光纤通信简介专业英语要点

OpticalFiberCommunication-introductionForewordTheuseoflighttosendmessagesisnotnew.Fireswereusedforsignalinginbiblicaltimes,smokesignalshavebeenusedforthousandsofyearsandflashinglightshavebeenusedtocommunicatebetweenwarshipsatseasincethedaysofLordNelson.TheideaofusingglassfibertocarryanopticalcommunicationsignaloriginatedwithAlexanderGrahamBell.Howeverthisideahadtowaitsome80yearsforbetterglassesandlow-costelectronicsforittobecomeusefulinpracticalsituations.Thepredominantuseofopticaltechnologyisfortransmissionofdataathighspeed.Opticalfibersreplaceelectricwireincommunicationssystemsandnothingmuchelsechanges.Perhapsthisisnotquitefair.Theveryspeedandqualityofopticalcommunicationssystemshasitselfpredicatedthedevelopmentofanewtypeofelectroniccommunicationsitselfdesignedtoberunonopticalconnections.ATM(AsynchronousTransferMode)andSDH(SynchronousDigitalHierarchy)technologiesaregoodexamplesofthenewtypeofsystems.Itisimportanttorealizethatopticalcommunicationsisnotlikeelectroniccommunications.Whileitseemsthatlighttravelsinafibermuchlikeelectricitydoesinawirethisisverymisleading.Lightisanelectromagneticwaveandopticalfiberisawaveguide.Everythingtodowithtransportofthesignaleventosimplethingslikecoupling(joining)twofibersintooneisverydifferentfromwhathappensintheelectronicworld.Thetwofields(electronicsandoptics)whilecloselyrelatedemploydifferentprinciplesindifferentways.Somepeoplelookaheadto“true”opticalnetworks.Thesewillbenetworkswhereroutingisdoneopticallyfromoneend-usertoanotherwithoutthesignaleverbecomingelectronic.Indeedsomeexperimentallocalarea(LAN)andmetropolitanarea(MAN)networkslikethishavebeenbuilt.In1998opticallyroutednodalwideareanetworksareimminentlyfeasibleandthenecessarycomponentstobuildthemareavailable.However,nosuchnetworkshavebeendeployedoperationallyyet.In1998the“happening”areainopticalcommunicationswasWavelengthDivisionMultiplexing(WDM).Thisistheabilitytosendmany(perhapsupto1000)independentopticalchannelsonasinglefiber.ThefirstfullycommercialWDMproductsappearedonthemarketin1996.WDMisamajorsteptowardfullyopticalnetworking.TransmittingLightonaFiberAnopticalfiberisaverythinstrandofsilicaglassingeometryquitelikeahumanhair.Inrealityitisaverynarrow,verylongglasscylinderwithspecialcharacteristics.Whenlightentersoneendofthefiber,ittravels(confinedwithinthefiber)untilitleavesthefiberattheotherend.Twocriticalfactorsstandout:Verylittlelightislostinitsjourneyalongthefiber.Fibercanbendaroundcornersandthelightwillstaywithinitandbeguidedaroundthecorners.Anopticalfiberconsistsoftwoparts:thecoreandthecladding.Thecoreisanarrowcylindricalstrandofglassandthecladdingisatubularjacketsurroundingit.Thecorehasa(slightly)higherrefractiveindexthanthecladding.Thismeansthattheboundary(interface)betweenthecoreandthecladdingactsasaperfectmirror.Lighttravelingalongthecoreisconfinedbythemirrortostaywithinit-evenwhenthefiberbendsaroundacorner.Whenlightistransmittedonafiber,themostimportantconsiderationis“whatkindoflight?”Theelectromagneticradiationthatwecalllightexistsatmanywavelengths.Thesewavelengthsgofrominvisibleinfraredthroughallthecoloursofthevisiblespectrumtoinvisibleultraviolet.Becauseoftheattenuationcharacteristicsoffiber,weareonlyinterestedininfrared“light”forcommunicationapplications.Thislightisusuallyinvisible,sincethewavelengthsusedareusuallylongerthanthevisiblelimitofaround750nanometers(nm).IfashortpulseoflightfromasourcesuchasalaseroranLEDissentdownanarrowfiber,itwillbechanged(degraded)byitspassagedownthefiber.Itwillemerge(dependingonthedistance)muchweaker,lengthenedintime(“smearedout”)，anddistortedinotherways.OpticalTransmissionSystemConceptsThebasiccomponentsofanopticalcommunicationsystemareopticaltransmitterandreceiver,Fiberjumpers,Optical,fibersplicetrayOpticalfiber.Aserialbitstreaminelectricalfromispresentedtoamodulator,whichencodesthedataappropriatelyforfibertransmission.Alightsource(laserorLightEmittingDiode—LED)isdrivenbythemodulatorandthelightfocusedintothefiber.Thelighttravelsdownthefiber(duringwhichtimeitmayexperiencedispersionandlossofstrength).Atthereceiverendthelightisfedtoadetectorandconvertedtoelectricalform.Thesignalisthenamplifiedandfedtoanotherdetector,whichisolatestheindividualstatechangesandtheirtiming.Itthendecodesthesequenceofstatechangesandreconstructstheoriginalbitstream.Thetimedbitstreamsoreceivedmaythenbefedtoausingdevice.Opticalcommunicationhasmanywell-knownadvantages.WeightandSizeFibercableissignificantlysmallerandlighterthanelectricalcablestodothesamejob.Inthewideareaenvironmentalargecoaxialcablesystemcaneasilyinvolveacableofseveralinchesindiameterandweighingmanypoundsperfoot.Afibercabletodothesamejobcouldbelessthanonehalfaninchindiameterandweighafewouncesperfoot.Thismeansthatthecostoflayingthecableisdramaticallyreduced.MaterialCostFibercablecostssignificantlylessthancoppercableforthesametransmissioncapacity.InformationCapacityTheidearateofsystemin1998wasgenerally150or620Mbpsonasingle(unidirectional)fiber.Thisisbecausethesesystemswereinstalledinpastyears.Theusualratefornewsystemsis2.4Gbpsoreven10Gbps.Thisisveryhighindigitaltransmissionterms.Intelephonetransmissiontermstheverybestcoaxialcablesystemsgiveabout2,000analogvoicecircuits.A150Mbpsfiberconnectiongivesjustover乙000digitaltelephone(64kbps)connections.Butthe150Mbpsfiberisataveryearlystageinthedevelopmentoffiberopticalsystems.Thecoaxialcablesystemwithwhichitisbeingcomparedismuchmorecostlyandhasbeendevelopedtoitsfullestextent.Fibertechnologyisstillinitsinfancy.Usingjustasinglechannelperfiber,researchershavetrialsystemsinoperationthatcommunicateatspeedsof100Gbps.Bysendingmany(“wavelengthdivisionmultiplexed”)channelsonasinglefiber,wecanincreasethiscapacityahundredandperhapsathousandtimes.RecentlyresearchersatNECreportedasuccessfulexperimentwhere132opticalchannelsof20Gbpseachwerecarriedover120km.Thisis2.64terabitspersecond!Thisisenoughcapacitytocarryabout30millionuncompressedtelephonecalls(at64kbpsperchannel).Thirtymillioncallsisaboutthemaximumnumberofcallsinprogressintheworldatanyparticularmomentintime.Thatistosay,wecouldcarrytheworld'speaktelephonetrafficoveronepairoffibers.Mostpracticalfibersystemsdon'tattempttodothisbecauseitcostslesstoputmultiplefibersinacablethantousesophisticatedmultiplexingtechnology.NoElectricalConnectionThisisanobviouspointbutneverthelessaveryimportantone.Electricalconnectionshaveproblems.Inelectricalsystemsthereisalwaysthepossibilityof“groundloops”causingaseriousproblem,especiallyintheLANorcomputerchannelenvironment.Whenyoucommunicateelectricallyyouoftenhavetoconnectthegroundstooneanotheroratleastgotoalotoftroubletoavoidmakingthisconnection.Onelittleknownproblemisthatthereisoftenavoltagepotentialdifferencebetween“ground”atdifferentlocations.Theauthorhasobservedasmuchas3voltsdifferenceingroundpotentialbetweenadjacentbuildings(thiswasafreaksituation).Itisnormaltoobserve1or2voltdifferencesoverdistanceofakilometerorso.Withshieldedcabletherecanbeaproblemifyouearththeshieldsatbothendsoftheconnection.Opticalconnectionisverysafe.Electricalconnectionsalwayshavetobeprotectedfromhighvoltagesbecauseofthedangertopeopletouchingthewire.Insometropicalregionsoftheworld,lightningposesaseverehazardeventoburiedtelephonecables!Ofcause,opticalfiberisn'tsubjecttolightningproblemsbutitmustberememberedthatsometimesopticalcablescarrywireswithinthemforstrengtheningortopowerrepeaters.Thesewirescanbeatargetforlightning.NoElectromagneticInterferenceBecausetheconnectionisnotelectrical,youcanneitherpickupnorcreateelectricalinterference(themajorsourceofnoise).Thisisonereasonthatopticalcommunicationhassofewerrors.Thereareveryfewsourceofthingsthatcandistortorinterferewiththesignal.Inabuildingthismeansthatfibercablescanbeplacedalmostanywhereelectricalcableswouldhaveproblems,(foeexamplenearaliftmotororinacableductwithheavypowercables).Inanindustrialplantsuchasasteelmill,thisgivesmuchgreaterflexibilityincablingthanpreviouslyavailable.Inthewideareanetworkingenvironmentthereismuchgreaterflexibilityinrouteselection.Cablesmaybelocatednearwaterorpowerlineswithoutrisktopeopleorequipment.DistancesbetweenRegeneratorsAsasignaltravelsalongacommunicationlineitlosesstrength(isattenuated)andpicksupnoise.Thetraditionalwaytoregeneratethesignal,restoringitspowerandremovingthenoise,istouseeitherarepeateroranamplifier.Indeeditistheuseofrepeaterstoremovenoisethatgivesdigitaltransmissionitshighquality.Inlong-lineopticaltransmissioncablesnowinusebythetelephonecompanies,therepeaterspacingistypically40kilometers.Thiscompareswith12kmforthepreviouscoaxialcableelectricaltechnology.Thenumberofrequiredrepeatersandtheirspacingisamajorfactorinsystemcost.OpenEndedCapacityThemaximumtheoreticalcapacityofinstalledfiberisverygreat(almostinfinite).Thismeansthatadditionalcapacitycanbehadonexistingfibersasnewtechnologybecomesavailable.Allthatmustbedoneischangetheequipmentateitherendandchangeorupgradetheregenerators.BetterSecurityItispossibletotapfiberopticalcable.Butitisverydifficulttodoandtheadditionallosscausedbythetapisrelativelyeasytodetect.Thereisaninterruptiontoservicewhilethetapisinterestedandthiscanalertoperationalstafftothesituation.Inaddition,therearefeweraccesspointswhereanintrudercangainthekindofaccesstoafibercablenecessarytoinsertatap.WavelengthDivisionMultiplexingWavelengthDivisionMultiplexing(WDM)isthebasictechnologyofopticalnetworking.Itisatechniqueforusingafiber(oropticaldevice)tocarrymanyseparateandindependentopticalchannels.TheprincipleisidenticaltothatusedwhenwetuneourtelevisionreceivertooneofmanyTVchannels.Eachchannelistransmittedatadifferentradiofrequencyandweselectbetweenthemusinga“tuner”whichisjustaresonantcircuitwithintheTVset.OfcoursewavelengthintheopticalworldisjustthewaywechoosetorefertofrequencyandopticalWDMisquiteidenticaltoradioFDM.TherearemanyvarietiesofWDM.Asimpleformcanbeconstructedusing1310nmasonewavelengthand1550astheotheror850and1310.ThistypeofWDMcanbebuiltusingrelativelysimpleandinexpensivecomponentsandsomeapplicationshavebeeninoperationforanumberofyearsusingthisprinciple.Wavelengthselectivecouplersareusedbothtomix(multiplex)andtoseparate(de-multiplex)thesignals.Thedistinguishingcharacteristichereistheverywideseparationofwavelengthsused(differentbandsratherthandifferentwavelengthsinthesameband).Therearemanyvariationsaroundonthisverysimpletheme.Somesystemsuseasignalfiberbidirectionallywhileothersuseseparatefibersforeachdirection.Othersystemsusedifferentwavelengthbandsfromthoseillustratedinthefigure(1310and1550forexample).Themostcommonsystemsrunatverylowdatarates.Commonapplicationareasareinvideotransportforsecuritymonitoringandinplantprocesscontrol.DenseWDMhoweverisanotherthing.DenseWDMreferstotheclosespacingofchannels.Sadly,"dense”isaqualitativemeasureandjustwhatdensemeansislargelyinthemindofthedescription.Othersusethetermtodistinguishsystemswherethewavelengthspacingis1nmperchannelorless.Eachopticalchannelisallocateditsownwavelength——orratherrangeofwavelengths.Atypicalopticalchannelmightbeinmwide.Thischannelisreallyawavelengthrangewithinwhichthesignalmuststay.Itisnormallymuchwiderthanthesignalitself.Thewidthofachanneldependsonmanythingssuchasthemodulatedlinewidthofthetransmitter,itsstabilityandthetolerancesoftheothercomponentsinthesystem.Inpracticaltermsthetransmitterisalwaysalaser.Itmusthavealinewidthwhich(aftermodulation)fitseasilywithinitsallocatedband.Itmustnotgooutsidetheallocatedbandsoitshouldhavechirpanddriftcharacteristicsthatensurethis.Dependingonthewidthoftheallocatedband,thesecharacteristicsdon'tneedtobethemostperfectobtainable.Howevertheydohavetobesuchthatthesignalstayswhereitissupposedtobe.Thereceiverisrelativelystraightforwardandisgenerallythesameasanon-WDMreceiver.Thisisbecausethesignalhasbeende-multiplexedbeforeitarrivesatthedetector.光纤通信简介前言使用光来传送信息并不新鲜。旧约时代就开始用火来传递信息，烟雾信号已使用千年。从纳尔逊勋爵时代开始，海上舰船间的通信就采用闪烁的灯光。AlexanderGrahamBell最先提出用玻璃纤维来传送光通信信号。但直到80多年后，有了更好的玻璃纤维及低成本电子设备，该想法才真正切实可行。光技术主要用于高速数据传输。除了用光纤代替电缆外，与其他通信系统没有什么区别。不过，这样说也许并不太公平。光通信系统的高速及高质量的传输预示了以光纤作为传输媒介的电通信系统的新的发展方向。异步传输模式（AsynchronousTransferMode,ATM）和同步数字体系（SynchronousDigitalHierarchy,SDH）技术就是这种新系统的很好的例子。认识到光通信不同于电通信很重要。人们很容易误解成光在光纤上传输就像电在电缆上传输一样。光是一种电磁波而光纤是一种波导。在光的世界里，任何的和信号的传输相关的方面（即使是诸如将两条纤维连在一起这样简单的事情）和电的世界中都完全不同。虽然电和光这两个领域紧密相关，但却在不同的方面采用不同的原理。有人预测未来将会出现“纯”光网络。这些网络中，完全通过光路由将信号从发射端传输到接收端，无须转换成电信号。实际上，已经建立了一些这样的实验局域网和广域网。1998年，光路由节点广域网已经完全可行，且建造该网络的必要部件都是现成的。不过，这样的网络至今都没有部署运营。1998年，光通信中的热门领域就是波分复用。该技术能够在一条单独的光纤上传输很多（也许超过1000个）独立的光信道。1996年，市场上出现了第一个完全商业化的波分复用设备。波分复用是迈向全光网络互联的重要的一步。光在光纤上传输光纤是非常细的硅玻璃线，很像人的头发。实际上，光线是具有特殊性质的又窄又长的玻璃柱体。光从光纤的一端进入后，将一直在光纤内传输，直到从另一端离开。光纤有两个关键因素：1、光在光纤中传输时损失非常小。2、光在拐弯处能够弯折，折弯后光仍旧在光纤中传输。光纤由两部分组成：纤芯和纤包。纤芯是一个狭窄的圆柱形玻璃纤，纤包是围绕在其上的管状套。纤芯的折射系数逼纤包大一些，这说明纤芯和纤包间的边界（接口）就像一个平面镜。在纤芯中传输的光因镜子的反射而始终限制在纤芯内，即使当光纤折弯时也如此。光在光线中传输时，最重要的是考虑使用哪种类型的光。那些我们称为光的电磁辐射存在于不同波长处，从不可见的红外光，到可以看见的各种颜色的频谱，一直到不可见的紫外光。由于光纤的衰减特性，我们只对通信应用中的红外''光”感兴趣。由于使用的波长通常比可见波长的极限（大概是75nm）长，因此这种光通常不可见。如果将光源（如激光器或者发光二极管）产生的短光脉冲送入光纤，它将在其传输路径上衰减。随着距离的不断增加，信号强度将逐渐减弱，传输时间变长，以及产生其他方面的失真。光传输系统概念光通信系统由光端机、光纤跳线、光纤熔接盒等组成。-调制器的输入是一串电比特流，编码后的数据适于在光纤中传输。-光源（激光器或发光二极管）由调制器驱动，将光送入光纤。光沿光纤传播（这时的光可能会发生散射和能量损失）。-在接收端，将光送入检测器并转换成电的形式。然后信号经放大并送入另一个检测器,其作用是将各自的状态变化及时序分开，然后对状态变化序列解码并还原成原始比特流。-由此所得的具有时间标记的比特流可供后续设备使用。光通信有很多众所周知的优点：重量和体积对于完成相同的功能而言，光缆比电缆要小得多且轻得多。在广域环境中，大的同轴电缆系统很容易电缆直径达到数英尺而重量高达每英尺好几磅。对于完成同样功能的光纤系统，光缆直径一般少于半英尺、重量仅为每英尺几盎司。这说明铺设光缆的成本远远低于电缆。材料成本对于同样的传输容量，光缆的成本大大低于铜制电缆。信息容量1998年，光纤系统的数据传输率一般为每根光纤（单向）150Mbps或620Mbps。这是由于这些系统是当时所建。现在新系统的数据传输速率是为2.4Gbps甚至是10Gbps。就数字传输而言这个数据传输率已经非常高。在电话传输中最好的同轴电缆系统能够支持大约2000路模拟语音电路。一条150Mbps的光缆仅能支持超过2000路的数字电话（64kbps）连接。不过，150Mbps光缆仅是光纤系统发展最早阶段的数据传输率。相比之下，同轴电缆系统要昂贵得多，且很难再有新的突破。光纤技术仍处于发展初期。仅在每根光纤上使用一个信道，研究人员就能建立通信速率为100Gbps的实验系统。如果在单根光纤上采用波分复用发送多路信号，可以将通信容量增加100甚至1000倍。最近NEC的研究人员披露了这样一个成功的实验，该实验中每根光纤上承载132个光信道，每个信道速率为20Gbps，传输距离可达120km，也就是每秒2.64万亿比特！这个容量足以传送三千万路未经压缩的电话呼叫（每路64kbps）。任何时候在世界上的任何地方，三千万路呼叫都可以认为是系统运行的最大峰值。也就是说，一对光缆就能承载世界上所有电话的峰值流量。但由于把多条光纤汇成光缆的成本比使用复杂多分复用技术低得多，因此大部分实用的光纤系统并没有这样做。无电气连接这一点显而易见但非常重要。电连接形式存在一些问题。在电气系统中总有出现“接地环路”的可能，这会导致严重的问题，尤其是在局域网或者是计算机系统中。当使用电通信的时候，通常要将地线连接起来，或者至少是尽可能避免出现这样的连接。一个不太为人所知的问题就是不同的地方的''地”之间通常存在电势差。作者曾在相邻建筑的地之间发现了最多有3V的电势差（奇怪的情形）。通常相距1公里左右存在1V或2V的电势差是正常的。当使用屏蔽电缆时，如果在连接的两端把屏蔽都接地