光纖知識(shí)點(diǎn)概念

1模式的概念ConceptofModes:PropagationoflightalongawaveguidecanbedescribedintermsofasetofguidedelectromagneticwavescalledModeofwaveguide.2模式的分布特性EachModeisapatternofelectricandmagneticfielddistributionthatisrepeatedalongthefiberatequalintervals.AcertaindiscretenumberofModesarecapableofpropagatingalongthewaveguide.本征值方程是一個(gè)關(guān)于b的以n為參量的超越方程，通常使用數(shù)值方法求解。對(duì)于不同的n，可存在一組離散的b解，每一個(gè)b對(duì)應(yīng)能在光纖中傳播的光場(chǎng)的一種空間分布，這種空間分布在傳播過程中只有相位的變化，而沒有分布形狀的變化，將這種空間分布稱為模式。根據(jù)不同的條件，光纖中可能存在許多模式35簡(jiǎn)并模方法：每個(gè)LP0m模由HE1m模導(dǎo)出；每個(gè)LP1m模由TE0m、TM0m、HE2m構(gòu)成；每個(gè)LPvm模（v>1）由HEv+1,m、EHv-1,m構(gòu)成。例如LP01模只包含基模HE11LP11模由TE01、TM01、HE21構(gòu)成；6階躍光纖中的功率分布當(dāng)V→∞時(shí)，它的能量將聚集在纖芯中；當(dāng)V→Vc時(shí)，能量的大部分是在包層里，這時(shí)的導(dǎo)波將成為輻射模。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單模光纖mmF多模光纖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光功率發(fā)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存在的主要問題（1）兩大體系，3種地區(qū)性標(biāo)準(zhǔn)，使國(guó)際間的互通存在困難。（2）無統(tǒng)一的光接口，無法實(shí)現(xiàn)橫向兼容。（3）準(zhǔn)同步復(fù)用方式，上下電路不便。（4）網(wǎng)絡(luò)管理能力弱，建立集中式電信管理網(wǎng)困難。（5）網(wǎng)絡(luò)結(jié)構(gòu)缺乏靈活性（6）面向話音業(yè)務(wù)所謂SDH是一套可進(jìn)行同步信息傳輸、復(fù)用、分插和交叉連接的標(biāo)準(zhǔn)化數(shù)字信號(hào)的結(jié)構(gòu)等級(jí)。SDH網(wǎng)絡(luò)則是由一些基本網(wǎng)絡(luò)單元（NE）組成的，在傳輸媒質(zhì)上（如光纖、微波等）進(jìn)行同步信息傳輸、復(fù)用、分插和交叉連接的傳送網(wǎng)絡(luò)3．SDH的特點(diǎn)（1）新型的復(fù)用映射方式：同步復(fù)用方式和靈活的映射結(jié)構(gòu)。（2）接口標(biāo)準(zhǔn)統(tǒng)一：全世界統(tǒng)一的NNI，體現(xiàn)了橫向兼容性。（3）網(wǎng)絡(luò)管理能力強(qiáng)：幀結(jié)構(gòu)中豐富的開銷比特。（4）組網(wǎng)與自愈能力強(qiáng)：采用先進(jìn)的ADM、DXC等組網(wǎng)。（5）兼容性好：具有完全的前向兼容性和后向兼容性。（6）先進(jìn)的指針調(diào)整技術(shù)：可實(shí)現(xiàn)準(zhǔn)同步環(huán)境下的良好工作。（7）獨(dú)立的虛容器設(shè)計(jì)：具有很好的信息透明性（8）系列標(biāo)準(zhǔn)規(guī)范：便于國(guó)內(nèi)、國(guó)際互連互通SDH采用一套標(biāo)準(zhǔn)化的信息結(jié)構(gòu)等級(jí)，稱為同步傳送模塊STM-N（N=1，4，16，64，…），相應(yīng)各STM-N等級(jí)的速率為 STM-1 155.520Mbit/s STM