Optical Resonators With Whispering-Gallery applications(4)

WGM谐振腔综述

18IEEEJOURNALOFSELECTEDTOPICSINQUANTUMELECTRONICS,VOL.12,NO.1,JANUARY/FEBRUARY2006

canberealizedwithtemperature.Theinsertionlossofthe lterwasatthe5dBlevel.

Tocharacterizetheabsolutetunabilityofanopticalresonator-basedphotonic lter,itisusefultointroducetheratiooftheresonatorFSRandlineartunabilityrangegivenbythehostma-terial.Tuningthe ltersdoesnotchangetheFSRinthe rstapproximation,butonlyshiftsthecomboftheopticalmodes,makingitoverlapwithitselfforeachfrequencyshiftpropor-tionaltotheFSR.Hence,the ltercanbetunedatanyprescribedsinglefrequencyifthelineartunabilityexceedstheFSR.Thelackofselectivityinasingle-resonator ltercanbecompen-satedwiththeapplicationofcoupled-resonator lters.Ifeachresonatorinthe ltercanbetunedbyitsFSR,thewhole l-tercanbetunedatanyfrequency,whereasthespectrumofthemultiresonator ltercanbeveryrareduetotheVerniereffect.Somephotonicapplicationscallfornarrowband lterssimul-taneouslypassingboththecarrierandsidebands.Forexample,thisisimportantforthegenerationofspectrallypuremicrowavesignalsinoptoelectronicoscillators[62],wherebeatingoftheopticalsidebandsandthecarrieronafastphotodiodegeneratesmicrowaves.Tunabilityofthemicrowavefrequencyoftheos-cillatorrequiresthatthefrequencydifferencebetweenthe lterpassbandschangecontrollably.Thispropertyislackinginex-istingtunable lters,wheretheentire lterspectrumshiftsasawholeasthetuningvoltageisapplied.Acriticalcomponentofanovelminiature lterwithelectro-opticallyrecon gurablespectrumwasrecentlyreported.The lterisbasedonaWGRfabricatedfromacommerciallyavailablelithiumniobatewaferhavingaspeciallyengineereddomainstructure[63].D.WGMFiltersinOptoelectronicOscillatorsandLasersforStabilization

1)WGMFiltersforOEO:Generationofspectrallypuresig-nalsat1to100GHzisrequiredincommunications,radar,andnavigation.Theadventofhighthroughputopticalcommunica-tionlinkspointstotheprospectsfornetworksoperatingatdataratesashighas160Gb/sandconsistingofmultiplesofchannelsseparatedbyafewGHz.Schemesforrealizingthistypeofcapa-bilityrelyonsourcescapableofprovidinghighfrequency,lowphasenoisesignals,withoutwhicherror-freehighdataratesys-temswouldnotbepossible.Similarly,highperformanceradarsystemsrequirelowphasenoiseoscillatorstoallowdetectionoffeeblesignalsfromadensebackgroundclutter.

Theoptoelectronicoscillator(OEO)isadevicethatproducesspectrallypuresignalsatmanytensofgigaltertgbasedonpho-tonictechniques,andthusovercomessomeoftheinherentlim-itationsoftheconventionalelectronicdevices[62],[64]–[69].TheOEOisagenericarchitectureconsistingofalaserasthesourceoflightenergy.Thelaserradiationpropagatesthroughamodulatorandanopticalenergystorageelement,suchasanoptical ber,beforeitisconvertedtotheelectricalenergywithafastphotodiode.TheRFelectricalsignalattheoutputofthephotodiodeisampli edand ltered,andthenfedbackintothemodulator,closingtheloop.Ifthetotalgainexceedslinearlosesoftheloop,thesystemoscillatesatthefrequencydeterminedbythe lter.TheuseofopticalstorageelementsallowsfortherealizationofextremelyhighQsandthusspectrallypuresignalsinopti-caloscillators,sincethenoiseperformanceofanoscillatorisdeterminedbytheenergystoragetime,orqualityfactorQ.Inparticular,along berdelayleadstorealizationofmicrosec-ondstoragetimes,correspondingtoQsofaboutamillionata10GHzoscillationfrequency.Thisisahighvaluecomparedtoconventionaldielectricmicrowavecavitiesusedinoscilla-tors[70],[71].The berdelaylinealsoprovidesforwidebandfrequencyoperationunhinderedbytheusualdegradationoftheoscillatorQwithincreasingfrequency.Thus,spectrallypuresignalsatfrequenciesashighas43GHz,limitedonlybythemodulatoranddetectorbandwidth,havebeendemonstrated.InagenericOEO[62],thelong berdelaylinesupportsmanymicrowavemodesimposedonanopticalwave.Anarrowbandelectrical ltershouldbeinsertedintotheelectronicseg-mentoftheOEOfeedbacklooptoachieveastablesinglemodeoperation.Thecenterfrequencyofthis lterdeterminestheop-erationalfrequencyoftheOEO.Whilethisapproachyieldsthedesiredspectrallypurehighfrequencysignals,thephysicalsizeoftheOEOisratherbulkybecauseofthekilometersof berdelayneeded.Moreover,thelong berdelayisverysensitivetothesurroundingenvironmentsotheOEOdoesnotproduceanoutputwithhighlongtermfrequencyaccuracyandstability.TheOEOistypicallyphaselockedtoastablereferenceforlongtermstability.

ThepropertiesoftheOEOwithahigh-QWGRinplaceoftheelectronic lter,aswellasthe berdelay,wasstudiedin[72].Itwasshownthatthemethodallowsonetochoosevirtuallyanarbitraryfrequencyofoscillationbytuningtheresonator.

2)WGMFiltersfortheLaserStabilization:InadditiontothestabilizationoftheOEO,WGRscanbeusedforlaserstabi-lization.Opticalfeedbackfromahigh-Qmicrosphereresonatorwasusedtonarrowthespectrumofaminiaturehigh-coherentdiodelaser,andanearlyhalf-pitchgradient-indexlensservedasacouplingelement[73].Aswasestimatedfromthevariationinfrequency-tuningrange(chirp-reductionfactor),thefastlinewidthofthelaserwasreducedbymorethanthreeordersofmagnitude.

Amodi cationofexternalopticalfeedbackthatincludesaWGRwasusedtonarrowthelineofadiodelaser[74].AWGMofahigh-Qmicrospherewasexcitedbymeansoffrustratedtotalinternalre ection,whilethefeedbackforopticallockingofthelaserwasprovidedbyintracavityRayleighbackscattering.Abeatnoteofthetwolaserdiodesopticallylockedtoapairoforthogonallypolarizedmodesofthesamemicroresonatorhadtheindicated 6aspectralwidthof20kHz,andthestabilityof2×10overaveragingtimesof10s.AtheoreticalmodelforthelaserstabilizationwithaWGRwaspresentedin[75].

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