光纖通信簡介專業(yè)英語要點

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“l(fā)ight”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.光纖通信簡介前言使用光來傳送信息并不新鮮。舊約時代就開始用火來傳遞信息，煙霧信號已使用千年。從納爾遜勛爵時代開始，海上艦船間的通信就采用閃爍的燈光。AlexanderGrahamBell最先提出用玻璃纖維來傳送光通信信號。但直到80多年后，有了更好的玻璃纖維及低成本電子設(shè)備，該想法才真正切實可行。光技術(shù)主要用于高速數(shù)據(jù)傳輸。除了用光纖代替電纜外，與其他通信系統(tǒng)沒有什么區(qū)別。不過，這樣說也許并不太公平。光通信系統(tǒng)的高速及高質(zhì)量的傳輸預(yù)示了以光纖作為傳輸媒介的電通信系統(tǒng)的新的發(fā)展方向。異步傳輸模式（AsynchronousTransferMode,ATM）和同步數(shù)字體系（SynchronousDigitalHierarchy,SDH）技術(shù)就是這種新系統(tǒng)的很好的例子。認(rèn)識到光通信不同于電通信很重要。人們很容易誤解成光在光纖上傳輸就像電在電纜上傳輸一樣。光是一種電磁波而光纖是一種波導(dǎo)。在光的世界里，任何的和信號的傳輸相關(guān)的方面（即使是諸如將兩條纖維連在一起這樣簡單的事情）和電的世界中都完全不同。雖然電和光這兩個領(lǐng)域緊密相關(guān)，但卻在不同的方面采用不同的原理。有人預(yù)測未來將會出現(xiàn)“純”光網(wǎng)絡(luò)。這些網(wǎng)絡(luò)中，完全通過光路由將信號從發(fā)射端傳輸?shù)浇邮斩?，無須轉(zhuǎn)換成電信號。實際上，已經(jīng)建立了一些這樣的實驗局域網(wǎng)和廣域網(wǎng)。1998年，光路由節(jié)點廣域網(wǎng)已經(jīng)完全可行，且建造該網(wǎng)絡(luò)的必要部件都是現(xiàn)成的。不過，這樣的網(wǎng)絡(luò)至今都沒有部署運營。1998年，光通信中的熱門領(lǐng)域就是波分復(fù)用。該技術(shù)能夠在一條單獨的光纖上傳輸很多（也許超過1000個）獨立的光信道。1996年，市場上出現(xiàn)了第一個完全商業(yè)化的波分復(fù)用設(shè)備。波分復(fù)用是邁向全光網(wǎng)絡(luò)互聯(lián)的重要的一步。光在光纖上傳輸光纖是非常細(xì)的硅玻璃線，很像人的頭發(fā)。實際上，光線是具有特殊性質(zhì)的又窄又長的玻璃柱體。光從光纖的一端進(jìn)入后，將一直在光纖內(nèi)傳輸，直到從另一端離開。光纖有兩個關(guān)鍵因素：1、光在光纖中傳輸時損失非常小。2、光在拐彎處能夠彎折，折彎后光仍舊在光纖中傳輸。光纖由兩部分組成：纖芯和纖包。纖芯是一個狹窄的圓柱形玻璃纖，纖包是圍繞在其上的管狀套。纖芯的折射系數(shù)逼纖包大一些，這說明纖芯和纖包間的邊界（接口）就像一個平面鏡。在纖芯中傳輸?shù)墓庖蜱R子的反射而始終限制在纖芯內(nèi)，即使當(dāng)光纖折彎時也如此。光在光線中傳輸時，最重要的是考慮使用哪種類型的光。那些我們稱為光的電磁輻射存在于不同波長處，從不可見的紅外光，到可以看見的各種顏色的頻譜，一直到不可見的紫外光。由于光纖的衰減特性，我們只對通信應(yīng)用中的紅外''光”感興趣。由于使用的波長通常比可見波長的極限（大概是75nm）長，因此這種光通常不可見。如果將光源（如激光器或者發(fā)光二極管）產(chǎn)生的短光脈沖送入光纖，它將在其傳輸路徑上衰減。隨著距離的不斷增加，信號強度將逐漸減弱，傳輸時間變長，以及產(chǎn)生其他方面的失真。光傳輸系統(tǒng)概念光通信系統(tǒng)由光端機、光纖跳線、光纖熔接盒等組成。-調(diào)制器的輸入是一串電比特流，編碼后的數(shù)據(jù)適于在光纖中傳輸。-光源（激光器或發(fā)光二極管）由調(diào)制器驅(qū)動，將光送入光纖。光沿光纖傳播（這時的光可能會發(fā)生散射和能量損失）。-在接收端，將光送入檢測器并轉(zhuǎn)換成電的形式。然后信號經(jīng)放大并送入另一個檢測器,其作用是將各自的狀態(tài)變化及時序分開，然后對狀態(tài)變化序列解碼并還原成原始比特流。-由此所得的具有時間標(biāo)記的比特流可供后續(xù)設(shè)備使用。光通信有很多眾所周知的優(yōu)點：重量和體積對于完成相同的功能而言，光纜比電纜要小得多且輕得多。在廣域環(huán)境中，大的同軸電纜系統(tǒng)很容易電纜直徑達(dá)到數(shù)英尺而重量高達(dá)每英尺好幾磅。對于完成同樣功能的光纖系統(tǒng)，光纜直徑一般少于半英尺、重量僅為每英尺幾盎司。這說明鋪設(shè)光纜的成本遠(yuǎn)遠(yuǎn)低于電纜。材料成本對于同樣的傳輸容量，光纜的成本大大低于銅制電纜。信息容量1998年，光纖系統(tǒng)的數(shù)據(jù)傳輸率一般為每根光纖（單向）150Mbps或620Mbps。這是由于這些系統(tǒng)是當(dāng)時所建?，F(xiàn)在新系統(tǒng)的數(shù)據(jù)傳輸速率是為2.4Gbps甚至是10Gbps。就數(shù)字傳輸而言這個數(shù)據(jù)傳輸率已經(jīng)非常高。在電話傳輸中最好的同軸電纜系統(tǒng)能夠支持大約2000路模擬語音電路。一條150Mbps的光纜僅能支持超過2000路的數(shù)字電話（64kbps）連接。不過，150Mbps光纜僅是光纖系統(tǒng)發(fā)展最早階段的數(shù)據(jù)傳輸率。相比之下，同軸電纜系統(tǒng)要昂貴得多，且很難再有新的突破。光纖技術(shù)仍處于發(fā)展初期。僅在每根光纖上使用一個信道，研究人員就能建立通信速率為100Gbps的實驗系統(tǒng)。如果在單根光纖上采用波分復(fù)用發(fā)送多路信號，可以將通信容量增加100甚至1000倍。最近NEC的研究人員披露了這樣一個成功的實驗，該實驗中每根光纖上承載132個光信道，每個信道速率為20Gbps，傳輸距離可達(dá)120km，也就是每秒2.64萬億比特！這個容量足以傳送三千萬路未經(jīng)壓縮的電話呼叫（每路64kbps）。任何時候在世界上的任何地方，三千萬路呼叫都可以認(rèn)為是系統(tǒng)運行的最大峰值。也就是說，一對光纜就能承載世界上所有電話的峰值流量。但由于把多條光纖匯成光纜的成本比使用復(fù)雜多分復(fù)用技術(shù)低得多，因此大部分實用的光纖系統(tǒng)并沒有這樣做。無電氣連接這一點顯而易見但非常重要。電連接形式存在一些問題。在電氣系統(tǒng)中總有出現(xiàn)“接地環(huán)路”的可能，這會導(dǎo)致嚴(yán)重的問題，尤其是在局域網(wǎng)或者是計算機系統(tǒng)中。當(dāng)使用電通信的時候，通常要將地線連接起來，或者至少是盡可能避免出現(xiàn)這樣的連接。一個不太為人所知的問題就是不同的地方的''地”之間通常存在電勢差。作者曾在相鄰建筑的地之間發(fā)現(xiàn)了最多有3V的電勢差（奇怪的情形）。通常相距1公里左右存在1V或2V的電勢差是正常的。當(dāng)使用屏蔽電纜時，如果在連接的兩端把屏蔽都接地