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. 38. W. Goralski, SONET/SDH,McGraw-Hi11, New York, 3rd ed., 2003. 39. A. Scavennec and O. Leclerc, "Toward highspeed 40-Gb/s transponders," Proc. IEEE, vol' 94, pp.986-996, MaY 2006. bai, C. R. Davidson, M. Nissov, H' Li, W. T. Anderson, Y. Cai, L. Liu, A. N. Pilipetskii' D. G. Foursa, W. W. Patterson, P. C. Corbett' A. J. Lucero, and N. S. Bergano, "Transmission of 40-Gb/s WDM signals over transoceanic 40. J.X. Couch fi.,Digital and'Analog CommuniSystem,s, Prentice Hall, Upper Saddle cation distance using conventional NZ-DSF with rrceiver dispersion slope compensation," /' Lightwave Technology, vol. 24, pp. I9l-200, River, NJ, 7thed.,2W. 29. F.P. Kapron, D. B. Keck, and R. D' Maurer, "Radiation losses in glass optical waveguides," Appl. Phys. Lett., vol. 17, pp. 423'425, Nov.1970. Jan. 2006. E. Le Rouzic and S. Gosselin, "160-Gb/s optical networking: A prospective techno-economical analysis," J. Lightwave Technology, vol' 23, 4t. 30. W. A. Gambling, "The rise and rise of optical fibers," IEEE J. Sel. Topics Quantum Electron', vol. 6, no. 6, pp. 1084-1093 Nov./Dec' 2000' 31. B. St. Arnaud, J. Wu, andB. Kalali,'Customer- pp. 3024-3033, Oct. 2005' S. Vorbeck, R. Leppla, E. Lach, M. Schmidt, S. B.Papemyi, andK. Sanapi, "Field M. S"ho"id"r., 42. controlled and -managed optical networks," J' Lightwave Technology,vol. 21, pp' 2804'2810, Nov. 2003. 32. transmission of 8 x 170 Gb/s over high-loss SSMF link using third-order distributed Raman amplification," l. Lighhuave kchnology,vol' 24' pp.175-182, Jan. 2006. D. Simeonidou,R.Nejabati,G. Zervas,D' Klonidis, A. Tzanakaki, M. J. O'Mahony, "Dynamic optical-netswork architectures and technologies for existing and emerging grid services," '/' Lighnvav e kchnology, vol. 23, pp. 1347 -3357' Oct.2005. 33. N. Taesombut, F. Uyeda, A. A. Chien, L. Smarr, T. A. DeFanti, P. Papadopoulos, J. Leigh, M' Ellisman, and J. Orcutt' '"The OptlPuter: Highperformance, QoS-guaranteed network service ior emerging e-science applications," IEEE Commun. Mag., vol. 44, pp. 38-45' May 2006' 43. M. L. Jon"., "Optical networking standards," J. Li ghtw av e 200,4. Co**un. Mag., vol. 23, pp. 46-57, May 1985' 35. ITU-T Recommendation G.Sup39, Optical Sy stem lar G.600 Series Recommendations for all aspects of optical fiber communications' 45. Spe-iat issue on "standards activities: Addressing the challenges of next-generation optical networks," K. Kazi, guest editor, Optical Networks Magazine, vol. 4, issue 1, Jan'/Feb' 2003. 46. J. Piprek, ed', Optoelectronic Devices: . Advanced Simulation and Analysis, Springer, New York, 2005. K.Kawano and T. Kitoh,Introductionto Optical Desi gn and Engineering Consi'derations, Feb. 2006. theorem36. applications: and Its various extensions A Tutorial Review," Proc. IEEE, vol' 65, A. J. Jeni, "The Shannon sampling pp. 1565-1596, Nov. 1977. chnolo gy, v ol. 22, pp. 27 5 -280, M. Telecommunications Sector-International Telecommunication Union (ITU-T), various 34. D. H. Rice and G. Kpiser, "Apptcations of fiber optics to tactical communication systems," IEEE Te 47 Waveguide Analysis: Solving Maxwell's Equation and the Schrddinger Equation,Wrley, Hoboken, NJ, 2002. A. J. Lowery "Phototric simulation t@Is," in 51. I. Kaminow and'T. I-i; edr.; Optical Fiber Telecommuniaotiansi IY*B' Systems and I mp ai rme nt s, Acdernie, 2002. 49. A. J. Lowery, *WDM sysGms sinnilations," in A. Gumaste and T. Anthony, eds.; DWDM Nen+tork Desigw and Engi*ering Solutions, Cisco Press, 2003. G. Guekos, ed., Photonic Telecommunicalions: II Devices for w to Model and Measure, SpringeqNew Yas*, 1998. N. Antoniades, I. Roudas, Q. Ellinaq, aild V. Grigoryan, Modeling and Computer-Aided Design of Optical Commwic,atiow Sys;*tas and ilerworlr,,Springer, New Yorlq 2007. 52. YPlsystems, Inc., Hohdel, Na*, Jersey USA, wwwvpisystems.com. 53. RSoft Design Group; Inc., Ossioing; New Yortq USA, www.rsoftdesign.com. 54. Optiwave Systems, [nc., Toronto, Ontario, Canada, www.optiwave.com. 55. The book website can be f,ouad at htp ://www. mhha comlkeiser/ofc4e. 3