光纤通信技术：Chapter 4 Optical Signal Detection and Recovery

Chapter4

OpticalSignalDetectionandRecoveryVocabularyopticalreceiver光接收机signaldistortion信号畸变BER(bit-errorrate)误码率APD雪崩光电二极管ISI码间串扰darkcurrent暗电流photoncurrent光生电流directdetection直接探测coherentdetection相干探测noisefigure噪声指数highsensitivity高灵敏度responsivity响应度thermalnoise热噪声shotnoise散粒噪声noiseequivalentpower噪声等效功率currentfluctuation电流波动excessnoisefactor过剩噪声因子quantumlimit量子受限automaticgaincontrol自动增益控制pre-amplifier前置放大器decisioncircuit判决电路decisionthresholdvalue判决阈值clockrecovery时钟恢复electron-holepair电子空穴对driftvelocity漂移速率Gaussianstatistics高斯统计high-passfilter高通滤波器linearchannel线性信道electro-opticaldelay电光延迟samplingtime采样时间powerfluctuation功率波动optimumgain最优增益complementaryerrorfunction互补误差函数multiplicationfactor倍增因子spectraldensity谱密度intensitynoise强度噪声Chapter42SOEI,HUSTtimingjitter定时抖动mode-partitionnoise模式分配噪声parasiticreflection寄生反射depletionregion耗尽区biasvoltage偏置电压reversebias反向偏置tunnelingbreakdown隧道击穿equalizer均衡器stimulatedabsorption受激吸收quantumefficiency量子效率straylight杂散光sensitivitydegradation灵敏度劣化diffusive扩散gainflatness增益平坦ionizationcoefficient电离系数gainregion增益区boundarycondition边界条件avalancheprocess雪崩过程pulsebroadening脉冲展宽stationaryrandomprocess平稳随机过程Poissonstatistics泊松统计Wiener-Khinchintheorem维纳-辛钦定理pulseshaping脉冲整形transferfunction传递函数rectangularpulse矩形脉冲one/two-sided单/双边带transparenttransmission透明传输wavelengthrouting波长路由point-to-point点对点topologystructure拓扑结构powerbudget功率预算powermargin功率余量powerpenalty功率代价Chapter43SOEI,HUSTChap.4OpticalSignalDetectionandRecovery4.1Opticaldetectordesign4.2Opticalreceiverstructure4.3Receivernoise4.4Receiversensitivity4.5CoherentopticalreceiverRequirementHighsensitivity,Highresponsespeed,Lowcost,Highreliability,robusttodistortions.Chapter45SOEI,HUSTopticalsignalvoltagesupplyO/Epre-amplifierautomaticgaincontrolamplifierfilterdecisioncircuitclockrecovery

PINorAPDelectricaldataFrontEndLinearChannelDataRecovery4.1.1Responsivity

andBandwidth1.StimulatedabsorptionSpontaneousEmissionStimulatedemissionStimulatedabsorptionLEDLDPDOpticalsourceChapter46SOEI,HUSTPhotodetector2.ResponsivityThephotocurrentgeneratedRistheresponsivityofthephotodetector(inunitsofA/W).3.HowtoincreaseR?Chapter47SOEI,HUSTwhenChapter48SOEI,HUST4.PDbandwidthChapter49SOEI,HUSTRCtimeconstantOpticalsignalPinVout(t)90%10%V0TrWTransittimeChapter410SOEI,HUST5.DarkcurrentOriginatesfromstraylightorthermallygeneratedelectron-holepairs4.1.2TypicalStructuresandPerformancesChapter411SOEI,HUSTP-NPhotodiodesareversebiasedp-njunctionFastSlowa)Ap-nphotodiodeunderreversebias;(b)variationofopticalpowerinsidethePD;(c)energy-banddiagramshowingcarriermovementthroughdriftanddiffusion.

thepresenceofadiffusivecomponentdistortsthetemporalresponseofaphotodiode.

solution:decreasingthewidthsofthep-andn-regions;increasingthedepletion-regionwidth.Chapter412SOEI,HUST

i-layer→highresistance→largeelectricfield→thedepletionregionthroughoutit→driftcomponentdominatesoverthediffusioncomponentButdiffusioncomponentmaystillexist!2.PINPhotodiodesChapter413SOEI,HUSTEg(i)<hv<Eg(p,n)Chapter414SOEI,HUSTDouble-heterostructurePINsStimulatedabsorptiononlyoccursinthei-layer.Thediffusivecomponentcurrentiseliminatedcompletely(Highresponsespeed).

InPInGaAsInPChapter415SOEI,HUSTChapter416SOEI,HUSTTypicalstructure(a)AnAPDtogetherwiththeelectric-fielddistributioninsidevariouslayersunderreversebias;(b)designofasiliconreach-throughAPD.3.AvalanchePhotodiodes(APDs)Chapter417SOEI,HUSTOperationprinciple:impactionizationAnacceleratingelectrongivesapartofitskineticenergytoanotherelectroninthevalencebandthatendsupintheconductionband,leavingbehindahole.aphoton asingleprimaryelectronelectronwithhighkineticenergymanysecondaryelectrons&holesi-layer:absorption;gain-layer:multiplicationthroughimpactionizationMainparametersimpact-ionizationcoefficients:(αe:electrons;αh:holes)dependonmaterialandontheelectricfield.

Chapter418SOEI,HUSTMultiplicationfactorthecurrentflowwithinmultiplicationlayerie,ih:electron&holecurrentrespectivelythetotalcurrent:Chapter419SOEI,HUSTassumingthat:,theelectricfieldacrossthegainregionisuniform(areconstants),onlyelectronscrosstheboundarytoenterthen-region.

Consideringtheboundarycondition

Chapter420SOEI,HUSTChapter421SOEI,HUSTResponsivityM:averageAPDgainavalanchebreakdown!whenwhereisionizationcoefficientratio.Bandwidth

WhereM0isthelowfrequencygain,τeistheeffectivetransittimeChapter422SOEI,HUSTChapter423SOEI,HUST

DifferentAPDstructuresChapter424SOEI,HUSTReach-throughAPD0.85um:Si,KA<<1M~100,B~100Mb/s1.31um,1.55um:Ge,InGaAsSolution:largebandgap→canbeappliedhighreversevoltage

Absorption&Multiplicationlayersusethesamematerial.(ilayer)Chapter425SOEI,HUSTSAM-APD:SeparateabsorptionandmultiplicationAPDp+-InPn+-InPn-InPi-InGaAsMA-+Problem:largebandgapdifferencebetweenInP&InGaAsEInp=1.35eVEInGaAs=0.75eV1)holesgeneratedaretrappedattheheterojunction.2)slowresponse&smallbandwidth.Solution:Chapter426SOEI,HUSTChapter427SOEI,HUSTp+-InPn+-InPn-InPInGaAsMA-+InGaAsPGSAGM-APD(SeparateAbsorptionGrading&Multiplication)p+-InPN+-InPn-InPInGaAsMA-+InGaAsPGN+-InPCChapter428SOEI,HUSTSAGCM-APD(C-chargelayer)Chap.4OpticalSignalDetectionandRecovery4.1Opticaldetectordesign4.2Opticalreceiverstructure4.3Receivernoise4.4Receiversensitivity4.5CoherentopticalreceiverTIA4.2.1FrontEndChapter430SOEI,HUSTEquivalentcircuitfor(a)high-impedanceand(b)transimpedancefrontendsinopticalreceivers.Thephotodiodeismodeledasacurrentsourceinbothcases.DesignRequirementsAlargeloadresistorReceiverbandwidth

UsingEqualizertoincreasethebandwidthbyattenuatingthelow-frequencycomponentsHigherOutputVoltageLowThermalNoiseImprovedsensitivity

Atrade-offbetweenreceiversensitivityandbandwidth!Chapter431SOEI,HUSTTransimpedanceamplifier(TIA)HighSensitivityandLargeBandwidth4.2.2LinearChannelChapter432SOEI,HUST1.ComponentsAmplifierwithAGC:limitingtheaverageoutputvoltageforacceptingawiderangeofinputACvoltagesandprovidingconstant-amplitudevoltage.Low-passfilter:reducingthenoise;determiningthereceiverBWandthusshapingthevoltagepulsetoreducetheISI.2.Optimizationofthelow-passfilterChapter433SOEI,HUSTTheinputsignal:Theoptimizedshapeoftheoutputpulsefordecision:Consecutiveraised-cosineimpulses,demonstratingzero-ISIpropertyChapter434SOEI,HUSTThetransferfunctionofthelinearchannelisgivenas:raised-cosineFormationofEyediagram(NRZ)Chapter435SOEI,HUST4.2.3DecisionCircuitChapter436SOEI,HUSTComponents:clockrecovery(CR)+decisioncircuit.TheCRisolatesaspectralcomponentatf=Bfromthereceivedsignalandprovidesinformationaboutthebitslottothedecisioncircuitandhelpstosynchronizethedecisionprocess.CRiseasyincaseofRZformatwhiledifficultincaseofNRZformat.Thebestsamplingtimecanbedeterminedfromtheeyediagram.Chap.4OpticalSignalDetectionandRecovery4.1Opticaldetectordesign4.2Opticalreceiverstructure4.3Receivernoise4.4Receiversensitivity4.5CoherentopticalreceiverShotnoise:electronsaregeneratedatrandomtimes,depictedasastationaryrandomprocesswithPoissonstatistics(approximatedbyGaussianstatistics)Chapter438SOEI,HUST4.3.1NoiseMechanismsphotodiodecurrentwithshotnoisetheautocorrelationfunctionisrelatedtothespectraldensitybytheWiener-Khinchintheoremconsideringthedarkcurrentthetwo-sidedspectraldensitytheone-sidedspectraldensityChapter439SOEI,HUST2.Thermalnoise:Atafinitetemperate,electronsmoverandomlyinaresistorintheabsenceofanappliedvoltage.ThecurrentfluctuationinducedbythermalnoiseisastationaryrandomprocesswithGaussianstatistics.photodiodecurrentwithshotnoisepowerspectraldensityFn:factorbywhichthermalnoiseisenhancedbyvariousresistorsusedinpre-andmain-amplifierthetwo-sidedspectraldensitytheone-sidedspectraldensityChapter440SOEI,HUST3.Totalnoisepower

Thephotodiodecurrentwithshotandthermalnoise:

Notethatis(t),iT(t)

areindependentrandomprocesses.Chapter441SOEI,HUST1.PINreceiver4.3.2SNRofOpticalReceiversChapter442SOEI,HUSTThermalnoiselimitNoise-equivalentpower

(NEP):opticalpowerperunitbandwidthrequiredtoproduceSNR=1Detectivity:(NEP)-1Shot-noiselimitSNRcanbewrittenintermofthenumberofphotonNpcontaininthe“1”bit.bychoosingChapter443SOEI,HUST2.APDreceiverFA(M):excessnoisefactor,Chapter444SOEI,HUSTShotnoiseenhancement:Secondaryelectron-holepairsgeneratesatrandomtimesthroughtheprocessofimpactionization.Thermal-noiselimitShot-noiselimitOptimumAPDgainChapter445SOEI,HUSTChap.4OpticalSignalDetectionandRecovery4.1Opticaldetectordesign4.2Opticalreceiverstructure4.3Receivernoise4.4Receiversensitivity4.5CoherentopticalreceiverBit-ErrorRate(BER)：probabilityofincorrectidentificationofabitbythedecisioncircuitofthereceiver.Chapter447SOEI,HUST4.4.1BERandSensitivity(assumingp(1)=p(0)=1/2)

(complementaryerrorfunction)Chapter448SOEI,HUSTDecisionthreshold:IDshouldbeoptimizedtominimizetheBER.TheminimumoccurswhenIDischosenas:QfactorChapter449SOEI,HUSTRelationbetweenQandBER

Chapter450SOEI,HUSTChapter451SOEI,HUST2.Sensitivity:theminimumaveragereceivedpowerPrecrequiredbythereceivertooperateataBERof10-9

.AveragereceivedpowerChapter452SOEI,HUSTPINreceiver

APDreceiverChapter453SOEI,HUST4.4.2BERandSNRShot–noiselimitThermal–noiselimitChapter454SOEI,HUST4.4.3SensitivityDegradationPowerpenalty:Theincreaseintheminimumaveragereceivedpowerbecauseofnon-idealconditions:extinctionratio;intensitynoise;timingjitter;mode-partitionnoise;parasiticreflectionsExtinctionratioideal:rex=0infact:rex≠0Ib<Ith,P0=0,relaxationoscillationandelectro-opticaldelayriseup.Ib>Ith,P0≠0,rex≠0,modulationbandwidthrisesup.ForaPINreceiver,inthethermalnoiselimit:Powerpenalty:ForaAPDreceiver,δexislargerbyafactorofabout2Chapter455SOEI,HUSTChapter456SOEI,HUSTPowerpenaltyversustheextinctionratioPowerpenaltyversustheintensitynoiseparameterIntensityNoisemodepartitionnoise;parasiticreflectionPowerfluctuationcurrentfluctuationChapter457SOEI,HUSTRZ,Error-freeTimingjitterChapter458SOEI,HUSTProblem光接收机灵敏度分析某PIN光接收机等效噪声带宽近似等于接收光信号比特速率的一半，工作在1550nm时量子效率为0.9，前端总电容为0.003pF，放大器噪声指数为3dB，忽略暗电流，分别计算下面受限情况下，该接收机工作在10Gb/s时误码率要求为10－12时对应的灵敏度（单位为dBm）：1）热噪声受限；2）散粒噪声受限。注：普朗克常数：；基本电荷电量：；玻尔兹曼常数：Chapter459SOEI,HUSTChap.4OpticalSignalDetectionandRecovery4.1Opticaldetectordesign4.2Opticalreceiverstructure4.3Receivernoise4.4Receiversensitivity4.5CoherentopticalreceiverDirectdetectionistheconventionalwaytodetectopticalsignal.QUADRATICdetectionofelectricfield.OnlyIntensity!4.5.1OperationPrinciplesAmplitude,phaseandpolarization!LOfrequency≈carrierfrequency：intradynedetectionCoherentdetection:LINEARdetectionoftheelectricfield,bybeatingwithlocaloscillator.1.Comparisonbetweendirectdetectionandcoherentdetection

Chapter461SOEI,HUSTRef1.Fiber-OpticCommunicationSystems(Edition4),pp158~1614.5.2TypicalStructure1.Phasediversity:ThephotocurrentsI1，I2，I3，I4providefullinformationonrealandimaginarypartsoftheincomingsignalfield.I1I4Chapter462SOEI,HUSTRef2.OpticalFiberTelecommunicationsVB,pp95~113Chapter463SOEI,HUSTPhotocurrentI1-4andresultingcurrentsi(t)andq(t)Phaseinformation(+phasenoise)AmplitudeFrequencyoffsetWiththeλ/4waveplate(QWP),thepolarizationoftheLObecomescircular,whilethesignalremainslinearlypolarizedandis45degreewithrespecttoprincipalaxesofthePBSs.Afterpassingthroughthehalfmirror(HM),thePBSsseparatethetwopolarizationcomponentsoftheLOandsignalwhiletwobalancedphotodiodesPD1andPD2detectthebeatbetweentheLOandsignalineachpolarization.WhenthecircularlypolarizedLOissplitwithaPBS,thephasedifferenceis90degree.Incontrast,thereisnophasedifferencebetweent