《光通信技术》课件：Lecture3

DepartmentofOpticalEngineering,2012Fiber-OpticCommunicationsTechnologyLecture3Instructor:X.WuDepartmentofOpticalEngineeringSinglemodefibers-basicsMakingafibersupportingonlyonemodeMultimodefibersuffersfromhighpulsespreading(dispersion)-intermodaldispersionV<2.405p113figure4.15How?Smallcore~10mSmallNA~0.125(small)GaussianbeamI(r)=I(0)exp(-2r2/w02)1/e2=0.135 I(w0)=0.135I(0)DepartmentofOpticalEngineeringSinglemodefibers-basicsMFD=modefielddiameterWhyuseMFDratherthancorediameter?CorediameterMFD:corediameter8.3m,MFD=2w0=9.3m~20%oflightcarriedbycladdingThefiber’seffectiverefractiveindex,neff,isacombinationofncoreandncladdingLossduetodifferentMFDincoupling:LosscouplingMFD(dB)=-10log[4/(MFD1/MFD2+MFD2/MFD1)2]Cutoffwavelength(whathappensaslowerthanc?)c=1.306d*NA(5.5onp.143)DepartmentofOpticalEngineeringAttenuation–bending,etc.BendinglossMacrobendinglossIncreasewithopwavelengthanddecreasewithbendradiusGaussiandistributionextendsoutonetailMACnumber=MFD/crangingfrom7to9(p.145)MicrobendinglossReason:microvariationofthefiber’scentralaxisarounditstheoreticalstraightposition

DepartmentofOpticalEngineeringChromaticdispersion(wavelength-dependentdispersion)MaterialdispersionandWaveguidedispersionMaterialdispersionLongerwavelength->faster->pulseshapedistortion(refractiveindexlessforlongerwavelength)tmat/L(ps/km)=Dmat()Howmuchtypically?10-20ps/nmkmDispersionandBandwidthDepartmentofOpticalEngineeringWaveguidedispersionCausedbystructure,doesnotexistinopenspaceNegligibleinmultimodefiber,butnotinsinglemodefiberDependsonthemode-fielddistributionbetweencoreandthecladding;MFD.MFDdependsonwavelengthWhy?TheLongerthe,largertheMFDandthelargerthefractionoftotalpulsepowertravelingincladdingThecladdingportiontravelsfaster(nclddingless)Thelongerthe,thegreaterthepulsespreadinginthenegativehalfofthetimeaxis(p.149)D()DepartmentofOpticalEngineeringD()ChromaticDispersion

D()=Dmat()+Dwg()Typically15ps@1550nm,0around1300nm:zerodispersionwavelengthHowtoimproveMaterialdispersioncannotchangemuchChangethewaveguide-dispersionHow?Redesigntherefractive-indexprofileMatchedcladdingv.s.DepressedcladdingDispersionShiftedDispersionFlattenedDepartmentofOpticalEngineeringPolarization-modeDispersion(PMD)Singlemodeactuallycarriestwomodes!PulsespreadSmall0.01-0.5ps/kmv.s.17ps/nmkmforD()Randomprocess,changewithtime,hardtocompensatePhysicalreasonsnxnyasymmetryFiberpinch,twist,bending,ect.EffectofPMDOC-192:192x51.84Mb/s(OC1)=10GMuchworseinthecaseof40GDepartmentofOpticalEngineeringDepartmentofOpticalEngineeringSinglemodeFibers--AdeeperlookModefielddistributiondeterminesattenuationanddispersioninSMfiberHowtocalculate?NeedtoknowrefractiveindexprofileNoperfectstepindexprofileRealprofile->effectivestep-index(ESI)GaussianmodelDepartmentofOpticalEngineeringNormalizedMFDMFD:

insidecore:Besselfunctioninsidecladding:Exponential

calculationW0inceaseswithoperationCutofffrequencyandV-numberCoreradiusandrelativeindexcannotbechangedBendinglossDispersioncompensationOnlyV-numbercanbeadjustedV2.405forsinglemode(p.113figure4.15)Inreality,V=1.8-2.2->cutoff1000nm–1250nmDepartmentofOpticalEngineeringHowtochoose

cutoffwavelengthorV-numberTheshorterthecutoffwavelength(thelargerV-number),theclosertobimodalstate->excessmodalnoiseIncreasingthecutoffwavelength(thesmallerV-number)ModedoesnotconfinesecurelywithinfiberBendinglossincreasesAbsorptionincreasesc<1250nmcandVareinterchangablefiberparametersMinimumoperatingwavelengthPowerconfinementDepartmentofOpticalEngineeringAttenuationandPowerleakageLossesIRabsorptionUVabsorptionRayleighscatteringBendingReducingbendinglossIncrease->highdoping->highscatteringIncrease->dispersionchangeIncreaseoperating->MFD->powerleakDepartmentofOpticalEngineeringDispersioncompensation:DCFOneofmeans:DCF(dispersion–compensatingfiber)Negativedispsersionslope-0.2ps/nmkmDepressedcladdingstructureHowtochoose/useDCFFindD(operating)ofthefibertobecompensated:D(1550nm)=17ps/nmkmforSMF-28FindlengthratioL/LDCFDDCF=-D*(L/LDCF)LookupaDCFwithDDCFDCFhighattenuation->FOM(figureofmerit):dispersionslope/attenuation(ps/nmdB)DepartmentofOpticalEngineeringCopingwithPMDCopewithPMDPolarization-maintaining(PM)fiberAsymmetry,highbirefringenceAsymmetrystructureincoreandcladdingPre-inducedmechanicalstressPolarizationcrosstalk(dB)=10(P/P║)hparameter=(P/P║)/LExtinctionratioER(dB)=-10log(P/P║)DepartmentofOpticalEngineeringCopingwithdispersionDispersiontypeFiberthataresubjecttomodaldispersionMethodsofcopingwithdispersionIntermodal(modal)1.Step-indexMM2.GI-indexMM1.Graded-indexMM2.Single-modeChromatic(intramodal)Single-modefiber1.Dispersion-shiftedfiber,DSF2.Narrowbandwidthdiode3.CompensationwithDCFandDCG4.Slopecompensation5.Pre-andpost-compensationtechniquesPMDAlltypesoffiberexceptPMfiber1.PMDcompensation2.PMfiberandPMcomponentsDepartmentofOpticalEngineeringHomeworkProblemsforchapter5Reviewthesummaryof