+ All Categories
Home > Documents > By Abdulla Ali Kassim - University of Technology, Iraq...Abdulla Ali Kassim B. Sc. Laser Eng. 2006...

By Abdulla Ali Kassim - University of Technology, Iraq...Abdulla Ali Kassim B. Sc. Laser Eng. 2006...

Date post: 28-May-2020
Category:
Upload: others
View: 10 times
Download: 1 times
Share this document with a friend
73
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.
Transcript

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.


Recommended