光纤通信第五版-第5章-光纤波导

1§5.1阶跃折射率光纤§5.2渐变折射率光纤§5.3损耗§5.4波导的耦合§5.5阶跃折射率光纤中的模式和场§5.6渐变折射率光纤中的模式和场§5.7光纤中的脉冲畸变和信息速率§5.8光纤的制造§5.8光缆第5章光纤波导2Section5.1阶跃折射率光纤Step-IndexFiberStepIndexFiber(SIFiber)n1n2包层折射率n2纤芯折射率n1n1n2如需满足全反射条件，则csin12nncn1n23Section5.1Step-IndexFiber121nnn在平板波导中，我们定义了相对折射率差光纤典型的值是0.01是否可以不要包层，直接将纤芯裸露在空气中？4STEP-INDEXFIBER纤芯和包层的损耗都要很小.消逝波。如果包层很厚(~20mormore),没有光能量能够到达包层的外缘.CommonSIWaveguides1.)GlasscoreGlasscladding2.)GlasscorePlasticcladding3.)PlasticcorePlasticcladding5全玻璃损耗最小.用在远距离传输(100km)玻璃/塑料损耗中等.(100m)全塑料光纤损耗高.短距离(10m)STEP-INDEXFIBER6STEP-INDEXFIBERFibern1n2NAoAll-glass1.481.460.2413.9Plastic-CladSilica1.461.40.4124.2All-plastic1.491.390.5332典型的光纤参数:7STEP-INDEXFIBERExample:全玻璃光纤NA=0.24.从平板波导分析可知2221sinnnnNAoo24.0sinoIfno=1(air),wehavethat8STEP-INDEXFIBER282o2oAcceptanceConeLostLight14o是接收角的一半o9STEP-INDEXFIBER包层模包层模也有可能在包层中以全反射形式传输.包层模损耗较大，不能长距离传输.n2n1TotalReflection10Section5.2渐变折射率光纤Graded-IndexFiberCladdingCorez2an1n2a0RayPathsIndex11Section5.2Graded-IndexFiber纤芯中包层中ararnrn,21)(1arnnrn,21)(21是确定折射率变化的参数.通常=2.12GRADED-INDEXFIBER对进行求解212222212121nnnn2122212122212nnnnn13GRADED-INDEXFIBER121nnn通常情况下n1n2.因此有Thisisjustthefractionalrefractiveindexchange(aswehaveseenbeforeforstep-indexanddielectricslabwaveguides).2112121212122)(2))((nnnnnnnnn14GRADED-INDEXFIBER光线分析aa00rrSnell’sLawn(r)12c1212sinsinnnn2n12211sinsinnn15GRADED-INDEXFIBERSincen2n1,thensin2sin1and21,Result:Raysarebentawayfromthenormal.Thustheraysarecontinuallybentastheyprogresstowardsthecladding.Atsomepoint,theincidentangleisgreaterthanthecriticalangleandtotalreflectionoccurs.16NUMERICALAPERTURESI光纤n2n2n100212221nnnNA接收角与入射光线的位置是相互独立的.17NUMERICALAPERTUREGRIN光纤01远离纤芯的入射光线相比靠近纤芯的入射光线接收角要小.(10)1018NUMERICALAPERTUREGRIN光纤01接收角随这轴心距离的增加而减小.(10)，NA也随之减小.n(r)19NUMERICALAPERTUREExample:1for211xxxLet=2arnrn21)(12121)(arnrn1假设考虑如下近似:20NUMERICALAPERTURE2112arnNAGRINfiber称为抛物线折射率剖面.对这个光纤，数值孔径有,(5.5)ararnrnfor,1)(21有arnrnfor,1)(121NUMERICALAPERTURE我们可以得到类似的SIformula:222)(nrnNA典型GRINFibersCoreDiameter(m)CladdingNA(axial)501250.262.51250.275851250.2622NUMERICALAPERTUREExample:PlotNAif24.00135.0248.12)0(1nNANA0.241ar0135.048.11n023NUMERICALAPERTUREGRIN光纤的耦合效率比阶跃折射率光纤小因为入射到纤芯边缘处的光线很难传输.24NUMERICALAPERTURE考虑纤芯折射率公式:ar,21)(1arnrn如果=2,变为抛物线分布.如果=,变为n(r)=n1,ra阶跃折射率分布.25Section5.3衰减如果接收端的信号很小，会很难被探测，信号不清晰或者发生传输错误。因此光纤的损耗是一个必须要研究的问题。ATTENUATION5.3.1玻璃通过在玻璃中掺杂其它离子来改变折射率分布结论：具有极高化学纯度的高石英含量的玻璃可以制作低损耗的光纤。2627Glass损耗主要由以下三种引起:Absorption吸收Scattering散射Geometriceffects几何效应5.3.2吸收1.IntrinsicAbsorption本征损耗所有材料都会吸收某些频段的波长玻璃在紫外会出现一个吸收峰。在红外7to12m区域也会有吸收峰.28291001010.10.01ATTENUATION(dB/km)WAVELENGTH(m)0.50.60.711.21.523510AbsorptionforaSilicaGlassFiberGlassAbsorptioninUVGlassAbsorptioninIRAbsorption302.Impurities杂质(a)Metalions金属离子Fe,Cu,andNi,在0.6-1.6m范围内吸收(b)Hydroxylion(OH)OH根离子OH根离子是最主要的损耗.OH根离子吸收损耗主要发生在2.73,1.37,1.23，0.95m.Absorption313.AtomicDefects原子缺陷Gammaray,x-ray,neutron,andelectronbombardmentcauseatomicchangeswhichareabsorbing.Somefibersdarkenwhenirradiatedandlaterclearupwhentheradiationisremoved.AbsorptionThemoleculesinglassarerandomlylocated.Why?Theglasswasformedusingheat,whichcausedarandommovementofthemolecules.Whentheglasssolidifiedthemoleculeswerefrozenintheirrandomlocations.Theresultisarandomdensity.Thisyieldsarandomrefractiveindexthroughthematerial.Thus,photonswillbescatteredattherandomboundariesofthechangingrefractiveindices.3233RayleighScattering：当对象尺寸与波长相比拟时，电磁波会发射散射，与波长相关Asdecreasesthescatteringincreasesgreatly.415.3.3RayleighScattering瑞利散射3440.85dB1.7,inmkmL考虑场的损耗公式.Itis)sin(zteEz41损耗可以被近似为RayleighScattering对于Rayleighscattering而言,)/1(685.8/kmL351001010.10.01ATTENUATION(dB/km)WAVELENGTH(m)0.50.60.711.21.523510AttenuationforaSilicaGlassFiberRayleighScatteringLossOHAbsorptionPeakRayleighScattering1001010.10.01ATTENUATION(dB/km)WAVELENGTH(m)0.50.60.711.21.523510TotalFiberLossAttenuationforaSilicaGlassFiberThelossisclearlyloweratthelongerwavelengths.ATTENUATION36375.3.4Inhomogeneities不均匀性光纤制作中无意引入的材料不均匀性也会导致散射损耗。但与波长无关，可以从工艺上来控制.385.3.5GeometricEffects几何损耗光纤的弯曲被导致损耗.弯曲分为宏弯和微弯.1.宏弯-LargescalefiberbendingwithcurvatureradiusR12Rn11cbut2cRadiation392.微弯纤芯中小的轴向弯曲，通常是在制造过程过产生。会导致各种导波模式之间的的耦合，甚至溢出GeometricEffects40Selectedlosscurvesfollow:3.02.5210.5ATTENUATION(dB/km)800900100011001200130014001500160017001.50AttenuationofaSilicaGlassFiberWAVELENGTH(nm)TotalLossRayleighScatteringOHAbsorptionPeak5.3.6TotalAttenuation总损耗41Low-water-contentsingle-modefiberloss低含水单模光纤，全波光纤0.50.40.20.10.30.01280132013601400144014801520156016001640ATTENUATION(dB/km)WAVELENGTH(nm)42SpectralBandClassificationSchemeBandDescriptorRange(nm)O-bandOriginal1260-1360E-bandExtended1360-1460S-bandShortwavelength1460-1530C-bandConventional1530-1565L-bandLongwavelength1565-1625U-bandUltra-longwavelength1625-1675434530201510ATTENUATION(dB/km)WAVELENGTH(nm)40060080010001200AttenuationofaHard