學(xué)習(xí)重點(diǎn)：（1）掌握有關(guān)光通信的基本單詞、詞組和縮略語的基本含義及用法（2）培養(yǎng)閱讀翻譯有關(guān)光通信的科技文獻(xiàn)和產(chǎn)品說明書的能力

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14OpticalCommunications生詞avalanche['?v?lɑ:n?]

n.雪崩attenuation[??tenju'ei??n]n.衰減cladding['kl?di?]n.金屬包層；電鍍

PassageA

OpticalFibreandit’sApplicationsinCommunicationcoaxial[k?u‘?ks?l]a同軸的distortion[dis't?:??n]n.扭曲；崎變dispersion[dis'p?:??n]n.散布,分散drawback['dr?:.b?k]n.不利點(diǎn)eliminate[i'limineit]v.除去；排除；消除incoherent[.ink?u'hi?r?nt]

a.不相干的interference[.int?'fi?r?ns]n.干涉index['indeks]n.索引；指針；指數(shù)modal['m?udl]a.模態(tài)的refractive[ri'fr?ktiv]a.折射的silica['silik?]n.矽土；二氧化硅sophisticate[s?'fistikeit]v.摻合；弄復(fù)雜spectrum['spektr?m]n.光譜stray[strei]a.雜散的n.[pl.]雜散電容；天電干擾Comparedwiththesystemsbasedonelectricalcables,thereareanumberofcompellingadvantages,whichleadtothewidespreadadoptionoffibreopticcablefortelecommunications,themostimportantare:(1)Muchlowerlevelsofsignalattenuation,≈0.2

dB/kmformodernsingle-modesilicafibers,sothatmanytensofkilometerscanbebridgedwithoutamplifyingthesignals.

翻譯：與電纜系統(tǒng)相比，光纖電纜具有一些極據(jù)竟?fàn)幮缘膬?yōu)點(diǎn)，這使得它被廣泛應(yīng)用于通信系統(tǒng)，這些優(yōu)點(diǎn)最重要的是：（1）低得多的衰減，現(xiàn)代的單模二氧化硅光纖信號衰減約為0.2

dB/km，這樣，無需放大信號就可跨越數(shù)萬公里。(2)Fibreopticcablingprovidesamuchhigherbandwidthallowingmoredatatobedelivered.Forexample,thewavelengthrangefrom1.3

μmto1.6

μmcorrespondstoabandwidthaslargeas43THz,whicharesomeordersofmagnitudehigherthanthatofanyelectricalcable.Evenso,thetheoreticalpotentialofthisbandwidthhasnotbeenfullyutilizedsofar.(3)Fibreopticcablesaremuchlighterthanthecoaxialcables,sothatthecostoflayingafiber-opticcableismuchlowerthanthecoaxialcables.

翻譯：2）光纜可提供高得多的帶寬，從而傳遞更多的數(shù)據(jù)。舉例來說，波長范圍從1.3μm至1.6μm所對應(yīng)的帶寬是43THz，比普通電纜高出了幾個(gè)數(shù)量級。即便如此，從理論講，此帶寬的潛力至今仍未被充分利用。（3）光纖電纜遠(yuǎn)輕于同軸電纜，這就使得鋪設(shè)光纖電纜的成本遠(yuǎn)低于同軸電纜。(4)Fibreopticsdoesnotsufferfromstrayinterference,suchasgroundloopsorelectromagneticinterferencethatoftenoccurswithcoaxialcabling.Justforreasonsoftheseadvantages,opticaldatatransmissionisincreasinglyusedinvariousareastoday,suchasforexampletelephony,Internettraffic,andcableTV,mostlyforlongertransmissiondistancesofatleastafewkilometers.Thereis,however,atendencyforopticalsystemstopenetrateareaswithsmallerandsmallertransmissiondistances.

翻譯：4）光導(dǎo)纖維不會遭受雜散干擾，而諸如接地環(huán)路或電磁干擾之類的雜散干擾卻在使用同軸電纜時(shí)常常發(fā)生。正是由于具有這些優(yōu)勢，光數(shù)據(jù)傳輸在今天被越來越多地應(yīng)用在各個(gè)領(lǐng)域，如電話、互聯(lián)網(wǎng)通訊、有線電視等，這些多數(shù)用于至少有幾公里的較長傳輸距離。然而，目前有將光系統(tǒng)以越來越小的傳遞距離滲透到各個(gè)領(lǐng)域的趨勢。

However,fibersystemsaresomewhatmoresophisticatedtoinstallandoperate,sothattheytendtobelesseconomicaliftheirfulltransmissioncapacityisnotrequired.Therefore,usuallytheconnectiontothehomesandofficesarestillbridgedwithelectricalcables,whilefiber-basedcommunicationsdothebulkofthelong-haultransmission.

Anyfibreopticdatatransmissionsystemiscomprisedbyanumberofdifferentelements,mainlyare:transmitter(lightsource),fibreopticcable,opticalrepeaterandamplifier,receiver(detector).翻譯：不過，光纖系統(tǒng)的安裝和操作有點(diǎn)復(fù)雜，以至于如果不充分利用它們的全部傳輸容量，使用它們往往就不太經(jīng)濟(jì)。因此，家里和辦公室的連接線通常還是使用電纜，而基于光纖的通信大部分用于遠(yuǎn)距離傳輸。任何光纖數(shù)據(jù)傳輸系統(tǒng)都由一些不同的部分組成，主要是：發(fā)射機(jī)（光源），光纖光纜，光中繼器和放大器，接收器（探測器）。Originally,largelasershavebeenusedforopticalfibretelecommunicationsystems.Today,however,avarietyofsemiconductordevicescanbeused.Themostcommonlyuseddevicesarelightemittingdiodes(LEDs),andothersemiconductorlaserdiodes.ThesimplesttransmitterdeviceistheLED,mainadvantageofwhichischeap,andthismakesitidealforlowcostapplicationswhereonlyshortrunsareneeded.翻譯：

最初，大型激光器用于光纖電信系統(tǒng)。但是現(xiàn)在可以使用各種各樣的半導(dǎo)體設(shè)備。最常用的器件是發(fā)光二極管（LED），以及其它半導(dǎo)體激光二極管。最簡單的發(fā)射裝置是LED。它的主要優(yōu)點(diǎn)在于價(jià)格便宜，因而，它成為低成本短距離應(yīng)用的理想選擇。

Howevertheyhaveanumberofdrawbacks.Thefirstisthattheyofferaverylowlevelofefficiency.Onlyabout1%oftheinputpowerenterstheopticalfibre.TheotherdisadvantageofLEDsisthattheyproduceincoherentlightthatcoversarelativelywidespectrum,typicallybetween30and60nm.Thismeansthatanychromaticdispersioninthefibrewilllimitthebandwidthofthesystem.

Inviewoftheirperformance,LEDsareusedmainlyinlocal-area-networkapplicationswherethedataratesaretypicallyintherange10-100Mb/sandtransmissiondistancesareafewkilometres.翻譯：不過，它們也有一些弊端：第一個(gè)問題是它們所提供的工作效率非常低，只有約10％的輸入功率進(jìn)入光纖；LED的另一個(gè)缺點(diǎn)是，他們所生產(chǎn)的非相干光覆蓋比較寬的頻譜，通常是30到60納米，這意味著在光纖中任何顏色的色散都將限制系統(tǒng)的帶寬。鑒于它們的性能，目前發(fā)光二極管主要用于局域網(wǎng)，這些地方數(shù)據(jù)傳輸率通常在10-100Mb/s，并且傳輸距離為幾公里遠(yuǎn)。

Anotheradvantageisthatsemiconductorlaserscanbemodulateddirectlyathighfrequenciesbecauseofshortrecombinationtimeforthecarrierswithinthesemiconductormaterial.Becausethelaserdiodesareoftendirectlymodulated,thisprovidesaverysimpleandeffectivemethodoftransferringthedataontotheopticalsignal,achievedbycontrollingcurrentapplieddirectlytothedevice.Thisinturnvariesthelightoutputfromthelaser.翻譯：另一個(gè)好處是半導(dǎo)體激光器可在高頻率直接調(diào)制，由于在半導(dǎo)體材料中載流子的復(fù)合時(shí)間很短。因?yàn)榧す舛O管往往直接進(jìn)行調(diào)制，這就提供了將數(shù)據(jù)轉(zhuǎn)換成光信號一個(gè)非常簡單而有效的方法，這一轉(zhuǎn)換是用控制直接加到激光設(shè)備上的電流來達(dá)到的。這反過來又改變了由激光器輸出的光信號。

Howeverforveryhighdataratesorverylongdistancelinks,itismoreeffectivetorunthelaserataconstantoutputlevel(continuouswave).Thelightisthenmodulatedusinganexternaldevice,theadvantageofwhichisthatitincreasesthemaximumlinkdistancebecauseaneffectknownaslaserchirpiseliminated.Thischirpbroadensthespectrumofthelightsignalandthisincreasesthechromaticdispersioninthefibreopticcable.

翻譯：但是對于非常高的數(shù)據(jù)傳輸速率或非常長距離的鏈接，以恒定的輸出（連續(xù)波）運(yùn)行激光更為有效。用外部設(shè)備進(jìn)行光調(diào)制，其優(yōu)點(diǎn)是增加了最大鏈路距離，因?yàn)橐环N被稱為激光啾聲的現(xiàn)象被消除了。這種激光啾聲拓寬了光信號的頻譜，增加了光纖電纜中的色散。Afibreopticcableconsistsofacore,acladding,andaprotectivecoatingoutsideofthecladding,asschematicallyshowninFigure14-1.Thecladdingisalayeraroundthecoreandtherefractiveindexofwhichisslightlylowerthanthatofthecore.Thismeansthatthelightpassingdownthecoreundergoestotalinternalreflectionwhenitreachesthecore/claddingboundary,anditistherebycontainedwithinthecoreoftheopticalfibre.翻譯：光纜由纜芯、包層以及外部有覆層的保護(hù)涂層組成，此保護(hù)涂層是在包層的外面，如圖14-1所示。包層是在纜芯的周圍且其折射率相對纜芯的要低。這意味著當(dāng)在纜芯中進(jìn)行傳輸?shù)墓獾竭_(dá)核心/包層邊界時(shí)要進(jìn)行全內(nèi)反射，因而光被包圍在纜芯中。Phototransistorsarenotusedbecausetheydonothavesufficientspeed.Oncetheopticalsignalfromthefibreopticcablehasbeenappliedtothephoto-detectorandconvertedintoanelectricalformat,itcanbeprocessedtorecoverthedata,whichthencanbepassedtoitsfinaldestination.

翻譯：沒有使用光電晶體管是因?yàn)樗鼈兯俣炔粔?。一旦光纖電纜中光信號傳遞給光電探測器并轉(zhuǎn)換成電氣格式，它就可被加工處理以恢復(fù)原始數(shù)據(jù)，然后傳給最終目的地。3.疑難解析1)Inmostcases,silicafibersareused,exceptforveryshortdistances,whereplasti