Chapter5

OpticFiberWaveguides5.1Step-IndexFiber(SIFiber)n1n2Claddingindexn2

Coreindexn1n1>n2Fortotalreflectionwerequire>cn1n2fractionalrefractiveindexchangeCommonSIWaveguides

Glasscore GlasscladdingGlasscore Plasticcladding3)Plasticcore PlasticcladdingAllGlassLowestloss.Usedforlongestdistances(uptoabout100km)

Glass/PlasticModerateloss.Moderatedistances(uptoabout100m)AllPlasticHighloss.Shortdistances(uptoabout10m)Typicalfiberproperties:Claddingmodesmayalsobetrappedinthefiberbytotalreflectionattheouterboundaryofthecladding.Thecladdingmodesarelossyanddonottravelfar.n2n1TotalReflection5.2Graded-IndexFiber(GRINFiber)CladdingCorez2an1n2a0RayPathsIndexInthecoreInthecladdingaisadesignparameter.Itisusuallyclosetoa=2.aa00rrSnell’sLawn(r)12cn2n12>1SIFibern2n2n100Theacceptanceangleisindependentofwheretheincidentrayentersthecore.GRINFiber01Arayenteringthecoreawayfromtheaxishasasmalleracceptanceanglethanarayenteringalongtheaxis.(a1<a0)a1<a001Theacceptanceangledecreaseswithradialdistancefromtheaxis.(a1<a0)ThustheNAdecreaseswithradialdistancefromtheaxis.n(r)Example:Let=2Usetheapproximation:ThisisaparabolicprofileGRINfiber.Forthisfiber,(5.5)RepresentativeGRINFibersCouplingefficiencyforaGRINfiberislessthenthatforastep-indexfiber.Considerthegeneralcoreindexequation:

Wesawthatif=2,thisbecomestheparabolicprofile.If=,thisbecomes n(r)=n1,rawhichisjustthestep-indexfiberprofile.Lossesmaybeclassifiedas: Absorption Scattering Geometriceffects5.3Attenuation

5.3.1 GlassAbsorptionforaSilicaGlassFiber1001010.10.01ATTENUATION(dB/km)WAVELENGTH(m)11.21.523510GlassAbsorptioninUVGlassAbsorptioninIR5.3.2Absorption1.IntrinsicAbsorption

2.Impurities

MetalionsHydroxylion(OH)

3.AtomicDefectsRayleighScatteringisthescatteringofelectromagneticwavesbyobjectshavingsizesmallcomparedtothewavelength.TheamountofscatteringvariesasAsdecreasesthescatteringincreasesgreatly.5.3.3 RayleighScatteringThelosscanbeexpressed(approximately)by1001010.10.01ATTENUATION(dB/km)WAVELENGTH(m)11.21.523510AttenuationforaSilicaGlassFiberRayleighScatteringLossOHAbsorptionPeak1001010.10.01ATTENUATION(dB/km)WAVELENGTH(m)11.21.523510TotalFiberLossAttenuationforaSilicaGlassFiberThelossisclearlyloweratthelongerwavelengths.5.3.4 Inhomogeneities

Unintentionalimperfectionsintheglasscauseadditionalscatteringlosses.Roughcoretocladdinginterfaceproducesmoreloss.Theselossesareminimizedbycarefulmanufacturingprocesses.5.3.5 GeometricEffects12Rn11>cbut2<cRadiationAttenuationofaSilicaGlassFiber5.3.6 TotalAttenuation

3.02.5210.5ATTENUATION(dB/km)800900100011001200130014001500160017001.50WAVELENGTH(nm)TotalLossRayleighScatteringOHAbsorptionPeakScatteringboundstheshort-wavelengthsideandinfraredabsorptionboundsthelong-wavelengthside.Low-water-contentsingle-modefiberloss1280132013601400144014801520156016001640ATTENUATION(dB/km)WAVELENGTH(nm)SpectralBandClassificationSchemeBandDescriptorRange(nm)O-bandOriginal1260-1360E-bandExtended1360-1460S-bandShortwavelength1460-1530C-bandConventional1530-1565L-bandLongwavelength1565-1625U-bandUltra-longwavelength1625-16754530201510ATTENUATION(dB/km)WAVELENGTH(nm)40060080010001200AttenuationofaHardCladSilicaFiber

403525505000500300200ATTENUATION(dB/km)WAVELENGTH(nm)400500600700800AttenuationofanAllPlasticFiber2000100010050GlassFibersWavelength Loss0.8m 3dB/km1.3m 0.5dB/km1.55m 0.2dB/kmPlasticFibersWavelength Loss570nm 100dB/km670nm 160dB/kmTestinglosses-cutbackcircuitCOUPLERFIBERLDdetectorL1L2PinPoutLPower/dBmPinPoutL1L2LfiberLength/kmkmFig.Testingusingopticaltime-domainreflectometerTestinglosses-OTDROpticaltime-domainreflectometerYokogawaAQ7270onthecalibrationstand

Step-IndexFiberModeChartn2neffn10 1 2 3 4 5 6Axial90°cHE11TE01TM01HE21HE12EH11HE31HE41EH21TM02TE02HE222.405CriticalAngle5.4MODESANDFIELDSINSTEP-INDEXFIBERS5.4.1 ModesFortheSIfiber,ifV>10,thenumberofmodesisapproximately:(5.8)Example:Letn1=1.48,n2=1.46,2a=50m,and=0.82m.FindVandN(thenumberofpropagatingmodes).

Vw/aSpotSizew/avs.V5.4.2SpotSizeThesingle-modeconditionis:(5.10)Solvefor5.4.3 Single-Mode（SM）OperationSMfiber(5.10)Thecutoffwavelength(thefiberismultimodeatallshorterwavelengths)orExample:Findthemaximumcoreradiusforsingle-modeoperationif: n1=1.465,n2=1.46,=1.250mTheresultingmaximumcorediameteris:Singlemodefiber(SM)單模光纖Originalfiber常規光纖（G.652）Dispersionshiftedfiber(DSF)色散位移光纖（G.653）Nonezerodispersionshiftedfiber(NZ-DSF)非零色散位移光纖（G.655）Dispersioncompensationfiber(DCF)色散補償光纖Dispersionflattenedfiber色散平坦光纖SubmarineFiber海底光纖（G.654）Low-water-contentfiber全波段光纖 1.3(m)16012080400-40MaterialDispersion1.3(m)02010TriangularProfileFiberWaveguideDispersion1.551.55MMgDispersionShiftedFiber(m)16012080400-40DispersionFlattenedFiber1.55M+Mg(ps/nm.km)DispersionCompensationTransmissionFiberCompensationFiberattenuationDispersionvalueatNondispersionwavelengthDCFNZ-DSF（G.655）DSF（G.653）Originalfiber（G.652）EETwoHE11ModesFiberCore5.4.4 BirefringenceandPolarization EllipticalCoreUnequalForcesFyFxPolarization-ModeDispersion(PMD)

(BirefringenceDispersion)Polarization-preservingfiberPolarizingfiberHE11modetransversepattern5.4.5ElectricFieldsintheStep-IndexFiber

5.5ModesInGraded-IndexFibersThenumberofmodesforlargeV(V>10)isapproximately:ThetransversepatternisGaussian0rSinglemodecondition5.6PulseDistortionAndInformationRateInFibersSignalsaredistortedby:MaterialDispersionWaveguideDispersionModalDistortionPolarization-ModeDispersionMultimode(Modal)Distortion5.6.1DistortioninSIFibersThisresultappliestothehighlymultimodeSIfibers(V>10).Example:n1=1.48,n2=1.46WaveguideDispersionFrom(4.24)Thetotaldispersivepulsespreadis:1.31.550.82110-20M(ps/nm.km)(m)MaterialDispersionforPureSilica5(m)031WaveguideDispersioninaStep-IndexFiber20510.20.1125102050100SOURCESPECTRALWIDTH(nm)PULSESPREADPERUNITLENGTH(ns/km)3-dBOPTICALBANDWIDTH-LENGHPRODUCT(GHz*km)=0.7m0.80.911.12LDLED52.50.050.250.5105.6.2DistortioninSingle-ModeFibers

0.6 1.31.8(m)16012080400-40MaterialDispersion

5(m)031WaveguideDispersionn(r)Higher-OrderedRayWhyareGRINfibersneeded?BecausethemodaldistortionislowerthaninmultimodeSIfibers.AxialRayn15.6.3 DistortioninGraded-IndexFibersRegionofFasterTravelaa00RecallthemodalspreadforaSIfiber:Thereductioninpulsespreadisbythefactor:TheGRINmodalspreadisapproximately:(5.19)Example:ConsideramultimodeGRINfiberhavingWefoundearlierthatUsing(5.19)fortheGRINfiber,wefind1.ConsidermultimodeGRINfibersinthefirstwindowregionof0.8-0.9m.Discussion:ifLEDsareused(>20nm),then

0.0250.012.5520510.20.1125102050100SOURCESPECTRALWIDTH(nm)PULSESPREADPERUNITLENGTH(ns/km)3-dBOPTICALBANDWIDTH-LENGHPRODUCT(GHz*km)=0.7m0.80.911.121.4ns/kmLEDExample:=0.85m,=30nm,n1=1.48,n2=1.46,multimodeGRINfiber.Computethetotalpulsespread.WepreviouslycalculatedthatThetotalpulsespreadisnow0.050.250.512.5520510.20.1125102050100SOURCESPECTRALWIDTH(nm)PULSESPREADPERUNITLENGTH(ns/km)3-dBOPTICALBANDWIDTH-LENGHPRODUCT(GHz*km)=0.7m0.85m0.911.122.7FromthefigurewefindD(t/L)mat=2.7ns/km.2.ConsidermultimodeGRINfibersinthesecondwindow,near1.3mm.Themodalspreadisontheorderof1ns/km.CharacteristicsofCommercialFibers

5.6.4LengthDependenceofPulseSpreadReason:modemixing.Ifwelet(/L)bethemodalpulsespreadinthelinearregion,thenandforL<LeforL>LeExample:(/L)=20ns/km,Le=1km(ns)0 1234L<LeL>LeLLe10203040500Recallthe3-dBopticalbandwidth:Whenmodaldistortionisdominant,thenSection5.7ConstructionOfOpticalFibersDefinitionsFiber:corewithcladding,havingthefinalfiberdimensions.Preform:oversizedcoreandcladding.Fibercable:fiberimbeddedwithinaprotectivesheath.FiberCoreMeltCladdingMeltPlatinumCrucibles5.7.1 DoubleCrucibleTechnique5.7.2 RodInTube5.7.3 DopedDepositedSilica(DDS)ChemicalVaporDeposition(externalCVD,InternalCVD)Vaporaxialdeposition(VPO)TorchSootFuelBCl4GeCl4SiCl4O2GlassFormingVaporsExternalCVDMandrel5.7.4 ExternalDDSBurnerSootFuelBCl4GeCl4SiCl4O2GlassFormingVapors5.7.5 AxialDepositionBCl4GeCl4SiCl4O2ChemicalVaporsTorchRotatingSilicaTubeExhaust5.7.6 InternalDeposition5.7.7 FiberDrawingPreformFurnaceCoatingApplicatorCuringFurnaceorLampsCapstanTake-upReelFiberDiameterMeasurementMovingFeedFiberonthedrumGeneralCableStructure

1.Core2.Cladding3.PrimaryBuffer4.SecondaryBuffer5.StrengthMember6.OuterCovering5.8OpticFiberCablesLoose-TubeConstruction

ProtectingTubeFiberLoose-Fiber,SlottedConstructionFiberTapeLight-Duty,Tight-BufferCableThedimensionsgivenarethediameters.Polyurethane,3.8mmKevlar,2mmHytrel,SecondaryBuffer,1mmSilastic,PrimaryBuffer,0