PHOTONICS IN CABLE

TELEVISION SYSTEMS

B Y T E D A R C I E

THE PHOTONIC OPPORTUNITY

During the last decade millions of miles of fiber optic cable and thousands of photonic components revolutionized the telecomshymunications industry Digital long-haul sysshytems using single-mode fiber and simple Fabry-Perot lasers offered unquestionable quality and bandwidth advantages over the antiquated microwave network Yet in spite of the success of digital lightwave techshynology the fidelity of even the finest early fiber links did not allow transmission of the complete video spectrum delivered by the cable television industry over coaxial cable

Fiber transmission of this multishychannel amplitude-modulated ( A M ) vestigial-sideband (VSB) signal had to wait nearly a decade for improveshyments mostly in laser technology These improvements have arrived and with them come a variety of analog systems and architectures that are now revolutionizing the cable television industry

Before lightwave A M - V S B systems were available cable operators used satellite or installed digital or F M -subcarrier fiber systems to transmit video between various head end locations Digital or F M video formats are much less demanding in terms of

transmission fidelity than AM-VSB but at the expense of increased bandwidth These formats are then converted to AM-VSB for bandwidth-efficient transmission over coax and so that the signal delivered to the home is compatible with cable-ready consumer video equipment Since multichannel format conversions require large racks of expensive equipment it is essential that this cost be shared among as many subscribers as possible It then becomes a priority to transmit the cable-ready AM-VSB format as far as possible from centralized head-end locations to large subscriber areas

C A B L E T V SYSTEMS

This priori ty shaped cable systems into the conventional tree and branch archishytecture2 as shown in Figure 1 Long trunks of heavy-gauge coaxial cable w i th high-power and highly-l inear amshyplifiers every 300 meters or so stretch up to 32 kilometers f rom a head end Passive taps and l ighter-gauge coax branch from the trunk through more amplifiers to ind iv idua l subscribers In spite of the enormous cascaded cable and tap loss high-fidelity transmission of at least 60 channels over a bandshywid th of 400 M H z was commonplace

Improved Service Quality Unfortunately although the bandwidth of the cable network was impressive it had problems operationally Failure of any one of dozens of amplif iers wou ld disrupt service to large numbers of subshyscribers Accumulat ion of noise and disshytortion from each amplif ier results in impaired performance for subscribers w i th the misfortune of l i v ing far f rom the head end The performance of these mostly aerial trunk systems given corshyrosion l ightning strikes storms and human hazards was a serious operashytional bottleneck

Lightwave A M - V S B trunk systems eliminate this bottleneck as shown in Figure 2 By using 10-20 k m analog f i shyber trunk systems w i th l ightwave reshyceiver nodes at strategic pos i t ions throughout the distr ibution area headshyend-quality v ideo can be interjected wi th in a few kilometers of every home 3

The degradation caused by long ampl i shyfier cascades is eliminated Careful conshysideration of fiber routes also reduces the number of subscribers disrupted by the failure of any one component

Increased Bandwidth A M - V S B fiber trunk systems also proshyvide dramatic increases i n the bandshywidth of a typical cable installation 4

The loss of coaxial cable increases as the square of the frequency transmitshyted The o ld coaxial cable network was l imited by practical amplif ication and equalization to somewhere near 500 M H z But w i th the introduction of fiber trunk systems the longest coaxial cable lengths f rom any l ightwave receiver node can be short Wi th fewer than three amplifiers between the head end and any subscriber coaxial-cable loss

is no longer the bandwidth- l imit ing facshytor Us ing suitable electronic ampl i f i shyers bandwidths approaching 1 G H z can be del ivered to each home

Cost Effectiveness Improvements i n service quali ty and bandwidth more than justify the cost of the l ightwave components required Since fiber is needed only in the trunk port ion of the network leaving the cost of the coaxial distr ibution unchanged the cost of each l ightwave l ink can be d iv ided among a thousand or more subshyscribers The total cost of the l ightwave system inc luding fiber is less than 10 of the total system cost N e w fiber-trunkcoaxial-distr ibut ion systems can

t ransmiss ion f ide l i ty l ike A M - V S B v ideo or bandwidth-ef f ic ient mu l t i shylevel digi tal modulat ion formats The ultimate transmission capacity is imshypressive This paper describes the deshyvelopments and requirements of the photonic technology that makes these A M - V S B l ightwave systems possible

A M - V S B S I G N A L S

The video format of choice both for broadcast and cable television distr ishybut ion has for decades been A M - V S B 5

Each channel consists of an RF carrier that is ampli tude modulated (AM) by v ideo information This approximately 4 -MHz-bandw id th v ideo signal conshytains the line-by-line video information

actually be cheaper to install and mainshytain whi le prov id ing far superior pershyformance These compel l ing economic and technical advantages resulted in the appearance seemingly overnight of a strong industry-wide demand for l ightwave technology

Photonic technology thereby proshymoted the cable industry f rom an unreshyliable provider of sub-500-MHz var ishyab le-qual i ty v i deo to a potent ia l ly highly reliable provider of 1-GHz bandshyw id th to the home This entire 1-GHz bandwid th has the potential to support services that require extremely h igh

separated by hor izontal and vertical synchronization pulses p lus audio and color subcarriers Single-sideband vesshytigial (VSB) f i l tering is used to min i shymize the bandwidth of the modulated spectrum The resultant RF spectrum is dominated by the remaining RF carshyrier wh ich is reduced by typical ly 56 dB by the A M and contains relatively low-level signal information over the 4 M H z bandwidth

For cable distr ibution many chanshynels are frequency-division mult iplexed (FDM) separated nominal ly by 6 M H z (8 M H z in Europe) over the bandwidth

COAXIAL-CABLE CATV DISTRIBUTION NETWORK WITHOUT FIBER TRUNK SYSTEMS LONG CASCADES OF COAX AND AMPLIFIERS RESULT IN POOR CARRIER-TO-NOISE RATIO

18 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

F I G U R E 1

C A B L E T V SYSTEMS

supported by the coaxial cable A typi shycal 60-channel domestic cable system operates between 5525 and 43925 M H z Given the large dynamic range required to transmit both the large reshymaining RF carrier and the low-level sidebands transmission of this mult i shychannel A M - V S B spectrum is by d ig i shytal l ightwave standards a hideous task

INTENSITY-MODULATION

FUNDAMENTALS

Direct-detection l ightwave technology is ideal for transmission of analog and microwave signals 6 Ideally the light output f rom a diode laser is a l inear function of the injected current By preshybiasing the laser to an average output power modulat ion appl ied to the bias current is converted l inearly to intenshysity modulat ion (IM) A t a photodetecshytor the detected current is l inearly proshyp o r t i o n a l to the rece i ved op t i ca l intensity The optical intensity is then analogous to voltage i n an all-electrical RF system Hence the laser-fiber-deshytector can be thought of s imply as a low loss piece of coaxial cable

The laser (optical carrier) is modushylated by the sum of the video channels that are combined to form the total RF signal spectrum The resultant optical spectrum contains sidebands f rom the I M superimposed on unintentional freshyquency modulat ion or chirp that genshyerally accompanies I M This complex optical spectrum must be understood if certain subtle impairments are to be avoided But for simple system design purposes we care only about the optishycal intensity and the RF power specshytrum the details of the optical specshytrum are unimportant

NOISE LIMITATIONS

We define the carrier-to-noise ratio (CNR) as the ratio of the RF power in the remaining carrier of one video chanshynel to the total RF noise power in the 4 M H z bandwidth around the carrier For high-qual i ty t ransmission the C N R must be in the order of 50 dB (electrical power ) In c o m p a r i s o n d i g i t a l l ightwave systems require C N R s less than 20 dB Thus it is important to consider al l sources of noise in designshying a h igh quality A M - V S B system 7

even those that do not affect digi tal l ightwave systems

Shot Noise The most important noise sources are shot noise receiver noise and relative-intensity noise (RIN) Shot noise is a fundamental consequence of the statisshytical nature of the photodetection proshycess It results i n a noise power specshytral density or electrical noise power per unit bandwidth ( d B m H z ) that is proport ional to the total received phoshytocurrent

Since many (60 or more) v ideo channels are combined to modulate the laser s imu l taneous ly the depth of modulat ion for each channel is less than typically 4 The modulat ion depth (m) is defined as the ratio of the peak I M for one channel d iv ided by the average

transmitters launch slightly less than 10 m W average optical power Hence the total optical loss margin al lowed is less than 10 dB

Receiver Noise Receiver noise is generated by the elecshytronic ampl i f ier used to boost the deshytected photocurrent to usable levels The easiest receiver to bu i l d consists of a p-i-n photodiode connected d i shyrectly to a low-noise 7 5 Ω ampli f ier This is often sufficient to make the reshyceiver noise smal l compared to shot noise In systems designed for lower C N R values where the received power (hence shot noise power) is smaller receiver noise must be reduced A va -

intensity W i th such smal l m the deshytected signal current is a small fraction of the total received current The total shot noise power wh ich is proportional to the total received current is then significant compared to the signal of one channel

Shot noise limits the C N R to a level referred to as the quantum limit Reshyceived powers near 1 m W are required if C N R s greater than 50 dB are to be achieved for 40-80 channel systems Obviously the transmitted power must be large or the system loss must be small Typica l commercial ly-available

r iety of impedance-match ing techshyniques and push-pu l l amplif ier designs can reduce the receiver noise current by at least a factor of three

Relative Intensity Noise R I N can originate f rom the laser or f rom reflections and Rayleigh backscatter in the fiber In the laser R I N is caused by spontaneous emiss ion i n the active layer Spontaneous emission drives ranshydom fluctuations in the number of phoshytons i n the laser wh ich appear as a random modulat ion of the output inshytensity w i th frequency components ex-

NEW CATV ARCHITECTURE USING AM-VSB FIBER TRUNK SYSTEMS TO DELIVER HIGH QUALITY VIDEO SIGNALS DEEP WITHIN THE DISTRIBUTION NETWORK EFFIshyCIENT USE CAN BE MADE OF THE EXISTING COAX TRUNK SYSTEM

FIGURE 2

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 19

tending to tens of G H z A l though deshytecting more optical power helps to overcome shot and receiver noise the ratio of signal-to-RIN remains constant Hence R I N can be dominant in high-C N R systems when the received power is large

R I N is also caused by component reflections and double-Rayleigh back-scatter in the fiber by a process called mu l t ipa th interference 8 Doub ly - re shyflected signals arr iv ing at the detector can interfere coherently w i th the deshysired unreflected signal Depending on the modulated optical spectrum of the laser this interference results i n noise that can be significant 9 This noise l ike R I N from the laser cannot be overcome by detecting more power

Figure 3 shows the noise l imits for a system w i th a received average phoshytocurrent of 1 m A Since good detecshytors have a responsivity near 08 A m p s Wthis corresponds to a received optishycal power of approximately 12 m W The C N R (or carrier-to-interference [CIR]) increases w i th increasing modushylation depth unti l nonlinearity becomes dominant as discussed later Typica l receiver noise levels (n) are for this relatively h igh received power insigshynificant R I N f rom either the laser or mult ipath interference may reduce the C N R by a few dB M a x i m i z i n g the modulat ion depth is crit ical to good C N R performance

LINEARITY R E Q U I R E M E N T S

Source linearity places a strict l imit on how much modulat ion can be appl ied Linearity in this case refers to the l inshyearity of the current-to-light-intensity (I-L) conversion in the laser or voltage-to-light (V-L) for an external modulashytor Nonl ineari ty results in the generashyt ion of distort ion products between combinations of video-channel carriers This is not to be confused wi th optical nonlinearities in the devices or fiber that cou ld produce unrelated d is tor t ion products in the presence of mult ip le optical carriers

Ideally the I-L or V - L charactershyistics w o u l d be perfectly l inear In acshytual i ty numerous nonl inear mechashynisms must be considered for direct modula t ion and no external modu la shytor has a l inear transfer funct ion For l ow modula t ion frequencies a Tayshy

lor-series expansion of the I-L or V - L character ist ic centered at the bias point results i n l inear quadrat ic cu shyb ic and higher-order terms The l i n shyear term describes the eff iciency w i t h w h i c h the app l ied s ignal is converted to l inear intensi ty modu la t ion The quadrat ic term results i n second-orshyder d is to r t ion the cubic p roduces th i rd-order d istor t ion and so on

Strict requirements on l inear i ty arise f rom the number of distort ion products generated by the many carrishyers i n the mult i-channel signal Second-order nonlinearity results in sum and difference mix ing products for every combination of two channels The large number of such combinations results i n as many as 20 second-order p rodshyucts w i th in a single channel If h igh-quality video requires that the total secshyond-order d is tor t ion or composi te second-order distort ion (CSO) is 60 dB less than the carrier power (mdash60 dBc) then each sum or difference product must be less than mdash73 dBc

Similar ly for third-order distortion products result f rom mix ing between a l l combinat ions of three channels However since the number of combishynations of three channels is much larger than for two up to 1000 third-order products can interfere w i th one chanshynel For composite third-order distorshyt ions or the composi te t r ip le-beat (CTB) to be less than - 6 0 dBc each product must be less than mdash90 dBc

Such strict l inearity requirements are diff icult to meet w i th the push-pul l or feedforward amplif iers that the inshydustry spent years developing for coshyaxial-cable trunk systemsmdash amplif iers that use complex circuits and carefully matched high-power transistors Yet we require such performance f rom a single diode laser By careful design and unshyderstanding of the various nonlinear mechanisms sufficiently linear distribshyuted-feedback (DFB) lasers have been developed Also L i N b O 3 external modushylators have been linearized to compete wi th the directly-modulated DFBs

F I G U R E 3

NOISE A N D LINEARITY LIMITS TO T H E P E R F O R M A N C E O F M U L T I - C H A N N E L L IGHTWAVE

A M - V S B T R U N K S Y S T E M S T H E C A R R I E R - T O - N O I S E RATIO IS B O U N D E D BY F U N D A shy

M E N T A L SHOT NOISE ( Q U A N T U M LIMIT) A N D CLIPPING ( S A L E H LIMIT) LIMITS

R E C E I V E R NOISE (N) A N D R E L A T I V E INTENSITY NOISE ( R I N ) T Y P I C A L L Y D E G R A D E

T H E C N R BY A F E W D B

20 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

C A B L E T V SYSTEMS

CABLE TV SYSTEMS

D F B L A S E R L INEARITY

Several factors limit the light-versus-current (L-I) linearity of directly-modushylated DFB lasers Early work on laser dynamics led to a complete understandshying of resonance-enhanced distortion (RD)1 0 1 1 RD arises from the same carshyrier-photon interaction within the laser that is responsible for the well-known relaxation-oscillation resonance For high-power lasers required for CATV applications and for frequencies less than 500 MHz RD is not important But as frequencies approaching 1 GHz are considered RD may be large

Within the frequency range beshytween 50 and 500 MHz nonlinear gain and loss inter-valence-band absorpshytion and more importantly spatial-hole burning (SHB) and carrier leakshyage can all be significant Carrier leakage prevents all of the current inshyjected into the laser bond wire from entering the active layer12 This leakage must be reduced to immeasurable levshyels Breakthroughs in the design and implementation of highly effective curshyrent-confining structures were critical in the development of lasers suitable for AM-VSB applications

SHB results from the nonuniform distribution of optical power along the length of the laser In DFB lasers beshycause of the grating feedback the lonshygitudinal distribution of optical power can be highly uniform The increased stimulated emission in regions where the optical power is large results in inshycreased local saturation of the gain Hence modulation of the current does more than just modulate the output power SHB results in modulation of the local gain which modulates the optical frequency (chirp) and the effishyciency with which light is emitted from the laser The net result is distortion13

that can add to or cancel other distorshytions making it in some cases a desirshyable effect

Clipping Even if all nonlinear processes were eliminated the allowable modulation would be limited by the fact that the minimum output power is zero Typishycal operating conditions with for exshyample 60 channels each with an avershyage modulation depth (m) of near 4 results in a peak modulation of 240

Since peak modulations of 100 modushylate the output power to zero clipping is inevitable

The effects of clipping were first approximated by Saleh14 who calcushylated the modulation level at which the total power contained in all orders of distortion became appreciable Figure 3 shows the total interference from clipshyping as a function of modulation depth Even for perfectly linear lasers the modulation depth is bounded to valshyues beyond which all orders of distorshytion increase rapidly Further work has provided a complete understanding of how clipping contributes to the specshytrally-resolved CSO and CTB distorshytions15 The Saleh limit remains an outshystanding approximation to the maximum modulation that can be apshyplied to an ideal laser

C H I R P - R E L A T E D IMPAIRMENTS

With direct modulation laser chirp can cause problems The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for fiber lengths of just a few km Dispershysion converts the frequency modulashytion into intensity modulation (IM) which mixes with the signal IM to proshyduce second-order distortion16 17

Chirp also causes problems with

any optical component that has a transshymission that is a function of optical freshyquency This can occur if two optical reflections conspire to form a weak inshyterferometer or in an erbium-doped fishyber amplifier (EDFA) that has a freshyquency-dependent gain18 Once again the chirp is converted to IM which mixes with the signal IM to form secshyond-order distortion

E X T E R N A L MODULATION

Laser-diode-pumped Y A G lasers with low RIN and output powers greater than 200 mW have been developed reshycently Combined with linearized exshyternal L iNbO 3 modulators these lasers have become a high-performance comshypetitor to directly-modulated DFB lashysers The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator

The only option available today for a low-loss modulator capable of hanshydling more than a few tens of mW is a L i N b O 3 Mach-Zehnder modulator These are available with insertion losses less than 3 dB modulation bandwidths greater than a few GHz and switching voltages near 5 V Ideally for our purshyposes the modulators have a single inshyput fiber and two complementary outshyput fibers This is accomplished using a

FIGURE 4

WAVELENGTH-DIVISION MULTIPLEXED SPLIT-DAND 112-CHANNEL SYSTEM USING DIRECT MODULATION AT 13 microM AND AN EXTERNALLY-MODULATED 155 microM DFB LASER A UNIQUE UPPER-FREQUENCY BAND OF CHANNELS IS NARROWCAST TO EACH SEPARATE TRUNK WHILE THE LOWER BAND IS AMPLIFIED BY AN ER-DOPEO FIBER AMPLIFIER AND BROADCAST TO SEVERAL TRUNKS THE OUTPUTS FROM THE TWO RECEIVERS WOULD BE COMBINED AND DISTRIBUTED OVER SHORT LENGTHS OF COAXIAL CABLE

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 21

C A B L E T V SYSTEMS

Y-branch splitter at the input and a voltshyage-controlled directional coupler at the output

The ou tpu t in t ens i ty of these modulators is a s inuso ida l funct ion of the bias voltage B y pre-bias ing the di rect ional coupler to the point where the power is equal i n both outshyput fibers modu la t ion app l i ed to the Mach-Zehnder results i n the most l i n shyear intensi ty modu la t ion This bias point w h i c h corresponds to the point of inf lect ion i n the s inuso ida l transshyfer function produces zero second-order dis tor t ion Unfor tunate ly the corresponding third-order d is tor t ion is approximate ly 30 d B worse than a typica l d i rec t ly-modula ted D F B l a shyser A means of l inea r i z ing this th i rd -order nonl inear i ty is essential

Various l inearization techniques have been explored mostly borrowed conceptually from early work i n radio and electronics The two most popular approaches are f e e d f o r w a r d a n d predistort ion Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to proshyvide an error signal This error signal is then used to modulate a second laser which is combined wi th the first laser such that the total instantaneous intenshysity of the two lasers is a replica of the applied voltage In principle this techshynique is capable of l inearizing any orshyder of distortion and correcting R I N from the laser In practice difficulties i n generating accurate error signals make feedforward techniques rather difficult to implement

Predistortion requires less circuit complexity than feedforward A careshyfu l ly -des igned nonl inear c i rcu i t is placed before the nonlinear modulator such that the combined transfer funcshytion of the predistorter-modulator is l i n shyear Various nonlinear electronic deshyvices or circuits can act as second- or third-order predistorters Difficulties include matching the frequency depenshydence of the predistorter w i t h that of the modulator hence achieving good linearity over a wide frequency range But if the frequency response of the modulator is flat numerous circuit deshysigns can provide good performance for cable television purposes

Y A G lasers wi th external modulashy

tion provide several advantages and disadvantages i n comparison to direct modulation The YAG-modula to r comshybination offers a considerable increase i n launched power over DFBs The low R I N of the Y A G laser translates into a slight C N R improvement

A l t h o u g h external ly-modula ted systems are immune to chirp related problems fiber nonl inear i tymdashin the form of stimulated Br i l lou in scattering (SBS)mdashplaces a l imit on the launched power SBS i n wh ich light is scattered from acoustic phonons i n the fiber causes a rap id decrease i n C N R for launched powers greater than approxishymately 10 m W 1 9 The chirp i n D F B sysshytems broadens the optical spectrum so that SBS is not a problem

Another disadvantage arises from architectural issues To be cost-competishytive w i t h DFBs the output of the Y A G -modulator is split to serve eight or more trunks The concept of broadcasting from one transmitter to a wide serving area is contrary to the direct ion of change wi th in the industry Narrowcast architectures where different informashytion can be sent to each trunk offer greater flexibility and diversity of sershyvices than do broadcast systems Bal shyancing these architectural issues w i t h the systems issues presents an interestshying challenge to system operators

W A V E L E N G T H - D I V I S I O N M U L T I P L E X I N G

Al though the coaxial-cable port ion of the new fibercoax system architectures can support a bandwidth approaching 1 G H z single-fiber systems that must feed the short lengths of coax can supshyport only 400 M H z Progressive operashytors seeking 150 channel A M - V S B sysshytems 2 0 must use mult iple fibers to feed one coaxial cable Clearly there is room for creative use of wavelength-division mul t ip lexing ( W D M ) to increase the bandwidth of the fiber

By combining direct-modulation at 13 μm wavelength wi th external modushylation at 155 μm W D M transmission of 112 channels has been d e m o n shystrated 2 1 A schematic of the system is shown i n Figure 4 The 155 μm source is an externally modulated D F B laser capable of launching several m W into an erbium-doped fiber amplifier A l shythough the E D F A is operated we l l into

saturation it provides enough gain to al low splitting the 155 μm signal eight ways Passive fiber W D M couplers are used to combine and separate the two wavelengths Externally modulating the 155 μm signal eliminates problems wi th C S O from chromatic dispersion and freshyquency-dependent E D F A gain A r c h i shytectures of this type combine the s imshyplici ty of broadcast distribution for the s t a n d a r d b a n d of channe l s w i t h narrowcast transmission of a special band of channels that is unique to each trunk Continued development of new W D M componen t s w i t h m u l t i p l e wavelengths i n each of the 15513 μm windows w i l l provide increased bandshywid th and flexibility

L O O K I N G A H E A D These technologies provide cable opshyerators w i t h the means to redesign sysshytem architectures for any foreseeable future broadband services W i t h a coshyaxial-cable bandwidth near 1 G H z opshyportunities for enhanced pay-per-view or video-on-demand services abound A l though many researchers may quesshytion the sociological consequences of a nation armed w i t h hundred-channel remote-control selectors 2 2 the technical and economic viability has been proven

Other non-enter ta inment-video services could certainly benefit from the cable indus t ry s b roadband analog transport capability Next-generation cellular telephone systems or personal communications networks could transshymit analog signals to and from remote antenna sites v ia fibercoax systems Architectures designed for narrowcast transmission have adequate bandwidth for interactive two-way video H i g h -definition television can be squeezed into bandwidths comparable to todays A M - V S B standard television W i t h conshytinued advances i n photonic technolshyogy the revitalized cable industry is poised for whatever information distrishybut ion opportunities might arise

T E D A R C I E is head of the Lightwave Comshymunications Research Department ATampT Bell Laboratories Crawford Hill Laboratory Holmdel NJ

2 2 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

CABLE TV SYSTEMS

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19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3

THE PHOTONIC OPPORTUNITY

During the last decade millions of miles of fiber optic cable and thousands of photonic components revolutionized the telecomshymunications industry Digital long-haul sysshytems using single-mode fiber and simple Fabry-Perot lasers offered unquestionable quality and bandwidth advantages over the antiquated microwave network Yet in spite of the success of digital lightwave techshynology the fidelity of even the finest early fiber links did not allow transmission of the complete video spectrum delivered by the cable television industry over coaxial cable

Fiber transmission of this multishychannel amplitude-modulated ( A M ) vestigial-sideband (VSB) signal had to wait nearly a decade for improveshyments mostly in laser technology These improvements have arrived and with them come a variety of analog systems and architectures that are now revolutionizing the cable television industry

Before lightwave A M - V S B systems were available cable operators used satellite or installed digital or F M -subcarrier fiber systems to transmit video between various head end locations Digital or F M video formats are much less demanding in terms of

transmission fidelity than AM-VSB but at the expense of increased bandwidth These formats are then converted to AM-VSB for bandwidth-efficient transmission over coax and so that the signal delivered to the home is compatible with cable-ready consumer video equipment Since multichannel format conversions require large racks of expensive equipment it is essential that this cost be shared among as many subscribers as possible It then becomes a priority to transmit the cable-ready AM-VSB format as far as possible from centralized head-end locations to large subscriber areas

C A B L E T V SYSTEMS

This priori ty shaped cable systems into the conventional tree and branch archishytecture2 as shown in Figure 1 Long trunks of heavy-gauge coaxial cable w i th high-power and highly-l inear amshyplifiers every 300 meters or so stretch up to 32 kilometers f rom a head end Passive taps and l ighter-gauge coax branch from the trunk through more amplifiers to ind iv idua l subscribers In spite of the enormous cascaded cable and tap loss high-fidelity transmission of at least 60 channels over a bandshywid th of 400 M H z was commonplace

Improved Service Quality Unfortunately although the bandwidth of the cable network was impressive it had problems operationally Failure of any one of dozens of amplif iers wou ld disrupt service to large numbers of subshyscribers Accumulat ion of noise and disshytortion from each amplif ier results in impaired performance for subscribers w i th the misfortune of l i v ing far f rom the head end The performance of these mostly aerial trunk systems given corshyrosion l ightning strikes storms and human hazards was a serious operashytional bottleneck

Lightwave A M - V S B trunk systems eliminate this bottleneck as shown in Figure 2 By using 10-20 k m analog f i shyber trunk systems w i th l ightwave reshyceiver nodes at strategic pos i t ions throughout the distr ibution area headshyend-quality v ideo can be interjected wi th in a few kilometers of every home 3

The degradation caused by long ampl i shyfier cascades is eliminated Careful conshysideration of fiber routes also reduces the number of subscribers disrupted by the failure of any one component

Increased Bandwidth A M - V S B fiber trunk systems also proshyvide dramatic increases i n the bandshywidth of a typical cable installation 4

The loss of coaxial cable increases as the square of the frequency transmitshyted The o ld coaxial cable network was l imited by practical amplif ication and equalization to somewhere near 500 M H z But w i th the introduction of fiber trunk systems the longest coaxial cable lengths f rom any l ightwave receiver node can be short Wi th fewer than three amplifiers between the head end and any subscriber coaxial-cable loss

is no longer the bandwidth- l imit ing facshytor Us ing suitable electronic ampl i f i shyers bandwidths approaching 1 G H z can be del ivered to each home

Cost Effectiveness Improvements i n service quali ty and bandwidth more than justify the cost of the l ightwave components required Since fiber is needed only in the trunk port ion of the network leaving the cost of the coaxial distr ibution unchanged the cost of each l ightwave l ink can be d iv ided among a thousand or more subshyscribers The total cost of the l ightwave system inc luding fiber is less than 10 of the total system cost N e w fiber-trunkcoaxial-distr ibut ion systems can

t ransmiss ion f ide l i ty l ike A M - V S B v ideo or bandwidth-ef f ic ient mu l t i shylevel digi tal modulat ion formats The ultimate transmission capacity is imshypressive This paper describes the deshyvelopments and requirements of the photonic technology that makes these A M - V S B l ightwave systems possible

A M - V S B S I G N A L S

The video format of choice both for broadcast and cable television distr ishybut ion has for decades been A M - V S B 5

Each channel consists of an RF carrier that is ampli tude modulated (AM) by v ideo information This approximately 4 -MHz-bandw id th v ideo signal conshytains the line-by-line video information

actually be cheaper to install and mainshytain whi le prov id ing far superior pershyformance These compel l ing economic and technical advantages resulted in the appearance seemingly overnight of a strong industry-wide demand for l ightwave technology

Photonic technology thereby proshymoted the cable industry f rom an unreshyliable provider of sub-500-MHz var ishyab le-qual i ty v i deo to a potent ia l ly highly reliable provider of 1-GHz bandshyw id th to the home This entire 1-GHz bandwid th has the potential to support services that require extremely h igh

separated by hor izontal and vertical synchronization pulses p lus audio and color subcarriers Single-sideband vesshytigial (VSB) f i l tering is used to min i shymize the bandwidth of the modulated spectrum The resultant RF spectrum is dominated by the remaining RF carshyrier wh ich is reduced by typical ly 56 dB by the A M and contains relatively low-level signal information over the 4 M H z bandwidth

For cable distr ibution many chanshynels are frequency-division mult iplexed (FDM) separated nominal ly by 6 M H z (8 M H z in Europe) over the bandwidth

COAXIAL-CABLE CATV DISTRIBUTION NETWORK WITHOUT FIBER TRUNK SYSTEMS LONG CASCADES OF COAX AND AMPLIFIERS RESULT IN POOR CARRIER-TO-NOISE RATIO

18 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

F I G U R E 1

C A B L E T V SYSTEMS

supported by the coaxial cable A typi shycal 60-channel domestic cable system operates between 5525 and 43925 M H z Given the large dynamic range required to transmit both the large reshymaining RF carrier and the low-level sidebands transmission of this mult i shychannel A M - V S B spectrum is by d ig i shytal l ightwave standards a hideous task

INTENSITY-MODULATION

FUNDAMENTALS

Direct-detection l ightwave technology is ideal for transmission of analog and microwave signals 6 Ideally the light output f rom a diode laser is a l inear function of the injected current By preshybiasing the laser to an average output power modulat ion appl ied to the bias current is converted l inearly to intenshysity modulat ion (IM) A t a photodetecshytor the detected current is l inearly proshyp o r t i o n a l to the rece i ved op t i ca l intensity The optical intensity is then analogous to voltage i n an all-electrical RF system Hence the laser-fiber-deshytector can be thought of s imply as a low loss piece of coaxial cable

The laser (optical carrier) is modushylated by the sum of the video channels that are combined to form the total RF signal spectrum The resultant optical spectrum contains sidebands f rom the I M superimposed on unintentional freshyquency modulat ion or chirp that genshyerally accompanies I M This complex optical spectrum must be understood if certain subtle impairments are to be avoided But for simple system design purposes we care only about the optishycal intensity and the RF power specshytrum the details of the optical specshytrum are unimportant

NOISE LIMITATIONS

We define the carrier-to-noise ratio (CNR) as the ratio of the RF power in the remaining carrier of one video chanshynel to the total RF noise power in the 4 M H z bandwidth around the carrier For high-qual i ty t ransmission the C N R must be in the order of 50 dB (electrical power ) In c o m p a r i s o n d i g i t a l l ightwave systems require C N R s less than 20 dB Thus it is important to consider al l sources of noise in designshying a h igh quality A M - V S B system 7

even those that do not affect digi tal l ightwave systems

Shot Noise The most important noise sources are shot noise receiver noise and relative-intensity noise (RIN) Shot noise is a fundamental consequence of the statisshytical nature of the photodetection proshycess It results i n a noise power specshytral density or electrical noise power per unit bandwidth ( d B m H z ) that is proport ional to the total received phoshytocurrent

Since many (60 or more) v ideo channels are combined to modulate the laser s imu l taneous ly the depth of modulat ion for each channel is less than typically 4 The modulat ion depth (m) is defined as the ratio of the peak I M for one channel d iv ided by the average

transmitters launch slightly less than 10 m W average optical power Hence the total optical loss margin al lowed is less than 10 dB

Receiver Noise Receiver noise is generated by the elecshytronic ampl i f ier used to boost the deshytected photocurrent to usable levels The easiest receiver to bu i l d consists of a p-i-n photodiode connected d i shyrectly to a low-noise 7 5 Ω ampli f ier This is often sufficient to make the reshyceiver noise smal l compared to shot noise In systems designed for lower C N R values where the received power (hence shot noise power) is smaller receiver noise must be reduced A va -

intensity W i th such smal l m the deshytected signal current is a small fraction of the total received current The total shot noise power wh ich is proportional to the total received current is then significant compared to the signal of one channel

Shot noise limits the C N R to a level referred to as the quantum limit Reshyceived powers near 1 m W are required if C N R s greater than 50 dB are to be achieved for 40-80 channel systems Obviously the transmitted power must be large or the system loss must be small Typica l commercial ly-available

r iety of impedance-match ing techshyniques and push-pu l l amplif ier designs can reduce the receiver noise current by at least a factor of three

Relative Intensity Noise R I N can originate f rom the laser or f rom reflections and Rayleigh backscatter in the fiber In the laser R I N is caused by spontaneous emiss ion i n the active layer Spontaneous emission drives ranshydom fluctuations in the number of phoshytons i n the laser wh ich appear as a random modulat ion of the output inshytensity w i th frequency components ex-

NEW CATV ARCHITECTURE USING AM-VSB FIBER TRUNK SYSTEMS TO DELIVER HIGH QUALITY VIDEO SIGNALS DEEP WITHIN THE DISTRIBUTION NETWORK EFFIshyCIENT USE CAN BE MADE OF THE EXISTING COAX TRUNK SYSTEM

FIGURE 2

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 19

tending to tens of G H z A l though deshytecting more optical power helps to overcome shot and receiver noise the ratio of signal-to-RIN remains constant Hence R I N can be dominant in high-C N R systems when the received power is large

R I N is also caused by component reflections and double-Rayleigh back-scatter in the fiber by a process called mu l t ipa th interference 8 Doub ly - re shyflected signals arr iv ing at the detector can interfere coherently w i th the deshysired unreflected signal Depending on the modulated optical spectrum of the laser this interference results i n noise that can be significant 9 This noise l ike R I N from the laser cannot be overcome by detecting more power

Figure 3 shows the noise l imits for a system w i th a received average phoshytocurrent of 1 m A Since good detecshytors have a responsivity near 08 A m p s Wthis corresponds to a received optishycal power of approximately 12 m W The C N R (or carrier-to-interference [CIR]) increases w i th increasing modushylation depth unti l nonlinearity becomes dominant as discussed later Typica l receiver noise levels (n) are for this relatively h igh received power insigshynificant R I N f rom either the laser or mult ipath interference may reduce the C N R by a few dB M a x i m i z i n g the modulat ion depth is crit ical to good C N R performance

LINEARITY R E Q U I R E M E N T S

Source linearity places a strict l imit on how much modulat ion can be appl ied Linearity in this case refers to the l inshyearity of the current-to-light-intensity (I-L) conversion in the laser or voltage-to-light (V-L) for an external modulashytor Nonl ineari ty results in the generashyt ion of distort ion products between combinations of video-channel carriers This is not to be confused wi th optical nonlinearities in the devices or fiber that cou ld produce unrelated d is tor t ion products in the presence of mult ip le optical carriers

Ideally the I-L or V - L charactershyistics w o u l d be perfectly l inear In acshytual i ty numerous nonl inear mechashynisms must be considered for direct modula t ion and no external modu la shytor has a l inear transfer funct ion For l ow modula t ion frequencies a Tayshy

lor-series expansion of the I-L or V - L character ist ic centered at the bias point results i n l inear quadrat ic cu shyb ic and higher-order terms The l i n shyear term describes the eff iciency w i t h w h i c h the app l ied s ignal is converted to l inear intensi ty modu la t ion The quadrat ic term results i n second-orshyder d is to r t ion the cubic p roduces th i rd-order d istor t ion and so on

Strict requirements on l inear i ty arise f rom the number of distort ion products generated by the many carrishyers i n the mult i-channel signal Second-order nonlinearity results in sum and difference mix ing products for every combination of two channels The large number of such combinations results i n as many as 20 second-order p rodshyucts w i th in a single channel If h igh-quality video requires that the total secshyond-order d is tor t ion or composi te second-order distort ion (CSO) is 60 dB less than the carrier power (mdash60 dBc) then each sum or difference product must be less than mdash73 dBc

Similar ly for third-order distortion products result f rom mix ing between a l l combinat ions of three channels However since the number of combishynations of three channels is much larger than for two up to 1000 third-order products can interfere w i th one chanshynel For composite third-order distorshyt ions or the composi te t r ip le-beat (CTB) to be less than - 6 0 dBc each product must be less than mdash90 dBc

Such strict l inearity requirements are diff icult to meet w i th the push-pul l or feedforward amplif iers that the inshydustry spent years developing for coshyaxial-cable trunk systemsmdash amplif iers that use complex circuits and carefully matched high-power transistors Yet we require such performance f rom a single diode laser By careful design and unshyderstanding of the various nonlinear mechanisms sufficiently linear distribshyuted-feedback (DFB) lasers have been developed Also L i N b O 3 external modushylators have been linearized to compete wi th the directly-modulated DFBs

F I G U R E 3

NOISE A N D LINEARITY LIMITS TO T H E P E R F O R M A N C E O F M U L T I - C H A N N E L L IGHTWAVE

A M - V S B T R U N K S Y S T E M S T H E C A R R I E R - T O - N O I S E RATIO IS B O U N D E D BY F U N D A shy

M E N T A L SHOT NOISE ( Q U A N T U M LIMIT) A N D CLIPPING ( S A L E H LIMIT) LIMITS

R E C E I V E R NOISE (N) A N D R E L A T I V E INTENSITY NOISE ( R I N ) T Y P I C A L L Y D E G R A D E

T H E C N R BY A F E W D B

20 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

C A B L E T V SYSTEMS

CABLE TV SYSTEMS

D F B L A S E R L INEARITY

Several factors limit the light-versus-current (L-I) linearity of directly-modushylated DFB lasers Early work on laser dynamics led to a complete understandshying of resonance-enhanced distortion (RD)1 0 1 1 RD arises from the same carshyrier-photon interaction within the laser that is responsible for the well-known relaxation-oscillation resonance For high-power lasers required for CATV applications and for frequencies less than 500 MHz RD is not important But as frequencies approaching 1 GHz are considered RD may be large

Within the frequency range beshytween 50 and 500 MHz nonlinear gain and loss inter-valence-band absorpshytion and more importantly spatial-hole burning (SHB) and carrier leakshyage can all be significant Carrier leakage prevents all of the current inshyjected into the laser bond wire from entering the active layer12 This leakage must be reduced to immeasurable levshyels Breakthroughs in the design and implementation of highly effective curshyrent-confining structures were critical in the development of lasers suitable for AM-VSB applications

SHB results from the nonuniform distribution of optical power along the length of the laser In DFB lasers beshycause of the grating feedback the lonshygitudinal distribution of optical power can be highly uniform The increased stimulated emission in regions where the optical power is large results in inshycreased local saturation of the gain Hence modulation of the current does more than just modulate the output power SHB results in modulation of the local gain which modulates the optical frequency (chirp) and the effishyciency with which light is emitted from the laser The net result is distortion13

that can add to or cancel other distorshytions making it in some cases a desirshyable effect

Clipping Even if all nonlinear processes were eliminated the allowable modulation would be limited by the fact that the minimum output power is zero Typishycal operating conditions with for exshyample 60 channels each with an avershyage modulation depth (m) of near 4 results in a peak modulation of 240

Since peak modulations of 100 modushylate the output power to zero clipping is inevitable

The effects of clipping were first approximated by Saleh14 who calcushylated the modulation level at which the total power contained in all orders of distortion became appreciable Figure 3 shows the total interference from clipshyping as a function of modulation depth Even for perfectly linear lasers the modulation depth is bounded to valshyues beyond which all orders of distorshytion increase rapidly Further work has provided a complete understanding of how clipping contributes to the specshytrally-resolved CSO and CTB distorshytions15 The Saleh limit remains an outshystanding approximation to the maximum modulation that can be apshyplied to an ideal laser

C H I R P - R E L A T E D IMPAIRMENTS

With direct modulation laser chirp can cause problems The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for fiber lengths of just a few km Dispershysion converts the frequency modulashytion into intensity modulation (IM) which mixes with the signal IM to proshyduce second-order distortion16 17

Chirp also causes problems with

any optical component that has a transshymission that is a function of optical freshyquency This can occur if two optical reflections conspire to form a weak inshyterferometer or in an erbium-doped fishyber amplifier (EDFA) that has a freshyquency-dependent gain18 Once again the chirp is converted to IM which mixes with the signal IM to form secshyond-order distortion

E X T E R N A L MODULATION

Laser-diode-pumped Y A G lasers with low RIN and output powers greater than 200 mW have been developed reshycently Combined with linearized exshyternal L iNbO 3 modulators these lasers have become a high-performance comshypetitor to directly-modulated DFB lashysers The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator

The only option available today for a low-loss modulator capable of hanshydling more than a few tens of mW is a L i N b O 3 Mach-Zehnder modulator These are available with insertion losses less than 3 dB modulation bandwidths greater than a few GHz and switching voltages near 5 V Ideally for our purshyposes the modulators have a single inshyput fiber and two complementary outshyput fibers This is accomplished using a

FIGURE 4

WAVELENGTH-DIVISION MULTIPLEXED SPLIT-DAND 112-CHANNEL SYSTEM USING DIRECT MODULATION AT 13 microM AND AN EXTERNALLY-MODULATED 155 microM DFB LASER A UNIQUE UPPER-FREQUENCY BAND OF CHANNELS IS NARROWCAST TO EACH SEPARATE TRUNK WHILE THE LOWER BAND IS AMPLIFIED BY AN ER-DOPEO FIBER AMPLIFIER AND BROADCAST TO SEVERAL TRUNKS THE OUTPUTS FROM THE TWO RECEIVERS WOULD BE COMBINED AND DISTRIBUTED OVER SHORT LENGTHS OF COAXIAL CABLE

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 21

C A B L E T V SYSTEMS

Y-branch splitter at the input and a voltshyage-controlled directional coupler at the output

The ou tpu t in t ens i ty of these modulators is a s inuso ida l funct ion of the bias voltage B y pre-bias ing the di rect ional coupler to the point where the power is equal i n both outshyput fibers modu la t ion app l i ed to the Mach-Zehnder results i n the most l i n shyear intensi ty modu la t ion This bias point w h i c h corresponds to the point of inf lect ion i n the s inuso ida l transshyfer function produces zero second-order dis tor t ion Unfor tunate ly the corresponding third-order d is tor t ion is approximate ly 30 d B worse than a typica l d i rec t ly-modula ted D F B l a shyser A means of l inea r i z ing this th i rd -order nonl inear i ty is essential

Various l inearization techniques have been explored mostly borrowed conceptually from early work i n radio and electronics The two most popular approaches are f e e d f o r w a r d a n d predistort ion Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to proshyvide an error signal This error signal is then used to modulate a second laser which is combined wi th the first laser such that the total instantaneous intenshysity of the two lasers is a replica of the applied voltage In principle this techshynique is capable of l inearizing any orshyder of distortion and correcting R I N from the laser In practice difficulties i n generating accurate error signals make feedforward techniques rather difficult to implement

Predistortion requires less circuit complexity than feedforward A careshyfu l ly -des igned nonl inear c i rcu i t is placed before the nonlinear modulator such that the combined transfer funcshytion of the predistorter-modulator is l i n shyear Various nonlinear electronic deshyvices or circuits can act as second- or third-order predistorters Difficulties include matching the frequency depenshydence of the predistorter w i t h that of the modulator hence achieving good linearity over a wide frequency range But if the frequency response of the modulator is flat numerous circuit deshysigns can provide good performance for cable television purposes

Y A G lasers wi th external modulashy

tion provide several advantages and disadvantages i n comparison to direct modulation The YAG-modula to r comshybination offers a considerable increase i n launched power over DFBs The low R I N of the Y A G laser translates into a slight C N R improvement

A l t h o u g h external ly-modula ted systems are immune to chirp related problems fiber nonl inear i tymdashin the form of stimulated Br i l lou in scattering (SBS)mdashplaces a l imit on the launched power SBS i n wh ich light is scattered from acoustic phonons i n the fiber causes a rap id decrease i n C N R for launched powers greater than approxishymately 10 m W 1 9 The chirp i n D F B sysshytems broadens the optical spectrum so that SBS is not a problem

Another disadvantage arises from architectural issues To be cost-competishytive w i t h DFBs the output of the Y A G -modulator is split to serve eight or more trunks The concept of broadcasting from one transmitter to a wide serving area is contrary to the direct ion of change wi th in the industry Narrowcast architectures where different informashytion can be sent to each trunk offer greater flexibility and diversity of sershyvices than do broadcast systems Bal shyancing these architectural issues w i t h the systems issues presents an interestshying challenge to system operators

W A V E L E N G T H - D I V I S I O N M U L T I P L E X I N G

Al though the coaxial-cable port ion of the new fibercoax system architectures can support a bandwidth approaching 1 G H z single-fiber systems that must feed the short lengths of coax can supshyport only 400 M H z Progressive operashytors seeking 150 channel A M - V S B sysshytems 2 0 must use mult iple fibers to feed one coaxial cable Clearly there is room for creative use of wavelength-division mul t ip lexing ( W D M ) to increase the bandwidth of the fiber

By combining direct-modulation at 13 μm wavelength wi th external modushylation at 155 μm W D M transmission of 112 channels has been d e m o n shystrated 2 1 A schematic of the system is shown i n Figure 4 The 155 μm source is an externally modulated D F B laser capable of launching several m W into an erbium-doped fiber amplifier A l shythough the E D F A is operated we l l into

saturation it provides enough gain to al low splitting the 155 μm signal eight ways Passive fiber W D M couplers are used to combine and separate the two wavelengths Externally modulating the 155 μm signal eliminates problems wi th C S O from chromatic dispersion and freshyquency-dependent E D F A gain A r c h i shytectures of this type combine the s imshyplici ty of broadcast distribution for the s t a n d a r d b a n d of channe l s w i t h narrowcast transmission of a special band of channels that is unique to each trunk Continued development of new W D M componen t s w i t h m u l t i p l e wavelengths i n each of the 15513 μm windows w i l l provide increased bandshywid th and flexibility

L O O K I N G A H E A D These technologies provide cable opshyerators w i t h the means to redesign sysshytem architectures for any foreseeable future broadband services W i t h a coshyaxial-cable bandwidth near 1 G H z opshyportunities for enhanced pay-per-view or video-on-demand services abound A l though many researchers may quesshytion the sociological consequences of a nation armed w i t h hundred-channel remote-control selectors 2 2 the technical and economic viability has been proven

Other non-enter ta inment-video services could certainly benefit from the cable indus t ry s b roadband analog transport capability Next-generation cellular telephone systems or personal communications networks could transshymit analog signals to and from remote antenna sites v ia fibercoax systems Architectures designed for narrowcast transmission have adequate bandwidth for interactive two-way video H i g h -definition television can be squeezed into bandwidths comparable to todays A M - V S B standard television W i t h conshytinued advances i n photonic technolshyogy the revitalized cable industry is poised for whatever information distrishybut ion opportunities might arise

T E D A R C I E is head of the Lightwave Comshymunications Research Department ATampT Bell Laboratories Crawford Hill Laboratory Holmdel NJ

2 2 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

CABLE TV SYSTEMS

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15 N J F r igo and G E Bodeep C l i p p i n g distorshyt ion i n subcarr ier m u l t i p l e x e d C A T V sysshyt e m s C o n f e r e n c e o n O p t i c a l F i b e r C o m m u n i c a t i o n ( O F C ) San Jose Cal i f 1992

16 GJ Meslener Chromat ic dispers ion induced dis tor t ion of modu la ted monochromat ic l ight e m p l o y i n g direct detection I E E E J Q u a n shyt u m Electron 20 1984 1208-1216

17 M R Ph i l l i p s et al Non l inea r d is tor t ion genshyerated by dispers ive t ransmiss ion of ch i rped in tensi ty-modulated s ignals I E E E Photonics Technol Lett 35 1991 481-483

18 C K u o and E E Bergmann E r b i u m - d o p e d fiber amplif ier second-order dis tor t ion i n anashylog l inks and electronic compensat ion I E E E Photon Tech Lett 3 1991 829

19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3

C A B L E T V SYSTEMS

This priori ty shaped cable systems into the conventional tree and branch archishytecture2 as shown in Figure 1 Long trunks of heavy-gauge coaxial cable w i th high-power and highly-l inear amshyplifiers every 300 meters or so stretch up to 32 kilometers f rom a head end Passive taps and l ighter-gauge coax branch from the trunk through more amplifiers to ind iv idua l subscribers In spite of the enormous cascaded cable and tap loss high-fidelity transmission of at least 60 channels over a bandshywid th of 400 M H z was commonplace

Improved Service Quality Unfortunately although the bandwidth of the cable network was impressive it had problems operationally Failure of any one of dozens of amplif iers wou ld disrupt service to large numbers of subshyscribers Accumulat ion of noise and disshytortion from each amplif ier results in impaired performance for subscribers w i th the misfortune of l i v ing far f rom the head end The performance of these mostly aerial trunk systems given corshyrosion l ightning strikes storms and human hazards was a serious operashytional bottleneck

Lightwave A M - V S B trunk systems eliminate this bottleneck as shown in Figure 2 By using 10-20 k m analog f i shyber trunk systems w i th l ightwave reshyceiver nodes at strategic pos i t ions throughout the distr ibution area headshyend-quality v ideo can be interjected wi th in a few kilometers of every home 3

The degradation caused by long ampl i shyfier cascades is eliminated Careful conshysideration of fiber routes also reduces the number of subscribers disrupted by the failure of any one component

Increased Bandwidth A M - V S B fiber trunk systems also proshyvide dramatic increases i n the bandshywidth of a typical cable installation 4

The loss of coaxial cable increases as the square of the frequency transmitshyted The o ld coaxial cable network was l imited by practical amplif ication and equalization to somewhere near 500 M H z But w i th the introduction of fiber trunk systems the longest coaxial cable lengths f rom any l ightwave receiver node can be short Wi th fewer than three amplifiers between the head end and any subscriber coaxial-cable loss

is no longer the bandwidth- l imit ing facshytor Us ing suitable electronic ampl i f i shyers bandwidths approaching 1 G H z can be del ivered to each home

Cost Effectiveness Improvements i n service quali ty and bandwidth more than justify the cost of the l ightwave components required Since fiber is needed only in the trunk port ion of the network leaving the cost of the coaxial distr ibution unchanged the cost of each l ightwave l ink can be d iv ided among a thousand or more subshyscribers The total cost of the l ightwave system inc luding fiber is less than 10 of the total system cost N e w fiber-trunkcoaxial-distr ibut ion systems can

t ransmiss ion f ide l i ty l ike A M - V S B v ideo or bandwidth-ef f ic ient mu l t i shylevel digi tal modulat ion formats The ultimate transmission capacity is imshypressive This paper describes the deshyvelopments and requirements of the photonic technology that makes these A M - V S B l ightwave systems possible

A M - V S B S I G N A L S

The video format of choice both for broadcast and cable television distr ishybut ion has for decades been A M - V S B 5

Each channel consists of an RF carrier that is ampli tude modulated (AM) by v ideo information This approximately 4 -MHz-bandw id th v ideo signal conshytains the line-by-line video information

actually be cheaper to install and mainshytain whi le prov id ing far superior pershyformance These compel l ing economic and technical advantages resulted in the appearance seemingly overnight of a strong industry-wide demand for l ightwave technology

Photonic technology thereby proshymoted the cable industry f rom an unreshyliable provider of sub-500-MHz var ishyab le-qual i ty v i deo to a potent ia l ly highly reliable provider of 1-GHz bandshyw id th to the home This entire 1-GHz bandwid th has the potential to support services that require extremely h igh

separated by hor izontal and vertical synchronization pulses p lus audio and color subcarriers Single-sideband vesshytigial (VSB) f i l tering is used to min i shymize the bandwidth of the modulated spectrum The resultant RF spectrum is dominated by the remaining RF carshyrier wh ich is reduced by typical ly 56 dB by the A M and contains relatively low-level signal information over the 4 M H z bandwidth

For cable distr ibution many chanshynels are frequency-division mult iplexed (FDM) separated nominal ly by 6 M H z (8 M H z in Europe) over the bandwidth

COAXIAL-CABLE CATV DISTRIBUTION NETWORK WITHOUT FIBER TRUNK SYSTEMS LONG CASCADES OF COAX AND AMPLIFIERS RESULT IN POOR CARRIER-TO-NOISE RATIO

18 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

F I G U R E 1

C A B L E T V SYSTEMS

supported by the coaxial cable A typi shycal 60-channel domestic cable system operates between 5525 and 43925 M H z Given the large dynamic range required to transmit both the large reshymaining RF carrier and the low-level sidebands transmission of this mult i shychannel A M - V S B spectrum is by d ig i shytal l ightwave standards a hideous task

INTENSITY-MODULATION

FUNDAMENTALS

Direct-detection l ightwave technology is ideal for transmission of analog and microwave signals 6 Ideally the light output f rom a diode laser is a l inear function of the injected current By preshybiasing the laser to an average output power modulat ion appl ied to the bias current is converted l inearly to intenshysity modulat ion (IM) A t a photodetecshytor the detected current is l inearly proshyp o r t i o n a l to the rece i ved op t i ca l intensity The optical intensity is then analogous to voltage i n an all-electrical RF system Hence the laser-fiber-deshytector can be thought of s imply as a low loss piece of coaxial cable

The laser (optical carrier) is modushylated by the sum of the video channels that are combined to form the total RF signal spectrum The resultant optical spectrum contains sidebands f rom the I M superimposed on unintentional freshyquency modulat ion or chirp that genshyerally accompanies I M This complex optical spectrum must be understood if certain subtle impairments are to be avoided But for simple system design purposes we care only about the optishycal intensity and the RF power specshytrum the details of the optical specshytrum are unimportant

NOISE LIMITATIONS

We define the carrier-to-noise ratio (CNR) as the ratio of the RF power in the remaining carrier of one video chanshynel to the total RF noise power in the 4 M H z bandwidth around the carrier For high-qual i ty t ransmission the C N R must be in the order of 50 dB (electrical power ) In c o m p a r i s o n d i g i t a l l ightwave systems require C N R s less than 20 dB Thus it is important to consider al l sources of noise in designshying a h igh quality A M - V S B system 7

even those that do not affect digi tal l ightwave systems

Shot Noise The most important noise sources are shot noise receiver noise and relative-intensity noise (RIN) Shot noise is a fundamental consequence of the statisshytical nature of the photodetection proshycess It results i n a noise power specshytral density or electrical noise power per unit bandwidth ( d B m H z ) that is proport ional to the total received phoshytocurrent

Since many (60 or more) v ideo channels are combined to modulate the laser s imu l taneous ly the depth of modulat ion for each channel is less than typically 4 The modulat ion depth (m) is defined as the ratio of the peak I M for one channel d iv ided by the average

transmitters launch slightly less than 10 m W average optical power Hence the total optical loss margin al lowed is less than 10 dB

Receiver Noise Receiver noise is generated by the elecshytronic ampl i f ier used to boost the deshytected photocurrent to usable levels The easiest receiver to bu i l d consists of a p-i-n photodiode connected d i shyrectly to a low-noise 7 5 Ω ampli f ier This is often sufficient to make the reshyceiver noise smal l compared to shot noise In systems designed for lower C N R values where the received power (hence shot noise power) is smaller receiver noise must be reduced A va -

intensity W i th such smal l m the deshytected signal current is a small fraction of the total received current The total shot noise power wh ich is proportional to the total received current is then significant compared to the signal of one channel

Shot noise limits the C N R to a level referred to as the quantum limit Reshyceived powers near 1 m W are required if C N R s greater than 50 dB are to be achieved for 40-80 channel systems Obviously the transmitted power must be large or the system loss must be small Typica l commercial ly-available

r iety of impedance-match ing techshyniques and push-pu l l amplif ier designs can reduce the receiver noise current by at least a factor of three

Relative Intensity Noise R I N can originate f rom the laser or f rom reflections and Rayleigh backscatter in the fiber In the laser R I N is caused by spontaneous emiss ion i n the active layer Spontaneous emission drives ranshydom fluctuations in the number of phoshytons i n the laser wh ich appear as a random modulat ion of the output inshytensity w i th frequency components ex-

NEW CATV ARCHITECTURE USING AM-VSB FIBER TRUNK SYSTEMS TO DELIVER HIGH QUALITY VIDEO SIGNALS DEEP WITHIN THE DISTRIBUTION NETWORK EFFIshyCIENT USE CAN BE MADE OF THE EXISTING COAX TRUNK SYSTEM

FIGURE 2

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 19

tending to tens of G H z A l though deshytecting more optical power helps to overcome shot and receiver noise the ratio of signal-to-RIN remains constant Hence R I N can be dominant in high-C N R systems when the received power is large

R I N is also caused by component reflections and double-Rayleigh back-scatter in the fiber by a process called mu l t ipa th interference 8 Doub ly - re shyflected signals arr iv ing at the detector can interfere coherently w i th the deshysired unreflected signal Depending on the modulated optical spectrum of the laser this interference results i n noise that can be significant 9 This noise l ike R I N from the laser cannot be overcome by detecting more power

Figure 3 shows the noise l imits for a system w i th a received average phoshytocurrent of 1 m A Since good detecshytors have a responsivity near 08 A m p s Wthis corresponds to a received optishycal power of approximately 12 m W The C N R (or carrier-to-interference [CIR]) increases w i th increasing modushylation depth unti l nonlinearity becomes dominant as discussed later Typica l receiver noise levels (n) are for this relatively h igh received power insigshynificant R I N f rom either the laser or mult ipath interference may reduce the C N R by a few dB M a x i m i z i n g the modulat ion depth is crit ical to good C N R performance

LINEARITY R E Q U I R E M E N T S

Source linearity places a strict l imit on how much modulat ion can be appl ied Linearity in this case refers to the l inshyearity of the current-to-light-intensity (I-L) conversion in the laser or voltage-to-light (V-L) for an external modulashytor Nonl ineari ty results in the generashyt ion of distort ion products between combinations of video-channel carriers This is not to be confused wi th optical nonlinearities in the devices or fiber that cou ld produce unrelated d is tor t ion products in the presence of mult ip le optical carriers

Ideally the I-L or V - L charactershyistics w o u l d be perfectly l inear In acshytual i ty numerous nonl inear mechashynisms must be considered for direct modula t ion and no external modu la shytor has a l inear transfer funct ion For l ow modula t ion frequencies a Tayshy

lor-series expansion of the I-L or V - L character ist ic centered at the bias point results i n l inear quadrat ic cu shyb ic and higher-order terms The l i n shyear term describes the eff iciency w i t h w h i c h the app l ied s ignal is converted to l inear intensi ty modu la t ion The quadrat ic term results i n second-orshyder d is to r t ion the cubic p roduces th i rd-order d istor t ion and so on

Strict requirements on l inear i ty arise f rom the number of distort ion products generated by the many carrishyers i n the mult i-channel signal Second-order nonlinearity results in sum and difference mix ing products for every combination of two channels The large number of such combinations results i n as many as 20 second-order p rodshyucts w i th in a single channel If h igh-quality video requires that the total secshyond-order d is tor t ion or composi te second-order distort ion (CSO) is 60 dB less than the carrier power (mdash60 dBc) then each sum or difference product must be less than mdash73 dBc

Similar ly for third-order distortion products result f rom mix ing between a l l combinat ions of three channels However since the number of combishynations of three channels is much larger than for two up to 1000 third-order products can interfere w i th one chanshynel For composite third-order distorshyt ions or the composi te t r ip le-beat (CTB) to be less than - 6 0 dBc each product must be less than mdash90 dBc

Such strict l inearity requirements are diff icult to meet w i th the push-pul l or feedforward amplif iers that the inshydustry spent years developing for coshyaxial-cable trunk systemsmdash amplif iers that use complex circuits and carefully matched high-power transistors Yet we require such performance f rom a single diode laser By careful design and unshyderstanding of the various nonlinear mechanisms sufficiently linear distribshyuted-feedback (DFB) lasers have been developed Also L i N b O 3 external modushylators have been linearized to compete wi th the directly-modulated DFBs

F I G U R E 3

NOISE A N D LINEARITY LIMITS TO T H E P E R F O R M A N C E O F M U L T I - C H A N N E L L IGHTWAVE

A M - V S B T R U N K S Y S T E M S T H E C A R R I E R - T O - N O I S E RATIO IS B O U N D E D BY F U N D A shy

M E N T A L SHOT NOISE ( Q U A N T U M LIMIT) A N D CLIPPING ( S A L E H LIMIT) LIMITS

R E C E I V E R NOISE (N) A N D R E L A T I V E INTENSITY NOISE ( R I N ) T Y P I C A L L Y D E G R A D E

T H E C N R BY A F E W D B

20 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

C A B L E T V SYSTEMS

CABLE TV SYSTEMS

D F B L A S E R L INEARITY

Several factors limit the light-versus-current (L-I) linearity of directly-modushylated DFB lasers Early work on laser dynamics led to a complete understandshying of resonance-enhanced distortion (RD)1 0 1 1 RD arises from the same carshyrier-photon interaction within the laser that is responsible for the well-known relaxation-oscillation resonance For high-power lasers required for CATV applications and for frequencies less than 500 MHz RD is not important But as frequencies approaching 1 GHz are considered RD may be large

Within the frequency range beshytween 50 and 500 MHz nonlinear gain and loss inter-valence-band absorpshytion and more importantly spatial-hole burning (SHB) and carrier leakshyage can all be significant Carrier leakage prevents all of the current inshyjected into the laser bond wire from entering the active layer12 This leakage must be reduced to immeasurable levshyels Breakthroughs in the design and implementation of highly effective curshyrent-confining structures were critical in the development of lasers suitable for AM-VSB applications

SHB results from the nonuniform distribution of optical power along the length of the laser In DFB lasers beshycause of the grating feedback the lonshygitudinal distribution of optical power can be highly uniform The increased stimulated emission in regions where the optical power is large results in inshycreased local saturation of the gain Hence modulation of the current does more than just modulate the output power SHB results in modulation of the local gain which modulates the optical frequency (chirp) and the effishyciency with which light is emitted from the laser The net result is distortion13

that can add to or cancel other distorshytions making it in some cases a desirshyable effect

Clipping Even if all nonlinear processes were eliminated the allowable modulation would be limited by the fact that the minimum output power is zero Typishycal operating conditions with for exshyample 60 channels each with an avershyage modulation depth (m) of near 4 results in a peak modulation of 240

Since peak modulations of 100 modushylate the output power to zero clipping is inevitable

The effects of clipping were first approximated by Saleh14 who calcushylated the modulation level at which the total power contained in all orders of distortion became appreciable Figure 3 shows the total interference from clipshyping as a function of modulation depth Even for perfectly linear lasers the modulation depth is bounded to valshyues beyond which all orders of distorshytion increase rapidly Further work has provided a complete understanding of how clipping contributes to the specshytrally-resolved CSO and CTB distorshytions15 The Saleh limit remains an outshystanding approximation to the maximum modulation that can be apshyplied to an ideal laser

C H I R P - R E L A T E D IMPAIRMENTS

With direct modulation laser chirp can cause problems The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for fiber lengths of just a few km Dispershysion converts the frequency modulashytion into intensity modulation (IM) which mixes with the signal IM to proshyduce second-order distortion16 17

Chirp also causes problems with

any optical component that has a transshymission that is a function of optical freshyquency This can occur if two optical reflections conspire to form a weak inshyterferometer or in an erbium-doped fishyber amplifier (EDFA) that has a freshyquency-dependent gain18 Once again the chirp is converted to IM which mixes with the signal IM to form secshyond-order distortion

E X T E R N A L MODULATION

Laser-diode-pumped Y A G lasers with low RIN and output powers greater than 200 mW have been developed reshycently Combined with linearized exshyternal L iNbO 3 modulators these lasers have become a high-performance comshypetitor to directly-modulated DFB lashysers The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator

The only option available today for a low-loss modulator capable of hanshydling more than a few tens of mW is a L i N b O 3 Mach-Zehnder modulator These are available with insertion losses less than 3 dB modulation bandwidths greater than a few GHz and switching voltages near 5 V Ideally for our purshyposes the modulators have a single inshyput fiber and two complementary outshyput fibers This is accomplished using a

FIGURE 4

WAVELENGTH-DIVISION MULTIPLEXED SPLIT-DAND 112-CHANNEL SYSTEM USING DIRECT MODULATION AT 13 microM AND AN EXTERNALLY-MODULATED 155 microM DFB LASER A UNIQUE UPPER-FREQUENCY BAND OF CHANNELS IS NARROWCAST TO EACH SEPARATE TRUNK WHILE THE LOWER BAND IS AMPLIFIED BY AN ER-DOPEO FIBER AMPLIFIER AND BROADCAST TO SEVERAL TRUNKS THE OUTPUTS FROM THE TWO RECEIVERS WOULD BE COMBINED AND DISTRIBUTED OVER SHORT LENGTHS OF COAXIAL CABLE

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 21

C A B L E T V SYSTEMS

Y-branch splitter at the input and a voltshyage-controlled directional coupler at the output

The ou tpu t in t ens i ty of these modulators is a s inuso ida l funct ion of the bias voltage B y pre-bias ing the di rect ional coupler to the point where the power is equal i n both outshyput fibers modu la t ion app l i ed to the Mach-Zehnder results i n the most l i n shyear intensi ty modu la t ion This bias point w h i c h corresponds to the point of inf lect ion i n the s inuso ida l transshyfer function produces zero second-order dis tor t ion Unfor tunate ly the corresponding third-order d is tor t ion is approximate ly 30 d B worse than a typica l d i rec t ly-modula ted D F B l a shyser A means of l inea r i z ing this th i rd -order nonl inear i ty is essential

Various l inearization techniques have been explored mostly borrowed conceptually from early work i n radio and electronics The two most popular approaches are f e e d f o r w a r d a n d predistort ion Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to proshyvide an error signal This error signal is then used to modulate a second laser which is combined wi th the first laser such that the total instantaneous intenshysity of the two lasers is a replica of the applied voltage In principle this techshynique is capable of l inearizing any orshyder of distortion and correcting R I N from the laser In practice difficulties i n generating accurate error signals make feedforward techniques rather difficult to implement

Predistortion requires less circuit complexity than feedforward A careshyfu l ly -des igned nonl inear c i rcu i t is placed before the nonlinear modulator such that the combined transfer funcshytion of the predistorter-modulator is l i n shyear Various nonlinear electronic deshyvices or circuits can act as second- or third-order predistorters Difficulties include matching the frequency depenshydence of the predistorter w i t h that of the modulator hence achieving good linearity over a wide frequency range But if the frequency response of the modulator is flat numerous circuit deshysigns can provide good performance for cable television purposes

Y A G lasers wi th external modulashy

tion provide several advantages and disadvantages i n comparison to direct modulation The YAG-modula to r comshybination offers a considerable increase i n launched power over DFBs The low R I N of the Y A G laser translates into a slight C N R improvement

A l t h o u g h external ly-modula ted systems are immune to chirp related problems fiber nonl inear i tymdashin the form of stimulated Br i l lou in scattering (SBS)mdashplaces a l imit on the launched power SBS i n wh ich light is scattered from acoustic phonons i n the fiber causes a rap id decrease i n C N R for launched powers greater than approxishymately 10 m W 1 9 The chirp i n D F B sysshytems broadens the optical spectrum so that SBS is not a problem

Another disadvantage arises from architectural issues To be cost-competishytive w i t h DFBs the output of the Y A G -modulator is split to serve eight or more trunks The concept of broadcasting from one transmitter to a wide serving area is contrary to the direct ion of change wi th in the industry Narrowcast architectures where different informashytion can be sent to each trunk offer greater flexibility and diversity of sershyvices than do broadcast systems Bal shyancing these architectural issues w i t h the systems issues presents an interestshying challenge to system operators

W A V E L E N G T H - D I V I S I O N M U L T I P L E X I N G

Al though the coaxial-cable port ion of the new fibercoax system architectures can support a bandwidth approaching 1 G H z single-fiber systems that must feed the short lengths of coax can supshyport only 400 M H z Progressive operashytors seeking 150 channel A M - V S B sysshytems 2 0 must use mult iple fibers to feed one coaxial cable Clearly there is room for creative use of wavelength-division mul t ip lexing ( W D M ) to increase the bandwidth of the fiber

By combining direct-modulation at 13 μm wavelength wi th external modushylation at 155 μm W D M transmission of 112 channels has been d e m o n shystrated 2 1 A schematic of the system is shown i n Figure 4 The 155 μm source is an externally modulated D F B laser capable of launching several m W into an erbium-doped fiber amplifier A l shythough the E D F A is operated we l l into

saturation it provides enough gain to al low splitting the 155 μm signal eight ways Passive fiber W D M couplers are used to combine and separate the two wavelengths Externally modulating the 155 μm signal eliminates problems wi th C S O from chromatic dispersion and freshyquency-dependent E D F A gain A r c h i shytectures of this type combine the s imshyplici ty of broadcast distribution for the s t a n d a r d b a n d of channe l s w i t h narrowcast transmission of a special band of channels that is unique to each trunk Continued development of new W D M componen t s w i t h m u l t i p l e wavelengths i n each of the 15513 μm windows w i l l provide increased bandshywid th and flexibility

L O O K I N G A H E A D These technologies provide cable opshyerators w i t h the means to redesign sysshytem architectures for any foreseeable future broadband services W i t h a coshyaxial-cable bandwidth near 1 G H z opshyportunities for enhanced pay-per-view or video-on-demand services abound A l though many researchers may quesshytion the sociological consequences of a nation armed w i t h hundred-channel remote-control selectors 2 2 the technical and economic viability has been proven

Other non-enter ta inment-video services could certainly benefit from the cable indus t ry s b roadband analog transport capability Next-generation cellular telephone systems or personal communications networks could transshymit analog signals to and from remote antenna sites v ia fibercoax systems Architectures designed for narrowcast transmission have adequate bandwidth for interactive two-way video H i g h -definition television can be squeezed into bandwidths comparable to todays A M - V S B standard television W i t h conshytinued advances i n photonic technolshyogy the revitalized cable industry is poised for whatever information distrishybut ion opportunities might arise

T E D A R C I E is head of the Lightwave Comshymunications Research Department ATampT Bell Laboratories Crawford Hill Laboratory Holmdel NJ

2 2 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

CABLE TV SYSTEMS

R E F E R E N C E S 1 R E Patterson et al L inea r i za t i on of m u l t i shy

c h a n n e l a n a l o g t r a n s m i t t e r s b y q u a s i shyfeedforward compensat ion technique IEEE Trans on C o m m u n Com-273 1979 582-588

2 N a t i o n a l C a b l e T e l e v i s i o n A s s o c i a t i o n ( N C T A ) Proceedings f rom Techn ica l Sesshysions annual meetings 1724 Massachusetts A v e N W W a s h i n g t o n D C

3 Society of Cable Te lev i s ion Engineers (SCTE) Proceeding f rom Technical Sessions b ienn ia l meetings Ex ton C o m m o n s Ex ton Pa

4 J A C h i d d i x et al A M v ideo on fiber i n C A T V systems need and implementa t ion I E E E Journ Selected Areas i n C o m m u n i c a shytions 8 1990 1229

5 C C I R Report X X I pt2 1986 215-216 6 T E D a r c i e Subcar r i e r m u l t i p l e x i n g for

l i g h t w a v e mul t ip le -access l i g h t w a v e netshyw o r k s J L i g h t w a v e Technol L T - 5 1987 1103-1110

7 T E Darc ie a n d G E Bodeep L i g h t w a v e subcarr ier C A T V T r a n s m i s s i o n Sys tems I E E E T r a n s o n M i c r o w a v e T h e o r y a n d Technol 385 1990 524-533

8 A F Judy Intensity noise f rom fiber Ray le igh backscatter and mechanica l spl ices i n Proc 15th European Conf Opt C o m m u n ( E C O C 89) Gotenberg Sweden 1989

9 T E Darcie et al Fiber-ref lect ion- induced impa i rments i n l i gh twave A M - S V B C A T V systems I E E E J L i g h t w a v e Techno l 98 1991 991-995

10 K Y L a u and A Y a r i v In termodula t ion d is shyto r t ion i n a di rect ly modu la ted semiconducshytor injection laser A p p l Phys Lett 45 1984 1034-1036

11 T E Darcie et al In te rmodula t ion and harshymonic dis tor t ion i n I a G a A s P lasers Electron Lett 21 665-666 erra tum ibid 22 1986 619

12 G P A g r a w a l and N K Dut ta L o n g - W a v e shylength Semiconductor Lasers V a n N o s t r a n d R e i n h o l d N e w Y o r k N Y 1986

13 A Takemoto et al Dis t r ibu ted feedback l a shyser d iode and m o d u l e for C A T V systems I E E E J Selected Areas i n C o m m u n i c a t i o n s 8 1990 1359

14 A A M Saleh Fundamenta l l i m i t on n u m shyber of channels i n subcarr ier m u l t i p l e x e d l igh twave C A T V systems Electron Lett 25 N o 12 1989 776-777

15 N J F r igo and G E Bodeep C l i p p i n g distorshyt ion i n subcarr ier m u l t i p l e x e d C A T V sysshyt e m s C o n f e r e n c e o n O p t i c a l F i b e r C o m m u n i c a t i o n ( O F C ) San Jose Cal i f 1992

16 GJ Meslener Chromat ic dispers ion induced dis tor t ion of modu la ted monochromat ic l ight e m p l o y i n g direct detection I E E E J Q u a n shyt u m Electron 20 1984 1208-1216

17 M R Ph i l l i p s et al Non l inea r d is tor t ion genshyerated by dispers ive t ransmiss ion of ch i rped in tensi ty-modulated s ignals I E E E Photonics Technol Lett 35 1991 481-483

18 C K u o and E E Bergmann E r b i u m - d o p e d fiber amplif ier second-order dis tor t ion i n anashylog l inks and electronic compensat ion I E E E Photon Tech Lett 3 1991 829

19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3

C A B L E T V SYSTEMS

supported by the coaxial cable A typi shycal 60-channel domestic cable system operates between 5525 and 43925 M H z Given the large dynamic range required to transmit both the large reshymaining RF carrier and the low-level sidebands transmission of this mult i shychannel A M - V S B spectrum is by d ig i shytal l ightwave standards a hideous task

INTENSITY-MODULATION

FUNDAMENTALS

Direct-detection l ightwave technology is ideal for transmission of analog and microwave signals 6 Ideally the light output f rom a diode laser is a l inear function of the injected current By preshybiasing the laser to an average output power modulat ion appl ied to the bias current is converted l inearly to intenshysity modulat ion (IM) A t a photodetecshytor the detected current is l inearly proshyp o r t i o n a l to the rece i ved op t i ca l intensity The optical intensity is then analogous to voltage i n an all-electrical RF system Hence the laser-fiber-deshytector can be thought of s imply as a low loss piece of coaxial cable

The laser (optical carrier) is modushylated by the sum of the video channels that are combined to form the total RF signal spectrum The resultant optical spectrum contains sidebands f rom the I M superimposed on unintentional freshyquency modulat ion or chirp that genshyerally accompanies I M This complex optical spectrum must be understood if certain subtle impairments are to be avoided But for simple system design purposes we care only about the optishycal intensity and the RF power specshytrum the details of the optical specshytrum are unimportant

NOISE LIMITATIONS

We define the carrier-to-noise ratio (CNR) as the ratio of the RF power in the remaining carrier of one video chanshynel to the total RF noise power in the 4 M H z bandwidth around the carrier For high-qual i ty t ransmission the C N R must be in the order of 50 dB (electrical power ) In c o m p a r i s o n d i g i t a l l ightwave systems require C N R s less than 20 dB Thus it is important to consider al l sources of noise in designshying a h igh quality A M - V S B system 7

even those that do not affect digi tal l ightwave systems

Shot Noise The most important noise sources are shot noise receiver noise and relative-intensity noise (RIN) Shot noise is a fundamental consequence of the statisshytical nature of the photodetection proshycess It results i n a noise power specshytral density or electrical noise power per unit bandwidth ( d B m H z ) that is proport ional to the total received phoshytocurrent

Since many (60 or more) v ideo channels are combined to modulate the laser s imu l taneous ly the depth of modulat ion for each channel is less than typically 4 The modulat ion depth (m) is defined as the ratio of the peak I M for one channel d iv ided by the average

transmitters launch slightly less than 10 m W average optical power Hence the total optical loss margin al lowed is less than 10 dB

Receiver Noise Receiver noise is generated by the elecshytronic ampl i f ier used to boost the deshytected photocurrent to usable levels The easiest receiver to bu i l d consists of a p-i-n photodiode connected d i shyrectly to a low-noise 7 5 Ω ampli f ier This is often sufficient to make the reshyceiver noise smal l compared to shot noise In systems designed for lower C N R values where the received power (hence shot noise power) is smaller receiver noise must be reduced A va -

intensity W i th such smal l m the deshytected signal current is a small fraction of the total received current The total shot noise power wh ich is proportional to the total received current is then significant compared to the signal of one channel

Shot noise limits the C N R to a level referred to as the quantum limit Reshyceived powers near 1 m W are required if C N R s greater than 50 dB are to be achieved for 40-80 channel systems Obviously the transmitted power must be large or the system loss must be small Typica l commercial ly-available

r iety of impedance-match ing techshyniques and push-pu l l amplif ier designs can reduce the receiver noise current by at least a factor of three

Relative Intensity Noise R I N can originate f rom the laser or f rom reflections and Rayleigh backscatter in the fiber In the laser R I N is caused by spontaneous emiss ion i n the active layer Spontaneous emission drives ranshydom fluctuations in the number of phoshytons i n the laser wh ich appear as a random modulat ion of the output inshytensity w i th frequency components ex-

NEW CATV ARCHITECTURE USING AM-VSB FIBER TRUNK SYSTEMS TO DELIVER HIGH QUALITY VIDEO SIGNALS DEEP WITHIN THE DISTRIBUTION NETWORK EFFIshyCIENT USE CAN BE MADE OF THE EXISTING COAX TRUNK SYSTEM

FIGURE 2

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 19

tending to tens of G H z A l though deshytecting more optical power helps to overcome shot and receiver noise the ratio of signal-to-RIN remains constant Hence R I N can be dominant in high-C N R systems when the received power is large

R I N is also caused by component reflections and double-Rayleigh back-scatter in the fiber by a process called mu l t ipa th interference 8 Doub ly - re shyflected signals arr iv ing at the detector can interfere coherently w i th the deshysired unreflected signal Depending on the modulated optical spectrum of the laser this interference results i n noise that can be significant 9 This noise l ike R I N from the laser cannot be overcome by detecting more power

Figure 3 shows the noise l imits for a system w i th a received average phoshytocurrent of 1 m A Since good detecshytors have a responsivity near 08 A m p s Wthis corresponds to a received optishycal power of approximately 12 m W The C N R (or carrier-to-interference [CIR]) increases w i th increasing modushylation depth unti l nonlinearity becomes dominant as discussed later Typica l receiver noise levels (n) are for this relatively h igh received power insigshynificant R I N f rom either the laser or mult ipath interference may reduce the C N R by a few dB M a x i m i z i n g the modulat ion depth is crit ical to good C N R performance

LINEARITY R E Q U I R E M E N T S

Source linearity places a strict l imit on how much modulat ion can be appl ied Linearity in this case refers to the l inshyearity of the current-to-light-intensity (I-L) conversion in the laser or voltage-to-light (V-L) for an external modulashytor Nonl ineari ty results in the generashyt ion of distort ion products between combinations of video-channel carriers This is not to be confused wi th optical nonlinearities in the devices or fiber that cou ld produce unrelated d is tor t ion products in the presence of mult ip le optical carriers

Ideally the I-L or V - L charactershyistics w o u l d be perfectly l inear In acshytual i ty numerous nonl inear mechashynisms must be considered for direct modula t ion and no external modu la shytor has a l inear transfer funct ion For l ow modula t ion frequencies a Tayshy

lor-series expansion of the I-L or V - L character ist ic centered at the bias point results i n l inear quadrat ic cu shyb ic and higher-order terms The l i n shyear term describes the eff iciency w i t h w h i c h the app l ied s ignal is converted to l inear intensi ty modu la t ion The quadrat ic term results i n second-orshyder d is to r t ion the cubic p roduces th i rd-order d istor t ion and so on

Strict requirements on l inear i ty arise f rom the number of distort ion products generated by the many carrishyers i n the mult i-channel signal Second-order nonlinearity results in sum and difference mix ing products for every combination of two channels The large number of such combinations results i n as many as 20 second-order p rodshyucts w i th in a single channel If h igh-quality video requires that the total secshyond-order d is tor t ion or composi te second-order distort ion (CSO) is 60 dB less than the carrier power (mdash60 dBc) then each sum or difference product must be less than mdash73 dBc

Similar ly for third-order distortion products result f rom mix ing between a l l combinat ions of three channels However since the number of combishynations of three channels is much larger than for two up to 1000 third-order products can interfere w i th one chanshynel For composite third-order distorshyt ions or the composi te t r ip le-beat (CTB) to be less than - 6 0 dBc each product must be less than mdash90 dBc

Such strict l inearity requirements are diff icult to meet w i th the push-pul l or feedforward amplif iers that the inshydustry spent years developing for coshyaxial-cable trunk systemsmdash amplif iers that use complex circuits and carefully matched high-power transistors Yet we require such performance f rom a single diode laser By careful design and unshyderstanding of the various nonlinear mechanisms sufficiently linear distribshyuted-feedback (DFB) lasers have been developed Also L i N b O 3 external modushylators have been linearized to compete wi th the directly-modulated DFBs

F I G U R E 3

NOISE A N D LINEARITY LIMITS TO T H E P E R F O R M A N C E O F M U L T I - C H A N N E L L IGHTWAVE

A M - V S B T R U N K S Y S T E M S T H E C A R R I E R - T O - N O I S E RATIO IS B O U N D E D BY F U N D A shy

M E N T A L SHOT NOISE ( Q U A N T U M LIMIT) A N D CLIPPING ( S A L E H LIMIT) LIMITS

R E C E I V E R NOISE (N) A N D R E L A T I V E INTENSITY NOISE ( R I N ) T Y P I C A L L Y D E G R A D E

T H E C N R BY A F E W D B

20 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

C A B L E T V SYSTEMS

CABLE TV SYSTEMS

D F B L A S E R L INEARITY

Several factors limit the light-versus-current (L-I) linearity of directly-modushylated DFB lasers Early work on laser dynamics led to a complete understandshying of resonance-enhanced distortion (RD)1 0 1 1 RD arises from the same carshyrier-photon interaction within the laser that is responsible for the well-known relaxation-oscillation resonance For high-power lasers required for CATV applications and for frequencies less than 500 MHz RD is not important But as frequencies approaching 1 GHz are considered RD may be large

Within the frequency range beshytween 50 and 500 MHz nonlinear gain and loss inter-valence-band absorpshytion and more importantly spatial-hole burning (SHB) and carrier leakshyage can all be significant Carrier leakage prevents all of the current inshyjected into the laser bond wire from entering the active layer12 This leakage must be reduced to immeasurable levshyels Breakthroughs in the design and implementation of highly effective curshyrent-confining structures were critical in the development of lasers suitable for AM-VSB applications

SHB results from the nonuniform distribution of optical power along the length of the laser In DFB lasers beshycause of the grating feedback the lonshygitudinal distribution of optical power can be highly uniform The increased stimulated emission in regions where the optical power is large results in inshycreased local saturation of the gain Hence modulation of the current does more than just modulate the output power SHB results in modulation of the local gain which modulates the optical frequency (chirp) and the effishyciency with which light is emitted from the laser The net result is distortion13

that can add to or cancel other distorshytions making it in some cases a desirshyable effect

Clipping Even if all nonlinear processes were eliminated the allowable modulation would be limited by the fact that the minimum output power is zero Typishycal operating conditions with for exshyample 60 channels each with an avershyage modulation depth (m) of near 4 results in a peak modulation of 240

Since peak modulations of 100 modushylate the output power to zero clipping is inevitable

The effects of clipping were first approximated by Saleh14 who calcushylated the modulation level at which the total power contained in all orders of distortion became appreciable Figure 3 shows the total interference from clipshyping as a function of modulation depth Even for perfectly linear lasers the modulation depth is bounded to valshyues beyond which all orders of distorshytion increase rapidly Further work has provided a complete understanding of how clipping contributes to the specshytrally-resolved CSO and CTB distorshytions15 The Saleh limit remains an outshystanding approximation to the maximum modulation that can be apshyplied to an ideal laser

C H I R P - R E L A T E D IMPAIRMENTS

With direct modulation laser chirp can cause problems The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for fiber lengths of just a few km Dispershysion converts the frequency modulashytion into intensity modulation (IM) which mixes with the signal IM to proshyduce second-order distortion16 17

Chirp also causes problems with

any optical component that has a transshymission that is a function of optical freshyquency This can occur if two optical reflections conspire to form a weak inshyterferometer or in an erbium-doped fishyber amplifier (EDFA) that has a freshyquency-dependent gain18 Once again the chirp is converted to IM which mixes with the signal IM to form secshyond-order distortion

E X T E R N A L MODULATION

Laser-diode-pumped Y A G lasers with low RIN and output powers greater than 200 mW have been developed reshycently Combined with linearized exshyternal L iNbO 3 modulators these lasers have become a high-performance comshypetitor to directly-modulated DFB lashysers The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator

The only option available today for a low-loss modulator capable of hanshydling more than a few tens of mW is a L i N b O 3 Mach-Zehnder modulator These are available with insertion losses less than 3 dB modulation bandwidths greater than a few GHz and switching voltages near 5 V Ideally for our purshyposes the modulators have a single inshyput fiber and two complementary outshyput fibers This is accomplished using a

FIGURE 4

WAVELENGTH-DIVISION MULTIPLEXED SPLIT-DAND 112-CHANNEL SYSTEM USING DIRECT MODULATION AT 13 microM AND AN EXTERNALLY-MODULATED 155 microM DFB LASER A UNIQUE UPPER-FREQUENCY BAND OF CHANNELS IS NARROWCAST TO EACH SEPARATE TRUNK WHILE THE LOWER BAND IS AMPLIFIED BY AN ER-DOPEO FIBER AMPLIFIER AND BROADCAST TO SEVERAL TRUNKS THE OUTPUTS FROM THE TWO RECEIVERS WOULD BE COMBINED AND DISTRIBUTED OVER SHORT LENGTHS OF COAXIAL CABLE

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 21

C A B L E T V SYSTEMS

Y-branch splitter at the input and a voltshyage-controlled directional coupler at the output

The ou tpu t in t ens i ty of these modulators is a s inuso ida l funct ion of the bias voltage B y pre-bias ing the di rect ional coupler to the point where the power is equal i n both outshyput fibers modu la t ion app l i ed to the Mach-Zehnder results i n the most l i n shyear intensi ty modu la t ion This bias point w h i c h corresponds to the point of inf lect ion i n the s inuso ida l transshyfer function produces zero second-order dis tor t ion Unfor tunate ly the corresponding third-order d is tor t ion is approximate ly 30 d B worse than a typica l d i rec t ly-modula ted D F B l a shyser A means of l inea r i z ing this th i rd -order nonl inear i ty is essential

Various l inearization techniques have been explored mostly borrowed conceptually from early work i n radio and electronics The two most popular approaches are f e e d f o r w a r d a n d predistort ion Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to proshyvide an error signal This error signal is then used to modulate a second laser which is combined wi th the first laser such that the total instantaneous intenshysity of the two lasers is a replica of the applied voltage In principle this techshynique is capable of l inearizing any orshyder of distortion and correcting R I N from the laser In practice difficulties i n generating accurate error signals make feedforward techniques rather difficult to implement

Predistortion requires less circuit complexity than feedforward A careshyfu l ly -des igned nonl inear c i rcu i t is placed before the nonlinear modulator such that the combined transfer funcshytion of the predistorter-modulator is l i n shyear Various nonlinear electronic deshyvices or circuits can act as second- or third-order predistorters Difficulties include matching the frequency depenshydence of the predistorter w i t h that of the modulator hence achieving good linearity over a wide frequency range But if the frequency response of the modulator is flat numerous circuit deshysigns can provide good performance for cable television purposes

Y A G lasers wi th external modulashy

tion provide several advantages and disadvantages i n comparison to direct modulation The YAG-modula to r comshybination offers a considerable increase i n launched power over DFBs The low R I N of the Y A G laser translates into a slight C N R improvement

A l t h o u g h external ly-modula ted systems are immune to chirp related problems fiber nonl inear i tymdashin the form of stimulated Br i l lou in scattering (SBS)mdashplaces a l imit on the launched power SBS i n wh ich light is scattered from acoustic phonons i n the fiber causes a rap id decrease i n C N R for launched powers greater than approxishymately 10 m W 1 9 The chirp i n D F B sysshytems broadens the optical spectrum so that SBS is not a problem

Another disadvantage arises from architectural issues To be cost-competishytive w i t h DFBs the output of the Y A G -modulator is split to serve eight or more trunks The concept of broadcasting from one transmitter to a wide serving area is contrary to the direct ion of change wi th in the industry Narrowcast architectures where different informashytion can be sent to each trunk offer greater flexibility and diversity of sershyvices than do broadcast systems Bal shyancing these architectural issues w i t h the systems issues presents an interestshying challenge to system operators

W A V E L E N G T H - D I V I S I O N M U L T I P L E X I N G

Al though the coaxial-cable port ion of the new fibercoax system architectures can support a bandwidth approaching 1 G H z single-fiber systems that must feed the short lengths of coax can supshyport only 400 M H z Progressive operashytors seeking 150 channel A M - V S B sysshytems 2 0 must use mult iple fibers to feed one coaxial cable Clearly there is room for creative use of wavelength-division mul t ip lexing ( W D M ) to increase the bandwidth of the fiber

By combining direct-modulation at 13 μm wavelength wi th external modushylation at 155 μm W D M transmission of 112 channels has been d e m o n shystrated 2 1 A schematic of the system is shown i n Figure 4 The 155 μm source is an externally modulated D F B laser capable of launching several m W into an erbium-doped fiber amplifier A l shythough the E D F A is operated we l l into

saturation it provides enough gain to al low splitting the 155 μm signal eight ways Passive fiber W D M couplers are used to combine and separate the two wavelengths Externally modulating the 155 μm signal eliminates problems wi th C S O from chromatic dispersion and freshyquency-dependent E D F A gain A r c h i shytectures of this type combine the s imshyplici ty of broadcast distribution for the s t a n d a r d b a n d of channe l s w i t h narrowcast transmission of a special band of channels that is unique to each trunk Continued development of new W D M componen t s w i t h m u l t i p l e wavelengths i n each of the 15513 μm windows w i l l provide increased bandshywid th and flexibility

L O O K I N G A H E A D These technologies provide cable opshyerators w i t h the means to redesign sysshytem architectures for any foreseeable future broadband services W i t h a coshyaxial-cable bandwidth near 1 G H z opshyportunities for enhanced pay-per-view or video-on-demand services abound A l though many researchers may quesshytion the sociological consequences of a nation armed w i t h hundred-channel remote-control selectors 2 2 the technical and economic viability has been proven

Other non-enter ta inment-video services could certainly benefit from the cable indus t ry s b roadband analog transport capability Next-generation cellular telephone systems or personal communications networks could transshymit analog signals to and from remote antenna sites v ia fibercoax systems Architectures designed for narrowcast transmission have adequate bandwidth for interactive two-way video H i g h -definition television can be squeezed into bandwidths comparable to todays A M - V S B standard television W i t h conshytinued advances i n photonic technolshyogy the revitalized cable industry is poised for whatever information distrishybut ion opportunities might arise

T E D A R C I E is head of the Lightwave Comshymunications Research Department ATampT Bell Laboratories Crawford Hill Laboratory Holmdel NJ

2 2 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

CABLE TV SYSTEMS

R E F E R E N C E S 1 R E Patterson et al L inea r i za t i on of m u l t i shy

c h a n n e l a n a l o g t r a n s m i t t e r s b y q u a s i shyfeedforward compensat ion technique IEEE Trans on C o m m u n Com-273 1979 582-588

2 N a t i o n a l C a b l e T e l e v i s i o n A s s o c i a t i o n ( N C T A ) Proceedings f rom Techn ica l Sesshysions annual meetings 1724 Massachusetts A v e N W W a s h i n g t o n D C

3 Society of Cable Te lev i s ion Engineers (SCTE) Proceeding f rom Technical Sessions b ienn ia l meetings Ex ton C o m m o n s Ex ton Pa

4 J A C h i d d i x et al A M v ideo on fiber i n C A T V systems need and implementa t ion I E E E Journ Selected Areas i n C o m m u n i c a shytions 8 1990 1229

5 C C I R Report X X I pt2 1986 215-216 6 T E D a r c i e Subcar r i e r m u l t i p l e x i n g for

l i g h t w a v e mul t ip le -access l i g h t w a v e netshyw o r k s J L i g h t w a v e Technol L T - 5 1987 1103-1110

7 T E Darc ie a n d G E Bodeep L i g h t w a v e subcarr ier C A T V T r a n s m i s s i o n Sys tems I E E E T r a n s o n M i c r o w a v e T h e o r y a n d Technol 385 1990 524-533

8 A F Judy Intensity noise f rom fiber Ray le igh backscatter and mechanica l spl ices i n Proc 15th European Conf Opt C o m m u n ( E C O C 89) Gotenberg Sweden 1989

9 T E Darcie et al Fiber-ref lect ion- induced impa i rments i n l i gh twave A M - S V B C A T V systems I E E E J L i g h t w a v e Techno l 98 1991 991-995

10 K Y L a u and A Y a r i v In termodula t ion d is shyto r t ion i n a di rect ly modu la ted semiconducshytor injection laser A p p l Phys Lett 45 1984 1034-1036

11 T E Darcie et al In te rmodula t ion and harshymonic dis tor t ion i n I a G a A s P lasers Electron Lett 21 665-666 erra tum ibid 22 1986 619

12 G P A g r a w a l and N K Dut ta L o n g - W a v e shylength Semiconductor Lasers V a n N o s t r a n d R e i n h o l d N e w Y o r k N Y 1986

13 A Takemoto et al Dis t r ibu ted feedback l a shyser d iode and m o d u l e for C A T V systems I E E E J Selected Areas i n C o m m u n i c a t i o n s 8 1990 1359

14 A A M Saleh Fundamenta l l i m i t on n u m shyber of channels i n subcarr ier m u l t i p l e x e d l igh twave C A T V systems Electron Lett 25 N o 12 1989 776-777

15 N J F r igo and G E Bodeep C l i p p i n g distorshyt ion i n subcarr ier m u l t i p l e x e d C A T V sysshyt e m s C o n f e r e n c e o n O p t i c a l F i b e r C o m m u n i c a t i o n ( O F C ) San Jose Cal i f 1992

16 GJ Meslener Chromat ic dispers ion induced dis tor t ion of modu la ted monochromat ic l ight e m p l o y i n g direct detection I E E E J Q u a n shyt u m Electron 20 1984 1208-1216

17 M R Ph i l l i p s et al Non l inea r d is tor t ion genshyerated by dispers ive t ransmiss ion of ch i rped in tensi ty-modulated s ignals I E E E Photonics Technol Lett 35 1991 481-483

18 C K u o and E E Bergmann E r b i u m - d o p e d fiber amplif ier second-order dis tor t ion i n anashylog l inks and electronic compensat ion I E E E Photon Tech Lett 3 1991 829

19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3

tending to tens of G H z A l though deshytecting more optical power helps to overcome shot and receiver noise the ratio of signal-to-RIN remains constant Hence R I N can be dominant in high-C N R systems when the received power is large

R I N is also caused by component reflections and double-Rayleigh back-scatter in the fiber by a process called mu l t ipa th interference 8 Doub ly - re shyflected signals arr iv ing at the detector can interfere coherently w i th the deshysired unreflected signal Depending on the modulated optical spectrum of the laser this interference results i n noise that can be significant 9 This noise l ike R I N from the laser cannot be overcome by detecting more power

Figure 3 shows the noise l imits for a system w i th a received average phoshytocurrent of 1 m A Since good detecshytors have a responsivity near 08 A m p s Wthis corresponds to a received optishycal power of approximately 12 m W The C N R (or carrier-to-interference [CIR]) increases w i th increasing modushylation depth unti l nonlinearity becomes dominant as discussed later Typica l receiver noise levels (n) are for this relatively h igh received power insigshynificant R I N f rom either the laser or mult ipath interference may reduce the C N R by a few dB M a x i m i z i n g the modulat ion depth is crit ical to good C N R performance

LINEARITY R E Q U I R E M E N T S

Source linearity places a strict l imit on how much modulat ion can be appl ied Linearity in this case refers to the l inshyearity of the current-to-light-intensity (I-L) conversion in the laser or voltage-to-light (V-L) for an external modulashytor Nonl ineari ty results in the generashyt ion of distort ion products between combinations of video-channel carriers This is not to be confused wi th optical nonlinearities in the devices or fiber that cou ld produce unrelated d is tor t ion products in the presence of mult ip le optical carriers

Ideally the I-L or V - L charactershyistics w o u l d be perfectly l inear In acshytual i ty numerous nonl inear mechashynisms must be considered for direct modula t ion and no external modu la shytor has a l inear transfer funct ion For l ow modula t ion frequencies a Tayshy

lor-series expansion of the I-L or V - L character ist ic centered at the bias point results i n l inear quadrat ic cu shyb ic and higher-order terms The l i n shyear term describes the eff iciency w i t h w h i c h the app l ied s ignal is converted to l inear intensi ty modu la t ion The quadrat ic term results i n second-orshyder d is to r t ion the cubic p roduces th i rd-order d istor t ion and so on

Strict requirements on l inear i ty arise f rom the number of distort ion products generated by the many carrishyers i n the mult i-channel signal Second-order nonlinearity results in sum and difference mix ing products for every combination of two channels The large number of such combinations results i n as many as 20 second-order p rodshyucts w i th in a single channel If h igh-quality video requires that the total secshyond-order d is tor t ion or composi te second-order distort ion (CSO) is 60 dB less than the carrier power (mdash60 dBc) then each sum or difference product must be less than mdash73 dBc

Similar ly for third-order distortion products result f rom mix ing between a l l combinat ions of three channels However since the number of combishynations of three channels is much larger than for two up to 1000 third-order products can interfere w i th one chanshynel For composite third-order distorshyt ions or the composi te t r ip le-beat (CTB) to be less than - 6 0 dBc each product must be less than mdash90 dBc

Such strict l inearity requirements are diff icult to meet w i th the push-pul l or feedforward amplif iers that the inshydustry spent years developing for coshyaxial-cable trunk systemsmdash amplif iers that use complex circuits and carefully matched high-power transistors Yet we require such performance f rom a single diode laser By careful design and unshyderstanding of the various nonlinear mechanisms sufficiently linear distribshyuted-feedback (DFB) lasers have been developed Also L i N b O 3 external modushylators have been linearized to compete wi th the directly-modulated DFBs

F I G U R E 3

NOISE A N D LINEARITY LIMITS TO T H E P E R F O R M A N C E O F M U L T I - C H A N N E L L IGHTWAVE

A M - V S B T R U N K S Y S T E M S T H E C A R R I E R - T O - N O I S E RATIO IS B O U N D E D BY F U N D A shy

M E N T A L SHOT NOISE ( Q U A N T U M LIMIT) A N D CLIPPING ( S A L E H LIMIT) LIMITS

R E C E I V E R NOISE (N) A N D R E L A T I V E INTENSITY NOISE ( R I N ) T Y P I C A L L Y D E G R A D E

T H E C N R BY A F E W D B

20 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

C A B L E T V SYSTEMS

CABLE TV SYSTEMS

D F B L A S E R L INEARITY

Several factors limit the light-versus-current (L-I) linearity of directly-modushylated DFB lasers Early work on laser dynamics led to a complete understandshying of resonance-enhanced distortion (RD)1 0 1 1 RD arises from the same carshyrier-photon interaction within the laser that is responsible for the well-known relaxation-oscillation resonance For high-power lasers required for CATV applications and for frequencies less than 500 MHz RD is not important But as frequencies approaching 1 GHz are considered RD may be large

Within the frequency range beshytween 50 and 500 MHz nonlinear gain and loss inter-valence-band absorpshytion and more importantly spatial-hole burning (SHB) and carrier leakshyage can all be significant Carrier leakage prevents all of the current inshyjected into the laser bond wire from entering the active layer12 This leakage must be reduced to immeasurable levshyels Breakthroughs in the design and implementation of highly effective curshyrent-confining structures were critical in the development of lasers suitable for AM-VSB applications

SHB results from the nonuniform distribution of optical power along the length of the laser In DFB lasers beshycause of the grating feedback the lonshygitudinal distribution of optical power can be highly uniform The increased stimulated emission in regions where the optical power is large results in inshycreased local saturation of the gain Hence modulation of the current does more than just modulate the output power SHB results in modulation of the local gain which modulates the optical frequency (chirp) and the effishyciency with which light is emitted from the laser The net result is distortion13

that can add to or cancel other distorshytions making it in some cases a desirshyable effect

Clipping Even if all nonlinear processes were eliminated the allowable modulation would be limited by the fact that the minimum output power is zero Typishycal operating conditions with for exshyample 60 channels each with an avershyage modulation depth (m) of near 4 results in a peak modulation of 240

Since peak modulations of 100 modushylate the output power to zero clipping is inevitable

The effects of clipping were first approximated by Saleh14 who calcushylated the modulation level at which the total power contained in all orders of distortion became appreciable Figure 3 shows the total interference from clipshyping as a function of modulation depth Even for perfectly linear lasers the modulation depth is bounded to valshyues beyond which all orders of distorshytion increase rapidly Further work has provided a complete understanding of how clipping contributes to the specshytrally-resolved CSO and CTB distorshytions15 The Saleh limit remains an outshystanding approximation to the maximum modulation that can be apshyplied to an ideal laser

C H I R P - R E L A T E D IMPAIRMENTS

With direct modulation laser chirp can cause problems The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for fiber lengths of just a few km Dispershysion converts the frequency modulashytion into intensity modulation (IM) which mixes with the signal IM to proshyduce second-order distortion16 17

Chirp also causes problems with

any optical component that has a transshymission that is a function of optical freshyquency This can occur if two optical reflections conspire to form a weak inshyterferometer or in an erbium-doped fishyber amplifier (EDFA) that has a freshyquency-dependent gain18 Once again the chirp is converted to IM which mixes with the signal IM to form secshyond-order distortion

E X T E R N A L MODULATION

Laser-diode-pumped Y A G lasers with low RIN and output powers greater than 200 mW have been developed reshycently Combined with linearized exshyternal L iNbO 3 modulators these lasers have become a high-performance comshypetitor to directly-modulated DFB lashysers The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator

The only option available today for a low-loss modulator capable of hanshydling more than a few tens of mW is a L i N b O 3 Mach-Zehnder modulator These are available with insertion losses less than 3 dB modulation bandwidths greater than a few GHz and switching voltages near 5 V Ideally for our purshyposes the modulators have a single inshyput fiber and two complementary outshyput fibers This is accomplished using a

FIGURE 4

WAVELENGTH-DIVISION MULTIPLEXED SPLIT-DAND 112-CHANNEL SYSTEM USING DIRECT MODULATION AT 13 microM AND AN EXTERNALLY-MODULATED 155 microM DFB LASER A UNIQUE UPPER-FREQUENCY BAND OF CHANNELS IS NARROWCAST TO EACH SEPARATE TRUNK WHILE THE LOWER BAND IS AMPLIFIED BY AN ER-DOPEO FIBER AMPLIFIER AND BROADCAST TO SEVERAL TRUNKS THE OUTPUTS FROM THE TWO RECEIVERS WOULD BE COMBINED AND DISTRIBUTED OVER SHORT LENGTHS OF COAXIAL CABLE

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 21

C A B L E T V SYSTEMS

Y-branch splitter at the input and a voltshyage-controlled directional coupler at the output

The ou tpu t in t ens i ty of these modulators is a s inuso ida l funct ion of the bias voltage B y pre-bias ing the di rect ional coupler to the point where the power is equal i n both outshyput fibers modu la t ion app l i ed to the Mach-Zehnder results i n the most l i n shyear intensi ty modu la t ion This bias point w h i c h corresponds to the point of inf lect ion i n the s inuso ida l transshyfer function produces zero second-order dis tor t ion Unfor tunate ly the corresponding third-order d is tor t ion is approximate ly 30 d B worse than a typica l d i rec t ly-modula ted D F B l a shyser A means of l inea r i z ing this th i rd -order nonl inear i ty is essential

Various l inearization techniques have been explored mostly borrowed conceptually from early work i n radio and electronics The two most popular approaches are f e e d f o r w a r d a n d predistort ion Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to proshyvide an error signal This error signal is then used to modulate a second laser which is combined wi th the first laser such that the total instantaneous intenshysity of the two lasers is a replica of the applied voltage In principle this techshynique is capable of l inearizing any orshyder of distortion and correcting R I N from the laser In practice difficulties i n generating accurate error signals make feedforward techniques rather difficult to implement

Predistortion requires less circuit complexity than feedforward A careshyfu l ly -des igned nonl inear c i rcu i t is placed before the nonlinear modulator such that the combined transfer funcshytion of the predistorter-modulator is l i n shyear Various nonlinear electronic deshyvices or circuits can act as second- or third-order predistorters Difficulties include matching the frequency depenshydence of the predistorter w i t h that of the modulator hence achieving good linearity over a wide frequency range But if the frequency response of the modulator is flat numerous circuit deshysigns can provide good performance for cable television purposes

Y A G lasers wi th external modulashy

tion provide several advantages and disadvantages i n comparison to direct modulation The YAG-modula to r comshybination offers a considerable increase i n launched power over DFBs The low R I N of the Y A G laser translates into a slight C N R improvement

A l t h o u g h external ly-modula ted systems are immune to chirp related problems fiber nonl inear i tymdashin the form of stimulated Br i l lou in scattering (SBS)mdashplaces a l imit on the launched power SBS i n wh ich light is scattered from acoustic phonons i n the fiber causes a rap id decrease i n C N R for launched powers greater than approxishymately 10 m W 1 9 The chirp i n D F B sysshytems broadens the optical spectrum so that SBS is not a problem

Another disadvantage arises from architectural issues To be cost-competishytive w i t h DFBs the output of the Y A G -modulator is split to serve eight or more trunks The concept of broadcasting from one transmitter to a wide serving area is contrary to the direct ion of change wi th in the industry Narrowcast architectures where different informashytion can be sent to each trunk offer greater flexibility and diversity of sershyvices than do broadcast systems Bal shyancing these architectural issues w i t h the systems issues presents an interestshying challenge to system operators

W A V E L E N G T H - D I V I S I O N M U L T I P L E X I N G

Al though the coaxial-cable port ion of the new fibercoax system architectures can support a bandwidth approaching 1 G H z single-fiber systems that must feed the short lengths of coax can supshyport only 400 M H z Progressive operashytors seeking 150 channel A M - V S B sysshytems 2 0 must use mult iple fibers to feed one coaxial cable Clearly there is room for creative use of wavelength-division mul t ip lexing ( W D M ) to increase the bandwidth of the fiber

By combining direct-modulation at 13 μm wavelength wi th external modushylation at 155 μm W D M transmission of 112 channels has been d e m o n shystrated 2 1 A schematic of the system is shown i n Figure 4 The 155 μm source is an externally modulated D F B laser capable of launching several m W into an erbium-doped fiber amplifier A l shythough the E D F A is operated we l l into

saturation it provides enough gain to al low splitting the 155 μm signal eight ways Passive fiber W D M couplers are used to combine and separate the two wavelengths Externally modulating the 155 μm signal eliminates problems wi th C S O from chromatic dispersion and freshyquency-dependent E D F A gain A r c h i shytectures of this type combine the s imshyplici ty of broadcast distribution for the s t a n d a r d b a n d of channe l s w i t h narrowcast transmission of a special band of channels that is unique to each trunk Continued development of new W D M componen t s w i t h m u l t i p l e wavelengths i n each of the 15513 μm windows w i l l provide increased bandshywid th and flexibility

L O O K I N G A H E A D These technologies provide cable opshyerators w i t h the means to redesign sysshytem architectures for any foreseeable future broadband services W i t h a coshyaxial-cable bandwidth near 1 G H z opshyportunities for enhanced pay-per-view or video-on-demand services abound A l though many researchers may quesshytion the sociological consequences of a nation armed w i t h hundred-channel remote-control selectors 2 2 the technical and economic viability has been proven

Other non-enter ta inment-video services could certainly benefit from the cable indus t ry s b roadband analog transport capability Next-generation cellular telephone systems or personal communications networks could transshymit analog signals to and from remote antenna sites v ia fibercoax systems Architectures designed for narrowcast transmission have adequate bandwidth for interactive two-way video H i g h -definition television can be squeezed into bandwidths comparable to todays A M - V S B standard television W i t h conshytinued advances i n photonic technolshyogy the revitalized cable industry is poised for whatever information distrishybut ion opportunities might arise

T E D A R C I E is head of the Lightwave Comshymunications Research Department ATampT Bell Laboratories Crawford Hill Laboratory Holmdel NJ

2 2 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

CABLE TV SYSTEMS

R E F E R E N C E S 1 R E Patterson et al L inea r i za t i on of m u l t i shy

c h a n n e l a n a l o g t r a n s m i t t e r s b y q u a s i shyfeedforward compensat ion technique IEEE Trans on C o m m u n Com-273 1979 582-588

2 N a t i o n a l C a b l e T e l e v i s i o n A s s o c i a t i o n ( N C T A ) Proceedings f rom Techn ica l Sesshysions annual meetings 1724 Massachusetts A v e N W W a s h i n g t o n D C

3 Society of Cable Te lev i s ion Engineers (SCTE) Proceeding f rom Technical Sessions b ienn ia l meetings Ex ton C o m m o n s Ex ton Pa

4 J A C h i d d i x et al A M v ideo on fiber i n C A T V systems need and implementa t ion I E E E Journ Selected Areas i n C o m m u n i c a shytions 8 1990 1229

5 C C I R Report X X I pt2 1986 215-216 6 T E D a r c i e Subcar r i e r m u l t i p l e x i n g for

l i g h t w a v e mul t ip le -access l i g h t w a v e netshyw o r k s J L i g h t w a v e Technol L T - 5 1987 1103-1110

7 T E Darc ie a n d G E Bodeep L i g h t w a v e subcarr ier C A T V T r a n s m i s s i o n Sys tems I E E E T r a n s o n M i c r o w a v e T h e o r y a n d Technol 385 1990 524-533

8 A F Judy Intensity noise f rom fiber Ray le igh backscatter and mechanica l spl ices i n Proc 15th European Conf Opt C o m m u n ( E C O C 89) Gotenberg Sweden 1989

9 T E Darcie et al Fiber-ref lect ion- induced impa i rments i n l i gh twave A M - S V B C A T V systems I E E E J L i g h t w a v e Techno l 98 1991 991-995

10 K Y L a u and A Y a r i v In termodula t ion d is shyto r t ion i n a di rect ly modu la ted semiconducshytor injection laser A p p l Phys Lett 45 1984 1034-1036

11 T E Darcie et al In te rmodula t ion and harshymonic dis tor t ion i n I a G a A s P lasers Electron Lett 21 665-666 erra tum ibid 22 1986 619

12 G P A g r a w a l and N K Dut ta L o n g - W a v e shylength Semiconductor Lasers V a n N o s t r a n d R e i n h o l d N e w Y o r k N Y 1986

13 A Takemoto et al Dis t r ibu ted feedback l a shyser d iode and m o d u l e for C A T V systems I E E E J Selected Areas i n C o m m u n i c a t i o n s 8 1990 1359

14 A A M Saleh Fundamenta l l i m i t on n u m shyber of channels i n subcarr ier m u l t i p l e x e d l igh twave C A T V systems Electron Lett 25 N o 12 1989 776-777

15 N J F r igo and G E Bodeep C l i p p i n g distorshyt ion i n subcarr ier m u l t i p l e x e d C A T V sysshyt e m s C o n f e r e n c e o n O p t i c a l F i b e r C o m m u n i c a t i o n ( O F C ) San Jose Cal i f 1992

16 GJ Meslener Chromat ic dispers ion induced dis tor t ion of modu la ted monochromat ic l ight e m p l o y i n g direct detection I E E E J Q u a n shyt u m Electron 20 1984 1208-1216

17 M R Ph i l l i p s et al Non l inea r d is tor t ion genshyerated by dispers ive t ransmiss ion of ch i rped in tensi ty-modulated s ignals I E E E Photonics Technol Lett 35 1991 481-483

18 C K u o and E E Bergmann E r b i u m - d o p e d fiber amplif ier second-order dis tor t ion i n anashylog l inks and electronic compensat ion I E E E Photon Tech Lett 3 1991 829

19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3

CABLE TV SYSTEMS

D F B L A S E R L INEARITY

Several factors limit the light-versus-current (L-I) linearity of directly-modushylated DFB lasers Early work on laser dynamics led to a complete understandshying of resonance-enhanced distortion (RD)1 0 1 1 RD arises from the same carshyrier-photon interaction within the laser that is responsible for the well-known relaxation-oscillation resonance For high-power lasers required for CATV applications and for frequencies less than 500 MHz RD is not important But as frequencies approaching 1 GHz are considered RD may be large

Within the frequency range beshytween 50 and 500 MHz nonlinear gain and loss inter-valence-band absorpshytion and more importantly spatial-hole burning (SHB) and carrier leakshyage can all be significant Carrier leakage prevents all of the current inshyjected into the laser bond wire from entering the active layer12 This leakage must be reduced to immeasurable levshyels Breakthroughs in the design and implementation of highly effective curshyrent-confining structures were critical in the development of lasers suitable for AM-VSB applications

SHB results from the nonuniform distribution of optical power along the length of the laser In DFB lasers beshycause of the grating feedback the lonshygitudinal distribution of optical power can be highly uniform The increased stimulated emission in regions where the optical power is large results in inshycreased local saturation of the gain Hence modulation of the current does more than just modulate the output power SHB results in modulation of the local gain which modulates the optical frequency (chirp) and the effishyciency with which light is emitted from the laser The net result is distortion13

that can add to or cancel other distorshytions making it in some cases a desirshyable effect

Clipping Even if all nonlinear processes were eliminated the allowable modulation would be limited by the fact that the minimum output power is zero Typishycal operating conditions with for exshyample 60 channels each with an avershyage modulation depth (m) of near 4 results in a peak modulation of 240

Since peak modulations of 100 modushylate the output power to zero clipping is inevitable

The effects of clipping were first approximated by Saleh14 who calcushylated the modulation level at which the total power contained in all orders of distortion became appreciable Figure 3 shows the total interference from clipshyping as a function of modulation depth Even for perfectly linear lasers the modulation depth is bounded to valshyues beyond which all orders of distorshytion increase rapidly Further work has provided a complete understanding of how clipping contributes to the specshytrally-resolved CSO and CTB distorshytions15 The Saleh limit remains an outshystanding approximation to the maximum modulation that can be apshyplied to an ideal laser

C H I R P - R E L A T E D IMPAIRMENTS

With direct modulation laser chirp can cause problems The interaction of chirp and chromatic dispersion in the fiber can cause unacceptable CSO levels for fiber lengths of just a few km Dispershysion converts the frequency modulashytion into intensity modulation (IM) which mixes with the signal IM to proshyduce second-order distortion16 17

Chirp also causes problems with

any optical component that has a transshymission that is a function of optical freshyquency This can occur if two optical reflections conspire to form a weak inshyterferometer or in an erbium-doped fishyber amplifier (EDFA) that has a freshyquency-dependent gain18 Once again the chirp is converted to IM which mixes with the signal IM to form secshyond-order distortion

E X T E R N A L MODULATION

Laser-diode-pumped Y A G lasers with low RIN and output powers greater than 200 mW have been developed reshycently Combined with linearized exshyternal L iNbO 3 modulators these lasers have become a high-performance comshypetitor to directly-modulated DFB lashysers The most challenging technical hurdle is to develop a linear low-loss optical intensity modulator

The only option available today for a low-loss modulator capable of hanshydling more than a few tens of mW is a L i N b O 3 Mach-Zehnder modulator These are available with insertion losses less than 3 dB modulation bandwidths greater than a few GHz and switching voltages near 5 V Ideally for our purshyposes the modulators have a single inshyput fiber and two complementary outshyput fibers This is accomplished using a

FIGURE 4

WAVELENGTH-DIVISION MULTIPLEXED SPLIT-DAND 112-CHANNEL SYSTEM USING DIRECT MODULATION AT 13 microM AND AN EXTERNALLY-MODULATED 155 microM DFB LASER A UNIQUE UPPER-FREQUENCY BAND OF CHANNELS IS NARROWCAST TO EACH SEPARATE TRUNK WHILE THE LOWER BAND IS AMPLIFIED BY AN ER-DOPEO FIBER AMPLIFIER AND BROADCAST TO SEVERAL TRUNKS THE OUTPUTS FROM THE TWO RECEIVERS WOULD BE COMBINED AND DISTRIBUTED OVER SHORT LENGTHS OF COAXIAL CABLE

OPTICS amp PHOTONICS NEWSSEPTEMBER 1992 21

C A B L E T V SYSTEMS

Y-branch splitter at the input and a voltshyage-controlled directional coupler at the output

The ou tpu t in t ens i ty of these modulators is a s inuso ida l funct ion of the bias voltage B y pre-bias ing the di rect ional coupler to the point where the power is equal i n both outshyput fibers modu la t ion app l i ed to the Mach-Zehnder results i n the most l i n shyear intensi ty modu la t ion This bias point w h i c h corresponds to the point of inf lect ion i n the s inuso ida l transshyfer function produces zero second-order dis tor t ion Unfor tunate ly the corresponding third-order d is tor t ion is approximate ly 30 d B worse than a typica l d i rec t ly-modula ted D F B l a shyser A means of l inea r i z ing this th i rd -order nonl inear i ty is essential

Various l inearization techniques have been explored mostly borrowed conceptually from early work i n radio and electronics The two most popular approaches are f e e d f o r w a r d a n d predistort ion Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to proshyvide an error signal This error signal is then used to modulate a second laser which is combined wi th the first laser such that the total instantaneous intenshysity of the two lasers is a replica of the applied voltage In principle this techshynique is capable of l inearizing any orshyder of distortion and correcting R I N from the laser In practice difficulties i n generating accurate error signals make feedforward techniques rather difficult to implement

Predistortion requires less circuit complexity than feedforward A careshyfu l ly -des igned nonl inear c i rcu i t is placed before the nonlinear modulator such that the combined transfer funcshytion of the predistorter-modulator is l i n shyear Various nonlinear electronic deshyvices or circuits can act as second- or third-order predistorters Difficulties include matching the frequency depenshydence of the predistorter w i t h that of the modulator hence achieving good linearity over a wide frequency range But if the frequency response of the modulator is flat numerous circuit deshysigns can provide good performance for cable television purposes

Y A G lasers wi th external modulashy

tion provide several advantages and disadvantages i n comparison to direct modulation The YAG-modula to r comshybination offers a considerable increase i n launched power over DFBs The low R I N of the Y A G laser translates into a slight C N R improvement

A l t h o u g h external ly-modula ted systems are immune to chirp related problems fiber nonl inear i tymdashin the form of stimulated Br i l lou in scattering (SBS)mdashplaces a l imit on the launched power SBS i n wh ich light is scattered from acoustic phonons i n the fiber causes a rap id decrease i n C N R for launched powers greater than approxishymately 10 m W 1 9 The chirp i n D F B sysshytems broadens the optical spectrum so that SBS is not a problem

Another disadvantage arises from architectural issues To be cost-competishytive w i t h DFBs the output of the Y A G -modulator is split to serve eight or more trunks The concept of broadcasting from one transmitter to a wide serving area is contrary to the direct ion of change wi th in the industry Narrowcast architectures where different informashytion can be sent to each trunk offer greater flexibility and diversity of sershyvices than do broadcast systems Bal shyancing these architectural issues w i t h the systems issues presents an interestshying challenge to system operators

W A V E L E N G T H - D I V I S I O N M U L T I P L E X I N G

Al though the coaxial-cable port ion of the new fibercoax system architectures can support a bandwidth approaching 1 G H z single-fiber systems that must feed the short lengths of coax can supshyport only 400 M H z Progressive operashytors seeking 150 channel A M - V S B sysshytems 2 0 must use mult iple fibers to feed one coaxial cable Clearly there is room for creative use of wavelength-division mul t ip lexing ( W D M ) to increase the bandwidth of the fiber

By combining direct-modulation at 13 μm wavelength wi th external modushylation at 155 μm W D M transmission of 112 channels has been d e m o n shystrated 2 1 A schematic of the system is shown i n Figure 4 The 155 μm source is an externally modulated D F B laser capable of launching several m W into an erbium-doped fiber amplifier A l shythough the E D F A is operated we l l into

saturation it provides enough gain to al low splitting the 155 μm signal eight ways Passive fiber W D M couplers are used to combine and separate the two wavelengths Externally modulating the 155 μm signal eliminates problems wi th C S O from chromatic dispersion and freshyquency-dependent E D F A gain A r c h i shytectures of this type combine the s imshyplici ty of broadcast distribution for the s t a n d a r d b a n d of channe l s w i t h narrowcast transmission of a special band of channels that is unique to each trunk Continued development of new W D M componen t s w i t h m u l t i p l e wavelengths i n each of the 15513 μm windows w i l l provide increased bandshywid th and flexibility

L O O K I N G A H E A D These technologies provide cable opshyerators w i t h the means to redesign sysshytem architectures for any foreseeable future broadband services W i t h a coshyaxial-cable bandwidth near 1 G H z opshyportunities for enhanced pay-per-view or video-on-demand services abound A l though many researchers may quesshytion the sociological consequences of a nation armed w i t h hundred-channel remote-control selectors 2 2 the technical and economic viability has been proven

Other non-enter ta inment-video services could certainly benefit from the cable indus t ry s b roadband analog transport capability Next-generation cellular telephone systems or personal communications networks could transshymit analog signals to and from remote antenna sites v ia fibercoax systems Architectures designed for narrowcast transmission have adequate bandwidth for interactive two-way video H i g h -definition television can be squeezed into bandwidths comparable to todays A M - V S B standard television W i t h conshytinued advances i n photonic technolshyogy the revitalized cable industry is poised for whatever information distrishybut ion opportunities might arise

T E D A R C I E is head of the Lightwave Comshymunications Research Department ATampT Bell Laboratories Crawford Hill Laboratory Holmdel NJ

2 2 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

CABLE TV SYSTEMS

R E F E R E N C E S 1 R E Patterson et al L inea r i za t i on of m u l t i shy

c h a n n e l a n a l o g t r a n s m i t t e r s b y q u a s i shyfeedforward compensat ion technique IEEE Trans on C o m m u n Com-273 1979 582-588

2 N a t i o n a l C a b l e T e l e v i s i o n A s s o c i a t i o n ( N C T A ) Proceedings f rom Techn ica l Sesshysions annual meetings 1724 Massachusetts A v e N W W a s h i n g t o n D C

3 Society of Cable Te lev i s ion Engineers (SCTE) Proceeding f rom Technical Sessions b ienn ia l meetings Ex ton C o m m o n s Ex ton Pa

4 J A C h i d d i x et al A M v ideo on fiber i n C A T V systems need and implementa t ion I E E E Journ Selected Areas i n C o m m u n i c a shytions 8 1990 1229

5 C C I R Report X X I pt2 1986 215-216 6 T E D a r c i e Subcar r i e r m u l t i p l e x i n g for

l i g h t w a v e mul t ip le -access l i g h t w a v e netshyw o r k s J L i g h t w a v e Technol L T - 5 1987 1103-1110

7 T E Darc ie a n d G E Bodeep L i g h t w a v e subcarr ier C A T V T r a n s m i s s i o n Sys tems I E E E T r a n s o n M i c r o w a v e T h e o r y a n d Technol 385 1990 524-533

8 A F Judy Intensity noise f rom fiber Ray le igh backscatter and mechanica l spl ices i n Proc 15th European Conf Opt C o m m u n ( E C O C 89) Gotenberg Sweden 1989

9 T E Darcie et al Fiber-ref lect ion- induced impa i rments i n l i gh twave A M - S V B C A T V systems I E E E J L i g h t w a v e Techno l 98 1991 991-995

10 K Y L a u and A Y a r i v In termodula t ion d is shyto r t ion i n a di rect ly modu la ted semiconducshytor injection laser A p p l Phys Lett 45 1984 1034-1036

11 T E Darcie et al In te rmodula t ion and harshymonic dis tor t ion i n I a G a A s P lasers Electron Lett 21 665-666 erra tum ibid 22 1986 619

12 G P A g r a w a l and N K Dut ta L o n g - W a v e shylength Semiconductor Lasers V a n N o s t r a n d R e i n h o l d N e w Y o r k N Y 1986

13 A Takemoto et al Dis t r ibu ted feedback l a shyser d iode and m o d u l e for C A T V systems I E E E J Selected Areas i n C o m m u n i c a t i o n s 8 1990 1359

14 A A M Saleh Fundamenta l l i m i t on n u m shyber of channels i n subcarr ier m u l t i p l e x e d l igh twave C A T V systems Electron Lett 25 N o 12 1989 776-777

15 N J F r igo and G E Bodeep C l i p p i n g distorshyt ion i n subcarr ier m u l t i p l e x e d C A T V sysshyt e m s C o n f e r e n c e o n O p t i c a l F i b e r C o m m u n i c a t i o n ( O F C ) San Jose Cal i f 1992

16 GJ Meslener Chromat ic dispers ion induced dis tor t ion of modu la ted monochromat ic l ight e m p l o y i n g direct detection I E E E J Q u a n shyt u m Electron 20 1984 1208-1216

17 M R Ph i l l i p s et al Non l inea r d is tor t ion genshyerated by dispers ive t ransmiss ion of ch i rped in tensi ty-modulated s ignals I E E E Photonics Technol Lett 35 1991 481-483

18 C K u o and E E Bergmann E r b i u m - d o p e d fiber amplif ier second-order dis tor t ion i n anashylog l inks and electronic compensat ion I E E E Photon Tech Lett 3 1991 829

19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3

C A B L E T V SYSTEMS

Y-branch splitter at the input and a voltshyage-controlled directional coupler at the output

The ou tpu t in t ens i ty of these modulators is a s inuso ida l funct ion of the bias voltage B y pre-bias ing the di rect ional coupler to the point where the power is equal i n both outshyput fibers modu la t ion app l i ed to the Mach-Zehnder results i n the most l i n shyear intensi ty modu la t ion This bias point w h i c h corresponds to the point of inf lect ion i n the s inuso ida l transshyfer function produces zero second-order dis tor t ion Unfor tunate ly the corresponding third-order d is tor t ion is approximate ly 30 d B worse than a typica l d i rec t ly-modula ted D F B l a shyser A means of l inea r i z ing this th i rd -order nonl inear i ty is essential

Various l inearization techniques have been explored mostly borrowed conceptually from early work i n radio and electronics The two most popular approaches are f e e d f o r w a r d a n d predistort ion Feedforward requires that a portion of the modulated output signal be detected and compared to the original applied voltage signal to proshyvide an error signal This error signal is then used to modulate a second laser which is combined wi th the first laser such that the total instantaneous intenshysity of the two lasers is a replica of the applied voltage In principle this techshynique is capable of l inearizing any orshyder of distortion and correcting R I N from the laser In practice difficulties i n generating accurate error signals make feedforward techniques rather difficult to implement

Predistortion requires less circuit complexity than feedforward A careshyfu l ly -des igned nonl inear c i rcu i t is placed before the nonlinear modulator such that the combined transfer funcshytion of the predistorter-modulator is l i n shyear Various nonlinear electronic deshyvices or circuits can act as second- or third-order predistorters Difficulties include matching the frequency depenshydence of the predistorter w i t h that of the modulator hence achieving good linearity over a wide frequency range But if the frequency response of the modulator is flat numerous circuit deshysigns can provide good performance for cable television purposes

Y A G lasers wi th external modulashy

tion provide several advantages and disadvantages i n comparison to direct modulation The YAG-modula to r comshybination offers a considerable increase i n launched power over DFBs The low R I N of the Y A G laser translates into a slight C N R improvement

A l t h o u g h external ly-modula ted systems are immune to chirp related problems fiber nonl inear i tymdashin the form of stimulated Br i l lou in scattering (SBS)mdashplaces a l imit on the launched power SBS i n wh ich light is scattered from acoustic phonons i n the fiber causes a rap id decrease i n C N R for launched powers greater than approxishymately 10 m W 1 9 The chirp i n D F B sysshytems broadens the optical spectrum so that SBS is not a problem

Another disadvantage arises from architectural issues To be cost-competishytive w i t h DFBs the output of the Y A G -modulator is split to serve eight or more trunks The concept of broadcasting from one transmitter to a wide serving area is contrary to the direct ion of change wi th in the industry Narrowcast architectures where different informashytion can be sent to each trunk offer greater flexibility and diversity of sershyvices than do broadcast systems Bal shyancing these architectural issues w i t h the systems issues presents an interestshying challenge to system operators

W A V E L E N G T H - D I V I S I O N M U L T I P L E X I N G

Al though the coaxial-cable port ion of the new fibercoax system architectures can support a bandwidth approaching 1 G H z single-fiber systems that must feed the short lengths of coax can supshyport only 400 M H z Progressive operashytors seeking 150 channel A M - V S B sysshytems 2 0 must use mult iple fibers to feed one coaxial cable Clearly there is room for creative use of wavelength-division mul t ip lexing ( W D M ) to increase the bandwidth of the fiber

By combining direct-modulation at 13 μm wavelength wi th external modushylation at 155 μm W D M transmission of 112 channels has been d e m o n shystrated 2 1 A schematic of the system is shown i n Figure 4 The 155 μm source is an externally modulated D F B laser capable of launching several m W into an erbium-doped fiber amplifier A l shythough the E D F A is operated we l l into

saturation it provides enough gain to al low splitting the 155 μm signal eight ways Passive fiber W D M couplers are used to combine and separate the two wavelengths Externally modulating the 155 μm signal eliminates problems wi th C S O from chromatic dispersion and freshyquency-dependent E D F A gain A r c h i shytectures of this type combine the s imshyplici ty of broadcast distribution for the s t a n d a r d b a n d of channe l s w i t h narrowcast transmission of a special band of channels that is unique to each trunk Continued development of new W D M componen t s w i t h m u l t i p l e wavelengths i n each of the 15513 μm windows w i l l provide increased bandshywid th and flexibility

L O O K I N G A H E A D These technologies provide cable opshyerators w i t h the means to redesign sysshytem architectures for any foreseeable future broadband services W i t h a coshyaxial-cable bandwidth near 1 G H z opshyportunities for enhanced pay-per-view or video-on-demand services abound A l though many researchers may quesshytion the sociological consequences of a nation armed w i t h hundred-channel remote-control selectors 2 2 the technical and economic viability has been proven

Other non-enter ta inment-video services could certainly benefit from the cable indus t ry s b roadband analog transport capability Next-generation cellular telephone systems or personal communications networks could transshymit analog signals to and from remote antenna sites v ia fibercoax systems Architectures designed for narrowcast transmission have adequate bandwidth for interactive two-way video H i g h -definition television can be squeezed into bandwidths comparable to todays A M - V S B standard television W i t h conshytinued advances i n photonic technolshyogy the revitalized cable industry is poised for whatever information distrishybut ion opportunities might arise

T E D A R C I E is head of the Lightwave Comshymunications Research Department ATampT Bell Laboratories Crawford Hill Laboratory Holmdel NJ

2 2 OPTICS amp PHOTONICS NEWSSEPTEMBER 1992

CABLE TV SYSTEMS

R E F E R E N C E S 1 R E Patterson et al L inea r i za t i on of m u l t i shy

c h a n n e l a n a l o g t r a n s m i t t e r s b y q u a s i shyfeedforward compensat ion technique IEEE Trans on C o m m u n Com-273 1979 582-588

2 N a t i o n a l C a b l e T e l e v i s i o n A s s o c i a t i o n ( N C T A ) Proceedings f rom Techn ica l Sesshysions annual meetings 1724 Massachusetts A v e N W W a s h i n g t o n D C

3 Society of Cable Te lev i s ion Engineers (SCTE) Proceeding f rom Technical Sessions b ienn ia l meetings Ex ton C o m m o n s Ex ton Pa

4 J A C h i d d i x et al A M v ideo on fiber i n C A T V systems need and implementa t ion I E E E Journ Selected Areas i n C o m m u n i c a shytions 8 1990 1229

5 C C I R Report X X I pt2 1986 215-216 6 T E D a r c i e Subcar r i e r m u l t i p l e x i n g for

l i g h t w a v e mul t ip le -access l i g h t w a v e netshyw o r k s J L i g h t w a v e Technol L T - 5 1987 1103-1110

7 T E Darc ie a n d G E Bodeep L i g h t w a v e subcarr ier C A T V T r a n s m i s s i o n Sys tems I E E E T r a n s o n M i c r o w a v e T h e o r y a n d Technol 385 1990 524-533

8 A F Judy Intensity noise f rom fiber Ray le igh backscatter and mechanica l spl ices i n Proc 15th European Conf Opt C o m m u n ( E C O C 89) Gotenberg Sweden 1989

9 T E Darcie et al Fiber-ref lect ion- induced impa i rments i n l i gh twave A M - S V B C A T V systems I E E E J L i g h t w a v e Techno l 98 1991 991-995

10 K Y L a u and A Y a r i v In termodula t ion d is shyto r t ion i n a di rect ly modu la ted semiconducshytor injection laser A p p l Phys Lett 45 1984 1034-1036

11 T E Darcie et al In te rmodula t ion and harshymonic dis tor t ion i n I a G a A s P lasers Electron Lett 21 665-666 erra tum ibid 22 1986 619

12 G P A g r a w a l and N K Dut ta L o n g - W a v e shylength Semiconductor Lasers V a n N o s t r a n d R e i n h o l d N e w Y o r k N Y 1986

13 A Takemoto et al Dis t r ibu ted feedback l a shyser d iode and m o d u l e for C A T V systems I E E E J Selected Areas i n C o m m u n i c a t i o n s 8 1990 1359

14 A A M Saleh Fundamenta l l i m i t on n u m shyber of channels i n subcarr ier m u l t i p l e x e d l igh twave C A T V systems Electron Lett 25 N o 12 1989 776-777

15 N J F r igo and G E Bodeep C l i p p i n g distorshyt ion i n subcarr ier m u l t i p l e x e d C A T V sysshyt e m s C o n f e r e n c e o n O p t i c a l F i b e r C o m m u n i c a t i o n ( O F C ) San Jose Cal i f 1992

16 GJ Meslener Chromat ic dispers ion induced dis tor t ion of modu la ted monochromat ic l ight e m p l o y i n g direct detection I E E E J Q u a n shyt u m Electron 20 1984 1208-1216

17 M R Ph i l l i p s et al Non l inea r d is tor t ion genshyerated by dispers ive t ransmiss ion of ch i rped in tensi ty-modulated s ignals I E E E Photonics Technol Lett 35 1991 481-483

18 C K u o and E E Bergmann E r b i u m - d o p e d fiber amplif ier second-order dis tor t ion i n anashylog l inks and electronic compensat ion I E E E Photon Tech Lett 3 1991 829

19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3

CABLE TV SYSTEMS

R E F E R E N C E S 1 R E Patterson et al L inea r i za t i on of m u l t i shy

c h a n n e l a n a l o g t r a n s m i t t e r s b y q u a s i shyfeedforward compensat ion technique IEEE Trans on C o m m u n Com-273 1979 582-588

2 N a t i o n a l C a b l e T e l e v i s i o n A s s o c i a t i o n ( N C T A ) Proceedings f rom Techn ica l Sesshysions annual meetings 1724 Massachusetts A v e N W W a s h i n g t o n D C

3 Society of Cable Te lev i s ion Engineers (SCTE) Proceeding f rom Technical Sessions b ienn ia l meetings Ex ton C o m m o n s Ex ton Pa

4 J A C h i d d i x et al A M v ideo on fiber i n C A T V systems need and implementa t ion I E E E Journ Selected Areas i n C o m m u n i c a shytions 8 1990 1229

5 C C I R Report X X I pt2 1986 215-216 6 T E D a r c i e Subcar r i e r m u l t i p l e x i n g for

l i g h t w a v e mul t ip le -access l i g h t w a v e netshyw o r k s J L i g h t w a v e Technol L T - 5 1987 1103-1110

7 T E Darc ie a n d G E Bodeep L i g h t w a v e subcarr ier C A T V T r a n s m i s s i o n Sys tems I E E E T r a n s o n M i c r o w a v e T h e o r y a n d Technol 385 1990 524-533

8 A F Judy Intensity noise f rom fiber Ray le igh backscatter and mechanica l spl ices i n Proc 15th European Conf Opt C o m m u n ( E C O C 89) Gotenberg Sweden 1989

9 T E Darcie et al Fiber-ref lect ion- induced impa i rments i n l i gh twave A M - S V B C A T V systems I E E E J L i g h t w a v e Techno l 98 1991 991-995

10 K Y L a u and A Y a r i v In termodula t ion d is shyto r t ion i n a di rect ly modu la ted semiconducshytor injection laser A p p l Phys Lett 45 1984 1034-1036

11 T E Darcie et al In te rmodula t ion and harshymonic dis tor t ion i n I a G a A s P lasers Electron Lett 21 665-666 erra tum ibid 22 1986 619

12 G P A g r a w a l and N K Dut ta L o n g - W a v e shylength Semiconductor Lasers V a n N o s t r a n d R e i n h o l d N e w Y o r k N Y 1986

13 A Takemoto et al Dis t r ibu ted feedback l a shyser d iode and m o d u l e for C A T V systems I E E E J Selected Areas i n C o m m u n i c a t i o n s 8 1990 1359

14 A A M Saleh Fundamenta l l i m i t on n u m shyber of channels i n subcarr ier m u l t i p l e x e d l igh twave C A T V systems Electron Lett 25 N o 12 1989 776-777

15 N J F r igo and G E Bodeep C l i p p i n g distorshyt ion i n subcarr ier m u l t i p l e x e d C A T V sysshyt e m s C o n f e r e n c e o n O p t i c a l F i b e r C o m m u n i c a t i o n ( O F C ) San Jose Cal i f 1992

16 GJ Meslener Chromat ic dispers ion induced dis tor t ion of modu la ted monochromat ic l ight e m p l o y i n g direct detection I E E E J Q u a n shyt u m Electron 20 1984 1208-1216

17 M R Ph i l l i p s et al Non l inea r d is tor t ion genshyerated by dispers ive t ransmiss ion of ch i rped in tensi ty-modulated s ignals I E E E Photonics Technol Lett 35 1991 481-483

18 C K u o and E E Bergmann E r b i u m - d o p e d fiber amplif ier second-order dis tor t ion i n anashylog l inks and electronic compensat ion I E E E Photon Tech Lett 3 1991 829

19 X P M a o et al B r i l l o u i n scattering i n extershyna l ly modu la ted l igh twave A M - V S B C A T V t ransmission systems I E E E Pho ton Tech Lett 43 1992 287-289

20 J A C h i d d i x et al The use of fiber optics i n

cable c o m m u n i c a t i o n s n e t w o r k s I E E E J L i g h t w a v e Technol special issue on Bro ad shyb a n d A n a l o g V i d e o Transmiss ion O v e r F i shybers to be pub l i shed

21 M R P h i l l i p s et al 112 C h a n n e l sp l i t -band W D M l igh twave C A T V system I E E E Phoshyton Technol Lett 47 1992

22 J Darcie pr ivate communica t ion

O P T I C S amp P H O T O N I C S N E W S S E P T E M B E R 1992 2 3