FiberOpticNetworkDesignX.Wu,M.Y.Li,C.S.YanDept.ofOpt.Engr.,ZJU2011Class8

BrillouinScatteringStimulatedBrillouinScatteringSpecificsinfiberEquationsScatteringGainExerciseWhat’shappening:Brillouinscattering,namedafterLéonBrillouin,occurswhenlightinamedium(suchasair,wateroracrystal)interactswithtimedependentopticaldensityvariationsandchangesitsenergy(frequency)andpath.Thedensityvariationsduetoacousticmodes,suchasphonons,magneticmodes,suchasmagnons,ortemperaturegradients.Electrostriction:whenthemediumis‘compressed’,itsindexofrefractionchanges,andafractionofthetravelinglightwave,interactingwiththeperiodicrefractionindexvariations,isdeflectedasinathree-dimensionaldiffractiongrating.Sincethesoundwave,too,istravelling,lightisalsosubjectedtoaDopplershift,soitsfrequencychanges.BrillouinScatteringPhenomenonElasticscatteringInelasticscatteringThephononspresentinsideasolidmoveinthermalequilibriumwithverysmallamplitudescreatingfluctuationsinthedielectricconstant,whichisviewedasamovingdiffractiongratingbyanincidentlightwave.ThereforeBrillouinscatteringcanbeexplainedbythetwoconceptsofBraggsreflectionandDopplershift:FluctuationindensityMovingObservationsinFiber

ForastandardSMFSRSthreshold:100W,SBSthreshold:1WTheprimarymethodofachievingthisinvolvesincreasingthemodeareaofthefibers(large-mode-area-fibers).Toaccomplishthisandstillmaintain-single-modeoperation,however,theNAofthecoremustbedecreased.ReducingtheNAofthefibercreatesasmallerdifferencebetweentherefractiveindexofthecoreandcladding.Fiberswithcorediametersof30μmandNAsaslowas0.06areavailable.Anothermethodofmaintainingsingle-modeoperationistouseabend-lossmechanismtofilterouthigher-ordermodes->lossBrillouinScatteringv.s.RayleighandRaman

Rayleighscatteringoccursfromnon-propagatingdensityfluctuations(randominhomogeneities)->scatteringfromentropyfluctuations.Ramanscatteringarisesfromtheinteractionoflightwiththevibrational/rotationalmodesoftheconstituentmoleculesinthescatteringmedium(glass)->scatteringoflightfromopticalphonons.Brillouinscatteringarisesfromtheinteractionoflightwithpropagatingdensitywavesoracousticphonons(inhomogeneities).Eachofthesescatteringprocessesarealwayspresentinopticalfiberasnofiberisfreefrommicroscopicdefectsorthermalfluctuationswhichdrivethethreeprocesses.ElasticscatteringInelasticscatteringBrillouinScatteringforsensing,datastorage,……StimulatedBrillouinScatteringinSilicateGlassEnergyandmomentumconservationPhononspectrumEvolutionEquationsDensityorpressurevariation:ElectrostrictionStimulatedBrillouinScatteringinSilicateGlass

Dispersionrelationofphonon->angulardependencemultiple-orderGaineffectPeakgainThresholdPowerforSBSDefine:Ps(0)=P0

=1or0.01Measurementdeviatedfromtheory:difficulttomodeldependsonmaterial,fiberlength,temperature,strain,…Measurement:thresholdpower

Bigdifferenceobservedbetweenbelowandabovethreshold!SignaldepletedMeasurementGainSpectrum:~Downshift11GHzfromspontaneoustostimulatedBrillouinscattering->gainnarrowingGainv.s.inputpowerCounter-propagationpumpingDifficulttomodelSBSEffectsinopticalfiberNonlinearinteractionbetweenthepumpandStokeswavethroughanacousticwaveOnlyobserveinbackscatteringinfiber(shiftinforwardscatteringiszero)ReducessignalstrengthbydirectingmostportionofthetransmittedlightbacktowardthetransmitterIntroducesdistortionandnoiseintobothbackscatteredandtransmittedlightmainimpairmentinlong-distanceopticalcomm.Systems->longdistancecantbeachievedbypowerincrease!A=11GHz;p=1.55mAnExampleAfterSBSLaunchedAnexampleBER,NoiseandEffectiveAreaEffectiveAreaIncreaseWithSBSThresholdandEffectiveArea,LengthThresholdpower:atwhichthescatteredpowergrowsaslargeastheinputpowerToavoidchannelpowerbelowthresholdBrillouinandPumpLinewidthEffectiveLengthEffectiveAreaNoise,Noise,NoiseSBS:aparametriccouplingamongabackscatteredStokeswave,pumpwave,andelectrostriction-inducedtravelingacousticwavesignificantlyincreasesintensitynoiseinboththebackscatteredandtransmittedlight.

Experimentalandtheoreticalstudyconfirm:Theintensitynoiseinthetransmittedlightisphasenoiseofthelasersource,convertedbySBSThephasetointensityconversionisattributedtomodificationofthepumpopticalspectrumbythefrequency-dependentimaginarypartoftheBrillouinlossspectrum.Anonlinearphaseshiftattheopticalcarrierisextendedtoconversionofphasemodulationtointensitymodulation(PM-IM)Ref:IntensitynoiseinducedbystimulatedBrillouinscatteringinopticalfibertransmissionsystems.Zhang,Jinye.Supervisor:Phillips,MaryR.NorthwesternUniversityMeasurestoreduceSBSnoiseCompensationofPM-IMconversionbychromaticdispersionispredictedbythemodelandsuccessfullyobservedinanexperimentwithadispersioncompensatingfiberBytransmittingtheSBS-degradedopticalsignalthroughaBrillouinamplifier,significantreductionofintensitynoise(improvementofmorethan6dBofthesignal-to-noiseratio(SNR)ofthetransmitteds