光纤知识点概念

1模式的概念ConceptofModes:PropagationoflightalongawaveguidecanbedescribedintermsofasetofguidedelectromagneticwavescalledModeofwaveguide.2模式的分布特性EachModeisapatternofelectricandmagneticfielddistributionthatisrepeatedalongthefiberatequalintervals.AcertaindiscretenumberofModesarecapableofpropagatingalongthewaveguide.本征值方程是一个关于b的以n为参量的超越方程，通常使用数值方法求解。对于不同的n，可存在一组离散的b解，每一个b对应能在光纤中传播的光场的一种空间分布，这种空间分布在传播过程中只有相位的变化，而没有分布形状的变化，将这种空间分布称为模式。根据不同的条件，光纤中可能存在许多模式35简并模方法：每个LP0m模由HE1m模导出；每个LP1m模由TE0m、TM0m、HE2m构成；每个LPvm模（v>1）由HEv+1,m、EHv-1,m构成。例如LP01模只包含基模HE11LP11模由TE01、TM01、HE21构成；6阶跃光纤中的功率分布当V→∞时，它的能量将聚集在纤芯中；当V→Vc时，能量的大部分是在包层里，这时的导波将成为辐射模。第三章Signalattenuation:isoneofthemostimportantpropertiesofanopticalfiber.Itlargelydeterminesthemaximumunamplifiedorrepeaterlessbetweenatransmitterandareceiver.Thebasicattenuationmechanismsinafiber:AbsorptionScatteringlossesRadioactivelossesmicroscopicvariationsinthematerialdensitycompositionalfluctuationsstructuralinhomogeneitiesdefectsoccurringduringfibermanufacture.Radiativelossesfiberundergoesabendoffiniteradiusofcurvature.Fibercanbesubjecttotwotypesofbends:1.macroscopicbends:havingradiithatarelargecomparedwiththefiberdiameter.2randommicroscopicbends:canarisewhenthefibersareincorporatedintocables.IntramodalDispersionIntramodalDispersionispulsespreadingthatoccurswithinasinglemode.Thespreadingarisesfromthefinitespectralemissionwidthofanopticalsource.Thisphenomenonisalsoknownasgroupvelocitydispersion.

1MaterialDispersion：arisesfromthevariationoftherefractiveindexofthecorematerialasafunctionofwavelength.2WaveguideDispersion：occursbecauseasingle-modelfiberconfinesonlyabout80%oftheopticalpowertothecore.3Theotherfactorgivingrisetopulsespreadingisintermodaldelay,whichisaresultofeachmodehavingadifferentvalueofthegroupvelocityatasinglefrequency.Ofthesethree,waveguidedispersionusuallycanbeignoredinmultimodefibers.However,thiseffectissignificantinsingle–modefibersSMF单模光纤mmF多模光纤Refractive-indexprofilesInthedesignofsingle-modefibers,dispersionisamajordistinguishingfeature,sincethisiswhatlimitslong-distanceandveryhighspeedtransmission.Insinglemodefibers:dispersionislowestat1300nm，ttenuationisaminimumat1550nmMethod:adjustthebasicfiberparameterstoshiftthezero-dispersionminimumtolongerwavelengths.第四章LD:Laser-DiodesOpticalOutputCoherent,Monochromatic,DirectionalLED:Light-EmittingDiodesOpticalOutputIncoherent,BroadSpectral,LargebeamLEDsaslightsourcefor100~200Mb/swithMMF1Simpledrivecircuitry2.Nothermaloropticalstabilizationcircuit3.BefabricatedlessexpensivelyAnLEDmusthave:1.Ahighradianceoutput2.Afastemissionresponsetime3.Ahighquantumefficiency光功率发射和耦合PowerLaunchingandCoupling Inanopticalfiberlink,twomajorsystemare:Howtolaunchopticalpowerintoaparticularfiber?Howtocoupleopticalpowerfromonefiberintoanother?Considerationinopticalpowerlaunching1NumericalapertureCoresizeRefractive-indexprofileCore-claddingindexdifferenceSize,radiance,angularpowerdistributionofsourceAsignificantfactorinanyfiberopticsysteminstallationistherequirementtointerconnectfibersinalow-lossmanner.Connection:permanentbondisgenerallyreferredtoasasplice,demountablejointisknownasaconnector.Everyjoiningtechniqueissubjecttocertainconditionswhichcancausevariousamountsofopticalpowerlossatthejoint.Theselossesdependonparameterssuchasinputpowerdistribution,lengthofthefiberbetweentheopticalsourceandthejoint,geometricalandwaveguidecharacteristicsofthetwofiberendsatthejoint,andthefiberend-facequalities.Character:HighsensitivityMinimumadditionnoiseFastresponsespeedTypesofphotodetectorsPhotomultiplierPyroelectric-detectorsPhototransistorPhotodiodesPhotodiodeisfitforfiberopticalsystemSmallsize,suitablematerial,highsensitivity,Fastresponsespeeddarknoisecurrent:Noincidentlight,currentcontinuesthroughthebiascircuit.bulkdarknoisecurrent:Fromelectronsandholeswhicharethermallygeneratedinthepnjunctionofthephotodiode.surfacedarknoisecurrent:Dependonsurfacedefect,cleanliness,biasvoltage,andsurfacearea.GainmechanismofAPDdependsonelectronandholeionizationrate:TemperaturesensitiveHighbiasvoltage第七章Typicalerrorratesrangesfrom10-9~10-12ThisBERdependsontheSNRatreceiver.SDH1．PDH存在的主要问题（1）两大体系，3种地区性标准，使国际间的互通存在困难。（2）无统一的光接口，无法实现横向兼容。（3）准同步复用方式，上下电路不便。（4）网络管理能力弱，建立集中式电信管理网困难。（5）网络结构缺乏灵活性（6）面向话音业务所谓SDH是一套可进行同步信息传输、复用、分插和交叉连接的标准化数字信号的结构等级。SDH网络则是由一些基本网络单元（NE）组成的，在传输媒质上（如光纤、微波等）进行同步信息传输、复用、分插和交叉连接的传送网络3．SDH的特点（1）新型的复用映射方式：同步复用方式和灵活的映射结构。（2）接口标准统一：全世界统一的NNI，体现了横向兼容性。（3）网络管理能力强：帧结构中丰富的开销比特。（4）组网与自愈能力强：采用先进的ADM、DXC等组网。（5）兼容性好：具有完全的前向兼容性和后向兼容性。（6）先进的指针调整技术：可实现准同步环境下的良好工作。（7）独立的虚容器设计：具有很好的信息透明性（8）系列标准规范：便于国内、国际互连互通SDH采用一套标准化的信息结构等级，称为同步传送模块STM-N（N=1，4，16，64，…），相应各STM-N等级的速率为 STM-1 155.520Mbit/s STM