CHAPTER I
Overview of Optflcal Fiber
Conmmrnlmflaatioms
Ever since ancient times, people had a principal need to communicate with one another. This need
created interests in devising cornrnunication systems for sending messages from one distant place to
people
another- Optical communicition rnethods were of special interest among the many systems that
by the
used
tried to ori. On, of the eafliest known optical transmission links was a fire-signal method
events.
Greeks in the eighth centgry,BC for sending alarms, ealls for help, or anRouncements of certain
Improvements oi these optical transmission systems were not pursued very actively because of technology
limitations at the time. For exampte, the speed of sending information over the communication link was
limited since the transmission rafe depended on how fast the senders could move theirhands, the optical
signal receiver was the error-prone human eye, Iine-of,sight transmission paths were required, and
atmospheric effects such as fog and rain made the transmission path unreliable. Thus it turned out to be
fastelmore efficient, and more dependable to send messages by a courier over the road network.
Subsequently, no significant advances for optical communications appeared until the invention of
the laser in the early 1-9@s and a series of technology developments related to optical fibers around
1970. These events fiaally.allowed practical lightwave communication systetns to start being fielded
worldwide in 1978. These systems operate in the near-infrared region of the electromagnetic spectrum
and use optical fibers as ttgr transmission medium. The goal of this book is to describe the various
technologts, implementationmethodologres, and performance measurement techniques that make optical
fiber corimunic;tion syste.ms possible. The reader can find additional information on the theory of light
propagation in fibers,.the derign of links and networks, and the evolution of optical fibers, photonic
devices, and optical fiber communication systems in a variety of books and conferen"" pro.""dingr''''
This chapter gives an overview of fundamental communications concepts and illustrates how optical
fiber transmissiJn systems operate. First, Sec. 1.1 gives the motivations behind developing optical fiber
transmission systems. Next Sec. t,2defines the different spectral bands which describe various operational
wavelength iegions used in optical communications. Section 1.3 explains fundamental data
communicationioncepts, encoding methods, channel capacity, and the decibel notation for expressing
optical power levels. Section 1.4 gives the basic hierarchy for multiplexing digitized information streams
uied on optical links and Sec. t.5 describes how wavelength division multiplexing can boost the
transmission capacity of an optical fiber significantly. Next, Sec.
introduces the functions and
implementation considerations of the key elements used in optical fiber systems.
An important aspect of realizing a smoothly interacting worldwide lightwave network is to have
well-established international standards for all aspects of components and neiworks. Section 1.7 discusses
the organizations that are involved with this standardization activity and lists the main classes of standards
related to optical communication components, system operations, and installation procedures. Finally,
Sec. 1.8 gives an introduction to modeling and simulation tools that have been developed to aid in the
design of optical fibers, passive and active devices, links, and networks.
Chapters 2 through 10 describe the purpose and performance characteristics of the major elements in an
optical link. These elements include optical fibers, light sources, photodetectors, passive optical devices,
a{rP_lifie_rs,,and active optoelectronic devices used in multiple-wavelength nitwork.. 'it
Chapters
9Pt:"t
I I through l4 show how the elements are put together to form links and networks, and
"o
explain measurement
methodologies used to evaluate the performance of lighrwave components and links.
t*
Motivations for Lightwave Communications
a
Prior to about 1980 most cornr,nunication technologies:involved some type of electrical transmission
mechanism. The era of electr,ical communications star,ted in 1837 with.the invention of the telegraph by
Samuel F. B. Morse. The telegraph system used the Morse code, which represents letters and numbers
by a coded series of dots and dashes. The encoded symbols were conveyed by sending short and long
pulses of electricity over a copper wire at a rate.of tens of pulses per second. More advanced telegrapl
schemes' such as the Baudot system invented in1874, enabled the information speeds to increise to
about 120 bits per second (b/s), but required the use of skilled operators. Shortly thereafter in 1876
Alexander Graham Bell developed a fundamentally different device that could tansmit the entire voice
signal in an analog form and which did not require any expertise to use.24,25
Both the telegraph and the analog voice signals were sent using a baseband transmission mode.
Basebandrcfers to the technology in which a signal is transmitted directly over a channel. For example,
this method is used on standard twisted-pair wire links running from an analog telephone to the nearest
switching interface equipment. The same baseband method is used widely iroptical communications,
that is, the optical output from a light source is turned on and off in response to the variations in voltage
levels of an information-bearing,electical signal.
In the ensuing years an increasingly larger portion of the electromagnetic spectrum was utilized to
develop and deploy progressively more sophisticated and reliable eleitrical communication systems
with larger capacities for conveyinginformation from one place to another. The basic motivations.behind
each new system application were to improve the transmission fidelity so that fewer distortions or errors
occur in the received message, to increase the data rate or capacity of a communication link so that more
information can be sent, or to inciease the transmission distance between in-line repeater or amplification
stations so that messages can be sent farther without the need to restore the signai amplitude;r fidelity
periodically along its path. These activities led to the birttr of a wide variety of communication systems
that are based on using high-capacity long-distance terrestrial and underseacopper-based wire lines and
wireless radio-frequency (RF), microwave, and satellite links.
t
r-I
iOueruieu of Optical F-lber Commnications
In these developments,tfte basiro ffend.for advancing the link capacity was to use increasingly higher
channel frequencies. 16s leassn,for this trend is that a,time:varying baseband information-bering
signal rnay be transferrod oy:€r &,cofllrnunication channel by superimposing it onto a sinusoidal
electromagnetic wave, which is,known,as the carrier wave or silmply canier. At the destination the
baseband inforrnatio4 signal'rg.removed from the carier wave 'and'processed as desired. Since the
amount of information that can be transmitted is directly related to the frequency range over which the
carrier operat€s,, incleasing.the carrier frequency theoretically increases the available transmission
bandwidth and consequentf, provides a larger information capacity.zfl8 For example, Fig. 1.tr shows
Designation. , Transmission
Applications
media
1015
1016
Hz
m
I01a Hz
100 GHz
Millimeter
waves
Navigation
Super higft
,frequency
l0
9pl
o
,
Wavesuide
I
Mrcrowave
radio
-
($m)
Ultra high"
fregieacy'r
. (UHF) r: j,
o
Veryhigh
6
frequency;
Satellite-to-satellite
Microwave relay
Earth-to:satellite
Radar
10
GHz
I GHz
I]I{F TV
>l
o
g
Mobile. Aeronautica
Shortwave
radio
(VHF)
VHF TVandFM
Mobile radio
10 m
HiCh
Business
frequeh.y
Amateur radio
(I$)
9
l00MHz S
rL
10
MHz
lntemational
100
"
Citizen's band
Medium
AM broad casting
frequency
I MHz
(MF)
Longwave
radid
Low
Aequency
100 kHz
Navigation
,,,:(I"F),
l0
Aetonautical
Submarine cable
Transoceanic radio
'Ve4rlcw.-
.,,Wire
frequenc,y,
pa!!s
10
kIIz
(vLF)
I
100
Audio
I
+
i
I
I
I
1
fE. 1.1 Tle regions oJ the etectronryneticspec'tntm
Telephone
Telegraph
I kHz
used Jor rodio and optical fiber
permission
clrlmmwrdlcntions. f{/sed uith
Jrom A. B. Carlson, Communication
rystems, @ 1986, M&ratu-Hilt B@k CompanA.)
theelecmtragretie spffil bandsmed fer radio transmission As the diverse r,adio tecbnologies move
from high frequeaey (FIF) to very-hig\frequenc)r (VHF) to ulra high frequency (UFIF) bands with
norniuatr curier frequencies of IO', 10P, and tA' Hz, respectively, increasingly higher infurmation
transmissie* speeds can k en$oyed to provide a higher link eapacity. Thus &e trend in eleetrical
sorrrnunication systefir developrrent$ was to, use prcgressively higher frequencies, which offer
comesponding ilrereases ia badwidth or inforrnation eapaeityAs Fig 1 .1 also shows, otricat firque,rcies are several orders of magnittrde higher than those used by
electrical comrnunication systeers. Thus the invention of the taser in the early 196Os aroused a curiosity
about the possibility of using the optical region of the electromagnetic spectrum for transmiuing
information. Of particulm interes is the near-infrared spectral band ranging from about 77O to 1675 nm,
since this is a low-Ioss regior* in silica glass fibers.lE-2o The technical breakthrough for optical fiber
eommunications started in 1970 when researchers at Corning demonstrated the feasibility of producing
2e
a glass fiber having an optical power loss that was low enough fbr a praetical transmission link.2o'
As research progressed, it became clear that many complex problerns made it extremely difficult to
extend the carrier coficept for achieving a super broadband optical communication link. Nevertheless,
the unique properties of optical fibers gave them a number of performance advantages compared to
copper wires, so that orptical lirks operating fur a simple on-off keyed baseband mode werc attractive
applications.
The first installed optical fiber tinks which appeared in the late 1970s were used for transmitting
telephony signals at abclt 6 trrfbls over distances of around l0 km. As research and development
progressed, the sophistication and capabilities of these systems increased.rapidly during the 1980s to
create links carrying aggregated data rates beyond terabits per second over distances of hundreds of
kilonreters without the need tc restore signal fidelity along the path length.
Starting in the 1990s there was aburgeoning demand on communication-network assets for bandwidthhungry services such as database queries, home shopping, highdefinition interactive video, remote
education, telernedicine ard e.health, high-rriglgdon editing of home vidoos, blogging, and large-scale
high-capacity e-science and Grid computing.3G33 This demand was fueled by the rapid proliferation of
personal computers (PCs) coupled with a phenomenal increase in their storage capacity and processing
capabilities, the widespread availability and continuous expansion of the Internet, and an extensive
choice of remotely accessible programs and information databases. To handle the ever-increasing demand
for high-bandwidth servises froxr ranging from home-based PC users to large businesses and research
organizations, telecomrnunication companircs worldwide greatly enhanced the capaeity of fiber lines by
adding more independent signal'c4q1dng wavelengths on individual fibers and increasing the transmission
speed of inforrnation being carried by each wavelength.
The advantages of optieal fitlers eompared to copper wires include the following:
Long Distance Transmhsian Optical fibers have lower ffansmission losses eompared to copper
wires. Consequently data can be sent over longer distances, thereby reducing the number of
intermediate repeaters needed to boost and restore signals in long spans. This reduction in equipment
and components decreases system cost and complexity.
Large Information Capacity Optical fibers have wider bandwidths than copper wires, so that
more information can be sent over a single physical line. This properry decreases the number of
physical lines needed for sending a given amount of information.
r
I
1-
A:a uAo gJ.Oedcal Flbq
furr,lors:unicafinrrs
Srnall Size ad law We@fit The low weight and the srnall dimEnsions sf fibers offer a distinc.t
advantage over heavy, brdky wix'e cables in crowded underground city ducts or in ceiling-mounted
of irnportance in aircraft, satel{ites, and ships where small, lighMeigts
cables are advantagmus, ard ia trctical military applications wllere large amounts .of cable nrust
be unreeled and reuieved rapidy.3a
cable trays. This feature also is
Imm*niq tu Elactt*:dl lac{wnn An especially important feature of an optical fiber relates
to the fact fiat it is a dieleetic material, which means it does not conduct electricity. This makes
optical fibers immurrc to Sre el€cfomagnetic interference effects seen in coppff wires, such as
inductive pickup from other adjacent signal-carrying wires or coupling of electrical noise into the
line from any type of reuby eryipment.
fulcly Wical
fibers offer a high degree 'of operational safety, since they ds not have
the problems of ground loops, qparks, and potentially high voltages inherent in coppa lines. Hswever,
precautions with respect to
ligh emissions need to be observed to prevent possitle eye damage.
Enhanccd
laser
Incrcascd Signal Secarity An optical fiber offers a high degree of data security, since the optical
signal is well-confinod within the fiber and an opaque coating around the fiber absorbs any sigaal
emissions. This feature is in contrast to copper wires where electrical signals potentially could be
tappd off easily. Thus fibers are atractive in applications where information security is important,
such as financial, legal, governmeal, and military systems.
l1"2x Opttcal spectral Baads
All *leommuaicatirm sys$ems use ffi ferr ofelecuomagnetic enerry to transmit signals. Tk spectrron
of electrcmftgnetic {EM)
.fum in Aig. L2. Elecoonugaetb erzrgy,h
"*l**m, b
Fiber optics
to 1675 nm
(-375 to 176Tl{z\
770
IIFIUI$ruISBadiio ffic*owaves
Iafrared ligbt
'1,-7
umts I
Frequancy
,w)
1012
Photoneaugy(eV)
Wavelengtb
I
(n)
I
rtg.
l0-2
l0-4
10-6
l0-8
1.2 Tte specfram of elrcfiomognetic radintion
a combiaation
of
microrryaves,:infrared lighL visible light'
electrical and magnetic fields and.includes power, radio waves,
electromagnetic
,uys. Each discipline takes up. a portion (or band) of the
ultraviolet light, x rays, *O
g*u
is that it can be'viewed as
spectrum. The fundamentll.natureLf an raaiation within thisipectrum
x
aboyt c = 3 108 m/s in a vacuum' Note
electromagneti" *ur", th*t travel at the speed of light, which is
factor r than' the speed c in a
that the speed of light s in a material is smaller U-y tfre refractive-index
silica glass, so that th9 speed of light in this
vacuum, as chapter 2 descriles. For example , n ='1.45 for
material is aboui s = 2 x108 m/s.
'"'i;;;;;ffi;r;ff*
,h; waves in different parts of the spectrum can be mealyred in several
in the wave' or
"f
irtioJut"i *uyr. fir"r" art tt " length of one period^of 1h9 waye, the energy conlained
frequency.to
to.use
i."n*"* of the wave. whereas electrical signal transmission tends to designate
il;;1il;,
the
generally nseswavelengrh
designate the signal operating bands, optical communication
signal
as
such
topics
discussing
operating region id photon ener_gy or optical poiff when
spectral
component performance'
,il"ngtt oi
the physical properties of a
"t.rtti-oitical
As can be seen mm rig. 12, there are three different ways to measure
units are 'related by some simple
wave in various ,"gio;r^ii #'EM tp;;,*"r. These meas,r€ment
frequency
to the wavelength
equations. First of AL in a vacuum the speed of light c is equal
% so
that
c --
'2'
times the
(l'1)
hv
hertz (Hz).
where the frequency y is measured in cycles per second.o t
(or wavelength) is determined by
frequency
its
and
pioton
of
a
energy
The relationship between the
- "
the equation known as Planck's
0'2)
E=
mewrs
J
ioules and
constant' The unit
where the parameter h = 6.63 x 10-3a J-s = 4.14eV-s is Pl.anck's
energy in
pm),
the
of
(measured in units
the unit ev stands for electron volts.lnterms of wavelength
'
law
:
hv
electron volts is given bY
r.2406
(1.3)
E(eV)=;-.----L\pm)
for
visible band. optical fiber
far-infrared radiation.:IJi"i*"", these liirits is the 400-to-700-nm
77O to 1675-nm'
nominally
from
tangqng
communications use the near-infrared. spectal band
bands for use in
spectral
six
designated
(ITU)
has
The International Telecommunications,Union
,egior.3s These long-wavelength band
optical fiber communi"u,i*, within the 1260-to-t6i5-nm
fibers and the performance behaviour
optical
of
designations arose from the attenuation characteristics
Figure 1.2 shows, the optical
tp".tto*
ranges from about 5 nm-11the
ultraviolet'"4o1t
1 mm
respectively' Figure 1'3
@DFA), which Chapters 3 and 10 describe,
,"giorr. *tti"t are known by the letters O, E, S, C, L, and U'
shows and Table 1.1 O"n
"r'tfr"
fiber systems. Thus this region is
The 770-to-g10 nm band is used for shorter-wavelength multimode
d".i;;;J;r ,rr" ,ioi-iorrt"ngth or multimode fibel band. Later chapters describe the operational
of an erbium-doped
m"r-"-piin"t
components, and otherpassive
performance characteristics and af,plications of optical fibers, electro-optic
'optical
bands.
devices for use in the short- and long-wavelength
Oueruiew of Optical Fiber Commwrjcatians
l
:U$ed in
Designation
opeal$ber,comrnitn&.&rs
Spectrum (nm)
Origin of Name
to
Extended band
O-band
E-band
1260
1360
to 1460
Original (first) region used for single-mode fiber links
Link use can extend into this region for fibers with low
Short band
S-band
1460 to 1530
Wavelengths are shorter than the C-band but higher
Conventional band
Long band
C-band
to
L-band
1530
1565
UltraJong band
U-band
1625
to 1675
Original band
136O
water content
1260
to 1625
E-Band
O-Band
1360
1565
than the E-band
Wavelength region used by a conventional EDFA
Gain decreases steadily to 1 at 1625 nm in this longer
wavelength band
Region beyond the response capability of an EDFA
C-nandl t-Band lu-eana
S-Band
t460
1530
1565
t625
1675
Wavelength (nm)
Fig. l.B Desigrutions
oJ
spectralbands usedJor opficalJiber @ffmlunico.tiotTs
l
)
\
m
f'undamental Data Communlcation Concepts
1.3.1
ElemeTtqr.y Commrurication Link
The exchange of information between any two devices across a communication channel involves using
some type of electrical or optical signal which carries this information. The channel could be a wire,
radio, microwave, satellite, atmospheric infrared, or optical fiber link. Each type of channel medium has
unique transmission performance characteristics associated with it, which ideally should match the
properties of the signai. However, regardless of its type themediumdegrades the fidelity of the transmitted
rignut U".uose of an imperfect reproduction of the signal format and because of the unavoidable presence
ofllectrical and opticai noise and interference. These impairments can lead to misinterpretations of the
signal by the electronics at the receivingend.
I
Figure 1.4 shows a block diagram of an elementary communication link. The user or device where the
is called the destination.The
-Jr.ug" originates is called i rouru and the final receiving user or device
Tl" function of the
a
transmittel'
to
input
message
as
the
serves
source
information
tlie
output-of
traisminer is to couple the message onto a transmission channel in the form of a time-varying signal
that matches the transfer properties of the channel. This process is known as encoding.
As the signal travels through the channel, various imperfect properties of the channel medium and of
various link-components induce impairments into the signal. These include electrical or optical noise
effects, signal distortions, and signal attenuation. In the presence of these impairments, the function of
it as
the receiveris to extract the weakened and distorted signal from the channel, amplify it, and restore
destination.
message
to
the
passing
it
on
it
and
decodirzg
form
before
encoded
its
original
possible
to
close as
AAAT
tttr
Message Encoded Weakeued Restored
the signal input and distorted and decoded
transmitter to the channel received signal message
input to
ng. 1.4 Tfe n:rcrin ompnents
bt an elementarg commtutirrrtlon ti,rir-
1.3.2 Analog Slgnals
The format of a signal is an important factor in efficiently and reliably sending the sigaal across a
network. The signals emittgd by informatioR sources can be classified into analog and digital formats.
An analog signal conveys information through a continuous and srnooth varintion in time of a physical
quantity such as optical, electrical, or acoustical intensities and frequencies. For example, voice, music,
electronic hum, and video inforrnation strearis are well-known analog signals.
A fundamental analog signal is the periodic sine wcve, shown in Fig. 1.5. Its thrce main characteristics
are its amplitude, period or frequency, and phase . The amplirude is the size or magnitude of the waveform.
llf
Ffg.
1.6
Period =
lf
llf
llequenq, pdod" ard" amplilude ctnraxtertstics
oJ
abastc silte wole
Amplitude is designated by the symbol A and is measured in volts, amperes, or watts, depending on the
signal type. Thefrequercy (designated by/) is the number of cycles per second that the wave undergoes
(i.e., the number of times it oscillates per second), which is expressed in units of hertz (Hz). A hertz
refers to a complete cycle of the wave. The pe riod of awave (represented by the symbol 7) is the inverse
of the frequency, that is period = T = llf. The terrr phase (designated by the symbol @) describes the
position of *re waveform'relative to time zero. Phase is measured in degrees or radians, where 180
degrees = zradians.
If the crests and roughs (high and low points) of two waves u,ith identical periods are aligned, they are
said to be in phase.For example, wave I and wave 2 shown in Fig. 1.6 are in phase. l.et wave t have an
Exam?le 1,1
A sine wave has a frequency/:= 100
kHz. Its pedod is I=/lOJ s = 0.01 ms.
A sine wave has period I = I ns. Its frequency is / =
qs)
l/( t0
= I GHz.
A sine wave is offset by 1/4 of a cycle with respect to
time zero. since one cycle if 360 degrees, the phase shit is
Q = 0.25 x 36O degrees = 90 degrees = nlZ radran
amplitude Ar and let wave 2 have an amplitude
Ar. If these two waves are added together, the
amplinrde A of the resulting wave will be the sum
A = At+ Ar. This effect is known as constructive
interference.
Figure 1.7 illustrates four phase shifis of a
wave relative to time zero. When two waves
differ slightly in their relative positions, they are
said to be out of phase. As an illustration, the
wave shown in Fig. 1.7(c) is 180' (zrradians)
out of phase with the wave shown in Fig. 1.7(a).
If the two waves in Fig. 1.7(a) and 1.7(c) have
identical periods, the resulting wave amplitude
will be the sum A = Ar + Ar.lf the two waves
also have the same amplitrr,rles, ithen they undergo
destructive interference so thatA = 0, that is they
will cancel each other out. These
interference
concepts are important when considering the
operation of devices such as laser diodes, thinfilm filters, and optical cbuPlers.
A1+ A2
fig. 1.6
Two in-pha,se u)aues usith tlw same
perid will
ll4 cycle
ll2
add" crn:.s,huctitselg
o) 90'
cycle
Addition of two waves that are
7r
radians out ofPhase Yields
zero final amplitude
fig. 1.2 lltustationoJJour
that are 78O" ortt
phase shifis oJausaue reWitse to time zcro. Ttuo identicollDaues
oJ
ptwse will interJere destrucfwelg.
Example 1.2
If the spectrumof a signal ranges from
its lowest frequencyJ"* = 10 kllz to its highest frequency
la1 =
100
llllzr
then
tk
f*** fw,,=90 kHz'
,.,
bandrryidth' B =
(o.rsimpli
Two further co]nmon characteristics in communications are the frequency spe:trum
;f a signal. The spectrun+ of a signal'i: r-n:.*q*:f fteQuencies that it
spectrurn) and the
sine waves of different
conrains. That is; the .p"it u; of u silnal is the combination of At tfre individual
refers'to.ths width of 'this
f,rdquencies which mak!-64;, .igi;r. T\e _bandwidrh (designated by B)
(kI'Iz),
megatrertz (Iv{I{zi' or
ThelqDdrvidth .otqsllgay. is specified in units,such as kilohertz
b;;iil
,p""*-.
gigahertz (GHz).
1.3.3 DiEitat Sfgnals
of discrete symbols' selected from a finite set of'elements, for
@, #;'or'vo: A eoaffdn disital
ffih6o""f}j.-niil;#;;;;b;, "rd other symbols socfi aspulse
shape'as
types of
twd
consists
wnich
signal configuration is ttw. binary waveprm,
--nown
:l:
of
segrlettce,
tt"
by
il
slsnat
a
digital
in
Partic.ulal
swT
;f; in Fii. 1.8, Th€ information contained
h"t."'
knyWrzey,,?r
absxiea(a
lfand
one
or
the presence (abinarry one,;orsirnply either
:ly-pll"i
loqic l) atfi a lagic Tero:(ot
one
or 0) of these pulses or D;{s, These iiis oten are referred to as aIoi*
,(ot
A digital signal is an,ordered
;;ffi;;
logi.c0),respictivelY.r..
sequence
,.,
l
-*1
Tt
F-
=llR:bitinterval
(a)
-,
l*To:bit
Bit duration
interval
(b)
Ftg.
1.8
tleir amplitude: wn4, and bit
binary uaueJorms
'r'r,yiy
(b) a 1 bttJills the
dwqtiotl" (o) fi\e bitfrlls tne eittre bit pe:riod.Jor ttrc t b-it ont4;
slot
bit frin7
fvst W qrtd.a,O bitfitls the seond ha{ of a
Exanqtbs oJ
tttso
period, or bit time' The bit
The time slot fi,,irr urhich abit occurs is called either the bit interval,bit
iecona'6ts\'ryhere
R
bits'per
of
a'rate
intervals are,spdcsd reguhr{y,ald occur every 1/R secoirds,:or'at
per second
asrkilobits
such
units
in
R is the bit rate o, tn" iotiiote.Tltedata rate commonly ii specffied
tai:!W
Oueruiew oJOpticalF-tber
Commtutications
,,:H
,
Y
(kb/s), megabits per second (Mb/s), or gigabits per second (Gb/s). As an example, a datarate of 2 x 10e
bls = 2 Gb/s. A bit can fill the entire bit interval or part of it, as shown in Fig. 1.8(a) and Fig. 1.8(b),
respectively. A block of eight bits often is used to represent an encoded symbol or word and is called an
octet ot a byte.
1.3.4 Dtgitizatton of Anatog
Signals
To send a signal in an analog format, the transmission channel typically cannot achieve perf'ect
reproduction of the signal at the destination, so there always will be some degree of distortion.
Furthermore, different,types of analog signals may require different channel responses. This puts a
strain on the design of amultipurpose analog link. In addition, noises that get added to the analog signal
it passes through multiple repeaters cannot be eliminated, which further degrades the signal.
In contrast, digital signals can undergo a gteat amount of distortion and still allow the information to
be exfiacted with a higf, degree of fidJity. to avoid the shortcomings of analog signals and to create
networks that can multiplex and switch any type of information, most informatipn is sent in a digital
format. To achieve this network flexibility, most analog signals need to be converted to a digital format.
as
However, as Chapter 9 describes, there are a number of situations in which it is advantageous to send highspeed analog signals in their native form over relatively short distances. One example is the transmission
of microwave signals from a satellite dish to a processing station located less than a, kilometer'away.
An analog signal can be transformed into a digital signal through a process of periodic sampling and
the assignment of quantized values to represent the intensity of the signal at regular intervals of time. To
convert an analog signal to a digital form, one starts by taking instantaneous measures of the height of
the signal wave at regular intervals, which is called sarzpling the signal. The simplest, but not necessarily
the bJst, way to convert these analog samples to a digital format is to divide the amplitude excursion of
the analog iignal into i/ equally spaced levels and assign a discrete binary word to each level. Each
analog sample is then assigned one'of these level values. This process is known as quantizatian. Since
the signal varies continuously in time, this process generates a sequence of real numbers.
Examole 1,3
Figure 1.9 shows an example of
digitization. Here the allowed voltage-amplitude excursion
is divided into eight equally spaced levels ranging from
zero to V volts. ln this figure, samples are taken every
microseiond and the'nearest discrete quantization level is
chosen as the one to be transntitted, according to the 3-bit
binary code listed next to the quantized levels shown in
Fig. 1.9. At the receiver this digital signal is then
demodulated. That is, the quantized levels are reassembled
into a continuously varying analog waveform.
Binary
code
number
111
6
6
Ec
=
9q
110
101
100
011
010
2
001
(a)
r0
000
l0
t2
12
(b)
Ftg. 1.9 Concept Jor digitizatian of anahag signals. (a) Originat signal uarying betuseen
and. V wlts: h) tumpled and qtntttized d$ital uersion
O
the digitization samples are taken frequently enough
can be recovered
relative to the rate at which the sign-al varies, then to a good approximation the signal
the reproduced
of
resemblance
The
points.
from the samples by drawing a stralght line between the sample
the
on
process
and
quantizing
g{ect of noise
signal to tfre iriginat .ijnuia"p"nd-s on the fineness of the
if
the sampling
the9r9;1t,
Ny4aist
and distortion added irito the Lansmission system. According to the
the
ieconstnrct
faithfully
can
device
rate is at least two times the highest frequency, then the receiving
reproduced
be
can
signal
the
if a sigial is limited to a bandwidth of Bhertz,then
;;;;;d;;iirtTh;,
by
at a rate of 28 times per second. These data samples are represented
it
is
sampied
if
distortion
without
be
V
can
Vr:
Vr:
bound?
""
a binary code. As noted in iig. t.l, eight quantized levels having upper
if
n
is,
That
levels.
sampling
give
finer
described by 3 binary Agrrr (ZY = S1. Vt-ore &gits can be used to
levels'
binary digits represent each saraple, then one can have 2" qtanization
Intuitively, one can see from Fig. 1.9 that
1.3.5 Channel
if
CaPaelty
This is the
m ,fr(, ,r"fysis of any com,rnunication network, an important factor is channel capacify'
to the user destination.
maximum rate at which data can be sent across a channel-from the message source
theorem states that
A fundamental and important theorem for this is the Sharuwn capacity formula.Tlis
capacity
if a channel has a bandwidth B (measured in hertz) then the maximrrm information-transmission
C of that channel is given trt bitS per secondby the relationship6-28'36
(1.4)
iBtogz(l + S/If)
C
signalpower and noise powel'
Here log2 r"pr"t"Utq the base-2 logarithm, and S and N are the average
point that t1gn4
i
it
,"rp"qtiu"ff fypicaUy'these powex are measured at the ryc.elv* since it is at this
relationship
following
the
of
calculation,
simplicity
For
Nore:
proeessed.
extracted f;om fie channel'ani
-QL
may be useful to find log2 "r:
logz v = (lo916
x)t(log*2) = (lo916
x)t0'3.
(1'5)
power in a signal to the
The parameter S/N is the signal-to*oise ratio (SNR), which is the ratio of the
expressed in decibels:
power contained in the noise a:t a particular measurement point. This ratio is often
sNRdB
= ro los
H#ffi
= ro ros
(1.6)
f,
achieved. In practice
The Shannon formula indicates the theoretical maximum capacity that can be
noise and does not
thermal
account
into
takes
this capacity cannot be reaehe{ since the formula only
intuitively
Furthellnore'
distortion.
or
delay
consider factors such as impulse noise, attenuation distortion,
raising
However,
strength.
signal
the
raising
by
it might seem that the capacity can be increased merely
powers'
Also
noise
highel
to
leads
the slgnal level also inciJasei'nonlinear effects fn the iysiem,_wt-rich
more
the
bandwidth
the
wider
the
since
ratio
S/N,
,"," ,irri increasing the bandwidtb B decreases the
noise is introduced into the system.
Examole 1.4
Supposewe'haveanoisychannelwith
which the signal-to-noise ratio is 1'
in
a l-MHz bandwidth
From Eq. (1.4), the maximum.capacity for this thanpel is
C =-Blogz(1 + S/N) - 106log, (1 + 1) = 106log, (2)
=
106 11.0;
= 1 Mb/s
Example 1.5
Let us,find the capacity of a channel
that operates between 3 MIIz and 4 MHz and in which the
signal-to-noise ratio is 20 dB. Then the bandwidth is
B = (4MIlz)-
(3 MHz)
=
1
MHz
and, from Bq. 11.6).
571g_1920/ro=100
Then
C
(1 + 100) = [106 logro (101)]/0'3
= 106 (2.0)/0.3 = 6.7 Mb/s
-
106 log2
Anruieu:
oJ
Opfiml F'Ibr bmmnications
1.3.6 Declbel Unlte
mechanisms in atransmissionmedium'
Reduction or attenuation of signal strengtharises from various loss
an
electric signal flows along a wire, and
as
For example, electric power Is lost thriugh heat generation
in a glass fiber or in an atmospheric
processes
optical power is attenuaied ttnough scatteing anO iUsorption
along a channel path to
periodically
channel. To compensate for the[ energy loises, amplifiers areused
boost the signal level, as shourn in Fig. 1'10'
Amplified
Original
signal
signal
Attenuated
signal
]-l
Amplifier
ru. 1.1O
Paiodicaltg placedamptifiers compensalefor energg losses alarE alink
a link o1-a device is to reference
A standard and convenient rnodrod for measuring attenuation through
fiber' the signal strengh
optical
as
the output signal level to the input level. For guidfo media such
P
it in terms of a logarithmic
normally decays exponentially. Thus, for con-venience one can designate
power ratio miasured in decibels (dB). The dB unit is defined by
I
Power ratio in
I
i
{r.
I
l_.
i'
I
dB
= 10 r"f
t
(1.7)
i
where Pr and P, are the elecrical or optical power
levels of a signal at points I and 2 in Fig' 1'11,
and log is the base- 10 logarithm. The logarithmic
naturs of the decibel allows a large ratio to be
expressed in a fairly simple manner. Power levels
ditrering by many orders of magnitude can be
compared easily when they are in decibel prm'
Anoiher attractive feature of,&edecibel is that to
measure the changes in ttre sfength of a signal,
one merely adds or subtracts th decibel numbers
between two different Points.
P1
Pz= 0.5Pt
[:l
Point
Ftg.
I
1.11
r_l
Point 2
F,:arnple oJpttlse attenuntionino'
tirlk P l and.P2are ttw Pwerleueb
oJ a stgnal at Polnts 1 and 2
..:
ExamPle 1.6 airo-"
thar after traveling a certain
asignal
rntdium,..tlrepower'of
in
sorire
translnissibn
distance
is reduced to half; that is, P2 +Q5 f1riaFig. t;l1. At this
poinq using,Eq; (1:7) the *tieu@bnorr less of,powe{ is
"n9
r0log
=t0log"
O't-4
Pl
=I0log0.5=10{-0.3)=-3dB
:. j
:
-'.:::";--:::
t;.
.:.
dB (or a 3-dB attenuation or loss) means that
the signal has,lost half its power- If,.an anplifier'is inserte.d
into the.ltrk at:rhis point to boost the signal back to its
origtgal,lgveJ tren,that amplifrer has 4-3-dB gain. H the
amplifier has a 6-dB gain then il boosts the signal.power
level to twicg tbe origrnat value.
Thus,
-3
Table 1.2 shows some sarnple values of power loss
given in decibels,and drrc percent of power remaiuing
after this loss. These types of numbers are innportant
when consideringfacbrs sueh as the effects of tapping
off a small part of an optical signal for monitoring
Power loss
(in dB)
purposes, for examining the power loss through some
optical element, or when calculating the signal
attenuation in a specific length of optical'fiber.
Percent
1
98
89
79
2
63
J
6
50
25
10
10
20
1
0.1
o.5
Example 1.7 Consider the transmission path from
point I to point 4 shown in Fig. 1.12. Here the signal is
attenuated by 9 dB between points 1 and 2. After getting a
14-dB boost from an amplifier at point 3, it is again
attenuated by 3 dB between points 3 and 4. Relative to
point l. the signal level ih dB ar point 4 is
dB levei at point 4
-
of
power left
(loss in line 1) + (ampliher gain)
+ (loss in line 2)
r
,
(-9dB)+(14dB)+(-3 dB)=+2dB
Thus the signal has a2-dB (afactor,of 100'2 = 1.58)
I to point 4.
gain in power in going from point
Transmission line 2
,::
E!g, i,.1?..
Point 3
Point 2
Psint I
i.
Nifiitb
-;-':':
..
- : :
:,,
oJ:rgitsl atterunfan atd. ompt!fication in d transmissfon Wfh
aueruiew of Optigal Fiber Commwtiaqtions
Since the decibel is used !9 rgf..-g{ 10 Iatlos or relative
units, it gives no indiCationofifhe absolute power level.
i
However, a derived unit can be used for this. Such
i.'n
a
unit that is particularly common in optical fiber
communications is the.dBrn (simply pronounced dee
bee em). This unit expresses the power level P as a
logarithmic ratio of P referred to 1 mW. In this case,
200 mW
100 mW
the power in dBm is an absolute value defined by
10
Power level (in dBm) = 161o*
r li"
tY)
(1.8)
An important'ru1e-+f.,thumb relationship to
m
mW
lmW
remember for optical fiber comrnunications is 0 dBm = I mW. Therefore, positive values of dBm are greater
than 1 mW and negative values are less than 1 mW.
Table 1.3 lists some examples of optical power levels
and their dBm equivalents.
I
Power
pW
10pW
100
lpw
100 nW
10
nW
lnW
dBm equivalent
23
"20
10
0
-10
-20
-30
-40
-50
-60
100 pW
10 pW
_:70
lpW
-90
-80
Network Informatlon Rates
To handle the continuously rising demand forhigh-bandwidth services fro:nusers ranging from individuais
to large businesses and.research organizations, le,Iecommulication companies worldwide are
implementing increasingly sophisticated digital multiplexing techniques that allow a larger number of
indipendent information sfreams to shale the same physical transmission channel. This section describes
some corlmon digital signal multiplexing techniques.
1.4.1 Telecom
fuUt" t.+ gives examples of infofmation rates for some rypical telecom services. To send these services
t'
1.!
from one user to another, network providers combine the signals from many different users and send the
aggregate signal over a single transmission line. This scheme is known as time-diuislstx-ruuh;p-lexing
dbt tl wherein N independertt infomiation stteams, each runriing at a data rate 9f R b/s, are interledved
etectricatty into:a si'ngie infor-mrition stream operating at a highlr rate of l/x R bls. To get aidetailed
perspective of this methodology, let us look at the multiplexing schemes used in telecommunications'
'"n'*fy uppfications of nU"r"opti" transmission linhs were mainly for large caplcity telephone lines.
fnese aigitA'tnks consisted of time-division-multiplexed 64-kb/s voice channels. The multiplexing was
Oevel@6a in *re-19@!' is-based'on what is known as the plesiochrorwus digital hierarchy (PDH).
t: 'shi
nigor"i.
-The
d'
s tfri;iAgita
i*r*arission hierarchy used in the-North fqrerican !el?,Pne network. _
^
funtlamentat'Uulfriihg.Uldak is a'I.544-MbA triinsmission i'ate tnown as aDS'|. ratp, where DS
stands for digital sys;tem.I1 is formed by time-division-multiplexing ffienff-four Voice chanflels, each
digitized at uZ+-UAirrate (rvhich isr"ferr"d to asDS0). Framingbirs, which indicate where an irrformation
uoit rtrut, and ends, are'added':alohg'with these voice channels to yield the 1.544-Mb/s bit stream.
Framing and other conffol bits that.may get added to an information unit in a digital stream are called
overheid.bits. At any multiplexlng ievifa signal at the designated input rate is c-ombined with other
input signals at the same rate.
Data rate
Type of service
Videci on demand/interactive TV
1.5 to 6 Mb/s
Video games
I to 2 Mbls
Remote education
1.5 to 3 Mb/s
Electronic shopping
Data ffansfer or telecomrhuting
1.5 to 6 Mb/s
1
to 3 Mb/s
0.384 to 2 Mb/s
Video conferencing
Voice (single telephone channel)
33.6to 56 kb/s
Six
I
44.736:lvlbls
lnputs
Seven
6.312-MbA
inputs
I
{7
t;--
274.l76lvIbls
T3
multiplexer
44.736lvIbls
T2
multiplexer
6.312 Mb/s
TI
multiplexer
1.544 }vlb/s
64-kbls
inputs
fE. 1.13 Wifat fa4gmissrior hierarchg used. in the North American tebphone netutork
DSr versus Tr In describing telephone network
data rates, one also sees terms_such as.Tl.,_I3, and
(e.g.,,71
or 73 and DS3) are used interchangeably. However
and
DS1
so on. Often ttre .terms Ti andDsle
there is a subtle difference in their meaning. Designations such as DSl, DS2, and DS3 refer to a service
type, fot example,'a user *ho wants to send information at a 1.544 Mb/s rate would subscribe to a DSl
seruice.,q,UUreviationsSuchas Tl,n,andT3refertothedataratewhichthetransmission-linetechnology
;;"r 6'J;iir*tn"t service overta physical link. For example, the DSI service is Eansported over a
physical wire oi optical fiber using elecrical or optical pulses sent at aTl = 1.544 Mb/s rate.
The TDM scherne is not restricted to multiplexing voice signals. For example, at the DSl level, any
64-kb/s digital signal of the appropriate format could be transmitted T on: of Og Z!_+put.channels
shown in Fig. 1.13. A_s nqted there and in Table 1.5, the main multiplexed rates for Norttr A.merican
applications-are designated as DSl (1.544 Mb/s), DSz (6.312 Mb/s), and DS3 (M.736 Mb/s). Europeln
,nd Japan"r" networks define similar hierarchies using different bit-rate levels as Table 1.5 shows. In
Euro@ the multiplexing hierarchy is labeled El, E2, E3, and so on.
Ouentiew of Optical Fiber Commrtrdcations
TebIs,I,,$-
rp&tfq*r.nul& g,ler .{s- usedinNorth
Digital multiplexing
Number of 64-kb/s
level
channels
Bit rate (Mbh)
North America
Europe
lapan
0.064
0.064
0.064
DS0
1
DS1
24
30
48
3.152
96
6.312
D52
Arrcrica"'fat'w$q qd1.&ffryl
r.544
r.544
2.O48
3.t52
6.3t2
8.448
t20
32.064
34.368
480
D53
44.736
91.053
672
r344
97.728
144A
139.264
1920
DS4
274.176
4032
5760
r.4.2
397.200
SONET/SDH
providers established
With the advent of high-capacity fiber optic transmission lines in the 1980s, service
and s,vnchronous
America
(SONET)
North
in
nerwort
optical
a standard signal format ciled iynchroious
frame
a
synchronous
deflne
standards
These
world.37'38
of
the
parts
digital hierirchy (SDH) in other
block
building
The
basic
lines.
trunk
fiber
optical
over
traffic
,frr.t r" for sending multiplexed digital
(STSl
lwel
Signal
Transport
the
Synchronous
is
called
and first level of the SONET signal hierarchy
by byte-interleaving N
1), which has a bit rate of St.g4 lvtUls. Higher-rate SONET signals are obtained
- l,evel N (OC-N)
Carrier
Optical
to
an
converted
and
scrambled
of these STS-1 frames, which then are
signal. For SDH
OC-l
of
an
that
N
times
exactly
line
rate
a
.rgrA. Thus the OC-N signal will have
- Level I
Module
Transport
Synchronous
155.52-Mbls
is
the
,f.t"*. the fundamental6uilding block
Ndifferent
multiplexing
generated
by
synchronously
are
streams
fiffrA-f l. Again, higher-rate information
STM-1 sign-als to form the STM-N signal. Table 1.6 shows commonly used SDH and SONET signal
Ievels, the line rate, and the popular numerical name for that rate.
teble 1.6
Cofia7:Ert SDH and.SOI\rET
SONET level
Electrical level
oc-l
STS-1
STS.3
oc-3
oc-12
OC-+8
oc-192
oc-768
srs-12
STS.48
STS-192
STS-768
line
rates and. their
popdar rwneriml
SDH level
Line fate (Mb/s)
STM-1
STM-4
STM-16
STM-64
STM-256
155.52
622.08
2488.32
9953.28
Popular rote
5r.84
398 1 3.
l2
rtalT"E
155 Mb/s
622 N{bls
2.5 Gb/s
10 Gb/s
40 Gb/s
non'te
m
wDM curcepts
The use of wavelength division multiplexing
(WDM) offers a further boost in fiber
transmission capacity. The basis of WDM is
to use multiple sources operating at slightly
different wavelengths to transmit several
indepehdent information streams simultaneously over the same fiber. Figure 1.14 shows
the basic WDM concept. Here N independent
optically formatted information streams, each
transmitted at a different wavelength, are
combined by means of an optical multiplexer
Individual
fiber lines
fig. f .f4
Optical
multiplexer
Basic corrcept oJtoauelengthdiuision
and sent over the sarne fiber. Note that each of
ntttltiplexing
these streams could be at a different data rate.
Each information stream maintains its individual data rate after being multiplexed with the other traffic
streams, and still operates at its unique wavelength. Conceptually. the WDM scheme is the same as frequency
division multiplexing (FDM) used in microwave radio and satellite systems.
Although researchers started looking at SIDM techniques in the I970s, during the ensuing years it
generally turned out to be easier to transmit only a single wavelength on a fiber using high-speed electronic
and optical devices, than t6'invoke the greater system complexity called for in WDM. However, a
dramatic surge in WDM popularity started in the early 1990s owing to several.factors. These include
new fiber types that provide better performance of multiple-wavelength operation at 1550 nm, advances
in producing WDM devices that can sepaJate closely spaced wavelengths, and the development of
optical amplifiers that can boost C-band optical signal levels completely in the optical domain.
tr.o I Key Elements of Optical Fiber Systems
Similar to electrical communication systems, the basic function of an optical fiber link is to transport a
signal from communication equipment (e.g. a computer, telephone, or video device) at one location to
corresponding equipment at ariotheJ location with a high degree of reliability and accuracy. Figure 1.15
shows the main constituen{s of an optical fiber communications link. The key sections are a transmitter
consisting of a-light'sourie'and'its associated drive circuitry, a cable offering mechanical and
environmental protection to the opticdl fibers contained inside, and a receiver consisting of
a
photodetector
plus amplification and signal-restoring circuitry. Additional components include optical amplifiers,
connectors, splices, couplers, regenerators (for restoring the signal-shape characteristics), and other
passive components and active photonic devices.
1.6.1
Ovenriew of Element Applications
The cabled fiber is one of the most important elements in an optical fiber link. In addition to protecting
the glass fibers during installation and seryice, the cable may contain copper wires for powering optical
amplifiers or signal regenerators, which are needed periodically in long-distance links for amplifying
and reshaping the signal. A variety of fiber types with different performance characteristics exist for a
wide range of applications. To protect the glass fibers during installation and service, there are many
different cable configurations depending on whether the cable is to be installed inside a building,
Oueruieu of Opfical Fiber Communications
Information
Optical connecters
sources
Video
Cabled optical fibers
camera
/
I
rE. 1.r5
Information
recipients
\
I
\
Passive or active oPtical devices
(optical filters, couplers, switches)
Main onstihrcnts
oJ
an opticalfrher coffarulnicdjfions ltnk
Very low-loss
underground in ddcts or through direct-burial methods, outside on poles' or under water.
joining
cables
for
networks
opticai connectors and splices-are needed in all categories of optical fiber
and for attaching one fiber to another.
from
As a result of installation and/or manufacturing limitations, individual cable lengths will range
weight
cable
and
size
reel
several hundred meters to several kilometers. Practical considerations such as
used when the
determine the actual length of a single cable section. The shorter segments tend to be
applications'
or
underwater
cables are pulled through-rlucts. Longir lengths are used in aerial, direct-burial,
in onassembled
are
hundreds of kilometers long. These cables
Transoceanic cable length. ur"
sections
cable
individual
shore factories and then-loaded into special cable-laying ships. Splicing together
forms continuous ftansmission lines for these long-distance links.
fiber. The
Once the cable is installed, a transrnitter can be used to launch a light signal into the
associated
and
core
transmitter consists of a light source that is dimensionally compatible with the fiber
(LEDs)
laser
diodes
and
electronic conrrol and moJuhtion circuitry. Semiconductor light-emitting diodes
simply
by
are suitable for this purpose. For these devices the light output can be modulated rapidly
varying the input .u.."nt at the desired transmission rate, thereby producing an optical signal. The
iJnput signals to the transmitter circuitry for driving the optical source can be either of an analog
the source
"tect
or digital io*.-fn" functions of the associated transmitter electronics are to set and stabilize
op"ruting point and output power level. For high-rate systems (usually greater than about 2.5 Gb/s),
an external
Oirect moautation.of the source can lead to unacc-ptable optical signal distortion. In this case,
In the
modulator is used to varJ the amplitude of a continuous light output from a laser diode source.
(126O
to
770+o.910-nm region thi light sources are generally alloys of GaAlAs. At longer wavelengths
1675 nm) an InGaAsP alloy is,the principat optical source material.
After an optical signal is launched into a fiber, it will become progressively attenulted and distorted
-*y
glass
with increasing distance because of scattering, absorption, and dispersion mechanisms.in.the
which
.;i"ri"f . At d;e destination of an optical fiber transmission line, there is a-receiving device
that detects
interprets the information contained in the optical signal. Inside the_ receiver is a photodiode
it to an
converts
and
fiber
optical
of
an
the
end
from
emerging
signal
optical
distorted
the weakened and
as a.piotocurreit). The receiver also contains electronic amplification
electrical signal (referred
signat fidelity. Silicon photodiodes are used in the 770+o-910-nm region'
to
restore
devices andiircuitry
region is an InGaAs alloy.
1260-to-1675-nm
in
the
The primary mat€rial
more complex than that of the transmitter, since it has
inherently
is
receiver
an
optical
ftr" a"rigp of
signal received by the photodetector. The principal
degraded
and
weakened
ofthe
to interpret the content
ti
power n::es:effy at the desired data rate to attain
figure of merit for a receiver is the minimum optical
a specified signal-to-noise ratio for an analog
either a given error probauility for digital ,yrt"*, or
level depends on the photodetector
p"tfo**19
system. The ability of a receiver to achieve a certain
successive amplification stages in
the
of
tn" system, and the characieristics
type, the effects of
the receiver.
noir"l,
devices that assist in controlling and
lncluded in any optical fiber link are various passive optical
that require no electronic'control for
g#irgA; figti.igriuf*. Passive devices *" optl.ut components
a nalrow spectrum of desired light'
only
their operation. Among these are opti.cal filteis that select
of different branches' optical
a
number
into
optical splitters that diiide the power in an optical signal
onto the same fiber (or that
wavelengths
multiplexers that combine signals from two * -o." iistinct
in multiple-wavelength optical fiber networks' and couplers
separate the waretengtt s at rfrZ t"""itt g end)
monitoring purposes'
;;!6r" tap off u ."ri=ui, p"r"entage of-iight, usually for performance
a wide range of active optical
contain
In addition, *oO"* Iopf*i.riJu,"A oftical fiber networks
include light signal modulators'
These
operation'
components, which require an electronic control for their
for adding and dropping
elements
tunable (wavelength-selectable) optical titiers, reconfigurable
switches'
ontical
nodes, i*iuUt" optical attenuators,lmd
wavelengths at intermediate
a fiber' ii becomes greatly weakened due
After an optical signat tras traveled a certairi distance along
up an optical link' engineers formulate a power
to power loss along ,n" nU"t Therefore, when setting
The
,tr". p*tl. for: ei1e1!s the available power margin'
budget and add umprffierc or,repearers
will
also
repeate(
a
whereas
boost'
power
optioal signal a
giu:
periodically placed
signal
for
rePeatert.**:111111e
o$1
t]o fggo,
attempt to restore the Jlgrratto its original *-t uf.l
p"tfottsphoton'to-electron conversion' elecffical
ui"p"uto
opi"uf-rignul,
ln""o*irri
un
amplification.
conversion' This process can be
amplification, retiming, pulse shaping, qd then elecfron-to-pfioto'
expended a gteat deal of
researchers
systems.
"diifi";;
#J),
*tr*
*"
ni*
f*
fairly complex for high-speed multiwavelength
]h}s,
light power level completely in the optical
effort to develop all-optical,arnplifiers, wniJfr boost the
links include the use of devices based on raredomain. Optical ampthcation mechanisms for WDM
of a stimulated Raman scattering
lengths JnU* and distribute6 amplification by means
earth-doped
effect.
system require measurement
The installation and operation of an optical fiber communication
the constituent components
of
characteristics
rcchniques for verifying ,fi;,h" specified performance
parameters, system engineers are interested in knowing
are satisfied. In addition to measuring optical fiber
such as
the characteristics of passive splitters,
r""to,*, and coupleis, and electro-optic components'
"on
Furthermoie' when a link is being installed and tested'
sources, photodetectoi., *O oiti"A amplifiers.
include bit error rate,' timing jitter' and signal-to-noise
operational parameters ihat should b1 m3asured
maintenance
During actual operation, measurements are needed for
ratio as indicated Uy tt".V.put
of remotely
status
the
and
"-.
fibers
in
as fault locations
and monitoring functionsio determine factors such
located oPtical arnPlifiers'
1-6.2 trIindows and SPectral Bands
systerns and the qharacteristics of the four key
Figure 1.16 shows the opelating rTge of optical fiber
Here the
tU"et tigtrt ,our."., photodetectors, and optical amplifiers'
componenrs of a link ,fia
bands of
wavelength
"pii"""f
main traditional operating
dashed vertical lines indicate the centers oi th" thr"e
of the
One
C-band'
region, the O-band' and the
optical tiber systemr,-*hi"h are the short-wavelength
shown at the
as a function of wavelength' as
principal characteristic* oiun optical fiber is its atte"nuation
topinFig.l.16.EarlyapplicationsinthelatelgT0smadeexclusiveuseoftheT7o-to.gl0-nmwavelength
Oueruiew oJ Optical Ftber Commtutications
optical sources and silicon photodetectors operating
band where there was alow-loss window and GaAlAs
window' since
available. Originally this region was referred to as the first
at these wavelengths were
absorption by water mo-11-11es' As a result of
around 1000 nm there was a large attenuation ,pit" ao"io
attenuation curve around 850 nm'
this spike, early fibers exhibitei a local minimum in the
impurities in the fiber material' in the
metallic
and
By reducing the .oo""routi"n of hydroxyl ions
very'low losJes in the 1260+o-1675-nm region'
1980s manufacrurers could fabricate optical fibers with
Optical fibers
1310 nm
)0
900
Optical sources
r7m77/7777V7777
lnGaAsP
GaAlAsI
I
I
Optical amPlifiers
m
RamanamPlification
r7v7v777777V7/77.?V7Vm
PDFA TDFA EDFA
Optical,fiber amplifiers
W
w4'v7xvvTVvz
1.0
-
i
o.s
q
o
&
900
700
Ctwracteristlcs
1500
1700
Wavelength (nm)
.
rE. 1.16
1300
1100
]d oprathg
This spectral band is called the
ranges
lo ng-wavelength
oJtteJour keg optielfiber tink componerts
region'
il'" glass still contained some water
.SiT:
1400 nm. This spike defined two low-loss
aiound
molecules, a third-ord#il"fi;-.'rtk" remalned
nm and the thirdwindow centered at 1550 nm'
1310
at
centercd
tn"i""i"a*iiiow
being
windows, these
the low-loss
caleil the o-band and C-band, respectively. The desire to use
These two windows
r"* ;
InGaAsP-based light sources and InGaAs
long-wavelength regions prompted the development of
doping optical fibers with rare-earth
photoderecror. tuut .* opJr.* ii r:ro and 1550 nm. In addition.
elernents such as Pr, Th,:and Er creates optical fiber amplifiers (PDFA; TDFA, and EDFA devices).
These devices. and the use,of,Raman amplification gave a further capacity boost to long-wavelength
WDM systems.
Special material-purification processes can eliminate almost all water molecules from the glass fiber
material, thereby dramatically reducing the water-attenuation peak around 140O nm. This process opens
the E-band (1360+o-1460 nm) tr.ansmission region to provide around,100,nm:more spectral bandwidth
in these specially fabricated fibers than in conventional single-mode fibers.
Systems operating at 1550 nm provide the lowest attenuation, but the signal dispersion in a standard
silica fiber is larger at 1550 qm than at 1310 nm. Fiber manufacturers overcame this limitation frst by
creating the dispersion-shifted fibers for single-wavelength operation and then by devising non-zero
dispersion-shifted fiber (NZDSF) for use with WDM implementations. The latter fiber type has led to
the widespread use of multiple-wavelength S-band and C-band systems for high-capacity, long-span
terrestrial and undersea hansmission links. These links routinely carry traffic at 10 Gb/s (OC-192ISTM.
64) over nominally 90-km distances between amplifiers or repeaters. By 2005 links operating at 4O Gb/s
were being installed and ?ield trials of I60-Gb/s long-distance transmission systems were tested
successfuliy.3e-a2
,
tz
.,.rcF
v
Standards for Opticat Fiber Communications
from different vendors to interface with one another, numerous
To allow components and equipment
-been
developed.a3-as The three basic classes for fiber optics are primary
intemational siandards have
standards, component testing standards, and system standards.
fuimary stolndolrds
refer to measuring and charaqterizing fundamental physical parameters such
of fibers, and optical power levels and spectral
widths. In the USA the main organization'involved in primary standards is the National Institute of
Standards and Technology (MST). This organization carries out fiber optic and laser standardization
work, and it sponsors an annual conference on optical fiber measurements. Other national organizations
include the National Physical Laboratory NPL) in the United Kingdom and the Physikalisch-Technische
Bundesanstalt (PTB) in Germany.
as attenuation, bandwidth, operatioual characteristics
t*ting
standardsdefine tests tbr fiber-optic component performance and they
Component
establish equipment-calibration procedures. Several different organizations are involved in formulating
testing standards, sorne,v€ry,active ones being the Telecommunications Industry Association (TIA) in
association with the Electrsn_ics Industries Alliance (EIA), the Telecommunication Sector of the
International Telecomrnunication Union (ITU-T), and the International Electrotechnical Commission
(IEC). The TIA has a list of over 120 fiberoptic test'standards and specifications under the general
designation TIA/EIA-455 XX-YY. where XX refers to a specific measurement technique and YY refers
to the publication year.'These standards are also called Fiber Optic kst Procedures (FOTP), so that
TIA/EIA-455-XX becomes FOTP-XX. These include awide variety of recommended lnethodsfortesting
the response of fibers, cables, passive devices, and electro-optic components to environmental factors
and operational conditions. For example, TIA/EIA-455-6A-L997, or FOTP-60, is a method published in
1997 for measuring fiber or cable length
Oueruiew
Opticetl Fiber Communieations
to measurement methods for links and networks The-rnajor organizations
and Electronic Engineers
ar; the American National Standards Institute (ANSI), the Institute for Electrical
optics system are test
fiber
for
interest
panicular
(IEEE), the ITU-T, and Telcordia Technologies. Of
range G.650 and
(in
number
the
G
series
the
standards and recorirmendations ffom the ITU-T. wiifrin
optical
multiplexing,
wavelength
amplifiers,
fiigfreri the recommendations r€late to fiber cables, optical
passive
for
coltrof
and
management
*d
,iirpon nerworks (oTN), system reliabiljty and availabilify,
recommendations deal with the ciinstruction,'installation'
optical nerworks (poN). Tli" L.seri"s
in the optical fiber outside
rfi?ointenance support,,monitoriiig, and testing of cable and other elements
sgstem standardsrefer
lru.t
Technologies provides a wide range of generic
fionr, ,frur is, tlhe fielded cablelsystem. Teicordia
the GR-3120
ieqoirements for telecommU*e[tion network components and systems: For.example'
necessary
the
describes
document
Ciiert, n"qutrements foi'Hatdened Fiber Optic Connectors
the field'
in
mated
be
can
and
hardened
specifications for opticJ connectors that are enl'ironmentally
'L
I
functions
io*prr.r-Uured simulation'and modeling tools that integrate componeni, 1ink, and network
-.u, ,i,uk" the design'process of'complex
optical links and networks more efficient, less,expensive, and
personal computers led to the development
i;;";Jr", rt
dr"liferatiorr andlncrease in capabilities of
"ripif
for predicting photonic component, link,
for
these
oi,,,*v rophirti.ut"d';ii";lafion programs
lachines
on well-established numerical models
based
are
tools
arJr[i;"ri p"*oi**a" behavioi. T[ese software
to geometric or position mismatches of fibers,
fibers, behaviors of passive and active
sources
light
of coupli*g.gplical pjwer from
intg
ufto can model many
networks.
"rn"i"r"i"*
optical
complex
of
opticat components,,*A lfr" peiformance
J[1V
waveguide grating
optical'filters,
wavlquiae
as
aet* {eviceg, .1uch
,r;nas-"i
.gouplers,
oflsophistiCation:
#ays, and optical ao***, to'a'ttgh degree
. '
anal can simulate factors such as connector losses due
rrJ;ir
1.8.1
SimulationTol"-Gharacteristics
-o*putir-afand
deqign
(CAy)lools
j
l
i
. :, , .i:
can ofler a powerf'ul method toassist in analyzing the'design of an
poinls to consider,
oprieJ C.rponent,'ci.Cuii, br irenVork before costly prototypes T",brl, Important
fri*"""i,*"'the approit'meitirjr 'and modeling assumptions made in the software design Since most
designed with ,ererai decibels of safety margin, approximations for
general necessary
operating Lehavior that areieasonably accurate,are not only acceptable but in
to allow uactable computation times'
the following characteristics:
The theoretical models used'in computer simulations nominally include
j^'"j|i,;;;^j""|i r" ,t all factors which could influence the performance of the component, circuit.
or neiwork can be apprcpnately evaluated'
g+iAete*.u ttrut simulated devices can be inteiconnected with each other to
telecommunication systems
;.ri",*g
#i
"
. ;;;;;;;'sEOf
f;; "ircuits or rietworts.
. Interfaces
tt ut paSs''s"ftrCiiAt information between the constituent components"so
o
I
o
that all possible
speed, so that quick
Computational effiGiehcy that allows a trade-off between accuracy and
a
design'
estimates of systein peifoimance can be made in the early stages of
The capability to simtilate devices over the desiied spectral bandwidth'
channels,
The ability to simJlate factors such as nonlinear eifects, crosstalk between optical
distortion in lasers, and dispersion in optical fibers'
l
l
il,
the simulation programs normally have
To enable a user to visualize and simulate a system quickly,
the following fearures:
graphical icons and a graphical user
The ability to create a system schematic based on a library of
(such as optical fibers' filters'
interface (GUI). The iCons represent various system components
and spectrum analyzers)'
meters'
power
amplifier9 and instrumentation, 1e.g. data sources,
For example' the user
simulation"
The ability for the user to interact *itf, ttt" program durilq.a
to
evaluate their effect'
in
order
may want to modify a parameter or some op".-uting condition
range of interest is
operating
the
when
This is especially-i-mpo.i*t in the early ,tug"S of a Jesign
o
o
.
o
being established.
A wide range of statistical-analysis, signal-processing,-and.display tools'
and optical specffa, eye diagrams'
Common airpruy forrrats, including tirie waveforms,ilectrical
and error-rate curves.
1.8.2
Commerciallyavailablesimulationtoolsforlightwaveapplications-":":1"1]."",':i::}:::::
(e'g' lasers' modulators'
r*g"ages. In these languages, system components
+^nlo
;;il#;;.##rg
p1?,11i:":ll,T-:::::'"li*,i1":
il1ilr;ffi;ffi;;i;ff;:;;;;;'*"'"q';'i"'*t".T9
ffit#i##,?i,,"'rr#","J-,;r;^rh;;;uiai.ectio"ur:pricalho
*::!:1::r,"i::':"*:T',:*
specifies the values of the owrali""lql'-"::^"^:-t
;ffiffi"r;i;; ; ff#;;;;;;i;;; ,h" userIn addition
to using preprogrammed modules, users can
,fr"
and
^-)
io ini"rfu..
"orrrpor,"nt
create their own custom
J*i"",
^7^e2--
ara ronrpccnterl
hv 2
characteristics.
programming
with either the underlying software code or the graphical
language.
of a complex component'
irliig'rr"t a set of glaphical icons, one ian put together a simulation
*:-,,+^^
A-o oimnlrr cclecte
^r
k, ;;;"fi'?#;;;il;fu1transmission puth,1'::111"111'1.1.3]i"'-**';^':l,Ti:
and connects them together
'#pili"t
;#'i":"Hffi#t,Iffiffi;J;;-;;;;;i; and measurement instruments,
,-- rlru^- rL^ .l^-irn
io
a
#tr*;*I;"r;;i;;?ate
ffi;;;""ffiilr
^nmnlcterl
when the design is completed'
a model of trre optical transmission system.
, r L--^a^.^^ ^- ^ +^^l bar'
tr
^a
tool
on
buttons
control
using
run
be
can
simulation
the
r"prarv
pP
"ro
and connected together, the complex and challengilg
d,l"rrh;;;;it,;*-f,";; s"te"ted
::3f,:
optical
and
of the electrical
for the user. This ir";;;';;;;sing rearistic ranges o1 the parameters
in an actual application'
----^1^-,
^-l
^L^ll^-^ian
nq
ud.res make sense
components and suu-mJdes. It is imlortant that the-p*u*"t"r
of vendors'
sheets
specification
the
In some cases this may entail examining
1.8.3
ExamPle Programs for Student Use
Several commercial vendors offer various suites
of software-based modeling-tool modules for optical
for use across all levels
*viili;';; rrr* o"tign and planning tools are intended
ranging from
comparisons
techno1ogy
and
of lightwave network ;;;;;t, p"rfor.-""-evaluations,
links, to entire optical networks'
fiber communication
passive and active .o*porr!nt*'and modules,.to complex ransmission
to offer a wide range of settable options
Familiar measurementTr-r*-*r are built into the software
sweeps across a parameter
multidimensional
when displaying data from multiple simulation runs and
laboratory setup'
any
mimic
to
modules allow data to be manipulated
network operators'
integrators,
system
Such tools *" in or"-Uy component and system manuficturers,
ofvarious
assessments
comparative
planning
and access service providlrs foifunctions such as capacity
system
WDM
of
analyses
and
syntheses
technologies, optimization of ffansport and service netwoikt,
tools
simulation
these
using
are
universities
many
and link designs, and component designs. In addition,
;;;.iG#-pio""*rinj
for both research and teaching purposes'
Owruteu oJ apdcal
F-tber Commurtlmtlorts
Abbreviated versions of several simulation modules may be downloaded for noncommercial
educational use from the websites of the tool vendors. These simplified versions contain predefined
component and link configurations that allow interactive concept demonstrations. Among the numerous
demonstration setups are optical amplifier structures, simple single-wavelength links, and WDM links.
The configurations are fixe4 but the reader has the ability to interactively change the operational parameter
values ofiomponents such as optical fibers, light sources, optical filters, and optical amplifiers. As part
of the results that are possible, images of what would appear on the display screen of a standard instrument,
such as a spectrum analyzer or an oscilloscope, enable the user to see the effects on link performance
when various component values change.
The book websiie (http://www.mhhe.com/keiser/ofc4e) describes current offerings available on vendor
websites of the demonstration modules that the reader may download and learn from.))
* pporr-,nt\{s
(ii)
are the energies in electron volts
v trl what
(eV) of light at wavelengths 850, 1310,
1.8
1490, and 1550 nm?
(b) Consider a l-ns pulse with a 100-nW
amplitude at each of these wavelengths.
-6 dBm,6 dBm, 17 dBm.
A signal travels from pointA to point B. (a)If
How many photons are in such a pulse at
each wavelength?
designed so that each channel has a spectral
width of 0.8 nm. How many wavelength
25
1rs,
250 ns, 125 ps. r$fhat are their
@
What is the total gain in dB? BY what
frequencies?
1.4 A sine wave is offset 1/6 of a cycle wittr respect
to time zero. What is its phase in degrees and
in radians?
1.5 Consider two signals that have the same
frequency. When the amplitude of the first
signal is at its maximum, the amplitr'rde of the
second signal is at half its maximum from the
zero level. What is the phase shift between
the two signals?
What is the duration of a bit for each of the
following three signals which have bit rates
of 64 kb/s, 5 Mb/s, and 10 Gb/s?
1,7 (a) Convert the following absolute power
gains to decibel power gains: 10". 0.3. l.
4, 10, 100,500,2".
(b) Convert ttre following deqibel power gains
to absolute power gains: -30 dB, 0 dB, 13
dB, 30 dB. lOn dB.
pointA and 0.125
mW at point B, what is the attenuation in dB?
(D) What is the signal power at point B if the
attenuation is 15 dB?
A signal passes through three cascaded
amplifiers, each of which has a 5-dB gain'
the signal power is 1.0 mW at
A WDM optical transmission system is
channels can be used in the C-band?
1.3 Three sine waves have the following periods:
(a) Convert the following absolute power
levels to dBm values: 1 PW 1 nW 1 mW,
l0 mW 50 mW.
(b) Convert the following dBm values to
power levels in units of mW: -13 dBm'
numerical factor is the signal amplified?
optical fiber has a total
@ A 50-km long
pW of
of 24 dB.If
*t@
"@
optical
500
attenuation
power get launched into the fiber, what is the
output optical power level in dBm and inpW?
Atransmission linehas abandwidth of 2 MHz.
Ifthe signal-to-noise ratio at the receiving end
is 20 dB, what is the maximum data rate that
l* L i'; i
this line can support? 4
(a) At the lowest TDM level of the digitaf'
service scheme, 24 channels of 64 kb/s
each are multiplexed into a 1.5'14-Mb/s
DSl channel. How much is the overhead
that is added?
(b) The next highermultiplexed level, the DS2
rate, is 6.312 Mb/s. How manY DSI
channels can be accommodated in theDS2
rate and what is the overhead?
=
'i=i !r
il
I
I
i
i
I
!
I
voice channels into a SONET frame, 84
columns in each SPE are divided into seven
(c) If the DS3 rate that is sent over a T3line
is 44,376 Mb/s, how many DSZ channels
can be accommodated on a 73 line and
groups of 12 columns. Each such group is called
avirtual tributary.
a) Whatis the bit rate of such a virtral tibutary?
(b) How many 64-kb/s voice channels can a
(d) Using the above results, find how many
(
DSO channels can be sent over a 73 line.
virtual tributaly accommodate?
What is the total added overhead?
.-.
insert
low-speed signals such as 64-kb/s
ro
(9
tr npr.nRENCES
1.
S. E.
Miller and A. G. Chynoweth, eds, Optical
Fiber Telecommunications, Academic, New
York, 1979. This book and the texts listed in
Refs. 2, 3, and 4 contain dozens of topics in all
areas of optical fiber iechnology presented by
researchers from AI&T Bell Laboratories over
a period of more than twenty Years.
2. S. E. Miller and I. P. Kaminow, eds., Optical
Fiber Telecommunications-Il, Academic, New
York, 1988.
-.,
Fibir
Telecommunications-Ill, vols. A and B'
I.
P.
Kaminow and T.L Koch , eds., Optical
Academic, New York, 1997.
4. I. P. Kaminow and T.Li, eds., Optical Fiber
Telecommuni.cations-IV, vols.
A
and' B,
Oxford University Press USA, New Yorlf;6tlr
ed.,2OA6.
tJ
t7. B. E. A. Saleh and M. C. Teich, Fundamentals
of Photonics, Wiley, Hoboken, NJ, 2nd ed.'
2007.
R. L. Freeman, Fiber-Optic Sysrc;s.f@i
Te lecommunications, Wiley, Hoboken' NJ,
Proceedings IEE, vol. 113, pp. 1151-1158, July
G. P. Agrawal Fiber Optic Communiution.
Systems,Wiley, Ho6oken, NJ, 3rd eA.,
8.
t6. A. Yariv and P. Yeh, Photonics: Optical
Electronics in Modern Communications,
Academic, New York, 2002.
M. Senior, Optical Fiber Communications,
Prentice-Hall, Englewood Cliffs. NJ, 2nd ed.,
t992.
2002.
7.
I. A. Buck, Fundamentals of Optical Fibers,
Wiley, New York,2004.
13. K.P. Ho, Phase-Modulated. Optical Communication System.s, Springer, New York, 2W ',
14. G. Keiser, Opticat Communications Es sehiial*,
McGraw-Hill, New York, 2003.
15. G. Keiser, FTIX Concepts and Applications,
Wiley, Hoboken, NJ, 2(X)6.
t2.
18. E. Snitzer, "Cylindrical dielectric waveguide
modes," J. Opt. Soc. Amer, vol. 51, pp.49l498, May 1961.
19. K. C. Kao and G. A. Hockman, "Dielectric-fibre
5. J.
6.
(c) What is the payload efficiencY?
2W/
R. Ramaswami and K. N. Sivarajan, Optical
Networlcs, Morgan Kaufrnann, San Francisco,
2nd ed.,2OO2.
9. E. Desurvire, Erbium-Doped Fiber Ampkfislpi
Wiley, Hoboken, NJ, 2002.
10. E. Desurvire, D. Bayart, B. Desthieux, and S'
Bigo, Erbium-Doped Fiber Amplifiers: Devices
and System Developmmts,'Wiley, New York,
2002.
11. B. Razavi, Design of Iniegiated Circuits for
Optic al Coitmunicartons, McGraw-Hill, New
York, 2003.
surface waveguides for optical frequencies,"
1966.
20. J. Hecht, City of Light,OxfordUniversity Press,
New York, 1999.
2t. Optical Fiber Communications (OFC) Conf. is
cosponsored annually by the Optical Society of
America (OSA), Washington, DC a4*'t{e
Institute of Electrical and Electronic Engiqgeis
(IEEE), New York, NY.
22. European Conference on Optical Fibre Communications (ECOC) is held annually inEurope;
sponsored by various European engineering
organizations.
23. Photonics Wesr held in the US
arrd
Asia'Pacific
Optical Communications (APOC) held in Asia
are two of a number of annual conferences
sponsored by SPIE, Bellingham, WA, USA-
Oueruiew of @ticalF'iber
24. Special Issue: "100 years of communication
piogress," IEEE Comm*n. Mag., vol.22,May
1984.
lWidjaja, Communicqtion
25. A. Iron.Garcia and I.
Networks, McGraw-Hill, New York, 2nd ed',
2004.
26.
J. G. Proakis, Digital Co-mmunications,
"McGraw-Hill, New'York, 4th €d., 2001.
27. A, B. Carlson, P. Crilly, and J. Rutledge,
Communication Systems, McGraw-Hill,' Burr
Ridge, IL, 4th ed., 2OO2'
28. L.
*.
Conuumiffi
37. H. G. Perros, Connection-Oriented Netvvorks:
SONET/SDH, ATM, MPLS and OPtical
Networlcs,WileY, Hoboken, NJ, 2005.
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