Anall-opticalswitchingtechniqueutilizingasilicamicro-sphereopticalresonatorcoatedbyaconjugatedpolymerwasdevelopedin[51].A250-µm-diametersilicamicrospherewascoatedbydippingintoatoluenesolutionofthepolymer.WGMresonantfrequencyshiftsaslargeas3.2GHzwereobservedwhen405nmpumplightwithapowerdensityontheorderof10W/cm2wasincidentonthemicrosphere.Thetimeconstantoftheobservedfrequencyshiftswasapproximately0.165s,lead-ingustoattributethefrequencyshifttothermo-opticeffects.SuchasystemiscapableofswitchingtheWGMresonantfre-quencyhaving2MHzlinewidthatspeedsontheorderof100ms.Finally,opticalmemoryelementsweredevelopedusingWGMdevices.Amemoryelementconstructedbyinterconnect-ingWGMmicroscopiclaserswasdemonstratedin[259].Thedeviceswitcheswithin20pswith5.5-fJopticalswitchingen-ergy.Ontheotherhand,itwasshowntheoreticallyanddemon-stratedexperimentallythatarandomdistributionofsphericalmicroparticlesmaybeusedasaspectralholeburningmem-ory[122],[123].
2)WGMModulators:Microwavecellularphonesystemsandpersonaldataassistantnetworksrequiredevicescapableofreceiving,transforming,andprocessingsignalsinmillime-terwavelengthdomain[260].Electroopticmodulatorsbasedonelectromagneticwaveinteractioninnonlinearopticalcavitieswithhigh-QWGMswillplayanenablingrolefortheseandsimilarapplications.
Anapproachtocreatecouplingbetweenlightandami-crowave eldinaWGRwasrecentlyproposed[80],[81].Inthatstudy,anef cientresonantinteractionofseveralopticalWGMsandamicrowavemodewasachievedbyengineeringtheshapeofamicrowaveresonatorcoupledtoamicrotoroidalopticalcavity.Basedonthisinteraction,anewkindofelectro-opticmodulator,aswellasphotonicmicrowavereceiver,wassuggestedandrealized[261]–[268].D.OptoelectronicElectronicOscillator
Besidesthesourcesofcoherentopticalradiation;i.e.,lasers,opticalWGRscanbeusedinsourcesofcoherentmicrowaveradiation.Anoptoelectronicoscillator(OEO)isanexampleofsuchasource.AnOEOproducesmicrowavesignalsusingpho-tonictechniques[62],[64]–[69].ThemodulatorisoneofthemainsourcesofpowerconsumptionintheOEObecauseofthelargepowerrequiredtodrivetheconventionalmodulators.BothbroadbandMach–Zehndermodulatorsandfreespacemi-crowavecavity-assistednarrow-bandmodulatorstypicallyre-quireonetoafewWattsofmicrowavepowertoachieveasig-ni cantmodulation.ThismeansthateitherthephotocurrentintheOEOsystemshouldbeampli edsigni cantly,orapowerfullasershouldbeusedasthesourceofthedrivepowerfortheOEO.AnOEObasedonaWGMresonantmodulatorwasrecentlyproposedandfabricated[269].Thedeviceischaracterizedbylowthresholdandlowpowerconsumption.Thedisadvantages
WGM谐振腔综述
ILCHENKOANDMATSKO:OPTICALRESONATORSWITHWGMs—PARTII:APPLICATIONS25
ofthedevicearelowsaturationandlowoutputpower,andapossibilityoftransformingthenoiseofthelight eldintothemicrowavesignal.Ingeneral,resonantandconventionalOEOshavenonoverlappingcharacteristicsandarebothuseful,dependingontheapplication.E.PulsePropagationandGeneration
Itisconvenienttodistinguishbetweentworegimesofopti-calpulsepropagationinaWGR:1)thepulsedurationexceedstheinverseoftheFSRofthecavity;and2)thepulsedura-tionisshorterthantheinversecavityFSR.Studiespresentedin[270]–[273]areprimarilyfocusedonthe rstregime.Specif-ically,thetransientbehavioroflightintensityinsideadielectricsphereexcitedbyalightpulsewasdiscussedin[270],[271].Longopticalpulseswereusedforpumpingofpolymermicro-lasers[272].Linearandnonlinearopticalpropertiesofwaveg-uidecoupledWGRshasalsobeenstudiedtheoretically[273].Thesecondcase,propagationofshortpulsesinWGRs,wasalsoexamined[274]–[276],[278],andageneraltheoreticalanalysisofthepropagationwaspresentedin[274].Timeresolvedmea-surementsofpicosecondopticalpulsespropagatingindielectricspheres[275]andsubpicosecondterahertzpulsepropagationinadielectriccylinder[276],[277]wererecentlyreported,andmicrocavityinternal eldscreatedbypicosecondpulseswasdiscussedtheoretically[278].Thebehaviorofultrashortlightpulsescoupledintotheresonantmodesofsphericalmicrocav-itieswasexploredin[279].Anoninvasivepulse-trackingtech-niquewasexploitedtoobservethetime-resolvedmotionofanultrashortlightpulsewithinanintegratedopticalmicrores-onator[280].
Theminimumpulsewidth,aswellastheperiodoftheop-ticalpulsetraingeneratedbyasystemthatinvolvesahigh-Qcavity,isdeterminedbytheresonatordispersion.Dependingonthedielectrichostmaterialandthegeometricsize,aWGRmaypossesseitherapositive,negative,orzerogroupvelocitydispersion(GVD)[281].ThisdispersionisimportantwhenthepulsedurationisshorterthantheinversecavityFSR.ResonatorspossessingapositivegroupvelocitydispersionmaybeusedforGVDcompensationinoptical berlinks.NegativeGVDcav-itieswithKerrnonlinearity(e.g.,fusedsilicacavities)sustainnonlinearSchrodingersolitonpropagation,andmaybeusedforpulseshapingandsolitonshorteninginconventionalmode-lockedlasers(see,e.g.,[282]–[284]).ZeroGVDcavitiesmaybeusedashigh- nesseetalonstostabilizeactivelymode-lockedlasers(asin[285]).IntegratedopticalWGMall-pass lterscanalsobeusedfortunabledispersioncompensationintheopticaltransmissionlineifthepulsedurationexceedstheinverseoftheFSRoftheresonator[251],[286].
Smallresonators,likeWGRs,areimportantforthestablegenerationofopticalpulseswithhighrepetitionrates.Thisiscon rmedbytheexperimentswithplanar,notWGM,smallres-onators.Forexample,2-pspulsesata16.3-GHzrepetitionratewereobtainedfora2.5-mm-longactivelymode-lockedmono-lithiclaser[287];420GHzsubharmonicsynchronousmodelockingwasrealizedinalasercavityoftotallengthofap-proximately174µm[288].Asigni cantsupermodenoisesup-