T

Aa

b

c

a

ARA

KFLFTT

1

tappmiecdfisotsmsoL

2

d

0h

Optik 125 (2014) 457– 460

Contents lists available at ScienceDirect

Optik

jou rn al homepage: www.elsev ier .de / i j leo

emperature sensitivity of long period fiber grating in SMF-28 fiber

mit Singha,∗, Derick Englesb, Anish Sharmac, Maninder Singhb

Department of Electronics and Communication Technology, Swami Sarvanand Institute of Engineering & Technology, Dinanagar, Gurdaspur, Punjab, IndiaDepartment of Electronics Technology, Guru Nanak Dev University, Amritsar, Punjab, IndiaDepartment of Electronics and Communication Technology, Punjab University, Chandigarh, India

r t i c l e i n f o

rticle history:eceived 5 January 2013ccepted 30 June 2013

a b s t r a c t

In this paper we have presented long period fiber grating (LPFG) as temperature sensor. Temperaturebased sensors have found a number of applications in commercial and industrial fields. In LPFG basedtemperature sensors, they respond to shift in various peak resonant wavelengths corresponding to var-ious attenuation bands of the transmission spectrum. Temperature effect on the various attenuation

eywords:iber opticsong period fiber gratingiber Bragg grating

bands of a LPFG have been investigated to create a highly sensitive measurement device. The tempera-ture sensitivities of various attenuation bands of a LPFG over the wavelength region of 1.1–1.7 �m, for agrating period of 280 �m period, are obtained by monitoring the wavelength shift of each peak resonantwavelength with temperature increment of 20 ◦C, ranging from 0 ◦C to 100 ◦C.

emperature sensorsemperature sensitivity

. Introduction

In recent years, long period fiber grating (LPFG) has been one ofhe most important fiber-optic devices. LPFG has found enormouspplications in optical communication and sensing systems. LPFGromotes the coupling between the propagating core mode and co-ropagating cladding modes. The high attenuation of the claddingodes results in the transmission spectrum of the fiber contain-

ng a series of attenuation bands centered at discrete wavelengths,ach attenuation band corresponding to the coupling to a differentladding mode. In optical communications, LPFG devices have beenemonstrated for numerous applications such as in band-rejectionlters [1], temperature and strain sensors [2] and refractive indexensors [3]. These sensors possess a number of unique advantagesver conventional sensors. For example, they possess low inser-ion loss, low back reflection, good sensitivity and good long-termtability. They are also free from corrosion attack [4], and electro-agnetic interferences [5] that seriously affect many conventional

ensors. Physical quantity changes are reflected as wavelength shiftf the peak resonant wavelength in the transmission spectrum ofPFG (Table 1).

. Theory

The basic principle of the LPFG is to couple light from the fun-amental guided core mode (i.e. the LP01 mode present in the core)

∗ Corresponding author.E-mail address: er.amitsingh07@gmail.com (A. Singh).

030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved.ttp://dx.doi.org/10.1016/j.ijleo.2013.06.037

© 2013 Elsevier GmbH. All rights reserved.

to other forward co-propagating cladding modes (LP0m mode withm = 1, 2, 3, 4 . . .) in the fiber with periodical variation of the RI isshown in Fig. 1.

The phase matching condition between the fundamental coremode and the forward co-propagating cladding modes for the long-period fiber grating (LPFG) is given by [7].

�res = (neff,co(�) − neff,clm (�))� (1)

where �res is the resonant wavelength, neff,co is the effective refrac-tive index of the core mode and neff,clm is the effective index of themth cladding mode. � is the grating period.

2.1. Core effective index

Each core mode propagation constant is found from a LP modedispersion relation in the form of an Eigen value equation. In thisapproach the inner cylinder is made up of core and outer cylindermade up of infinite and uniform cladding [8].

uco

(J1(uco)J0(uco)

)= wco

(K1(wco)K0(wco)

)(2)

where J0(uco) and J1(uco) are Bessel functions of first kind, of zeroand first order respectively and K represents the modified Besselfunction of second kind. Where uco and wco are normalized trans-verse wave numbers that can also be written in terms of the fiber’sV-number. The relation between uco and wco is as follows:

w2co = V2 − u2

co

V =(

2�aco

�

)√n2

co − n2cl

458 A. Singh et al. / Optik 125 (2014) 457– 460

Table 1Parameters used for simulation.

Symbol Parameter used Value used

aco Core radius 4.15 �macl Cladding radius 62.5 �mnco Core refractive index 1.4640ncl Cladding refractive index 1.46next External refractive index 1� Free space wavelength 1.310 �m

Fp

wr

a

ˇ

n

p

2

eatiic

ig. 1. Coupling of a fundamental guided core mode to a cladding mode in a long-eriod fiber grating [6].

here aco = core radius; nco = core refractive index; ncl = claddingefractive index; � = free space wavelength.

Thus using graphical approach we can find the intersection pointnd corresponding value of uco.

co =√

(pnco)2 −(

uco

aco

)2(3)

eff,co = ˇco

p

is free space propagation constant (Fig. 2).

.2. Cladding effective indices

There are two different techniques for determining claddingffective indices. One technique employ two layer geometry [1,11]nd second technique employ three layer geometry [9]. Calcula-

ion of cladding mode effective indices using two layer geometrys rather simple because in this approach the presence of the cores ignored so that the fiber geometry once again comprises twooncentric cylinders with a step-index profile. This method is very

Fig. 2. Calculation of core mode [8].

Fig. 3. Calculation of various cladding mode effective indices using two layer geom-etry [8].

similar to the procedure for determining the core effective refrac-tive index. Calculation of various cladding mode effective indicesusing two layer geometry is shown in Fig. 3.

Approximations used in above method are corrected by usingthree layer geometry. In this method core–cladding interface is notignored, and the required dispersion relation incorporates the exactmodes corresponding to the three-layer fiber structure [9], shownin Fig. 4.

Using three layer geometry we can directly obtain claddingmodes effective indices as shown in Fig. 4.

3. LPFG as temperature sensor

Taking the derivative of phase matching condition given in Eq.(1) with respect to temperature, we yield [10]:

d�res

dT= �

(dneff,co

dT− dneff,clm

dT

)+ (neff,co − neff,clm )

d�

dT(4)

The most reformed form of the temperature differentiatedphase-matching condition is the following [10]:

d�res

dT= d�0

d(ıneff)

(dneff,co

dT− dneff,clm

dT

)+ �

d�res

d�

1L

dL

dT(5)

where (1/�)(d�/dT) = (1/L)(dL/dT), and L is the length of long periodfiber grating. Since silica has a small thermal expansion coefficient,(1/L)(dL/dT) = 4.1 × 10−7/◦C. The right-hand side of Eq. (5) contains

Fig. 4. Calculation of the various cladding mode effective indices using three layergeometry.

A. Singh et al. / Optik 125 (2014) 457– 460 459

Fig. 5. Transmission spectrum of long period fiber grating, the location of peak res-onant bands are at 1.6278 �m, 1.3159 �m, 1.1978 �m and 1.1283 �m with 0 ◦C ast

sLcgctatttpoo(iftuaiatofrtr

hat

aTwg

t(1(c0nls

Fig. 6. Shift in a peak resonant wavelength of a long-period fiber grating withtemperature corresponding to fourth attenuation band. The spectra correspond totemperatures of 0 ◦C, 20 ◦C, 40 ◦C, 60 ◦C, 80 ◦C and 100 ◦C from left to right [2]. Theresonant wavelength shift from 1.6278 �m at 0 ◦C to 1.6378 �m at 100 ◦C (referencetemperature = 0 ◦C).

Fig. 7. Shift in the peak loss wavelengths with temperature corresponding to var-ious resonance bands of the grating. The location of the bands A, B, C, D are at1.6278 �m, 1.3159 �m, 1.1978 �m and 1.1283 �m with 0 ◦C as the reference tem-perature. The dashed line (E) is the shift for a Bragg grating at 1.550 �m with atemperature coefficient 0.013 nm/◦C (reference temperature = 0 ◦C) [2].

We proposed a model of long period fiber grating based temper-

he reference temperature.

eparate terms that contribute to the thermal sensitivity of thePFG: the first term represents thermo-optic effects (the materialontribution), and the second term mainly denotes the change inrating periodicity (waveguide contribution) [2,11]. From Eq. (5),hanges in the LPFG transmission spectrum arising from tempera-ure are therefore dependent on the physical parameters of the fibers well as the order of the relevant cladding mode and the period ofhe grating. The waveguide contribution is either positive or nega-ive, depending on the cladding mode’s d�res/d� polarity. Differentemperature-induced spectral behavior can be observed when cou-ling occurs with lower-order cladding modes as opposed to modesf higher order [2]. For a fixed resonant wavelength, the lower-rder cladding modes will be accessible with a large grating periodin excess of 100 �m), and in this case the material contributions the dominating effect [13]. The material contribution is strongunction of the difference between the thermo-optic coefficients ofhe core dnco/dT and cladding dncl/dT. Since for the standard fibernder analysis, the cladding is fabricated from pure silica, we willpproximate the cladding thermo-optic coefficient with that of sil-ca, dncl/dT = 7.8 × 10−6. The core of the fiber contains Germaniand the presence of external dopants modifies its thermal proper-ies [12]. The average value of thermo-optic coefficient for the coref the SMF-28 fibers was calculated to be dnco/dT = 7.97 × 10−6. Soar, the material effect is based on assuming that the grating periodemains unchanged under temperature variations. In order to keephe period constant, the phase-matching condition dictates that theatio �res/ıneff should remain unchanged.

The transmission spectrum is shown in Fig. 5. In this figure, weave four attenuation bands, whose peak resonant wavelengths aret 1.6278 �m, 1.3159 �m, 1.1978 �m and 1.1283 �m with 0 ◦C ashe reference temperature.

By using Eq. (5), the shifted transmission spectrum with temper-ture corresponding to fourth attenuation band is shown in Fig. 6.his figure illustrates the thermally induced shift in the peak lossavelength from 1.6278 �m at 0 ◦C to 1.6378 �m at 100 ◦C, thus

iving sensitivity of nearly 0.10 nm/◦C.Fig. 7 gives the wavelength shift of four attenuation bands with

emperature for a grating written in SMF-28 fiber for � = 280 �mnext = 1). For a peak index change, the bands are located at.6278 �m (A), 1.3159 �m (B), 1.1978 �m (C) and 1.1283 �mD) with 0 ◦C as the reference temperature. The temperatureoefficients of band A, B, C, D are determined to be0.10 nm/◦C,.045 nm/◦C, 0.036 nm/◦C, 0.032 nm/◦C respectively. Thus for theormal region of operation the higher order attenuation bands at

onger wavelengths are shown to possess higher temperature sen-itivity [2].

Fig. 8. Wavelength shift as a function of period for SMF-28 fiber.

In Fig. 8, shows the wavelength shift as a function of gratingperiod for different sets of attenuation bands of SMF-28 fiber. If wechoose higher period we will get higher wavelength shift. Fig. 8 alsoshows that for a fixed period the higher order attenuation bands arefound to have a larger temperature induced shifts [2].

4. Conclusion

ature sensor and studied various parameters which are helpful inenhancing the sensitivity of long period fiber grating. The fourth

4 tik 12

asattaiasss

R [

[tics and applications, Meas. Sci. Technol. 14 (5) (2003) R49–R61.

60 A. Singh et al. / Op

ttenuation band appearing over 1.62–1.65 �m wavelength hashown nearly three times higher sensitivity as compared to lowestttenuation band appearing over 1.12–1.15 �m and fourth attenua-ion band shows nearly seven times higher sensitivity as comparedo fiber Bragg grating at 1.550 �m. So, we concluded that higherttenuation bands possess large wavelength shift for same increasen temperature and by choosing higher grating period we canchieve better sensitivity and also long period grating temperatureensors are highly sensitive as compared to fiber Bragg grating sen-ors. Thus it shows significant role for using LPFG as temperatureensor.

eferences

[1] A.M. Vengsarkar, P.J. Lemaire, J.B. Judkins, V. Bhatia, T. Erdogan, J.E. Sipe, Long-period fiber gratings as band-rejection filters, J. Lightwave Technol. 14 (1)

(1996) 58–65.

[2] V. Bhatia, Applications of long-period gratings to single and multi-parametersensing, Opt. Express 4 (11) (1999) 457–466.

[3] H.J. Patrick, A.D. Kersey, F.K. Bucholtz, J.J. Ewing, B.A. Judkins, M. Vengsarkar,Chemical sensors based on long-period fibre grating response to index of

[

[

5 (2014) 457– 460

refraction, in: Conf. on Lasers and Electro-Optics, Baltimore, MD, CThQ5, 1997,pp. 420–421.

[4] K.R. Cooper, J. Elster, M. Jones, R.G. Kelly, Optical fiber-based corrosion sensorsystems for health monitoring of aging aircraft, in: IEEE Systems ReadinessTechnology Conference, 2001, pp. 847–856.

[5] J.A. Besley, T. Wang, L. Reekie, Fiber cladding mode sensitivity characterizationfor long-period gratings, J. Lightwave Technol. 21 (3) (2003) 848.

[6] H. Ke, J. Peng, C. Fan, Design of long-period fiber gratings with fast-varyingparameters, IEEE Photon. Technol. Lett. 13 (11) (2001) 1194–1196.

[7] A.M. Vengsarkar, P.R. Pedrazzani, J.B. Judkins, P.J. Lemaire, N.S. Bergano, C.R.Davidson, Long-period fiber – grating-based gain equalizers, Opt. Lett. 21 (5)(1996) 336–338.

[8] J.A. Buck, Fundamentals of Optical Fibers, John Wiley and Son Inc., New York,1995.

[9] T. Erdogan, Cladding-mode resonances in short-and long-period fiber gratingsfilter, J. Opt. Soc. Am. 14 (8) (1997) 1760–1773.

10] M.N. Ng, K.S. Chiang, Thermal effects on the transmission spectra of long-periodfiber gratings, Opt. Commun. 208 (2002) 321–327.

11] S.W. James, R.P. Tatam, Optical fibre long-period grating sensors: characteris-

12] N.P. Bansal, R.H. Deromus, Handbook of Glass Properties, Academic Press,Florida, 1986.

13] V. Bhatia, A.M. Vengsarkar, Optical fiber long-period grating sensors, Opt. Lett.21 (9) (1996) 692–694.