avoid light entering directly through the tips. Then,temperature sensing measurement was carried out by varying tem-perature from 20 ◦C to 100 ◦C. Five readings were recorded for eachmeasurement. The sensitivity (S) was obtained through the slopeof sensing response for spirally patterned and unpatterned ZnOnanorod coated POF devices.
4. Result and Discussion
ZnO nanorods coating: Fig. 3 shows the SEM image of unpat-terned and spirally patterned coating of ZnO nanorods fortemperature sensing. The SEM image in Fig. 3(a) and (b) clearlyshows the unpatterned and spiral patterned ZnO nanorods coat-ing on POF, respectively. In Fig. 3(c), ZnO nanorods can be seengrowing perpendicular to the surface of the POF, an importantgeometry to enhance the light scattering mechanism for lightside coupling into the POF. Moreover, the uniform high densitygrowth of ZnO nanorods (85 nanorods/3.62 × 10−12 m2 ∼ 23 × 106
nanorods/�m2) on POF surface can be observed from Fig. 3(c) andFig. 3 (d).
4.1. Temperature Sensing
The real time responses of the ZnO nanorod coated optical fibersensor to temperature changes from 20 ◦C to 100 ◦C were recordedtowards light side coupling. The measurements were conductedby exposing the spirally patterned and unpatterned ZnO nanorodscoated POFs to temperature under visible light illumination. It wasfound that the both coating schemes showed obvious output volt-age changes upon exposure to temperature as depicted in Fig. 4. It iswell known that the thermo-optic coefficient of the POF is an orderof magnitude higher than that of GOF, and the refractive index (RI)
H.R.B.A. Rahim et al. / Sensors and Actuators A 257 (2017) 15–19 17
Fig. 2. Experimental setup for the proposed temperature sensor towards light side coupling.
Fig. 3. (a) ZnO continuous coating on unpatterned POF, (b) ZnO spirally patternedcoating on POF, (c) the morphology of ZnO nanorod growth at low magnificationand (d) morphology of ZnO nanorods at higher magnification.
of POF is affected by temperature variation. Therefore, the tem-perature dependence must be taken into account for POF basedRI sensors. Several reports are available studying the temperaturedependence of the RI sensors based on GOF technology [19–21].As explained in our previous work [4], the spirally patterned ZnOnanorod coating leads to an increase in coupling of the light sourcecompared to the unpatterned POF’s due to the higher interfacialZnO regions on the POF.
Fig. 4. The response of spiral patterned coating and unpatterned coating in temper-ature sensing.
Earlier work had showed that absorptivity of ZnO has a lineardependence on temperature using reflection measurements [22]and refractive index of ZnO coating layer changed with temperature[23]. Fig. 5 illustrates the sensing mechanism of the temperaturesensor in this work due to the attenuation of light by scatteringand absorption as it traverses through the ZnO nanorods [24]. Thevisible light incident on the ZnO coating layer scatters into the POFand the guided light is detected. When the POF coated with ZnOnanorods was brought in proximity to the heated aluminum rodfollowing the continuous light illumination, the absorption of lightinside the ZnO nanorods coating increased and the amount of lightscattering into the POF relatively reduced with increasing temper-ature within the range as depicted in Fig. 5(c). Consequently, theintensity of guided light inside the POF decreased due to lower lightcoupling.
Fig. 6 shows the sensitivities of the optical sensor coated withZnO nanorods. In the case of spirally patterned coating, the sensorresponse to temperature changes was found to be 0.0623 mV/◦C.However, when the unpatterned coating was exposed to temper-ature changes, the sensitivity decreased to 0.0484 mV/◦C due toabsorption of light in ZnO coating layer resulting less light couplinginside the POF’s. Moreover, spirally patterned coating consists oftwo exposed elements; ZnO nanorods coating and uncoated regions(polymer). Thermal effect can be effectively sensed by spiral pat-terned POF because the uncoated regions contributes also to opticalloss which is dependent on temperature. The loss is reported toincrease with increasing temperature [25,26].
The temperature sensing was found to improve by a factor of1.3 using the spirally patterned coatings as compared to the unpat-
18 H.R.B.A. Rahim et al. / Sensors and Actuators A 257 (2017) 15–19
Fig. 5. The temperature sensing mechanism (a) before light illumination (b) upon light illumination and (c) aluminum rod in close proximity to ZnO nanorods coating layer.
Fig. 6. The sensitivity of spiral patterned and unpatterned coating in temperature sensing.
terned coatings. Moreover, this temperature sensor demonstrateda higher sensitivity compared to other optical fiber temperaturesensors which usually use common sensing method by introduc-ing light from one end and collected at the other end of the fibers[27,28]. It was observed that the spiral patterned coating is ableto monitor temperature to 0.1284 ◦C resolution and unpatternedcoating has a resolution of 0.2273 ◦C.
5. Conclusions
We have successfully demonstrated light side coupling withoptimized spiral patterned zinc oxide nanorod coatings for tem-perature. The inherent advantages of optical fibers paired withapparent transparency of ZnO nanorods in the visible wavelengthregion were applied to design a simple and cost effective opticalsensor. With increasing temperature, the coupled light was foundto reduce, which was used for calibrating as a temperature sensor.The spirally patterned ZnO nanorods coating demonstrated signif-icant improvement (1.3 times better sensitivity) in the coupledlight power compared to unpatterned coatings, since scatteringof light is dependent on the refractive index of ZnO surface andhighly sensitive to ZnO absorption. These robust but simple temper-ature sensors can find wide ranging applications for environmentalmonitoring [29], biomedical purposes [30] as well as in environ-ments with electromagnetic pollution and/or explosive conditions.For instance, it can be also used as a visible light photosensitive
indicator for those who have photosensitivity diseases related tosunlight or interior lighting. This proposed sensor is able to indicatethe intensity level of light at that a particular area due to excitationof ZnO upon receiving photons to scatter light into POF.
Acknowledgements
Authors would like to acknowledge the University of Malaya fortheir financial support under a High Impact Research Grant (GrantNo. D000009-16001) and thank Dr. Gabor Louis Hornyak of CoENat the Asian Institute of Technology, Thailand for allowing the useof the facilities for some of the experiments.
References
[1] H. Fallah, M. Chaudhari, T. Bora, S.W. Harun, W.S. Mohammed, J. Dutta,Demonstration of side coupling to cladding modes through zinc oxidenanorods grown on multimode optical fiber, Optics letters 38 (18) (2013)3620–3622.
[2] T. Bora, H. Fallah, M. Chaudhari, T. Apiwattanadej, S.W. Harun, W.S.Mohammed, J. Dutta, Controlled side coupling of light to cladding mode ofZnO nanorod coated optical fibers and its implications for chemical vaporsensing, Sensors and Actuators B: Chemical 202 (2014) 543–550.
[3] H. Fallah, S.W. Harun, W.S. Mohammed, J. Dutta, Excitation of core modesthrough side coupling to multimode optical fiber by hydrothermal growth ofZnO nanorods for wide angle optical reception, JOSA B 31 (9) (2014)2232–2238.
[4] H.R.B.A. Rahim, S. Manjunath, H. Fallah, S. Thokchom, S.W. Harun, W.S.Mohammed, J. Dutta, Side coupling of multiple optical channels by spiral
H.R.B.A. Rahim et al. / Sensors and Actuators A 257 (2017) 15–19 19
patterned zinc oxide coatings on large core plastic optical fibers, IET Micro &Nano Letters 11 (2) (2016) 122–126.
[5] R. Aneesh, S.K. Khijwania, Zinc oxide nanoparticle based optical fiberhumidity sensor having linear response throughout a large dynamic range,Applied optics 50 (27) (2011) 5310–5314.
[6] F.M. Dickey, L.S. Weichman, R.N. Shagam, Laser beam shaping techniques, in:Paper presented at the High-Power Laser Ablation, 2000.
[7] H.A. Rahman, S.W. Harun, N. Saidin, M. Yasin, H. Ahmad, Fiber opticdisplacement sensor for temperature measurement, Sensors Journal IEEE 12(5) (2012) 1361–1364.
[8] Z. Zhang, K. Grattan, A. Palmer, Fiber optic temperature sensor based on thecross referencing between blackbody radiation and fluorescence lifetime,Review of scientific Instruments 63 (5) (1992) 3177–3181.
[9] S. Harun, K. Lim, S. Damanhuri, H. Ahmad, Microfiber loop resonator basedtemperature sensor, Journal of the European Optical Society-Rapidpublications 6 (2011).
[10] T. Kurashima, T. Horiguchi, M. Tateda, Distributed-temperature sensing usingstimulated Brillouin scattering in optical silica fibers, Opt Lett 15 (18) (1990)1038–1040.
[11] I. Hernández-Romano, D. Monzón-Hernández, C. Moreno-Hernández, D.Moreno-Hernandez, J. Villatoro, Highly sensitive temperature sensor based ona polymer-coated microfiber interferometer, IEEE Photonics TechnologyLetters 27 (24) (2015) 2591–2594.
[12] J.E. Antonio-Lopez, Z.S. Eznaveh, P. LiKamWa, A. Schülzgen, R.Amezcua-Correa, Multicore fiber sensor for high-temperature applications upto 1000C, Optics Letters 39 (15) (2014) 4309–4312.
[13] X. Li, S. Lin, J. Liang, Y. Zhang, H. Oigawa, T. Ueda, Fiber-optic temperaturesensor based on difference of thermal expansion coefficient between fusedsilica and metallic materials, Photonics Journal IEEE 4 (1) (2012) 155–162.
[14] R. Mishra, M. Keimasi, D. Das, The temperature ratings of electronic parts,Electronics Cooling 10 (1) (2004) 20.
[15] M. Ramakrishnan, G. Rajan, Y. Semenova, G. Farrell, Overview of Fiber OpticSensor Technologies for Strain/Temperature Sensing Applications inComposite Materials, Sensors 16 (1) (2016) 99.
[16] G. Hvedstrup Jensen, Temperature Dependence of the Band Gap in ZnO fromReflection Data, physica status solidi (b) 64 (1) (1974) K51–K54, http://dx.doi.org/10.1002/pssb.2220640159.
[17] S. Baruah, J. Dutta, Effect of seeded substrates on hydrothermally grown ZnOnanorods, Journal of Sol-Gel Science and Technology 50 (3) (2009) 456–464.
[18] S. Baruah, C. Thanachayanont, J. Dutta, Growth of ZnO nanowires onnonwoven polyethylene fibers, Science and Technology of AdvancedMaterials (2016).
[19] P. Wang, Y. Semenova, Q. Wu, J. Zheng, G. Farrell, Temperature performanceof a macrobending single-mode fiber-based refractometer, Applied optics 49(10) (2010) 1744–1749.
[20] P. Wang, Y. Semenova, G. Farrell, Temperature dependence of macrobendingloss in all-fiber bend loss edge filter, Optics Communications 281 (17) (2008)4312–4316.
[21] Y.G. Han, J. Lee, S. Lee, Discrimination of bending and temperaturesensitivities with phase-shifted long-period fiber gratings depending oninitial coupling strength, Optics Express 12 (14) (2004) 3204–3208.
[22] H. Wang, A. Yang, L. Tang, Wide measurement-range fiber-optic temperaturesensor based on ZnO thin film, Optics and Lasers in Engineering 60 (2014)49–53.
[23] S. Sanjeev, D. Kekuda, Effect of Annealing Temperature on the Structural andOptical Properties of Zinc Oxide (ZnO) Thin Films Prepared by Spin CoatingProcess. Paper presented at the, IOP Conference Series: Materials Science andEngineering (2015).
[24] R. Near, S. Hayden, M. El-Sayed, Extinction vs absorption: which is theindicator of plasmonic field strength for silver nanocubes? The Journal ofPhysical Chemistry C 116 (43) (2012) 23019–23026.
[25] K. Minakawa, N. Hayashi, Y. Shinohara, M. Tahara, H. Hosoda, Y. Mizuno, K.Nakamura, Wide-range temperature dependences of Brillouin scatteringproperties in polymer optical fiber, Jpn. Appl. Phys. 53 (4) (2014) 042502.
[26] L.R. Husdi, K. Nakamura, S. Ueha, Sensing characteristics of plastic opticalfibres measured by optical time-domain retlectometry, Meas. Sci. Technol. 15(8) (2004) 1553–1559.
[27] S. Ju, P.R. Watekar, W.-T. Han, Highly sensitive temperature sensor using fiberBragg grating on Pb/Ge-codoped fiber, Paper presented at the National FiberOptic Engineers Conference (2009).
[28] H.A. Rahman, S.W. Harun, N. Saidin, M. Yasin, H. Ahmad, Fiber opticdisplacement sensor for temperature measurement, Sensors Journal IEEE 12(5) (2012) 1361–1364.
[29] R.J. Schroeder, T. Yamate, E. Udd, High pressure and temperature sensing forthe oil industry using fiber Bragg gratings written onto side hole single modefiber, in: Proceedings-SpieThe International Society For Optical Engineering,Spie International Society For Optical, 1999, pp. 42–45.
[30] V.A. Korolyov, Biomedical Fiber-Optic Temperature and Pressure Sensors,Biomedical Engineering (2012) 1–4.
Biographies
Hazli Rafis Abdul Rahim is currently pursuing his Ph.D inoptical fiber sensor at Photonics Engineering, Departmentof Electrical Engineering, University of Malaya, Malaysiasince 2014. His Ph.D research focuses on the structuredgrowth of zinc oxide nanorods on optical fiber and lightside coupling towards sensing applications. He is also anacademician at Faculty of Electronic and Computer Engi-neering, Universiti Teknikal Malaysia Melaka, Malaysia
Muhammad Quisar Lokman received his B.Eng(<BIO>Hons) degree in Telecommunication Engineeringfrom University of Malaya, Malaysia in 2015. He iscurrently pursuing his Master in Engineering Science(Photonics) in University of Malaya.
Sulaiman Wadi Harun received his B.Eng degree in Elec-trical and Electronics System Engineering from NagaokaUniversity of Technology, Japan in 1996, and M.Sc. andPh.D degrees in Photonics from University of Malaya in2001 and 2004, respectively. Currently, he is a full profes-sor at the Faculty of Engineering, University of Malaya. Hisresearch interests include fiber optic active and passivedevices.
Joydeep Dutta received his B.Sc. (Hons) and M. Sc. inPhysics from St. Edmund’s College, North Eastern Hill Uni-versity, Shillong, India in 1983 and 1985, respectively. Hejoined Calcutta University, India and received his Ph.D inPhysics in 1990. Now, he is the Chair of Functional Mate-rials Division at the KTH Royal Institute of Technology,Stockholm, Sweden.
Waleed Mohammed received his B.Eng (Hons) degreein Electronics and Electrical Communications and M.Sc.in Computer Engineering in Cairo University, Egypt. Hejoined the College of optics and photonics, University ofCentral Florida, USA and received M.Sc in optics in 2001.In 2004 he completed his Ph.D work. Currently, he is aresearch scholar at the School of Engineering, BangkokUniversity. His research includes optoelectronics, nano-technology, sensor and sensors network.
BOON IP ENTERPRISERegistered Patent & Trade Mark Agent
PATENT FILING REPORT(Our Reference: BIP160010, Your Reference: UM.TNC2/UMCIC/603/1151)
1. Country : Malaysia
2. Application number : PI 2016701346
3. Filing date : 13 Apr 2016
4. Title : COATED PLASTIC OPTICAL FIBER
5. Applicant : Universiti Malaya,50603 Kuala Lumpur, Malaysia
6. Inventor (s) : 1) Sulaiman Wadi Harun (A citizen of Malaysia)2) Hazli Rafis Abdul Rahim (A citizen of Malaysia)
Faculty of Engineering, Universiti Malaya,50603 Kuala Lumpur
3) Waleed Mohammed (A citizen of Egypt)School of Engineering, Bangkok University-Rangsit Campus, 12120 Bangkok
7. Dateline for PCT orforeign filling
: 13 Apr 2017
8. Dateline to requestsubstantive examination
: 13 Oct 2017
9. Abstract:
The present invention
provides a coated plastic
optical fiber (20) for
improving side coupling of
extended light. The
proposed fiber has a core
(10) and a coat of ZnO
nanorods to form a spiral
line (16) along and around
the elongated surface of the core (10). The gap between each adjacent ZnO nanaorods spiral line (x) is
twice the width of the ZnO nanorods spiral line (y). A method to coat the fiber is described. It was found
that spiral line coat of ZnO nanorods fiber (20) manage to double the side coupling of extended light
compared to uniform coating of ZnO nanorods fiber. Different incident wavelength excitation is achievable
at each loop of ZnO nanorods spiral line.
Patents Form No.1PATENTS ACT 1983
REQUEST FOR GRANT OF PATENT(Regulations 7(1))
To: The Registrar of Patents Patents Registration Office Kuala Lumpur, Malaysia
For Official Use
APPLICATION NO: PI 2016701346 Filing Date: 13/04/2016 Fee received on: 13/04/2016 Amount: RM260
Please submit this Form in duplicate together with theprescribed fee
Applicant's file reference: BIP160010
THE APPLICANT(S) REQUEST(S) THE GRANT OF A PATENT IN RESPECT OF THE FOLLOWING PARTICULARS: I. TITLE OF INVENTION: COATED PLASTIC OPTICAL FIBERII. APPLICANT(S) (the data concerning each applicant must appear in this box or , if the space insufficient, in the space below):
Name: Universiti MalayaI.C./Passport No: Address: Lembah Pantai, 50603 WILAYAH PERSEKUTUAN KUALA LUMPUR MALAYSIA Nationality:
Address for service in Malaysia: C/O BOON IP ENTERPRISE, 32A, JALAN 17/155C, BUKIT JALIL 57000 KUALA LUMPURMALAYSIA * Permanent resident or principal place of business: Telephone Number (if any) Fax Number (if any) Additional Infomation (if any)
Additional Infomation (if any)
III.INVENTOR:
Applicant is the inventor: Yes No √If the applicant is not the inventorName: Sulaiman Wadi HarunAddress: Faculty of Engineering, Universiti Malaya, 50603 WILAYAH PERSEKUTUAN KUALA LUMPUR MALAYSIA
Applicant is the inventor: Yes No √If the applicant is not the inventorName: Hazli Rafis Abdul RahimAddress: Faculty of Engineering, Universiti Malaya, 50603 WILAYAH PERSEKUTUAN KUALA LUMPUR MALAYSIA
Applicant is the inventor: Yes No √If the applicant is not the inventorName: Waleed MohammedAddress: School of Engineering, Bangkok University-Rangsit Campus, 12120 Bangkok THAILAND A statement justifying the applicant's to the patent accompanies this Form
Yes No √
Additional Information (if any)
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Applicant has appointed a patent agent in accompanying Form No. 17 Yes √ No Agent Registration No: PA/2007/0176Applicant has appointed to be their representative: TAN BOON LENG
V. DIVISIONAL APPLICATION:
This application is a divisional application The benefit of the filing date priority date of the initial application is claimed in as much as the subject-matter of the present application is contained in the initialapplication identified below :Initial Application No: Date of filing of initial application: Additional Information (if any)
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[email protected], cn=Boon Leng Tan, ou=Contact Number -0122831763,ou=Identity Card / Passport No -##############, ou=Terms of useat www.msctrustgate.com/rpa (c)00, ou=Bahagian Teknologi Maklumat V2,o=Perbadanan Harta Intelek Malaysia, l="10-2 Resource Centre,Technology Park Malaysia,", st="57000 Kuala Lumpur, WilayahPersekutuan", c=MY, CertSerialNo=57b936ab9f5d93922b4b8836137cf795|
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1521312
Attached DocumentD037-786003-1151 pofabs.pdf
D038-786003-Fig pof.pdfD062-786003-1151 pofdesc.pdfD063-786003-1151 pofcla.pdf
8
ABSTRACT
COATED PLASTIC OPTICAL FIBER
The present invention provides a coated plastic optical fiber (20) for improving side
coupling of extended light. The proposed fiber has a core (10) and a coat of ZnO nanorods to
form a spiral line (16) along and around the elongated surface of the core (10). The gap5
between each adjacent ZnO nanaorods spiral line (x) is twice the width of the ZnO nanorods
spiral line (y). A method to coat the fiber is described. It was found that spiral line coat of ZnO
nanorods fiber (20) manage to double the side coupling of extended light compared to uniform
coating of ZnO nanorods fiber. Different incident wavelength excitation is achievable at each
loop of ZnO nanorods spiral line.10
FIG. 2 to accompany abstract
1 / 3
Fig. 1
Fig. 2
xy
20
1012
14
1012
16
2 / 3
Fig. 3
(Prior Art)
Fig. 4
P(z1)
P(z2)
z
Inte
nsity
P(z1) P(z2)
Inte
nsity
P(z1)
P(z2)
P(z2)
zP(z1)
18
12 10
22
24
2628
12
22
26
10
242816
20
3 / 3
Fig. 5
Fig. 6
P(λ1)
P(z3)
Inte
nsity
P(λ2) P(λ3)
λ
P(z2)P(z1)
12
22
2616
2428
20
1
COATED PLASTIC OPTICAL FIBER
The present invention relates to optical fibers, particularly to optical fibers with improved
extended light coupling.
BACKGROUND ART
Optical fiber is usually made of glass or plastic. Light is introduced at one part of fiber,5
guided through the fiber and collected at another part of the fiber. Optical fiber is widely used in
telecommunication and sensor applications. Optical fiber sensor is developed to resolve the
limitations of electrical sensors. The advantages of fiber optic sensing are its higher sensitivity,
geometrical versatility and its immunity to electromagnetic interference. These advantages
made optical fiber sensors suitable to be deployed in harsh environment.10
Plastic optical fiber (POF) uses polymethyl methacrylate resin, a general purpose resin
as the core material. Around 96% of the cross section of the core facilitates transmission of light.
Being plastic, the fiber is rugged and easy to install. However, conventional POF has lower
transmission performance compared to glass fiber.
Sensitivity of POF can be improved by enhancing the extension of the evanescent tail15
and the interaction of fiber surface area. Fallah et al introduced a concept of fiber side coupling
by guiding extended light and leakage by coating POF surface with ZnO nanorods. The
presence of ZnO nanorods coat scatters the incident light at angles larger than the critical angle
between the surrounding and cladding. Excitation of cladding mode is achieved in the fiber. The
nanorods coat cause leakage from these modes, allowing light to exit from the side which can20
be collected by side probe other than fiber end. This configuration can be implemented as
multiple sensing probe and multiple channel combiner. One of the embodiments of optical
sensor was reported by Bora et al.
ZnO have higher refractive index compared to glass fiber. Thus, ZnO nanorods grown
on cladding of POF allows light coupling into nanorod waveguides. Although optical guidance is25
enhanced by ZnO nanorods, light leakage happens due to scattering phenomena. It was found
that only a small portion of the fiber could be used for signal collection. This situation is
undesirable for sensor or telecommunication receivers where extended light sources are
required. Extended light source leads to less guidance of light inside the fiber resulting in low
efficiency and sensitivity.30
2
SUMMARY
The aim of the present invention is to increase the magnitude of light collection of plastic
optical fiber. The present invention provides a coated plastic optical fiber with improved side
coupling of extended light. The plastic optical fiber comprises a core; characterized in that, a
coat of ZnO nanorods form a spiral line along and around the elongated surface of the core. The5
gap between each adjacent ZnO nanorods spiral line is twice the width of the ZnO nanorods
spiral line. The coated fiber can be used for side coupling multiple extended light source having
different wavelength.
A method of making the plastic optical fiber, comprising preparing a core with exposed
elongated surce; masking a spiral line along and around the core elongated surface; dipping the10
core in ZnO seed solution; evaporating the seed solution from the core; repeating the step of
dipping the core in seed solution and evaporating the solution a number of fimes; and placing
the core in a solution of zinc nitrate hexahydrate and hexamethyleneteramine.
BRIEF DESCRIPTION OF DRAWINGS15
Further characteristics and advantages of the present invention will become better apparent
from the following detail description of a preferred but not exclusive embodiment thereof,
illustrated by way of non-limiting example in the accompanying drawings, wherein:
FIG. 1 is a side diagram showing a core surface with spiral mask line according to a method of20
making the invention.
FIG. 2 is a side diagram showing a core surface with coated spiral line according to the
invention.
FIG. 3 is a side diagram showing a uniform coated core and side coupling intensity of two light
sources along the longitude of the core.25
FIG. 4 is a side diagram showing the fiber of Fig. 2 and side coupling intensity of two light
sources along the longitude of the core.
FIG. 5 is a side diagram showing the fiber of Fig. 2 and side coupling intensity of three light
sources of different wavelength.
30
3
DESCRIPTION OF EMBODIMENTS
Reference will now be made in detail to the preferred embodiments of the invention
which is intended to provide a thorough understanding of the present invention.
A structured scattering layer coated on the surface of plastic optical fiber (POF) is
proposed. The scattering layer consists of ZnO nanorods as fiber coating. A method to make5
the plastic optical fiber will be described. Typical standard multimode SK-80 POF fiber can be
used. The fiber core (10) consists of polymethyl metacrylate resin with diameter ranging from
1840 to 2080 µm. The core (10) is surrounded by fluorinated polymer jacket (12) with inner-
outer diameter of 1880 to 2120 µm.
The jacket (12) of the POF is stripped to expose around 7 cm of core fiber (10). A tape10
mask (14) is applied to form spiral line along and around the core elongated surface as shown
in Fig. 1. Plastic tape of 0.4 cm width is used to create spiral mask (14). Spiral mask (14) is
used to pattern ZnO spiral line (16) on core fiber (10). The width of intended spiral ZnO
nanorods line (16) or space between each spiral mask loop is around 0.2 cm.
Next, ZnO seeding procedure is described. ZnO seed particles can be synthesised by15
dissolving ca. 0.0044 g zinc acetate dihydrate [Zn(O2CCH3)2·2H2O] in 20 ml of ethanol to form a
1 mM solution. The solution serves as nucleation centres for ZnO nanorod growth. The masked
core (10) is dipped into ZnO seed solution for 30 s and placed on a hotplate set at 70 0C for 2
min. The solvent is allowed to evaporate. The dipping and drying of masked core (10) is
repeated ten times to ensure optimal seed distribution on the surface of the POF.20
ZnO nanorods are grown following the seeding process. About 2.97 g of zinc nitrate
hexahydrate [Zn(NO3)2·6H2O] and 1.40 g of hexamethyleneteramine [(CH2)6N4] are dissolved in
400 ml of deionised water to form 10mM solutions of each compound. The seeded core (10) is
then vertically placed in 200 ml of synthesis solution and heated at 90 0C. Following 5 hours of
heating, the solution is discarded and replaced with a new solution in order to maintain constant25
growth condition. Growth time varies from 8 to 20 hours. Good growth range exist between 10
to 14 hours at temperature range of 80 to 99 0C. The growth time was performed using 12 hours
at 90 0C. After that the POF (20) is removed and rinsed in deionised water.
The spiral pattern coated POF (20) is created and shown in Fig. 2. ZnO nanorods can be
observed with scanning electron microscopy (SEM). Magnification of 25.00 kX shows vertical30
alignment, dense and uniform distribution of ZnO nanorods pattered on POF.
4
Fig. 3 shows the mechanism of light scattering of ZnO coated POF by Fallah et al. ZnO
nanorods is uniformly coated (18) on the surface of the fiber (10). Two light sources, P(z1) and
P(z2), at position z1 and z2, are illuminated simultaneously. The light sources go through a
narrow aperture before hitting the core (10). Light scattering is induced by the presence of ZnO
nanorods on the surface excitation location (18) of POF. A portion of the scattered light is5
guided when scattering angles are greater than the critical angle between the surrounding and
the core. The coupled light propagates thorough the POF to the photodetector (22). The
presence of nanorods also causes light leakage (24) through the side of fiber. Due to the
nanorods induced leakage, the intensity of the guided light decreases exponentially to the ZnO
nanorod interface. Light from P(z2) that reaches photodetector (26) is greatly reduced. A digital10
oscilloscope and computer (not shown) is used to analyse data from photodetector (22). A part
of the light also reaches the tip (28). Hence, power coupled from Z2 provides small contribution
to light guidance. The contribution need to be increased by reducing light leakage.
Fig. 4 shows the mechanism of light scattering of patterned coated POF (20) according
to the invention. Light leakage (24) is reduced by using spiral line coating of ZnO nanorods (16).15
The reduced scattering layer contributes the increase of Z2 intensity (26) that reaches
photodetector (22). Light coupled inside the fiber (10) leaks exponentially inside the coated
region (16). The intensity remains steady in uncoated region until the next ZnO patterned spiral
(16). The intensity from point z2 is not reduced much due to a balance between the optimised
side coupling from the ZnO nanorods coat and the reduction of the leakage due to the reduced20
ZnO nanorods region. Light coupling is enhanced due to the use of extended light source.
The spiral patterned POF can also be used for multi-channel excitation, ie. multiple light
source having different frequencies. Multi-wavelength excitation with unpatterned growth
provides loss according to distance of light source from fiber. This effect can be reduced as
shown in Fig. 5. Different wavelength of light source P(z1), P(z2) and P(z3), at location z1, z225
and z3, are individually excited at different spiral patches of ZnO nanorods (16). The peaks of
the coupled light are relatively higher in this setup. This setup can be used for wavelength
division multiplexing. The coupling efficiency of each channel depends on the spacing between
the scattered domain.
Fig. 6 shows the transmittance of visible white light for spiral patterned coated and30
unpatterned uniform coated POF when an extended source is used. The result shows that the
spiral pattern coating is able to increase coupling of light source compared to unpatterned
uniform coating. The spiral patterned coating provides higher light transmittance by factor of 2.2.
5
The present invention provides an improved side coupling of POF by coated ZnO
nanorod spiral.
Non-patent citations:
Bora, Tanujjal, et al. "Controlled side coupling of light to cladding mode of ZnO nanorod coated5
optical fibers and its implications for chemical vapor sensing." Sensors and Actuators B:
Chemical 202 (2014): 543-550.
Fallah, H., et al. "Demonstration of side coupling to cladding modes through zinc oxide
nanorods grown on multimode optical fiber." Optics letters 38.18 (2013): 3620-3622.
10
6
CLAIMS
1. A plastic optical fiber [20], comprising:
a core [10];
characterized in that,
a coat of ZnO nanorods form a spiral line [16] along and around the elongated surface of5
the core [10].
2. The fiber [20] according to claim 1, wherein the gap between each adjacent ZnO
nanorods spiral line (x) is twice the width of the ZnO nanorods spiral line (y).
3. The fiber [20] according to claim 1, wherein the core [10] is polymethyl methacrylate
resin.10
4. The fiber [20] according to claim 1, wherein one end of the core [10] is covered with
jacket [12].
5. The fiber [20] according to any of preceding claims for side coupling of extended light
source.
6. A method of making a plastic optical fiber, comprising:15
preparing a core [10] with exposed elongated surface;
masking a spiral line [14] along and around the core elongated surface [10];
dipping the core [10] in ZnO seed solution;
evaporating the seed solution from the core [10];
repeating the step of dipping the core [10] in seed solution and evaporating the solution20
a number of times; and
placing the core [10] in a solution of zinc nitrate hexahydrate and
hexamethyleneteramine.
7. The method of claim 6, wherein the step of placing the core [10] in solution of zinc nitrate
hexahydrate and hexamethyleneteramine is performed for 10 to 14 hours at temperature25
of 80 to 99 0C.
8. The method of claim 6, wherein the step of masking is performed to create a gap
between each adjacent ZnO nanorods spiral line which is twice the width of the ZnO
nanorods spiral line.
7
9. The method of claim 6, wherein the step of preparing a core [10] with exposed elongated
surface is performed by preparing polymethyl methacrylate resin core.
