光纤通信技术：Chapter 2 Optical Signal Generation

Chapter2OpticalSignalGenerationVocabularyChapter22SOEI,HUSTOpticaltransmitter：光发射机LED:发光二极管LD：激光二极管Spontaneousemission：自发辐射Stimulatedemission：受激发射Stimulatedabsorption：受激吸收Boltzmanstatistics：玻尔兹曼统计分布Thermalequilibrium：热平衡Spectraldensity：光谱密度Populationinversion：粒子数反转Fermi-Diracdistribution：费米狄拉克分布Conductionband：导带Valenceband：价带Forward-biased：正向偏置Junction：结Fermilevel：费米能级Bandgap：带隙Heavydoping：重掺杂Homojunction：同质结Heterojunction：异质结Doubleheterostructure：双异质结recombination：复合Claddinglayer：包层Augerrecombination：俄歇复合Kineticenergy：动能Nonradiativerecombination：非辐射复合Surfacerecombination：表面复合Internalquantumefficiency：内量子效率Directbandgap：直接带隙Indirectbandgap：非直接带隙Carrierlifetime：载流子寿命Latticeconstant：晶格常数Ternaryandquaternarycompound：三元系和四元系化合物Substrate:衬底LPE：液相外延VPE：汽相外延MBE：分子束外延MOCVD：改进的化学汽相沉积MQW:多量子阱Electron-holepairs电子空穴对Externalquantumefficiency外量子效率Chapter23SOEI,HUSTFresneltransmissivity菲涅耳透射率Power-conversionefficiency功率转换效率Wall-plugefficiency电光转换效率Responsivity响应度Rateequation速率方程Surface-emitting表面发射Beamdivergence光束发散Edge-emitting边发射Resonantcavity谐振腔Gaincoefficient增益系数Differentialgain微分增益Laserthreshold激光阈值Thresholdcurrent阈值电流Groupindex群折射率Externalcavity外腔VCSEL:verticalcavitysurface-emittinglasers垂直腔表面发射激光器Photonlifetime光子寿命Slopeefficiency斜率效率Differentialquantumefficiency微分量子效率Linewidthenhancementfactor线宽加强因子Broadarea宽面Stripegeometry条形Diffusion扩散Index-guided折射率导引Ridgewaveguidelaser脊波导激光器Buriedheterostructure掩埋异质结Lateral侧向Transverse横向SLM:SingleLongitudinalmode单纵模MSR:Modesuppressionratio模式抑制比DFB:DistributedFeedback分布式反馈Braggdiffraction布拉格衍射Braggcondition布拉格条件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特征温度OOK开关键控DPSK差分相移键控QPSK正交相移键控QAM正交幅度调制Dualpolarization双偏振态（偏振复用）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.以下论述正确的是：（）A、非辐射复合会影响发光器件的发光效率；B、正向偏置的PN结中导带和价带的准费米能级趋于一致；C、半导体材料要发光，必须实现粒子数的反转；D、LD中最初的光子来源于内部的自发辐射；E、电子与空穴复合不一定产生光子；F、双异质结结构提高了半导体光源的量子效率；G、工作于1.55m处的半导体光源有源层材料为InP；

H、温度升高发光器件的发光效率会下降；

I、间接带隙半导体材料中非辐射复合效率高于辐射复合效率，不适合用作光源材料。Chapter251SOEI,HUST1.以下论述正确的是：（）

A、非辐射复合会影响发光器件的发光效率；

B、正向偏置的PN结中导带和价带的准费米能级趋于一致；C、半导体材料要发光，必须实现粒子数的反转；

D、LD中最初的光子来源于内部的自发辐射；

E、电子与空穴复合不一定产生光子；

F、双异质结结构提高了半导体光源的量子效率；G、工作于1.55m处的半导体光源有源层材料为InP；

H、温度升高发光器件的发光效率会下降；

I、间接带隙半导体材料中非辐射复合效率高于辐射复合效率，

不适合用作光源材料。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的激射波长一定是自发辐射的峰值波长；B、条形激光器中也存在双异质结结构；C、双异质结中对载流子的限制作用是因为存在内建折射率波导；D、通过选择合适的组分x和y，基于In1-xGaxAsyP1-y的半导体光源可设计工作于0.85m处；E、LD有谐振腔，而LED没有；F、LD的P-I曲线有阈值，而LED的P-I曲线没有阈值；G、LD和SOA中最初的光子均来源于自发辐射；H、激光器的小信号调制带宽会随着偏置电流的增加而增大；I、偏置电流选择合理可适当减小张驰振荡和电光延时效应的影响；J、单纵模LD用作光源时，色散容限大。

2.以下关于半导体材料和发光机理论述错误的是：TrueorFalse?Chapter261SOEI,HUST

A、LD的激射波长一定是自发辐射的峰值波长；B、条形激光器中也存在双异质结结构；

C、双异质结中对载流子的限制作用是因为存在内建折射率波导；

D、通过选择合适的组分x和y，基于In1-xGaxAsyP1-y的半导体光源可设计工作于0.85m处；E、LD有谐振腔，而LED没有；F、LD的P-I曲线有阈值，而LED的P-I曲线没有阈值；

G、LD和SOA中最初的光子均来源于自发辐射；H、激光器的小信号调制带宽会随着偏置电流的增加而增大；I、偏置电流选择合理可适当减小张驰振荡和电光延时效应的影响；J、单纵模LD用作光源时，色散容限大。

2.以下关于半导体材料和发光机理论述错误的是：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轮流截止和导通，避免载流子恢复时间的影响，可工作于高速率；射极耦合电路为恒流源，总电流可保持不变，噪声小；由于T2和T3导通电压的负温度特性，可另加两个二极管D1、D2对T2、T3进行补偿，使温度变化时驱动电流保持恒定。Chapter268SOEI,HUST热敏电阻RT接在电桥的一个臂上；在设定温度下，电桥处于平衡状态，制冷器没有电流流过；由于热敏电阻具有负的温度系数，温度升高时电桥平衡被破坏，制冷器开始工作，从而可使得LD的结温不超过设定温度。由于VT的单向导通特性，图示电路中的制冷器只能工作在单一模式（制冷或加热）2.ATCCircuitChapter269SOEI,HUSTChap.2OpticalSignalGeneration2.1ComponentsofOpticalTransmitters2.2FundamentalofLightEmittedbySemiconductor2.3Semiconductorlasers(LaserDiodes)andTheirCharacteristics2.4TransmitterDesign2.5ExternalModulationandAdvancedModulationFormatsChapter270SOEI,HUST2.5.1ExternalModulationandModulatorElectro-absorptionModulator（EAM）1.ExternalModulatorEAMmakesuseoftheFranz–Keldysheffect(夫兰兹-凯耳什效应),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