Principles of Fiber Optic Communication

5/19/2018 Principles of Fiber Optic Communication

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Principles of FiberOptic Communication

Module 4

of

Course 2, Elements of Photonics

OPTICS AND PHOTONICS SERIES

STEP (Scientific and Technological Educationin Photonics), an NSF ATE Project


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2008 CORD

This document was produced in conjunction with the STEP projectScientific and

Technological Education in Photonicsan NSF ATE initiative (grant no. 0202424). Any

opinions, findings, and conclusions or recommendations expressed in this material are those of

the author(s) and do not necessarily reflect the views of the National Science Foundation.

For more information about the project, contact either of the following persons:

Dan Hull, PI, Director

CORD

P.O. Box 21689

Waco, TX 76702-1689

(245) 741-8338

(254) 399-6581 fax

[email protected]

Dr. John Souders,

Director of Curriculum Materials

P.O. Box 21689

Waco, TX 76702-1689

(254) 772-8756 ext 393

(254) 772-8972 fax

[email protected]

Published and distributed by

CORD Communications

601 Lake Air Drive, Suite E

Waco, Texas 76710-5841

800-231-3015 or 254-776-1822

Fax 254-776-3906

www.cordcommunications.com

ISBN 1-57837-392-1


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PREFACE

This is the fourth module in Course 2 (Elements of Photonics) of the STEP curriculum.

Following are the titles of all six modules in the course:

1. Operational Characteristics of Lasers

2. Specific Laser Types

3. Optical Detectors and Human Vision

4. Principles of Fiber Optic Communication

5. Photonic Devices for Imaging, Storage, and Display

6. Basic Principles and Applications of Holography

The six modules can be used as a unit or independently, as long as prerequisites have been met.

For students who may need assistance with or review of relevant mathematics concepts, astudent review and study guide entitledMathematics for Photonics Education(available from

CORD) is highly recommended.

The original manuscript of this document was authored by Nick Massa (Springfield Technical

Community College) and edited by Leno Pedrotti (CORD). Formatting and artwork were

provided by Mark Whitney and Kathy Kral (CORD).


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CONTENTS

Introduction ...................................................................................................................................................1

Prerequisites ..................................................................................................................................................1

Objectives......................................................................................................................................................2

Scenario .........................................................................................................................................................3

Basic Concepts ..............................................................................................................................................4

Historical Introduction ..............................................................................................................................4

Benefits of Fiber Optics ............................................................................................................................6

The Basic Fiber Optic Link.......................................................................................................................7

Fiber Optic Cable Fabrication...................................................................................................................7

Preform fabrication ...............................................................................................................................7

Outside vapor deposition (OVD)..........................................................................................................9

Inside vapor deposition (IVD) ............................................................................................................10

Vapor axial deposition (VAD)............................................................................................................10

Total Internal Reflection (TIR) ...............................................................................................................11

Transmission Windows...........................................................................................................................12The Optical Fiber ....................................................................................................................................14

Numerical aperture..............................................................................................................................15

Fiber Optic Loss Calculations.................................................................................................................16

Power budget ......................................................................................................................................19

Types of Optical Fiber ............................................................................................................................21

Step-index multimode fiber ................................................................................................................21

Step-index single-mode fiber..............................................................................................................22

Graded-index fiber..............................................................................................................................23

Polarization-maintaining fiber ............................................................................................................23

Fiber Optic Cable Design........................................................................................................................24

Dispersion ...............................................................................................................................................27

Calculating dispersion.........................................................................................................................28Intermodal dispersion .........................................................................................................................29

Chromatic dispersion ..........................................................................................................................29

Fiber Optic Sources.................................................................................................................................32

LEDs ...................................................................................................................................................32

Laser diodes ........................................................................................................................................33

Fiber Optic Detectors ..............................................................................................................................34

Connectors ..........................................................................................................................................36

Fiber Optic Couplers...............................................................................................................................37

Star couplers .......................................................................................................................................37

T-couplers ...........................................................................................................................................39

Wavelength-division multiplexers......................................................................................................39

Fiber Bragg gratings ...........................................................................................................................41Erbium-doped fiber amplifiers (EDFA)..............................................................................................42

Fiber Optic Sensors.................................................................................................................................42

Extrinsic fiber optic sensors................................................................................................................43

Intrinsic sensors ..................................................................................................................................45

Laboratory ...................................................................................................................................................48

Problems......................................................................................................................................................51

References ...................................................................................................................................................52


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1

COURSE 2: ELEMENTS OF PHOTONICS

Module 2-4

Principles of Fiber OpticCommunication

INTRODUCTION

The dramatic reduction of transmission loss in optical fibers coupled with equally important

developments in the area of light sources and detectors have brought about a phenomenal

growth of the fiber optic industry during the past two decades. Its high bandwidth capabilities

and low attenuation characteristics make it ideal for gigabit data transmission and beyond. The

birth of optical fiber communication coincided with the fabrication of low-loss optical fibers

and room-temperature operation of semiconductor lasers in 1970. Ever since, the scientific and

technological progress in this field has been so phenomenal that within a brief span of 30 years

we are already in the fifth generation of optical fiber communication systems. Recent

developments in optical amplifiers and wavelength division multiplexing (WDM) are taking us

to a communication system with almost zero loss and infinite bandwidth. Indeed, optical

fiber communication systems are fulfilling the increased demand on communication links,

especially with the proliferation of theInternet. In this module,Principles of Fiber Optic

Communication,you will be introduced to the building blocks that make up a fiber opticcommunication system. You will learn about the different types of optical fiber and their

applications, light sources and detectors, couplers, splitters, wavelength-division multiplexers,

and other components used in fiber optic communication systems. Non-communications

applications of fiber optics including illumination with coherent light bundles and fiber optic

sensors will also be covered.

PREREQUISITES

Prior to this module, you are expected to have covered Modules 1-1,Nature and Properties of

Light;Module 1-3,Light Sources and Laser Safety;Module 1-4,Basic Geometrical Optics;and

Module 1-5,Basic Physical Optics;Module 1-6,Principles of Lasers;and Module 2-3, Optical

Detectors and Human Vision. In addition, you should be able to manipulate and use algebraic

formulas involving trigonometric functions and deal with units.


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2 Optics and Photonics Series, Course 2: Elements of Photonics

OBJECTIVES

Upon completion of this module, you will be able to:

Identify the basic components of a fiber optic communication system

Discuss light propagation in an optical fiber

Identify the various types of optical fibers

Discuss the dispersion characteristics for various types of optical fibers

Identify selected types of fiber optic connectors

Calculate numerical aperture (N.A.), intermodal dispersion, and material dispersion.

Calculate decibel and dBm power

Calculate the power budget for a fiber optic system

Calculate the bandwidth of an optical fiber Describe the operation and applications of fiber optic couplers

Discuss the differences between LEDs and laser diodes with respect to performancecharacteristics

Discuss the performance characteristics of optical detectors

Discuss the principles of wavelength-division multiplexing (WDM)

Discuss the significance of the International Telecom Union grid (ITU grid)

Discuss the use of erbium-doped fiber amplifiers (EDFA) for signal regeneration

Describe the operation and applications of fiber Bragg gratings

Describe the operation and application of fiber optic circulators

Describe the operation and application of fiber optic sensors.


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Module 2-4: Principles of Fiber Optic Communication 3

SCENARIO

Dante is about to complete a bachelors degree in fiber optic technology, a field that has

interested him since high school. To prepare himself for the highly rewarding careers that fiber

optics offers, Dante took plenty of math and science in high school and then enrolled in an

associate degree program in laser electro-optics technology at Springfield Technical

Community College (STCC) in Springfield, Massachusetts. Upon graduation from STCC he

accepted a position as an electro-optics technician at JDS Uniphase Corporation in Bloomfield,

Connecticut. The company focuses on precision manufacturing of the high-speed fiber optic

modulators and components that are used in transmitters for the telecommunication and cable

television industry.

As a technician at JDS Uniphase, Dante was required not only to understand how fiber optic

devices work but also to have an appreciation for the complex manufacturing processes that are

required to fabricate the devices. The background in optics, fiber optics, and electronics that

Dante received at STCC proved to be invaluable in his day-to-day activities. On the job, Dante

routinely worked with fusion splicers, optical power meters, and laser sources and detectors, aswell as with optical spectrum analyzers and other sophisticated electronic test equipment.

After a few years as an electro-optics technician, Dante went on to pursue a bachelors degree in

fiber optics. (The courses he had taken at STCC transferred, so he was able to enroll in his

bachelors program as a junior.) Because of his hard work on the job at JDS Uniphase, Dante

was awarded a full scholarship and an internship at JDS Uniphase. This allowed Dante to

complete his degree while working for JDS Uniphase part time. According to Dante, the

experience of working in a high-tech environment while going to school really helps you see the

practical applications of what you are learningwhich is especially important in a rapidly

changing field like fiber optics.


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BASIC CONCEPTS

Historical Introduction

Communication implies transfer of information from one point to another. When it is necessaryto transmit information, such as speech, images, or data, over a distance, one generally uses the

concept of carrier wave communication. In such a system, the information to be sent modulates

an electromagnetic wave such as a radio wave, microwave, or light wave, which acts as a

carrier. (Modulationmeans to vary the amplitude or frequency in accordance with an external

signal.) This modulated wave is then transmitted to the receiver through a channel and the

receiver demodulates it to retrieve the imprinted signal. The carrier frequencies associated with

TV broadcast (50900 MHz) are much higher than those associated with AM radio broadcast(600 kHz20 MHz). This is due to the fact that, in any communication system employingelectromagnetic waves as the carrier, the amount of information that can be sent increases as the

frequency of the carrier is increased.1Obviously, TV broadcast has to carrymuch more

information than AM broadcasts. Since optical beams have frequencies in the range of10

14to 10

15Hz, the use of such beams as the carrier would imply a tremendously large increase

in the information-transmission capacity of the system as compared to systems employing radio

waves or microwaves.

In a conventional telephone system, voice signals are converted into equivalent electrical

signals by the microphone and are transmitted as electrical currents through metallic (copper or

aluminum) wires to the local telephone exchange. Thereafter, these signals continue to travel as

electric currents through metallic wire cable (or for long-distance transmission as

radio/microwaves to another telephone exchange) usually with several repeaters in between.

From the local area telephone exchange, at the receiving end, these signals travel via metallic

wire pairs to the receiver telephone, where they are converted back into corresponding soundwaves. Through such cabled wire-pair telecommunication systems, one can at most send

48 simultaneous telephone conversations intelligibly. On the other hand, in an optical

communication system that uses glass fibers as the transmission medium and light waves as

carrier waves, it is distinctly possible today to have 130,000 or more simultaneous telephone

conversations (equivalent to a transmission speed of about 10 Gbit/s) through one glass fiber no

thicker than a human hair. This large information-carrying capacity of a light beam is what

generated interest among communication engineers and caused them to explore the possibility

of developing a communication system using light waves as carrier waves.

The idea of using light waves for communication can be traced as far back as 1880 when

Alexander Graham Bell invented the photophone (see Figure 4-1) shortly after he invented the

telephone in 1876. In this remarkable experiment, speech was transmitted by modulating a lightbeam, which traveled through air to the receiver. The flexible reflecting diaphragm (which

1The information-carrying capacity of an electromagnetic carrier is approximately proportional to the difference

between the maximum and the minimum frequencies (technically known as bandwidthof the channel) that can be

transmitted through the communication channel. The higher one goes in the electromagnetic spectrum in frequency

scale, the higher the bandwidth and hence the information-carrying capacity of such a communication system. That

is why historically the trend in carrier wave communication has been always toward bandwidths of higher and

higher frequencies.


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could be activated by sound) was illuminated by sunlight. A parabolic reflector placed at a

distance of about 200 m received the reflected light. The parabolic reflector concentrated the

light on a photoconducting selenium cell, which formed a part of a circuit with a battery and a

receiving earphone. Sound waves present in the vicinity of the diaphragm vibrated the

diaphragm, which led to a consequent variation of the light reflected by the diaphragm. The

variation of the light falling on the selenium cell changed the electrical conductivity of the cell,

which in turn changed the current in the electrical circuit. This changing current reproduced thesound on the earphone.

Figure 4-1Schematic of the photophone invented by Bell. In this system, sunlight was modulated by avibrating diaphragm and transmitted through a distance of about 200 meters in air to a receiver

containing a selenium cell connected to the earphone.

After succeeding in transmitting a voice signal over 200 meters using a light signal, Bell wrote

to his father: I have heard a ray of light laugh and sing. We may talk by light to any visible

distance without any conducting wire. To quote from MacLean: In 1880 he [Graham Bell]

produced his photophone which to the end of his life, he insisted was . the greatest

invention I have ever made, greater than the telephone Unlike the telephone, though, it had

no commercial value.

The modern impetus for telecommunication with carrier waves at optical frequencies owes its

origin to the discovery of the laser in 1960. Earlier, no suitable light source was available that

could reliably be used as the information carrier.2At around the same time, telecommunication

traffic was growing very rapidly. It was conceivable then that conventional telecommunication

systems based on coaxial cables, radio and microwave links, and wire-pair cable, could soon

reach a saturation point. The advent of lasers immediately triggered a great deal of investigation

aimed at examining the possibility of building optical analogues of conventional communicationsystems. The very first such modern optical communication experiment involved laser beam

transmission through the atmosphere. However, it was soon realized that shorter-wavelength

laser beams could not be sent in open atmosphere through reasonably long distances to carry

signals, unlike, for example, the longer-wavelength microwave or radio systems. This is due to

2We may mention here that, although incoherent sources like light-emitting diodes (LED) are also often used in

present-day optical communication systems, it was the discovery of the laser that triggered serious interest in the

development of optical communication systems.


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the fact that a laser light beam (of wavelength about 1 m) is severely attenuated and distortedowing to scattering and absorption by the atmosphere. Thus, for reliable light-wave

communication under terrestrial environments it would be necessary to provide a guiding

medium that could protect the signal-carrying light beam from the vagaries of the terrestrial

atmosphere. This guiding medium is the optical fiber,a hair-thin structure that guides the light

beam from one place to another through the process of total internal reflection(TIR), which

will be discussed in the next section.

Benefits of Fiber Optics

Fiber optic communication systems have many advantages over copper wire-based

communication systems. These advantages include:

Long-distance signal transmissionThe low attenuation and superior signal quality of fiber optic communication systems allow

communications signals to be transmitted over much longer distances than metallic-based

systems without signal regeneration. In 1970, Kapron, Keck, and Maurer (at Corning Glassin USA) were successful in producing silica fibers with a loss of about 17 dB/km at a

wavelength of 633 nm. Since then, the technology has advanced with tremendous rapidity.

By 1985 glass fibers were routinely produced with extremely low losses (< 0.2 dB/km).

Voice-grade copper systems require in-line signal regeneration every one to two kilometers.

In contrast, it is not unusual for communications signals in fiber optic systems to travel over

100 kilometers (km), or about 62 miles, without signal amplification of regeneration.

Large bandwidth, light weight, and small diameterTodays applications require an ever-increasing amount of bandwidth. Consequently, it is

important to consider the space constraints of many end users. It is commonplace to install

new cabling within existing duct systems or conduit. The relatively small diameter and lightweight of optical cable make such installations easy and practical, saving valuable conduit

space in these environments.

NonconductiveAnother advantage of optical fibers is their dielectric nature. Since optical fiber has no

metallic components, it can be installed in areas with electromagnetic interference (EMI),

including radio frequency interference (RFI). Areas with high EMI include utility lines,

power-carrying lines, and railroad tracks. All-dielectric cables are also ideal for areas of

high lightning-strike incidence.

Security

Unlike metallic-based systems, the dielectric nature of optical fiber makes it impossible toremotely detect the signal being transmitted within the cable. The only way to do so is by

accessing the optical fiber. Accessing the fiber requires intervention that is easily detectable

by security surveillance. These circumstances make fiber extremely attractive to

governmental bodies, banks, and others with major security concerns.


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The Basic Fiber Optic Link

Figure 4-2 shows a typical optical fiber communication system. It consists of a transmitting

device Tthat converts an electrical signal into a light signal, an optical fiber cable that carries

the light, and a receiverRthat accepts the light signal and converts it back into an electrical

signal. The complexity of a fiber optic system can range from very simple (i.e., local areanetwork) to extremely sophisticated and expensive (i.e., long-distance telephone or cable

television trunking). For example, the system could be built very inexpensively using a visible

LED, plastic fiber, a silicon photodetector, and some simple electronic circuitry.

On the other hand, a system used for long-distance, high-bandwidth telecommunication that

employs wavelength-division multiplexing, erbium-doped fiber amplifiers, external modulation

using distributed feedback (DFB) lasers with temperature compensation, fiber Bragg gratings,

and high-speed infrared photodetectors can be very expensive. The basic question is how much

information is to be sent and how far does it have to go?With this in mind we will first examine

the basic principles of fiber optics. We will then move on to the various components that make

up a fiber optic communication system, and finally look at the considerations that must be taken

into account in the design of a simple fiber optic link

Figure 4-2A typical fiber optic communication system: T, transmitter; C, connector; S, splice;R, repeater; D, detector, and coils of fibers

Fiber Optic Cable Fabrication

The fabrication of fiber optic cable consists of two processes: preform fabricationandfiber

draw. Preform fabrication involves manufacturing a glass perform consisting of a core and

cladding with the desired index profile of the fiber. Fiber draw involves heating the preform to

about 2000C and drawing it down to the desired diameter and adding a protective buffercoating.

Preform fabricationThe fabrication process for creating the glass preform (Figure 4-3) from which fiber optic cable

is drawn involves forming a glass rod that has the desired index profile and core/cladding

dimension ratio. This process, known as chemical vapor deposition or CVD, was developed by

Corning scientists in the 1970s and has made it possible to create ultra-pure glass fiber suitable

for optical transmission over very long distances. Using the CVD method, the ultra-pure glass

that makes up the preform is synthesized from ultra-pure liquid or gaseous reactants, typically,

silicon chloride (SiCl4), germanium chloride (GeCl4), oxygen, and hydrogen. This reaction


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produces a very fine soot of silicon and germanium oxide, which is then vitrified forming

ultra-pure glass.

Figure 4-3Two views of fiber optic preform fabrication (Sources: UpperFibercore Limited ofChilworth UK, a wholly-owned subsidiary of Scientific Atlanta Inc. of Lawrenceville, Georgia; used by

permission. LowerOFS; used by permission)

There are three processes commonly used to manufacture glass preforms:

1. Outside vapor deposition (OVD): Silicon and germanium particles are deposited on thesurface of a rotating target rod.

2. Inside vapor deposition (IVD): A soot consisting of silicon and germanium particles is

deposited on the inside walls of hollow glass tube.

3. Axial vapor deposition (AVD): Deposition is done axially, directly in the glass preform.

Inside vapor deposition (IVD) and outside vapor deposition (OVD) require a collapse stage to

close the hollow gap in the center of the preform after the soot is deposited. Outside vapor


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deposition (OVD) and axial vapor deposition (AVD) require sintering to vitrify the soot after

they have been deposited.

Outside vapor deposition (OVD)

The OVD process for manufacturing optical fiber typically consists of three stages:

1. Laydown Depositing the glass soot which will eventually form the glass preform

2. Consolidation Heating the glass soot in a furnace to solidify the glass preform

3. Draw Heating up the glass preform and drawing the glass into a fine strand of fiber

In the laydown stage (see Figure 4-4), many fine layers of silicon and germanium soot are

deposited onto a ceramic rod. During the laydown stage, SiCl4and GeCl4 vapors are passed over

the rotating rod and react with oxygen to generate SiO2and GeO2. A traversing burner flame

forms fine soot particles of silica and germania on the rod forming the core and cladding layers

of the fiber. The GeCl4serves as a dopant to increase the index of refraction of the core.

Figure 4-4Outside vapor deposition

The OVD process is distinguished by the method of depositing the soot on the ceramic rod. The

core material is deposited first, followed by the cladding material. Since the core and cladding

materials are deposited using vapor deposition, the entire resulting preform is extremely pure.

When the deposition process is complete, the ceramic rod is removed from the center of the

porous preform and the hollow preform is placed into a consolidation furnace. During the high

temperature consolidation process, water vapor is removed from the preform and sintering

condenses the preform into a solid, dense, transparent rod.


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During the draw process (see Figure 4-5), the

finished glass preform is placed in a draw

tower and drawn into a single continuous

strand of glass fiber. A draw tower consists of

a furnace to heat up the glass preform into

molten glass, a diameter-measuring device

(typically a laser micrometer), a coatingchamber for applying a protective coating, and

a take-up spool for winding the finished fiber.

A typical draw tower can be several stories

tall. The glass preform is first lowered into the

draw furnace. The tip of the preform is then

heated to about 2000C until a piece ofmolten glass (called a gob), begins to fall

due to the force of gravity. When the gob falls,

it pulls behind it a fine glass fiber and cools. A

draw tower operator then cuts off the gob and

threads the fine fiber strand into a tractor

assembly. The tractor assembly speeds up or

slows down to provide tension to the fiber

stand, which controls the diameter of the fiber.Figure 4-5Fiber draw process

The laser-based diameter monitor measures the diameter of the fiber hundreds of times per

second to ensure that the outside diameter of the fiber is held to acceptable tolerance levels

(typically 1 um). As the fiber is drawn, a protective coating is applied and cured using UVlight.

Inside vapor deposition (IVD)

Inside vapor deposition uses a process

known as modified chemical vapor

deposition(MCVD) to deposit the soot

on the inside wallsof a tube of ultra-

pure silica. (See Figure 4-6.) In this

method, a tube of ultra-pure silica is

mounted on a glass-working lathe,

equipped with an oxygen-hydrogen

burner. The chlorides and oxygen are

introduced from one end of the tube,

and caused to react by the heat of theburner. The resulting soot (submicron

particles of silica and germania) isFigure 4-6Inside vapor deposition

deposited inside the tube through a phenomenon known as thermophoresis. As the burner passes

over the deposits, they are vitrified into solid glass. By varying the ratio of silicon and

germanium chloride, the refractive index profile is built layer after layer, from the outside to the

core. The more germanium, the higher the refractive index of the glass. When the deposition

process is complete, the preform is heated to collapse the hollow tube into a solid preform.


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Vapor axial deposition (VAD)

The vapor axial deposition process involves the deposition of glass soot on the end of a rotating

pure silica boule. (See Figure 4-7.) The initial soot deposit forms the core of the preform.

Additional layers of soot are then added radially outward until the final desired refractive index

profile is achieved. The benefit of vapor axial deposition is that no hole is created. This

eliminates the need for both a central ceramic rod (as in OVD) and the need to collapse thepreform to eliminate the hole as in IVD.

Figure 4-7Vapor axial deposition

Total Internal Reflection (TIR)

At the heart of an optical communication system is the optical fiber that acts as the transmission

channel carrying the light beam loaded with information. As mentioned earlier, the guidance of

the light beam (through the optical fiber) takes place because of the phenomenon of total

internal reflection (TIR), which we will now discuss. You learned about critical angles, TIR,

etc. in Module 1-4,Basic Geometrical Optics. You need now to refresh your memory and apply

these ideas more directly to the physics of optical fibers. We first define the refractive index (n)

of a medium:

n=cv (4-1)

where c(3 108m/s) is the speed of light in free space and v represents the velocity of lightin that medium. For example, for light waves, n 1.5 for glass and n 1.33 for water.

Figure 4-8(a) A ray of light incident on a denser medium (n2> n1). (b) A ray incident on a rarermedium (n2< n1). (c) For n2< n1,if the angle of incidence is greater than the critical angle, the incident

ray will undergo total internal reflection.


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As you know, when a ray of light is incident at the interface of two media (like air and glass),

the ray undergoes partial reflection and partial refraction as shown in Figure 4-8a. The vertical

dotted line represents the normal to the surface. The angles 1, 2, and rrepresent the anglesthat the incident ray, refracted ray, and reflected ray make with the normal. According to Snells

law and the law of reflection,

n1sin 1= n2sin 2(Snells law) (4-2)1= r(Law of reflection)

Further, the incident ray, reflected ray, and refracted ray lie in the same plane. In Figure 4-8a,

we know from Snells law that since n2> n1, we must have 2 c), there is no refracted rayand we have total internal reflection TIR. (See Figure 4-8c and Figure 4-10b).

Example 1

For a glass-air interface, n1= 1.5, n2= 1.0, and the critical angle is given by

c= sin1

(1.0/1.5) 41.8

On the other hand, for a glass-water interface, n1= 1.5, n2= 1.33, and

c= sin1(1.33/1.5) 62.5.

Transmission Windows

Optical fiber communication systems transmit information at wavelengths that are in the near-

infrared portion of the spectrum, just above the visible, and thus undetectable to the unaided

eye. Typical optical transmission wavelengths are 850 nm, 1310 nm, and 1550 nm. Both lasers

and LEDs are used to transmit light through optical fiber. Lasers are usually used primarily for

1310 and 1550-nm single-mode applications. LEDs are used for 850 nm multimode

applications.


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Figure 4-9Typical wavelength dependence of attenuation for a silica fiber. Notice that the lowestattenuation occurs at 1550 nm [adapted from Miya, Hasaka, and Miyashita].

Figure 4-9 shows the spectral dependence of fiber attenuation(i.e., dB loss per unit length) as a

function of wavelength of a typical silica optical fiber. The losses are caused by variousmechanisms such as Rayleigh scattering, absorption due to metallic impurities and water in the

fiber, and intrinsic absorption by the silica molecule itself. The Rayleigh scattering loss varies

as 1/04, i.e., longer wavelengths scatter less than shorter wavelengths. (Here 0represents the

free space wavelength.) As we can see in Figure 4-9, Rayleigh scatter causes the dB loss/km to

decrease gradually as the wavelength increases from 800 nm to 1550 nm. The two absorption

peaks around 1240 nm and 1380 nm are primarily due to traces of OHions and metallic ions in

the fiber. For example, even 1 part per million (ppm) of iron can cause a loss of about

0.68 dB/km at 1100 nm. Similarly, a concentration of 1 ppm of OH

ion can cause a loss of 4

dB/km at 1380 nm. This shows the level of purity that is required to achieve low-loss optical

fibers. If these impurities are removed, the two absorption peaks will disappear. For

0> 1600 nm, the increase in the dB/km loss is due to the absorption of infrared light by silicamolecules. This is an intrinsic property of silica, so no amount of purification can remove this

infrared absorption tail.

As you see, there are two windows at which the dB/km loss attains its minimum value. The first

window is around 1300 nm (with a typical loss coefficient of less than 1 dB/km) where,

fortunately (as we will see later), the material dispersion is negligible. However, the loss

coefficient is at its absolute minimum value of about 0.2 dB/km around 1550 nm. The latter

window has become extremely important in view of the availability of erbium-doped fiber

amplifiers.


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The Optical Fiber

An optical fiber (Figure 4-10) consists of a central glass core of radius a surrounded by an

outer cladding made of glass with a slightly lower refractive index. The corresponding

refractive index distribution (in the transverse direction) is given by:

1

2

for

for

n n r a

n n r a

=

(4-4)

Figure 4-10(a) A glass fiber consists of a cylindrical central core surrounded by a cladding material of

slightly lower refractive index. (b) Light rays impinging on the core-cladding interface at an angle greater than the critical angle care trapped inside the core of the fiber and reflected back and forth (A,C, B, etc.) along the core-cladding interface.

Figure 4-10 shows a light ray incident on the air-core left interface at an angle i. The rayrefracts at angle in accordance with Snells law and then strikes the core-cladding interface atangle . In the drawing shown, the angle is greater than the critical angle cdefined inEquation 4-3, thus leading to total internal reflection atA. The reflected ray is totally internally

reflected again at CandBand so on, remaining trapped in the fiber as it propagates along the

core axis.The core diameter d= 2aof a typical telecommunication-grade multimode fiber is

approximately 62.5 m with an outer cladding diameter of 125 um. The cladding indexn2is approximately 1.45 (pure silica), and the core index n1, barely larger, around 1.465. The

cladding is usually pure silica while the core is usually silica doped with germanium, which

increases the refractive index slightly from n2to n1. The core and cladding are fused together

during the manufacturing process and typically not separable. An outside plastic buffer is

usually added to protect the fiber from environmental contaminants.


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Numerical aperture

One of the more important parameters associated with fiber optics is the numerical aperture.

The numerical aperture of a fiber is a measure of its light-gathering abilityand is defined by

N.A. = Sin (a)max (4-5)

where (a)maxis the maximum half-acceptance angle of the fiber, as shown in Figure 4-11.

Figure 4-11Numerical aperture

The larger the numerical aperture, the greater the light gathering ability of the fiber. Typical

values for N.A. are between 0.2 and 0.3 for multimode fiber and 0.1 to 0.2 for single-mode

fiber. The numerical aperture is an important quantity because it is used to determine the

coupling and dispersion characteristics of a fiber. For example, a large numerical aperture

allows for more light to be coupled into the fiber but at the expense of modal dispersion, whichcauses pulse spreading and ultimately bandwidth limitations.

The numerical aperture (N.A.) is related to the index of refraction of the core and cladding by

the following equation:

2 2max 1 2N.A. sin( )a n n= = (4-6)

As can be seen, the larger the difference between the core and cladding index, the larger the

numerical aperture and hence more modal dispersion.

The N.A. may also be expressed in terms of the relative refractive index differencetermed ,

where2 2

1 2

21

( )

2

n n

n

(4-7)

so that, with Equation 4-6, we get Equation 4-8.

2

21

(N.A.)

2n = (4-8)


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Combining Equations 4-6 and 4-8, we obtain a useful relation, Equation 4-9.

max 1N.A. sin( ) 2a n= = (4-9)

In short, a large N.A. represents a large difference in refractive index, leading to a large

acceptance angle and hence a large numerical aperture. However, this can lead to serious

bandwidth limitations. Typical values for range from 0.01 to 0.03 or 1 to 3 %

Example 2

For a typical step-index (multimode) fiber with core index n11.45 and 0.01, we get

sin(a)max= 1 2 1.45 2 (0.01) 0.205n = =

so that (a)max12. Thus, all light entering the fiber must be within a cone of half-angle 12. Thefull acceptance angle is 2 12 = 24.

Fiber Optic Loss Calculations

Lossin a fiber optic system is expressed in terms of the optical power available at the output

end with respect to the optical power at the input. As follows:

Loss = out

in

P

P (4-10)

wherePinis the input power to the fiber andPoutis the power available at the output end of the

fiber. For convenience,fiber optic lossis often expressed in terms of decibels (dB) where

LossdB= 10 logout

in

P

P (4-11)

Fiber optic cable manufacturers usually specify loss in optical fiber in terms of decibels per

kilometer (dB/km), as discussed earlier in connection with Figure 4-9.

Example 3

A fiber of 50-km length hasPin= 10 mW andPout= 1 mW. Find the loss in dB/km.

From Equation 4-11

dB

1 mWLoss 10log 10dB

10 mW

= =

(The negative sign indicates a loss.)

And so the loss per unit length of fiber, dB/km, is equal to

Loss(dB/km) = (10 dB/50 km) = 0.2 dB/km


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Example 4

A 10-km fiber optic communication system link has a fiber loss of 0.30 dB/km. Find the output

power if the input power is 20 mW.

Solution

From Equation 4-11, making use of the relationship thaty = 10xifx = logy,

outdB

in

dB out

in

Loss 10 log

Loss log

10

P

P

P

P

=

=

which becomes, then,

dBLoss

out10

in

10P

P=

.

So, finally, we have

dB

Loss

10out in 10P P= (4-12)

For fiber with a 0.30-dB/km loss characteristic, the lossdBfor 10 km of fiber becomes

LossdB= 10 km (0.30 dB/km) = 3 dB

Plugging this back into Equation 4-12 we get,

3

10out 20 mW 10 10 mWP

= =

Optical power in fiber optic systems is often expressed in terms of dBm, a decibel term that

references power to a 1 mWatt (milliwatt) input power level. Optical power here can refer to the

power of a laser source or just to the power somewhere in the system. IfPin in Equation 4-11 is

given as 1 milliwatt, then the power in dBm can be determined using equation 4-13:

out(dBm) 10 log1 mW

PP

=

(4-13)

With optical power expressed in dBm, output power anywhere in the system can be determined

simply by expressing the input power in dBm and then subtracting the individual component

losses, also expressed in dB. It is important to note that an optical source with a power input of1 mW can be expressed as 0 dBm, as indicated by Equation 4-13, since

10log1 mW

1 mW

= 10log(1) = (10)(0) = 0.

The use of decibels provided a convenient method of expressing optical power in fiber optic

systems. For example, for every 3 dB loss in optical power, the power in milliwatts is cut in

half. Consequently, for every 3-dB increase in optical power, the optical power in milliwatts is

doubled. For example, a 3-dBm optical source has aPof 2 mW, whereas a 6-dBm source has a


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Pof 0.25 mW, as can be verified with Equation 4-13. Furthermore, every increase or decrease

of 10 dB in optical power corresponds to a 10-fold increase or decrease in optical power in

expressed in milliwatts. For example, whereas 0 dBm corresponds to 1-milliwatt of optical

power, 10 dBm would be 10 milliwatts, and 20 dBm would be 100 milliwatts. Similarly,

10 dBm corresponds to 0.1 milliwatt, and 20 dBm would be 0.01 milliwatts, etc.

Example 5

A 3-km fiber optic system has an input power of 2 mW and a loss characteristic of 2 dB/km.

Determine the output power of the fiber optic system in mW.

Solution

Using Equation 4-13, we convert the source power of 2 mW to its equivalent in dBm:

dBm

2 mWInput power 10log 3 dBm

1 mW

= = +

The lossdBfor the 3-km cable is,LossdB= 3 km 2 dB/km = 6 dB

Thus, power in dB is

(Output power)dB= +3 dBm 6 dB = 3 dBm

Using Equation 4-13 to convert the output power of 3 dBm back to milliwatts, we have

(mW)(dBm) = 10 log

1 mW

PP

or(dBm) (mW)

log10 1 mW

P P= ,

or(dBm)

10(mW)

= 101(mW)

PP

so that(dBm)

10(mW) = 1 mW 10P

P

Plugging in forP(dBm) = 3 dBm, we get for the output power in milliwatts

310

out(mW) = 1 mW 10 = 0.5 mWP

Note that one can also use Equation 4-12 to get the same result, where nowPin= 2 mW andLossdB= 6 dB:

P Pout in

LossdB

10 = 10

or6

10

out =2 mW 10P = 0.5 mW, the same as above.


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Power budget

When designing a fiber optic communication system, one of the main factors that must be

considered is whether or not there is enough power available at the receiver to detect the

transmitted signal after all of the system losses have been accounted for. The process for

accounting for all of the system losses is called apower budget.

The power arriving at the detector must be sufficient to allow clean detection with few errors.

Clearly, the signal at the receiver must be larger than the noise level. The power at the detector,

Pr, must be above the threshold level or receiver sensitivityPs.

PrPs (4-14)

The receiver sensitivityPsis the signal power, in dBm, at the receiver that results in a particular

bit error rate (BER). Typically the BER is chosen to be at most one error in 1012

bits or 1012

.

Example 6

A receiver has sensitivityPsof 45 dBm for a BER of 1012

. What is the minimum power that mustbe incident on the detector?

Solution

Use Equation 4-13 to find the source power in milliwatts, given the power sensitivity in dBm. Thus,

45 dBm = 10 log1 mW

P

or45 dBm

10101 mW

P

= ,

so thatP= (1 mW) 104.5= 3.16 105mW = 31.6 microwatts

for a probability of error of 1 in 1012.

The received power at the detector is a function of:

1. Power emanating from the light source (PL)

2. Source-to-fiber loss (Lsf)

3. Fiber loss per km (FL) for a length of fiber (L)

4. Connector or splice losses (Lconn)5. Fiber-to-detector loss (Lfd)

Thepower marginis the difference between the received powerPrand the receiver sensitivityPs

by some marginLm.

Lm=PrPs (4-15)

whereLmis the loss margin in dB,Pris the received power, andPs is the receiver sensitivity in

dBm.


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If all of the loss mechanisms in the system are taken into consideration, the loss margin can be

expressed as Equation 4-16.

Lm=PLLsf (FLL) LconnLfdPs (4-16)

All units are in dB and dBm.

Example 7

A system has the following characteristics:

Source power (PL) = 2 mW (3 dBm)

Source to fiber loss (Lsf) = 3 dB

Fiber loss per km (FL) = 0.5 dB/km

Fiber length (L) = 40 km

Connector loss (Lconn) = 1 dB (one connector between two 20-m fiber lengths)

Fiber to detector loss (Lfd) = 3 dB

Receiver sensitivity (Ps) = 36 dBm

Find the loss marginLm.

Solution

Lm= 3 dBm 3 dB (40 km 0.5 dB/km) 1 dB 3 dB (36 dBm) = 12 dB

This particular fiber optic loss budget is illustrated in Figure 4-12, with each loss graphically

depicted.

Figure 4-12Fiber optic loss budget


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Types of Optical Fiber

There are three basic types of fiber optic cable used in communication systems: step-index

multimode, step-index single-mode, and graded-index multimode. These are illustrated in

Figure 4-13.

Figure 4-13Types of fiber

Step-index multimode fiber

Step-index multimode fiber has an index of refraction profile that steps from low-to-high-to-

low as measured from cladding-to-core-to-cladding. A relatively large core diameter 2aand

numerical aperture N.A. characterize this fiber. The core/cladding diameter of a typical

multimode fiber used for telecommunication is 62.5/125 m (about the size of a human hair).The term multimode refers to the fact that multiple modesorpathsthrough the fiber are

possible, as indicated in Figure 4-13a. Step-index multimode fiber is used in applications that

require high bandwidth (< 1 GHz) over relatively short distances (< 3 km) such as a local area

network or a campus network backbone.

The major benefits of multimode fiber are: (1) it is relatively easy to work with; (2) because of

its larger core size, light is easily coupled to and from it; (3) it can be used with both lasers and

LEDs as sources; and (4) coupling losses are less than those of the single-mode fiber. The

drawback is that because many modes are allowed to propagate (a function of core diameter,

wavelength, and numerical aperture) it suffers from intermodal dispersion, which will be

discussed in the next section. Intermodal dispersion limits bandwidth, which translates into

lower data rates.


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Step-index single-mode fiber

Single-mode step-index fiber (Figure 4-13b) allows for only one path, or mode, for light to

travel within the fiber. In a multimode step-index fiber, the number of modesMnpropagating

can be approximated by

2

n2

VM = (4-17)

Here Vis known as the normalized frequency, or the V-number, which relates the fiber size, the

refractive index, and the wavelength. The V-number is given by Equation 4-18.

2N.A.

aV

= (4-18)

or by Equation 4-19.

1

22

aV n

= (4-19)

In either equation, ais the fiber core radius, is the operating wavelength, N.A. is thenumerical aperture, n1is the core index, and is the relative refractive index difference betweencore and cladding.

The analysis of how the V-number is derived is beyond the scope of this module. But it can be

shown that by reducing the diameter of the fiber to a point at which the V-number is less than

2.405, higher-order modes are effectively extinguished and single-mode operation is possible.

Example 8

What is the maximum core diameter for a fiber if it is to operate in single mode at a wavelength of

1550 nm if the N.A. is 0.12?From Equation 4-18,

2N.A.

aV

=

Solving for a yields

a=2 (N.A.)

V

For single-mode operation, Vmust be 2.405 or less. The maximum core diameter occurs whenV= 2.405. So, plugging into the equation, we get

amax= (2.405)(1550 nm)(2 )(0.12)

= 4946 109m = 4.95 m

or, dmax= 2 a= 9.9 m

The core diameter for a typical single-mode fiber is between 5 and 10 m with a 125-m

cladding. Single-mode fibers are used in applications such as long distance telephone lines, wide-

area networks (WANs), and cable TV distribution networks where low signal loss and high data


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rates are required and repeater/amplifier spacing must be maximized. Because single-mode fiber

allows only one mode or ray to propagate (the lowest-order mode), it does not suffer from

intermodal dispersion like multimode fiber and therefore can be used for higher bandwidth

applications. At higher data rates, however, single-mode fiber is affected by chromaticdispersion,

which causes pulse spreading due to the wavelength dependence on the index of refraction of

glass (to be discussed in more detail in the next section). Chromatic dispersion can be overcome

by transmitting at a wavelength at which glass has a fairly constant index of refraction (~1300 nm)or by using an optical source such as a distributed-feedback laser (DFB laser) that has a very

narrow output spectrum. The major drawback of single-mode fiber is that compared to step-index

multimode fiber, it is relatively difficult to work with (i.e., splicing and termination) because of its

small core size and small numerical aperture. Because of the high coupling losses associated with

LEDs, single-mode fiber is used primarily with laser diodes as a source.

Graded-index fiber

In a step-index fiber, the refractive index of the core has a constant value. By contrast, in a

graded-index fiber, the refractive index in the core decreases continuously(in a parabolic

fashion) from a maximum value at the center of the core to a constant value at the core-claddinginterface. (See Figure 4-13c.) Graded-index fiber is characterized by its ease of use (i.e., large

core diameter and N.A.), similar to a step-index multimode fiber, and its greater information

carrying capacity, as in a step-index single-mode fiber. Light traveling through the center of the

fiber experiences a higher index of refraction than does light traveling in higher modes. This

means that even though the higher-order modes must travel farther than the lower order modes,

they travel faster, thus decreasing the amount of modal dispersion and increasing the bandwidth

of the fiber.

Polarization-maintaining fiber

Polarization-maintaining (PM) fiber is atype of fiber that only allows light of a

specific polarization orientation to

propagate. It is often referred to as high

birefringence single-mode fiber. (A

birefringent material is one in which the

refractive index is different for two

orthogonal orientations of the light

propagating through it.) In birefringent

fiber, light polarized in orthogonal

directions will travel at different speeds

along the polarization axes of the fiber. PM

fibers utilize a stress-induced birefringence

mechanism to achieve high levels of

birefringence. These fibers embed a stress-

applying region in the cladding area of the

fiber. (See Figure 4-14.) Placed

symmetrically about the core, it gives theFigure 4-14Polarization maintaining fiber


28/57


fiber cross-section two distinct axes of symmetry. The stress region squeezes on the core along

one axis, which makes the core birefringent. As a result, the propagation speed is polarization-

dependent, differing for light polarized along the two orthogonal symmetry axes. Birefringence

is the key to polarization-maintaining behavior. Because of the difference in propagation speed,

light polarized along one symmetry axis is not efficiently coupled to the other orthogonal

polarizationeven when the fiber is coiled, twisted or bent. PM fibers can be designed with

high stress levels to create birefringence sufficient to resist depolarization under harshmechanical and thermal operating conditions.

Fiber Optic Cable Design

In most applications, optical fiber is protected from the environment by using a variety of

different cabling types based on the type of environment in which fiber will be used. Cabling

provides the fiber with protection from the elements, added tensile strength for pulling, rigidity

for bending, and durability. As fiber is drawn from the preform in the manufacturing process, a

protective coating, a UV-curable acrylate, is applied to protect against moisture and to provide

mechanical protection during the initial stages of cabling. A secondary buffer then typically

encases the optical fibers for further protection.

Fiber optic cable can be separated into two types: indoorand outdoorcables. (See Table 4-1.)

Table 4-1. Indoor and Outdoor Cables

Indoor Cable Description

Simplex Cables Contains a single fiber for one-way communication

Duplex Cables Contains two fibers for two-way communications

Multifiber Cables Contains more than two fibers. Fibers are usually in pairs for duplex operation.For example, a twelve-fiber cable permits six duplex circuits.

Breakout Cables Typically have several individual simplex cables inside an outer jacket. Theouter jacket includes a ripcord to allow easy access

Heavy, Light,Plenum-Duty, andRiser Cable

Heavy-duty cables have thicker jackets than light duty cable for rougherhandling

Plenum cables are jacketed with low-smoke and fire retardant materials

Riser cables run vertically between floors and must be engineered to preventfires from spreading between floors

Outdoor Cables Outdoor cables must withstand more harsh environment conditions thanindoor cables.

Overhead Cables strung from telephone lines

Direct Burial Cables placed directly in a trench

Indirect Burial Cables placed in a conduitSubmarine Underwater cable, including transoceanic applications

Most telecommunication applications employ either a loose-tube, tight buffer, or ribbon-cable

design.Loose tube cable is used primarily in outside-plant applications that require high pulling

strength, resistance to moisture, large temperature ranges, low attenuation, and protection from

other environmental factors. (See Figure 4-15.) Loose-tube buffer designs allow easy drop-off

of groups of fibers at intermediate points. A typical loose-tube cable can hold up to 12 fibers,

with a cable capacity of more than 200 fibers. In a loose-tube cable design, color-coded plastic


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buffer tubes are filled with a gel to provide protection from water and moisture. The fact that the

fibers float inside the tube provides additional isolation from mechanical stress such as pull

force and bending introduced during the installation process. Loose-tube cables can be either all

dielectric, or armored. In addition, the buffer tubes are stranded around a dielectric or steel

central member which serves as an anti-buckling element. The cable core is typically

surrounded by aramid fibers to provide tensile strength to the cable. For additional protection, a

medium-density outer polyethylene jacket is extruded over the core. In armored designs,corrugated steel tape is formed around a single-jacketed cable with an additional jacket extruded

over the armor.

Figure 4-15Loose tube direct burial cable

Tight buffer cableis typically used for indoor applications where ease of cable termination and

flexibility are more of a concern than low attenuation and environmental stress. (SeeFigure 4-16.) In a tight-buffer cable, each fiber is individually buffered (direct contact) with an

elastomeric material to provide good impact resistance and flexibility, while keeping size at a

minimum. Aramid fiber strength members provide the tensile strength for the cable. This type of

cable is suited for jumper cables, which typically connect loose-tube cables to active

components such as lasers and receivers. Tight-buffer fiber may introduce slightly more

attenuation due to the stress placed on the fiber by the buffer. However, because tight-buffer

cable is typically used for indoor applications, distances are generally much shorter that for

outdoor applications allowing systems to tolerate more attenuation in exchange for other

benefits.


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Figure 4-16Tight buffer simplex and duplex cable

Ribbon cableis used in applications where fibers must be densely packed. (See Figure 4-17.)

Ribbon cables typically consist of up to 18-coated fibers that are bonded or laminated to form a

ribbon. Many ribbons can then be combined to form a thick, densely packed fiber cable that can

be either mass-fusion spliced or terminated using array connectors that can save a considerable

amount of time as compared to loose-tube or tight-buffer designs.

Figure 4-17Loose tube ribbon cable


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Cabling example

Figure 4-18 shows an example of an interbuilding cabling scenario.

Figure 4-18Interbuilding cabling scenario

Dispersion

In digital communication systems, information to be sent is first coded in the form of pulses.

These pulses of light are then transmitted from the transmitter to the receiver, where the

information is decoded. The larger the number of pulses that can be sent per unit time and still

be resolvable at the receiver end, the larger will be the transmission capacity, or bandwidthof

the system. A pulse of light sent into a fiber broadens in time as it propagates through the fiber.

This phenomenon is known asdispersion,and is illustrated in Figure 4-19.


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Figure 4-19Pulses separated by 100 ns at the input end would be resolvable at the output end of 1 kmof the fiber. The same pulses would not be resolvable at the output end of 2 km of the same fiber.

Calculating dispersion

Dispersion, termed t, is defined as pulse spreading in an optical fiber. As a pulse of lightpropagates through a fiber, elements such as numerical aperture, core diameter, refractive index

profile, wavelength, and laser linewidth cause the pulse to broaden. This poses a limitation on

the overall bandwidth of the fiber as demonstrated in Figure 4-20.

Figure 4-20Pulse broadening caused by dispersion

Dispersion tcan be determined from Equation 4-20.

t= (tout tin)1/2 (4-20)

Dispersion is measured in units of time, typically nanoseconds or picoseconds. Total dispersion

is a function of fiber length, ergo, the longer the fiber, the more the dispersion. Equation 4-21

gives the total dispersion per unit length.ttotal= L (Dispersion/km) (4-21)

The overall effect of dispersion on the performance of a fiber optic system is known as

intersymbol interference, as shown in Figure 4-19. Intersymbol interference occurs when the

pulse spreading due to dispersion causes the output pulses of a system to overlap, rendering

them undetectable. If an input pulse is caused to spread such that the rate of change of the input

exceeds the dispersion limit of the fiber, the output data will become indiscernible.


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Intermodal dispersion

Intermodal dispersion is the pulse spreading caused by the time delay between lower-order

modes (modes or rays propagating straight through the fiber close to the optical axis) and

higher-order modes (modes propagating at steeper angles). This is shown in Figure 4-21. Modal

dispersion is problematic in multimode fiber and is the primary cause for bandwidth limitation.

It is not a problem in single-mode fiber where only one mode is allowed to propagate.

Figure 4-21Mode propagation in an optical fiber

Chromatic dispersion

Chromatic dispersion is pulse spreading due to the fact that different wavelengths of light

propagate at slightly different speeds through the fiber. All light sources, whether laser or LED,

have finite linewidths, which means they emit more than one wavelength. Because the index of

refraction of glass fiber is a wavelength-dependent quantity, different wavelengths propagate at

different speeds. Chromatic dispersion is typically expressed in units of nanoseconds or

picoseconds per (km-nm).

Chromatic dispersion consists of two parts: material dispersionand waveguide dispersion.

tchromatic= tmaterial+ twaveguide (4-22)

Material dispersionis due to the wavelength dependency on the index of refraction of glass.

Waveguide dispersionis due to the physical structure of the waveguide. In a simple step-index-

profile fiber, waveguide dispersion is not a major factor, but in fibers with more complex index

profiles, waveguide dispersion can be more significant. Material dispersion and waveguide

dispersion can have opposite signs (or slopes) depending on the transmission wavelength. In the

case of a step-index single-mode fiber, these two effectively cancel each other at 1310 nm

yielding zero-dispersion, which makes high-bandwidth communication possible at this

wavelength. The drawback, however, is that even though dispersion is minimized at 1310 nm,

attenuation is not. Glass fiber exhibits minimum attenuation at 1550 nm. Glass exhibits its

minimum attenuation at 1550 nm, and optical amplifiers (known as erbium-doped fiberamplifiers [EDFA]) also operate in the 1550-nm range. It makes sense, then, that if the zero-

dispersion property of 1310 nm could be shifted to coincide with the 1550-nm transmission

window, very high-bandwidth long-distance communication would be possible. With this in

mind, zero-dispersion-shifted fiberwas developed.

Zero-dispersion-shifted fibershifts the zero dispersion wavelength of 1310 nm to coincide

with the 1550 nm transmission window of glass fiber by modifying the waveguide dispersion

slope. Modifying the waveguide dispersion slope is accomplished by modifying the refractive


34/57


index profile of the fiber in a way that yields a more negative waveguide-dispersion slope.

When combined with a positive material dispersion slope, the point at which the sum of two

slopes cancel each other out can be shifted to a higher wavelength such as 1550 nm or beyond.

(See Figure 4-22.)

Figure 4-22Single-mode versus dispersion-shifted fiber

An example of a zero-dispersion-shifted fiber is the W-profile fiber, named because of the

shape of the refractive index profile which looks like a W. This is illustrated in Figure 4-23.

By splicing in short segments of a dispersion-shifted fiber with the appropriate negative slope

into a fiber optic system with positive chromatic dispersion, the pulse spreading can be

minimized. This results in an increase in data rate capacity.

Figure 4-23W-profile fibers: (a) step-index, (b) triangular profile

In systems where multiple wavelengths are transmitted through the same single-mode fiber,

such as in dense wavelength division multiplexing(DWDM, discussed in a later section), it is

possible for three equally spaced signals transmitted near the specified zero-dispersionwavelength to combine and generate a new fourth wave, which can cause interference between

channels. This phenomenon is calledfour-wave mixing,which degrades system performance. If,

however, the waveguide structure of the fiber is modified so that the waveguide dispersion is

further increased in the negative direction, the zero-dispersion point can be pushed out past

1600 nm (outside the EDFA operating window). This results in a fiber in which total chromatic

dispersion is still substantially lower in the 1550 nm range without the threat of performance

problems. This type of fiber is known as nonzero dispersion-shifted fiber.


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The total dispersion of an optical fiber, ttot, can be approximated using

2 2

total modal chromatict t t = + (4-23)

where tmodalrepresents the dispersion due to the various components that make up the system.The transmission capacity of fiber is typically expressed in terms of bandwidthdistance. For

example, the (bandwidth distance) product for a typical 62.5/125-m (core/cladding diameter)multimode fiber operating at 1310 nm might be expressed as 600 MHz km.

The approximate bandwidth BW of a fiber can be related to the total dispersion by the following

relationship:

BW (Hz) = 0.35/ttotal (4-24)

Example 9

A 2-km-length multimode fiber has a modal dispersion of 1 ns/km and a chromatic dispersion of

100 ps/km

nm. It is used with an LED of linewidth 40 nm. (a) What is the total dispersion?(b) Calculate the bandwidth (BW) of the fiber.

(a) tmodal= 2 km 1 ns/km = 2 ns

tchromatic = (2 km) (100 ps/kmnm) (40 nm) = 8000 ps = 8 nsNow, from Equation 4-23,

ttotal = ([2ns]2 + [8 ns]2)1/2= 8.25 nsAnd from Equation 4-24,

(b) BW = 0.35/ttotal= 0.35/8.25 ns = 42.42 MHz

Expressed in terms of the product (BWkm), we get (BWkm) = (42.4 MHz)(2 km) 85 MHzkm.

Example 10

A 50-km single-mode fiber has a material dispersion of 10 ps/km nm and a waveguide dispersion

of 5 ps/kmnm. It is used with a laser source of linewidth 0.1 nm. (a) What is tchromatic? (b) Whatis ttotal? (c) Calculate the bandwidth (BW) of the fiber.

(a) With the help of Equation 4-22, we get

tchromatic= 10 ps/kmnm 5 ps/kmnm = 5 ps/kmnm

(b) For 50 km of fiber at a linewidth of 0.1 nm, ttotalis

ttotal = (50 km) (5 ps/kmnm) (0.1 nm) = 25 ps

(b) BW = 0.35/ttotal= 0.35/25 ps = 14 GHz

Expressed in terms of the product (BW km), we get

(BW km) = (14 GHz)(50 km) = 700 GHz km

In short, the fiber in this example could be operated at a data rate as high as 700 GHz over a one-

kilometer distance.


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Fiber Optic Sources

Two types of light sources are commonly used in fiber optic communications systems:

semiconductor laser diodes(LD) and light-emitting diodes(LED). Each device has its own

advantages and disadvantages as listed in Table 4-2.

Table 4-2. LED Versus Laser

Characteristic LED Laser (LD)

Output power Lower Higher

Spectral width Wider Narrower

Numerical aperture Larger Smaller

Speed Slower Faster

Cost Less More

Ease of operation Easier More difficult

Fiber optic sources must operate in the low-loss transmission windows of glass fiber. LEDs are

typically used at the 850-nm and 1310-nm transmission wavelengths, whereas lasers areprimarily used at 1310 nm and 1550 nm.

LEDs

LEDs are typically used in lower-data-rate, shorter-distance multimode systems because of their

inherent bandwidth limitations and lower output power. They are used in applications in which

data rates are in the hundreds of megahertz as opposed to GHz data rates associated with lasers.

Two basic structures for LEDs are used in fiber optic systems:surface-emittingand edge-

emittingas shown in Figure 4-24.

Figure 4-24Surface-emitting versus edge-emitting diodes

LEDs typically have large numerical apertures, which makes light coupling into single-modefiber difficult due to the fibers small N.A. and core diameter. For this reason LEDs are most

often used with multimode optical fiber. LEDs are used in lower-data-rate, short-distance

(


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LEDs are available with connector-ready housings that allow a connectorized fiber to be

directly attached and are relatively inexpensive compared to laser diodes. LEDs are used in

applications including local area networks, closed-circuit TV, and where transmitting electronic

data in areas where EMI may be a problem.

Laser diodesLaser diodes are used in applications in which longer distances and higher data rates are

required. Because an LD has a much higher output power than an LED, it is capable of

transmitting information over longer distances. Consequently, and given the fact that the LD has

a much narrower spectral width, it can provide high-bandwidth communication over long

distances. The LDs smaller N.A. also allows it to be more effectively coupled with single-mode

fiber. The difficulty with LDs is that they are inherently nonlinear, which makes analog

transmission more difficult. They are also very sensitive to fluctuations in temperature and drive

current, which causes their output wavelength to drift. In applications such as wavelength-

division multiplexing in which several wavelengths are being transmitted down the same fiber,

the wavelength stability of the source becomes critical. This usually requires complex circuitry

and feedback mechanisms to detect and correct for drifts in wavelength. The benefits, however,of high-speed transmission using LDs typically outweigh the drawbacks and added expense.

In high-speed telecommunications applications, specially designed single-frequencydiode lasers

that operate with an extremely narrow output spectrum (< .01 nm) are required. These are

known as distributed-feedback (DFB)laser diodes (Figure 4-25). In DFB lasers, a corrugated

structure, or diffraction grating, is fabricated directly in the cavity of the laser, allowing only

light of a very specific wavelength to oscillate. This yields an output wavelength spectrum that

is extremely narrowa characteristic required for dense wavelength division-multiplexing

(DWDM) systems in which many closely spaced wavelengths are transmitted through the same

fiber. Distributed-feedback lasers are available at fiber optic communication wavelengths

between 1300 nm and 1550 nm.

Figure 4-25Fourteen-pin butterfly mount distributed feedback laser diode (Source: JDS UniphaseCorporation; used by permission)


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Fiber Optic Detectors

The purpose of a fiber optic detector is to convert light emanating from the optical fiber back

into an electrical signal. The choice of a fiber optic detector depends on several factors

including wavelength, responsivity, and speed or rise time. Figure 4-26 depicts the various types

of detectors and their spectral responses.

Figure 4-26Detector spectral response

The process by which light energy is converted into an electrical signal is the opposite of the

process by which an electrical signal is converted into light energy. Light striking the detector

generates a small electrical current that is amplified by an external circuit. Photons absorbed in

the PN junction of the detector excite electrons from the valence band to the conduction band,resulting in the creation of an electron-hole pair. Under the influence of a bias voltage these

carriers move through the material and induce a current in the external circuit. For each

electron-hole pair created, the result is an electron flowing in the circuit. Current levels are

usually small and require some amplification as shown in Figure 4-27.

Figure 4-27Typical detector amplifier circuit


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The most commonly used photodetectors in fiber optic communication systems are the PIN and

avalanche photodiodes (APD). The material composition of the device determines the

wavelength sensitivity. In general, silicon devices are used for detection in the visible portion of

the spectrum. InGaAs crystals are used in the near-infrared portion of the spectrum between

1000 nm and 1700 nm. Germanium PIN and APDs are used between 800 nm and 1500 nm.

Table 4-3 gives some typical photodetector characteristics:

Table 4-3. Typical Photodetector Characteristics

Photodetector Wavelength (nm) Responsivity (A/W) Dark Current (nA) Rise Time (ns)

Silicon PIN 2501100 0.11.0 110 0.07

InGaAs PIN 13101625 0.30.85 0.11 0.03

InGaAs APD 13101625 0.71.0 30200 0.03

Some of the more important detector parameters listed below in Table 4-4 are defined and

described in Module 1-6, Optical Detectors and Human Vision.

Table 4-4. Photodetector Parameters

Parameter Description

Responsivity The ratio of the electrical power to the detectors output optical power

Quantumefficiency

The ratio of the number of electrons generated by the detector to thenumber of photons incident on the detector

Quantum efficiency = (Number of electrons)/Photon

Dark current The amount of current generated by the detector with no light applied.

Dark current increases about 10% for each temperature increase of 1Cand is much more prominent in Ge and InGaAs at longer wavelengthsthan in silicon at shorter wavelengths.

Noise floor The minimum detectable power that a detector can handle. The noisefloor is related to the dark current since the dark current will set the lowerlimit.

Noise floor = Noise (A)/Responsivity (A/W)

Response Time The time required for the detector to respond to an optical input. Theresponse time is related to the bandwidth of the detector by

BW = 0.35/tr

where tris the rise time of the device. The rise time is the time requiredfor the detector to rise to a value equal to 63.2% of its final steady-statereading.

Noise equivalentpower (NEP)

At a given modulation frequency, wavelength, and noise bandwidth, NEPis the incident radiant power that produces a signal-to-noise ratio of oneat the output of the detector


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Connectors

In the 1980s, there were many different types and manufacturers of connectors.Some remain in

production, but much of the industry has shifted to standardized connector types, with details

specified by standards organizations such as the Telecommunications Industry Association, the

International Electro-technical Commission, and the Electronic Industry Association. Today,

there are many different types of connectors available for fiber optics depending on theapplication. Some of the more common types are shown in Table 4-5:

Table 4-5. Fiber Optic Connector Types(Source of photos: JDS Uniphase Corporation; used by permission)

Type Description Diagram

SC Snap-in Single-Fiber Connector: A squarecross section allows high packing density onpatch panels and makes it easy to package ina polarized duplex form that assures the fibersare matched to the proper fibers in the matedconnector. Used in premise cabling, ATM,fiber-channel, and low-cost FDDI. Available insimplex and duplex configurations.

ST Twist-on Single-Fiber Connector: The mostwidely used and broadly used type ofconnector for data communicationsapplications. A bayonet-style twist and lockcoupling mechanism allows for quick connectsand disconnects, and a spring-loaded 2.5 mmdiameter ferrule for constant contact between

mating fibers

LC Small Form Factor Connector: Similar to SCconnector but designed to reduce systemcosts and connector density.

FC Twist-on Single-Fiber Connector: Similar to the

ST connector and used primarily in thetelecommunications industry. A threadedcoupling and tunable keying allows ferrule tobe rotated to minimize coupling loss.


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Regardless of the type of connector used, compatibility between connectors produced by different

manufacturers is essential. This does not necessarily mean that the connectors are identical.

Connectors produced by different manufacturers may differ in the number of parts, ease and

method of terminations, material used, and whether epoxy is used. Connectors may also differ in

their performance involving insertion loss, durability, return loss, temperature range, etc.

Single-mode and multimode connectors may also vary in terms of ferrule bore tolerance. Astandard 125-um single-mode requires a more exacting fit to minimize insertions loss, whereas a

multimode fiber with its larger core may be more forgiving. A typical multimode connector may

have a bore diameter as large as 127 um to accommodate the largest fiber size. A single-mode

connector, however, may be specified with a bore diameter of 125, 126, or 127 um to ensure a

more precise fit.

Fiber Optic Couplers

A fiber optic coupleris a device used to connect a single (or multiple) fiber to many other

separate fibers. There are two general categories of couplers: Star couplers (Figure 4-28a)

T-couplers (Figure 4-28b)

Figure 4-28(a) Star coupler (b) T-coupler

Star couplers

In a star coupler, each of the optical signals sent into the coupler are available at all of the

output fibers (Figure 4-28a). Power is distributed evenly. For an n nstar coupler (n-inputs andn-outputs), the power available at each output fiber is 1/nthe power of any input fiber.

The output power from a star coupler is simply

Po=Pin/n (4-25)

where n= number of output fibers.

The power division(or power splitting ratio)PDstin decibels is given by Equation 4-26.

PDst(dB) = 10 log(1/n) (4-26)


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The power division in decibels gives the number of decibels apparently lost in the coupler from

single input fiber to single fiber output. Excess power loss(Lossex) is the power lost from input

to totaloutput, as given in Equation 4-27 or 4-28.

outex

in

(total)Loss

P

P= (4-27)

outex/dB

in

(total)Loss 10log

P

P= (4-28)

Example 11

An 8 8 star coupler is used in a fiber optic system to connect the signal from one computer toeight terminals. The power at an input fiber to the star coupler is 0.5 mW. Find (1) the power at each

output fiber and (2) the power division in decibels.

Solution

(1) The 0.5-mW input is distributed to eight fibers. Each has (0.50 mW)/8 = 0.0625 mW.

(2) The power division, in decibels, from Equation 4-26 is

PDst= 10 log(1/8) = 9.0 dB

Example 12

A 10 10 star coupler is used to distribute the 3-dBm power of a laser diode to 10 fibers. Theexcess loss (Lossex) of the coupler is 2 dB. Find the power at each output fiber in dBm and W.

SolutionThe power division in dB from Equation 4-26 is

PDst= 10 log (1/10) = 10 dB

To findPoutfor each fiber, subtract PDstand LossexfromPinin dBm:

Pout= 3 dBm 10 dB 2 dB = 9 dBm

To findPoutin watts we use Equation 4-13:

9 = 10 log out1 mW

P

out

1 mWP =

9

1010

Pout = (1 mW)(100.9)

Pout = (103)(0.126) W

Solving, we get

Pout

Principles of Fiber Optic Communication

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