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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
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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
fiber communications
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
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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
fiber communications
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
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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
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in-
the
es.
a
ire.
er
ca-
re-
pli
Io-
.,
E
=sE
=c
=-
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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)