光纖通信技術(shù)：Chapter 2 Optical Signal Generation

Chapter2OpticalSignalGenerationVocabularyChapter22SOEI,HUSTOpticaltransmitter：光發(fā)射機(jī)LED:發(fā)光二極管LD：激光二極管Spontaneousemission：自發(fā)輻射Stimulatedemission：受激發(fā)射Stimulatedabsorption：受激吸收Boltzmanstatistics：玻爾茲曼統(tǒng)計(jì)分布Thermalequilibrium：熱平衡Spectraldensity：光譜密度Populationinversion：粒子數(shù)反轉(zhuǎn)Fermi-Diracdistribution：費(fèi)米狄拉克分布Conductionband：導(dǎo)帶Valenceband：價(jià)帶Forward-biased：正向偏置Junction：結(jié)Fermilevel：費(fèi)米能級(jí)Bandgap：帶隙Heavydoping：重?fù)诫sHomojunction：同質(zhì)結(jié)Heterojunction：異質(zhì)結(jié)Doubleheterostructure：雙異質(zhì)結(jié)recombination：復(fù)合Claddinglayer：包層Augerrecombination：俄歇復(fù)合Kineticenergy：動(dòng)能Nonradiativerecombination：非輻射復(fù)合Surfacerecombination：表面復(fù)合Internalquantumefficiency：內(nèi)量子效率Directbandgap：直接帶隙Indirectbandgap：非直接帶隙Carrierlifetime：載流子壽命Latticeconstant：晶格常數(shù)Ternaryandquaternarycompound：三元系和四元系化合物Substrate:襯底LPE：液相外延VPE：汽相外延MBE：分子束外延MOCVD：改進(jìn)的化學(xué)汽相沉積MQW:多量子阱Electron-holepairs電子空穴對(duì)Externalquantumefficiency外量子效率Chapter23SOEI,HUSTFresneltransmissivity菲涅耳透射率Power-conversionefficiency功率轉(zhuǎn)換效率Wall-plugefficiency電光轉(zhuǎn)換效率Responsivity響應(yīng)度Rateequation速率方程Surface-emitting表面發(fā)射Beamdivergence光束發(fā)散Edge-emitting邊發(fā)射Resonantcavity諧振腔Gaincoefficient增益系數(shù)Differentialgain微分增益Laserthreshold激光閾值Thresholdcurrent閾值電流Groupindex群折射率Externalcavity外腔VCSEL:verticalcavitysurface-emittinglasers垂直腔表面發(fā)射激光器Photonlifetime光子壽命Slopeefficiency斜率效率Differentialquantumefficiency微分量子效率Linewidthenhancementfactor線寬加強(qiáng)因子Broadarea寬面Stripegeometry條形Diffusion擴(kuò)散Index-guided折射率導(dǎo)引Ridgewaveguidelaser脊波導(dǎo)激光器Buriedheterostructure掩埋異質(zhì)結(jié)Lateral側(cè)向Transverse橫向SLM:SingleLongitudinalmode單縱模MSR:Modesuppressionratio模式抑制比DFB:DistributedFeedback分布式反饋Braggdiffraction布拉格衍射Braggcondition布拉格條件DBR:distributedBraggreflector分布式布拉格反射器Phase-shiftedDFBlaser相移DFB激光器Gaincoupled增益耦合Coupledcavity耦合腔Characteristicstemperature特征溫度OOK開(kāi)關(guān)鍵控DPSK差分相移鍵控QPSK正交相移鍵控QAM正交幅度調(diào)制Dualpolarization雙偏振態(tài)（偏振復(fù)用）Chapter24SOEI,HUSTChap.2OpticalSignalGeneration2.1ComponentsofOpticalTransmitters2.2FundamentalofLightEmittedbySemiconductor2.3Semiconductorlasers(LaserDiodes)andTheirCharacteristics2.4TransmitterDesign2.5ExternalModulationandAdvancedModulationFormatsChapter25SOEI,HUST2.1.1SchematicDiagramofOpticalTransmittersBinarytosingleCoding/linecodingModulatorOpticalSourceDrivingCircuitPCMChannelcouplerOpticalsignaloutputChapter26SOEI,HUSTBiasedcurrentModulationcurrent(≥10Gbit/s)ModulationcurrentBiasedcurrent(≤2.5Gbit/s)DirectModulationExternalModulationChapter27SOEI,HUSTstability:power&wavelengthreliability:>25years(PouttoPout/2)smallemissiveareacompatiblewithfibercoredimensionsrightwavelengthrange0.85μm:GaAlAs/GaAs1.31μm,1.55μm:InP/InGaAsPnarrowlinewidth→dispersion,phasenoiseeasytorealizedirectmodulationhighefficiency&lowthreshold:MQW-LD,Ith~10mAMQWDFBLD2.1.2RequirementsforOpticalSourceChapter28SOEI,HUSTChap.2OpticalSignalGeneration2.1ComponentsofOpticalTransmitters2.2FundamentalofLightEmittedbySemiconductor2.3Semiconductorlasers(LaserDiodes)andTheirCharacteristics2.4TransmitterDesign2.5ExternalModulationandAdvancedModulationFormatsChapter29SOEI,HUST1.ThreeFundamentalTransitionProcesses

SpontaneousEmission→LED

StimulatedEmission→LD,SOA

30-受激吸收.swf

→PIN,APD

LightEmission2.2.1EnergyBandsinSemiconductorStimulatedEmissionSpontaneousEmissionStimulatedAbsorptionChapter210SOEI,HUSTE2N2N1E1:spectraldensityInthermalequilibrium,accordingtoBoltzmannstatistics:kB:BoltzmannconstantT:absolutetemperatureAccordingtoPlanck’sformula:2.EmissionandAbsorptionRatesChapter211SOEI,HUSTvisibleornear-infraredregion,roomtemperatureN2>N1,Rstim>Rabs(populationinversion)Thermalequilibrium laseroperation ?Operationconditionforlaser:Externalpumpingsourceisneeded:injectioncurrent,pumpinglightetc.

Einstein’scoefficientsChapter212SOEI,HUST3.RecombinationbetweenElectronsandHolesEfc,EfvaretheFermilevelsinconductionbandandvalenceband,respectivelyTheoccupationprobabilityforelectronsintheconductionandvalencebandsisgivenbytheFermi-Diracdistributionsrespectively:Conductionandvalencebandsofasemiconductor.Chapter213SOEI,HUSTρcv:jointdensityofstates,whichdescribesthenumberofstatesperunitvolumeperunitenergyrangeEg:bandgapmr:reducedmassmc,mv:effectivemassesofelectrons&holesinconductionandvalencebands,respectivelyChapter214SOEI,HUSTpopulation-inversioncondition:inthermalequilibrium:pumpingenergyintosemiconductorbyinjectingcurrent:Togetlaseroutput,Chapter215SOEI,HUST2.2.2p-nJunctions1.TypeofSemiconductorIntrinsicsemiconductor:undoped,Fermilevelislyinginthemiddleofthebandgap.n-typesemiconductor:Fermilevelmovestowardtheconductionbandasthedopantconcentrationincreases.p-typesemiconductor:Fermilevelmovestowardthevalencebandasthedopantconcentrationincreases.Chapter216SOEI,HUSTinthermalequilibriumunderforwardbiased2.p-nJunctionsunderforwardbiased:built-inelectricfieldisreduceddiffusionofelectronsandholesacrossthejunctionelectronsandholesarepresentsimultaneouslyindepletionregiongeneratelightthroughspontaneousemissionorstimulatedemissioninthermalequilibrium:

theFermilevelmustbecontinuousacrossthep–njunctionachievedthroughdiffusionofelectronsandholesacrossthejunction.Chapter217SOEI,HUSTHomojunction:equalbandgapsthesamesemiconductormaterialwideregionforelectron-holerecombinationdifficulttoobtainhighcarrierdensityHeterojunction:differentbandgapsDouble-heterojunction:sandwichingathinlayerbetweenthep-typeandn-typelayers,andthebandgapofthesandwichlayerissmallerthanthelayerssurroundingit.3.Homojunction&HeterojunctionChapter218SOEI,HUSTEnergy-banddiagramof(a)homostructureand(b)double-heterostructurep–njunctionsinthermalequilibrium(top)andunderforwardbias(bottom).Chapter219SOEI,HUSTActivelayer:lightisgeneratedinsideitasaresultofelectron-holerecombinationsmallerbandgap→largerrefractiveindex→waveguide(1D)Heterojunction:confinementofcarriers&opticalfield0.85μm:cladding/active:GaAlAs/GaAs1.31μm,1.55μm:cladding/active:InP/InGaAsPSimultaneousconfinementofchargecarriersandopticalfieldinadoubleheterostructuredesign.Chapter220SOEI,HUST1.Electron-holeRecombinationDefects&surfacerecombinationAugerNonradiativerecombination2.2.3NonradiativeRecombinationChapter221SOEI,HUST2.InternalQuantumEfficiencyRrr:radiativerecombinationrateRnr:nonradiativerecombinationrateRtot:totalrecombinationrateτ:recombinationtimeNonradiativerecombination,especiallyAugerrecombination(temperaturedependent)isharmfultodevices!positivefeedback

Chapter222SOEI,HUSTE0E0k1k2direct-bandgap(GaAs,InP)indirect-bandgap(Si,Ge)3.CarrierLifetimeA:defects&surfaceB:spontaneousradiationC:Augercoefficientk1=k2Chapter223SOEI,HUSTChap.2OpticalSignalGeneration2.1ComponentsofOpticalTransmitters2.2FundamentalofLightEmittedbySemiconductor2.3Semiconductorlasers(LaserDiodes)andTheirCharacteristics2.4TransmitterDesign2.5ExternalModulationandAdvancedModulationFormatsChapter224SOEI,HUST2.3.1AmplitudeandPhaseConditionsAdvantages(comparedtoLED):emittingrelativelyhighpower(to100mW)narrowangularspreadnarrowspectralwidthdirectmodulationathighfrequency(to10GHz)1.ComponentsandAdvantagesofLDs:Components:Chapter225SOEI,HUSTPeakgainofmedium:

when :differentialgain(gaincrosssection) :injectedcarrierdensity :transparentcarrierdensity:thresholdcarrierdensityNTisequaltoNth?2.OpticalGainChapter226SOEI,HUSTGainspectrumofa1.3-μmInGaAsPlaseratseveralcarrierdensitiesN.Variationofpeakgaingp

withN.Thedashedlineshowsthequalityofalinearfitinthehighgainregion.Chapter227SOEI,HUSTFeedbackR1R2n0=1n3.FeedbackandLaserThresholdChapter228SOEI,HUSTThreshold

modelofLDsChapter229SOEI,HUSTAmplitudeconditionPhaseconditionspacingbetweenoscillatingfrequenciesoscillatingfrequenciesthresholdgainMLMChapter230SOEI,HUST2.3.2LDStructures1.Broad-areaLDsAbroad-areasemiconductorlaser.Theactivelayer(hatchedregion)issandwichedbetweenp-typeandn-typecladdinglayersofahigher-bandgapmaterial.Lightconfinementmechanisminthedirectionperpendiculartothejunctionplaneintroducedbydoubleheterostructure

XYdistributioninnearfieldChapter231SOEI,HUSTnosuchlight-confinementmechanisminthelateraldirectionparalleltothejunctionplane.thelightgeneratedspreadsovertheentirewidthofthelaser.arelativelyhighthresholdcurrentandaspatialpatternthatishighlyellipticalandthatchangesinanuncontrollablemannerwiththecurrent.Spatialmodedistributioninfarfield?Chapter232SOEI,HUST2.StripeLDsGain-guidedsemiconductorlasersCrosssectionoftwostripe-geometrylaserstructuresusedtodesigngain-guidedsemiconductorlasersandreferredtoas(a)oxidestripeand(b)junctionstripe.

XYChapter233SOEI,HUSTsolvethelight-confinementproblembylimitingcurrentinjectionoveranarrowstripe.thespotsizeisstillnotstableasthelaserpowerisincreased.Chapter234SOEI,HUSTIndex-guidedsemiconductorlasersCrosssectionoftwoindex-guidedsemiconductorlasers:(a)ridge-waveguidestructureforweakindexguiding;(b)buriedheterostructureforstrongindexguiding.

XYChapter235SOEI,HUSTWhenlightisconfinedintoacavitysmallerthanitswavelength(~1μm),itbehavesasaparticle(quantum)ratherthanasawave.3.Multi-Quantum-WellLDsInMQWstructure,oftenanumberofquantumwellsareusedoneontopofanother.Theseparatinglayersbetweenthemareverythin(~10nm)andhavedifferentbandgaps.TheMQWstructurecanreducethelasingthreshold,andpreventslateralmodesforming.AndtheMQWlasershaveanarrowerlinewidththanconventionalstructures.Chapter236SOEI,HUSThomojunctionDoubleheterostructureStripegeometryMulti-quantum-wellRelativelystrongerconfinementofinjectedcarriersandoutputphotons,thuslowerthresholdcurrentandhigherslopeefficiency!Chapter237SOEI,HUSTSideModeSuppressionRatio(SMSR):orMLMLossSLM2.3.3ControlofLongitudinalModesChapter238SOEI,HUSTFeedbackisnotlocalizedatthefacetsbutisdistributedthroughoutthecavitylength.Anditcanbeachievedthroughaninternalbuilt-ingratingthatleadstoaperiodicvariationofthemodeindex.FeedbackoccursbymeansofBraggdiffraction,aphenomenonthatcouplesthewavespropagatingintheforwardandbackwarddirections.ThereforemodeselectivityoftheDFBlaseroccursonlyforwavelengthsλBsatisfying

theBraggcondition:1.DistributedFeedback(DFB)LasersChapter239SOEI,HUST2.SampledGratingDBRLasersDBR:distributedBraggreflectorChapter240SOEI,HUST3.Cleaved-coupledCavityLasersBycleavingaconventionalmulti-longitudinal-modesemiconductorlaserinthemiddle,itisdividedintotwosectionsandseparatedbyanarrowairgap(~1μm).Thereflectivityofcleavedfacets(~30%)allowsenoughcouplingbetweenthetwosectionsaslongasthegapisnottoowide.Possibletotunetheoutputwavelengthoveratuningrange~20nmbyvaryingthecurrentinjectedintooneofthecavitysectionsactingasamodecontroller.Tuningisnotcontinuous,sinceitcorrespondstosuccessivemodehops.Chapter241SOEI,HUST4.ExternalCavityLasers

Consistingofalaserdiode(LD),adiffractiongrating,afocuslensandamirror.Bychangingtheangleofthemirror,thelasingwavelengthistuned.Byoptimallyaligningthesecomponents,alasingcavityiscreatedthathasnomodehopswhenthewavelengthischanged.conventionalChapter242SOEI,HUSTR>99%5.VCSELs(VerticalCavitySurfaceEmittingLasers)Chapter243SOEI,HUSTThemirrorstacksaremadeofalternatinglayersofmaterialofdifferentrefractiveindices,formingaBragggratingtoobtainthewavelengthselection.Theactiveregionisveryshort,whichmeansthatthemirrorsshouldhavearelativelyhighreflectivity.Theoxideconfinementtechniqueinwhichaninsulatingaluminum-oxidelayer,actingasadielectricaperture,confinesboththecurrentandtheopticaltransversemodes.Thelowdivergencecircularlightbeamallowsforeasyandefficientcouplingtoafiber.Typicalcoupledoutputpowerisafewmilliwatts.Chapter244SOEI,HUST2.3.4NoiseandLinewidth1.NoiseMechanismsinLDsIntensity,phase,andfrequencyofLDswillfluctuateevenwhenbiasedataconstantcurrent.Noisemechanisms：eachspontaneouslyemittedphotonaddstothecoherentfield(establishedbystimulatedemission)asmallfieldcomponentwhosephaseisrandom,andthusperturbsbothamplitudeandphaseinarandommanner.Intensityfluctuationsleadtoalimitedsignal-to-noiseratio(SNR),whilephasefluctuationsleadtoafinitespectrallinewidth.Chapter245SOEI,HUST2.LinewidthandRelatedMeasurementThemodifiedScholow-Townsformulagivestherelationshipbetweenthelinewidthandspontaneousemission:

whereP,photondensityinsidethelasercavity;Rsp,spontaneousemissionfactor;βc,linewidthenhancementfactor.SpectralwidthandlinewidthChapter246SOEI,HUSTCoherencetimeandcoherencelengthcanallberelatedtothelinewidth.Coherencelengthdescribesthepropagationdistanceoverwhichalightwavesignalmaintainsitscoherence,wherevgisthegroupvelocityoftheopticalsignal.Foralightsourcewiththelinewidthof10kHz,thecoherencelengthisapproximately~30km.Coherencetimeisthetimeintervalwithinwhichthephaseofalightwaveisstillpredictable.Thelinewidthmeasurementisimplementedbythedelayedself-heterodynetechniques.IfthedifferentialdelayoftheMach-Zehnderinterferometerismuchlongerthanthecoherencetimeoftheopticalsignal,thecorrespondingcomponentsviadifferentpathscancombine

incoherentlyatthesecondopticalcoupler(OC).Itresemblesthemixingbetweenlightsfromtwoindependentlasersourceswithidenticalspectrallinewidth.Chapter247SOEI,HUSTESALinewidthmeasurementsetupOCOCChapter248SOEI,HUSTFrequencytranslationandlinewidthrelationsindelayedself-heterodynedetectionAnacousto-opticfrequencymodulator(AOFM)isusedasafrequencyshifterinonearmtoavoidthehighnoiselevelsinlow-frequencyregionofmostelectricalspectrumanalyzer(ESA).TheAOFMcancauseafrequencyshiftoffIFontheorderofafewhundredmegahertz.Thustheheterodynedetectionoftheopticalsignalsarerealizedinthephotodetector.Chapter249SOEI,HUSTIfthenormalizedRFspectraldensitymeasuredbyESAisSIF(f),theopticalsignalpowerspectraldensitySp,s(f)willsatisfythefollowingauto-convolution:TrueorFalse?Chapter250SOEI,HUST1.以下論述正確的是：（）A、非輻射復(fù)合會(huì)影響發(fā)光器件的發(fā)光效率；B、正向偏置的PN結(jié)中導(dǎo)帶和價(jià)帶的準(zhǔn)費(fèi)米能級(jí)趨于一致；C、半導(dǎo)體材料要發(fā)光，必須實(shí)現(xiàn)粒子數(shù)的反轉(zhuǎn)；D、LD中最初的光子來(lái)源于內(nèi)部的自發(fā)輻射；E、電子與空穴復(fù)合不一定產(chǎn)生光子；F、雙異質(zhì)結(jié)結(jié)構(gòu)提高了半導(dǎo)體光源的量子效率；G、工作于1.55m處的半導(dǎo)體光源有源層材料為InP；

H、溫度升高發(fā)光器件的發(fā)光效率會(huì)下降；

I、間接帶隙半導(dǎo)體材料中非輻射復(fù)合效率高于輻射復(fù)合效率，不適合用作光源材料。Chapter251SOEI,HUST1.以下論述正確的是：（）

A、非輻射復(fù)合會(huì)影響發(fā)光器件的發(fā)光效率；

B、正向偏置的PN結(jié)中導(dǎo)帶和價(jià)帶的準(zhǔn)費(fèi)米能級(jí)趨于一致；C、半導(dǎo)體材料要發(fā)光，必須實(shí)現(xiàn)粒子數(shù)的反轉(zhuǎn)；

D、LD中最初的光子來(lái)源于內(nèi)部的自發(fā)輻射；

E、電子與空穴復(fù)合不一定產(chǎn)生光子；

F、雙異質(zhì)結(jié)結(jié)構(gòu)提高了半導(dǎo)體光源的量子效率；G、工作于1.55m處的半導(dǎo)體光源有源層材料為InP；

H、溫度升高發(fā)光器件的發(fā)光效率會(huì)下降；

I、間接帶隙半導(dǎo)體材料中非輻射復(fù)合效率高于輻射復(fù)合效率，

不適合用作光源材料。Chapter252SOEI,HUST2.3.5CWCharacteristicsofLDs

1.RateEquationsForaSLMlaser,therateequations:P,N:numberofphotons&carriersNetrateofstimulatedemission—opticalgain:Photonlifetime:gm:peakgainofmaterial:gaincrosssection,ordifferentialgain:transparentcarriernumberChapter253SOEI,HUSTForI>Ith,R1=R22.CWOperationConditionsChapter254SOEI,HUSTThresholdofP-IcurvesSpontaneousemissionStimulatedemissionI0:constant,T0:characteristictemperatureGaAs:T0=120K,InGaAsP:T0=50~70KBendingofP-Icurves

Rnr:mainlydependingonAugerrecombinationinInGaAsPLDsSolution:built-inthermoelectriccoolerisusedtodealwithtemperaturesensitivitiesofInGaAsPLDs3.P-ICurvesChapter255SOEI,HUSTInternalquantumefficiency:Slopeefficiency:Differentialquantumefficiency:Externalquantumefficiency:wall-plugefficiency:GaAslasers:InGaAsPlasers:4.EfficienciesChapter256SOEI,HUST2.3.6ModulationResponseofLDsSmall-signalmodulation:Frequencyresponse:1.Small-SignalModulationModulationbandwidthChapter257SOEI,HUSTModulationresponseofalaserasafunctionofmodulationfrequencyatseveralbiaslevels.theefficiencyisreducedwhenthemodulationfrequencyexceedsΩR

byalargeamount.Chapter258SOEI,HUST2.Large-SignalModulationExternalmodulationforhighspeedtransmission!Frequencychirp

βc:amplitude-phasecouplingparameter,forbulkmaterial:4~8,MQW:~3.frequencyshift:leadingedge:thelongitudinalmodefrequencyshiftstowardtheblue

side.trailingedge:shiftstowardtheredside.

Chapter259SOEI,HUSTElectro-opticaldelay&relaxationoscillation

Whenpumpingpowerisappliedtothelaser,theupperenergystatepopulationbuildsupuntilaninversionoccursandlasingcancommence.Lasingcandepletetheupperenergystateveryquickly.Andifpumpingisn'tquitefastenough,lasingwillmomentarilystop.Verysoonafterwardsitwillstartagainasthepumpbuildsupapopulationinversionagain.Chapter260SOEI,HUSTA、LD的激射波長(zhǎng)一定是自發(fā)輻射的峰值波長(zhǎng)；B、條形激光器中也存在雙異質(zhì)結(jié)結(jié)構(gòu)；C、雙異質(zhì)結(jié)中對(duì)載流子的限制作用是因?yàn)榇嬖趦?nèi)建折射率波導(dǎo)；D、通過(guò)選擇合適的組分x和y，基于In1-xGaxAsyP1-y的半導(dǎo)體光源可設(shè)計(jì)工作于0.85m處；E、LD有諧振腔，而LED沒(méi)有；F、LD的P-I曲線有閾值，而LED的P-I曲線沒(méi)有閾值；G、LD和SOA中最初的光子均來(lái)源于自發(fā)輻射；H、激光器的小信號(hào)調(diào)制帶寬會(huì)隨著偏置電流的增加而增大；I、偏置電流選擇合理可適當(dāng)減小張馳振蕩和電光延時(shí)效應(yīng)的影響；J、單縱模LD用作光源時(shí)，色散容限大。

2.以下關(guān)于半導(dǎo)體材料和發(fā)光機(jī)理論述錯(cuò)誤的是：TrueorFalse?Chapter261SOEI,HUST

A、LD的激射波長(zhǎng)一定是自發(fā)輻射的峰值波長(zhǎng)；B、條形激光器中也存在雙異質(zhì)結(jié)結(jié)構(gòu)；

C、雙異質(zhì)結(jié)中對(duì)載流子的限制作用是因?yàn)榇嬖趦?nèi)建折射率波導(dǎo)；

D、通過(guò)選擇合適的組分x和y，基于In1-xGaxAsyP1-y的半導(dǎo)體光源可設(shè)計(jì)工作于0.85m處；E、LD有諧振腔，而LED沒(méi)有；F、LD的P-I曲線有閾值，而LED的P-I曲線沒(méi)有閾值；

G、LD和SOA中最初的光子均來(lái)源于自發(fā)輻射；H、激光器的小信號(hào)調(diào)制帶寬會(huì)隨著偏置電流的增加而增大；I、偏置電流選擇合理可適當(dāng)減小張馳振蕩和電光延時(shí)效應(yīng)的影響；J、單縱模LD用作光源時(shí)，色散容限大。

2.以下關(guān)于半導(dǎo)體材料和發(fā)光機(jī)理論述錯(cuò)誤的是：Chapter262SOEI,HUSTChap.2OpticalSignalGeneration2.1ComponentsofOpticalTransmitters2.2FundamentalofLightEmittedbySemiconductor2.3Semiconductorlasers(LaserDiodes)andTheirCharacteristics2.4TransmitterDesign2.5ExternalModulationandAdvancedModulationFormatsChapter263SOEI,HUST2.4.1BasicConcepts1.DigitalModulationLDdigitalmodulationFordirectlymodulatedLD,biasednearthreshold!Tomitigatetheelectro-opticaldelayandrelaxationoscillation.Tosuppressthepatterneffect.Inducingrelativelylowextinctionratioandlargeshotnoise.Chapter264SOEI,HUST2.DigitalLogicElectricalLevel

0 1 TTL:0~0.8V 2.0~5.0V (-5V)ECL:-1.75V -0.85 V (+5V)PECL:+3.25V +4.15 V3.ExtinctionRatioPP1P00tChapter265SOEI,HUST4.Source-fiberCoupling

5.Packaging

sourcefiberRfcoatinglensedfiberdiesubmountPDheatsinkTECcoolerfibermetalshellTEC(ThermallyExpandCore)FiberChapter266SOEI,HUSTChapter267SOEI,HUST2.4.2Drivingandmodulationcircuits1.DigitalModulationCircuitwithAPCforLDT1和T2輪流截止和導(dǎo)通，避免載流子恢復(fù)時(shí)間的影響，可工作于高速率；射極耦合電路為恒流源，總電流可保持不變，噪聲小；由于T2和T3導(dǎo)通電壓的負(fù)溫度特性，可另加兩個(gè)二極管D1、D2對(duì)T2、T3進(jìn)行補(bǔ)償，使溫度變化時(shí)驅(qū)動(dòng)電流保持恒定。Chapter268SOEI,HUST熱敏電阻RT接在電橋的一個(gè)臂上；在設(shè)定溫度下，電橋處于平衡狀態(tài)，制冷器沒(méi)有電流流過(guò)；由于熱敏電阻具有負(fù)的溫度系數(shù)，溫度升高時(shí)電橋平衡被破壞，制冷器開(kāi)始工作，從而可使得LD的結(jié)溫不超過(guò)設(shè)定溫度。由于VT的單向?qū)ㄌ匦?，圖示電路中的制冷器只能工作在單一模式（制冷或加熱）2.ATCCircuitChapter269SOEI,HUSTChap.2OpticalSignalGeneration2.1ComponentsofOpticalTransmitters2.2FundamentalofLightEmittedbySemiconductor2.3Semiconductorlasers(LaserDiodes)andTheirCharacteristics2.4TransmitterDesign2.5ExternalModulationandAdvancedModulationFormatsChapter270SOEI,HUST2.5.1ExternalModulationandModulatorElectro-absorptionModulator（EAM）1.ExternalModulatorEAMmakesuseoftheFranz–Keldysheffect(夫蘭茲-凱耳什效應(yīng)),accordingtowhichthebandgapofasemiconductordecreaseswhenanelectricfieldisappliedacrossit.Thus,atransparentsemiconductorlayerbeginstoabsorblightwhenitsbandgapisreducedelectronicallybyapplyinganexternalvoltage.Chapter271SOEI,HUSTCharacteristics:relativelylowdrivevoltages(typ.2V)cost-effectiveinvolumeproductionandeasytorealizeintegrationwavelength-dependentabsorptionrelativelylowdynamicextinctionratios(<10dB)residualchirplimitedopticalpower-handlingcapabilitiesChapter272SOEI,HUSTMach–ZehnderModulator(MZM)OpticalwaveguideMach-ZehnderInterferometerTravelling-WaveImpedancematchedelectrodestructureMZMsworkbytheprincipleofinterference,controlledbymodulatingtheopticalphase.Therefractiveindexofelectro-opticmaterialssuchasLiNbO3canbechangedbyapplyinganexternalvoltage.

Thereforephaseshiftcanbeintroducedthroughvoltage-inducedindex.Chapter273SOEI,HUST2.OperationPrincipleofMZMsPhaseshiftinthecorrespondingarm:Outputopticalfield:Themodulationvoltagethatisrequiredtochangethephaseinonemodulatorarmbyπ,andtherebyletstheMZMswitchfromfulltransmissiontofullextinction,iscalledswitchingvoltageVπ.Chapter274SOEI,HUSTV1(t)=-V2(t),thephasetermcanbeeliminatedinEout(t),knownasbalanceddrivingorpush–pulloperation.Outputopticalintensity:sinusoidalpowertransferfunctionChapter275SOEI,HUSTBiasedandmodulation(data)voltage:Single-driveMZMElectricalNRZdataChapter276SOEI,HUST2.5.2OpticalSignalGeneration1.NRZFormatReverseloadingForwardloading1111100000WaveformEyediagramChapter277SOEI,HUST2.

RZFormat33%RZNRZ67%CSRZChapter278SOEI,HUSTPulsecarvingDifferentRZformatscanbeimplementedby

pulsecarving:50%RZ--SinusoidallydrivingaMZMatthedatarateBbetweentheminimumandthemaximumtransmission,i.e.theamplitudeofclockisVπ/2andthebiasedvoltageis-Vπ/2.The