微波光電子學(xué)

謝康光電信息學(xué)院微波光電子學(xué)

在當(dāng)代社會和經(jīng)濟發(fā)展中，信息容量與日俱增，隨著高容量和高速度信息的發(fā)展，電子學(xué)和微電子學(xué)遇到了一定的困難。光頻在1014—1015Hz，而且激光束的頻寬可窄至103Hz，因而光纖能承載傳送大量的信息。在許多領(lǐng)域中，凡涉及到超大、超精、超微、超功率、超高速及復(fù)雜圖像的有關(guān)應(yīng)用中都常常要求助于光電子技術(shù)。 一方面，隨著頻率升高和帶寬加大，調(diào)制、探測過程中信號頻率已進(jìn)入微波頻率范疇，不得不采用微波技術(shù)來處理光學(xué)問題；另一方面，光電子技術(shù)的發(fā)展使人們可以采用最新的光學(xué)技術(shù)從本質(zhì)上改進(jìn)系統(tǒng)結(jié)構(gòu)，處理微波信號的傳輸與控制。因此研究微波與光波的相互作用十分必要。由于集成光學(xué)和微波集成電路的發(fā)展,以及兩者在半導(dǎo)體材料和工藝方面的兼容性,使得原來各自獨立發(fā)展的光波和微波兩門學(xué)科開始緊密結(jié)合,隨著這兩個領(lǐng)域的交融，一門新興學(xué)科微波光電子學(xué)應(yīng)運而生。微波光電子學(xué)研究光波與微波的相互作用，主要包括光的微波調(diào)制，外差光生微波源，微波信號的探測，微波器件的光學(xué)控制等領(lǐng)域的機理和技術(shù)。微波光電子學(xué)的主要應(yīng)用領(lǐng)域包括光信息處理，微波的光載傳輸，相控陣天線波束光學(xué)實時延遲控制及波束合成。光對微波信號產(chǎn)生、放大與交換的調(diào)控作用，主要是利用光對微波半導(dǎo)體器件有源層中載流子濃度和運動的激發(fā)與控制；微波對光傳輸、折射偏振及信號傳遞的調(diào)控作用則利用導(dǎo)光媒質(zhì)的極化與載流子分布受微波場變化而導(dǎo)致光導(dǎo)率、折射與偏振特性的改變。微波光電子學(xué)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微波光電子學(xué)的主要技術(shù)A.SourceTechnologies

DirectlyModulatedSemiconductorLasers:

ExternalModulators:

HeterodyneSources:

B.DetectionTechnologiesPhotodetectors:

OpticalControlofMicrowaveDevices:

DirectlyModulatedSemiconductorLasers:主要優(yōu)點：simplicity最近進(jìn)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-?thick,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.

微波光電子學(xué)的主要技術(shù)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.馬赫-曾德爾(Mach-Zehnder)(MZ)干涉型調(diào)制器示意圖如下圖所示。由兩個線偏振的調(diào)相波相干合成而實現(xiàn)強度調(diào)制功能。在LiNbO3晶體的襯底上制成Ti擴散分叉條狀波導(dǎo)。條狀波導(dǎo)中間和兩側(cè)制作表面電極。在外加電場的作用下，在分叉的波導(dǎo)中傳輸?shù)膶?dǎo)模由于受到大小相等、符號相反的電場的作用，分別產(chǎn)生

和-

的相位變化。在輸出的第二個分叉匯合處，相干合成的光強將隨相位差的不同而異，從而得到強度調(diào)制。在MZ干涉儀型強度調(diào)制器中，提高調(diào)制深度及降低插入損耗，必須采取以下措施：

(1)分支張角不宜太大（一般為1o左右），因為張角越大，輻射損耗越大。

(2)波導(dǎo)必須設(shè)計成單模，防止高階模被激勵。

(3)波導(dǎo)和電極在結(jié)構(gòu)上應(yīng)嚴(yán)格對稱，使兩個調(diào)相波的固定相位差等于零。用Ti擴散LiNbO3波導(dǎo)制成的MZ干涉型調(diào)制器，其調(diào)制深度可達(dá)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.微波光電子學(xué)的主要技術(shù)A.SourceTechnologies

DirectlyModulatedSemiconductorLasers:

ExternalModulators:

HeterodyneSources:

B.DetectionTechnologiesPhotodetectors:

OpticalControlofMicrowaveDevices:

微波信號的光外差合成

(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.采用一個光學(xué)梳狀頻率發(fā)生器控制兩個光注入鎖定激光器，將激光器的輸出照射在一個單一載流子光二極管上進(jìn)行外差。該方法可以產(chǎn)生輸出功率在毫瓦量級，頻率在10-110GHz范圍可調(diào)的微波。Blockdiagramofopticalmillimeter-wavesynthesissystemTheOFCGemitsanopticalcombwithexactfrequencyspacingfRF.Frommorethan100comblines,eachinjection-lockedlaserselectsonlyone.ThetwolinesarecombinedandarethenfedintoanEDFA.Thedifferencefrequencyisanintegralmultipleofthereferencefrequencyn

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