《光通信技术》课件：Lecture5

DepartmentofOpticalEngineeringAReviewchecklistFiberFibermodeWhatisit?Whyitisdiscrete?ConditionforsinglepropagationmodeAttenuationFunctionofwavelengthCalculationCausesDispersionIntramodaldispersionIntermodaldispersionPolarizationmodedispersionHigherorderdispersion

FiberCablesConstructionelementsOpticalcharacteristicsFiberconnecterizationConnectorsConnectortypes/structureSplicesDepartmentofOpticalEngineeringLightsourceandtransmittersAtransmitterconsistsofthreeparts:LightsourceLEDLDCouplingopticsElectronicsDepartmentofOpticalEngineeringClassificationofmaterialsDepartmentofOpticalEngineeringDepartmentofOpticalEngineeringLightradiationbyasemiconductorEnergyBand•Inapurecrystalatlowtemperatures,theconductionbandisemptyandvalencebandisfull•Thesetwoenergybandisseparatedbybandgap•Whentemperatureisraised,someelectronsareexcitedcrossthegap

ThermalpopulationDepartmentofOpticalEngineeringHolesDepartmentofOpticalEngineeringBothfreeelectronsandholesconductcurrentsDepartmentofOpticalEngineeringElectrons:particleandwaveParticlenatureEffectivemassm*Momentump=m*Vm*meinteractionwithlatticeWavenaturep=m*V=h/Wavevectork=2/p=hk/2KeneticenergyK.E.=p2/2m*k2DepartmentofOpticalEngineeringDirectorindirectbandgap:ifminlinesupwithmaxDepartmentofOpticalEngineeringHowtochoosek’s:electronconfinedinaboxDepartmentofOpticalEngineeringDiscretekvaluesDepartmentofOpticalEngineeringMorethanonestatehavethesameenergy

-degeneracyordensityofstateDepartmentofOpticalEngineeringDepartmentofOpticalEngineering

ExtrinsicSemiconductors

n-typesiliconImpuritywithdonoratomsMajoritycarrieriselectronsMinoritycarrierisholesp-typesiliconImpuritywithacceptorwithatomsMajoritycarrierisholesMinoritycarrieriselectronsDepartmentofOpticalEngineeringP-NJunctionNregion –+vecharge•pregion –-vecharge•Electricalneutral•DiffusionpotentialCurrentdensityp(x)orn(x)DiffusionDriftDepartmentofOpticalEngineeringContactpotential:

-balancebetweendiffusionanddriftDriftcurrent=diffusioncurrentDepartmentofOpticalEngineeringE-kdiagram:Si&GaAsDepartmentofOpticalEngineeringIndirectdirectCarrierRecombinationDepartmentofOpticalEngineeringDiffusionofmajoritycarriersacrossthedepletionregion•Carrierinjection•InjectedminoritycarriersrecombinewithmajorityRadiative/non-radiativerecombinationWhatisapossiblemeasuretomakeithappen?That’swhatwewantProducesacurrentandinterruptstheequilibriumofcarrierpopulationReduceselectrostaticbarrieratthejunction,thusdiffusionoccuragainSplittingFermilevel

ApplyaE-fieldtoreducebuilt-infield:+onp-type,-onn-type=forward-biasingEffectofforward-biasingDepartmentofOpticalEngineeringEffectofforwardbiasDepartmentofOpticalEngineeringRadiativerecombinationDepartmentofOpticalEngineeringConservationlawsDepartmentofOpticalEngineeringK-selectionrule:k1k2Twotypesofbandgaps,againDirectbandgapIndirectbandgapDepartmentofOpticalEngineeringPhotonemissionunlikely:10-2–10-4sRadiativerecombiationpossible:10-8–10-10sPopularmaterialsDepartmentofOpticalEngineeringDepartmentofOpticalEngineeringLight-emittingDiodesTheforwardcurrentinjectselectronsintothedepletionregionandrecombineswithholesradiativelyandnon-radiativelyEmitterCharacteristics,

(a)

LED,

(b)LaserDepartmentofOpticalEngineeringPowerv.s.drivingcurrentPowerv.s.

photonnumbers->excited(injected)electronsNintinternalquantumefficiency:electron->photonRadiationwavelength-energygapDepartmentofOpticalEngineeringSomepopularIII-VcompoundsDepartmentofOpticalEngineeringDepartmentofOpticalEngineeringBlueLED:achallengeDepartmentofOpticalEngineeringBlueLED:achallengeDepartmentofOpticalEngineeringWhiteLED:amagicDepartmentofOpticalEngineeringRadiationPatternsSurface-emittingLED(SLED)ALambertiansourceP=P0cosEdge-emittingLED(ELED)ALambertiansourceinaplaneTwotypesofpackagingDepartmentofOpticalEngineeringCoupling,spectralwidthandefficienciesDepartmentofOpticalEngineeringCouplingofSLEDintoastep-indexfiberPin=P0(NA)2 62.5/125MMfiber:NA=0.275P0=100W->Pin=7.56WSpectralwidthSLED170nmELED65nm(LD1nm)WavelengthIncreasewithtemperature0.38nm/ºCIncreasewithdrivingcurrent0.69nm/mARisetime0.4-10nsDeterminedbyrecombinationtimeModulationbandwidthBW=1/RefractiveindexchangeswithtemperatureDirect-couplingefficiencyDepartmentofOpticalEngineeringSource-FiberCoupling–LambertianSourcesGeneralized

CoupledPowerLambertianSourceradiancedistributionSourceFiberCoupling-IISchematicofatypicalassemblyofcouplingopticsTransmittersemployinga)butt-couplingandb)lens-couplingdesignsTemperatureeffectDepartmentofOpticalEngineeringLaserReliabilityandAgingTraditionalLaserTransmitterApproachesUseatransmissionlineandimpedancematchKeepitcloseanddon’tworryaboutthematchDepartmentofOpticalEngineeringDrivingCircuitsLaserDriverStabilizationAverageandPeakPowerStabilizationAveragePower,MarkDensityandModulationAvarietyoffeedbackapproachesareavailabletocompensateforlaserimperfectionsandtheconsequencesoftemperaturevariationandagingPackagingBosticaet.al.,IEEETransactionsonAdvancedPackaging,Vol.22,No3,August1999DrawingofPackagingApproachOpticalModule(a),Electricalmodule(b)Close-upofassembledmoduleCompletedmoduleintegratedontestboard10Channels12.5Gb/saggregatebandwidth1300nmcommerciallaserarray50/125Multimodefiberribbon130mW/channelCMOSDriverArrayBER<10-141.2kmtransmissionwithnoBERdegradationExampleCommercialTransmitterModulePalomarTechnologiesHomostructurev.s.heterostructureDepartmentofOpticalEngineeringHomostructure:

semiconductorwiththesameenergygapDrawbacks:Diffuseactiveregion(loweff.)Radiatesabroadlightbeam

Heterostructure:

semiconductorwithdifferentenergygapsPurposes:Confinementofelectron-holeinarestrictedregionConductionofradiatedlightinonedirectionTop:Homojunction.Middle:Singleheterojunction.Bottom:DoubleheterojunctionDepartmentofOpticalEngineeringOpticalconfinementindoubleheterostructureTypicalLDpackageDepartmentofOpticalEngineeringLongitudinalModesDepartmentofOpticalEngineeringToformastanding-wavepattern:Foralongcavity,NisabignumberSpacingbetweentwoadjacentlongitudinalmodes

GainbandwidthDepartmentofOpticalEngineeringEmissionspectra↔gainprofileDepartmentofOpticalEngineeringDepartmentofOpticalEngineeringAcloselookDepartmentofOpticalEngineeringConventionalLDQuantum-wellLDDepartmentofOpticalEngineeringAthinactiveregionRecombinationeasierLessforwardcurrentHighopticalgainMoreefficientConditionsforlasingE2-E1<Fc-Fv(populationinversion)g(1/L)ln(1/R)+(netgain)=2nL/p,paninteger(phasecoherence)ReflectivityLongitudinalmodespacingLaserDiodeStructureandOpticalmodesConditionsforcontinuouslasing(steadystate)Netrateofchangeofdensityofconductionbandelectronsiszero(injectionminusrecombinationanddepletion)Netrateofchangeofdensityofphotonscreatediszero(stimulatedemissionminusleakageandspontaneousemission)LaserElectricalModelsSmallsignalmodel(Hitachi)Steady-statelasingconditionsTurn-ondelayTurn-onDelayToreducetheturnondelay:•UsealowthresholdlaserandmakeIplarge•BiasthelaseratorabovethresholdIb=0Ib=0.9IthIb=0.5IthTurnonDelay(ns)RelaxationoscillationDecaysase-t/2,whereandwithafreqency,whereModulationfrequencyDifferencebetweenopticaloutputatmodulationfrequencymandsteady-stateoutputisproportionaltoResonanceFrequencySemiconductorlasersexhibitaninherentsecondorderresponseduetoenergy“sloshing”back-and-forthbetweenexcitedelectronsandphotonsLargeSignalTransientResponseEffectsofcurrentandtemperatureApplyingabiascurrenthasthesameeffectasapplyingapumplaser;electronsarepromotedtoconductionband.FcandFvgetfartherapartaswellIncreasingthetemperaturecreatesapopulationdistributionratherthanasharpcutoffneartheFermilevelsFabryPerotLaserCharacteristics(HitachiOptoDataBook)QuantumefficiencyInternalquantumefficiencyi

：photonsemittedperrecombinationevent,determinedempiricallytobe0.650.05fordiodelasersExternalquantumefficiencyegivenbyTotalquantumefficiency Equaltoemittedopticalpowerdividedbyappliedelectricalpower,orhe/qVForGaAslasers,TQE50%ForInGaAsPlasers,TQE20%ChirpingCurrentmodulationcausesbothintensityandfrequencymodulation(chirp)Astheelectrondensitychangesthegain(imaginarypartofrefractiveindexni)andtherealpartoftherefractiveindex(nr)bothchange.Thesusceptibilityofalasertochirpingischaracterizedbythealphaparameter.1-3isexpectedforonlytheverybestlasers.Chirpinggetsworseathighfrequencies:Relaxationoscillationswillproducelargedp/dtwhichleadstolargechirpingDampingofrelaxationoscillationswillreducechirpCorrectlyadjustingthematerialcompositionandlasermodevolumecanreduce.ReflectionSensitivityR.G.F.Baets,UniversityofGhent,BelgiumProblemSolutionExampleAGaInAsdiodelaserhasthefollowingproperties:Peakwavelength:1.5337mSpacingbetweenpeaks:1.787x10-3

mJ/Jth=1.2Whataretheturn-ondelaytime,thecavitylength,thethresholdelectrondensity,andthethresholdcurrent?Turn-ondelaytime=3.7ln(1.2/1.2-1)=6.63nsCavitylengthL=(1.5337)2/(2)(3.56)(1.787x10-3) =184.9mThresholdelectrondensityR=0.3152g(1/L)ln(1/R)+gth=1/.01849ln(1/.3152)+100=162.4cm-1Fromfigure,N=1.8x1018cm-3ThresholdcurrentJ/2de=I/2deLWIth=(0.5x10-4)(1.6x10-19)(1.8x1018)(.01849)(4x10-4)/(3.7x10-9)Ith=29mALaserDiodeStructuresMostrequiremultiplegrowthstepsThermalcyclingisproblematicforelectronicdevicesDepartmentofOpticalEngineeringVCSEL(vertical-cavitysurfaceemittinglasers)Hottestareaoftransmitter!Shortcavity;2m,adjacentlongitudinalmodespacing=72nmnaturallysingle-modeoperationSmallfootprint;beabletopackagedintodensearraysSmallactiveregion->highcurrentdensity->lowpowerconsumptionHighswitchtime?DepartmentofOpticalEngineeringVCSEL(vertical-cavitysurfaceemittinglasers)Hottestareaoftransmitter!Shortcavity;2m,adjacentlongitudinalmodespacing=72nmnaturallysingle-modeoperationSmallfootprint;beabletopackagedintodensearraysSmallactiveregion->highcurrentdensity->lowpowerconsumptionHighswitchtime?DepartmentofOpticalEngineeringE-OperformanceSpontaneousemission:photonsareemittedinrandomdirectionwithnophaserelationshipamongthem•Stimulatedemission:

initiallybyanphoton,andtheemittedphotonmatchestheoriginalphotonnotonlyinenergybutalsoinothercharacteristics,suchasthedirectionofpropagation.•Allthelasers,includingsemiconductorlasersemitlightthroughtheprocessofstimulatedemission•LEDemitslightthroughthespontaneousemission.DepartmentofOpticalEngineeringTypicalVCSELDepartmentofOpticalEngineeringGainprofileandMMoperationDepartmentofOpticalEngineeringVCSELlifetimeDepartmentofOpticalEngineeringUniformityona3-inwaferAmapofthresholdcurrent(mA)forasingle3-inchwaferDepartmentofOpticalEngineeringDFBLDNarrowspectrallinewidth