光纖通信(OpticalFiberCommunication)AnalogSystems課件

CHAPTER9

ANALOG

SYSTEMS9.1OVERVIEWOFANALOGLINKS

9.2CARRIERTO-NO1SERAT1O

9.2.1CarrierPower

9.2.2PhotodetertorandPreamplifierNoises

9.2.3RelativeIntensityNoise(RIN)

9.2.4ReflectionEffectsonRIN

9.2.5LimitingConditions

9.3MULTICHANNELTRANSMISSIONTECHNIQUES

9.3.1MultichannelAmplitudeModulation

9.3.2MultichannelFrequencyModulation

9.3.3SubcarrierMultiplexingCHAPTER9ANALOGSYSTEMS91Intelecommunicationnetworksthetrendhasbeentolinktelephoneexchangeswithdigitalcircuits.Amajorreasonforthiswastheintroductionofdigitalintegrated-circuittechnologywhichofferedareliableandeconomicmethodoftransmittingbothvoiceanddatasignals.Sincetheinitialapplicationsoffiberopticsweretotelecommunicationnetworks,itsfirstwidespreadusagehasinvolveddigitallinks.However,inmanyinstances,itismoreadvantageoustotransmitinformationinanalogforminsteadoffirstconvertingittoadigitalformat.Someexamplesofthisaremicrowave-multiplexedsignals,subscriberservicesusinghybridfiber/coax(HFC),videodistributionantennaremoting,andradarsignalprocessing.Formostanalogapplications,oneuseslaserdiodetransmitters,soweshallconcentrateonthisopticalsourcehere.Whenimplementingananalogfiberopticsystem,themainparametersoneneedstoconsiderarethecarrier-to-noiseratio,bandwidth,andsignaldistortionresultingfromnonlineartiesinthetransmissionsystem.Section9.1describesthegeneraloperationalaspectsandcomponentsofananalogfiberopticlink.Traditiona1ly,inananalogsystem,acarrier-to-noiseratioana1ysisisusedinsteadofasignal-to-noiseratioanalysis,sincetheinformationsignalisnormallysuperimposedonaradio-frequency(RF)carrier.Thus,inSec.9.2weexaminecarrier-to-noiseratiorequirements.Thisisfirstdoneforasinglechannelundertheassumptionthattheinformationsignalisdirectlymodulatedontoanopticalcarrier.Intelecommunicationnetworks2Fortransmittingmultiplesignalsoverthesamechannel,onecanuseasubcarriermodulationtechnique.Inthismethod,whichisdescribedinSec.9.3,theinformationsignalsarefirstsuperimposedonancillaryRFsubcarriers.Thesecarriersarethencombinedandtheresultingelectricalsignalisusedtomodulatetheopticalcarrier.Alimitingfactorinthesesystemsisthesignalimpairmentarisingfromharmonicandintermodulationdistortions.Fortransmittingmultiplesign39.lOVERVIEWOFANALOGLINKS

Figure9-lshowsthebasicelementsofananaloglink.ThetransmittercontainseitheranLEDoralaserdiodeopticalsource.AsnotedinSec.4-4andshowninFig.4-35,inanalogapplications,onefirstsetsabiaspointonthesourceapproximatelyatthemidpointofthelinearoutputregion.Theanalogsignalcanthenbesentusingoneofseveralmodulationtechniques.Thesimplestformforopticalfiberlinksisdirectintensitymodulation,whereinsimplyvaryingthecurrentaroundthebiaspointinproportiontothemessagesignallevelmodulatestheopticaloutputfromthesource.Thus,theinformationsignalistransmitteddirectlyinthebaseband.Asomewhatmorecomplexbutoftenmoreefficientmethodistotranslatethebasebandsignalontoanelectricalsubcarrierpriortointensitymodulationofthesource.Thisisdoneusingstandardamplitude-modulation(AM),frequency-modulation(FM),orphase-modulation(PM)techniques.Nomatterwhichmethodisimplemented,onemustpaycarefulattentiontosignalimpairmentsintheopticalsource.Theseincludeharmonicdistortions,intermodulationproducts,relativeintensitynoise(RIM)inthelaser,andlaserclipping.Inrelationtothefiber-opticelementshowninFig.9-l,onemusttakeintoaccountthefrequencydependenceoftheamplitude,phase,andgroupdelayinthefiber.Thus,thefibershouldhaveaflatamplitudeandgroup-delayresponsewithinthepassbandrequiredtosendthesignalfreeoflineardistortion.Inaddition,sincemodal-distortion-limitedbandwidthisdifficulttoequalize,itisbesttochooseasingle-mode.9.lOVERVIEWOFANALOGLINKSF49.lOVERVIEWOFANALOGLINKS

fiber.Thefiberattenuationisalsoimportant,sincethecarrier-to-noiseperformanceofthesystemwillchangeasafunctionofthereceivedopticalpowerTheuseofanopticalamplifierinthelinkleadstoadditionalnoise,knownasamplifiedspontaneousemission(ASE),asisdescribedinChap.11:Inthe

opticalreceiver,theprincipalimpairmentsarequantumorshotnoise,APDgainnoise,andthermalnoise.9.lOVERVIEWOFANALOGLINKSf5InanalyzingthePerformanceofanalogsystems,oneusuallycalculatestheratioofrmscarrierpowertormsnoisepowerattheinputoftheRFreceiverfollowingthephotodetectionprocess.Thisisknownasthecarrier-noiseratio(CNR).LetuslookatsometypicalCNRvaluesfordigitalandanalogdata.Fordigitaldata,considertheuseoffrequency-shiftkeying(FSK).Inthismodulationscheme,theamplitudeofasinusoidalcarrierremainsconstant,{butthephaseshiftsfromonefrequencytoanothertorepresentbinarysignals.ForFSK,BERsof10-9and10-15translateintoCNRvaluesof36(l5.6dB)and64(l8.0dB),respectively.Theanalysisforanalogsignalsismorecomplex,sinceitsometimesdependsonuserperceptionofthesignalquality,suchasinviewingatelevisionpicture.Awidelyusedanalogsignalisa525-linestudio-qualitytelevisionsignal.Usingamplitudemodulation(AM)forsuchasignalrequiresaCNRof56dB,sincetheneedforbandwidthefficiencyleadstoahighsignal-to-noiseratio.Frequencymodulation(FM),ontheotherhand,onlyneedsCNRvaluesofl5-l8dB.IfCNRirepresentsthecarrier-to-noiseratiorelatedtoaparticularsignalcontaminant(e.g.,shotnoise),thenforNsignal-impairmentfactorsthetotalCNRisgivenby9.2CARRIERTO-NO1SERAT1OInanalyzingthePerformanceo69.2CARRIERTO-NO1SERAT1OForlinksinwhichonlyasingleinformationchannelistransmitted,theimportantsignalimpairmentsincludelaserintensitynoisefluctuations,laserdipping,photoreceptornoise,andoptical-amplifiernoise.Whenmultiplemessagechannelsoperatingatdifferentcarrierfrequenciesaresentsimultaneouslyoverthesamefiber,thenharmonicandintermodulationdistortionsarise.Furthermore,theinclusionofanopticalamplifiergivesrisetoASEnoise.Inprinciple,thethreedominantfactorsthatcausesignalimpairmentsinafiberlinkaresh0tnoise,optical-amplifiernoise,andlaserclipping.Mostotherdegradationeffectscanbesufficientlyreducedoreliminated.Inthissection,weshallfirstexamineasimplesingle-channelamplitudemodulatedsignalsentatbasebandfrequencies.Section9.3addressesmultichannelsystemsinwhichintermodulationnoisebecomesimportant.Problem9-l0givesexpressionsfortheeffectsoflaserclippingandASEnoise.9.2CARRIERTO-NO1SERAT1OForl79.2.lCarrierPower

Tofindthecarrierpower,letusfirstlookatthesignalgeneratedatthetransmitter.AsshowninFig.9-2,thedrivecurrentthroughtheopticalsourceisthesumofthefixedbiascurrentandatime-varyingsinusoid.Thesourceactsasasquare-lawdevice,sothattheenvelopeoftheoutputopticalpowerP(t)hasthesameformastheinputdrivecornet.Ifthetime-varyinganalogdrivesignaliss(t),thenwherePtistheopticaloutputpoweratthebiascurrentlevelandthemodulationindexmisdefinedbyEq.(4-54).Intermsofopticalpower,themodulationindexisgivenbywherePpeakandPtaredefinedinFig.9-2.Typicalvaluesofmforanalogapplicationsrangefrom0.25to0.50.Forasinusoidalreceivedsignal,thecarrierpowerCattheoutputofthereceiver(inunitsofA2)iswhereistheunitygainresp0nsivityofthephotodetector,Misthephotodetectorgain(M=lforpinphotodiodes),andPistheaveragereceivedopticalpower.9.2.lCarrierPowerTofindth89.2.lCarrierPower9.2.lCarrierPower99.2.2PhotodetertorandPreamplifierNoisesTheexpressionsforthephotodiodeandpreamplifiernoisesaregivenbyEqs.(6-l6)and(6-l7),respectively.Thatis,forthephotodiodenoisewehaveHere,asdefinedinChap.6,istheprimaryphotocurrent,IDisthedetectorbulkdarkcurrent,MisthephotodiodegainwithF(M)beingitsassociatednoisefigure,andBisthereceiverbandwidth.Then,theCNRforthephotodetectoronlyisCNRdet=C/σ2N

GeneralizingEq.(6-l7)forthepreamplifiernoise,wehaveHere,Rqeistheequivalentresistanceofthephotodetectorloadandthepreamplifier,andFtisthenoisefactorofthepreamplifier.Then,theCNRforthepreamplifieronlyisCNRdet=C/σ2N

9.2.2PhotodetertorandPreamp109.2.3RelativeIntensityNoise(RIN)Withinasemiconductorlaser,fluctuationsintheamplitudeorintensityoftheoutputproduceopticalintensitynoise.Thesefluctuationscouldarisefromtemperaturevariationsorfromspontaneousemissioncontainedinthelaseroutput.Thenoiseresultingfromtherandomintensityfluctuationsiscalledrelativeintensitynoise(RIN),whichmaybedefinedintermsofthemean-squareintensityvariations.Theresultantmean-squarenoisecurrentisgivenbyThen,theCNRduetolaseramplitudefluctuationsonlyisCNRRIN=C/σ2RINHere,theRIN,whichismeasuredindB/Hz,isdefinedbythenoise-to-signalPowerratio.where<(ΔPL)2>isthemean-squareintensityfluctuationofthelaseroutputandPL

istheaveragelaserlightintensity.Thisnoisedecreasesastheinjection-currentlevelincreasesaccordingtotherelationship9.2.3RelativeIntensityNoise119.2.3RelativeIntensityNoise(RIN)SubstitutingtheCNRsresultingfromEqs.(9-4)through(9-7)intoEq.(9-l)yieldsthefollowingcarrier-to-noiseratioforasingle-channelAMsystem:9.2.3RelativeIntensityNoise12Inimplementingahigh-speedanaloglink,onemusttakespecialprecautionstom1nlmlzeopticalreflectionsbackintothelaser.2Back-reflectedsignalscanincreasetheRINbyl0-20dBasshowninFig.9-5.Thesecurvesshowtheincreaseinrelativeintensitynoiseforbiasp0intsrangingfroml.24tol.62timesthethreshold-curentlevel.ThefeedbackpowerratioinFig.9-5istheamountofopticalpowerreflectedbackintothelaserrelativetothelightoutputfromthesource.Asanexample,thedashedlineshowsthatat.l.33Iththefeedbackratiomustbelessthan-60dBinordertomaintainanRINoflessthan-l40dB/Hz.9.2.4ReflectionEffectsonRINInimplementingahigh-speeda13Letusnowlookatsomelimitingconditions.Whentheopticalpowerlevelatthereceiverislow,thepreamplifiercircuitnoised0ndnateSthesystemnoise.Forthese,wehave9.2.5LimitingConditionsInthiscase,thecarrier-to-noiseratioisdirectlyproportionaltothesquareofthereceivedoptica1power,sothatforeachl-dBvariationinreceivedopticalpower,C/Nwillchangeby2dB.Forwell-designedphotodiodes,thebulkandsurfacedarkcurrentsaresmallcomparedwiththeshot(quantum)noiseforintermediateopticalsignallevelsatthereceiver.Thus,atintermediatepowerlevelsthequantum-noisetermofthephotodiodewilldominatethesystemnoise.Inthiscase,wehavesothatthecarrier-to-noiseratiowillvarybyldBforeveryl-dBchangeinthereceivedopticalpower.IfthelaserhasahighRINvaluesothatthereflectionnoisedominatesoverothernoiseterms,thenthecarrier-to-noiseratiobecomesLetusnowlookatsomelimiti149.2.6LimitingConditions

whichisaconstant.Inthiscase,thePerformancecannotbeimprovedunlessthemodulationindexisincreased.9.2.6LimitingConditionswhic15Sofar,wehaveexaminedonlythecaseofasinglesignalbeingtransmittedoverachannel.Inbroadbandanalogapplications,suchascabletelevision(CATV)supertrunks,oneneedstosendmultipleanalogsignalsoverthesamefiber.Todothis,onecanemployamultiplexingtechniquewhereanumberofbasebandsignalsaresuperimposedonasetofNsubcarriersthathavedifferentfrequenciesf1,f2...fN.Thesemodulatedsubcarriersarethencombinedelectricallythroughfrequency-divisionmultiplexing(FDM)toformacompositesignalthatdirectlymodulatesasingleopticalsource.Methodsforachievingthisincludevestigialsidebandamplitudemodulation(VSB-AM),frequencymodulation(FM),andsubcarriermultiplexing(SCM).0fthese,AMissimpleandcost-effectiveinthatitiscompatiblewiththeequipmentinterfacesofalargenumberofCATVcustomers,butitssignalisverysensitivetonoiseandnonlineardistorti0n.AlthoughFMrequiresalargerbandwidththanAM,itprovidesahighersignal-to-noiseratioandislesssensitivetosourcenonlinearities.MicrowaveSCMoperatesathigherfrequenciesthanAMorFMandisaninterestingapproachforbroadbanddistributionofbothanaloganddigitalsignals.Tosimplifytheinterfacewithexistingcoaxialcablesystems,currentfiberlinksinCATVnetw0rksprimarilyusetheAM-VSBSchemedescribedinSec.MULTICHANNELTRANSMISSIONTECHNIQUESSofar,wehaveexaminedonly16Theinitialwidespreadapplicationofanalogfiberopticlinks,whichstartedinthelatel980s,wastoCATVnetworks.Thesecoax-basedtelevisionnetworksoperateinafrequencyrangefrom50to88MHzandfroml20to550MHz.Thebandfrom88tol20MHzisnotused,sinceitisreservedforFMradiobroadcast.TheCATVnetworkscandeliverover80amplitude-modulatedvestigial-sideband(AM-VSB)videochannels,eachhavinganoisebandwidthof4MHzwithinachannelbandwidthof6MHz,withsignal-to-noiseratiosexceeding47dB.Toremaincompatiblewithexistingcoax-basednetworks,amultichannelAM-VSBformatwasalsochosenforthefiberopticsystem.Figure9-7depictsthetechniqueforcombiningNindependentmessages.Aninformationbearingsignalonchanneliamplitude-modulatesacarrierwavethathasafrequencyfi,wherei=l,2...,N.AnRFpowercombinerthensumstheseNamplitude-modulatedcarrierstoyieldacompositefrequency-division-multi-plexed(FDM)signalwhichintensity-modulatesalaserdiode.Followingtheopticalreceiver,abankofparallelbandpassfiltersseparatesthecombinedcarriersbackintoindividualchannels.TheindividualmessagesignalsarerecoveredfromthecarriersbystandardRFtechniques.ForalargenumberofFDMcarrierswithrandomphases,thecarriersaddonapowerbasis.Thus,forNchannelstheopticalmodulationindexmisrelatedtotheper-channelmodulationindexmiby9.3.1MultichannelAmplitudeModulationTheinitialwidespreadapplica179.3.1MultichannelAmplitudeModulation9.3.1MultichannelAmplitudeM189.3.1MultichannelAmplitudeModulationIfeachchannelmodulationindexesmihasthesamevaluemc,thenAsaresult,whenNsignalsarefrequency-multiplexedandusedtomodulateasingleopticalsource,thecarrier-to-noiseratioofasinglechannelisdegradedbyl0logN.Ifonlyafewchannelsarecombined,thesignalswilladdinvoltageratherthanpower,sothatthedegradationwillhavea20logNcharacteristic.Whenmultiplecarrierfrequenciespassthroughanonlineardevicesuchasalaserdiode,signalproductsotherthantheoriginalfrequenciescanbeproduced.AsnotedinSec.4.4,theseundesirablesignalsarecalledintermodulationproductsandtheycancauseseriousinterferenceinbothin-bandandout-ofbandchannels.Theresultisadegradationofthetransmittedsignal.Amongtheintermodulationproducts,generallyonlythesecond-orderandthird-ordertermsareconsidered,sincehigher-orderproductstendtobesignificantlysmaller.Third-orderintermediation(IM)distortionproductsatfrequenciesfI+fj-fk(whichareknownastriple-beatIMproducts)and2fI-fj(whichareknownastwo-tonethird-orderIMproducts)arethemostdominant,sincemanyofthesefallwithinthebandwidthofamultichannelsystem.Forexample,a50-channelCATVnetworkoperatingoverastandardfrequencyrangeof55.25-373.25MHzhas39second-orderIMproductsat54.0MHzand786third-orderIMtonesat229.25MHz.The9.3.1MultichannelAmplitudeM199.3.1MultichannelAmplitudeModulationamplitudesofthetriple-beatproductsare3dBhigherthanthetwo-tonethird-orderIMproducts.Inaddition,sincethereareN(N-l)(N-2)/2triple-beattermscomparedwithN(N-l)two-tonethird-orderterms,thetriple-beatproductstendtobethemajorsourceofIMnoise.Ifasignalpassbandcontainsa1argenumberofequallyspacedcarriers,severalIMtermswillexistatornearthesamefrequency.Thisso-calledbeat

stackingisadditiveonapowerbasis.Forexample,forNequallyspacedequal-amplitudecarriers,thenumberofthird-orderlMproductsthatfallrightontherthcarrierisgivenbyfortwo-tonetermsofthetype2fI-fj,andby

fortriple-beattermsofthetypefI+fj-fk

9.3.1MultichannelAmplitudeM209.3.1MultichannelAmplitudeModulationWhereasthetwo-tonethird-ordertermsarefairlyevenlyspreadthroughtheoperatingpassband,thetriple-beatproductstendtobeconcentratedinthemiddleofthechannelpassband,sothatthecent6rcarriersreceivethemostintermodulationinterference.Tables9-land9-2showthedistributionsofthethird-ordertriple-beatandtwo-toneIMproductsforthenumberofchannelsNrangingfromltoMultichannelAmplitudeM219.3.1MultichannelAmplitudeModulationTheresultsofbeatstackingarecommonlyreferredtoascompositesecondorder(CSO)andcompositetriplebeat(CTB,andareusedtodescribetheperformanceofmultichannelAMlinks.Thesearedefinedas9.3.1MultichannelAmplitudeM229.3.1MultichannelAmplitudeModulation9.3.1MultichannelAmplitudeM239.3.2MultichannelFrequencyModulationTheuseofAM-VSBsignalsfortransmittingmultipleanalogchannelsis,inprinciple,straightforwardandsimple.However,ithasaC/Nrequirement(or,equivalently,forAM,anS/Nrequirement)ofatleast40dBforeachAMchannel,whichplacesverystringentrequirementsonlaserandreceiverlinearity.Analternativetechniqueisfrequencymodulation(FM),whereineachsubcarrierisfrequency-modulatedbyamessagesignal.Thisrequiresawiderbandwidth(30MHzversus4MHzforAM),butyieldsasignal-to-noiseratioimprovementoverthecarrier-to-noiseratio.TheS/NattheoutputofanFMdetectorismuchlargerthantheC/Nattheinputofthedetector.TheimprovementisgivenbywhereBistherequiredbandwidth,fppisthepeak-to-peakfrequencydeviationofthemodulator,fv

isthehighestvideofrequency,andwisaweightingfactorusedtoaccountforthenouniformresponseoftheeyepatterntowhitenoiseinthevideobandwidth.ThetotalS/Nimprovementdependsonthesystemdesign,butisgenerallyintherange36-44dB.ThereducedC/NrequirementsthusmakeanFMsystemmuchlesssusceptibletolaserandreceivernoisesthananAMsystem.9.3.2MultichannelFrequencyM249.3.3SubcarrierMultiplexing

ThereisalsogreatinterestinusingRFormicrowavesubcarriermultiplexforhigh-capacity1ightwavesystems.Thetermsubcarriermultiplex(SCM)isusedtodescribethecapabilityofmultiplexingbothmultichannelanaloganddigitalsignalswithinthesamesystem.Figure9-l2showsthebasicconceptofanSCMsystem.TheinputtothetransmitterconsistsofamixtureofNindependentanaloganddigitalbasebandsignals.Thesesignalscancarryeithervoice,data,video,digitalaudio,high-definitionvideo,oranyotheranalogordigitalinformation.Eachincomingsignalsi(t)ismixedwithalocaloscillator(LO)havingafrequencyfi.Thelocaloscillatorfrequenciesemployedareinthe2-to-8-GHzrangeandareknownasthesubcarriers.Combiningthemodulatedsubcarriersgivesacompositefrequency-divisionmultiplexedsignalwhichisusedtodrivealaserdiode.Atthereceivingend,theopticalsignalisdirectlydetectedwithahigh-speedwidebandInGaAspinphotodiodeandreconvertedtoamicrowavesignal.Forlong-distancelinks,onecansalsoemployawidebandInGaAsavalanchephotodiodewitha50-to80-GHzgain-bandwidthproductoruseanopticalpreamplifier.Foramplifyingthereceivedmicrowavesignal,onecanuseacommerciallyavailablewidebandlow-noiseamplifierorapin-FETreceiver.9.3.3SubcarrierMultiplexing259.3.3SubcarrierMultiplexing9.3.3SubcarrierMultiplexing26CHAPTER9

ANALOG

SYSTEMS9.1OVERVIEWOFANALOGLINKS

9.2CARRIERTO-NO1SERAT1O

9.2.1CarrierPower

9.2.2PhotodetertorandPreamplifierNoises

9.2.3RelativeIntensityNoise(RIN)

9.2.4ReflectionEffectsonRIN

9.2.5LimitingConditions

9.3MULTICHANNELTRANSMISSIONTECHNIQUES

9.3.1MultichannelAmplitudeModulation

9.3.2MultichannelFrequencyModulation

9.3.3SubcarrierMultiplexingCHAPTER9ANALOGSYSTEMS927Intelecommunicationnetworksthetrendhasbeentolinktelephoneexchangeswithdigitalcircuits.Amajorreasonforthiswastheintroductionofdigitalintegrated-circuittechnologywhichofferedareliableandeconomicmethodoftransmittingbothvoiceanddatasignals.Sincetheinitialapplicationsoffiberopticsweretotelecommunicationnetworks,itsfirstwidespreadusagehasinvolveddigitallinks.However,inmanyinstances,itismoreadvantageoustotransmitinformationinanalogforminsteadoffirstconvertingittoadigitalformat.Someexamplesofthisaremicrowave-multiplexedsignals,subscriberservicesusinghybridfiber/coax(HFC),videodistributionantennaremoting,andradarsignalprocessing.Formostanalogapplications,oneuseslaserdiodetransmitters,soweshallconcentrateonthisopticalsourcehere.Whenimplementingananalogfiberopticsystem,themainparametersoneneedstoconsiderarethecarrier-to-noiseratio,bandwidth,andsignaldistortionresultingfromnonlineartiesinthetransmissionsystem.Section9.1describesthegeneraloperationalaspectsandcomponentsofananalogfiberopticlink.Traditiona1ly,inananalogsystem,acarrier-to-noiseratioana1ysisisusedinsteadofasignal-to-noiseratioanalysis,sincetheinformationsignalisnormallysuperimposedonaradio-frequency(RF)carrier.Thus,inSec.9.2weexaminecarrier-to-noiseratiorequirements.Thisisfirstdoneforasinglechannelundertheassumptionthattheinformationsignalisdirectlymodulatedontoanopticalcarrier.Intelecommunicationnetworks28Fortransmittingmultiplesignalsoverthesamechannel,onecanuseasubcarriermodulationtechnique.Inthismethod,whichisdescribedinSec.9.3,theinformationsignalsarefirstsuperimposedonancillaryRFsubcarriers.Thesecarriersarethencombinedandtheresultingelectricalsignalisusedtomodulatetheopticalcarrier.Alimitingfactorinthesesystemsisthesignalimpairmentarisingfromharmonicandintermodulationdistortions.Fortransmittingmultiplesign299.lOVERVIEWOFANALOGLINKS

Figure9-lshowsthebasicelementsofananaloglink.ThetransmittercontainseitheranLEDoralaserdiodeopticalsource.AsnotedinSec.4-4andshowninFig.4-35,inanalogapplications,onefirstsetsabiaspointonthesourceapproximatelyatthemidpointofthelinearoutputregion.Theanalogsignalcanthenbesentusingoneofseveralmodulationtechniques.Thesimplestformforopticalfiberlinksisdirectintensitymodulation,whereinsimplyvaryingthecurrentaroundthebiaspointinproportiontothemessagesignallevelmodulatestheopticaloutputfromthesource.Thus,theinformationsignalistransmitteddirectlyinthebaseband.Asomewhatmorecomplexbutoftenmoreefficientmethodistotranslatethebasebandsignalontoanelectricalsubcarrierpriortointensitymodulationofthesource.Thisisdoneusingstandardamplitude-modulation(AM),frequency-modulation(FM),orphase-modulation(PM)techniques.Nomatterwhichmethodisimplemented,onemustpaycarefulattentiontosignalimpairmentsintheopticalsource.Theseincludeharmonicdistortions,intermodulationproducts,relativeintensitynoise(RIM)inthelaser,andlaserclipping.Inrelationtothefiber-opticelementshowninFig.9-l,onemusttakeintoaccountthefrequencydependenceoftheamplitude,phase,andgroupdelayinthefiber.Thus,thefibershouldhaveaflatamplitudeandgroup-delayresponsewithinthepassbandrequiredtosendthesignalfreeoflineardistortion.Inaddition,sincemodal-distortion-limitedbandwidthisdifficulttoequalize,itisbesttochooseasingle-mode.9.lOVERVIEWOFANALOGLINKSF309.lOVERVIEWOFANALOGLINKS

fiber.Thefiberattenuationisalsoimportant,sincethecarrier-to-noiseperformanceofthesystemwillchangeasafunctionofthereceivedopticalpowerTheuseofanopticalamplifierinthelinkleadstoadditionalnoise,knownasamplifiedspontaneousemission(ASE),asisdescribedinChap.11:Inthe

opticalreceiver,theprincipalimpairmentsarequantumorshotnoise,APDgainnoise,andthermalnoise.9.lOVERVIEWOFANALOGLINKSf31InanalyzingthePerformanceofanalogsystems,oneusuallycalculatestheratioofrmscarrierpowertormsnoisepowerattheinputoftheRFreceiverfollowingthephotodetectionprocess.Thisisknownasthecarrier-noiseratio(CNR).LetuslookatsometypicalCNRvaluesfordigitalandanalogdata.Fordigitaldata,considertheuseoffrequency-shiftkeying(FSK).Inthismodulationscheme,theamplitudeofasinusoidalcarrierremainsconstant,{butthephaseshiftsfromonefrequencytoanothertorepresentbinarysignals.ForFSK,BERsof10-9and10-15translateintoCNRvaluesof36(l5.6dB)and64(l8.0dB),respectively.Theanalysisforanalogsignalsismorecomplex,sinceitsometimesdependsonuserperceptionofthesignalquality,suchasinviewingatelevisionpicture.Awidelyusedanalogsignalisa525-linestudio-qualitytelevisionsignal.Usingamplitudemodulation(AM)forsuchasignalrequiresaCNRof56dB,sincetheneedforbandwidthefficiencyleadstoahighsignal-to-noiseratio.Frequencymodulation(