1光纤通信系统OpticalFiberCommunications英文资料及中文翻译Communicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistanceacommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiorequitesaswellasmicrowaveandmillimeterwavefrequencies.Thecarriermaybemodulatedbyusingeitheropticalananalogdigitalinformationsignal..Analogmodulationinvolvesthevariationofthelightemittedfromtheopticalsourceinacontinuousmanner.Withdigitalmodulation,however,discretechangesinthelengthintensityareobtained(i.e.on-offpulses).Althoughoftensimplertoimplement,analogmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseratioatthereceiverthandigitalmodulation.Also,thelinearityneededforanalogmodulationismotalwaysprovidedbysemiconductoropticalsource,especiallyathighmodulationfrequencies.Forthesereasons,analogopticalfibercommunicationslinkaregenerallylimitedtoshorterdistancesandlowerbandwidthsthandigitallinks.Initially,theinputdigitalsignalfromtheinformationsourceissuitablyencodedforopticaltransmission.Thelaserdrivecircuitdirectlymodulatestheintensityofthesemiconductorlastwiththeencodeddigitalsignal.Henceadigitalopticalsignalislaunchedintotheopticalfibercable.Theavalanchephotodiodedetector(APD)isfollowedbyafront-endamplifierandequalizerorfiltertoprovidegainaswellaslinearsignalprocessingandnoisebandwidthreduction.Finally,thesignalobtainedis2decodedtogivetheoriginaldigitalinformation.GeneratingaSerialSignalAlthoughaparallelinput-outputschemecanprovidefastdatatransferandissimpleinoperation,ithasthedisadvantageofrequiringalargenumberofinterconnections.Asanexampletypical8bitparalleldataportuses8datalines,plusoneortwohandshakelinesandoneormoregroundreturnlines.Itisfairlycommonpracticetoprovideaseparategroundreturnlineforeachsignalline,soan8bitportcouldtypicallyusea20coreinterconnectioncable.Whilstsuchamultiwaycableisquiteacceptableforshortdistancelinks,uptoperhapsafewmeters,itbecomestooexpensiveforlongdistancelinkswhere,inadditiontothecostofthemultiwordcable,separatedriverandreceivercircuitsmayberequiredoneachofthe10signallines.Wherepartofthelinkistobemadeviaaradiolink,perhapsthroughaspacesatellite,separateradiofrequencychannelswouldberequiredforeachdatabitandthisbecomesunacceptable.Analternativetotheparalleltransferofdataisaserialinwhichthestatesoftheindividualdatabitsaretransmittedinsequenceoverasinglewirelink.Eachbitisallocatedafixedtimeslot.Atthereceivingendtheindividualbitstatesaredetectedandstoredinseparateflip-flopstages,sothatthedatamaybereassembledtoproduceaparalleldataword.Theadvantageofthisserialmethodoftransmissionisthatitrequiresonlyonesignalwireandagroundreturn,irrespectiveofthenumberofbitsinthedatawordbeingtransmitted.Themaindisadvantageisthattherateatwhichdatacanbetransferredisreducedincomparisonwithaparalleldatatransfer,sincethebitsaredealtwithinsequenceandthelargerthenumberofbitsintheword,theslowerthemaximum...
