By Abdulla Ali Kassim - الجامعة التكنولوجية...Supervisor Certification I certify...
Transcript of By Abdulla Ali Kassim - الجامعة التكنولوجية...Supervisor Certification I certify...
Republic of Iraq
Ministry of Higher Education and Scientific Research
University of Technology
Laser and Optoelectronics Engineering Department
INTERMODEL DISPERSION OF LASER
PULSES TRANSMITTED THROUGH
SILICA OPTICAL FIBER
A Thesis
Submitted to the Laser and Optoelectronics Engineering
Department, University of Technology in Partial Fulfillment
of the Requirements for the Degree of Master of Science in
Laser Engineering
By
Abdulla Ali Kassim B. Sc. Laser Eng.
2006
Supervised by
Asst.Prof. Dr. Mohamed Saleh Ahmed
August 2008 A. D. Shaban 1429A. H.
جمھورية العراق مي لوزارة التعليم العالي والبحث الع
الجامعة التكنولوجية قسم ھندسة الليزر والبصريات االلكترونية
ألتشتت الموجي المكاني لنبضات ليزرية
مرسلة عبر ليف زجاجي بصري
رسالة مقدمة الى
قسم ھندسة الليزر والبصريات االلكترونية الجامعة التكنولوجية الليزرنيل درجة الماجستير علوم في ھندسة من متطلبات كجزء
المھندس تقدم بھا
عبداهللا علي قاسم
بإشراف
حممد صاحل أمحدالدكتور
ھ١٤٢9 شعبان م ٢٠٠8 بآ
Acknowledgment
First of all, praise and thank be to ALLAH the most beneficent, the
most merciful who enabled me to achieve this research.
Who has given me the greatest pride to carry out my research work
under the supervision of Asst. Prof. Dr. Mohamed Saleh Ahmed whose
valuable advice, guidance, constructive criticism, encouragement and
cooperation throughout all the stages of preparing this study are gratefully
appreciable. I am greatly indebted to him and wish to express my deep
gratitude and sincere thanks to my supervisor.
Special greater thank to Asst. Lecturer Jassim K. H. for his help and
his scientific advice during the project.
A great thank to Asst. Lecturer Wail Yass and Aseel Abdul-Ameer for
their help and good advice.
I would like to thank Dr.Kaddim Abd, Dr.Sami A. Hatff and Dr.Evan
for their advice. And special thank to Jumana Basher manager of internet
unit.
Greater Full thanks to all staff of Laser and Optoelectronic
Engineering department special Head of the department Asst.Prof.
Dr.Mohamed Hssain.
I would like to thank my Family for their encouragement and help me
by invoke for me, specially my aunt (Awtif) who encourages me to continue in
my study.
I am deeply indebted to my friends and colleagues for their
encouragement and help.
Abdulla Ali
Supervisor Certification I certify that this thesis entitled (Characteristics of Fiber
Transmitted Pulsed laser) was prepared by (Abdulla Ali Kassim)
under my supervision at the Laser and Optoelectronics Engineering
Department University of Technology as a partial fulfillment of
the requirements for the degree of Master of Science in Laser
Engineering. Signature:
Supervisor: Dr. Mohamed Saleh Ahmed
Title: Asst. Professor Date: / 8 / 2008
Certification of the Linguistic Supervisor I certify that this thesis entitled (Characteristics of Fiber
Transmitted Pulsed laser) was prepared under my linguistic
supervision.
Its language was amended to meet the style of the English Language
Signature:
Name: Sabah Aziz Dhahir
Title: Lecturer
Date: / 8 / 2008
VIII
List of Abbreviations
APD Avalanche Photodiode CDMA Code Division Multiple Access
EMI electromagnetic interference EMP electromagnetic pulse FBL fly-by-light FBW fly-by-wire HERF high energy radio frequency
InGaAsP Indium Gallium Arsenide Phosphide ISI Intersymbol Interference
LANs Local Area Networks LD Laser diode
LED Light Emitting Diode MANs Metropolitan Area Networks MMF Multi Mode Fiber NRZ Non Return to Zero NVP Nominal Velocity of Propagation O/E Optical- to-Electrical
OOK On –Off Key OPD Optical Path Different p-i-n p- and the n-doped material, (i) intrinsic (undoped or lightly
doped) PMD Polarization-Mode Dispersion PMs Principal modes RZ Return to Zero Sc Subscriber Connector
SDH Synchronous Digital Hierarchy SLM Spatial light modulator SMA Sub Miniature Assembly
ST WDM Wave Division Multiplexing
NA Numerical Aperture TTL Transistor Transistor Logic gate TCP Terminal Count Up o/p Output i/p Input
O/E Optical to Electrical converter
IX
List of Symbols
cφ Critical angle. 1φ Incident angle. 2φ Refracted angle.
B Bit rate. Bi Input bit rate.
bits-1 bit per second. Bo Output bit rate. Bs Bit rats. c Velocity of light. d Diameter.
dB Decibel. f Focal length. f Frequency.
Gb Giga Bit. GHz Giga Hertz. Hz Hertz. km Kilometer. L Length of the optical fiber. m Meter.
MHz Mega Hertz. ms Millisecond. n1 Refractive index of the core. n2 Refractive index of cladding. NA Numerical Aperture. nm Nanometer. nλ Refractive index. Pin Input power. Pout Output power. Ps Pico second. R Data rate. Rc Critical radius. tf Fall time.
THz Tera Hertz. Ti Input pulse width.
Tmax Maximum delay time. Tmin Minimum delay time.
X
To Output pulse width. tr Rise time. V Volt. α Attenuation. αmax Acceptance angle. Δ Relative Refractive Index. δTg The delay difference between the fastest and slowest modes. δTs The delay difference between the extreme meridional ray and the
axial ray. λ Wavelength. μF Micro Farad. μm Micrometer. μs Microsecond. σ The rms pulse broadening σg The rms pulse broadening of a near parabolic index profile graded
index fiber. σs The rms pulse broadening at the fiber output due to intermodal
dispersion. τ Pulse duration.
Abstract
Laser pulses transmitted through optical fibers suffer intramodal
(color) and intermodal dispersion. Intermodal dispersion affects data
launched into optic fiber communication systems. This problem has been
dealt with theoretically by most reported works. However, few practical
studies have been mentioned.
This research aims at the examination of optical fiber transmitted
laser pulse experimentally. For the purpose of making this study possible
an optic fiber guidance system prototype has been designed and built
herein. It consists of laser circuit drive, optical fiber, detector circuit and a
decoder. Four signals (λ=680nm and power= 0.1mW) of different
frequencies (138.889, 277.778, 645.16, and 1369.863 Hz) (each indicates
assumed direction), of pulse widths (7.2, 3.6, 1.55, and 0.73 ms)
respectively are sent through 400m graded index fiber. A p-i-n detector is
used to receive output coded signals that are decoded afterward by a
decoder. Intermodal dispersion has been noticed and the pulse width
broadening for each frequency is recoded. They are (7.22, 3.61, 1.555, and
0.732 ms) that lead to frequencies of (138.504, 277.008, 643.08, 1366.120
Hz) respectively.
Effects of imperfect laser to fiber coupling and degraded laser power
on transmitted pulses have also been investigated.
Chapter One Introduction and Historical Review 1
Chapter One Introduction and Historical Review
1.1 Introduction One of the principal needs of people has been to communicate these
needs may be broadly defined as the transfer of information from one point to
another. When the information is to be conveyed over any distance a
communication systems is usually required [1].
This information transfer most often is accomplished by modulating the
information onto an electromagnetic wave (carrier).The modulated carrier is
then transmitted (propagated) to the destination, where the electromagnetic
wave is received and information recovered (demodulated) [2]. Techniques
have been developed for this process using electromagnetic carrier waves
operating at radio frequencies as well as microwave and millimeter wave
frequency .However "communication" may also be achieved using an
electromagnetic carrier which is selected from the optical range of
frequencies[1].
Today optical communication systems are used in many applications
such as synchronous digital hierarchy (SDH), wavelength division
multiplexing (WDM) network systems, Local Area Networks (LANs),
Metropolitan Area Networks (MANs), and board-to-board interconnections,
all of which utilize optical fiber as the means of conveying data [3].
Optical fiber is the medium in which communication signals are
transmitted from one location to another in the form of light guided through
thin fibers of glass or plastic. These signals are digital pulses or continuously
modulated analog streams of light representing information. These can be
Chapter One Introduction and Historical Review 2
voice information, data information, computer information, video information,
or any other type of information [4].
One of the most important components in any optical fiber system is the
optical fiber itself, since its transmission characteristics play a major role in
determining the performance of the entire system [5].
The important characteristic of optical fiber is the bandwidth. This is
limited by the signal dispersion within the fiber, which determines the number
of bits of information transmitted in a given time period. Therefore once the
attenuation was reduced to acceptable levels attention was directed towards
the dispersive properties of fibers [1]. Intermodal dispersion is one of these
properties which acts as a critical factor in optical fiber data transmission. The
light is able to take many different paths or “modes” as it travels within the
multimode fiber. The distance traveled by light in each mode is different from
the distance traveled in other modes. When a pulse is sent, parts of that pulse
take many different modes (usually all available modes). Therefore, some
components of the pulse will arrive before others. The difference between the
arrival times of light taking the fastest mode versus the slowest obviously gets
greater as the distance gets greater [6].
1.2 Historical Background of Optical Fibers Optical fiber material, hence the used wavelengths are critical
parameters, when attenuation problem is encountered. Below is a historical
review on the development of optical fibers.
The principle of total internal reflection was studied by Tyndall during
the 1850s. Development of optical fibers occurred in the 1950s driven by
imaging applications in the medical and nondestructive testing fields [7].
Optical fibers were first envisioned as optical elements in the early
1960s. It was perhaps those scientists well-acquainted with the microscopic
Chapter One Introduction and Historical Review 3
structure of the insect eye who realized that an appropriate bundle of optical
waveguides could be made to transfer an image, and the first application of
optical fibers to imaging was conceived [8].
In 1970s when Corning Glass Works announced an optical fiber loss
less than the benchmark level of 10dB/km that commercial applications began
to be realized. The revolutionary concept which Corning incorporated and
which eventually drove the rapid development of optical fiber
communications was primarily a materials one-it was the realization that low
doping levels and very small index changes could successfully guide light for
tens of kilometers before reaching the detection limit [9]. By reducing the
impurities in the source materials and improving the homogeneity of the glass
processing, an attenuation of 0.2dB/km was obtained by Miya, Hosaka, and
Miyashita. In 1978, Pinnow, Van Uitert, and Goodman et al proposed that an
ultra-low-loss fiber with a loss less than 0.01dB/km for non-silica based fibers
was theoretically possible. This announcement motivated many researchers to
study and discover other infrared materials [10].
The first-generation system is 850 nm could transmit light several
kilometers without repeaters, but were limited by loss of about 2dB/km in the
fiber. A second generation soon appeared, using new indium gallium arsenide
phosphide (InGaAsP) lasers that emitted at 1300nm, where fiber attenuation
was as low as 0.5 dB/km, and pulse dispersion was somewhat lower than at
850 nm. Development of hardware for the first transatlantic fiber cable
showed that single mode systems were feasible, so when deregulation opened
the long-distance phone market in the early 1980s. The third generation
operates at 1550nm, where fiber loss is 0.2 to 0.3dB/km, allowing even longer
repeater spacing [4].
Chapter One Introduction and Historical Review 4
The bandwidth of a light based system was so high that a relatively low
frequency could be tolerated in order to get lower losses and hence more
transmission range. The lower frequency or red end of the visible spectrum
and then even further down into the infrared are explored. Infrared light
covers a fairly wide range of wavelengths and is generally used for all fiber
optic communications. Visible light is normally used for very short range
transmission, Figure (1-1) [11].
Figure (1-1) Fiber optics use visible and infrared light.[11]
1.3 Literature Survey In 1998, Fiber Optic Guided Video Missiles are developed and
manufactured in several countries. These missiles are fitted with video
cameras, and the video signal is transmitted back to the control station via an
optical fiber that tethers behind the missile when it has been fired. The control
signals from the control station are transmitted along the same optical fiber
Chapter One Introduction and Historical Review 5
from the control station to the missile. A human operator controls the missile
with a joystick [12].
P. Hlubina, et al. (2003), reported a work in which low-resolution
spectrometer is used in measurement of the intermodal dispersion in optical
fibers. The technique utilizes a tandem configuration of a compensated
Michelson interferometer and a few-mode optical fiber and the resolving of
so-called equalization wavelengths at which the optical path difference (OPD)
in the interferometer is the same as the intermodal group OPDs. The
intermodal dispersion in three different optical fibers was measured and the
results for two of them were compared with the results of an adequate
theoretical analysis using a model of a weakly guiding, step-index optical fiber
[13].
T. D. Engeness et al. (2003) presented a method for dispersion-tailoring
of OmniGuide and other photonic band-gap guided fibers based on weak
interactions (“anticrossings”) between the core-guided mode and a mode
localized in an intentionally introduced defect of the crystal. Because the core
mode can be guided in air and the defect mode in a much higher-index
material, it has been able to obtain dispersion parameters in excess of 500,000
ps/nm-km. Furthermore, because the dispersion is controlled entirely by
geometric parameters and not by material dispersion, it is easily tunable by
structural choices and fiber-drawing speed. So, for example, it is demonstrated
how the large dispersion can be made to coincide with a dispersion slope that
matches commercial silica fibers to better than 1%, promising efficient
compensation. Other parameters are shown to yield dispersion-free
transmission in a hollow OmniGuide fiber that also maintains low losses and
negligible nonlinearities, with a nondegenerate TE01 mode immune to
polarization-mode dispersion (PMD) [14].
Chapter One Introduction and Historical Review 6
H. Wu, et al. (2003), indicated that intersymbol interference (ISI)
caused by intermodal dispersion in multimode fibers is the major limiting
factor in the achievable data rate or transmission distance in high-speed
multimode fiber-optic links for local area networks applications. Compared
with optical-domain and other electrical-domain dispersion compensation
methods, equalization with transversal filters based on distributed circuit
techniques presents a cost-effective and low power solution [15].
S. Fan and Joseph M. Kahn, (2005), generalized the concept of
principal states of polarization and prove the existence of principal modes in
multimode waveguides. Principal modes do not suffer from modal dispersion
to first order of frequency variation and form orthogonal bases at both the
input and the output ends of the waveguide. They show that principal modes
are generally different from eigenmodes, even in uniform waveguides, unlike
the special case of a single-mode fiber with uniform birefringence. The
difference is most pronounced when different eigenmodes possess similar
group velocities and when their field patterns vary as a function of frequency.
The work may provide a new basis for analysis and control of dispersion in
multimode fiber systems [16].
W. Mason, et al. (2005), developed an aircraft flight control system.
This is a fly-by-light (FBL) adaptation to a current Hamilton-Sundstrand fly-
by-wire (FBW) configuration. Because FBL systems use fiber-optics to
replace conventional electrical wiring, FBL systems are lighter in weight,
capable of faster and larger data transfer, and resistant to electromagnetic
interference (EMI), electromagnetic pulse (EMP), lightning, and high energy
radio frequency (HERF) [17].
R. A. Panicker et al. (2007), presented a work in which transmitter-
based adaptive optics and receiver-based single-mode filtering are combined
Chapter One Introduction and Historical Review 7
to compensate modal dispersion in multimode fiber (MMF). A liquid-crystal
spatial light modulator controls the launched field pattern for ten 10-Gb/s
nonreturn-to-zero channels, wavelength-division multiplexed on a 200-GHz
grid in the C-band. Error-free transmission through 2.2 km of 50-μm graded-
index MMF is achieved for launch offsets up to 10μm and for worst-case
launched polarization. A ten-channel transceiver based on parallel integration
of electronics and photonics is employed. [18].
R. A. Panicker et al. (2007), proposed a provably optimal technique for
minimizing intersymbol interference (ISI) in multimode fiber (MMF) systems
using adaptive optics via convex optimization. Spatial light modulator (SLM)
is used to shape the spatial profile of light launched into an MMF. An
expression is derived for the system impulse response in terms of the SLM
reflectance and the field patterns of the MMF principal modes (PMs). Finding
optimal SLM settings to minimize ISI, subject to physical constraints, is posed
as an optimization problem. It is observed that the problem can be cast as a
second-order cone program, which is a convex optimization problem. Its
global solution can, therefore, be found with minimal computational
complexity, and can be implemented using fast, low-complexity adaptive
algorithms. Simulation results are included, which show that this technique
opens up an eye pattern originally closed due to ISI. That can be seen,
contrary to what one might expect, the optimal SLM settings do not
completely suppress higher-order PMs. [19].
P. C. Pandey and A. Mishra, (2007), showed that with the use of scalar
field approximation an analytical study of a dielectric waveguide whose core
cross-section is bounded by two spirals of the form 1/r = ξθ can be made. This
waveguide is similar to that of a distorted slab waveguide in which both a
curvature and a flare are present. The modal characteristic equation is derived
Chapter One Introduction and Historical Review 8
by analytical analysis under the weak guidance approximation. Modal
dispersion curve is found, which support only single mode propagation and
the same compared with the same kind of waveguide with metal claddings
[20].
I. Kamitsos and N. K. Uzunoglu, (2007), showed that multimode fibers
can be characterized by multipath propagation of optical signals and this leads
to severe intersymbol interference at the output of the fiber. An approach
based on the Rake receiver is proposed to overcome this drawback. An
optimization algorithm was developed and appropriate software was employed
to apply the proposed methodology on specific multimode fiber. Extensive
simulation results were produced and are presented. The numerical results
have shown that the order of magnitude of the maximum data rate, R,
supported at different CDMA gains, in order to achieve a Bit Error Rate value
smaller or equal to a convergent point, is related to the length of the
multimode fiber, L, by the expression R = dL−1 with d increasing from 106 to
107 (Kbps. m) when CDMA gain increases from 50 to 500 [21].
Chapter One Introduction and Historical Review 9
1.4 Aim of the Work This work is aimed at studying and analysing the characteristics of laser
pulses that are launched through optical fibers. The study will focus on the
evaluation of intermodal dispersion introduced when laser signals of different
frequencies are lunched into multimode graded index fiber. An optical fiber
guidance system will be designed and built for the purpose of making the
present study possible.
Chapter One Introduction and Historical Review 10
1.5 Thesis Layout Chapter two: Focuses on the fundamental of optical fiber systems:
describing generally their main parts namely the optical sources, optical fiber
cables, and detectors.
Chapter three: Presents and describes the procedures followed to
implement the experimental work.
Chapter four: Introduces results of optical fiber coupling, intermodal
dispersion, and optical fiber guidance system performance.
Chapter five: Presents conclusions extracted out of the present work in
addition to a suggestion for future work.
Chapter Two Fiber Optic Transmission Fundamentals 11
Chapter Two Fiber Optic Transmission Fundamentals 2.1 Introduction
A fiber optic transmission system has the same basic elements as the
metallic cable system: the transmitter, cable and the receiver, but the cable is
an optical fiber and the signal is converted to light before transmission into the
fiber, as illustrated in Figure (2-1). This chapter gives an introduction to
optical transmission systems describing generally their main parts namely the
optical sources, optical fiber cables and detectors. Functional basic principles
of each of these parts are also introduced.
Figure (2-1) Parts of a fiber optic data link.
2.2 Optical Transmitter The transmitter converts an electrical analog or digital signal into a
corresponding optical signal. Fiber-optic communication systems often use
semiconductor optical sources such as light-emitting diodes (LED) and
semiconductor lasers because of several inherent advantages offered by them.
Some of these advantages are compact size, high efficiency, good reliability,
right wavelength range, small emissive area compatible with fiber core
dimensions, and possibility of direct modulation at relatively high
frequencies [22]. A light source such as a LED or a laser is placed at one end
of the fiber, the light source emits short but rapid pulses of light that enter the
core at different angles. The laser produces a very pure and narrow beam. It
also has a high-power output, allowing the light to propagate further that
Chapter Two Fiber Optic Transmission Fundamentals 12
produced by the LED. The LED produces less concentrated light consisting of
many wavelengths [6].
2.2.1 Light Emitting Diodes (LEDs) Almost all light sources used in communications today are made from
semiconductors [6]. Semiconductor LEDs emit incoherent light: spontaneous
emission of light in semiconductor produces light waves that lack a fixed-
phase relationship. The use of LEDs in single mode systems is severely
limited because they emit unfocused incoherent light. Even LEDs developed
for single mode systems are unable to launch sufficient optical power into
single mode fibers for many applications. LEDs are the preferred optical
source for multimode systems because they can launch sufficient power at a
lower cost than semiconductor lasers (LDs) [23].
2.2.2 Semiconductor Laser Diode (LD) Semiconductor lasers emit light through stimulated emission. As a
result of the fundamental differences between spontaneous and stimulated
emission, they are not only capable of emitting high powers, but also have
other advantages related to the coherent nature of emitted light. A relatively
narrow angular spread of the output beam compared with LEDs permits high
coupling efficiency into single-mode [22]. Figure (2-2) demonstrates the
difference between laser and LED intensity profiles .Table (2-1) summarizes
the LD and LED differences.
Chapter Two Fiber Optic Transmission Fundamentals 13
Figure (2-2) Shows the difference between LD and LED profiles [11]
Table (2-1) LEDs and LDs comparison [8].
Light Emitting Diode (LED) Laser (LD)
1. Spontaneous emission. 1. Stimulated emission.
2. Produces incoherent light. 2. Produces a narrow beam of coherent
light.
3. Low cost. 3. High cost.
4.Modulation to several hundred MHz. 4. Modulation to several tens GHz
5. Coupling sufficient optical power into a
fiber is difficult.
5. High optical output power available and
low loss coupling to fiber possible.
6. Restricted to large core fibers. 6. Suitable for single mode fibers.
7. Eye safe light output. 7. Safety: light output can harm eyes.
8. Longer life time than laser. 8. Shorter lifetime than LEDs
2.3 Modulation of Laser The most commonly used modulation format in optical communication
is the non-return-to-zero (NRZ) format shown in Figure (2-3 (b)) .This format
is a form of on-off keying (OOK): the signal is on to transmit a one bit and is
off to transmit a zero bit. When the signal (i.e., the laser light) is on, it stays on
for the entire bit period. In high-speed and long-haul transmission (e.g., fiber
links between two continents), the return-to-Zero (RZ) format, shown in
Chapter Two Fiber Optic Transmission Fundamentals 14
Figure (2-3(a)), generally is preferred. In this format, the pulses, which
represent the one bit, occupy only a fraction (e.g., 50%) of the bit period [24].
Figure (2-3) Showing (a) return-to-zero (RZ) and (b) non return to-
zero (NRZ) formats [24].
2.4 Drive Circuits The type of electrical circuit in a fiber optic transmitter depends on the
application for which the device will be used, the data format (such as analog
or digital), and the light source inside. LEDs and lasers use similar types of
driving circuits and are best driven by electrical current sources [22]. The
circuit of Figure (2-4) is one of the simplest possible driving circuits [25].
Chapter Two Fiber Optic Transmission Fundamentals 15
Figure (2-4) A common-emitter saturating switch [25]
2.5 Fiber optic Fiber optic communication systems became the preferred means for
high-speed data transmission because of the achievable wide bandwidth and
low loss in the optical fiber. It is estimated that 50 THz bandwidth is available
in fiber with a corresponding loss of less than 0.16 dB/km [26].
In its simplest form an optical fiber consists of a cylindrical core of
silica glass surrounded by a cladding whose refractive index is lower than that
of the core. Because of an abrupt index change at the core–cladding interface,
such fibers are called step-index fibers. In a different type of fiber, known as
graded-index fiber, the refractive index decreases gradually inside the
core [22].
Single mode and multimode fibers can have a step-index or graded-
index refractive index profile. The performance of multimode graded-index
fibers is usually superior to multimode step-index fibers. However, each type
of multimode fiber can improve system design and operation depending on the
intended application. Performance advantages for single mode graded-index
fibers compared to single mode step-index fibers are relatively small.
Therefore, single mode fiber production is almost exclusively step-index.
Figure (2-5) shows the refractive index profile for a multimode step-index
Chapter Two Fiber Optic Transmission Fundamentals 16
fiber and a multimode graded-index fiber. Figure (2-5) also shows the
refractive index profile for a single mode step index fiber. Since light
propagates differently in each fiber type, Figure (2-5) shows the propagation
of light along each fiber [23].
Figure (2-5) The refractive index profiles and light propagation in multimode step-
index, multimode graded-index, and single mode step-index fibers [8].
2.6 Basic Fiber Parameters
2.6.1 Refractive index All materials that allow the transmission of electromagnetic radiation
have an associated refractive index. In copper cables this is analogous to the
nominal velocity of propagation (NVP).
The refractive index of materials used within an optical fiber has a
direct influence upon the basic properties of the fiber. A more comprehensive
definition of refractive index can be given as defined in equation (2-1).
Chapter Two Fiber Optic Transmission Fundamentals 17
material in the gth at wavelenradiation eticeletromagn ofvelocity vacuumain gth at wavelenradiation eticeletromagn ofvelocity
λλ
λ =n . . (2-1)
It can therefore be seen that the refractive index of a material may vary
across the electromagnetic radiation spectrum. Table (2-2) provides further
information regarding the electromagnetic spectrum [27]. Table (2-2) Pure silica: refractive index variation with wavelength [27].
Wavelength λ (nm) Refractive index n
600 1.4580
700 1.4553
800 1.4538
900 1.4518
1000 1.4504
1100 1.4492
1200 1.4481
1300 1.4469
1400 1.4458
1500 1.4466
1600 1.4434
2.6.2 The Principle of Total Internal Reflection (TIR)
Reference to Figure (2-6), as the angle of incidence 1φ in an optically
denser material becomes larger, the refracted angle 2φ approaches π/2. Beyond
this point no refraction into the adjoining material is possible, and the light
rays become totally internally reflected.
A light ray gets bent toward the glass surface as it leaves the glass in
accordance with Snell’s law. If the angle of incidence 1φ is increased, a point
will eventually be reached where the light ray in air is parallel to the glass
surface. This point is known as the critical angle of incidence cφ as
determined by equation (2-2).
Chapter Two Fiber Optic Transmission Fundamentals 18
1
2sinnn
c =φ ,………………………………………………… .. (2-2)
where n1 and n2 are the refractive indices of medium1 (core) and medium
(cladding) respectively.
When 1φ is greater than cφ , the condition for total internal reflection is
satisfied; that is, the light is totally reflected back into the glass with no light
escaping from the glass surface [28].
Figure (2-6) Representation of the critical angle and total internal reflection at a glass-
air interface [1].
2.6.3 Numerical Aperture Numerical aperture is a characteristic of an optical fiber that depends
on the refractive indices of the core and the cladding and measures light
gathering ability of the core.
22
21 nnNA −= ,…………………………………………………….… (2-3)
where NA is numerical aperture; n1 and n2 are refractive indicies of core and
cladding in fiber optic respectively [29]
Chapter Two Fiber Optic Transmission Fundamentals 19
2.6.4Acceptance Angle Acceptance angle or the maximum acceptance angle is the largest
possible light launch angle from the fiber axis. Light waves within the
acceptance angle that enter the fiber become guided along the fiber core. The
maximum acceptance angle αmax , Figure (2-7), is given by
NA1max sin −=α ……………………………………….…………...……… (2-4)
max
Figure (2-7) The acceptance angle when launching light into an optical fiber [1]. Total acceptance angle is twice the maximum acceptance angle and is
the total angle around the fiber axis within which all light rays can be
launched into the fiber [29]. Table (2-3) shows numerical aperture and
acceptance angle of the available optical fiber geometries. Table (2-3) Available optical fiber Geometries silica [27]
Core/Cladding
diameter
Numerical
aperture
Acceptance angle
(αmax)(degrees)
Critical angle
(θc)(degrees)
8/125 0.11 6.32 85.74
50/125 0.2 11.54 82.25
62.5/125 0.275 15.96 79.90
85/125 0.26 15.07 79.90
100/140 0.29 16.86 78.73
Chapter Two Fiber Optic Transmission Fundamentals 20
2.7 Optical Fiber Loss Fiber loss (attenuation) is one of the fundamental limiting factors.
Since optical receiver requires a certain minimum amount of power to obtain
information, the transmission distance is limited by fiber loss. Fiber loss can
be found as follows:
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛−=
in
out
PP
LkmdB log10α ,………………………………………………… (2-5)
where α is the attenuation, L is the length of fiber, Pout and Pin are output and
input powers respectively [26].
2.7.1 Bending Losses Two classes of fiber losses arise from either large-radii bends or
small fiber curvatures with small periods. These bending effects are called
macrobending and microbending losses, respectively. It is crucial to be able to
characterize these losses because they are important if one wishes to be able to
wrap fiber around a mandrel or storage spool [10].
Large bending losses tend to occur at a critical radius of curvature Rc
which may be estimated from [1].
3
21
43
NAnRc πλ
= ,……………………………………………..………… (2-6)
Figure (2-8) An illustration of the radiation loss at the fiber bends [1]
Chapter Two Fiber Optic Transmission Fundamentals 21
2.8 Dispersion The width (duration) of the pulse propagating in an optical fiber
increases with distance of propagation. The pulse of light is composed of
wavelengths; the propagation velocity is not the same for all wavelengths.
This phenomenon is called dispersion [30]. Dispersion affects the bandwidth
of the system, hence maintaining low dispersion is of equal importance for
ensuring increased system information capacity, versatility and cost
effectiveness [31].
In order to appreciate the reasons for different amounts of pulse
broadening within the various types of optical fiber, it is necessary to consider
the dispersive mechanisms involved. These include intramodal dispersion, and
intermodal dispersion.
For no overlapping of light pulses down on an optical fiber link the
digital bit rate B must be less than the reciprocal of the broadened (through the
dispersion) pulse duration (τ).
τ1
=B ,…………………………………………..………………...… (2-7)
For amount signal overlap on the channel the maximum bit rate is
given approximately by
σ2.0
=B bit s-1…………………………………………………...…….(2-8)
Figure (2-9) illustrates the effect of dispersion on the transmitted
signal for short and long distances.
Chapter Two Fiber Optic Transmission Fundamentals 22
Figure (2-9) Effect of dispersion [6].
2.8.1 Intramodal Dispersion Intramodal or chromatic dispersion causes pulse broadening and is
the main limitation to increasing channel counts, bit rates and transmission
distances in fiber–optic lines. Dispersion becomes a problem when the optical
pulses in transport fiber begin to overlap. Pulse interference depends on
dispersion value, data bit rates, optical source spectral width and fiber
length [31].The delay differences may be caused by dispersive properties of
the waveguide material (material dispersion) and also guidance effects within
the fiber structure (waveguide dispersion) [1].
(A) Material Dispersion
Material dispersion is caused by variations of refractive index of the
fiber material with respect to wavelength. Since the group velocity is a
function of the refractive index, the spectral components of any given signal
will travel at different speeds causing deformation of the pulse [1-31].
Chapter Two Fiber Optic Transmission Fundamentals 23
(B) Waveguide Dispersion Waveguide dispersion occurs because different spectral components
of a pulse travel with different velocities by the fundamental mode of the
fiber. It is as a result of axial propagation constant being a function of
wavelength due to the existence of one or more boundaries in the structure of
the fiber. Without such boundaries, the fiber reduces to a homogeneous
medium, the fundamental mode becomes a uniform plane-wave, and the
waveguide dispersion effect is eliminated [1-31].
2.8.2 Intermodal Dispersion Intermodal dispersion results from the propagation delay differences
between modes within a multimode fiber (MMF). As the different modes
which constitute a pulse in multimode fiber travel along the channel at
different group velocities, the pulse width at the output is dependent upon the
transmission times of slowest and fastest modes.
Fortunately for step index and parabolic-index MMF, the modal
impulse response turns out to be a square pulse that is, when an impulse
excitation is launched into the fiber, the propagating modes are distributed
evenly between the fastest mode and the slowest mode. Intermodal dispersion
does not occur in single-mode fibers, but is a significant effect in multimode
fibers .Figure (2-10) is a schematic diagram illustrating the pulse broadening
due to intermodal dispersion in three different optical fiber [1-31].
Chapter Two Fiber Optic Transmission Fundamentals 24
Multimode Step index
Multimode graded index
Single mode fiber
Figure (2-10) Schematic diagram showing a multimode step index fiber, multimode
graded index and single mode step index fiber, and illustrating the pulse broadening
due to intermodal dispersion in each fiber type [1].
(A)Multimode Step Index Fiber Multipath dispersion can be understood by referring to Figure (2-11),
where different rays travel along paths of different lengths. As a result, these
rays disperse in time at the output end of the fiber even if they were coincident
at the input end and traveled at the same speed inside the fiber [22].
Chapter Two Fiber Optic Transmission Fundamentals 25
Figure (2-11) The paths taken by the axial ray and an extreme meridional ray in a
perfect multimode step index [22].
As both rays (axial ray and an extreme meridional ray) are traveling
at the same velocity within the constant refractive index fiber core then the
delay difference is directly related to their respective path lengths within the
fiber. Hence the time taken for the axial ray to travel along a fiber of length L
gives the minimum delay time TMin and:
cLn
ncLTMin
1
1
=⎟⎠⎞⎜
⎝⎛
= ……………………………………………… (2-9)
where n1 is the refractive index of the core and c is the velocity of light in a
vacuum .
The extreme meridional ray exhibits the maximum delay time TMax:
θθ
coscos 1
1c
Ln
nc
LTMax == ……………………………………………… (2-10)
Using Snell’s law of refraction at the core-cladding interface
following equation (2-11):
Chapter Two Fiber Optic Transmission Fundamentals 26
θφ cossin1
2 ==nn
c …………………………………………………… (2-11)
where n2 is the refractive index of the cladding. Further more, substituting
into equation (2-10) for cos θ gives:
2
21
cnLnT Max = ………………………………………………………… (2-12)
The delay difference δTs between the extreme meridional ray and the
axial ray may be obtained by subtracting equation (2-9):
2
21
1
21
2
21
1
2
21
cnLn
nnn
cnLnT
cLn
cnLnTTT
s
MinMaxs
Δ≅⎟⎟
⎠
⎞⎜⎜⎝
⎛ −=
−=−=
δ
δ
……………………………………….(2-13)
where Δ is the relative refractive index difference. However, when Δ<<1, the
relative refractive index difference may also be given approximately by:
2
21
nnn −
=Δ ……………………………………………….……………..(2-14)
Hence rearranging equation (2-13):
cLn
nnn
cLnTs
Δ≈
−= 1
2
211δ …………………………………………….(2-15)
Also substituting for Δ:
cnNALTs
1
2
2)(
=δ ………………………………………………………… (2-16)
Equation (2-17) below shows the rms pulse broadening at the fiber
output due to intermodal dispersion for multimode step index fiber σs , [1-22].
cnNAL
cLn
s1
21
34)(
32=
Δ=σ ………………………………………..……… (2-17)
Chapter Two Fiber Optic Transmission Fundamentals 27
(B) Multimode Graded Index Fibers The refractive index of the core in graded-index fibers is not constant
but decreases gradually from its maximum value n1 at the core center to its
minimum value n2 at the core–cladding interface [2].Intermodal dispersion in
multimode fibers is minimized with the used graded index fiber .Hence
multimode graded index shows substantial bandwidth improvement over
multimode step index fibers. The reason for improvement performance for
graded index fiber may be observed by considering the ray diagram for graded
index fiber shown in Figure (2-12) [1].
Axial ray
Figure (2-12) Ray trajectories in a graded-index fiber [22].
The dramatic improvement in multimode fiber bandwidth achieved
with parabolic or near parabolic refractive index profile is highlighted by
consideration of reduced delay difference between the fastest and slowest
modes for this graded index fiber δTg. Using a ray theory approach the delay
difference is given by:
cLnTg 8
21Δ=δ ………………..….………………..…………………….. (2-18)
Chapter Two Fiber Optic Transmission Fundamentals 28
The rms pulse broadening of a near parabolic index profile graded
index fiber σg is reduced compared to the similar broadening for the
corresponding step index fiber σs [1].
cLn
g 320
21Δ=σ …………………………………………………………. (2-19)
2.9 Source to Fiber Coupling A design goal for any transmitter is to couple as much light as possible
into the optical fiber. In practice, coupling efficiency depends on the type of
optical source (LED versus laser) as well as on the type of fiber (multimode
versus single mode).
Two approaches have been used for source–fiber coupling. In one
approach, known as direct or butt coupling, the fiber is brought close to the
source and held in place by epoxy. In the other, known as lens coupling, a lens
is used to maximize the coupling efficiency. Each approach has its own
merits, and the choice generally depends on the design objectives. An
important criterion is that the coupling efficiency should not change with time;
mechanical stability of the coupling scheme is therefore a necessary
requirement [22-33].
2.10 Optical Receiver An optical receiver converts the optical signal received at the output
end of the fiber link back into the original electrical signal. Semiconductor
photodiodes are used as photodetectors because of their compact size and
relatively high quantum efficiency. In practice, a p-i-n or an avalanche
photodiode produces electric current that varies with time in response to the
incident optical signal [23-33].
Chapter Two Fiber Optic Transmission Fundamentals 29
2.10.1 p-i-n Photodiode A p-i-n diode is the most popular method of converting the received
light into an electronic signal. Their appearance is almost identical to LEDs
and LD. Indeed the diagrams in Figure (2-13) would serve equally well for
p-i-n diodes if the labels were changed. They can be terminated with SMA,
ST, SC, biconic and a variety of other connectors or a pigtail [11].
Figure (2-13) p-i-n photodiode and SMA housing [19-23]
A p-i-n photodetector consists of a p-n junction with a layer of intrinsic
(undoped or lightly doped) semiconductor material sandwiched in between the
p- and the n-doped material. The junction must be reverse biased to create a
strong electric field in the intrinsic material. Photons incident on the i-layer
create electron-hole pairs, which become separated by the electric drift field.
As a result, a photocurrent appears at the terminals. A p-i-n photodetector can
be operated at a voltage of about 5 to 10V [24].
Chapter Three
Chapter Three 3.1 Introduction
This chapter contains all the details of the experimental work .It
includes circuit design (mainly optical fiber transmitter and receiver circuit),
equipment description and work procedures.
3.2 Optical Fiber Guidance System
In order to make the study of the pulsed laser signal characteristics
transmitted through optical fiber possible an optical fiber guidance
system is designed, built and utilized for this purpose. The main
parts of guidance system a frequency module, laser drive, optical
fiber, optical receiver, and digital frequency detector as shown in
figure (3-1).
Figure (3-1) block diagram of optical fiber guidance system
Chapter Three
3.2.1 Optical Fiber Transmitter Circuit Figure (3-2) shows the diagram of the transmitter circuit. It consists of
laser of wavelength (λ) of 680nm. This circuit works at pulsed laser mode at
four different frequencies that can be change from frequency to another by a
selector. LM7805CT
LINE VREG
COMMON
VOLTAGE
1e-014 V 3.3V
1kΩ 2N2222A
101 Ω
12 V
1N4148
Key = 4Key = 3Key = 2
1uF 2.2uF 4.7uF
10KΩ_LINKey = R
50%
Key = A 1kΩ 50%
10nF
555_VIRTUALGND
DIS
OUTRST
VCC
THR
CON
TRI
Key = 1
10uF
Figure (3-2) Transmitter circuit diagram
The transmitter circuit is divided into two parts, the first part is an
astable 555 timer IC and the second part is a laser diode drive.
The 555 timer IC is made of a combination of linear comparators and
digital flip-flop as demonstrated in figure (3-2). A series connection of three
resistors sets the reference level input to the two comparators at 2/3 Vcc and
1/3 Vcc, the output of these comparators is setting or resetting the flip-flop
unit.
Chapter Three
Figure (3-2) Details of 555 timers IC.
The following analysis illustrates the operation of the 555 as an astable
circuit using external resistor and capacitor to set the timing interval of the
output signal, see figure (3-3).
Figure (3-3) Astable multivibrator using 555IC
Chapter Three
Capacitor C2 charges toward Vcc through the external resistors R1 and
R2. Referring to figure (3-3) the capacitor voltage rises until it goes above
2/3Vcc. This voltage is the threshold voltage at pin 6, which drive comparator
1to trigger the flip-flop so that the output at pin 3 goes low. In addition, the
discharge transistor is driven on, causing the output at pin 7 to discharge the
capacitor through resistor R2. The capacitor voltage then decreases until it
drops below the trigger level (Vcc/3). The flip –flop is triggered so that the
output goes back high and the discharge transistor is turned off, so that the
capacitor can again charge through resistors R1and R2 toward Vcc [1].
In the present application four frequencies are required; therefore four
capacitors at the values 1 μF, 2.2 μF, 4.7 μF, and 10 μF are used instead of C2
to get different frequencies and the change from one frequency to another by
employing a selector as shown in figure (3-2).
The calculation of time intervals during which the output is high and
low can be made using relations:
CRRThigh )(7.0 21 += ………………………………………… (3-1)
CRTLow 27.0= ………………………………………………….. (3-2)
figure (3-4) shows the time high and time low [1].
Figure (3-4) Timing astable mode [1].
The total period is
Lowhigh TTTperiod +== …………………………...……….… (3-4)
Chapter Three
The frequency of the astable circuit is then calculated using [1].
CRRT )2( 21 +f 44.11
== ………………………………………………… (3-5)
In the second part a laser drive circuit is connected with laser diode at
wavelength 680nm. A regulator LM7805 is connected to convert the voltage
from 12v to 5v and a zener diode is used to cut the voltage at 3.3 volts which
supplies the laser diode which supplied signal by astable timer 555 IC circuit
as square wave at the bias of the transistor 2N2222. Figure (3-5) is a
photograph of drive circuit of laser diode (transmitter).
Four capacitors R2
R1
Timer 555
Capacitor 10nF Selector
zener Regulator
Transistor 2N2222
Figure (3-5) Transmitter circuit.
Chapter Three
3.2.2 Optical Fiber Receiver Circuit Figure (3-5) shows the diagram of the receiver circuit that uses p-i-n
photodiode as the detector, which is connected to a current to voltage
converter.
V112 V
R11.0kΩ
C1100uF
D2DIODE_VIRTUALphto diode
R2
10kΩR3270kΩ
1
741
3
2
4
7
6
51
RF
1.0MΩ
1
DRD6.2
Figure (3-6) Optical fiber receiver
The current from the p-i-n detector is usually converted to a voltage
before the signal is amplified. The current to voltage converter is perhaps the
most important section of any optical receiver circuit. An improperly designed
circuit will often suffer from excessive noise associated with ambient light
focused onto the detector. In this circuit IC 741 comparator is used with
feedback resistance as shown in figure (3-6).
3.2.3 The Optical Fiber A duplex optical fiber of length 200m with core / cladding diameter
62.5 /125 μm , numerical aperture 0.275 ( from table (2-3) ) , and Sc connector
is utilized here in order to obtain a single fiber of 400m length. This is done
simply by connecting two adjacent ends of the duplex fiber together using Sc
adapter to get single fiber, figure (3-7) shown the Sc adapter.
Chapter Three
Figure (3-7) Sc adapter
3.3 Laser to Fiber Coupling The design objective for any transmitter is to couple as much light as
possible into the optical fiber. In practice, the coupling efficiency depends on
the type of optical source (LED or LD) as well as on the type of fiber
(multimode or single mode).
The coupling efficiency is improved by etching a well and bringing the
fiber close to the emissive area. The power coupled into the fiber depends on
many parameters, such as the numerical aperture of the fiber and the distance
between fiber and laser. In this work the laser beam is lunched into the fiber.
The diameter of laser diode is 1mm that incident on a micro collimated
lens at focal length 2mm which used in the coupling laser to fiber , this lens is
choosing after many calculation to numerical aperture of it that approximated
equal to the numerical aperture of fiber where:
Radius of laser spotsin α = Focal length of lens
25.025.0sin ==mmmmα …………………….... ……………………. (3-6)
Chapter Three
Figure (3-8) shows the diode laser incident on the collimated lens
To get efficient coupling good aliment must be achieved for microscope
used in this case it has good aliment. The laser is mounted on the stage of the
microscope and along the same line the lens is placed in front of the laser.
After the optical fiber is illustrated to receive the light pencil emanating for
the lens by change the distance between the optical fiber and the lens far or
near to get good coupling as shown in figure (3-9).
Figure (3-9) Microscope that is used in optical fiber coupling.
Chapter Three
The original connector of the optical fiber is Sc connector but used the
place of the lenses in microscope as connector instead of original connector to
get good aliment and coupling as shown in figure (3-10).
Figure (3-10) (a) The place of lenses in microscope used as connector of optical
fiber (b) the original Sc connector of optical fiber
The microscope objective housing is exploited to fix the fiber so that an
excellent laser- lens- optical fiber part alignment is obtained fine adjustment
for obtaining as perfect coupling as possible is achieved by moving the
microscope near toward and backward the lens. This would obviously vary the
numerical aperture to the desired volue.
The purpose of making the examiner of the effect optical penanue (laser beam
characters, numerical aperture, misalignment, ete) on coupling efficiency
possible a small microscope assembly has been exploited then adopter
covalent coupling scheme. Figure (3-9) shows a microscope assemble after
being modified to suit experiment demands. A coupling lens in the middle of
the object stay optical fiber is implemented in the object however a l
By done o perfect on axis lan optical fiber part alignment has been
implemented. The prea arrangement allows control the laser pencil – optical
fuls end coupling van simply moving the object housing focal and
bachnisl the lens using microscope fine adjacent node.
Chapter Three
A transmitter-receiver optical fiber communication system has been design
implemented and utitiyil for carrying on the study.
3.4 Digital Frequency Detector After the signal is being by received the optical receiver this signals at
frequency entering to digital guidance system this system will explain in the
block diagram as shown in figure (3-12).
Figure (3-12) block diagram of digital frequency detector
As shown in block diagram the signal from optical receiver is connected
to with clock at control gate to get synchronous at counter started work, and it
is connected with control circuit to control the reset and enable latch at D latch
flip flop.
Chapter Three
The timing circuit consists of two stages; the first stage is control the
enable latch in D latch, when the D latch will not respond to a signal input if
the enable input is 0 it simply stays latched in its last state, when the enable
input is 1, however, the output follows the D input. This circuit doses on
strengthening and inversion the signal, IC 4011 NAND gate is used in this
circuit as NOT gate as shown in figure (3-13).
U4A
4011BD_5V
U5A
4011BD_5V
C2
1.0nF
R31.0kΩ
connect to latch enable
signal from optical receiver
strengthening and inversion the signal
Figure (3-13) First stage that controlled the enable latch
The second part of the timing circuit is used to control the clear counter,
to reset all outputs to zero the clear input is taken high .the over–riding clear is
independent of load and count inputs, therefore the control circuit of the clear
is designed as strengthening ,inversion , thinning the signal and again
inversion it. As shown in figure (3-14) IC 4011 NAND gate is used as NOT
gate.
signal from optical receiver
U6A
4011BD_5V
U7A
4011BD_5VR41.0kΩ
connect to clear inthe counter
C3
10nF
U8A
4011BD_5V
U9A
4011BD_5V
Figure (3-14) Second stage that controlled the clear of the counter
Chapter Three
IC 7408 AND gate is used as control gate, the clock circuit is designed
as shown in the figure (3-15) by use IC 4011 NAND gate as not gat when
connect the two edge end of the IC as one end.
R1100kΩ
U1A
4011BD_5V
U2A
4011BD_5V
C1
10nF
R21.0kΩ
R510KΩ_LINKey = R
50%
U3A
4011BD_5V
connect to control gate
Figure (3-15) clock circuit
The signal from control gate input to up counter which used two stage
of counter 4-bit to get 8-bit, which used two of IC 74193 4-bit counter, when
the counter overflows the carry output produces a pulse of equal pulse width
to that of the count up input pulse, the two counters are connect by connect the
output of carry of the first counter to up counter of the second counter, the
output of the 8-bit counter connect to D latch. The IC 74373 D latch is a 1-bit
memory circuit; the D latch is active at 0 and store the signal at 1.
The output signal from D latch is connect to four stage of comparators
every one of these comparators consist of two comparator, most and least
comparator, IC 7485the comparator is used, this signal is connect after some
calculation and programming the IC of comparator.
These calculations are depended on the pulse duration of the signal of
optical receiver divided on 50 µm and convert it from decimal to the binary
and program the comparator, in the comparator A>B.
Chapter Three
This calculation for A frequency
1st code =A1 = 111010002321050106.11
6
3
==××
−
−
2nd code = A2 = 011101001161050108.5
6
3
==××
−
−
3rd code =A3 = 00110000481050104.2
6
3
==××
−
−
4th code =A4 = 00011000241050102.1
6
3
==××
−
−
These frequencies input to the comparators from the D latch, in the
comparator must A>B there for B is programmed as least than A.
The codes of B are:
1st code =227=11100011
2nd code =108 = 01101100
3rd code = 35 = 00011110
4th code = 15= 00001111
The output of the first code must be greater than all codes, the output of
the second code least than first code but greater than third and fourth codes,
the third is greater than fourth code.
Therefore IC 7421 four input AND gate is used, the output of the four
stages comparator entering to this IC when choosing the first frequency. And
the other three frequencies are entering to IC 7486 two input Exclusive OR
which used the four frequencies in this circuit which used as decoder ,three
output are gotten from IC 7486 and one output at high frequency from
IC 7421, see figure (3-16) and (3-17).
Chapter Three
U1
74193N
A15B1C10D9
UP5
QA 3QB 2QC 6QD 7
DOWN4
~LOAD11 ~BO 13~CO 12CLR14
U2
74193N
A15B1C10D9
UP5
QA 3QB 2QC 6QD 7
DOWN4
~LOAD11 ~BO 13~CO 12CLR14
U3
74LS373N
1D32D43D74D85D136D147D178D18
~OC1ENG11
1Q 22Q 53Q 64Q 95Q 126Q 157Q 168Q 19
U4
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U5
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U6
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U7
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U8
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U9
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U10
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U11
74LS85N
A213B214
A112B111
OAGTB 5
A010B09
A315B31 OAEQB 6
OALTB 7
AEQB3ALTB2
AGTB4
U12A
74HC21N_2V
VCC5V
U13A74S08D
Timing circuit
Signal from optical receiver
U14A
7486_VHDLU15A
7486_VHDLU16A
7486_VHDL
Figure (3-16) circuit of digital detector
Chapter Three
Figure (3-17) Photograph of digital detector
Chapter Five Conclusions and Future Works 56
Chapter Five Conclusions and Future Works
5.1 Conclusions The important facts derived from the practical results at this work can
be summarized as follows:
1. By using short distance optical fiber the intermodal dispersion is
minimized to the case where it can be used in certain application.
2. When high bit rate is sent the intermodal dispersion is reduced, as it
is inversely proportional to the total broadening time.
3. Using optical fiber graded index leads to the decreasing of the
intermodal dispersion, due to the parabolic refractive index geometry
of the fiber.
4. Input laser power stability should be maintained during launching:
degraded laser leads to degraded power and shape of the transmitted
laser pulses.
5. The optical fiber guidance system does not require the unbroken
line-of-sight, the link directionality, and human eye safety on an
interrogating laser.
6. Efficient fiber to laser coupling must be achieved and maintained
through out the launching process: inefficient coupling leads to
degraded output power signal.
Chapter Five Conclusions and Future Works 57
5.2 Future works A number of future works can be suggested depending on the practical
results of this thesis, these include the following points:
1. Measurement of intermodal dispersion by using different types of
optical fibers (multimode step index, multimode graded index),
for different distances.
2. Measurement of intermodal dispersion in optical fiber by using
triangle or sin wave.
3. Using joystick instead of manual selector to reduce the delay
time.
4. Utilizing the optical fiber guidance system in controlling and
mobilizing a robot, or on the control of airplane.
Experimental work Introduction
An optical fiber communication system is consisting of transmitter,
fiber, and receiver as shown below.
Receiver Connector Fiber Connector Transmitter
(Laser or LED)
Transmitter
The transmitter source is laser or LED the circuit of transmitter is
common emitter circuit for laser or LED as shown
But to send the signal at variable bit rats therefore designed the circuit
as shown in below by using IC 555 timer circuit to send different bit rats this
circuit is depended in the project.
1e-014 V 3.3V
1kΩ 2N2222A
101 Ω
BJT_NPN_VIRTUAL7805
12 V
1N4148
Key = 4Key = 3Key = 2
1uF 2.2uF 4.7uF
10KΩ_LINKey = R
50%
Key = A 1kΩ 50%
10nF
555_VIRTUALGND
DIS
OUTRST
VCC
THR
CON
TRI
10uF
Key = 1
The important parts of the circuit are explaining briefly
This IC consist of eight legs this legs is connected as shown up and
explain the important operation legs .
The legs No (8) connect to the +Vcc, No (1) to the ground.
No (2) trigger input is used to initiate a monostable timing period.
Triggering occurs on the negative going edge AB of the pulse shown in the
diagram below, at a voltage level less that 1/3 of the VCC or V+ supply rail.
The trigger pin must be returned to a level above 1/3 of the OV or V– supply
rail before the end of the set timing period ‘T’. Should the trigger pulse
interval‘t’ be greater than the timing period ‘T’ then the output will remain in
the active state (output high) for time ‘t’. Once triggered the trigger input is
disabled and any trigger pulses occurring during the timing period ‘T’ have no
effect on the set time.
The No(4) reset function is used to return the timer output to the steady
state (output low) when interruption of a monostable timing period is required.
When not required the reset should be connected to VCC or V+. This avoids a
false reset occurring.
The No (5) control voltage: open +circuit voltage at the control pin is set at
2 /3 VCC or V by the internal resistors R. This resistor network sets the
threshold comparator trip level at 2/3 supply and the trigger comparator at 1/3
supply. By imposing an external voltage on this pin the comparator reference
levels may be shifted above or below the nominal levels hence affecting the
timing in both the monostable and stable modes. [1]
The four capacitors are connected in the circuit to change the bit rats by
using selector when the selector selects any one of the capacitors the signal of
bit rate change. The regulator 7805 is connected to decrease the voltage of
power supply from 12v to 5v.and the zener diode is used to keep and product
the voltage at 3.3v to operate the source of the transmitter. This is top view of
transmitter circuit.
Selector
IC555
Capacitors
LED
III
CONTENTS Acknowledgment I Abstract II Contents III List of Figures V List of Tables VII List of Abbreviations VIII List of Symbols IX
CHAPTER ONE : Introduction and Historical Review 1.1 Introduction 1 1.2 Historical Background of Optical Fibers 2 1.3 Literature Survey 4 1.4 Aim of the Work 9 1.5 Thesis Layout 10
CHPTER TWO : Fiber Optic Transmission Fundamentals 2.1 Introduction 11 2.2 Optical Transmitter 11 2.2.1 Light Emitting Diode (LED) 12 2.2.2 Semiconductor Laser Diode (LD) 12 2.3 Modulation of Laser 13 2.4 Derive Circuit 14 2.5 Fiber Optic 15 2.6 Basic Fiber Parameter 16 2.6.1 Refractive Index 16 2.6.2 The Principle of Total Internal Reflection (TIR) 17 2.6.3 Numerical Aperture 18 2.6.4 Acceptance Angle 19 2.7 Optical Fiber Loss 20 2.7.1 Bending Loss 20 2.8 Dispersion 21 2.8.1 Intramodal Dispersion 22 2.8.2 Intermodal Dispersion 23 2.9 Source to Fiber Coupling 28 2.10 Optical Receiver 28 2.10.1 p-i-n Photodiode 29
CHAPTER THREE: Experimental Work 3.1 Introduction 30 3.2 Optical Fiber Guidance System 30
3.2.1 Optical Fiber Transmitter Circuit 31 3.2.2 Optical Fiber Receiver Circuit 34 3.2.3 The Optical Fiber 35
IV
3.3 Laser to Fiber Coupling 35 3.4 Digital Frequency Detector 37 3.4.1 Timing Circuit 38 3.4.2 Clock Circuit Generator 39 3.4.3 The Digital Frequency Decoder Circuit 40
CHAPTER FOUR : Results and Discussion 4.1 Introduction 45 4.2 Signal Transmission Measurement 45
4.2.1 Critical Radius 45 4.2.2 Fiber Coupling 46 4.2.3 Effect of Deteriorated Laser Power 49 4.2.4 Measurement of Intermodal Dispersion 49 4.2.5 Measurement of the Rise Time and Fall Time 52
4.4 Selector Delay Time Measurements 54 4.5 Decoder Output Indicators 54
CHAPTER FIVE: Conclusions and Future works 5.1 Conclusions 56 5.2 Future Works 57 REFRENCES 58
Lens
d
Tolerance Coupling
Laser
f
Laser Beam
α
Misalignment Coupling
Optical fiber
Lens
Optical fiber
d Laser
f
α
Lens
dLaser
Optical fiber α
f
Lens
Optical fiber d Laser
f
α
Optical fiber
Lens
d Laser
f
Laser beam
Perfect Coupling
Tolerance Coupling
Lens
d Laser
f
Optical fiber
Lens
d Laser
f
Optical fiber α
LD
LED
Wavelength (λ)
Intensity
Laser
Base
Lens Optical
fiber
Objective housing Stage
Examination Committee Certification We certify that we have read the thesis entitled “Intermodal
Dispersion of Laser Pulses Transmitted Through Silica Optical Fiber”,
and as an examination committee examined the student “Abdulla Ali
Kassim” in its content, and that in our opinion it is adequate for the partial
fulfillment of the requirement for the degree of Master of Since in Laser
Engineering.
Signature: Name: Dr. Shatha M. Abdul Hakim Tile: Assist Professor Address: Physics Dep. Collage of Science. Baghdad University. (Member) Date: / 10 / 2008
Signature: Name: Dr. Mohammed Hussain Ali Tile: Assist Professor Address: Head of Laser and Optoelectronics Eng. Dep. University of Technology (Chairman) Date: / 10 / 2008
Signature: Name: Dr. Mohamed Saleh Ahmed Tile: Assist Professor: Address: Applied Science.Dep. University of Technology (Supervisor) Date: / 10 /2008
Signature: Name: Dr. Anwaar A. Al-Dergazly Tile: Lecturer Address: Laser and Optoelectronics Eng. Dep. Naharian University (Member) Date: / 10 / 2008
Approved by Department of Laser and Optoelectronics Engineering,
University of Technology.
Signature: Name: Dr. Mohammed Hussain Ali Tile: Assist Professor Address: Head of Laser and Optoelectronics Eng. Dep. University of Technology. Date: / 10 / 2008
V
List of Figures
(1-1) Fiber optics use visible and infrared light. 4 (2-1) Parts of a fiber optic data link. 11 (2-2) Shows the difference between LD and LED profiles. 13 (2-3) Showing (a) return-to-zero (RZ) and (b) non return …… 14 (2-4) A common-emitter saturating switch. 15 (2-5) The refractive index profiles and light propagation in….. 16 (2-6) Representation of the critical angle and total internal …… 18 (2-7) The acceptance angle when launching light into an ….. 19 (2-8) An illustration of the radiation loss at the fiber bends. 20 (2-9) Effect of dispersion. 22
(2-10) Schematic diagram showing a multimode step index ……. 24 (2-11) The paths taken by the axial ray and an extreme …. 25 (2-12) Ray trajectories in a graded-index fiber. 27 (2-13) p-i-n photodiode and SMA housing. 29 (3-1) Block diagram of optical fiber guidance system. 30 (3-2) Transmitter circuit diagram. 31 (3-3) Details of 555 timer IC. 32 (3-4) Astable multivibrator using 555IC. 32 (3-5) Timing astable mode. 33 (3-6) Optical fiber receiver. 34 (3-7) A microscope assembly is utilized to implement laser….. 36 (3-8) Block diagram of digital frequency detector. 37 (3-9) First part clear of the counter. 38
(3-10) First stage that control the enable latch. 38 (3-11) Clock circuit. 39 (3-12) Timing diagram of digital frequency decoder. 39 (3-13) Circuit of digital detector. 43 (3-14) Photograph of all system. 44 (4-1) Shows the relationship between the refractive index of …. 46 (4-2) Demonstrates the optical coupling process where …… 47 (4-3) Efficient coupling, (a) the transmitted…… 47 (4-4) Demonstrates geometries of imperfect coupling where ….. 48 (4-5) Imperfect coupling: (a) the transmitted signal and…… 48 (4-6) Output signal for deteriorated laser power input. 49 (4-7) Illustrates the (a) unlunched signal 7.2ms (138.889Hz)…. 50 (4-8) Demonstrates the (a) input signal before being ……. 50
VI
(4-9) Shows the unlaunched and transmitted signals (a) the …. 51 (4-10) Pulse shape of (a) the unlaunched signal of (0.73ms, ….. 51 (4-11) Shows the rise time (tR) (1V/div and 0.5ms/div). 53 (4-12) Shows the fall time (tf) (1V/divand 0.5ms/div). 53 (4-13) Shows a delay time introduced when moving from one …. 54
VII
List of Tables (2-1) LEDs & LDs comparison. 13 (2-2) Pure silica: refractive index variation with wavelength. 17 (2-3) Available optical fiber Geometries. 19 (3-1) Output of the digital comparator. 41 (3-2) Demonstrates chosen frequency, value leading to the output..... 42 (4-1) Gives the value of input and output signal. 52 (4-2) Demonstrates decoder outputs. 55
REFRENCES 58
REFRENCES
1. J. Senior, Optical Fiber Communications: Principles and Practice, 1st, Prentice
Hall International,Inc. 1985
2. R. M. Gayliard and S. Karp, Optical Communication system, John wiely and
sonce ING.1979.
3. M. Hirose, K. Kishine, H. Ichino, and N. Ishihara, "Low-Power 2.5-Gb/s Si-
Bipolar IC Chipset for Optical Receivers and Transmitters Using Low-Voltage
and Adjustment-Free Circuit Techniques" IEICE Trans. Electron., vol. E82-C,
1999.
4. . J. Hayes, Fiber Optics Technician’s Manual, 2nd ed, McGraw-Hill.
5. G. Keiser, Optical Fiber Communications, 3rd ed., McGraw-Hill, New York,
2000.
6. J. R. Dutton, Understand Optical Communication, 1st ed International Business
Machines, 1998.
7. J .Power, An Introduction To Fiber Optic Systems, 2nd ed, McGraw-Hill, 1996.
8. M. Bass, Handbook of Optics Volume II Devices, Measurements, and
Properties. 2nd ed, McGraw-Hill, 1995.
9. M. Bass, Fiber Optics Handbook Fiber, Devices, and Systems for Optical
Communications, 1st ed, McGraw-Hill, 2002.
10. R. W. Waynant, Electro-Optics Handbook, 2nd ed, McGraw-Hill, 2000.
11. J. Crisp, Introduction to Fiber Optics, 2nd ed, Newnes, 2001.
12. P. Olckers, “5DT releases a Virtual Reality Training Simulator for Fiber Guided
Video Missile Operators” DEXSA ’98, Pretoria, South Africa, November 1998.
13. P. Hlubina, T. Martynkien and W. Urba´nczyk, “Measurements of intermodal
dispersion in few-mode optical fibers using a spectral-domain white-light
interferometric method” Meas. Sci. Technol, vol 14, pp 784-789, April 2003.
14. T. D. Engeness, M. Ibanescu, S. G. Johnson, O. Weisberg, M. Skorobogatiy, S.
REFRENCES 59
Jacobs, and Y. Fink, “Dispersion tailoring and compensation by modal
interactions in OmniGuide fibers” Optics Express, vol. 11, No. 10, pp.1175-
1196, May 2003.
15. H. Wu, J. A. Tierno, P. Pepeljugoski, J. Schaub, S. Gowda, J. A. Kash, and A.
Hajimiri, “Integrated transversal equalizers in high-speed fiber optic systems”
IEEE J. Solid-State Circuits, vol. 38, pp. 2131–2137, Dec. 2003.
16. S. Fan and J. M. Kahn, “Principal modes in multi-mode waveguides” Optics
Letters, vol. 30, no. 2, pp. 135–137, Jan. 2005.
17. C. Benjamin, J. Collins, D. Diefenderfer, M. LaPierre, C. Legendre, A. Tharp, J.
Wallace and J. Worrell, “Advanced Gunship Proposal” Virginia Polytechnic
Institute and State University, 2005.
18. R. A. Panicker, J. P. Wilde, J. M. Kahn, D. F. Welch and I. Lyubomirsky, “10 x
10 Gb/s DWDM transmission through 2:2 km multimode fiber using adaptive
optics” IEEE Photon. Technol. Lett., vol. 19, no. 15, pp.1154-1156, August 1,
2007.
19. R. A. Panicker, J. M. Kahn, and S. P. Boyd, “Compensation of Multimode Fiber
Dispersion using Adaptive Optics via Convex Optimization” Lightwave
Technology, May 2007.
20. P. C. Pandey and A. Mishra, “Modal Dispersion Characteristics of a Single
Mode Dielectric Optical Waveguide with a Guiding Region Cross-Section
Bounded by Two Involuted Spirals” Progress In Electromagnetics Research,
PIER. 76, pp. 1-13, 2007.
21. I. Kamitsos and N. K. Uzunoglu, “Improvement of Transmission Properties of
Multimode Fibers Using Spread Spectrum Technique and a Rake Receiver
Approach” Progress In Electromagnetics Research, PIER 76,pp. 413–425, 2007.
22. G. P. Agrawal, Fiber-Optic Communications Systems, 3rd ed. John Wiley &
Sons, Inc., 2002.
REFRENCES
60
23. D. Jones, Module 24-Introduction to Fiber Optics, Naval Education Training
Professional Development and Technology Center, 1998.
24. E. Sackinger, Broadband Circuits for Optical Fiber Communication, 1st ed, John
Wiley & Sons,Inc.,2005.
25. M. Banu, B. Jalali, R. Nottenburg, D. A. Humphrey, R. K. Montgomery, R. A.
Hamm, M. B. Panish, “10 Gbit/s Bipolar Laser Driver” Electronics Letters, Vol.
27, No. 3, January , 1991.
26. Y. Song, “Optical Communication Systems for Smart Dust” Thesis of M.Sc,
Virginia Polytechnic Institute and State University, 2002.
27. B. Elliott, Fiber Optic Cabling, 2nd ed, Newnes 2002.
28. G.Keiser, Optical Communication Essentials, McGraw-Hill, 2004.
29. S. Kasap, H. Ruda, and Y. Boucher, An Illustrated Dictionary of
Optoelectronics and Photonics: Important Terms and Effects, 2000.
30. W. Hamid, “Analysis, Design and Performance Evaluation of Multi Layer
Single Mode Optical Fiber” M.Sc thesis, University of Technology, 2007.
31. T. Barake, “A Generalized Analysis of Multiple-Clad Optical Fibers with
Arbitrary Step-Index Profiles and Applications” M.Sc thesis, Virginia
Polytechnic Institute and State University, 1997.
32. W. H. Park, “Fluorescence lifetime sensor using optical fiber and optical signal
processing” M.Sc thesis, University of Toronto, 1998.
33. G. P. Agrawal, Light Wave Technology Telecommunication Systems, John
Wiley and Sons, Inc.2005.
34. R. Boylestad, Electronic Devices and Circuit Theory, Englewood Cliffs,
NewJersey, 1987.