信息與通信工程專業(yè)科技英語課件

TechnicalEnglishForInformationandCommunicationEngineering2024/6/250UnitSixteenOpticalCommunicationComponents12概述光纖作為通信傳輸媒介的基本原理均芯光纖和漸變折射率光纖傳輸引起的失真和誤碼多模和單模光纖光纖作為調(diào)制和檢測元件3terrestrial地面的celestial天體的transparent透明的opaque不透明的refraction折射cladding包層exponentially按指數(shù)規(guī)律地homogeneous均勻的，同質(zhì)的gradient梯度graded-indexfiber漸變折射率光纖evanescent短暫的，很快消失的impinge撞擊traverse跨越，橫過incident入射intermodal模態(tài)間的intramodal模態(tài)內(nèi)的dispersion色散，散開4axis軸coaxial同軸的radius(pl.radii)半徑repeater中繼器overlap重疊diameter直徑silica硅石，二氧化硅conduit管道immune免疫crosstalk竄音hazard危險fusion熔合，核聚變detector檢波器，檢測器on-offmodulate啟閉調(diào)制bias偏置threshold閾值incoherent非相干的5emission發(fā)射sensitivity靈敏度ambient周圍的aging老化emanate散發(fā)，發(fā)出birefringence雙折射polarization極化，偏振acousto-optic聲光的diffract衍射，繞射grating光柵photodiode光二極管avalanche雪崩（效應(yīng)）bulky笨重的valence（原子的）價accelerate加速quantum量子6一個中心透明的稱為纖芯的區(qū)域和一個環(huán)繞纖芯的稱為包層的折射率較低的區(qū)域Asdiscussedearlier,theatmospherecannotbeusedasatransmissionchannelforterrestrialcommunicationsusinglightbeams.Themostpromisingchannelistheopticalfiberwaveguide.Anopticalfiberessentiallyconsistsofacentraltransparentregioncalledthecorewhichissurroundedbyaregionoflowerrefractiveindexcalledthecladding(seeFigure16.1).Unit16TheOpticalFiber

7傳播能量的大部分被限制在纖芯內(nèi)，包層中的場按指數(shù)律衰減。Figure16.1Atypicalopticalfiberconsistingofatransparentmaterialofrefractiveindexn1

andsurroundedbyacladdingofaslightlylowerrefractiveindexn2.Typicaldimensionsofthecoreandcladdingare50and125mm,respectively.Mostofthepropagatingenergyisconfinedtothecoreregionandthefielddecaysexponentiallyinthecladding.

Unit16CaptionofFigure16.18由光束朝纖芯中央連續(xù)折射而產(chǎn)生光導(dǎo)從中心向外具有遞減梯度的由于在纖芯包層的界面處的全內(nèi)反射現(xiàn)象而形成光導(dǎo)

Thecorecouldeitherbehomogeneousorcouldhaveagradientinrefractiveindexwiththerefractiveindexdecreasingawayfromthecenterofthecore.Intheformertypeoffiber,alsoreferredtoashomogeneouscorefibers(seeFigure16.2),theguidanceoflightoccursthroughthephenomenonoftotalinternalreflectionatthecore-claddinginterface.Inthelattertypeoffibers,alsoreferredtoasgraded-indexfibers(seeFigure16.3),theguidanceoflightoccursthroughcontinuousrefractionoflightraystowardsthecenterofthecore.Unit169以大于臨界角射到纖芯－包層界面處與軸線成較大角度傳播的光線比起那些以較小角度傳播的光線來，要經(jīng)過較長的路徑，因此需要用較多的時間Unit16Figure16.2Ahomogeneouscoreopticalfiberinwhichtherefractiveindexinthecoreisconstant.Lightraysimpingingonthecore-claddinginterfaceatananglegreaterthanthecriticalanglearetrappedinsidethecoreofthewaveguide.Insuchafiberraystravelingatlargeranglestotheaxishavetotraversealargerpathandhencetakealongertimethanthoserayswhichpropagatewithlesseranglestotheaxis.1Thisleadstoasubstantialamountofbroadeninginapulsepropagatingthroughthefiber.CaptionofFigure16.210即使與軸線夾角較大的光線要經(jīng)過較長的路徑，但是在折射率較低光速較大的區(qū)域內(nèi)傳播，…Unit16…光線朝纖芯中央連續(xù)折射而被束縛在纖芯內(nèi)Figure16.3Agradedindexopticalfiberinwhichtherefractiveindexinthecoredecreasescontinuouslyawayfromtheaxis.Figure16.3(a)showsatypicalvariationofrefractiveindexacrossthecoreoftheopticalfiber.Lightraysinsuchafiberaretrappedbyacontinuousrefractiontowardsthecenterofthecore,whichleadstoaperiodicfocusingoftherays,asillustratedin(b).Insuchafiber,eventhoughraysmakinglargerangleswiththeaxistraversealongerpathlength,

theydosoinaregionwithalowerrefractiveindexandhenceatahigherspeedofpropagation:thisleadstoasmallervalueofpulsedispersioninsuchfibersascomparedtohomogeneouscorefibers.2

CaptionofFigure16.311存在著不改變場結(jié)構(gòu)并以固定的相位和群速傳播的特殊的場分布Inanopticalwaveguide,thereexistspecificfielddistributionswhichpropagatewithoutchangingtheirformandwithadefinitephaseandgroupvelocity.Thesefieldconfigurationsarereferredtoasthemodesoftheopticalwaveguide.Thesemodesarecharacterizedbydifferentpropagationconstantsanddifferentgroupvelocities.Inamultimodewaveguide,thereexistalargenumberofthesepropagatingmodeswhileinasingle-modewaveguidethereexistsonlyonemode.Unit1612可使傳播模式的場在包層－空氣界面處很弱，使得光纖便于處置和支撐而不會嚴重地擾亂傳播模式Eachmodehasmostoftheenergyinsidethecore,butduetotheevanescentfieldsoutsidethecore,apartoftheenergyisalsotravelinginthecladding.Bymakingthecladdingsufficientlythick,thefieldsofthemodeatthecladding-airboundarycanbemadesmall,thusmakingiteasytohandleandsupportwithoutcausingmuchdisturbancetothemodes.Unit1613各個脈沖必須能在時間上被正確分辨Asalreadydiscussed,inafiberopticcommunicationsystemtheinformationiscodedintheformofdiscretepulseswhicharetransmittedthroughthefiber.Theinformationcapacityofthesystemwillbedeterminedbythenumberofpulsesthatcanbesentperunittime.Fortheinformationtoberetrievedattheoutputend,thevariouspulsesmustbewellresolvedintime.Unit1614在光纖中由于不同模式之間的群速度不等，以及各模式的傳輸常數(shù)依賴于波長等因素，光脈沖會在光纖傳輸過程中變寬。Inanopticalfiberduetovariousfactorslikethedifferencesingroup-velocitybetweenthedifferentmodesandthedependenceofthepropagationconstantofamodeonwavelength,apulseoflightbroadensasitpropagatesthroughthefiber.3

Unit1615脈沖之間必須以一個最小的時間間隔分開，這個時間間隔就確定了系統(tǒng)的最大信息容量Hence,eventhoughtwopulsesmaybewellresolvedattheinputend,becauseofbroadeningofthepulsestheymaynotbesoattheoutput(seeFigure16.4).Insuchacasenoinformationcanberetrievedattheoutput.Thusforagivenbroadening,thepulseshavetobeseparatedbyaminimumtimeintervalwhichwoulddeterminetheultimateinformation-carryingcapacityofthesystem.Unit1616脈沖展寬決定了相鄰脈沖間的最小間隔，也決定了光纖的最大信息容量在光纖輸入端每個脈沖寬度為t1的一串脈沖經(jīng)傳輸后，變成一串寬度為t2(>t1)的脈沖。Figure16.4Aseriesofpulseseachofwidtht1(attheinputendofthefiber)aftertransmissionthroughthefiberemergesasaseriesofpulsesofwidtht2(>t1).Thebroadeninginthepulsesiscausedbythedifferentgroupvelocitiesofthevariousmodesandbythedependenceofthepropagationconstantonwavelength.Ifthebroadeningislarge,thenadjacentpulseswilloverlapattheoutputendandmaynotberesolvable.Thus,pulsebroadeningdeterminestheminimumseparationbetweenadjacentpulses,whichinturndeterminesthemaximuminformation-carryingcapacityofthefiber.Unit16CaptionofFigure16.417每一種模式一般都以不同的特征群速度傳播Whenapulseofradiationisinjectedintoafiber,itexcitesvariousmodesofthefiber.Sinceeachmodepropagateswith,ingeneral,adifferentcharacteristicgroupvelocity,theincidentpulseoflightbroadensasitpropagatesthroughthefiber.Thisisreferredtoasintermodalbroadening.Whenthefibercancarryonlyonepropagatingmode,i.e.,inasingle-modefiber,thisbroadeningisabsent,butduetothedependenceofthepropagationconstantonwavelength,thereisstillsomebroadening;thisisreferredtoasintramodalbroadening.Unit1618后者是由于光源為有限帶寬，以及對不同波長的光有不同的折射率而產(chǎn)生的一種效應(yīng)Bothintermodalandintramodaldispersionsariseasaresultof(a)waveguideeffectsand(b)materialeffects;thelatterduetothefinitebandwidthofthesourceandthefactthatatdifferentwavelengthstherefractiveindicesaredifferent.4

Unit1619使用激光器的系統(tǒng)與使用LED的系統(tǒng)相比其材料色散更小

Itmaybementionedherethatsincelasershavemuchsmallerspectralwidthascomparedtolight-emittingdiodes,thematerialdispersionismuchlowerinasystememployinglasersascomparedtooneusingLEDs.Forexample,withanLED,thepulsebroadeningduetomaterialdispersionmaybe~4nsec/km,whereaswithalaserthiswouldbelessthan0.2nsec/km.Unit1620由于與軸線夾角較大的光線必須經(jīng)過較長的光程，因此它們要用更長的時間到達輸出端Thebroadeningofapulseoflightasitpropagatesthroughanopticalfibercanalsobevisualizedbyusingtheconceptofgeometricaloptics.Whenapulseoflightisinjectedintoahomogeneouscoreopticalfiber,itexcitesraystravelingatdifferentangleswiththeaxis.AscanbeseenfromFigure16.2sinceraysmakinglargerangleswiththeaxishavetotraversealongeropticalpathlength,theytakealongertimetoreachtheoutputend.Unit1621盡管與軸線夾角較大的光線必須通過較長的光程，但它們是在折射率較低的媒介中傳播Consequentlythepulseoflightbroadensasitpropagatesthroughthefiber.Incontrast,inagradedindexfiber,eventhoughraysmakinglargerangleswiththeaxishavetotraverselongerpathlengthstheydosoinamediumwithalowervalueofrefractiveindex(seeFigure16.3).Unit1622Unit16Thusthelongerpathlengthcanbepartiallycompensatedbypropagationatahighervelocity.Hencethebroadeningofapulsemustbemuchlowerinagradedindexfiberascomparedtoahomogeneouscorefiber.Infactthisisindeedthecase,andforhigh-bandwidthapplications,gradedindexfibersaremoresuitablethanhomogeneouscorefibers.23可以制成只存在一種傳播模式的光纖Itmaybementionedherethatinopticalfibershavingverysmallcoreradiiandsmallindexdifferencebetweenthecoreandcladding,itcanbesoarrangedthatonlyonemodeofpropagationexistsinthefiber.Suchfibersarethereforereferredtoassingle-modefibers.Becauseofthepresenceofjustonemode,thedispersioninthesefibersisverysmallandisonlyduetointramodalbroadening.Suchfibersareindeedexpectedtobeusedinfuturesuperhighbandwidthsystems.Unit1624與同軸電纜等傳統(tǒng)金屬系統(tǒng)相比，通過光纖通信或傳輸還有另一些優(yōu)點Inadditiontotheextremelylargeinformation-carryingcapacityofasystemusinglightwaves,communicationortransmissionthroughopticalfibers

hasseveralotheradditionaladvantagesovertheconventionalmetallicsystemslikethecoaxialcable,etc.

Unit1625因為實際可獲得的光纖傳輸損耗極低直徑大約是100微米(i)Becauseof

theextremelylowtransmissionlossofpracticallyavailablefibers,onecanhavemuchgreaterdistancebetweenrepeaterstations,resultinginsubstantialcostsavings.(ii)Opticalfibersaretypicallyabout100mmindiameterandarebasicallymadeofsilicaorglass.Thisresultsinaheavyreductioninweightandvolumeofspacerequired,whichisanimportantconsiderationforlayinginalreadycrowdedavailableconduits.Thissavinginweightandvolumeisalsoimportantforshipboardapplicationsanddatahandlingusingopticalfibersinaircrafts.Unit1626不受電磁干擾影響

由于不存在任何由短路等原因而造成的危險(iii)Opticalfibersareimmunetoelectromagneticinterferenceandthereisnocrosstalk.Thisisanimportantconsiderationforsecurecommunicationsindefense.(iv)Opticalfiberscanbeusedinexplosiveaswellashigh-voltageenvironmentsduetotheabsenceofanyhazardduetoshortcircuits,etc.Unit1627光纖已經(jīng)被用來…傳送數(shù)據(jù)和控制信息，也用于…監(jiān)控地下核爆炸。Unit16Inadditiontothemainapplicationintelecommunications,opticalfibersarealsoexpectedtoplayanimportantroleincomputerlinks,spacevehicles,industrialautomationandprocesscontrol,etc.Infact,recentlyopticalfibershavebeenused

tocarrydataandcontrolinformationwithinbigfusionlasersatLawrenceLivermoreLaboratoryandLosAlamosScientificLaboratoryandalsoformonitoring

undergroundnuclearexplosionsattheNevadatestsite.Theadditionaladvantagesofusingopticalfibersincludelowercostandimmunityfromnoise.28人們還需要能把信息編碼成為光波的調(diào)制器和能在接收端檢測光脈沖并把光脈沖解譯還原成信息的檢測器Whatwehavediscussedaboveisjustoneofthecomponentsofalightwavecommunicationsystem.Inadditiontothis,onerequiresmodulators,whichwouldcodetheinformationintothelightwave,anddetectors,whichcoulddetectthepulsesoflightatthereceiveranddecodetheinformation.Wewilldiscussbrieflytheprinciplebehindmodulatorsanddetectors.Unit16ModulatorsandDetectors

29Unit16通過改變其某個輸入?yún)?shù)如輸入電流來直接調(diào)制Lightsourcescaneitherbemodulateddirectlybyvaryingsomesourceinputparameterlikeinputcurrent,orthelightoutputcanbemodulatedexternallybypassingitthroughdevicesknownasmodulators.Themostpromisingsourcetobeusedinopticalfibercommunicationsystems,namely,thesemiconductorlasersources,canbemodulatedeasilybyvaryingtheinputcurrent.30Unit16光源必須被鍵控調(diào)制鍵控調(diào)制是通過把激光二極管偏置在稍低于門限值上來實現(xiàn)的，門限值一般為100毫安左右。Infact,indigitalsystems,thesourcehastobeon-offmodulated,andpractically,thesemiconductorsourcescanbeon-offmodulatedathighspeedswithrisetimesoflessthanananosecond.Theon-offmodulationisdonebybiasingthelaserdiodeslightlybelowthethresholdvalue,whichistypically~100mA.Atthisstage,thelaserdiodeoperatesasanLEDandemitsincoherentlightatalowopticaloutputpower.31Unit16將二極管激光器從非相干光發(fā)射狀態(tài)轉(zhuǎn)換成具有較大輸出光功率的相干光發(fā)射狀態(tài)這一延遲必須不大于比特之間的間隔從而使光脈沖能精確重建輸入信號Anadditionalcurrent(~20mA)isaddedbyahigh-speeddriver,whichswitchesthelaserdiodefromincoherentemissionstatetoacoherentemissionwithlargeoutputopticalpower.Bykeepingthe“offstate”slightlybelowthreshold,thedelaybetweentheappliedelectricalpulseandtheresultingopticaloutputpulseisminimized;thisdelaymustindeednotbemorethanthebitintervalsothattheopticalpulsesaccuratelyreproducetheinputsignal.5

32Unit16以便兼顧環(huán)境溫度的緩慢變化和激光器本身逐漸老化這兩種因素Animportantfactortobetakencareofisthetemperaturesensitivityoftheoutputopticalpower.Intheabove-mentionedschemeofoperation,thisfactcanbetakencareofbyvaryingtheDCbiasthroughanopticalfeedbackcircuitsoastotakecareofbothslowchangesinambienttemperatureandthegradualagingofthelaseritself.6

33通常由收集激光器背面射出的光來實現(xiàn)輸出功率監(jiān)控，激光器前面發(fā)出的光則全部耦合到光纖中去。Themonitoringoftheoutputpowerisusuallydoneby

collectingthelightemanatingfromthebacksideofthelaser,thelightfromthefrontsurfacebeingcoupledintothefiberitself.7

Unit1634Unit16放在正交的偏振鏡之間Fornonsemiconductorlasersources,anexternalmodulatorisusedformodulation.Theexternalmodulatorsmakeuseofvariouspropertiespossessedbydifferentmaterials.Thus,certaincrystalshaveabirefringencewhichchangesinthepresenceofanappliedelectricfield.Thus,thestateofpolarizationofabeamcanbechangedbypassingitthroughsuchacrystal.Ifthecrystalisplacedbetweencrossedpolarizers,onewouldhaveanintensitymodulation.35Unit16傳播的聲波產(chǎn)生一個折射率光柵，反過來使光波發(fā)生衍射。Similarly,acousto-opticmodulatorsarebasedontheinteractionofanacousticbeamwiththelightwave.Thepropagatingacousticwavecreatesarefractiveindexgratingwhichinturndiffractstheopticalwave.36Unit16光波通信中應(yīng)用的三種重要檢測器是光電倍增管，PIN光二極管和雪崩光二極管。

Atthereceivingterminalsoratrepeaterstations,onerequiresopticaldetectorswhichreceivetheinputopticalsignalandconvertitintoelectricalsignals.Thethreeimportantdetectortypesthatfinduseinlightwavecommunicationarethephotomultiplier,thePINphotodiode,andtheavalanchephotodiode.Eventhoughphotomultiplierspossesslargegains,thelattertwoareexpectedtofindmorewidespreadapplicationbecausetheyarelessbulky,donotneedhighbiasvoltages,andaremuchcheaper.37Unit16由一個具有開闊中心區(qū)域的反偏P-N結(jié)構(gòu)成，為了接收入射光，該區(qū)域涂有抗反射的涂層由此產(chǎn)生的電子和空穴被外加電場分離，產(chǎn)生通過P-N結(jié)的光電流。

Thesimplestsolid-statephotodetectorconsistsofareverse-biasedp-njunction

withanopencenterareathatisanti-reflectioncoatedtoreceivetheincidentlight.Theabsorbedphotonsexciteelectronsfromthevalencebandintotheconductionband.Theelectronsandholessogeneratedareseparatedbyanappliedelectricfieldtoinduceaphotocurrentacrossthejunction.

38Unit16光子產(chǎn)生的電子-空穴對在這種器件中被加速，釋放出更多的電子-空穴對，以此獲得增益。Inordertodetectverylowopticalpowersoneusestheavalanchephotodetector.Inthisdevice,theelectron-holepairproducedbyaphotonoflightisacceleratedinthedeviceandismadetoreleasemoreelectron-holepairs,thusleadingtoagain.39Unit16為了適應(yīng)系統(tǒng)的信息率所產(chǎn)生的一次光電子與入射在檢測器上的光子之比Thephotodetectorsrequiredinafiberopticcommunicationsystemmusthaveahighresponsivityattheoperatingwavelengthandmustalsohavesufficientbandwidthinordertoaccommodatetheinformationrateofthesystem.Themostpromisingphotodetectorforthe0.80mmwavelengthregionseemstobesiliconphotodiodes.Theyhaveveryfastresponsetimes(

0.1nsec).Thequantumefficiency(theratioofprimaryphotoelectronsgeneratedtothephotonsincidentonthedetector)isalsolarge.40結(jié)合課文的思考題Whatarethedifferencesbetweenhomogeneouscorefiberandgradedindexopticalfiber?Whataretheadvantagesofgradedindexopticalfiber?Howdoesinter-symbolinterference(ISI)occurinanopticalfibercommunicationsystem?Inadditiontobeingtransmissionmedia,whatelsecanopticalfibersbeusedfor?41ExercisesTogetherwiththealreadyprovenmodulationbandwidthcapabilityoffibercablethispropertyprovidesatotallycompellingcasefortheadoptionoffibercommunicationinthemajorityoflong-haultelecommunicationapplications.A.anabsolutelyconvincingexampleB.anentirelyconceivableconditionC.averyimportantsituationD.ahighlycompetitivecase42ExercisesUnfortunately,wewillseethatitisnotaseasytodistinguishbetweenpsychologicallyvalidandinvalidAIprogramsasitistodistinguishbetweenphysicallyvalidandinvalidjumpermakingprocedures.AtellwhetherornotAIprogramsareofimportancetopsychologyBprovidepsychologicaldifferencesbetweenavalidAIprogramandaninvalidAIprogramCgivevalidreasonstodistinguishpsychologicallydifferentAIprogramsDseparatepsychologicallyvalidAIprogramsfrompsychologicallyinvalidones43ExercisesWearetodevelopacodesuchthatfewerbitsareassignedtocodewordsrepresentinggraylevelshavinghigherprobabilityofoccurrence,andviceversa.A.inotherwords,graylevelsthatmorefrequentlyoccuraregivenmorebitsB.andmorebitsareusedforgraylevelsoflowerprobabilityofoccurrenceC.otherwise,morebitsareneededinthesimilarcaseD.inotherwords,moreprobablegraylevelsrequireshortercodewords44Weshallusethisparameterasameasureofdegradationduringtheevaluationofdifferentquantizationsystemslaterinthischapter.A.anactionofprocessingB.adegreeofenhancementC.anamountofimpairmentD.aneffectofinterferenceExercises45ExercisesWhatisfascinatingaboutallthisfromtheperspectiveofcomputerarchitectureisthat,ontheonehand,thebasicbuildingblocksfortoday’scomputermiraclesarevirtuallythesameasthoseoftheIAScomputerfrom50yearsago,whileontheotherhand,thetechniquesforsqueezingthelastiotaofperformanceoutofthematerialsathandhavebecomeincreasinglysophisticated.A.inviewofcomputerarchitectureB.withproposedcomputerarchitectureC.takingcomputerarchitectureintoaccountD.fromvariousaspectsofcomputerarchitecture46Figure16.1Atypicalopticalfiberc