《电子信息类专业英语(第二版)第一课

UnitOneOpticalFiberCommunicationUnitOneOpticalFiberCommunicationPassageATheGeneralSystemPassageBAdvantagesofOpticalFiberCommunicationPassageCFiber-opticsCommunicationUnitOneOpticalFiberCommunicationPassageATheGeneralSystemCommunicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistance,acommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiofrequenciesaswellasmicrowaveandmillimeterwavefrequencies.However,“communication”mayalsobeachievedbyusinganelectromagneticcarrierwhichisselectedfromtheopticalrangeoffrequencies.UnitOneOpticalFiberCommunicationAnopticalfibercommunicationsystemissimilarinbasicconcepttoanytypeofcommunicationsystem.AblockschematicofageneralcommunicationsystemisshowninFigure1.1(a),thefunctionofwhichistoconveythesignalfromtheinformationsourceoverthetransmissionmediumtothedestination.Thecommunicationsystemthereforeconsistsofatransmitterormodulatorlinkedtotheinformationsource,thetransmissionmedium,andareceiverordemodulatoratthedestinationpoint.Inelectricalcommunicationstheinformationsourceprovidesanelectricalsignal,usuallyderivedfromamessagesignalwhichisnotelectrical(e.g.sound),toatransmittercomprisingelectricalandelectroniccomponentswhichconvertsthesignalintoasuitableformforpropagationoverthetransmissionmedium.Thisisoftenachievedbymodulatingacarrier,which,asmentionedpreviously,maybeanelectromagneticwave.Thetransmissionmediumcanconsistofapairofwires,acoaxialcableoraradiolinkthroughfreespacedownwhichthesignalistransmittedtothereceiver,whereitistransformedintotheoriginalelectricalinformationsignal(demodulated)beforebeingpassedtothedestination.UnitOneOpticalFiberCommunicationHowever,itmustbenotedthatinanytransmissionmediumthesignalisattenuated,orsuffersloss,andissubjecttodegradationsduetocommunicationbyrandomsignalsandnoise,aswellaspossibledistortionsimposedbymechanismswithinthemediumitself.Therefore,inanycommunicationsystemthereisamaximumpermitteddistancebetweenthetransmitterandthereceiverbeyondwhichthesystemeffectivelyceasestogiveintelligiblecommunication.Forlong-haulapplicationsthesefactorsnecessitatetheinstallationofrepeatersorlineamplifiersatintervals,bothtoremovesignaldistortionandtoincreasesignallevelbeforetransmissioniscontinueddownthelink.UnitOneOpticalFiberCommunicationForopticalfibercommunicationssystemshowninFigure1.1(a)maybeconsideredinslightlygreaterdetail,asgiveninFigure1.1(b).Inthiscasetheinformationsourceprovidesanelectricalsignaltoatransmittercomprisinganelectricalstagewhichdrivesanopticalsourcetogivemodulationofthelightwavecarrier.Theopticalsourcewhichprovidestheelectrical-opticalconversionmaybeeitherasemiconductorlaserorlightemittingdiode(LED).Thetransmissionmediumconsistsofanopticalfibercableandthereceiverconsistsofanopticaldetectorwhichdrivesafurtherelectricalstageandhenceprovidesdemodulationoftheopticalcarrier.Photodiodes(p-n,p-i-noravalanche)and,insomeinstances,phototransistorsandphotoconductorsareutilizedforthedetectionoftheopticalsignalandtheoptical-electricalconversion.Thusthereisarequirementforelectricalinterfacingateitherendoftheopticallinkandatpresentthesignalprocessingisusuallyperformedelectrically.UnitOneOpticalFiberCommunicationInformationsourceTransmissionmediumReceiver(demodulator)Transmitter(modulator)Communicationsystem(a)DestinationUnitOneOpticalFiberCommunicationInformationsourceElectricaltransmiterDestination(b)OpticalsourceOpticalfibercableOpticaldetectorOpticalfibercommunicationsystemElectricalreceiverFigure1.1(a)Thegeneralcommunicationsystem;(b)TheopticalfibercommunicationsystemUnitOneOpticalFiberCommunicationTheopticalcarriermaybemodulatedusingeitherananalogordigitalinformationsignal.InthesystemshowninFigure1.1(b)analogmodulationinvolvesthevariationofthelightemittedfromtheopticalsourceinacontinuousmanner.Withdigitalmodulation,however,discretechangesinthelightintensityareobtained (i.e.on-offpulses).Althoughoftensimplertoimplement,analogmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseratioatthereceiverthandigitalmodulation.[1]Also,thelinearityneededforanalogmodulationisnotalwaysprovidedbysemiconductoropticalsources,especiallyathighmodulationfrequencies.Forthesereasons,analogopticalfibercommunicationlinksaregenerallylimitedtoshorterdistancesandlowerbandwidthsthandigitallinks.UnitOneOpticalFiberCommunicationFigure1.2showsablockschematicofatypicaldigitalopticalfiberlink.Initially,theinputdigitalsignalfromtheinformationsourceissuitablyencodedforopticaltransmission.Thelaserdrivecircuitdirectlymodulatestheintensityofthesemiconductorlaserwiththeencodeddigitalsignal.He