微波光电子学

谢康光电信息学院微波光电子学

在当代社会和经济发展中，信息容量与日俱增，随着高容量和高速度信息的发展，电子学和微电子学遇到了一定的困难。光频在1014—1015Hz，而且激光束的频宽可窄至103Hz，因而光纤能承载传送大量的信息。在许多领域中，凡涉及到超大、超精、超微、超功率、超高速及复杂图像的有关应用中都常常要求助于光电子技术。 一方面，随着频率升高和带宽加大，调制、探测过程中信号频率已进入微波频率范畴，不得不采用微波技术来处理光学问题；另一方面，光电子技术的发展使人们可以采用最新的光学技术从本质上改进系统结构，处理微波信号的传输与控制。因此研究微波与光波的相互作用十分必要。由于集成光学和微波集成电路的发展,以及两者在半导体材料和工艺方面的兼容性,使得原来各自独立发展的光波和微波两门学科开始紧密结合,随着这两个领域的交融，一门新兴学科微波光电子学应运而生。微波光电子学研究光波与微波的相互作用，主要包括光的微波调制，外差光生微波源，微波信号的探测，微波器件的光学控制等领域的机理和技术。微波光电子学的主要应用领域包括光信息处理，微波的光载传输，相控阵天线波束光学实时延迟控制及波束合成。光对微波信号产生、放大与交换的调控作用，主要是利用光对微波半导体器件有源层中载流子浓度和运动的激发与控制；微波对光传输、折射偏振及信号传递的调控作用则利用导光媒质的极化与载流子分布受微波场变化而导致光导率、折射与偏振特性的改变。微波光电子学早期工作Opticalsourcescapableoffastmodulation;Suitabletransmissionmedia;Fastopticaldetectors;Thedevelopmentofthefirstlasers,includingin1960boththepulsedrubylaseratHughesResearchLaboratoriesandthecontinuouslyoperatingheliumneonlaseratBellLaboratories,canbesaidtohavestartedtheopticalcommunicationsera.Theimportantissueofhowtomodulatetheoutputofthesesourcesathighratesbecamethesubjectofintenseactivity.Electroopticmodulatorsachievedfrequenciesashighas11GHzbytheearly1970s.Opticalsourcescapableoffastmodulation;Greatercompactnesswasofferedbythesemiconductorlaser,andwiththedevelopmentofdoubleheterostructuredevicescapableofroom-temperaturecontinuousoperationin1970,thisbecamethepreferredsourceforopticalcommunication.Afurtheradvantageofthesemiconductorlaserwasitscapabilityfordirectmodulationviatheinjectedcurrent,andmicrowavebandwidthsweresoonrealized.Earlyplanswerebasedonfree-spaceopticsandgaslenses.Followingrealizationoflow-losstransmissioninsilicaopticalfiber,thisrapidlybecamethepreferredtransmissionmedium.Multimodefiberoperatingatawavelengthof850nm

single-modefiber,lowerdispersionat1300nmandlowlossat1550nm.2.Suitabletransmissionmedia;Fordetection,fastdepletionandavalanchedetectorsweredevelopedatanearlystageandsubsequentlydevelopedtogiveusefulmicrowavebandwidthresponse.3.Fastopticaldetectors微波光电子学的主要技术A.SourceTechnologies

DirectlyModulatedSemiconductorLasers:

ExternalModulators:

HeterodyneSources:

B.DetectionTechnologiesPhotodetectors:

OpticalControlofMicrowaveDevices:

DirectlyModulatedSemiconductorLasers:主要优点：simplicity最近进展：reducingelectricalparasiticsoflaserstructures；optimizinglaserparametersforhigh-speedoperation.Limitationstohigh-speedoperationofsemiconductorlaserscanbecategorizedintwogroupsConsiderationsotherthantheactivelayercanbecollectedinonegroup,including:thedesignoftheopticalcavity,reductionoftheparasitics,efficientremovaloftheexcessheat.Intrinsicpropertiesoftheactivelayer,whichpresenttheultimatelimitonthespeedofoperationandhasledtoutilizationofquantumwells(QW’s).Usingrateequations,small-signalmodulationresponseofadirectlymodulatedsemiconductorlasercanbeexpressedaswhereAisanamplitudefactor,

istheangularmodulationfrequency,

pistheangularrelaxationresonancefrequency,and

isthedampingfactor.Thedampingfactorandresonancefrequencyarerelated:

=K

p2+

0Fromlowtohighfrequencies,theresponsestartsoutflat,sharplypeaksaround

p,anddropsratherfastforfrequenciesextendingbeyond

p.Theamountofpeakingdependsontherelativemagnitudeof

.As

pincreases,sodoes

,andtheresponseflattensoutduetoexcessivedamping.Small-signalCWmodulationresponseofalaseratvariousbiascurrents(10,20,40,60,80,and100mA)Modulationbandwidthislimitedbythephoton–electronresonancefrequency

p,abovewhichtheundampeddetectedelectricalresponsefallsas1/4.pcanbeapproximatedbyFasterresponserequireshigh

p：reducingthephotonlifetime

p

shortopticalcavity,reducedfacetreflectivityincreasingthedifferentialgaing0

reduceddimensionalityincreasingthephotondensitiesS0

smallcavityvolumesorshortandnarrowopticalwaveguides

PhotondensitiesS0:High-speedlasersrequireshortandnarrowcavitieswithstrongopticalconfinementStrongindexguidingintheopticalwaveguideisimportanttoconfinetheopticalmodetoanarrowregion.Ridgewaveguidelasers,inwhichthewaveguideissurroundedeitherbyairorlowdielectricconstantdielectric,arecommonlyused.PhotondensitiesS0:Thelaseroperatesinthefundamentalspatialmode,nottopartitionthephotondensityamonghigherordermodesandreducetheopticalconfinement.TypicallengthsoflasersbasedontheInGaAssystemareinthe100–150-

mrangeduetocomparativelyhighermaterialgain.ForInGaAsP-basedlasers,lengthsareoftheorderof400mduetolowermaterialgain.

Highdifferentialgaing0:HighdifferentialgainistypicallyobtainedusingQW’sandstrain.Comparedtoabulkmaterial,carrierdensityrisesverysharplywithcarrierinjectionintoaQWbecauseofmodifiedstep-likedensityofstates.Materialdesignisveryimportant.Gaincompressionfactor: animportantlimitationwhichhasthusfarlimitedreliable1.55-

mroom-temperatureoperationlaserstobandwidthslessthan30GHzdespitemuchresearcheffort.Twoprocessescontributetothegaincompression:ThespectralholeburningThecarrierheating.ThespectralholeburningCausedbythedepletionofcarriersatandaroundthelasingenergy.Ascarriersaredepleted,gainisreduced,whichdampenstheresponseofthelaser.Carriersconsumedbylasingaresuppliedbyinjectedcarriersrelaxingfromtheirinjectionenergiesbyintrabandrelaxationprocesses.Theserelaxationtimescanbereducedbyincreasingthecarriertocarrierscatteringrates,whichcanbeachievedbypdopingtheactivelayer.CarrierheatingCarriersatthebandedgeareconstantlyremovedduetostimulatedemission.Theremainingdistributionhasaneffectivetemperaturehigherthanthelatticetemperature,whichreducesthedifferentialgain.Carrierheatingishigherforstraineddevicesduetoincreasedvalencebandcurvature.Adevicedesignedusingtheaboveprinciples:Four5.7-nm-wideIn0.35Ga0.65AsQW’sseparatedby20.1-nmundoped

GaAsbarriers,itsdeepQW’sprovidesstrongconfinementandsuppressesthecarrierescape.Undoped

GaAsconfinementlayersonbothsidesoftheMQWregionareonly400-Åthick,thinenoughtominimizecarrier-transport-relatedproblems.TheupperandlowercladdinglayersareAl0.8Ga0.2As,creatingstrongopticalconfinement.Thequalityofthematerialwasimprovedoverpreviousdesigns.Thedevicewasashort-ridgewaveguidelaseroperatingat1.1m.

pincreaseswithI.AtlowI,bandwidthislimitedbyp.AsIincrease,dampingstartstolimitthebandwidth.Atabiascurrentof155mA,the3-dBmodulationbandwidthexceeds40GHzandislimitedbythedamping.Thesmall-signalCWmodulationresponseofaridgewaveguidelaseratabiascurrentof155mA.Anotherveryimportantconcernforlinkdesignistherelativeintensitynoise(RIN)ofthelaser.RINdirectlyaffectsthenoisefigureofthelink.TypicalRINvaluesofhigh-speedlasersareinthe125–150-dB/Hzrange.Distributedfeedback(DFB)deviceshavelowerRINvaluesthanFabry–Perotdevices.ThetypicalRINspectrumofasemiconductorlaserresemblesitsfrequencyresponse:peakingat

plevelingoffoneithersideThelow-noisefrequencyrangeislessthanitsbandwidthsincetheresponseislimitedatfrequencieshigherthan

pnoiseisenhancedcloseto

pSignal-to-noiseratio(SNR)attheoutputofalinkcanbeexpressedasSNR=m2/(2

RIN

BW),whereBWisthesystembandwidth,andmisthemodulationindex.Improvementsin

preduceRINandconsequentlyincreaseSNRofthelink.Activeresearchareaatthecurrenttimeenhancingmodulationbandwidths.improvingtheslopeefficiency.Typicalslopeefficiencyorexternaldifferentialquantumefficiencyofhigh-speedlasersisinthe0.1–0.3W/Arange.Asignificantlimitationtoslopeefficiencyisthecouplinglossbetweenthelaserchipandsingle-modefiber.Recentdevelopmentsinhigh-speedlasersresultedinsmall-signalbandwidthsexceeding40and25GHzat1.1-and1.55-

mwavelengths,respectively.FurtherimprovementsintheslopeefficiencyandRINoftheselasersmaymakethemattractivecandidatesfordirectlymodulatedlinks.

微波光电子学的主要技术A.SourceTechnologies

DirectlyModulatedSemiconductorLasers:

ExternalModulators:

HeterodyneSources:

B.DetectionTechnologiesPhotodetectors:

OpticalControlofMicrowaveDevices:

ExternalModulators:ThemostcommonmaterialsforexternalmodulatorsareLiNbO3,III–Vcompoundsemiconductors,andelectroopticpolymers.LiNbO3andIII–Vcompoundsemiconductorsoffermaturematerialtechnology.Polymersoffersignificantpotential,butmaterialtechnologyisstillundergoingdevelopment.Atthecurrenttime,thereisasignificantamountofresearchefforttodevelophigh-performanceelectroopticpolymers.Modulatorsclassifiedintolumpedelementsmodulatorstraveling-wavemodulatorsModulatorsrealizedthroughtheelectro-absorptionmechanismtheinterferometricmechanismThemodulatedcomponentoftheopticalpoweroutputcanbewrittenasTheamountofmodulatedopticalpowerforagivenmodulatingsignalcanbeincreasedbyincreasingtheopticalinputpower.Microwavephotoniclinkscoulddisplaygainwithouttheuseofelectricalamplification.A.LumpedModulatorsElectro-absorptionmodulatorsoperatebyconvertingtheincidentlightintophoto-currentintheirabsorbingstate.WaveguidemodulatorsusingtheFranz–KeldysheffectinbulksemiconductormaterialsorthequantumconfinedStarkeffectinquantum-wellmaterials.ThequantumconfinedStarkeffect(QCSE)InaQW,electronsandholesareconfinedinthesamephysicalQW.Overlappingandinteractingstronglyandformabondcalledanexciton.Hasastrongabsorptionsomewhatsimilartoanatomicabsorption.Spectraofabsorptionisverysharp,andislocalizedinthevicinityofwavelengthscorrespondingtothebandgapoftheQW.Whenanexternalelectricfieldisapplied,electronandholeareforcedtooppositeendsoftheQW

physicallyseparated.Spatialoverlapoftheelectronandtheholeisreduced

excitonicabsorptionisdecreasedandbroadened.possibletomodulatetheabsorptionverystronglywithexternalfieldsaroundanarrowwavelengthrange

knownastheQCSE.EmbeddingsuchQW’sinawaveguide applyinganelectricfieldchangingtheabsorptionoftheQW’sthroughtheQCSEmodulatingtheinsertionlossofthewaveguideTypically,MQW’sareusedtoincreaseabsorptionandareembeddedintheiregionofareversebiasedp-i-ndiode.ThephotocurrentspectraofanunstrainedMQWmaterialasafunctionofwavelengthatdifferentappliedvoltages.Twopeaksareresolvedintheabsorptionspectra,duetoexcitonsformedbetweenelectronsandhh’sandelectronsandlh’s.Transitionenergiesofhhandlh

excitonsaredifferent.Thehh

excitonsinteractwithTEpolarizedlightandlh

excitonsinteractwithbothTEandTMpolarizedlight.Asbiasvoltageincreases,absorptioncharacteristicsbroadenandpeakabsorptiondecreasesandmovestowardlongerwavelengths.At1.55-mabsorptionismodulatedstronglywhenbiaschangesbetween0and3V.Possibletomakeaverysimplemodulator,whichisaveryshortwaveguide.ThetransmissionthroughsuchamodulatorasafunctionofappliedvoltagecanbeexpressedasTheon/offratioofanEAmodulatorindecibelscanbeexpressedasOpticalpropagationlossofEAmodulatorsislarge,typicallyinthe15–20-dB/mmrange.Maincomponentsofthislossarethefreecarrierabsorption,especiallyintheplayers,andband-to-bandabsorption.Thesecondlosscomponentcanbemadesmallerbyincreasingtheseparationbetweenthewavelengthofoperationandtheabsorptionpeak,whichiscalleddetuning.Typicaldetuningvaluesareabout20–50nm.Typical

/

valuesareinthe3–10range.

Largeon/offratiodevicescanbeobtainedusinglongdevices,butthatalsoincreasestheinsertionloss.ForthetypicalEAmodulatorlengthsin50–300-

mrangepropagationlossis1–3dB.Togetlargeextinctionratioswithlowdeviceinsertionloss,

/

shouldbemaximized.

Fiber-to-fiberinsertionlossofaEAmodulatorasafunctionofexternalbiasatdifferentwavelengths.QCSEismostpronouncedforphotonenergiesnearthebandgapofthematerialandshowsastrongwavelengthdependence.Atshorterwavelengths,modulationbecomesmoreefficient,butinsertionlossalsoincreases.ForalumpedelementthespeedofoperationislimitedbytheRCtimeconstantofthecircuit.EAmodulatorsareveryshortdevicesand,hence,havesmalldevicecapacitance.Typically,a2.5-

m-wideand150-

m-longdevicehasacapacitanceofabout0.33pF,whichislowenoughfor20-GHzbandwidth.Bulkmodulatorsat1.55

mhaveachieved-3-dBelectricalbandwidthsof50GHzwith4.5-Vdrive20-dBextinction8dBfiber-to-fiberinsertionlossToobtainsufficientlylowcapacitanceforsuchhigh-speedoperation,theactivesectionofthewaveguidemustbekeptveryshort,50

minthisexample,limitingthemodulationsensitivity.

Measuredfrequencyresponseofa1.55-

mmodulator,showinga3-dBelectricalbandwidthof50GHz.Opticalpower-handlingcapabilityForlargeopticalpower-handling,photo-generatedcarriersshouldescapefromtheQW’sandshouldbeeasilycollectedbytheohmiccontacts.ThecarrierpileupscreenstheelectricfieldinsidetheQW.Thedecreasedfieldfurtherinhibitsthecarrierescapeandenhancescarrierpileup.Theendeffectisthesaturationoftheabsorptionanddegradationinthemodulationresponse.Materialdesignswithlowerhhmassandlowervalencebanddiscontinuity.Tensilestrainreducesthehhmassandbarrierheightsforelectronsandhh’s

reductionofphoto-generatedcarriersweepouttimes

decreasingthecarrierpileupinthewells.OpticalsaturationperformanceoftheEAmodulatorimproves.Anattractivefeatureofelectro-absorptionmodulatorsisthattheycanbeintegratedwithsemiconductorlaserstoformcompactopticalsourcescapableofultrafastmodulation.CurrentdeviceresearchonEAmodulatorsisonreducingthedrivevoltage,whileincreasingtheon/offratioandthebandwidth.Forlumpedoperation,widebandwidthrequiresashortdevice,whereasforashortdevice,on/offratioislowandoperationvoltageishigh.Inoneapproach,adouble-passmodulatorwasproposedanddemonstrated.Devicecapacitanceremainsunchanged,butabsorptionlengthisdoubled.AnotherrecentresearchdirectionistousetheEAmodulatorasatraveling-wavedevice.马赫-曾德尔(Mach-Zehnder)(MZ)干涉型调制器示意图如下图所示。由两个线偏振的调相波相干合成而实现强度调制功能。在LiNbO3晶体的衬底上制成Ti扩散分叉条状波导。条状波导中间和两侧制作表面电极。在外加电场的作用下，在分叉的波导中传输的导模由于受到大小相等、符号相反的电场的作用，分别产生

和-

的相位变化。在输出的第二个分叉汇合处，相干合成的光强将随相位差的不同而异，从而得到强度调制。在MZ干涉仪型强度调制器中，提高调制深度及降低插入损耗，必须采取以下措施：

(1)分支张角不宜太大（一般为1o左右），因为张角越大，辐射损耗越大。

(2)波导必须设计成单模，防止高阶模被激励。

(3)波导和电极在结构上应严格对称，使两个调相波的固定相位差等于零。用Ti扩散LiNbO3波导制成的MZ干涉型调制器，其调制深度可达80％，功耗35

W/MHz左右。Interferometricmodulatorsusinglithium–niobatetechnologieshavebeenrealizedwith-3-dBelectricalbandwidthofover70GHzmodulatorlength2-cmanextinctionvoltageV

of5.1V.Fiber-to-fiberinsertionlosswas5.6dB.FortheGaAssystem-3-dBelectricalbandwidths50GHzV

of13Vfora1-cm-longmodulatorThesmallsizeoftheopticalguidesinGaAsleadstosignificantfiber-to-modulatorcouplinglossessothatthefiber-to-fiberlossforsuchmodulatorsisoforder10dB.

ThegainofanexternallymodulatedlinkusingaMach–Zehnder-typemodulatorisproportionalto(tP/V

)2Ptheinputopticalpowertisthefiber-to-fiberinsertionlossV

theon/offvoltageofthemodulator.LowV

,capabilityofhandlinglargeamountsofopticalpowerandlowfiber-to-fiberinsertionlossareessentialtogethighgainlinks.ElectroopticpolymermodulatorsOperationat110GHzhasbeendemonstrated.Problemsofopticalpowerhandling,stability,andhigh-temperatureoperationarebeingovercome

makingpolymertechnologyoneofconsiderableinterest.B.Traveling-waveModulatorsAnapproachtoobtainverywidebandwidthmodulators.Theelectrodeisdesignedasatransmissionline.ElectrodecapacitanceisdistributedanddoesnotlimitthemodulatorspeedduetoRCtime-constantlimitations.Themodulatingelectricalsignalontheelectrodetravelsinthesamedirectionasthemodulatedopticalsignal.Whentravelwiththesamevelocity,thephasechangeinducedbytheelectricalsignalisintegratedalongthelengthoftheelectrode.Theelectrodecanbemadeverylong,typicallythousandsofwavelengths.Longelectrodeallowsaverysmallphasechangeoverawavelengthtoaccumulatetoanappreciablevalue.Drivevoltagerequirementsissignificantlyrelaxed,withoutsacrificingelectricalbandwidth.Thepropertiesoftheelectrodedeterminethemainpropertiesofthemodulatorsuchasbandwidthanddrivevoltage.Thisapproachismostefficientifthegroupvelocitiesoftheelectricalandopticalsignalsarematched.characteristicimpedanceoftheelectrode(almostuniversally50)ismatchedtothatofthedriver.Iftheterminationisdifferentthanthecharacteristicimpedance,afrequencydependentstanding-wavepatternwillbegeneratedontheelectrode.Thesmall-signalmodulationresponseofatraveling-wavemodulatorwithacharacteristicimpedancematchedtoboththedriverandloadimpedanceisgivenasAlow-loss,velocity-andimpedance-matchedelectrodeisessentialfortherealizationofaverywide-bandwidthtraveling-wavemodulator.Toreducethedrivevoltageofthemodulatorarequirementfortheelectrodeistogenerateastrongelectricfieldoverlappingwellwiththeopticalmodeanddirectedinacertaindirectiondictatedbytheelectroopticmaterial.1)LiNbO3Traveling-WaveModulators:LiNbO3traveling-wavemodulatorshavethemostmaturetechnology.TheopticalstructureisaMach–Zehnderinterferometer.ThemicrowavestructureisaCPWtransmissionline.AschematicofatypicalLiNbO3traveling-wavemodulatorTheeffectivemicrowaveindexofaCPWonLiNbO3islargerthan4.TheeffectiveindexofanopticalmodeinaTiindiffusedLiNbO3opticalwaveguideisabout2.15.Anelectricalsignalappliedtotheelectrodewilltravelslowerthantheopticalwave.VelocitymatchinginLiNbO3modulatorsrequiresincreasingthevelocityofpropagationofthemicrowaveontheelectrode.ThemostcommonwayofachievingvelocitymatchingistouseaSiO2bufferlayerundertheelectrodeandtoincreasethethicknessoftheconductors.Atraveling-wavemodulatorwith-3-dBopticalbandwidthexceeding110GHz,employingridgestructure,

hasbeenrecentlydemonstrated.modulatorlengthL=2cmV

is5.1V.Ifthelengthofthesamedeviceisincreasedto3cm,V

decreasesto3.5V,theopticalbandwidthsdecreaseto45GHz,Traveling-waveLiNbO3modulatorsofferstabledevicesthatcanhandlelargeopticalpowers.Suchmodulatorswithfiber-to-fiberinsertionlossaround5dB.V

reductionisbeingpursued.

2)GaAs/AlGaAsTraveling-WaveModulators:III–VcompoundsemiconductorssuchasGaAs,InP,andtheiralloyspossesselectroopticcoefficientThemostcommonlyusedopticalstructureisaMach–Zehnderinterferometer.OnewayofmakingopticalwaveguidesinIII–Vcompoundsemiconductorsistoadjusttheindexofrefractionbycontrollingthecompositionoftheiralloys.IncreasingAlcompositionxinAlxGa1-xAscompoundsemiconductordecreasesitsindexofrefraction.AlxGa1-xAsislatticematchedtoGaAsforallxvalues.BygrowingAlxGa1-xAslayersepitaxially,ahigherindexmaterialissandwichedbetweentwolowerindexmaterials,provideswaveguidingintheverticaldirection.Aribisetchedtoprovidelateralwaveguiding.Theeffectiveindexundertheribishigherthantheeffectiveindexoutsidetherib,providingalateralindexstepandatwo-dimensionalwaveguideisformed.ThecrystalstructureofIII–VmaterialsisZincBlende.Theelectroopticcoefficientis1.4pm/V,about20timeslessthanthatofLiNbO3.Netindexchangeforagivenelectricfieldisonlyaboutfivetimeslessduetohigherindexofrefractionofthesemiconductor.Theopticalindexofrefractionisaround3.4.Themicrowaveindexisabout2.65.Velocitymatchingrequiresslowingdownofthemicrowavesignal.Themostcommonlyusedtechniquetoslowthemicrowavesignalistouseaslow-wavetransmissionline.Suchlinesareobtainedbyperiodicallyloadingauniformtransmissionline.Thesmall-signalresponseofaGaAs/AlGaAstraveling-wavemodulatorsTheelectricalbandwidthofaGaAs/AlGaAstraveling-wavemodulatorsat1.55

misinexcessof40GHz.Flatupto20GHzandstartstorolloffgraduallyandbecomesabout1.5–2dBdownat40GHz.Extrapolatingthecurvefitthebandwidthwasestimatedtobebetween50to60GHz.GaAsmodulatorscanhandleverylargeamountsofopticalpowerssincetheyareverysimilartosemiconductorlasersthatgenerateveryhighopticalpowers.Practicallimittothepower-handlingcapabilityisthecatastrophicfacetdamage.GaAsmodulatorsalsoofferstableoperation.V

valuesarearound15VFiber-to-fiberinsertionlossisinthe10–15-dBrange.Effortstoreducethedrivevoltageandfiber-to-fiberinsertionusingnovelprocessingtechniquesareunderway.

3)PolymerTraveling-WaveModulators:Organicpolymershavemanyattractivefeaturesforintegratedopticalapplications.possibletoformmultilayerpolymerstacks.canbepatternedusingseveraldifferenttechniques.canbemadeelectroopticthroughhigh-temperaturepoling.Organicpolymerspresentgoodopticalpropertieslowpropagationlosslowindexofrefractionveryclosetothatofthesingle-modefiberThesepropertiesresultedinpassivelow-losspolymeropticalwaveguidesthatcancoupletosingle-modeopticalfibersveryefficiently.Electroopticcoefficientr33valuesrangebetween1–20pm/V.Possibletogetverygoodvelocitymatchingusingamicrostripelectrode.PolymermodulatorsoperatingatW-band(75–110GHz)havebeenreported,V

valuesarearound10Vandfiber-to-fiberinsertionlossisabout10dB.Polymermodulatorsofferhigh-frequencyresponseandflexible、low-costtechnology.Atthecurrenttimetheelectroopticpolymerisaveryintenseresearcharea.toimprovematerialstabilityandelectroopticpropertiesofpolymers.theeliminationofhigh-temperatureelectric-fieldpoling.Biasstabilityandpower-handlingcapabilityissuesarealsounderexamination.4).Traveling-WaveEAModulators:ThespecialstructureofanEAmodulatorpresentsinterestingissues.TheopticalpropagationlossoftheEAdeviceisratherhigh,increasingthelengthoverafewhundredmicrometersintroducesexcessiveloss.Largedevicecapacitanceperunitlengthmakesitdifficulttomakea50-

transmissionlinewithmatchedvelocity.ForTW-EAmodulatorelectrodes,measuredmicrowavelosscoefficientsofabout60–80dB/cmat40GHzwerereported.Rathershortdevicehastobeused,velocitymatchingisnotanissueuptofrequencieswellintothesubmillimeter-waverange.Thebandwidthistypicallylimitedbythemicrowaveloss.A200-

m-longTW-EAdevicewithabandwidthover64GHzwasreported.Thedevicewaspolarizationinsensitiveandoperatedat1.55

m.For20-dBon/offratio,drivevoltagewas3V.Comparedwithlumpedoperation(bandwidthsof50GHzwith4.5-Vdrive)drivevoltageandbandwidthisimprovedusingTW-EAmodulatorapproach.Inalltraveling-wavedesigns,broad-bandlinearizationofthetransfercharacteristicsandpolarization-independentoperationremainaschallengesfordeviceresearchers.Linearityisanimportantrequirementforanaloglinks.Allmodulatorshavenonlineartransferfunctions.Thedeviceisbiasedatspecificpointsontheelectrical-to-opticaltransferfunctiontoassurelinearity.Thestabilityofthebiaspointarealsoveryimportant.微波光电子学的主要技术A.SourceTechnologies

DirectlyModulatedSemiconductorLasers:

ExternalModulators:

HeterodyneSources:

B.DetectionTechnologiesPhotodetectors:

OpticalControlofMicrowaveDevices:

微波信号的光外差合成

(HeterodyneSources):Theuseofopticalgenerationofmicrowavesignalshasattractedconsiderableattentions.Amethodofproducingthedesiredmicrowavesignalistheheterodynemixingoftwoopticalcarriers.Considertwomonochromaticopticalsourcesemittingatfrequencies

1and2,where|1-2|<<1,2:theiropticalfieldsareoverlappedwithcommonpolarizationilluminateaphotodetectorofresponsivityRtheresultingphotocurrentisgivenbyNotethetermatthedifferencefrequencybetweenthetwosources.Lasersforthirdwindowopticalcommunicationstypicallyemitatfrequenciesoforder200THzSlightdetuningofthesourcesenablesfrequencieslimitedonlybythephotodetectorbandwidthtobegenerated.Thefree-runninglinewidthofsemiconductorlasersislarge(typically1–50MHz)Thetemperatureandcurrentdependenceoftheiremissionfrequencyisstrong(typically10GHz/Kand1GHz/mA,respectively)Theapplicationofspecialcontroltechniquesisrequiredtoobtainaspectrallypuremicrowaveheterodynesignal.Injectionlockingtwosemiconductorslavelaserstodifferentfrequencymodulationsidebandsofasemiconductormasterlasercouldcorrelatingthephasenoiseoftheslavelasers.Heterodynefrequencyof35GHzwithlinewidthslessthan10Hzwerereported.Injectionlockingtospectrallinesfromanopticalcombgeneratorhasbeenusedtogeneratefrequenciesupto110GHz.采用一个光学梳状频率发生器控制两个光注入锁定激光器，将激光器的输出照射在一个单一载流子光二极管上进行外差。该方法可以产生输出功率在毫瓦量级，频率在10-110GHz范围可调的微波。Blockdiagramofopticalmillimeter-wavesynthesissystemTheOFCGemitsanopticalcombwithexactfrequencyspacingfRF.Frommorethan100comblines,eachinjection-lockedlaserselectsonlyone.ThetwolinesarecombinedandarethenfedintoanEDFA.Thedifferencefrequencyisanintegralmultipleofthereferencefrequencyn

f