光纤通信系统课件：第六讲 半导体激光二极管

第6章

半导体激光二极管

LaserDiode(LD)Contents

LaserPrinciplesLaserDiodes(LD)

LDOperatingCharacteristicsNarrow-linewidthandTunableLDLaserLightAmplificationbyStimulatedEmissionofRadiationSection6.3LASERPRINCIPLES=filledstate=formerlyfilledstateLowerLevelHigherLeveloriginalphotonnewphoton

OpticalStimulatedEmissionAlaserneedsthiscondition,referredtoaspopulationinversion.LaserBecauseoftheneedforamplificationbeyondanyattenuationthatoccurs,thelaserwillnotlaseuntiltheappliedpowerisbeyondathresholdvalue.Inotherwords,notuntilthegainishighenough.One(orboth)ofthemirrorsispartiallytransmittingtoallowsomeoftheopticalpowertoescapefromthecavity.Longitudinalmodes:1500MHzffffGainCavityResonancesOutputSpectrumShownarethethreelongitudinalmodesofthelaser.Thesingle-transverse-modeoutputhasaGaussianbeamintensitypattern.0.1351rI/I0w=10mm0.5Transversemodes:Section6.4LaserDiode10m~0.1-0.3m300mJunctionRegionnpMetalStripeContactLaserDiodeMetalVToproduceoscillationweneed:

Feedback

Amplification

TuningFeedback

Cleavethefrontandbackfacesofthecrystal.Thisprovidesmirror-likereflections.Example:AlGaAs,n=3.6Amplification

Wg---------------------------------+++++++++++++++++++++HoleEnergyElectronEnergyOriginalPhotonFreeElectronsFreeHolespStimulatedPhotonnIncomingPhotonForwardBiasedpnJunctionTuningThebandgapenergydeterminestheoutputwavelength.Previously,wefound(6.2)whereisinmandWgisineV.Theoutputspectrumisarangeofwavelengths,notasinglewavelength,becausetheenergytransitionoccursbetweenbandsofenergy(notdiscretelinesasinagaslaser).-------------------

--------------+++++++++++++++++++++WgThecavityalsoaffectstheoutputspectrum.Example:Thecavityresonantwavelengthspacingis(3.26)(3.25)Assume:

0=0.82m,L=300m,n=3.6 =2nm(laserlinewidth)ThenThenumberoflongitudinalmodesisapproximatelyInthisexample:Weconcludethattherearesixlongitudinalmodes.819 820 821Gain0.311nm(nm)(nm)CavityResonancescForthelaserdiode,wehave:(nm)cOutputSpectrumHomojunction:Apnjunctionformedwithasinglesemiconductor.Forexample:GaAsorAlGaAs.Heterojunction:Apnjunctionformedwithdifferentsemiconductors.++++++++++Considerthefollowingheterojunctionlaserdiode:ElectronEnergynnp1.8eV1.55eV1.8eV----------------------------AlxGa1-xAsAlyGa1-yAsAlxGa1-xAsElectronBarrierHoleBarrierInjectedHolesInjectedElectronsSection6.5LaserDiodeOperatingCharacteristicsP(mW)50I(mA)ITH=thresholdcurrentExample:ITH=75mADiodeVoltages1.2-2Volts

ActualIdealITH100Onlywhenthegainexceedsthelosses,oscillationoccurs.OpticalPowerOpticalPower111100InputCurrentorSignalttDigitalModulationisIdcITHOpticalPowerOpticalPowerInputCurrent(Signal)AnalogModulationtPdcIdciPspIspITHTemperatureDependenceP(mW)50i(mA)30°C80°C70100Thethresholdcurrentincreaseswithtemperature.Thermalgenerationofholesinthenmaterialandelectronsinthepmaterialproduceunwantedrecombinationsoutsidetheactivelayer.

Thisreducesthegain,andthusincreasesthethresholdcurrent.Thisingenerallowerstheoutputpower.Thelaserdiodewavelengthchangeswithtemperaturebecausethediodematerial’sindexofrefractionistemperaturedependent.Thus,thecavityresonant-frequencychangeswithtemperature.Recallthecavityresonancefrequencies:(3.24)Example:WavelengthShift=0.1nm/oCT1=27°CGainCavityResonanceOutputCavityResonanceOutputT2=30°CLaserDiodeBandwidthTypicallaserdioderisetimestr:0.1-1ns

Bandwidth f3-dB=0.35/trExample:Lettr=0.1ns!!!

Lightfromthelaserdiodecanbemodulatedatratesover40GHz(40Gb/s).SpectralWidthMultimode(longitudinal)laserdiodes: Dlistypically1to5nmSingle-mode(longitudinal)laserdiodes: Dlistypically0.1to0.2nmHalf-PowerBeamwidths

||=10Parallel=35PerpendicularRadiationPattern||7m0.2mRecallthedivergenceofaGaussianbeam:(2.17)Ingeneral,thesmallerthespotsize,thelargerthebeamdivergence.Section6.6Narrow-Spectral-WidthandTunableLaserDiodesInthissectionwediscussthedistributedfeedback(DFB)laserdiode,whichproducesanarrowlinewidthandthedistributedBraggreflector(DBR)laserdiode,whichproducestunablewavelength.pnDistributedFeedback(DFB)LaserDiodeTheDFBwasdevelopedtoobtainasingle-longitudinal-modelaserdiode.DifferentMaterialsGratingThegratingactsasafilter,permittingonlyoneofthecavity’smodestopropagate.CleavedFacetActiveLayerMetallizationLaserGainCavityResonancesGratingResonancesLaserOutput0DISTRIBUTED-FEEDBACK(DFB)LDThegratingresonances,accordingtoBragg’slaw,arethosewavelengthsforwhichthegratingperiodL(illustratedonaprecedingslide)isanintegralnumberofhalf-wavelengths.Thatis:(6.10)isthewavelengthinthediode,misaninteger0isthefree-spacewavelengthThegratingperiodthensatisfiesandtheresonantwavelengths,asmeasuredinfreespace,aregivenby:(6.11)Example:ConsideranInGaAsPDFBLD 0=1.55m,n=3.5,letm=1(firstorder)Determinethegratingperiod.Letm=2(secondorder)TunableDistributedBraggReflector(DBR)LaserDiodeThegain,phase,andreflectorregionsareseparateasindicated.Theactiveregionisinblue.Eachregionhasitsindependentdrivecurrent.pnIPIBIGGAINPHASEBRAGGCLEAVEDFACETTunableDBRLaserDiodesGaincurrentIG:Determinestheamplificationintheactivelayerasitwouldinaconventionallaserdiode.PhasecurrentIP:ControlsthephaseofthewavereflectedfromtheBraggregionbyheatingthephaselayerwhichchangesitrefractiveindex.

BraggcurrentIB:Controlsthegratingperiod(L)bychangingthetemperature(andthustherefractiveindex)intheBr