Lightwave Primer Henry IEEE 1985

8/7/2019 Lightwave Primer Henry IEEE 1985

1/18

1862 IEEE J OUR NAL OF Q U A N T U M ELECTRONICS, VOL. QE-21, NO. 12 , DECEMBER

PapersLightwave Primer

(Invited Paper)Abstract-This paper presents an introduction to the principles of

lightwave system engineering. The treatment is historical rather thancategorical-lightwave systems are described in terms of theirevolutionthrough four generations of technology, from a first generation oper-ating at 0.85 pm wavelength over multimode fiber to a fourth generationemploying coherent techniques at 1.55 pm. Basic engineering consid-erations such asfiber dispersion and receiver sensitivity are introducedearly, then refined as the discussion progresses toward higher-perfor-mance, more sophisticated systems. The fundamental mechanisms thatlimit the performance of agiven technology are quantified, and a figureof merit, the product of bit rate times maximum repeater spacing, isestimated. Values of this product range from about 2 Gbitsis . km forfirst-generation technology to roughly 900 Gbits/s . km for coherentsystems.

I.INTRODUCTIONA . General DiscussionAscant decade ago most activity in optical-fiber com-munications was confined to laboratory experiments .ments. Since that time the field.hasdeveloped rapidly, andnow lightwave fiber systems find broad application bothon land and under the sea [11-[3], [1021, [1031. The en-gineering principles of these systems are the subject ofthispaper.Ratherthanstudylightwavesystemsfromaglobal point of view, where the goal isto elucidate a smallnumber of very general principles, our approach will begenerallyhistorical-thedevelopment of lightwave sys-tems from the past to the foreseeable future will beviewedin terms of evolution through four generations of technol-ogy. The first of these, discussed in Section 11, uses mul-timode fibers and an operat ing wavelength of about 0.85pm. First-generationsystems have enjoyedwidespreadpopularity and ar e only now beginning to feel competitionfrom the second generation, which operates at a longerwavelength, 1.3 pm. As described in Section 111, the shiftto this wavelength permits greater separation between therepeaters in along-distancetransmissionsystem. Toachieve the highest level of performance, most second-generationsystemsusesingle-modefiber,ratherthanmultimode. The path of lightwave evolution beyond the

Manuscript received Apri l 29, 1985; revised September 9, 1985.The author is with AT&T Bell Laboratories, Crawford Hill Laboratory.Holmdel , NJ 07733.

second generation is unclear because 1.3 pm systemsstill relatively new and there are many practical lessoyet to be learned. It seems probable,however, that the generation, described in Section IV, will use a still lonwavelength, 1.55 pm, where the attenuationof silica fiisataminimum.Severallaboratoryexperiments hdemonstratedthehighsystemperformancethatcanbeachieved at this wavelength. Further improvementin liwave systems beyond the thirdgenerationmightcomefromtheintroduction of coherentopticaltechniques,which are discussed in Section V. Practical applicationthese techniques could make possible a fourth generatof lightwave technology.B. Basic Lightwave Link

The focus of our attention in this paper will beon liwave systems for transmission of digital information oa distance of a kilometer or more. While there are mapplications for analog lightwave systems, a discussionthem here would needlessly complicate the presentationand enhance only slightly our understanding of basic pciples. Similar reasoning applies to our neglect of shlength systems: they are very important, but do not lethemselves to simple description and straightforwardysis.The basic digital lightwave transmission link is shin Fig. l(a). The input data stream modulates a transter, whose output typically is simple binary intensity ulation, also called on-off keying (OOK). (In Sectionwewill discussmoresophisticatedmodulationtech-niques.) After it is launched into an optical fiber, thenal may travel tens or even hundreds of kilometers bearriving at the receiver, where it is detec ted. The tramission link shown in Fig. l( a) might be a complete tem in itself, or, what is more likely, it might be part repeatered line [Fig. l(b)] consisting of several links ccaded together to cover a longer distance than could bespanned by a single transmitter-receiver pair.The primary measure of performance of a digital stem is the bit error rate (BER), which is the probabthat an error will be made in the detection of a receibit. The maximum tolerable BER depends on the syst

0018-919718511200-1862$01.000 1985 IEEE


2/18

HENRY: LIGHTWAVE PRIMER 1863

TRANSMITTER RECEIVERmDATA OUT

(a)

TRANS.(b )

Fig. 1. (a)Digita ll ightwavetransmissionlink.(b )Lightwaverepeateredline composed of a cascade of simple links.

\

cB 4 -z ABSORPTION /

FIBER

2 1.0: ....ATTENUATIONVIBRATIONAL

ln 0.8z 0.60.4

z 0.2-- e....- RAYLEIGH 2 s . .2 SCATTERING

t- 0.13 0.08..*......L.

z 0.06I I I I I l l 1 I L,U

jl- SINGLE-MODE FIBER-+FIBERI- - MULTIMODEwc0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.8 2

WAVELENGTH (pm)Fig. 2. Attenuation constant as a function of wavelength for high-qu alityoptical fiber. Fiber loss, which is dominated by Rayleigh scattering andvibrational absorption, reaches a minimum in the vicinity of X = 1.55p m . OH absorption peaks are shown by d ash es .

application, but for our purposes we may use BER 5 lop9as the criter ion for satisfactory performance.C . Fiber A ttenuationAs an optical signal propagates along a fiber, it is at-tenuated. The power level at a distance 1 kilometers fromthe transmitter is

P(I) = PT * 10-A/10 (1)where PT is the power launchedby the transmitter intothefiber and A is the attenuation constant of the fiber ex-pressed in decibels per kilometer. It is the extremely lowattenuation of optical fibers (of order 1 dB/km), coupledwith their small size (- 0.1 mm diameter), which makesthem so attractive for communications purposes. The de-velopment of practical techniques for making fibers withlosses only slightly greater than the theoretical minimumrepresentsanextraordinaryengineeringachievement.Some of the early fibers , which were used to provide il-lumination in confined areas, showed attenuation in theneighborhood of 5000 dB/km [4]. Even as recently as1971, an attenuation of 20 dB/km was the best that couldbe achieved [5]. Today, fiber loss close to 0.2 dBikm iscommonplace. Fig. 2 showstheattenuation of todayshigh-quality (although not necessarily champion) fusedsilica fibers for wavelengths between 0.8 and 1.8pm, whichis the region of interest for communications systems [6]-[8]. The discontinuity in the figure at 1.1 pm wavelength

reflects the fact that the diagram representslosses for twodifferent types of fiber: multimode for the shorter wave-lengths and single-mode for the longer. The difference be-tween the fibers, and the reasonsfor preferring one or theother, will become clear later on.The fundamental processes which cause attenuation inoptical fibers are Rayleigh scattering, which dominates atthe shorter wavelengths, and vibrational absorption in sillica (and other fiber constituents), which is important atlong wavelengths. The lower limit to Rayleigh scatteringis set by randomdensityfluctuations frozen into thefused silica during manufacture; these lead to a fiber at-tenuation shown by the dotted line in Fig. 2 :0.6A - - d B / hh4

where X is the free-space wavelength in micrometers [9].(Rayleighscattering formultimodefibers is actuallysomewhat higher ( - 50 percent) than that givenby (2) be-cause of enhanced scattering from fiber dopants.) Thesil-*ita vibrational absorpt ion peak is at h - 9 pm: what isshown by the chained linein Fig. 2 is the short-wavelengthtail of this resonance [lo], 1111. The magnitude of the at-tenuation is a sensitive function of fiber composition; thebehavior shown in Fig. 2 represents typical fiber charac-teristics. The important lesson to be drawn from Fig. 2 isthat Rayleigh scattering and vibrational absorption com-bine to produce a low-loss window in silica fiber. Theabsolute loss minimum is near 1.55pm, where each of theloss mechanisms contributes -0.1 dB/km to fiber atten-uation.The loss peaksshown by thedashedportions of thecurve in Fig. 2aredue to absorption by OH ions.Al-though this loss mechanism is not intrinsic to optical fi-bers, the OH ion is ubiquitous, and removing it from thesilicaduring fibermanufacture isextremelydifficult.High-qualityfiber now inproductionshowsexcess OHattenuation at1.39pm in therange 0.1-2 dB/km 1121-

Reducing fiber attenuationis important because a light-wave receiver requires at least a minimum amount of op-tical signal power P , in order to detect a transmitted bitwith an acceptable error rate. The maximumlength L overwhich a signal can be transmitted before it is too weak tobe detected is obtained from (1):

~ 4 1 .

10 PTL = - loglo -.A PRL given by this equation is called t h e loss-limited trans-mission distance. Equation (3) , as simple as it is, makesan important point: the loss-limited transmission distancedepends strongly on fiber attenuation,but only weakly ontransmitter and receiver power levels. For example, mov-ing the operating wavelength from 0.8 to 1.55 pm reducesA by roughly a factor of 10 (see Fig. 2), and thereby in-creases L by the same factor . A similar improvement inPT or P,, however, yields only a 25 percent improvement


3/18

1864 IEEEJOURNAL OF QUANTUMELECTRONICS. V O L . QE-21. NO . 12,DECEMBERA

6001 . 5 5 ~ m QUANTUMLIMIT ~ F I B E RB2L= 4000(G b /s )2 . km40 0

- SIN G LE-MO D E

20 0 - 1.55pm(A.0.25 d B / k m ) / I ) 0 .n \J$100 - 1'3Pm ( A . O . 4 0d e / k m ) .-,-x 80 - '\& \v 60 -Wu 40 - SL eza e.. e. 0 . 8 5 p m Op..t- 20 -2n \ 0\ ..... \ F-400M \\ \ *. \*. B L = 2 5 0G b/s - k rn \10 - \ *.

8 -

4 -6 -

\ '.. \\ \F T 3 C \\ COAXIAL \STEP-'\ CABLE e . \ GRADED-I N D E X \\ \ \ IN D EX

2 - \ \'\I \10.1 1 10 100

I I b1000 ' 10,000BIT RATE (Mb/s)

Fig. 3. Repeater spacing versusbit rate for various lightwave technologies.The loss l imits (solid l ines) and dispersion limits (dashed lines) indicateapproximately where trqnsmission impairments become significant. Thepoints correspond to actual achievements:0 X = 1.3 pm,0 X = 1.5 km , control led-dispersionfiber,H X = 1.55 p m, DF Blaser,A X = 1.55 pm, C' laser,0 commercialsystems.The performance of coaxial cable (dot ted l ine) is shown for comparison.T h e 1.55 km quantum limit represents the performance that would beachieved with the ultimate lightwave receiver (ideal homodyne PSK) an da fiber with A = 0.25 dBikm.

in L for typical system parameters. This disparityin payoffis worth remembering,especially when contrastedwiththe case of radio transmission, where a factor of 10 im-provement in PT or P , yields more than a factor of 3 im-provement in transmission distance.Beforeapplying (3) tolightwavesystems, let usgainsome perspective by studying a different guided-wave me-dium-metallic coaxial cable. Typical parameters for a 100Mbit/s system might be A = 30 dB/km, P T = +30 dBm(1W), and P , = -75 dBm, yielding L = 35 km [15]. Las a function of bit rate is shown in Fig. 3 , where it isassumed thatA a (frequency)"2. The transmissionperfor-mance of coaxial cable depicted in Fig. 3 will be a usefulstandard against which to compare lightwave system per-formance to be calculated in later sections.D. Fiber Dispersion

As the bit rate (or, more precisely, the symbol rate) isincreased, dispersive effects in the fiber become more im-portant. Fiber dispersion causes the transmitted pulses tospread and overlap as they propagate, so they become in-distinct at the receiver,which leads to increased BER. Thevarious causes of fiber dispersion will be discussed later;for now it is sufficient just to consider the system shownin Fig. 4 and recognize that the pulse broadening associ-ated with dispersion manifests itself as a reduction in thehigh-frequency input-output response of the system. As-

TRANSMITTER RECEIVERDISPERSIVEFIBER

" ( O LSYSTEMRESPONSE

FREQUENCYFig. 4. Effect of fiber dispersion on end-to-end lightwave system respoH ( w ) . As dispersion increases, the bandwith of H ( w ) decreases.

sume a constant-amplitude sine wave applied to the trmitter input. As its frequency is increased, the amplituof the output signal at the receiver decreases, resultina low-pass system response. The deleterious effectsofpersion can be corrected after the receiver by usingequalizertoboosttheattenuatedhigh-frequencysignalcomponents. Alongwith the signal enhancement,hoever, comes anunavoidable increase in noise at the boofrequencies, and hence a higher BER. By increasingtransmitter power we can compensate fortheenhancenoise and return the BER to the value it would have in the absence of dispers ion. The added transmitterporequired to accomplish this is called the dispersion pealty, animportantquantity in lightwavesystemdesignTypically, systems operate in a regime where the disp


4/18


sion penalty is small ( 5 1 dB). For this case the penaltyis simply related to fiber dispersion, as we now show. Letasufficientlynarrow(intime)optical pulse be injectedinto a fiber; the time dependence of the pulse detected atthe far endof the fiber is calledh ( t ) ,a readily measurablequantity. The corresponding baseband frequency responseis the Fourier transform of h ( t ) :W

H ( w ) = h ( t ) ejwfdt. (4)For the simple case of modest dispersion and symmetrich ( t ) , a Taylors series expansion of h ( t ) yields

-a

H ( w ) - H ( 0 ) { 1 - p1 2 2e , } ( 5 )e,is the mean-square width of h ( t ) , sometimes called themean-squareimpulsespread.Asthefrequency w in-creases,thebasebandfrequencyresponsedecreases,asexpected. Not surprisingly, the effect of this decrease onthe error-rate performance of a digital system shows upmost strongly in data sequences which have the greatesthigh-frequency content; that is, sequences which have lotsof alternations between binary marks and spaces. Indeed,to estimate the effect of dispersion on error rate we needonlyconsiderthedottingsequence: 101010 * * . Mostof the power in this sequence is carried by the Fouriercomponent at w = a B where B is the bit rate. To com-pensate for the dispersive attenuation of this component,the transmitter power must be increasedby [ H ( n B ) ] - ,sothe dispersion penalty is

Po - -10 loglo H ( 9 B ) - 2 1 ( 0 , B ) ~ .( 6 )Asanapproximaterule wecansaythatthedispersionpenalty is about 1 dB when the rms impulse spread is one-quarter of a bit period, and grows rapidly as pulse-widthincreases [ 1 6 ] , [ 1 7 ] .For system design purposes we mayuse

a f B I4 ( 7 )as a condition for avoiding excessive dispersion penalty.We expect to see e, increase with fiber length, reflectingthe fact that a pulse continues to spread as it propagatesalong the fiber. Thus, the condition given by (7) places aconstraint on maximum fiber length, independent of theloss considerations discussed earlier. The maximum fiberlengththatsatisfies ( 7 ) is calledthedispersion-limitedtransmission distance. This limit and theloss limit definedearlier are useful indexes for comparingdifferentlight-wave systems.E. Bit Rate-Distance Product

A single figureof merit that could characterize theover-all performance of a digital link would be a most usefultool. Alas, it appears impossible to define a measure thatOur impl ici t assumpt ion, that the receiver output signal i s l inear i n th eoptical power input, is valid for most lightwave systems.

would be fair to all. Nonetheless, the information-carry-ing capacity of a link, given by the product of bit rate ( B )and distance betwen transmitter and receiver( L ) ,providesat least a crude basis for comparisons of different light-wave technologies. Consider a system in which a total bitrate BT >> B is to be transmitted over a total distanceL T>> L. To span this distance requires LTIL repeater sec-tions in cascade. To carry BT requires BTiB links operatingin parallel. Thus, the total number of repeaters requiredfor the system isBT * LTN R ==7.B L

The higher the B . L product of an individual link, thesmaller the required number of repeaters. All other thingsbeing equal, the fewer repeaters a system has, the moredesirable it is. The concept of bit rate-distance productwill be useful in later sections.11. FIRST-GENERATIONLIGHTWAVE:X - 0.85 pmA . SystemOverview

The first generation of practical lightwave systems usedAlGaAslasersandlight-emittingdiodes(LEDs) assources (X - 0.85 pm), multimode fiber as the transmis-sion medium,and siliconp-i-nsandavalanchephoto-diodes (APDs) as detectors. Multimode fibers, althoughnot capable of the low-loss, low-dispersion performanceof single-mode fibers, were (and are) extremely attractivebecause their large core size ( - 50 pm versus - 8 pm)eases manufacturing tolerances and makes fiber splicingfairly easy.We saw in Section I that fiber loss and dispersion placebounds on the maximum fiber length from transmitter toreceiver. Let us now evaluate those limits for first-gener-ation lightwave systems.

B. Loss LimitA reasonable value for fiber attenuation at h = 0.85 pmis A = 2.5 dBikm; this is slightly higher than the valueshown in Fig. 2 to allow for the inevitable splicing lossesthat occur in practical systems. To evaluate the loss-lim-ited transmission distance from ( 3 ) , only crude estimatesof PT and P , are needed because the logarithm is such aweak function of its argument. For PT a value of 1 mW is

reasonable. To estimate P R we need to know the bit ratebecause higher-speed receivers need more optical power.The optical energy per bit, however, is roughly constant,independent of bit rate. An Si APD receiver requires aphoton flux of about 300 photons per bit to achieve BER< thus, P R - 7 X B mW where B is the bitrate [181-[20]. Theloss-limitedtransmissiondistancecorresponding to these parameters is shownin Fig. 3 as afunction of system bit rate. For speeds above- 2 Mbits/s,optical fiber allows longer loss-limited transmission dis-tance than coaxial cable. The advantage of fibergrowsas the bit rate is increased because with cable the distance


5/18


6/18


semiconductor devices used as sources and detectors. Inorder to calculate the system performance bounds in Fig.3, no further detail was necessary. But device character-istics are an important considerationin system design, andsome modest familiarity with them is essential to an un-derstanding of lightwave system capabilities.1) Sources: Althoughthereare twousefultypes ofsource at X = 0.85 pm, lasers and LEDs, the former arethe more popular for transmission distances beyond 1 kmor so, which is the region of interest in this paper. Com-pared to the LED, th e laser can launch more powerinto afiber, can be modulated at higher speeds, and has a nar-rower spectral ,width. (The narrow spectral width of thelaser minimizes pulse spreading due to material disper-sion,whichwillbediscussed in Section 111.) Forfirst-generation high-capacity transmission systems, these ad-vantages offset the increased ruggedness and lowercost ofthe LED.The characteristics of the AlGaAs double-heterostruc-ture injection laser areso beautifully matched to the needsof lightwave communications systems that very little spe-cialized modification of the basic device is needed. Typi-cal lasersdeliver a few milliwatts and can be directly mod-ulated(byvaryingthedrivecurrent)up to gigabit persecond rates. Achieving adequatereliability, however, wasa very difficult problem, whose solution required an inten-sive engineering effort. With the early lasers a lifetime ofan hour was sufficient for a laboratory demonstration, butfor practical communications applications a lifetimeof 10-100 years is needed [27]. Improving the lifetime by fiveorders of magnitude was accomplished by painstakinglyidentifying failure mechanisms, especially dark-line de-fects, and devising ways to eliminate them [28], [29].

Although the relativelynarrow spectralwidth of thelaser (- few nanometers) minimizes material dispersioneffects in the fiber, it is not an unmitigated blessing. Theoptical intensity distribution across the core of a fiber isgranular, rather than smooth. This so-called speckle pat-tern becomes more pronounced as the laser spectral widthdecreasesbecause it depends oncoherentinterferenceamong the modes in the fiber. If there is any spatial filterin the fiber, such as an imperfectly mated connector or adirectional coupler, the transmission through the fiberwilldepend on the details of the speckle pattern at the filterand will exhibit random variation withtime as the specklesmove about due to inter ference effects. This system im-pairment is called modal noise; its effects are most severein analog systems where high (40-60 dB) optical signal-to-noise ratios (SNRs) are required [30], [31]. In digitalsystems the degradation is not as great, but it is still se-rious enough that lasers with extremely narrow spectralwidth are to be avoided.2) Detectors: Silicon, the mainstay of the integrated-circuit industry, is also an excellent photodetector in thewavelength range 0.4-1.0 pm [32]1 As with sources, thereare two choices for the detector structure: a p-i-n diodeor an APD. Typically, Si APD receivers enjoy a 10-15 dBsensitivity advantage over Si p-i-n receivers because the

APD has internal gain, which makes the noise of the am-plifier stages after the detector relatively less important.Details of receiver design and performance cannot be dis-cussed here because of space limitations. It will suffice tosay that the state-of-the-art sensitivity for Si APD receiv-ers at speeds up to several hundred megabits per secondcorresponds to a photon flux of roughly 300 photons perbit.Two disadvantages of the use of the APD are its highsupply-voltage requirements ( 2 5 0 V versus 5-10 V forthe p-i-n) and its temperature sensitivity. Both these draw-backs necessitate more complicated (and therefore moreexpensive) support and control electronics, but for high-capacity systems the enhanced performance appearsto beworth the price [26].111. SECOND-GENERATIONLIGHTWAVE:X - 1.3 pm

A . Transition to a Lon ger WavelengthFig. 2 shows that increasing the operating wavelength

from 0.85 pm to the 1.3-1.6 pm region is a promising routeto enhanced system performance. Forvalues of P, and PRcomparable to those used for first-generation systems, thereduced fiber loss at the longer wavelengths increases theloss-limited transmission distance by a factor of 5-10, asshown in Fig. 3. Only recently has it been possible to ex-ploit this low-loss region; suitable sources and detectorsbased on InGaAsP and Ge had to be developed first. Thepractical implementation of these devices marked the be-ginning of second-generation lightwave technology.Some of the earliest system designs to take advantageof reduced fiber attenuation at1.3 pm wavelength usedmultimode fibers and LEDs rather than lasers [33], [34].The low fiber loss (-0.6 dB/km) roughly compensatedforthe modest output power of the LED, resul ting in a loss-limited transmission distance comparable to that of lasersystems at X = 0.85 pm. The advantages of the LED sys-tem are simplicity of design, low cost, and very high re-liability. Thesefeatures have madeLED-basedsystemsattractive for commercial applications.The 1.3 pm LED system represents a transition betweenfirst- and second-generation lightwave systems. While itoperates at anew wavelength using newly developed semi-conductor components, its bit rate-distance performanceis no better than that of 0.85 pm systems already in exis-tence. The big leap in performance comes with the intro-duction of single-mode fibers, which have both lower dis-persion and lower loss than multimode designs.B . Single-ModeFibers

A single-mode fiber, as its name implies, supports justa single waveguide mode. The simplest single-mode fiberstructure is similar to thestep-indexmultimodefibershown in Fig. 5 , except that the core diameter is muchsmaller,typically 8 pm, so higher-ordermodescannotpropagate. An impulse injected into such a fiber propa-gates as a single waveguide mode, so modal dispersion isnonexistent.Dispersive effects arenot entirely absent ,


7/18


8/18


t - cw

- 1 nm UWAVELENGTH

Fig. 8. Spec t rum of a typical InGaAsP injection laser; CW (solid line) andmodulated (dashed l ine). The spect ral broadening caused by modulationleads to increased dispersion pena lty.to laser oscillation, i.e., high optical gain and/orlow cav-ity loss. If this region is small enough, only the funda-mentaltransversemode will have sufficientlysmall netloss to sustain laser oscillation.A second important property of the laser, although per-haps not a strict requirement, is the capability of beingdirectly modulated at high speeds. Although the earliestsecond-generationsystemsoperated atspeeds of a fewhundredmegabitspersecond,thehighest bit rate-dis-tance products will be achieved only at speeds above 1Gbit/s, as shown in Fig. 3. Not all lasers are capable ofmodulation at these speeds. Indeed, there is a correlationbetween the structure used for transverse mode controland the speed capability of the laser; some designs showpoorperformance at 500 Mbits/s, while othersoperatewith only moderate degradation up to at least 8 Gbits/s[451>V6I.The final item in our discussion of laser characteristicsis spectrum control : a laser which oscillates in a singletransverse mode and has high-speed modulation capabil-ity (our first two requirements) may still not be suitablefor use in single-mode systems if its spectrum is not wellbehaved. Under CW conditions a typical laser spectrummight consist of a few lines spaced about 1 nm apar t, asshown by the solid curves in Fig. 8. (These lines corre-spond to different longitudinal modes of the laser struc-ture.) When the laser is modulated, the weak lines in thewings of the spectrum tend to become stronger, increas-ing T ~and reducing the maximum bit rate-distance prod-uct given by (15). To ameliorate this effect the laser canbe kept well abovethresholdforbothbinarysymbols(marks and spaces). This technique itself causes systemperformancedegradation(although usuallyminor)be-cause the full dynamic range of the laser is not used.Another spectrum-behavior problem is mode partitionnoise [47], [48]. Even though the total output power of alaser is nearly constant, the relative intensities of the var-ious lines in the lasers spectrum can vary considerablyfrom one pulseto the next. The combinationof these spec-tral fluctuations and fiber dispersion produces random dis-tortion of the received pulses, leading to an increase inBER,asshownschematically inFig. 9. Mode partitionnoise is a complicated phenomenon, but its effect on theperformance of high-speed ( B z 100 Mbits/s) lightwavesystems can be summarized simply: for a fixed dispersionpenalty(say 1 dB),thedispersion-limited bitrate-dis-tance product in the presence of mode partition noise is

TRANSMITTEDPULSE SPECTRAPULSE1AxPULSE 2_ILL

RECEIVEDPULSE WAVEFORMS

DISPERSIVEFIBER

TIME

x TIMEFig. 9. Laser m ode partit ion noise. Pu lse-to-pulse fluctuations in the trans-mit ted spect rum, combined wi th fiber dispersion, produce random dis-tortion of the received pulses.roughly one-half of what it would have been if the laserspectrum were not fluctuating. Thus , (15) might be mod-ified to read

and the dispersion limit in Fig. 3 shifted accordingly. Var-ious calculationsandmeasurements of mode partitionnoise yield a spread of results, but most of them are withina factor of 2 of (16) [49]-[51].2) Detectors: Both p-i-n and APD detectors are avail-able at X = 1.3 pm. The most popular detector materialshave been InGaAs for p-i-ns and Gefor APDs [52], [53].(Silicon cannot be used because it is transparent at thiswavelength.) The choiceof which detector to use is a dif-ficult one because the Ge APD is far more temperature-sensitiveandless ruggedthantheInGaAsp-i-n, but itdoes offer a modest sensitivity advantage ( - 3 dB at 420Mbi tds ). In principle, the best features of both types ofdetectors can be combined in the InGaAs APD. Early at-tempts at building such a device were plaguedby high darkcurrent and/or slow response. A new heterojunction de-vice, stillinresearch stage,appearsto have overcomethese problems [54]. Itmay be the prototypefor a practicalInGaAs APD.The best Ge andInGaAsAPDsrequire1000-2000photons per bit to achieve BER s lop9 ; the lower valuewas used for PR in computing the loss-limited transmis-sion distance inFig. 3. ThefactthatAPDs at 1.3 pmrequire roughly four times the photon flux of Si APDs at0.85 pm [see Section 11-D2)] does not significantly reducethe distance advantage stemming from lower fiber loss at1.3 pm.IV. THIRD-GENERATIONLIGHTWAVE:X - 1.55 p m

A . Pros and Cons of 1.55 pm TransmissionTo achievemaximumloss-limitedtransmissiondis-tance,alightwavesystemshouldoperateatthe wave-length where fiber attenuation is smallest . For fused-silicafibers (see Fig. 2) this occurs in the region of X = 1.55pm, where the attenuation constant for high-quality sin-gle-mode fiber is -0.25 dB/km, slightly more than halfas large as the attenuationat 1.3 pm. (Champion fiberswith A - 0.16 dB have been produced in laboratory quan-tities [55].) Since optoelectronic components at 1.55 pm


9/18


10/18

HENRY: LIGHTWAVE PRIMER 187 I

\

MIRRORFig. 11. Structures for single-frequency lasers. (a) Basic Fabry -Perot cav-ity. (b) External cavity . (c) Distributed feedback (DFB ). (d) Cleaved cou-pled-cavity (C3).

t----,O nm--/WAVELENGTH

Fig, 12. Sp ec t ru m of a C laser modulated at 1 Gbitis (solid line). Th e 1A linewidth leads to dispersive effects at 1.55 pm comparable to thoseseen with multimode lasers at 1.3 p m . In addition, mode partition fluc-tuations (dashed lines) can cause bit-detection errors.

behavior, even under modulation, has been observed, withunwanted cavity modes suppressed 20-30 dB relative totheprimarymode.Somesystemdemonstrations usingDFB lasers are indicated by the solid squares ( W ) in Fig.3 1651, [661.Another technique for producing single-frequency op-erationisembodiedinthecleavedcoupled-cavity (C3)laser shown in Fig. 11(d), which uses the interaction be-tween twocoupled laser cavities to enhance a desired modeof oscillation[67].Aswiththe DFB structure, the Claser has shown excellent single-frequency behavior. Theexperimentsmarkedwithopentriangles (A) inFig. 3,each using a C 3 laser, established world-class standards[68], [69]. The 4 Gbit/s experiment, which holds the cur-rent B * L record of 468 Gbits/s * km, is particularly in-teresting because it used an integrated-optic modulatorrather than direct modulation of the laser diode itself. Itseems likely that this technique will be extended to evenhigher bit rates.The relative advantages of the DFB and C 3 designs forpractical applications have not yet been established. Therapid progress made with the C 3 laser since its inventionsuggests that it is easy to make, whereas the DFB struc-ture requires extremely tight process control during fab-rication. On the other hand, the DFB laser is attractivebecause its mode-selection mechanism is integrated deepwithin the device structure; once the basic semiconductordevice is built, no additional construction or tuning is re-quired. A practical repeater using a DFB laser transmit terhas been successfully designed and tested [70].

The term single-frequency laser is a convenient butnot altogether precise description of C and DFB devices.For example, the spectrum of a C 3 laser modulated at 1Gbit/s , shown by the solid line in Fig. 12, shows two sig-nificant departures from ideal single-frequency operation:a linewidth of - I A FWHM, which is orders of magni-tude greater than the intrinsic linewidth, and small butsignificant sidemodes approximately 30 dB weaker thanthe primary mode [71]. Both these features represent po-tentially important limitations on the performanceof third-generation lightwave systems. The observed line broaden-ingisafrequency chirp associated with modulation-induced changes in the carrier density, which leadsto sig-nificant dispersion effects [72]. If we take q,= 0.5 andD = 15 ps/kmnm, we find from (15) that B L islim-ited to roughly 300 Gbitsis . km. That is, the dispersion-limited bit rate-distance product using such a single-fre-quency laser at 1.55 pm is about the same as the disper-sion limit for second-generation systems at 1.3 pm. Fur-therresearchisneeded toproducelasers whichhavesignificantly reduced chirp.The second nonideality of single-frequency lasers, re-sidual sidemodes, leads to mode partit ion noise similar tothat described in Section II I-C l) , but with an importantdifference. At 1.3 pm the difference in propagation delaybetween adjacent laser modes after, say, 100 km of fiberis - 100 ps, which is a small fraction of the pulse periodB - I . Thus, the effect of mode partition fluctuations is tocause random distortion of the received pulses. At 1.55pm, on the other hand, the delay difference between lasermodes can be several nanoseconds, so a mode partitionfluctuationtypicallywill causesome of the power in atransmitted pulse to arrive at the receiver in the wrongtime-slot altogether. Even if the average power in a side-mode is extremely small, the instantaneous intensity canoccasionallyfluctuateuptoalargevalue, say, half theaverage power of the primary mode (dashed curve in Fig.12), and thereby cause a bit detection error at the receiver.The degreeof sidemode suppression needed to.ensureBER5 lo-- is not well understood for all types of lasers, butin the case of simple Fabry-Perot devices [Fig. ll (a )] 17-20 dBsuppressionappearstoberequired[73],[74].Transmission experiments with C 3 and DFB lasers sug-gestthatsubstantiallymorethan 20 dBsuppressionisneeded.D. Fiber Nonlinearities

With present 1.55 pm technology, laser transmitters canlaunch at most a few milliwatts into a fiber. That situationmay change, however; after all, AlGaAs lasers at0.85 pmcan generate close to 100 mW. Perhaps tomorrows lasersat 1.55 pm will have similar or even greater capability. Letus examinethe--systemsimplications of suchaperfor-mance improvement.The most obvious consequenceof increased laser poweris, of course, greater loss-limited transmission distance(3) . A laserwith P, = 1 W mightnearlydouble theachievable repeater spacing in a 1 Gbit/s system. If im-


11/18

1872 l EEE J O U R N A L O F Q U A N T U M ELECTRONICS, VOL. QE-21, N O . 12. DECEMBER

plementation of repeaters with high-power lasers werenottoo difficult, this added transmission distance would be asignificant improvement. The flaw in this scenario is thatnonlinear effects in fibers make it impossible (or at leastverydifficult) tomakeeffective use of high transmitterpower. The most important of these effects are stimulatedBrillouin scattering (SBS) and stimulated Raman scatter-ing(SRS)[75].Bothmanifestthemselves as power-de-pendentexcessfiberloss, which increasesrapidly fortransmi tter power above a threshold level, while remain-ing negligible at lower power levels. For a nearly mono-chromatic transmitter (linewidth d 50 MHz), SBS is themost important nonlinear process; the threshold power isonly about 10 mW. The impairment due toSBScan beeliminated, however, by broadening the transmitted spec-trum so its spectral density is lessthan about 0.1 mWiMHz [76]. This condition can be satisfied either with anoisy laser having sufficient incidental frequency modu-lation or by using modulation techniques which producethe desired spectral characteristics [77]-[79]. In the longrun, then, SBS is not likely to be a serious problem.Unlike SBS, SRS cannot be easily eliminated. It is anincoherent process depending on total transmitted powerand is relatively independent of transmitter spectral width.The power threshold atwhich significant added loss beginsto appear is about 1 W for a single-channel system, butcan be as low as 20 mW per channel for a two-channelWDM system [SO], 1811. This may be a serious limitationfor future lightwave systems because WDM appears al-most certain to be a popular technique [82]. The systempower penalty due to SR S in a WDM system depends onmany details, but as aguidelinewecan say that if thesystem channels are spread uniformly overa bandwidth of40-100 nm, then significant added loss ( 2 1 dB) due toSRS will be encountered when the total launched powerexceeds roughly 40 mW. (More on SRSwill follow in Sec-tion V-C.)

V. FOURTH-GENERATIONLIGHTWAVE:COHERENTSYSTEMSA . TheCloudyCrystal Ball

Predicting the course of lightwave evolution beyond the1.55 pm systems discussed in Section IV is a risky busi-nessbecause it depends so sensitivelyontechnologicalbreakthroughs yet to be made. For example, new optical-fiber materials for use at wavelengths beyond 2 pm can inprinciple have attenuation constants ranging from0.06 dB/km all the way down to 0.0001 dB/km [83]. If these lossescanberealized in actualfibers,the focus of lightwaveresearch might shift away from 1.55 pm systems towardsstilllongerwavelengths,necessitatingentirely new re-search programs to develop suitable semiconductor mate-rials. For the time being, however, the most promising di-rection for lightwave evolution is toward coherent systemsat 1.55 pm, whichwetentativelycallfourth-generationtechnology.The term coherent lightwave can be defined in a va-

riety of ways. For our purposeswe take the broadest vthecharacteristicthatsetscoherentlightwavesystemsapartfromearliergenerations is theiruse of optoetronic devices to process photons in sophisticated wrather than merely detect them [84]-[86]. Not only doherent techniques offer the possibility of improved sysperformance(as weshallseebelow),theyalsoallowamuchrichervariety of systemconfigurationsandfunc-tions. One application of coherent techniques, for exple, makes use of simple optical amplifiers instead ofgenerativerepeaters in long-distancetransmission tems. The simplicity and flexibility of such an approaclikely tobeanattractivefeaturetotomorrowssystemplanners.Underlying the many potential applications of cohetechniques are two fundamental processes: homodyne heterodyne) detection and direct optical amplification. Ithis section we will developsimple models for these pcesses, and use them to quantify the potential advantaand shortcomings of coherent lightwave systems.B. Coherent Optical Reception

A coherent optical receiver is the lightwave analog superheterodyne radio set. Unlike direct detection, wthe optical signal is converted directly into a demodulelectricaloutput,thecoherentreceiverfirstaddstothesignal a locally generated optical wave and then detecthe combination. The resulting photocurrent carries alinformation of the original signal, but is at a frequenclow enough (5gigahertz) so that further signal procescan be performed using conventional electronic circuitrThis method offers significant improvements in recesensitivity and wavelength selectivity compared to diredetection. In the 1.3-1.6 pm lightwave band, for examan ideal coherent receiver requires a signal energyof o10-20 photons per bit to achieve a BER of lo-, far than the roughly 1000photons required by todays APDAndbecause of itsimprovedselectivity,acoherentre-ceiver might permit WDM systems with channel spacof only, say, 100 MHz, instead of the 100 GHz requiwith conventional optical multiplexing technology. A ther advantage of coherent reception, not often cited potentially very important, is thatit allows the use of etronic equalization to compensate for the effects of optpulse dispersion in the fiber. The receiver sensitivity improvement, which for the near term is probably the mimportant of these advantages, is the subject of this stion.Conceptually, the simplest type of coherent receptioalthough not necessarilythemostpractical,isachievedwith a homodyne receiver, in which the local oscillatorphase locked to the incoming optical carrier, as shownFig. 13. The optical signal isfirst combined with the mstronger local oscillator wave using a partially reflectinplate called a beam splitter. (A fiber directional couplmight also be used for this function.) Usually, signal pois more precious than local oscillator power, so the besplitter is made almost completely transparent, and con-


12/18

H E N R Y : LIGHTWAVE P R I M E R 1873

SPLITTER

LOCAL OSCILLATORt ---AN--FREQUENCYOFOCCURRENCE

- -No 1 N 1 N-DECISIONTHRESHOLD

Fig. 13. Homodynereceiver. Theincomingsignal is combinedwithth elocal oscillator wave and then detected by the mixer photodiode . Shotnoise from the mixer causes fluctuations in the in tegrator output, whichcan lead to bit-detection errors.

sequently it reflects only weakly. For our idealization wewill assume negligible transmission loss in the beam split-ter. The combined signal and local oscillator waves illu-minate a photodetector, called the mixer, whose averageoutput current is proportional to the total optical poweraveragedovermanyopticalcycles (Popt).Fora p-i-nmixer,

where R is the average generation rate of photoelectrons,9 is the quantum efficiency of the detector, and hv is thephoton energy. E L and E,, which are proportional to theenvelopes of the local oscillator and signal fields incidenton the photodiode, are defined in normalized units suchthat the simple square-law relation of (18) holds. (We as-sume these fields have the same polarization.) The outputof the photodetector is integratedfor the durationof a databit T, and the result is compared to a threshold to deter-mine if a binary 0 or 1 was transmitted.We now calculate the sensit ivity of the homodyne re-ceiver for OOK, in which a binary 1 is transmitted asan optical pulse ( E , # 0), and a binary 0 is repre-sented by the absence of optical energy (E , = 0 ) . Theexpected number of counts at the integrator outputfor the0 and 1 signal states is

-N o = E i TN,= (EL + ES)2 . T - ( E : + 2E,E,) . T (19)

where the approximate equality is based on the assump-tion EL >> E,.When an individual bit is detected, the integrator out-put, of codrse, will not be precisely No or R , because thephotoelectronsaregenerated at random intervals. (Thisrandomness is the basis of shot noise, which is seen when-ever current flows through a diode.) The integrator out-puts will therefore be distributed around x. and x,, inaccordance with Poisson statistics, as shown by the his-togram in Fig. 13. The error probability (BER) of the re-

TABLE IA V E R A G EN U M B E ROF PHOTONSPE R BI TTO A C H I E V EB E R < W I T HIDEALB I N A R Y R E C E I V E RRECEIVER HEERODYNE DIRECI HOMODYNEMODULATION D?ZETION

ON-OFFKEYING (COK) 36 10 18

ceiver is related to the fraction of each distribution on thewrong side of the threshold level. The narrower the dis-tributions are relative to their separation, thelower the er-ror rate will be. Under our assumption of a strong localoscillator ( E Z T > loo), thedistributions are approxi-mately Gaussian with peak-to-peak separationA N = N,- No = 2ELEsT Goa)

and widtha - * - 6 . (20b)

Thus, we can use tabulated values of the error function tocalculateerrorrates [87]. To achieve BER requiresA N l a - 12, so from (20a) and (20b) we findE ; T = 36. (2 1)

But from (18) E i T is simply the expected number of pho-toelectrons per bit when the mixer is used as a simple di-rectdetector(i.e., EL = 0 ) .Thatis, toachieve BERwith OOK homodyne, the average energy of each opticalpulsemustbe sufficient toproduce 36 direct-detectionphotoelectrons. We can take this result a step further bynoting that for antireflection-coated InGaAsP p-i-n diodes,the quantum efficiency TJ approaches unity, so lo- BERcorresponds to an average received optical energy of 36photons per pulse. For the usual case where 1 s and0s are equally probable, anOOK data stream is ononlyhalf the time. Thus, the required number of photons perbit of information is half the required number per pulse,or 18, as shown in Table I.If phase shift keying (PSK) is used instead of OOK,even greater receiver sensitivity can be achieved. In thiscase information is impressed on the phase of the trans-mitted wave and we write the two states of the receivedwave as E , cos w t and E , cos (w t + x ) . Using argumentssimilar to those in the OOK case, we findAN = (EL + T - (EL - T = 4 E L E s T (22a)

andis = W T .

The condition A N l a = 12 then implies


13/18

1874 IEEEJOURNAL OF QUANTUMELECTRONICS, VOL. QE-21.NO. 12, DECEMBER

Thus, for PSK homodyne detection with an ideal photo-diode ( q = 1) an average signal energy of 9 photons perbit is requiredtoachieveBER.(For PSK theopticalsignal is on all the time so we need not distinguish be-tween photons per pulse and photons per bit, as we did intheOOKcase.)The improvedsensitivity ofPSK com-pared to OOK stems from the fact that for a given signalfield E s the value of A N using PSK is twice as large aswithOOK. PSK with homodyne detection provides thebest sensitivity that can be achieved with the simple co-herent receiver structure of Fig. 13. The system perfor-mance associated with this quantum-limited sensitivityis shown in Fig. 3. The transmission distance at 1 Gbitisis about 50 percent greater than with conventional inten-sity modulation and APD detection.Homodyne receivers, although they are the most sensi-tive, are also the most difficult to build because the localoscillator must be controlled by an optical phase-lockedloop. The heterodyne receiver, in which the local oscilla-torfrequency is deliberately offset fromthesignalfre-quency, is considerably easier to implement. In this casethe useful portion of the mixer output appears at a fre-quency called the intermediate frequency (IF), whichisthe difference between the optical signal and local oscil-latorfrequencies.Thedesireddatastream is extractedfrom the IF signal with standard radio-frequency demod-ulation techniques. The price paid for elimination of theoptical phase-locked loop in the heterodyne receiver is a3 dB degradation in sensitivity compared to homodyne. Asemiclassical way to view this impairment is to recognizethat in a heterodyne receiver thesignal and local oscillatorare constantly slipping in phase. The mixer output (18) ismost sensitive to the incoming signal when the signal andlocal oscillator are aligned in phase, either parallel or an-tiparallel. When they are in quadrature, mixer sensitivityis negligible. The IF signal,which carries the desired data,is an average over these good and bad conditions.With homodyne, on the other hand, the signal is alwaysaligned with the local oscillator, so mixer response to thesignal is maximized. Since the homodyne receiver takesfull advantage of the incoming signal , it is not surprisingthat its sensitivity is greater. More precisely, for given sig-nal and local oscillator powers, the power available fromthe mixer output in a heterodyne receiver is just half thatavailable in the homodyne case. Since the shot-noise powerfor both is the same (because the local oscillator powersare equal), the sensitivity of the heterodyne receiver is 3dB poorer.Receiver sensitivities for several common typesof mod-ulation are shown in Table I. Those not derived above werecalculated using conventional communication theory cou-pled withtheGaussiannoiseapproximationmentionedearlier [87]. The values shown are approximate becausethere are usually several possible demoduiation schemesfor a given modulation technique, and these can have sig-nal power requirements differing by 20 percent or so. Theimportant point is that all the sensitivities shown in TableI are roughly equal, so the choice ofwhich modulation

scheme to use in practical systems will probably depmost strongly on considerations other than receiver sen-sitivity.For purposes of comparison the right-hand column Table I shows receiver sensitivity for OOK and an iddirect-detection(photon-counting)receiver,for whBER - (1/2) exp (-2?i,,) where flp is the average numof photons received during each bit interval. It is imptant to recognize that the required average flux, 10 phtondbit, is smaller than the requirement for all heteroand homodyne schemes except homodyne PSK. In otwords, coherent techniques do not have a significant ssitivity advantage over ideal direct-detection schemes.key question is, which technique (coherent or direct) be made to perform closesttoitsidealsensitivitylimitwhen incorporated into a practical system? At this timethere is no generally accepted answer.To date, the best published coherent transmission ex-periments report shorter transmission distances than tachievedwithconventionallightwavetechniques [8This is probably just a reflection of the infancy of the herent art; improvements can be expected to occur rap-idly. Coherent techniques will become increasingly vaable as the trend toward higher bit rates continues. Forlaser transmitte r of fixed average output power, a higbit rate means fewer photons perbit delivered to the fiand consequently reduced fiber loss margin. Moreover, sensitivity of APD receivers deteriorates at bit rates byond - 4 Gbits/s due to limited diode bandwidth. Thetwoeffectsconspire to makemultigigabittransmissionvery difficult withtodaystechnology.Coherenttech-niques can help with both of these problems. First, improved receiver sensitivity helps compensate for reducedtransmitter energy per bit. And second, the wider bandwidth of the p-i-la mixer diode (compared to an APD) ofers hope for quantum-limited performance at very higspeeds, at least in the homodyne case. (For heterodynereceivers, where the IF is likely to be several times therate, the situation is not so clear.)It should be emphasized that in this discussion we haassumed that the onlynoise in a coherent receiver is shnoise generated by the photodiode; other noise contributions such as electronic noise in the integrator have beneglected. The justification for this assumptionis that snoise increases withlocaloscillatorpower level, somaking the local oscillator strong enough, we can evtually overwhelm any other sourcesof noise. This is cashot-noise-limited operation. In practice, however, the quiredlocaloscillatorpowerisoften not available.Fexample, if the photodiode drivesa conventional 50 fi aplifier with a noise temperature of 300 K, the requirepower on the diode is roughly 4 mW [89]. Given normoptical coupling losses, this impliesa local oscillator lagenerating in excess of10mW, animpracticallyhighvalue. With very careful low-noise design it is possibleoperate at much lower power levels. A low-capacity p-photodiode followedby amicrowave FET amplifier,fexample,canachieveshot-noise-limitedoperationwith


14/18


only about -20 dBm local oscillator power, a level wellwithin the range of practicality.C . OpticalAmplijicntion

Coherent lightwave techniques can be used to constructlong-distance digital transmission systems without regen-erative repeaters. Instead of repeaters, optical amplifiersare placed at intervals along the fiber, much as conven-tional amplifiers are used in analog coaxial-cable systems.The difference between regeneration and amplification isnot merely one of nomenclature. A regenerative repeaterrequires optoelectronic devices for source and detector, aswell as substantial circuitry for pulse slicing, retiming, andreshaping. Theoptical amplifier, ontheotherhand,ismuch simpler; it is a single component which delivers atits output a linearly amplified replica of the optical inputsignal. In addition to simplicity, the advantage to this ap-proach is flexibility. The same amplifier can be used forany modulation scheme atany bit rate. Indeed, if the am-plifier is sufficiently linear, a single device can simulta-neouslyamplifyseveralsignalsatdifferent wavelengthsand bit rates.There are two promising approaches to optical ampli-fication:semiconductoramplifiers, which utilizestimu-latedemissionfrominjectedcarriers,andfiberampli-fiers,in which gainisprovided by stimulatedRamanscattering[90], [91].Bothtypes have attractiveadvan-tages, but at present semiconductor amplifiers seem closerto practical implementation, so we will confine our dis-cussion to them.The structu re of asemiconductoropticalamplifierissimilar to that of a conventional injection laser. In act,laserspumpedjust below threshold have been used todemonstrate optical amplification. The drawback to usinglasers as amplifiers is that they can only achieve a rela-tively small gain-bandwidth product, typically & * B -50 GHz where G is the power gain and B is its 3 dB band-width. Thus, to achieve a useful gain ( 520 dB) the band-width of the amplifier can be no greater than 5 GHz, orabout 1/2 A at 1.55 pm, which might be uncomfortablynarrow in many applications. A more promising optical-amplifier structureisthe traveling-waveamplifier [92],[93]. Again, the device is similar to a laser, except in thiscase the end facets are antireflection-coated to eliminatecavity resonances. Photons incident on a traveling-waveamplifier do not bounce back and forth once inside thecavity; they make just a single pass through the device,stimulating enough emission during that transit to provideuseful gain. The elimination of cavity resonances yields avery broad bandwidth for the traveling-wave amplifier-10 A at 20 dB gain has been observed in an AlGaAs de-vice.In addition to providing high gain and wide bandwidth,thetraveling-wavedevice is potentiallycapable of ex-tremely low-noise amplification. A simple model for thenoisebehavior of traveling-waveamplifiersisshowninFig. 14; it is valid under conditions of high SNR (whichis always the case with properly functioninglightwave sys-

AFig. 14. Noise model for an optical am plifier. For typical lightwave appli-cations, quantum fluctuations in the amplifier can be represented as sim-ple additive noise with spectral density 2 h v .

...RECEIVER

Fig. 15. Lightwavetransmissionsystemusingopticalamplifiers.Co m-pared with a conventional regenerative repeater, the optical amplifier issimpler (it requires no high-speed digital circuitry) and more flexible (itcan amplify simultaneously one or more signals at different wavelengthsand bit rates).

tems) and high gain. The model consistsof an ideal noise-lessamplifier of gain G preceded by anadditive noisesource of spectral densityS = nsp hv (24)

where nsp,the spontaneous emission factor, is a measureof the amplifier population inversion, andhv, as before, isthe photon energy. The minimum value of nSpis unity,which occurs for the ideal case of complete inversion. Inthis situation, S = hv. That is, it is theoretically impos-sible to have a noiseless amplifier. In practice, nSp- 2 hasbeen achieved, indicating that amplifiers only 3 dB noiserthan the theoretical limit should be realizable [94].The use of optical amplifiers in a long-distance trans-mission system is shown in Fig. 15. After each section offiber of length L there is an amplifier whose gain G justcompensates for fiber loss:G = l@Lo. (25)

As an optical pulse travels through this cascade of ampli-fiers, it gets noisier and noisier because the additive noisefrom each amplifier is cumulative. At the end of the chainthe SNR isp T 10 -ALI 1,oSNR = [L,,,/L] n Sp hvB

where L,,, is the total system length,so (L,,,/L) is the totalnumber of repeaters, and the. noise bandwidth equals thebit rate B . For any given total system length, the SNR ismaximized by using an optimum link length L - 4 .3 /A .(This corresponds to an optimum gain G = e for eachamplifier.) This optimization is not useful, however, be-cause it requires impractically short fiber links: - 17 kmfor A = 0.25 dB/km. But even with a suboptimal choiceof L , impressive system performance can be achieved, aswe show below.


15/18

1876 IEEEJOURNAL OF QUANTUMELECTRONICS,VOL.QE-21,NO. 12 , DECEMBER

The maximum transmission distance for the system ofFig. 15 is reached when the accumulated noise reduces theSNR ( 2 6 ) to a value just large enough to ensure reliablebit detection at the receiver. Let us assume a I Gbit/s sys-tem with P T = 1 mW. Taking L = 100km, A = 0.25 dB/km, n.7p= 2 , and SNR = 13 dB [87], we find from (26)that L,,, - lo5 km, which is greater than the circumfer-ence of the earth! Thus, it appearspossible to join any twopoints on earth witha chain of nonregenerative repeaters.(Amoreprecisecalculation, which takesintoaccountmany details not mentioned here, gives maximum systemlengths in the vicinity of 20 000 km-still long enough toconnect any points on earth [84].) We can thus imagine aworldwide network of light pipes providing transmis-sion of signals at essentially arbitrary bit rates betweenany locations in the world. But alas, standing in the wayof this exciting possibility is fiber dispersion, which im-poses serious limitations on system performance. To thissubject we now turn.D. Dispersion LimitationsTo extractmaximumusefulnessfromsingle-mode fi-bers,thetransmittedspectralwidth q, mustbemini-mized. We saw in Section IV-C that linewidths as narrowas 1 A are still broad enough to cause significant disper-sive effects. In theory, however, smaller linewidths are at-tainable (although external modulators might be requiredto achieve them). The lower limit to transmitter spectralwidth is set by the modulation rate B :

X22 ~ h.- - B.CSubstituting this into (15) we obtain the ultimate disper-sion limit:

B ~ L5 ~ c -2 D X 2 At X = 1.55ym where D - 15 ps/km * nm, B2LI4000(Gbits/s)2 . km, which is shown in Fig. 3. The intersec-tion of this limit with the quantum limit (homodyne PSK)corresponds to the highest bit rate-distance product thatcan be achieved with the coherent techniques describedinthispaper: - 900 Gbits/skm.Forstillbetter perfor-mance a low-loss dispersion-controlled fiber or sophisti-cated equalizer might be used.Dispersiveeffects are especiallyserious in the lightpipe discussed earlier because a transmitted pulse getsprogressively broader as it propagates. (With regenerativerepeaters pulse spreading doesnot accumulate because thepulses are reshaped at each repeater.) Thus, in the caseofthe light pipe the appropriate value of L to use in ( 2 8 )is the entire system length, not the individual link length.For L,,, = lo4 km (28) limits B to - 600 Mbitsis. Thismight turn out to be a significant drawback to the use ofoptical amplifiers in long-distance transmission systems.

E.EngineeringChallengesThere are manydifficult engineering problems that mbe solved before coherent systems can be considered srious candidates for commercial applications. Perhaps most important of these is stabilizing the output of intion lasers. Instead of a pure monochromatic sine wavlaser at 1.55 ym generates an output signal seriously d

graded by phase noise and intensity fluctuations. (Laseralso suffer from long-term frequency drif t, but solutito this problem are available [ 9 5 ] . )Phase noise is partularly troublesome in PSK systems, where the rms phajit ter from one bit to the next must be held to 0.15 radso [1041. In the injection laser the relatively strong spotaneous emission and low cavity Q lead to significant phnoise, which manifests itself as a broadening of the lasspectrallinewidth.Theaccumulatedrmsphasefluctua-tion of a laser transmitter during onebit interval is relto the linewidth by Iwhere Af is the laser linewidth in Hertz (FWHM) [7To satisfy the constraint A+ I0.15 rad, we require

Af 5 0.004 * B. (For a 1 Gbit/s system, then, the linewidth must be lthan 4 MHz, which is very difficult to obtain with convtional (300 ym length) lasers [96]. It appears that speclong-cavity or external-cavitylaserswillbeneededtoachieveadequatelynarrowlinewidths [ 9 7 ] . (Inthisdiscussion we have assumed that the transmitterwas the ssource of system phase noise. If the local oscillator is anoisy, the constraint on transmitter spectral purity is evtighter.)Laser intensity fluctuations, like phase fluctuations, driven by spontaneous emission. Their effect is most se-riousinreceiverapplications because these fluctuatiare detected by the mixer diode, causing excess noise aits output and thereby degrading receiver sensitivity. Tamound of degradationdependsonthedetails of laconstructionandoperatingconditions, butit is usuaseveral decibels. Fortunately, there is an elegant solutito this problem: the optical balanced mixer [98]. This vice uses two photodiodes and combines their outputssuch a way that local oscillator intensity noise is cancewhile the desired signal remains unaffected. In practicebalanced mixer can effectively suppress as much as 20excesslasernoise.Thismeansthatessentiallyideal(quantum-limited) receiver performance can be achievedeven with relatively noisy lasers.In addition to the task of reducing laser noise, coherlightwavepresentsseveralotherengineeringchallengesthat must be overcome. To cite just one example, therethe problem of controlling optical polarization, either a polarization-maintaining fiber or an adaptive polariza-tion compensator, in order to ensure that the signal and


16/18

HENRY: LIGHTWAVE PRIMER

local oscillator fields are in proper alignment at the mixer[99]-[ 1011. Both these techniques have been successfullyimplemented in research experiments, but their applica-tion to practical systems has not been demonstrated.Theroadahead forcoherentlightwavewillnotbesmooth. The problems already discussed, plus a host ofothers not even mentioned, will provide ample challengeto the most dedicated researchers. We cannot predict whatsuccess they will have, but the history of the lightwave artthus far tells us that betting against them is an unwiseinvestment.

R E F E R E N C E S[ I ] T. Li, Adva nces in optical fiber communications: An historical per-spective, IEEE J. Select .AreasCommun., vol.SAC-I, pp. 356-372,Apr. 1983.[2] Y. Suematsu, Long-wavelength optical fiber communication, Proc.IEEE, vol. 71, pp. 692-721, June 1983.131 S. E. Miller and A. G. Chynoweth , OpticalFiberTelecommunica-t ions. New York: Academic, 1979, chs. 1, 21.[4] J . Strong, Concepts of ClassicalOpt ics. Sa nFrancisco:Freeman,1958, appendix N.[ 5 ] F. P. Kapron, D. B. Keck, and R. D. Maurer , Radiation losses inglass optical waveguides, Appl . Phys. Let t . , vol. 17, pp. 423-425,Nov. 1970.161 S. R. Nagel, K. L. Walker, and J. B. M acChesney, Current statusof MCVD: Process and performance, in Tech. Dig. Top. Mee t. Opt.Fiber Commun., Phoenix , AZ, Apr. 1982, Paper TuCC2.[7] Y. Suematsu ,Ed . , OpticalDevicesan dFibers. Amste rd am,T h eNetherlands: North-H olland, 1982, ch. 3.1.[SI J . Irven and A. P. Harr ison, Single-mode and mult imode f ibers co-doped with fluorine, in Ech. Dig. Int. Con$ Integrated Opt., Opt.Fiber Comm un., San Francisco , CA, Apr. 1981, Paper TuC4.[9 ] T. Okoshi, OpticalFibers. New York: Academic,1982,ch .2.[101 J . E. M idwinter , Optical Fibers for Pansmission. New York: Wiley,[ I 11 M . Garb u n y, OpticalPhysics. New York: Academic, 1965, ch. 5.[12] Y. Suematsu ,E d . , OpticalDevices and Fibers. Amste rd am,TheNetherlands:North-Holland,1982,ch .3.2.[131 H.Murataan dN.Inagaki,Low-losssingle-modefiberdevelop-ment and splicing research in Japan, IEEE J . Quantum E lectron..vol. QE-17, pp. 835-849, June 1981.[I41 J. Stone and P. J. Lemaire , Reduction of loss due to OH i n opticalf ibers by a two-step OH-O D exchange process , Electron. Lett .,vol. 18, no. 21, pp , 78-80, Jan. 1982.11.51 Reference Dat afor R adio Engineers, H. W. Sa m , Indianapolis, IN,1975 ch . 24.[I61 S. D. Personick , Receiver design for d ig ita l f iber optical commu-nication system-Part 1, Bell Syst. Tech. J ., vol. 5 2 , pp. 843-874,JulyiAug. 1973.[I71 J. E. Midwinter , Optical Fibersfor Transmission. New York: Wiley,1979, ch . 14.[18] J. E. Goell, An optical repeater with h igh-impedance input ampli-fier, Bel l Syst. Ec h. J ., vol. 5 3 , pp. 629-643 , Apr. 1974.[19] P. K.Runge.A 5 0 -Mb isrepeaterfo rafiber-opticPC M experi-men t , in Con$ Rec. In t. Comm un. Con$, Minneapolis , MN, June

1974, Paper 17B .[20] S . D.Personick, OpticalFiberTransmissionSystems. New York:Plenum. 1981, ch. 3.1211 S. E. Miller and A. G. C hynoweth , OpticalFiber Elecommunica-f i o n s . New York: Academic, 1979, ch. 2.1221 D . Gloge and E. A. J. Marcati l i , Multimode theory of graded-coref ib e r s , Bell Sys t . E c h . J . , vol. 52, pp. 1563-1578, Nov. 1973.1231 L. G. Co h en , I. P. Kaminow, H. W. A stle, and L. W. Stulz, Profiledispersion effects on transmission bandwidthsin graded index opticalf ibers , IEEE J. QuantumElectron., vol.QE-14,pp. 37-41, Jan.1978.1241 D . P. Jablonowski, D. D. Pad gette , and J . R. Merten , Performanceof the MCVD preform process in mass production condit ions, inPaper TuEE2.Tech. Dig. Top. Me et. Opt. Fiber Comm un., Pho enix , AZ , Apr. 1982,

ch . 8 .

1877S. D. Pe rso n ick ,Timedispersio n in dielectricwaveguide, BellSyst. Tech. J . vol. 50, pp. 843-859, Mar. 1971.J. R. Stauffer, FT3C-A lightwave system for metropolitan and in-tercity applications. IEEE J. Select . Areas Commun., vol SAC-1,pp. 413-419, Apr. 1983.C. K. Kao. Optical Fiber Systems. New York: M cGraw-H ill, 1982,ch. 5.H . C. Casey, Jr . and M. B. Panish , Heterostructure Lasers, Part B.New York: Academic, 1978, ch. 8.R. L. Hartman, N. E. Schumaker, and R . W. Dixon, Continuouslyoperated AlGaAs DH lasers with 70 l ife t imesas long as two year s,App l . Phys. Let t . , vol. 31, pp. 756-759, Dec . 1977.K . 0 .Hill , Y. Tremblay, and B. S. Kawasaki, Modal noise i n mul-t imode f iber l inks , Opt . Let t . , vol. 5 , pp. 270-272, June 1980.K. Petermann, Nonlinear d is tort ions and noise in optical commu-nication systems due to fiber connectors, IEEE J. Quantum Elec-t ron . , vol. QE- 16, pp. 761-770, July 1 980.J. C .Daly ,Ed. , FiberOptics. BocaRaton,FL: CRCPress, 1983.ch. 5.D. Glo g e, A . Alb anese , C .A . Bu r ru s , E. L. Chinnock, J. A . Cope-land, A. G. Denta i , T. P. Lee, T. Li ,an dK.Ogawa,High-speeddigital lightwave communications using LEDs and PIN photodiodesat 1.3 p m , Bell Syst. Tech. 1,vol 59, pp. 1365-1382, O ct. 1980.R. D. Martin-Royle and H. G. Bennett, Optical-fiber transmissionsystemsin th eBritish telecom network: An overview, Brit. Tele-commun. Eng., vol. 1, pp. 190-199, Jan. 1983.AmericanInstituteofPhysicsHandbook, 3rd ed . New York:McC raw-Hill, 1972, Table 6b-17.D . N. Payne and A. H. Hartog, Determination of the wavelengthof zero m aterial dis persion in optical fibers by pulse-delay measure-ments. Electron. Lett .. vol. 13. no . 13. DD. 627-629. O ct. 1977.[37] A. Sugimura, K. Daikoku, N. Imoto , and T. Miya, Wavelength dis-persioncharacteristics of single-modefibers in low-lossregion,ZEEE J. Quantum Electron. , vol. QE-16, pp. 215-225, Feb. 1980.[38] T. Li , Structures . parameters and transmission propert iesof opticalf ib e r s , Proc. IEEE, vol. 68, pp. 1175-1180, Oct. 1980 .[39] L. G . Cohen, F. V. DiMarcello , J. W. Fleming, W. G. French, J. R.Simpson, and E. Weiszmann, Pulse d ispers ion propert ies of f iberswith various material constituents, Bell Syst. Tech. J . vol. 57, pp.1653-1662, May/June 1978.1401 K. Nakagawa and T. Ito, Detailed evaluation of an attainable repea-te rspacingfo rfibretransmissionat 1.3 pmand 1.55 pm wave-lengths , Electron. Lett ., vol. 15, no. 22, pp. 776-777, Nov. 1979.[41] P. K. Runge, C. A. Brackett , R. F. Gleason, D . Kalish, P. D. Lazay,T. R. Meeker, D. G. Ro ss , C. B . S w a n , A . R . W a h l , R . E . Wagner,an d J. C. Willia ms, 101-km lightwave undersea system experimentat 274 M bis , in Tech. Dig. Top. Mee t. Opt . Fiber Comm un., Phoe-nix, AZ, Apr. 1982, Paper PD7.[42] J. I. Yamanda, S. Machida, and T. Kimura, 2 Gbit is optical t rans-mission experiments at 1.3 pm with 44 km single-mode f iber , Elec-tron. Lett ., vol. 17, no. 2 5, pp. 479-480 , June 1981.[43] S. M. Ab b o t t ,R . E. Wagner,and P. R.Trischit ta ,S Lundersealightwave system experiments, in Con$ Rec. IEEE Int. Con$ Com-m u n . , Boston, MA, June 1983, Paper C3.5 .[44] E . Iwahashian d H. Fukutomi,F-400Msystemoverview, Rev.Electron. Commun. Lab. vol. 31, pp. 237-243, May 1983 .[45] R. A. Linke, Direct g igabit modulation of in jection lasers : Struc-turedependentspeedlimita t ions, IEEE/OSA J. LightwaveTech-nol., vol. LT-2, Feb. 1984.[46] U. Koren, G. Eisenstein. J. Bowers . A. H . Gnauck, and P. K . Tien ,Improvedbandwidth of threechannelburiedcrescentlasers,Electron. Lett ., submitted for publication.[47] Y. Okano, K . Nakagawa,and T. Ito,Lasermodepartitionnoiseevaluation for optical fiber transmission. IEEETrans. Commun.,vol. COM-28, pp. 238-243 , Feb. 1980.[48] K. Ogawa, A nalysis of mode partition noise in laser transmissionMay 1982.sy s tems , IEEE J. Quanrum Electron. , vol. QE-18, pp. 849-855.[49] K. Iwashita and K. Nakagaw a, Mode partition noise characteristicsin high-speed modulated laser diodes, IEEE J. Quuntum E lectron.,vol. QE-18, pp. 2000-2004, Dec. 1982.[ S O ] S. Yamamoto, H . Sakaguchi, and Norio Seki, Repeater spacing of28 0Mbit/ssingle-modefiber-optictransmissionsystemusing 1.55 %pm laser d iode source , IEEE J. Quantum Elecrron., vol. QE-18,pp. 264-273, Feb. 1982.[ S l ] N . K. Cheung, S. Davidow, and D. G. Duff, A 90 Mb/s transmis-

i L .


17/18

1878 IEEEJOURNAL O F QUANTUMELECTRONLCS, VOL. QE-21, N O . 12,DECEMBERsion experimen t in single-mode fibers using 1.5 pm multi-longitudi-nal mode InGaAsPIInP lasers, IEEE/OSA J. Lightwave Technol.,vol. LT-2, pp. 1034-1039, Dec . 198 4.[52] M. C.Brain,Comp arison of availabledectorsfo rdigitalopticalfiber systems for the 12-1.55 pm wavelength range, IEEE J. Quan-tum Electron. , vol. QE-1 8, pp. 219-224, Feb. 1982.[53] J. Yamada, A. Kawana, T. Miya, H . Nagai, and T. Kimura, Giga bit/s optical receiver sensitivity and zero-dispersion single-m ode fibertransmission at 1.55 p m , IEEE J. Quantum Electron. , vol. QE-18,pp. 1537-1547, Oct. 1982.

[54]J. C.Campbel l ,A .G .Dentai , W. S. Holden,an dB. L. Kasper,High-perfo rmance avalanche photodiode with separate absorption(InGaAs),grading(InGaAsP)an dmultiplication(InP)regions.Electron. Let t . , vol. 19, no. 29, pp. 818-820, Sept. 1983.[55] K. C. Nelson, D . L. Brownlow, L. G. Cohen , F. V. DiMarcel lo, R.G. Huff, J. T. K rause, W. A. Reed, D. S. Shenk, E. A. Sigety, J . R .Simpson, and K . L. Walker, Fabricat ion and performance of longlengths of sil icacorefiber,in Proc. Con$ Opf . F iberCornmun. ,San Die go, CA , F:b. 1985 , Pape r WH 1.[56] T. D. Crof t, J. E. Ritter, and V. A . Bh agavatula, Low-loss disper-sion-shifted single-mode fiber manufac tured by the OVD process,W D 2 .in Proc. Opt . Fiber Commun., Sa nDiego, CA , Feb. 1985,Paper[57] N. Imoto, A . Kawana, S. Machida, and H. Tsuchiya, Cha racteris-tics of dispersion free single-mode fiber in the 1.5 pm wavelengthOct .1980.region, IEEE J. QuantumElectron. , vol. QE-16, pp. 1052-1058,

[58]L. C. Blank, L. Bickers, and S. D. Walker, 220 km and 233 kmtransmission experiments over low-loss dispersion-shifted fibre at 140Mb/s and 34 Mb/ s , i n Proc. Con$ Opt. Fiber Cornmun., San Diego,CA , Feb. 1985, Paper PD-7.[59] L. G. Cohen , W. L. Mammel , and S. J. Jang, Low-loss quadruple-clad single-mode lightguides with dispersion below 2 psik m. nm overth e1 . 2 8 ~ m - 1 . 6 5 pm wavelength range, Elec t ron .Le f t . , vol. 18,no . 2 5 , pp. 1023-1024, Nov. 1982.[60] K.H. Came ron, P. J . Chidgey, and K. R. Preston, 102 km opt icalfiber transmission experim ents at 1.52 pm using an external cavitycontrolled laser transmission module, Elec t ron . Le f t . , vol. 18. no.I611 C. Lin and C. A. Burrus, CW and high-speed single-longitudinaloperation of a short InGaAsP injection laser with external coupledcavity, in Tech. Dig. 7i1pMeet . Opt . Fiber Commun. New Orleans,LA, Feb.-Mar. 1983, Paper PDS.[62] H. Kressel and T. K. Butler, Semiconductor Lusers and Heterojunc-

t i onL E D s . New York: Academic, 1977, ch . 15.[63] K.Sakai , K. Utaka, S. Akiba,and Y. Matsush ima, 1.5 pm rangeInGaAsPiInP dist ributed feedback laser, IEEE J. Qu a n tu m Elec-t ron . , vol. 18, QE-18, pp . 1272-1278. Aug. 1982.[64] Y. Suematsu, A. Arai. and K. Kishino, Dynamic single-mod e semi-conductor lasers with a distributed reflector, IEEEIOSA J. Light-wave Echnol., vol. LT-1,pp.161-175, Ma r. 1983[65j V. J. Mazurcyk. N. S. Bergano, R. E. Wagner. K. L. Walker, N. A.Ol sson , L. G . Cohen . R . A . Logan , and J . C . C ampbel l , 420 Mb/s transmission through 20 3 km using silica-core fiber and DFB laser,in Proc. 10th Eur. Con$ Opt . Commun., Stuttgart. West Germany,Sept. 1984, Paper PD-7.[66]A .H .Gnauck , B . L. Kasper, R.A .L inke , R. W. Dawson, T. L .Koch, T. J. Bridges, E. G. Burkhard t, R . T. Yen, D. P. Wilt , J. C .Campbel l ,K .C .Nelson,an dL.G .Cohen ,4 Gbist ransmissionover 103 km of opticalfiberusinga novel electronicmultiplexer/demultiplexer, in Con& Opt. Fiber Commun., San Diego. CA, Feb.1985, Paper PD2.[67] W. T. Tsang, N . A. Olsson, and R. A. Logan, High-speed di rectsingle-frequency modulation with large tuning rate and frequency ex-cursion in cleaved-coupled-cavity semico nducto r lasers. Appl. Phys .L e f t . , vol. 42, no. 15, pp . 650-652, Apr. 1983.[68] B. L.Kasper,R .A .Linke, J. C.Campbel l ,A .G .Dentai, R . S.Vodhanel, P. S. Henry,I. P. Kaminow,and J . 3 . KO, A 161.5 kmtransmissionexperimentat42 0 Mbis, in Proc. 9th Euro. Con$Opt . Commun., Geneva , Switzerland, Oct. 1983, Pape r PD #7.1691 S. K. Korotky, G.E i sens t e in ,A .H .Gnauck , B . L . Kasper ,J. J.Veselka, R . C. A l fe rness , L . L . Buhl , C . A . Burrus , T . C . D . Huo,L. W. Stul tz, K. C . Nelson. L. G. Coh en, R . W. Dawson, and J. C.Campbel l , 4 GbIs transmission experimen t over 117 km of opticalf i ber us ing a T i :L iNb03 ex t e rna l modula to r , in Proc. Con$ Opt.F iber Comm un. , San Diego, CA , Feb. 1985, Paper PD I.[70] K. Nakagawa, N. O hta, and K . Hagimoto, Design and performance

22, pp. 650-651, July 1982.

of anexperimenta! 1.6 Gb/s opt icalrepeater, in Proc. Int. CInt. Opt. Fiber Commun., Tokyo, Japan, June 1983, Paper 29C2[71] C . H. Henry, Theory of the linewidth of semiconductor laser, IJ. Quurztum Electron, vol. QE-18, pp. 259-264. Feb. 1982.(721 R. A. Linke, Modulat ion induced t ransient chi rping in singlequency lasers, I E E E J . Quantum Electron. , vol. QE-21. June 1[73] R. A. Linke, B. L. Kasper, C. A. Burrus . 1. P. Kaminow, J.-Sand T. P. Lee, M ode powerpartitionevents in nearlysingle-quency lasers, IEEEIOSA J. Ligh twave Echnol . , vol. LT-3, 1[ 7 4 jC . H. Henry, P. S. Henry.an d M . Lax,Partit ionfluctuations nearlysingle-longitudinal-mode lasers, IEEEIOSA J. LightTechnol., vol. LT-2. pp. 209-216, June 1984 .[75] S. E. Mil ler and A. G. Chynoweth, Optical Fiber EAecommufions. New York: Academ ic, 1979, ch. 5.[76] R. G . Smith, Optical power handling capacity of low loss opfibers as determined by stimulated Raman and Brillouin scatteApp l . Op t . , vol. 11, pp. 2489-2494, Nov. 1972.[77]D.Cotter,Observation of stimulatedBrillouinscatte ring in 496, June 1982.loss silica fibre at 1.3 pm Elec tron . Le f t . , vol. 18, no. 10, pp. 41781 -, Transient stimulated Brillouin scattering in long single-mfibres. Electron. Let t . , vol. 18, no . 10. pp. 504-506, June 1981791 -, Suppress ion of stimulated Brillouin scattering during transmission of high-powernarrowbandlaserlight in monomfibre,Elect ron. Let t . , vol. 18, no. 22, pp. 638-640, July 1982[SO] A. R. Chr aplyv y and P. S. Henry, Performance degradat ion duestimulate d Raman scatterin g in w avelength-division multiplexedticalfibersystems, Elec t ron .Le t t . , vol. 19, no. 4, pp. 641-Aug. 1983.1811 A. Tomita,Crosstalkcaused by stimulatedRamanscattering single-[node wavelength division multiplexing systems, Ech.Top. Meet . Opt . Fiber Commun. , New Orleans, LA, Feb.-M ar. 1Paper TUHS.[82] N. A. Olsson, J. Hegarty, R. A. Logan, L. F. Johnson , K. L. WaL. G. Cohen , B. L. Kasper, and J. C. Campb ell, Transm ission 1.37 Tbit .kmisec capacity using ten wavelength division multiplasers at 1.5 pm, in Proc. Con$ Opt. Fiber Commun., San DiCA . Feb. 1985, Paper WB6.[831 T. Miyashitaand T. Mana be,Infraredopticalfibers, IEEQuantum Electron. , vol. QE-18, pp. 1432-1449, Oct. 1982.1841 Y. Yamamoto and T. Kimura, Cohere nt optical fiber transmissiosys t ems, IEEEJ. Quuntum Elec f ron . ,vol. QE-17. pp. 919-935, 1981.[85] T. Okoshi, Heterodyne and coherent optical fiber communicatioRecent progress, IEEE Trans . Microwurz 7heory Tech . ,vol. M

[86] P. S. Henr y, The prom ise of coherent l ightwave communicationin Proc.Nut . Commun. Forum, Rosemont ,IL, pp. 608-613, S1984.1871 M . Schwartz, W. R. Benn ett, and S. Stein, Communicution Sysan d Techniques. New York: McGraw-Hill ,1966,esp.ch. 7.[SS] R. Wyatt, T. G. Hodgkinson, and D. W. Smith, 1.52 pm PSKerodyne experiment featuring an external cavi ty diode laser lcillator, Electron. Let t . , vol. 19, no. 7, pp. 550 -552 , July 198[89] T. G . Hodgkinson, R. Wyat t , D. W. Smith, D. J . Malyon, and RHarmon, Studies of cohcrent transmission systems operating oinstalled cable links. in Proc. Globecom 83 , San Diego. CA, 1983, Paper 21.3.[90] T. Mukai. Y. Yamamoto, and T. Kimura , SIN and error rate formance in AlGaAs semiconductor laser prcamplifier and lineapeater system, IEEE J. Quuntum Electron. , vol. QE-18, pp. 11568, Oct , 1982.[91] A . R. Chraplyvy, J . Stone, and C. A. Burrus, Opt ical gain exceing 35 dB at 1.56 pm due to stimulated Raman scattering by mular D2 in a solid silica fiber, Opt . Let t . , vol. 8, pp. 415-417, 1983.1921 J. C. Simon , Sem icond uctor laser amplifier for singlc mode opf ibercommunica t i ons , J. Opt . Comrnun., vol. 4, pp. 51-62, 1983.1931 D. M . Fye,Practicallimitations on opticalamplifierperfor-mance. IEEE/OSA J. Liglztwuve Exhnol., vol. LT-2, pp. 403-Aug. 1984.[94] T. Mukai , T. Sai toh, 0 . Mikami, and T. Kimura, Fabry-Perot city type InGaAsP BH-laser amplifier with small optical-mode con

19.51 M . Shikad a, K. Em ura, s. Fuj ita. M . Ki tamura, M . Arai , M . Kofinement, Electron. Let t . , vol. 19, no. 21. pp. 582-58 3, July 1and K. Minemura, 100 MbIs ASK heterodyne detect ion experi

30 , pp. 1138-1148, A u ~ .1982.


18/18

HENRY: LIGHTWAVE 18 79using 1.3 pmDFB- la se rdiodes, Electron.Lett ., vol. 20. no. 16,pp. 164-165, Feb. 1984.[96]T. P. Lee, C. A. Burrus , K. Y. Liou, N . A . Olsson. R. A. Logan,an dD. P. Wilt.Measuredspectrallinewidth of single-frequency1. 3and 1.5 pm in jectionlasers , Electron.Lett ., vol. 20, no. 22,

[97] M . R . Matthews, K . H. Camero n ,R .Wy a t t ,an d W. J. Devlin ,Packagedfrequency-stabletunable20 kH z linewidth 1.5 pm In -GaAsPexternalcavitylaser, Electron.Lett ., vol. 21, no. 31, pp.113-115, Jan. 1985.1981 G. L . Abbas, V. W. S. Ch an , an d T . K . Yee, Cancellation of localoscillatorintensitynoisecaus ed by therelaxationoscillation ofGaAlA s lasers with a dual-detector heterodyne receiver , in Proc.

Con$ Op t . Fiber Commun., New Orleans,L A ,Jan.1984,Pap e rTUA2 .[99] N . Shibata, Y. Sasaki, K. Okam oto, and T. Hosaka, Fabrication ofpolarization-maintaining and absorption-reducing fibers, IEEEIOSAJ. Lightwave Technol., vol. LT-1, pp. 38-43, M ar. 1983.[loo] T. Katsuyama, H. Matsumu ra, and T. Suganuma,Low-losssinglepolarization fibers, A ppl . Op t . , vol. 2 2 , no. 1, pp. 1741-1747, June1983.[ lo l l R. C. Alferness and L. L. Buhl, New low-loss waveguide electro-opticpolarizationcontrollerfo r X = 1 .32 p m , in Tech. D i g , Int.Con$ Integrated O p t . , Opt. Fiber Commun., Tokyo, Japan,June1983, Paper 29C5-4PD.[lo23 I. Garrett and C. J . Todd, Components and systems for long-wave-length monomode fibre transmission, Opt. Quantum Electron., vol.14, pp. 95-143, Mar. 1982.[I031 P. K. Rung e and P. R. Trisch itta, Eds., Joint Special Issue on Un-derseaLightwaveCommunications, IEEE/OSA J. LightwaveTech-nol. , vol. LT-2, Dec. 1984.

pp. 1011-1012, NOV.1984.

[I041 K. Kikuchi, T. Okoshi, M . Nagamatsu ,an d N . Hen mi , Deg rad a -tion of bit-erro r rate in coherent o ptical com munications due to spec-tra l spread of the transmitter and the local oscil la tor , IEEE/OSA J.Lightwave Technol., vol. LT-2, pp. 1024-1033, Dec . 1984 .

Documents

Lightwave Primer Henry IEEE 1985