Chapter 1 Optical Fiber Transmission Media - TOMASI

8/10/2019 Chapter 1 Optical Fiber Transmission Media - TOMASI

1/46

1

Optical Fiber

Transmission

Media

--:]TER

OUTLINE

.:::.rduction

-:.tor)

of Optical

Fiber Communications

-,:;ical

Fibers

versus Metallic Cable Facilities

-

:;tromagnetic Spectrum

:

..k

Diagram

of

an Optical Fiber

--

-

:'nmunications System

::r.al Fiber Types

'

-r:ht

Propagation

I

E

Optical

Fiber Configurations

l-9

Optical

Fiber Classifications

I l0

Losses

in

Optical

Fiber Cables

I ll

Light Sources

1- ll

Optical

Sources

I

l-l

Light Detectors

l-

1-1 Lasers

I

'

I

5

Optical

Fiber

System Link

Bud-set

-:

-::TIVES

|

-.-.'-rc

optical comnrunications

I

:-:renr

an overview ofthe

history

ofoptical

tibers and optical

fiber communications

I

:-.pare the advantages and disadvantages

of optical

fibers over metallic cables

:::ne

electromagnetic

frcquency

and wavelenqth

spectrunt

I

-

:.:nbe several types

of optical

fiber construction

t

:

,

:.,iin

the

physics

of light

and the

following

terms: velocity of

propagation. refraction. refractir e

index. critical

I

I

--:-e.

acceptance angle, acceptance

cone. and

numerical aperture

-:.-ribe

how light

waves

propagate

through

an

optical fiber cable

-.-..te

ntocles

of

propugtttion

and irtdex

profile

:

t. ribe

the

three types

of optical fiber configurations:

single-mode

step

index. multimode

step

index.

and

mul-

'Je

-craded

index

:..:rbe

the

various losses incurred

in optical fiber

cables

-:

:e

liqht source and optical

power

-:i.rbe

lhe following

light sources: tight-emitting

diodes and

injection

diodes

-

..-:rbe

the following light detectors:

PIN diodes and

avalanche

photodiodes

:.-::be

the operation

ofa

laser

.:..:n

ho$ to calculate

a link budget

for an optical fiber system

1

I

I

I

I

a


2/46

1.1

INTRODUCTION

1-2

HISTORY

OF

Optical

fiber

cables

are the

newest

and

probably

the

most

promising

type

of

guided trans-

mission

medium

for

virtually

all

forms

ot'digital

and

data

communications

applications'

in

cluding

local,

metropolitan'

and

wide

area

networks

With

optical

fibers'

electromagnetic

waves"are

guided thiough

a

media

composed

of a

transparent

material

without

using

elec-

trical

cuneirt

t'low.

With optical

fibers,

electromagnetic

light

waves

propagate

through

the

media

in much

the

same

way that

radio

signals

propagate through

Earth's

atmosphere'

In essence,

an

oPtic'(tl

cotttttlLotic4'i;rts

J-)

ste''

is one

that

uses

light

as

the carier

of

information.

Propagating

light

waves

thrcugh Earth's

atmosphere

is

difficult

and often

im-

prr.tl.ul.

Cun."quJntly.-opiical

fiber

communications

systems

use

glass or

plastic

fiber

ca-

Lles

to

'colralri:

the

light

waves

and

guide them

in a manner

similar

to the

way

electro-

magnetic

\i'aves

are

guidecl through

a

metallic

transmission

medium

.

The

itdbrnutiort-carning

cayrcin

of

any electronic

communications

system

is

di-

rectly

proportional

to

bandwidth.

Optical

fiber cables

have'

for

all

practical

purposes'

an in-

nnite

ilaniwidth.

Therefore.

they

have

the

capacity

to

carry

much

more

information

than

their

metallic

counterparts

or. for

that

matter,

even

tAe

most

sophisticated

wireless

com-

munications

sYStems.

For

comparison

purposes.

it is

common

to

express

the

bandwidth

of

an analog

com-

munications

system

as

a

Percenlage

of

its

carier

frequency

This

is

sometimes

called

the

bandtidth

utiiizatio,?

r4ti;.

For inatance.

a

VHF raclio

communications

system

operating

at

acarrierfrequencyofl00MHzwithl0-MHzbandwidthhasabandwidthutiliZationratio

of

10olr.

A

microwave

rcdio

system

operating

at

a carrier

frequency

of

l0 GHz

with

a l07r

handwirlth utilization

ratio

would

hive

I GHz

of

bandwidth

available

Obviously'

the

higher

the

caniel

fiequency.

the more

bandwidth

available'

and the

greater the information-

ca-rrvins

caoacitl.

Lighr

frequencies

used

in

optical


systems

are

be-

* i,i"

io'' ir,"no+

.l0'rHz(100.000GHzto400,000GHz)

A bandwidth

utiliza-

tion

ratio

of

107.

would

be a

bandwidth

between

10,000

GHz

and

40'000

GHz'

OPTICAL

FIBEB

COMMUNICATIONS

In

1880.

Alexander

Graham

Bell

experimented

u

ith

an appalatus

he called

a

photophone'

The

photophone

was

a device

constructed

t'rom

mirrors

and

selenium

detectors

that

fians-

,oitt.,i

,ornd

*u.'.,

over

a

beam

of

light

The

photophone

was awkward

and

unreliable

and

hacl

no

real

practical

application.

Actuall\.

Iisual light

was a

primary means

ofcommuni-

cating

long

;efore

eleciionic

communications

came

about

Smoke

signals

and

minors

were

ur.d-ag.r-ago

to

conrel

shon.

simple

messages

Bell's

contraption'

however'

was the

tirst

attempt

at using

a

beam

of light

for

carrying

information'

Transmission

of

light

waves

for

any

useful

distance

through

Earth's

atmosphere

is

impractical

because

water

vapor,

oxygen.

and

particulates

in

the air

absorb and

attenuate

the

signals

at

light

frequencie.s.

Consequently.

the only

practical

type

of optical

communi-

catiois

system

is one

that

uses

a fiber

guide

ln

1930' J'

L.

Baird'

an English

scientist'

and

c. w. Hansell.

a

scientist

from

the

u;ircd

States,

were

granted

patents

for

scanning

and

transmitting

television

images

through

uncoated

fiber

cables

A

few

years later' a

German

scientist

named

H. Lamm

successfully

transmitted

images

through

a

single

glass

fiber

At

that time,

most

people

considered

fiber

optics

more

of

a toy

or a laboratory

stunt

and'

con-

sequently. it

was

not

until

the

early

1950s

that

any substantial

breakthrough

was'.qnade

in

the

field of

tiber

oPtics.

In t951. A.

b.

S.

van Heel of

Holland

and

H H

Hopkins

andN'

S

Kapany

ofEn-

glandexperimentedwithlighttransmissionthroughDundlesoffibers.Theirstudies]edto

t-h.

d.r"iop..rt

of the

fle;ible fberscope,

which

is used

extensively

in the

medical

held'

It

uas Kapany

who

coined

the teIm

"fiber

optics"

in

1956'

Chapter

1


3/46

ln 1958. Charles

H. Townes. an

American. and Afthur

L.

Scharvlou.

a Canadian.

wrote

a

paper

describing how

it was

possible

to use

stimulated emission

for amplifying light

waves

(laser)

as

well as microwaves

(maser).

Two

years later. Thcodore H. Maintan.

a

sci

entist

with Hughes Aircraft Company,

built the

first optical maser

The laser

(Iight

amplification

by .rtimulated emission

of

radiatir)n)

was

invented in

1960. The

laser's relatively high output

po*'er.

high

tiequcncy of operation.

and

capabilitl

of carrying

an

extremely

wide bandwidth signal

make

it

ideally suited

for

high-capacity

communications

systems. The

invention of the laser

-sreatly

accelerated

research efforts in

fiber-optic coinmunications.

although it

was not

until

I967 that

K.

C.

Kao and C.

A. Bock-

hanr

of

the Standard

Telecommunications Laboratory in

England proposed

a

new

conrmu-

nications

medium using

c

larlded fiber

cables.

The fiber

cables

available in

the 1960s

were extrenre)y

1tr.r.ir'

(more

than

1000

dB/km),

which limited optical transmissions

to short distances.

ln

1970. Kapron.

Keck. and

Maurer

of

Corning Glass

Works in Corning,

New

York.

developed

an optical

tiber

with

losses less than 2 dB/km.

That was the

"big"

breakthrough

needed to

pcrnlit practical

flber

optics

communications systems. Since

l9?0, fiber optics

technology

has

grown

exponen-

tially. Recently.

Bell Laboratories

succ'essfully transmitted

I

billion

bps

thlough

a fiber ca-

ble for 600 miles

without a regenerator

In the late 1970s and early

1980s. the refinement

ofoptical cables and the development

ofhigh-quality, affordable

light sources and

detectors opened the door

to the development

of

high-quality, high-capacity,

etficient, and affordable

optical fiber communications

systems. By

the

late

1980s,

losses

in optical fibers

were reduced to

as

low

as 0.16 dB/krn. and in

1988

NEC

Corporation

set a

new long-haul transmission

record by transrnitting

I0

-uigabytes

per second

over 80.1 kilometers

ofoptical

fiber Also in 1988, the

American National Standards

Institute

(ANSI) published

th e St

trchntnous

Opricdl

Nenrork

(.SON

ET). By the mid-

I

990s. opticnl

voice

and data networks

were commonplace throughout

the United States and

much ofthe world.

OPTICAL FIBERS VERSUS

CABLE

FACILITIES

Communications through

glass

or

plastic

fibers

has several advantages

ovel conven-

tional metallic transmission

media for both telecommunication

and computer

rretworking

applications.

1-3-1

Advantages

of

Optical Fiber

Cables

The advantagcs

of

using optical

fibers include the tbllou

ing:

l. Wider

bandridtlt

and

grcdter

iDformLltiott

('lPttit\'.

Optical

fiberr hirr

e

treater

in-

formation capacity

than metallic

cabies becalrse of

lhe

inherentl)

s

idel bands idth: lr

ail-

able

with

optical

t'requencies. Optical

libers ure arailable

\\ith

band\\idlh\

up

lo

\e\eral

thousand

gigahertz. The

pri,ran

eleclritttl

tottslunrs

(lesi\tan -e.

inductance.

and capaci-

tance)

in metallic cables cause

them to act

like lo\\

-prss

iille[s.

$

hich

lintit

iheir

triir]\nlis-

sion

frequencies,

band$,idth.

bit

rate. and intbrmttion-carq

ing clpircil).

\lode:n

optical


systems arc capable

of transmitting

ser elal

gigrbitr

per

second

over

hundreds

of

miles, allowing

literally millions

of

indi\ idLral

\

oice .1nd clata channels

to be

combined

and

propagated

over

one optical tiber

cable.

2. Inmwtitv

to


4/46

noise sources

(most

of

which

are man-made).

For

the same

reason'

fiber

cables

do

not ra-

diate

electromrg.netic

energl

4.

Entirotlma

t.tl

inrlrlnin.

Optical

fiber cables

are more

resistant

to

environmen-

ul

extremes

(including

weather

variations)

than

metallic

cables

Optical

cables

also

oper-

ate

over

a

wider temperature

range

and are

less

aftected

by

corrosive

liquids

and

gases'

5.

Sa/en

anrl contefiien('e.

Oplicdl

fiber cables

are sat'er

and

easier

to

install

and

maintain

than

metallic

cables.

Because

glass and

plastic fibers

are

noncondrrdors-

there

are

no electrical

currents

or

voltages associated

with

them

Optical

fibers

can

be used

around

volatile

liquids

and

gasses

without

worying

about

their

causing

explosions

or

fires

Opti-

cal tibers

are

also

smaller

and much

more

lightweight

and compact

than

metallic

cables'

Consequently.

they are

more

f-lexible.

easier

to

work with'

require

less

storage

space'

cheaper

to

transport.

and

easier

to install

and

maintain.

6. Lrnter

trctnsmi.isiorr

/oss

Optical

libers

have

considerably

less

signal

loss

than

their

metallic

counterparts.

Optical

tibers

are

cuffently

being

manufactured

with

as

lit-

tle

as a

few

tenths-of-a-decibel

loss

per kilometer.

Consequently'

optical

regenerators

anit

amplifiers

can

be spaced

considerably

farther

apart

than

$ith

metallic

transmission

lines.

7. Secrrill.

Optical

fiber

cables

are

more

secure

than metallic

cables

lt is

virtuall)

impossible

to tap

into a

fiber cable

without

the

user's

knowledge'

and

optical

cables

cannot

be detected

with metal

detectors

unless

they

are

reintbrced

with steel

for

strength'

8.

Durat:tilitl

(tnd

rcliabilitt

Optical

fiber

cables

last longer

and

are

more reliable

than

metallic

facilities

because

fiber

cables have

a

higher tolemnce

to

changes in

environ-

mental

conditions

and are

immune

to colrosive

materials'

9. Econontics.

The cost

of optical

fiber cables

is

approximately

the same

as metalli'

cables.

Fiber cables

have

less

loss and

require

fewer

repeaters'

\

'hich

equates

to lower

in-

stallation

and

overall

systen

costs

and

improved

reliability'

1-3-2

Disadvantages

of

Optical

Fiber

Cables

Although

the advantag;s

of optical

tiber cables

far exceed

the disadvantages

it

is impor-

tant

to know

rhe

limirations

of

the fiber.

The

disad antages

of optical

fibeIS include

Ihe

following:

l.

lntetf(kittg

cost.t

Optical

fiber

cable s)

stems

are

virtually useless

by themseh

e

Tobepracticalanduseful.the},muslbeconnectedtostandardelectronicfacilities.whic]:

often require

expensir

e interf'aces

2. Strengih.

Optical

ilbers

bl

themsehes

have

a significantly

lower

tensile

sffensti

than coaxial

cabie.

This

can

be improred

by coating

the

fiber

with standard

Ker'lar

and

"

protective

jacket

of PVC.

In

addition.

glass

fiber

is much

more

tiagile

than copper

\\ iri'

making

fiber

less attractive

where hardwarc

portability

is required'

i.

Renu)te

electrical

por|er

Occasionally.

it is necessary

to

provide electrical

po* e:

to remote

interface

or

regenerating

equipment.

This

cannot

be accomplished

with the

opt:'

cal cable.

so additional

metallic

cables

must

be included

in

the cable

assembly'

4. OptiutlJiber

utbles

are more

susceptible

to Losses

iriroducetl

by bending

tlte c':'

b1c.

Electromagnetic

waves

propagate through

an

optical

cable

by either

refraction

or re'

flection.

Thereibre.

bending

the

cable

causes

irregularities

in the

cable

dimensions'

rcsu::-

ing

in

a

loss

of signal

power. Optical

fibers

are

also

more

prone to

manufacturing

defec:'

as even

the most

minor

detect can

cause

excessive loss

of

signal

power'

5, Speciali:ed

kx

s. equiltnent.

trnd

truining'

Optical

fibcr

cables

require

spec:'

turls

to splice

anrl

repair cables

and

special

test

equipment

to make

routine

measuremen:'

Not

only

is repairing

fiber

cables

difficult

and expensive,

but technicians

working

on

op:--

cal cables

also

require

special

skills and

training.

tn addition'

sometimes

it is difTicult

to

-

cate taults

in optical

cables

because

there

is

no electrical

continuity'

Chapter'l

:l

i

i'l

j


5/46

in-

the

es.

a

ire.

er

ca-

re-

pli

Io-

.,

E

=sE

=c

=-

I

g

g

9 o oE

.g*E

:

"Ef;iAi5e=

eE *

E;

-Eg;;gEEfit

s5

i 5E

103

1oa

k{z

105 106

N4Hz

(mega)

GHz

(sisa)

nt 1d

1o"lo-

1d' 1d'

10" 1oj4 1or5

1016

1oj7

loro

1ole

1020 1021 10'z2

THz

Ptlz

EHz

(tera)

(penta)

(exa)

(kilo)

Chapter 1 Optical Fiber Transmission Media - TOMASI

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