信号系统与电子线路-复试专业英语

TechnicalEnglishForInformationScienceandElectronicEngineeringUnit8OpticalCommunicationsPartIElectromagneticSpectrum3NewWordsperusal精读，细读gigahertz吉赫（109Hz）impress印，铭刻，加载directionality方向性resonator谐振器tTerahertz特赫（1012Hz）atwill随心所欲incur招致，蒙受，引起synchronous同步的interleave交织throughput吞吐量nonlinearity非线性femtosecond毫微微秒(1015sec)coherent相干的obviate避免，使成为不必要micron微米miniature小型的miniaturization小型化counterpart对等物，对等的角色cladding光纤包层4NewWordsproximity接近，亲近fruition结果实，成就photon光子granular颗粒状bias偏置benign良性的51VariousobservationsrelatingtothepotentialofopticaltechnologyfortransmissionofinformationcanbemadefromperusalofthefrequencylineofTable8.1.与用于信息传输的光学技术的潜力有关的仔细研究表8.1中的频率表可以看到各种用于信息传输的光学技术的潜力。61Table8.1Afrequencylinewhichgivesthewavelengthsλ,thefrequenciesν,andthephotonenergyεpforthevariousregionsofthefrequencyspectrumNameaudioradioμwavemmwaveIROPT/UVx-rayGammaλ300km300m30cm3mm30μm0.3μm30Å0.03Åν(Hz)10310610910111013101510171020εp3μeV0.3meV30meV3eV300eV0.3MeV72Theinformationratesinwhichoneisinterestedinconventional“modern-day”communicationssystemsgenerallycorrespondtoaudioratesintelephonesystems,radioratesincommercialbroadcastsystems,ordigitaltelevisionratesinthemostadvancedvideodistributionsystems.1

…

的信息传播速率通常相应于

…“现代”常规通信系统中人们所感兴趣的信息传输速率通常对应于电话系统中的音频，商用广播系统中的无线电频率，或是在最先进的视频节目分配系统中的数字电视信息速率。82TheseratesaregenerallybelowseveralGHz.Ifoneweretotransmitsuchinformationwithoutimpressingitonanopticalcarrierbutinsteadonaradiofrequency(RF)carrierabithigherthanthemaximumrate,thetransmissionwavelengthoftheRFcarrierwouldbecentimetersorlarger.2

如果要传输这样的信息而不将它加载于光纤上，而是加载在略高于最大速率的射频载波上，则此射频载波就会是厘米波或波长再长一些的波。

92Therecan,however,begreatadvantagestousingopticalcarriers.Anobviousoneisthelowlossanddirectionalityoftheopticalfiber.Clearly,thecarriermusthaveahigherratethantheinformationrate.Amajorprinciplethathasappearedincommunicationssystemshasbeenthatthehigherthefrequency,thegreaterthetechnicalcomplexity.Microwavesarehardertohandlethanareradiowaves.通信系统的一个基本原则是102Aswavelengthsdecreasetoapproachthesizeofcircuitcomponents,circuitelementsarenolongerlumped,andleadscanactasreflectivecomponentsand/orantennasandlumpedelementsaselectromagneticresonators.3

随着……,电路单元就不再是集总的，导线可起到反射元件以及（或）天线的作用，集总单元则成为电磁谐振器。

112Thishasgenerallymeantthatsendingmoreinformationwouldcostmoreandtherewasthereforeacostperbit/sec(bps)oftransmittedinformationinthesensethatgoingtoahigherinformationraterequiresahigherfrequency.Thus,thefirstobservationfromthefrequencylinewouldbethat,foropticalcarriers,whichhavefrequenciesinthehundredsofTHz,informationbandwidthisinsomesensefree.因此在较高的信息率要求较高的频率这层意义上，要考虑传输信息的每个bps成本问题信息的带宽在某种意义上是免费的122Thatistosay,theopticalwavelengthissosmallcomparedtomostdevicesthatthetechnologyhaschangeddrasticallyfromelectricalandmicrowave.Onceweassumethatwehavesuchtechnology,nomatterhowhighaninformationrateonemightwantitwillnotbenecessarytochangethecarrier,asthecarrierfrequencyishigherthananyrealisticinformationratecoulde.所用技术与电和微波有根本的不同无论信息率有多高，再也没有必要改变载波了132Bandwidthisnotcompletelyfree,though,asencodersanddecodersmustnecessarilyoperateattheinformationrate,butmuchoftherestofthesystemmustnecessarilyhandleonlythecarrierplusmodulation.而系统其余部分大都只需要处理载波和调制142Ifacomponentcanhandleafrequencyof5×1014Hz,aninformationshiftinthatfrequencyofapartinathousand(correspondingtoa500GHzinformationrate)willhavelittleornoeffectondeviceperformance.在这个频率，信息偏移千分之一（相应于500吉赫兹的信息率）对器件的性能将没有什么影响

152Therefore,oncethesystemisalreadysetup,onecanupgradesystemspeedmoreorlessatwillwithoutthekindofcostsincurredbychangingtheelectromagneticcarrierinconventionalsystems.就可以随意升级系统而不会涉及常规系统中改变电磁载波所需付出的那种代价163Aconsequenceofthesizeoftheopticalbandwidthisthattheopticalcarriercanbeusedtocarrymanydifferenttelephoneconversations,televisionprograms,etc.,simultaneously.Theprocessbywhichthisisgenerallycarriedout(atleastinsynchronousformat)iscalledtimedivisionmultiplexing(TDM).通常实现这种同时传输多路信息的过程（至少以同步格式实现）称为时分复用173Theideaisthat,ifonewishestomultiplex16differentchannelseachtransmittingat1Mbps,onecouldperformthisbydividingeachbitperiodby16andtheninterleavingthebitsintoacomposite1μsecbit(1Mbpsrate)whichactuallycarries16bitsofinformationonit.4

其原理是：如果要复用16个1Mbps的不同信道，可将每一比特所占时间除以16，然后将16个数据比特交织成一个持续1微秒的复合比特（即比特率为1Mbps），这一复合比特实际上带有16比特的信息。183Withtelephoneconversationsrepresentingarateof64kbps,the100sofTbpsbandwidthoftheopticalcarrierholdsgreatpromiseforTDM.Ofcourse,TDMisnottheonlymultiplexingschemeonecanimagineusing.Onecouldimagineimpressinganumberofsubcarriers,spacedbyperhapssomeGHz,ontotheopticalcarrier.使实现TDM有了极大的可能也许相隔几个吉赫兹193Eachofthesecarrierscouldthenbemodulatedataninformationrateandthenre-separatedaccordingtotheirdifferentcarrierwavelengthsattheoutput.Suchaschemeisreferredtoaswavelength-divisionmultiplexing(WDM)orsubcarriermodulation,dependingontheimplementation.在输出端按其不同的载波波长重新分离203Manyofthepresent-dayschemesforincreasinglinkthroughputwithincreasingtrafficinvolvecombiningmanyTDMsignalsontoWDMcarriers.Infact,thelimitationondensityofWDMturnsouttobenotbandwidthbutpower.Thatis,eachchannelrequiressomeamountofpower.Themorechannels,then,thehigherthepowerrequirement.涉及到将许多TDM信号与WDM载波结合的技术213Atsomepowerlevel,opticalfibernonlinearityesimportant,andthisnonlinearitytendstomixthesignalstogether.Thereispresentlymucheffortgoingonintryingtofindwaystoequalizesuchnonlinearities.目前正在进行大量的研究，努力寻求对这种非线性的均衡处理。224Thehighcarrierfrequencyoftheopticalcarrieralsohasdrawbacks,especiallyasitrelates,throughthespeedoflight,totheopticalwavelength.Theopticalperiodcorrespondstolessthantwofemtoseconds.Thismeansthatphasecontrolcorrespondstomanipulationofsub-femtosecondperiodsoftime.当它通过光速与光的波长相联系时尤其如此毫微微秒（1015秒）234Althoughtechniquestodosuchareemerging,theyarecomplicated–muchmorecomplicatedthanmanipulatingmicrowaveorradiofrequencywaveforms.Forthisreason,coherentopticalreceptionisstillalaboratorytechnology.Thedevelopmentoftherareearth-dopedopticalfiberamplifierseemstohaveobviatedtheneedforcoherenttechniquesinmunicationasfarasimprovedsignal-to-noiseratiogoes.随着信噪比的提高，看来稀土元素掺杂光纤放大器的发展使通信中不再需要用相干技术。245Theshortperiodoftheopticalwavealsoimpliesashortwavelengthcenteredaroundhalfofamicron.Thesmallnessoftheopticalwavelength,therefore,allowsfortheminiaturizationoftransmitandreceivemodules,whichshouldallowconsiderablereductioninsize,weight,andcostofopticalcommunicationsystemswithrespecttomicrowave/radiowavecounterparts.5

光波波长之小使发射和接收模块得以小型化，这就使光通信系统的尺寸、重量以至价格与相应的微波、无线电波通信系统相比都大为降低。255Inthecaseofmicrowaves,thehigherthepackagingdensityofopenmicrowavechannels,theworsethecrosstalk.Nomatterhowtightlyonepacksfiber,ontheotherhand,thecrosstalkisessentiallyzeroifthecladdingisproperlydesigned.Thisleadstothecharacteristicthatfiberisanexcellentmediumforspacedivisionmultiplexing(SDM)–thatis,packaginganumberofchannelswithdifferentinformationstreamsincloseproximity.开放的微波信道排列的密度愈高，窜音就愈严重将多个传输不同信息流的信道紧密地封装在一起266Althoughalltheadvantagesofcoherentopticalcommunicationsystemshaveyettobebroughttofruition,anotherpropertyofopticalradiationhasmadetoday’sopticalcommunicationsystemsnotdesirableforapplications.6Theimportantpropertyhereisthatofphotonenergy.虽然相干光通信系统的所有优点还有待于落实在具体成果中，光辐射的另一性质却使目前的光通信系统不利于应用。276AsisseenfromTable8.1,thephotonenergyrangesfromroughly2eVtoroughly4eV.Thiswouldseemtobeanadvantageinefficiency.However,thereisapenaltytobepaidforhavingsuchphotonenergy.Becausesinglephotonsaredetectable,theemission/receptionprocessmusttakeonagranularnature.具有这样的光子能量需要付出高昂的代价286Asiswell-known,eveninasteadyrain,theprobabilityofaraindroplanding(asafunctionoftime)followsaPoissondistribution,implyingthatthereisraindropbunching.7

如所周知，即使在一场稳定的降雨中，雨滴落地的概率（作为时间的函数）服从Poisson分布，意味着有成串的雨滴。296

Araindropwouldratherfallrightaftertheonebefore.Raindropsareimpatientanddon’tliketowait.Inmuchthesamemanner,alaserlikestospitbunchesofphotonsevenunderconstantbiascurrent.Suchbehaviorleadstoatypeofnoisecommonlyreferredtoasshotnoiseorquantumnoise.即使在恒定偏置电流条件下，激光也发出光子束306Ontheemission/detectionprocess,whichturnsouttobequiteseriousforanalogcommunicationsalthoughmuchmorebenigninthedigitalcase.在发射/检测过程中，这一问题对于模拟通信变得相当严重，尽管在数字通信中要轻微得多317Opticalquantumdetectorscanoperateatroomtemperature,assinglephotonsaremeasurable.Therefore,opticaldirectdetectioncanbequitesensitiveifshotnoise-limited.Directdetection,further,istotallycompatiblewithintensitymodulationschemes–schemesinwhichthesourceisessentiallyjustturnedonandoff.8Suchmodulationschemesaretheeasiesttoimplement.进而言之，直接检测与强度调制方案完全兼容，在这些方案中光源实质上只是简单地接通和断开。327Whencoupledwithlight’sshortwavelengthwhichallowsforminiaturesourcesanddetectorsandmicron-sizedwaveguides,directdetectionschemeshaveallowedforsmall,lightweight,highbandwidthsystemswhicharecompetitiveinmanyareas,mostnotablytothepresentmunicationstransmission,althoughamyriadofotherapplicationsarecontinuallyopeningup.9光波长很小，可以用小型的光源和检测器以及微米级的波导，于是用直接检测方案可实现在许多领域具有竞争力的小巧的宽带系统。特别引人注目的是当前在电信传输中的应用，尽管无数其他应用也在不断涌现出来。337Asmentionedpreviously,theseapplicationshavetendedtoopenupmoreslowlythanoriginallypredicted,ascostwasreallynotmuchofaconsiderationinmunications,whereequipmentcostsareswampedbyotherconsiderations.10如前所述，（线路）成本并非电信系统中真正重要的考虑因素，通信设备的成本主要受到其他因素的制约，因此这些应用比预料的出现得慢。

347Withconsumerelectronics,oneneednotworryaboutrightofwayorinstallation.Atpresent,thecostofconnectingtopersonalcomputersafewmetersfromeachother

opticallyissoexpensivethatfiberhasnotyetcometotheconsumermarket.用光技术将相距几米的个人计算机连接起来357Thehighcostofthelinkinsuchacase,though,isnotfundamentalbutmorehistorical.Present-daydevelopmentsinmillimetercoreplasticisanexampleofamuchcheapertechnologythan,forexample,glassfiber.Thecostsofcomponentstogointofiberlinksaswellas

packagingcostsarepresentlybeingreducedandnewapplicationsareopeningup.PartIIOpticalFiber37NewWordscore纤芯diameter直径confinement限制waveguide波导graded-index渐变（梯度）折射率step-index阶跃（突变）折射率singlemodefiber单模光纤dispersion色散multimodefibers多模光纤polarization偏振，极化crystal晶体splice结合，焊接melt熔化doped掺杂质的preform预制品flexible柔顺的，灵活的bundle捆，包扎attenuation衰减repeater中继器，转发器circa(c.,ca.,cir.circ.)大约38NewWordsduct管道immune免疫的，不受影响的wiretapping搭线窃听concentric同心的tap-proof防窃听的ignition燃烧，点火strain应力hydrophone水听器seismic地震的sonar声纳towedstreamer拖曳飘带式水听器阵gyroscope陀螺仪co-located位于同一地方的endoscope内窥镜，内诊镜invasive入侵的，侵略性的erbium铒cylindrical圆柱形的dielectric介电的，电介质的abrupt突然的indexofrefraction折射率39NewWordsnormal垂直的，法线的aperture孔径，光圈refractive折射的profile轮廓，分布parabolic抛物线的speckle散斑，斑点coherent相干的transverse横向的evanescent渐逝的micron微米401Anopticalfiber(orfiber)isaglassorplasticfiberdesignedtoguidelightalongitslengthbyconfiningasmuchlightaspossibleinapropagatingform.1Infiberswithlargecorediameter,theconfinementisbasedontotalinternalreflection.Insmallerdiametercorefibers,(widelyusedformostcommunicationlinkslongerthan200meters)theconfinementreliesonestablishingawaveguide.光纤是一种玻璃的或塑料的纤维，用来沿其长度方向引导光，把尽可能多的光限于一种传播形式。411Fiberopticsistheoverlapofappliedscienceandengineeringconcernedwithsuchopticalfibers.Opticalfibersarewidelyusedinfiber-opticcommunication,whichpermitstransmissionoverlongerdistancesandathigherdataratesthanotherformsofwiredandwirelesscommunications.Theyarealsousedtoformsensors,andinavarietyofotherapplications.数据速率比有线和无线通信的其它形式更高422Thetermopticalfibercoversarangeofdifferentdesignsincludinggraded-indexopticalfibersandstep-indexopticalfibers.Basedonthewayinwhichlightsarepropagatedthoughthefiber,therearesinglemodefibersincludingnonzerodispersion-shiftedfibersanddispersion-shiftedfibers,andmultimodefibers.渐变折射率光纤和阶跃折射率光纤非零色散位移光纤和色散位移光纤432Afibermayeitherbesingle-modeormultimode,dictatedbythespecificdesignandthewavelengthofthelightpropagatinginthefiber.2Becauseofthemechanicalpropertiesofthemorecommonglassopticalfibers,specialmethodsofsplicingfibersandofconnectingthemtootherequipmentareneeded.一根光纤根据其设计和在其中传播的光的波长，可以是单模的也可以是多模的。

需要用特别的方法熔接光纤以及把它们连接到其它设备442Manufactureofopticalfibersisbasedonpartiallymeltingachemicallydopedpreformandpullingtheflowingmaterialonadrawtower.3Fibersarebuiltintodifferentkindsofcablesdependingonhowtheywillbeused.光纤制造过程是将化学掺杂的预制棒部分融化，并在一个抽丝塔上拉长流动的原料。453Opticalfibercommunication

Opticalfibercanbeusedasamediumformunicationandnetworkingbecauseitisflexibleandcanbebundledascables.Itisespeciallyadvantageousforlong-distancecommunications,becauselightpropagatesthroughthefiberwithlittleattenuationcomparedtoelectricalcables.Thisallowslongdistancestobespannedwithfewrepeaters.由于光纤的柔韧性以及可捆扎成光缆463Additionally,thelightsignalspropagatinginthefibercanbemodulatedatratesashighas40Gbps,andeachfibercancarrymanyindependentchannels,eachbyadifferentwavelengthoflight(wavelength-division-multiplexWDM).4

此外在光纤中传播的光信号可以调制到高达40Gbps的速率，每一根光纤能作为许多独立的信道，每个信道用不同波长的光来调制（波分复用）。473Intotal,asinglefiber-opticcablecancarrydataatratesashighas14.4Pbps(circa14millionGbps).Overshortdistances,suchasnetworkingwithinabuilding,fibersavesspaceincableductsbecauseasinglefibercancarrymuchmoredatathanasingleelectricalcable.总体上，单根光缆可携带数据高达14.4Pbps（大约1400万Gbps）483Fiberisalsoimmunetoelectricalinterference,whichpreventscross-talkbetweensignalsindifferentcablesandpickupofenvironmentalnoise.5

光纤也不会受电干扰的影响，防止了不同光缆中信号之间的串话以及环境噪声的介入。493Also,wiretappingismoredifficultcomparedtoelectricalconnections,andthereareconcentricdualcorefibersthataresaidtobetap-proof.Becausetheyarenon-electrical,fibercablescanbridgeveryhighelectricalpotentialdifferencesandcanbeusedinenvironmentswhereexplosivefumesarepresent,withoutdangerofignition.6

因为光纤是不用电的，它能横跨很高的电位差，能用于存在爆炸烟雾的环境下而没有燃烧的危险。504Althoughfiberscanbemadeoutoftransparentplastic,glass,oracombinationofthetwo,thefibersusedinlong-distancemunicationsapplicationsarealwaysglass,becauseoftheloweropticalattenuation.Bothmulti-modeandsingle-modefibersareusedincommunications,withmulti-modefiberusedmostlyforshortdistances(upto500m),andsingle-modefiberusedforlongerdistancelinks.光纤可用透明塑料、玻璃、或两者的结合制成514Becauseofthetightertolerancesrequiredtocouplelightintoandbetweensingle-modefibers(corediameterabout10micrometers),single-modetransmitters,receivers,amplifiersandothercomponentsaregenerallymoreexpensivethanmulti-modecomponents.7

由于将光耦合到单模光纤或在单模光纤之间耦合（纤芯直径大约为10微米）的允差较小，单模发射器、接受器、放大器和其它元件的价格通常比多模元件的贵。525Fiberopticsensors

Opticalfiberscanbeusedassensorstomeasurestrain,temperature,pressureandotherparameters.Thesmallsizeandthefactthatnoelectricalpowerisneededattheremotelocationgivethefiberopticsensoradvantagestoconventionalelectricalsensorincertainapplications.小的尺寸以及在偏远位置不需要电力536Opticalfibersareusedashydrophonesforseismicorsonarapplications.Hydrophonesystemswithmorethan100sensorsperfibercablehavebeendeveloped.Hydrophonesensorsystemsareusedbytheoilindustryaswellasafewcountries’navies.Bothbottommountedhydrophonearraysandtowedstreamersystemsareinuse.TheGermancompanySennheiserdevelopedamicrophoneworkingwithalaserandopticalfibers.

船底装有水听器阵列和拖缆系统已投入使用547Opticalfibersensorsfortemperatureandpressurehavebeendevelopedfordownholemeasurementinoilwells.Thefiberopticsensoriswellsuitedforthisenvironmentasitisfunctioningattemperaturestoohighforsemiconductorsensors.

因为它能工作在对半导体传感器而言太高的温度下558AnotheruseoftheopticalfiberasasensoristheopticalgyroscopewhichisinuseintheBoeing767andinsomecarmodels(fornavigationpurposes)andtheuseinhydrogenmicrosensors.

光学陀螺569Fiber-opticsensorshavebeendevelopedtomeasureco-locatedtemperatureandstrainsimultaneouslywithveryhighaccuracy.Thisisparticularlyusefultoacquireinformationfromsmallcomplexstructures.以很高的精确度同时测量同一点的温度和张力5710Otherusesofopticalfibers

Fibersarewidelyusedinilluminationapplications.Theyareusedaslightguidesinmedicalandotherapplicationswherebrightlightneedstobeshoneonatargetwithoutaclearline-of-sightpath.Insomebuildings,opticalfibersareusedtoroutesunlightfromtherooftootherpartsofthebuilding.Opticalfiberilluminationisalsousedfordecorativeapplications,includingsigns,art,andartificialChristmastrees.

需要明光照亮目标而不需要一个清晰的直线路径5811Opticalfiberisalsousedinimagingoptics.Acoherentbundleoffibersisused,sometimesalongwithlenses,foralong,thinimagingdevicecalledanendoscope,whichisusedtoviewobjectsthroughasmallhole.Medicalendoscopesareusedforminimallyinvasiveexploratoryorsurgicalprocedures(endoscopy).Industrialendoscopesareusedforinspectinganythinghardtoreach,suchasjetengineinteriors.

医学内窥镜用于微创探测或外科手术（内窥镜检查法）5912Anopticalfiberdopedwithcertainrare-earthelementssuchaserbiumcanbeusedasthegainmediumofalaseroropticalamplifier.Rare-earthdopedopticalfiberscanbeusedtoprovidesignalamplificationbysplicingashortsectionofdopedfiberintoaregular(undoped)opticalfiberline.掺杂某些稀土元素如铒通过一小段掺杂光纤熔接到正常（非掺杂）光纤6012Thedopedfiberisopticallypumpedwithasecondlaserwavelengththatiscoupledintothelineinadditiontothesignalwave.8Bothwavelengthsoflightaretransmittedthroughthedopedfiber,whichtransfersenergyfromthesecondpumpwavelengthtothesignalwave.Theprocessthatcausestheamplificationisstimulatedemission.

除了信号波外，耦合到光纤线路的另一个激光波长被泵浦（注入）到掺杂介质的光纤中。6113Opticalfibercanbeusedtosupplyalowlevelofpower(aroundonewatt)toelectronicssituatedinadifficultelectricalenvironment.Examplesofthisareelectronicsinhigh-poweredantennaelementsandmeasurementdevicesusedinhighvoltagetransmissionequipment.

光纤可用于给处于用电困难环境的电子设备提供低的功率（约一瓦特）6214PrincipleofoperationAnopticalfiberisacylindricaldielectricwaveguidethattransmitslightalongitsaxis,bytheprocessoftotalinternalreflection.Thefiberconsistsofacoresurroundedbyacladdinglayer(Figure8.1).Toconfinetheopticalsignalinthecore,therefractiveindexofthecoremustbegreaterthanthatofthecladding.光纤是一个圆柱形的电介质波导，通过全内反射沿其轴传播光光纤由一个被包层包围的纤芯组成6314Theboundarybetweenthecoreandcladdingmayeitherbeabrupt,instep-indexfiber,orgradual,ingraded-indexfiber.

Figure8.1Atypicalopticalfiberconsistingofatransparentmaterialofrefractiveindexn1andsurroundedbyacladdingofaslightlylowerrefractiveindexn2.Mostofthepropagatingenergyisconfinedtothecoreregionandthefielddecaysexponentiallyinthecladding.Cladding(n2)Core(n1)6415Multimodefiber

Fiberwithlarge(greaterthan10μm)corediametermaybeanalyzedbygeometricoptics.Suchfiberiscalledmultimodefiber,fromtheelectromagneticanalysis.Inastep-indexmultimodefiber(Figure8.2),raysoflightareguidedalongthefibercorebytotalinternalreflection.由于全内反射光线沿光纤纤芯传播6515Figure8.2Astep-indexmultimodefiberinwhichtherefractiveindexinthecoreisconstant.Lightraysimpingingonthecore-claddinginterfaceatananglegreaterthanthecriticalanglearetrappedinsidethecoreofthewaveguide.Insuchafiberraystravelingatlargeranglestotheaxishavetotraversealargerpathandhencetakealongertimethanthoserayswhichpropagatewithlesseranglestotheaxis.Thisleadstoasubstantialamountofbroadeninginapulsepropagatingthroughthefiber.xn(x)qCore:n1Cladding:n2(<n1)z6615Raysthatmeetthecore-claddingboundaryatahighangle(measuredrelativetoalinenormaltotheboundary),greaterthanthecriticalangleforthisboundary,arecompletelyreflected.9

当光线射到纤芯和包层之间界面的角度（与垂直于边界的直线之间的夹角）大于临界角时，光线被完全反射。

6715Thecriticalangle(minimumanglefortotalinternalreflection)isdeterminedbythedifferenceinindexofrefractionbetweenthecoreandcladdingmaterials.Raysthatmeettheboundaryatalowanglearerefractedfromthecoreintothecladding,anddonotconveylightandhenceinformationalongthefiber.10

以小角度射到分界面上的光线被折射，从纤芯进入包层，它们并不沿着光纤传输光，因而也不传输信息。6815Thecriticalangledeterminestheacceptanceangleofthefiber,oftenreportedasanumericalaperture.Ahighnumericalapertureallowslighttopropagatedownthefiberinraysbothclosetotheaxisandatvariousangles,allowingefficientcouplingoflightintothefiber.11

大的数值孔径使光能以接近于轴线的方式沿光纤传播，也能以不同的角度传输，从而使得光能有效地耦合进入光纤。

6915However,thishighnumericalapertureincreasestheamountofdispersionasraysatdifferentangleshavedifferentpathlengthsandthereforetakedifferenttimestotraversethefiber.12Alownumericalaperturemaythereforebedesirable.

然而，大的数值孔径增加了色散的总量，因为以不同角度传播的光线具有不同的光程长度，因而化了不同的时间穿过光纤。7016Inagraded-indexfiber(Figure8.3),theindexofrefractioninthecoredecreasescontinuouslybetweentheaxisandthecladding.Thiscauseslightraystobendsmoothlyastheyapproachthecladding,ratherthanreflectingabruptlyfromthecore-claddingboundary.这使得光线到达包层时是平滑地弯曲，而不是在纤芯包层边界上突然反射7116Theresultingcurvedpathsreducemulti-pathdispersionbecausehighanglerayspassmorethroughthelower-indexperipheryofthecore,ratherthanthehigh-indexcenter.Theindexprofileischosentominimizethedifferenceinaxialpropagationspeedsofthevariousraysinthefiber.Thisidealindexprofileisveryclosetoaparabolicrelationshipbetweentheindexandthedistancefromtheaxi