电子信息与通信工程专业英语课件chapter10

电子信息与通信工程专业英语Chapter10OpticalFiber

Communications10.1TheBasicCommunicationsSystem1Abasiccommunicationssystemconsistsof2atransmitter,areceiver,andaninformationchannel.Atthetransmitter,themessageisgeneratedandputintoaformsuitablefortransferovertheinformationchannel.Theinformationtravelsfromthetransmittertothereceiveroverthischannel.Informationchannelscanbedividedinto3twocategories:unguidedchannel,andguidedchannel.Theatmosphereisanexampleofanunguidedchanneloverwhichwavescanpropagate.Systemsusingatmosphericchannelsincludecommercialradioandtelevisionbroadcastsandmicrowaverelaylinks.Guidedchannelsincludeavarietyofconductingtransmissionstructures.Therearetwo-wireline,coaxialcable,andrectangularwaveguide.Guidedlinescostmoretomanufacture,install,andservicethando4atmosphericchannels.Guidedchannelshavetheadvantagesofprivacy,weatherindependence,andtheabilitytoconveymessageswithin,under,andaroundphysicalstructures.Fiberwaveguideshavetheseadvantagesandothers.Atthereceiver,themessageisextractedfromtheinformationchannelandputintoitsfinalform.Amoredetailed,butstillquitegeneralblockdiagramappearsinFigure10.1.Abriefdiscussionofeachblockinthisfiguregivesusagoodfeel5forthemainelementsofacommunicationssystem.Descriptionsoftheseelementsemphasizethosesuitableforfibersystems.Manyoftheconcisedescriptionsgiveninthissectionareexpandedlater.Fornow6,wepresentanoverviewofthesubject.Figure10.1Ageneralizedfiberopticcommunicationssystem10.1.1MessageOriginThemessageoriginmaytakeseveralphysicalforms.Quiteoftenitisatransducerthatconvertsanonelectricalmessageintoanelectricalsignal.Commonexamplesincludemicrophonesforconvertingsoundwavesintocurrentsandvideo(TV)camerasforconvertingimagesintocurrents.Insomecases,suchasdatatransferbetweencomputersorpartsofacomputer,themessageisalreadyinelectricalform.Thissituationalsoariseswhenafiberlinkconstitutesaportionofsomelargersystem.Examplesincludefibersusedinthegroundportionofasatellitecommunicationssystemandfibersusedinrelayingcabletelevisionsignals.Inanycase7,theinformationmustbeinelectricalformbeforetransmissionforeitherelectronicoropticalcommunications.10.1.2ModulatorThemodulatorhastwomainfunctions.First,itconvertstheelectricalmessageintotheproperformat.Second,itimpressesthissignalontothewavegeneratedbythecarriersource.Twodistinctcategoriesofmodulationformatareanaloganddigital.Ananalogsignaliscontinuousandreproducestheformoftheoriginalmessagequitefaithfully.Forexample,supposeasoundwavecontainingasingletoneistobetransmitted.Theelectricalcurrentproducedwhenamicrophonepicksupthiswavehasthesameshapeasthewaveitself.Inthiscase,themodulatorneednotchangetheformatofthesignal.Itcouldbeappropriatetoamplifythissignal,however,sothatthesignalwillbestrongenoughtodrivethecarriersource.Digitalmodulationinvolvestransmittinginformationindiscreteform.Thesignaliseitheronoroff.Theonstaterepresentsadigital1,theoffstateadigital0.Thesestatesarethebinarydigits(orbits)ofthedigitalsystem.Thedatarateisthenumberofbitspersecond(b/s)transmitted.Thesequenceofonandoffpulsesmightbeacodedversionofananalogmessage.Ananalog-to-digitalconverterdevelopsthedigitalsequencefromtheanalogmessage.Thereverseprocessoccursatthereceiver,wherethedigitalsignalisreturnedtoitsanalogform.Toimpressadigitalsignalontoacarrier,themodulatorneedonlyturnthesourceonoroffattheappropriatetime.Theeaseofconstructingdigitalmodulatorsmakesthisformatveryattractiveforfibersystems.10.1.3CarrierSourceThecarriersourcegeneratesthewaveonwhichtheinformationistransmitted.Thiswaveiscalledthecarrier.Thecarrierisproducedbyanelectronicoscillatorinradio-frequencycommunicationsystems.Forfiberopticsystems,alaserdiode(LD)oralight-emittingdiode(LED)isused.Thesetwodevicescancorrectlybecalledopticoscillators.Ideally,theyprovidestable,single-frequencywaveswithsufficientpowerforlong-distancepropagation.Actuallaserdiodesandlight-emittingdiodesdiffersomewhatfromthisideal.Theyemitarangeoffrequenciesandgenerallyradiateonlyafewmilliwatts8ofaveragepower.Thispowerissufficientinmanycases,becausereceiversaresosensitive.However,transmissionlossescontinuallydecreasethepowerlevelalongthefiber,sothelackofasufficientsourcepowerlimitsthelengthofanycommunicationslink.Thelackofatruesingle-frequencysourcealsodegradesthesystem.Thisdegradationlimitstheamountofinformationthatcanbecarriedalongagivenpathlength.LEDsandlaserdiodesaresmallandlightandconsumeonlymoderateamountsofpower.Itisrelativelyeasytomodulatethem—thatis,toimpresstheinformationontheirradiation.Bothofthesedevicesareoperatedbypassingcurrentthroughthem.Theamountofpowerthattheyradiatecanbemadeproportionaltothiscurrent.Inthisway,theopticoutputpowertakestheshapeoftheinputcurrentcomingfromthemodulator.Itshouldbeemphasizedthattheinformationbeingtransmittediscontainedinthevariationoftheopticpower.Thisiscalledintensitymodulation(IM).Althoughthesignalcurrenthasbothpositiveandnegativeparts,theoutputpowerofalightemitterisalwayspositive.Toachievelinearity,theactualmodulationcurrentinananalogsystemmustbeentirelypositive.ADCbiascurrent,addedtothedesiredinformationsignal,plishesthisresult.Similarly,themodulatingcurrentforadigitalsystemisalwayspositive.Becausealaserdiodedoesnotturnon(thatis,itdoesnotradiate)untilsomethresholdcurrentisapplied,themodulationcurrentcouldincludeaDCoffsetequaltothisthresholdvalue9.Thepresenceofabinary1drivesthecurrentbeyondthresholdandmakesthediodeemitlight.Abinary0leavesthecurrentatthreshold,wherenoradiationoccurs.AnLEDdoesnothaveathresholdandturnsonwheneverpositivecurrentflowsthroughit.LaserdiodesandLEDshavebeenconstructedthatradiateatfrequencieswhereglassfibersareefficienttransmittersoflight—thatis,wherefibershavelowattenuation.Thisisindeedfortunate,becausesuitablesourcesemittingatarbitraryfrequenciesaredifficulttoobtain.Withoutthismatchbetweensourcefrequencyandfiberlow-lossregion,fiberopticalcommunicationswouldnotexist.10.1.4ChannelCouplerNextweconsiderthecoupler,whichfeedspowerintotheinformationchannel.Inaradioortelevisionbroadcastingsystem,thiselementisanantenna.Theantennatransfersthesignalsfromthetransmitterontotheinformationchannel—inthiscase,theatmosphere.Inaguidedsystemusingwires,suchasatelephonelink,thecouplerneedonlybeasimpleconnectorforattachingthetransmittertothetransmissionlineusedastheinformationchannel.Foranatmosphericopticsystem,thechannelcouplerisalensusedforcollimatingthelightemittedbythesourceanddirectingthislighttowardthereceiver.Inourfibersystem,thecouplermustefficientlytransferthemodulatedlightbeamfromthesourcetotheopticfiber.Unfortunately,itisnoteasytoplishthistransferwithoutrelativelylargereductionsinpowerorsomewhatcomplicatedcouplerdesigns.Onedifficultyarisesbecauseofthesmallsizeofconventionalfibers,whichhavediametersoftheorderof50millionthsofameter.However,thelargelossbasicallyoccursbecauselightsourcesemitoveralargeangularextent.Fiberscapturelightonlywithinmorelimitedangles.Thelightemitterismerelybuttedagainstthefiber.Asindicated,evenifthefiberisbigenoughtointerceptallthelightraysemittedbythesource,thelightwillnotbeentirelycollected,becauseofthedifferencebetweentheradiationandacceptanceconeangles.Moreefficient,butalsomorecomplex,couplerscanbeconstructed.Thechannelcouplerisanimportantpartofthedesignofafibersystembecauseofthepossibilityofhighlosses.10.1.5InformationChannelTheinformationchannelreferstothepathbetweenthetransmitterandreceiver.Infiberopticcommunications,aglass(orplastic)fiberisthechannel.Desirablecharacteristicsoftheinformationchannelincludelowattenuationandalargelight-acceptance-coneangle.Lowattenuationandefficientlightcollectionareparticularlynecessaryfortransmissionoverlongpathlengths.Althoughhighlysensitivereceiversareavailable,thepowerdeliveredtothereceivermustbeabovesomelimitingvaluetoconveythedesiredmessagewithappropriateclarity.Opticalamplifiersboostthepowerlevelsofweaksignals.Theyareneededinverylonglinks(hundredsandthousandsofkilometers)toprovidesufficientpowertothereceiver.Repeaters(alsoknownasregenerators)convert10weakanddistortedopticalsignalstoelectricalonesandthenregeneratetheoriginaldigitalpulsetrainsforfurthertransmission.Repeaterscanbeusedonlyfordigitalsystems;opticalamplifiersaresuitableforbothanaloganddigitalsignals.Opticalamplifierscompensateonlyforsignalattenuation,butrepeatersreconstituteboththeamplitudeandwaveshapeofadigitalsignal.Inalongsystem,numerousamplifiersandsomerepeatersmaybeutilized.Beingmuchmorecomplex,repeatersaremoreexpensivethanamplifiers.Anotherimportantpropertyoftheinformationchannelisthepropagationtimeofthewavestravelingalongit.Ingeneral,thetraveltimedependsonthelightfrequencyandonthepathtakenbythelightrays.Asignalpropagatingalongafibernormallycontainsarangeofopticfrequencies(becauseopticsourcesemitarangeoffrequencies)anddividesitspoweralongseveralraypaths.Thisresultsinadistortionofthepropagatingsignal.Inadigitalsystem,thisdistortionappearsasaspreadinganddeformingoftheonpulses.Thespreadingincreaseswiththedistancetraveled.Eventually,thespreadingissogreatthatadjacentpulsesbegintooverlapandeunrecognizableasseparatebitsofinformation.Errorsinthetransmissionnowresult.Tokeepthisfromoccurring,pulsesmustbetransmittedlessfrequently.This,ofcourse,limitstherateatwhichthepulsescanbesent.Thewave-velocitydependenceonfrequencyandpathresultsinalimitationontheinformationrate,whetherthemodulationisdigitaloranalog.Therequirementsforlargelight-acceptanceangleandlowsignaldistortionarecontradictory.Practicalfibersrepresentadesigncompromisebetweenthesetwoqualities.Forsystemshavingmoderatepathlengthsandinformationrates,fiberswithsuitablevaluesofacceptanceangleandsignaldistortioncanbeobtained.Inanatmosphericelectroniccommunicationssystem,anantennacollectsthesignalfromtheinformationchannelandroutesittotherestofthereceiver.Inthefibersystem,theoutputcouplermerelydirectsthelightemergingfromthefiberontothelightdetector.Thislightisradiatedinapatternidenticaltothefiber'sacceptancecone.Becausecommonphotodetectorshavelargesurfaceareasandlargeacceptanceangles,lightcanbeefficientlycoupledfromthefiberbyasimplebuttconnection.10.1.6DetectorTheinformationbeingtransmittedmustnowbetakenoffthecarrierwave.Inanelectronicsystem,thisistheprocessofdemodulatingthesignalandisperformedbytheproperelectroniccircuit.Inthefibersystem,theopticwaveisconvertedintoanelectriccurrentbyaphotodetector.Semiconductorphotodiodesofvariousdesignsaremostcommonlyused.Thecurrentdevelopedbythesedetectorsisproportionaltothepowerintheincidentopticwave.Becausetheinformationiscontainedintheopticpowervariation,thedetectoroutputcurrentcontainsthisinformation.Thiscurrentisareplicaofthatusedtodrivethecarrierlightsource.Importantpropertiesofphotodetectorsincludesmallsize,economy,longlife,lowpowerconsumption,highsensitivitytoopticsignals,andfastresponsetoquickvariationsintheopticpower.Fortunately,lightdetectorshavingthesecharacteristicsarereadilyavailable.10.1.7SignalProcessorForanalogtransmission,thesignalprocessorincludesamplificationandfilteringofthesignal.Inadditiontofilteringoftheconstantbias,anyotherundesiredfrequencies,shouldbeblockedfromfurthertravel.Anidealfilterpassesallfrequenciescontainedinthetransmittedinformationandrejectsallothers.Thisimprovestheclarityoftheintendedtransmission.Properfilteringmaximizestheratioofsignalpowertounwantedpower.Randomfluctuationsinthereceivedsignalarereferredtoasnoise.Noiseispresentinallcommunicationsystems.Foradigitalsystem,theprocessorcanincludedecisioncircuitsinadditiontoamplifiersandfilters.Thedecisioncircuitdecideswhetherabinary1or0wasreceivedduringthetimeslotofanyindividualbit.Becauseofunavoidablenoise,therewillalwaysbesomeprobabilityoferrorinthisprocess.Thebit-errorrate(BER)shouldbeverysmallforqualitycommunications.Thedigital-signalprocessormustalsodecodetheingsequenceof0sand1siftheoriginalmessagewasanalog.Thisisdonebyadigital-to-analogconverter,whichrecreatestheoriginalelectricalformoftheinformation.Ifthecommunicationswerebetweenmachines,thenthedigitalformmightbesuitableforusewithoutdigital-to-analogconversion.10.1.8MessageOutputWeareconcernedwithtwodifferentsituationsatthispoint.Inonecase,themessageispresentedtoaperson,whoeitherhearsorviewstheinformation.Toachievethis,theelectricalsignalmustbetransformedintoasoundwaveoravisualimage.Suitabletransducersforplishingthistransformationaretheloudspeakerforaudiomessagesandthecathoderaytube(similartotheonesusedinatelevisionset)forpictures.Inthesecondcase,theelectricalformofthemessageemergingfromthesignalprocessorisdirectlyusable.Thissituationoccurs,forexample,whencomputersorothermachinesareconnectedthroughafibersystem.Italsooccurswhenthefibersystemisonlyapartofalargernetwork,asinafiberlinkbetweentelephoneexchangesorafibertrunklinecarryinganumberoftelevisionprograms.Intheselasttwosystems,theprocessingincludesdistributionoftheelectricalsignalstotheproperdestinations.Themessageoutputdeviceissimplyanelectricalconnectorfromthesignalprocessortothesucceedingsystem.Weareinterestedonlyperipherallyinsignal-processingcircuitsandmessage-outputdevices,becausethesecomponentsarethesameasthosealreadydevelopedforsystemsthatdonotuseoptics.Newwordsandphraseselectrical [iˈlektrikəl] adj. 与电有关的，电学的，电的microphone [ˈmaikrəfəun] n. 麦克风；话筒；传声器current [ˈkʌrənt] adj.n. 现在的，现行的；通用的，通行的，被普遍接电流portion [ˈpɔ:ʃən] n.vt. 一部分，一份把……分成份额，分配proper [ˈprɔpə] adj. 适合的；适当的continuous [kənˈtinjuəs] adj. 连续的，没有中断的appropriate [əˈprəupriət] adj. 合适的，适合的，适宜的converter [kənˈvə:tə] n. 转换器；变频器；整流器；变流器occur [əˈkə:] vi. 发生；举行；存在attractive [əˈtræktiv] adj. 有吸引力的oscillator [ˈɔsileitə] n. 振荡器sufficient [səˈfiʃənt] adj. 足够的；充足的propagation [prˈɔpəˈɡeiʃən] n. 繁殖；增殖radiate [ˈreidieit] vt. 发射出moderate [ˈmɔdərit] adj. 中等的，适度的，不是很大的arbitrary [ˈɑ:bitrəri] adj. 任意的；武断的；随心所欲的region [ˈri:dʒən] n. 地区；地带；区域；范围lens [lenz] n. 透镜，镜头；镜片Notes1.本节涉及光纤通信领域，标题可译为：光纤通信。2.consistsof由……组成，由……构成。此句译为：基本通信系统由发送机、接收机和信道构成。同义词组：form;makeupof，compose，formation，composition，configuration。3.bedividedinto被分成。此句译为：信道可以分为两类，即非导向信道和导向信道。例句：Theseimprovementscanbedividedintogeneralfeaturecategories:synchronization,thestorageengine,andthequeryprocessor.这些改进可划分为以下常规功能类别：同步、存储引擎和查询处理器。4.do用于加强语气和倒装语序的句中。此句译为：在制造、铺设和服务方面，被导引线路要比大气信道的成本更高。例句：Theaideadded:"Itdependsonhowyouseebenefits:aretheyarightoraprivilege?Sanctionslikethisdosendastrongmessage."该顾问补充说：“这取决于你如何看待福利：它们是一种权利还是一种特权？这种制裁措施确实传递了一条强有力的信息。”5.goodfeel深入的，很好的。此句译为：通过对图中的每个模块进行简单讨论，可以对通信系统的主要单元有很好的了解。例句：Trialanderror,andagoodfeelfortheproblem,arethebestwaysaroundthisissue.尝试、出错，以及深入的思考是解决这个问题的最佳途径。6.fornow眼下，目前，暂时。此句译为：现在就对该内容做概要介绍。例句：Fornow,thedegreetowhichtabletsarecannibalizingPCsalesisamatteroffiercedebate.目前为止，平板计算机挤占个人计算机市场的程度成为激烈争论的焦点。7.inanycase无论如何，总之。同类词语：whateverhappens,comewhatmay,anyway,ineithercase。此句译为：在任何情况下，无论是在电通信还是在光通信中，信息在传输之前都必定是电形态的。8.milliwatts【电】毫瓦(特)。milli，千分之一。例如：millisecond毫秒，millimeter毫米,millinile毫反应性单位。9.Becausealaserdiodedoesnotturnon(thatis,itdoesnotradiate)untilsomethresholdcurrentisapplied,themodulationcurrentcouldincludeaDCoffsetequaltothisthresholdvalue.turnon开启，turnoff关闭。DC=directcurrent的缩写，直流电。thresholdvalue，门限值。此句译为：半导体激光器在未达到阈值电流以前是不会开启的(也就是说，不辐射)，调制电流中应该包括一个约等于这个阈值的直流偏置。10.convert…to…，将……转换为……。此句译为：中继器(也就是所谓的再生器)将微弱的并且已失真的光信号转换成电信号，然后还原成原来的数字脉冲串，以便进行进一步传输。10.2AdvantageofFibers1Wearenowreadytodiscusstheadvantagesofopticfibers.Beforedoingso2,letusmentionafewwordsofcaution.Fibersystemsarenotperfect.Theyhavetechnicalandeconomiclimitations.Foranydesiredsystem,therelativemeritsofguidedchannelversusunguidedchannelandmetallicconductorversusfibermustbeevaluated.Thefollowingdiscussionofdesirablefiberpropertiescanbeusefulinthatevaluation.Thebasicmaterialforglassfibersissilicondioxide,whichisplentiful.Someopticfibersaremadeoftransparentplastic,anotherreadilyavailablematerial.Costsareoftenthemostimportantconsiderationinasystem.Comparisonsbetweenfiberandmetalliccablesmustbedonewithcare.Therearemanyfibercablesavailable,someofwhicharecheaperthantheirwireequivalents.Thesavingsmayeparticularlyapparentwhenthecomparisonismadeonthebasisofcostperunitofinformationtransfer.Forexample,avalidcomparisonforatelephonelinkwouldbeonthebasisofcostper3meterpertelephonechannel,ratherthanjustcostpermeter.Thisconsiderationarisesbecausefibershavegreaterinformation-carryingcapacitiesthandometallicchannels.Economiccomparisonsshouldalsoincludethecostsofinstallation,operation,andmaintenance.Somegeneralitiesabouttheseconcernsareworthpresenting.Forlongpaths,fibercablesarecheapertotransportandeasiertoinstallthanmetalcables.Thisisbecausefibersaresmallerandlighter.Onecabledesignhasafiber125µmindiameterenclosedinaplasticsheathof2.5mminouterdiameter.Theweightofthiscableis6kg/km;thelossis5dB/km.LetuscomparethiscablewiththeRG-19/Ucoaxialcable,whichhasanattenuationof22.6dB/kmwhencarryinga100MHzsignal.Itsouterdiameteris28.4mm,anditsweightis1110kg/km.Smallerandlightercoaxialcablesareavailable,buttheyhavehigherlossesthantheRG-19/U.Thesignificantsizeandweightadvantagesoffibercablesareapparentfromthisexample.Therearenogreatdifferencesbetweentheoperationoffibersystemsandthatofmetallicsystems.Thecostshereshouldbethesame.Maintenanceoffibercablesdoesdiffer,however.Ifalineisbroken,asaresultofeitheranaccidentorasystemmodification,splicesmustbemadeornewconnectorsattached.Theseoperationsrequiremoretimeandskillforfibersthanforwires.Asaresult,maintenancecostsshouldbeconsideredwhendesigningasysteminwhichmanychangesarelikelytobemade.Fibersandfibercableshaveturnedouttobesurprisinglystrongandflexible.Somefibersaresoslenderthattheydonotbreakwhenwrappedaroundcurvesofonlyafewcentimetersradius.Fibersareoftenstoredandtransportedwhiletightlywrappedaroundspoolshavingthissmallcurvature4.Fiberflexibilityisattractiveforinstallationscontainingmanyturnsalongthetransmissionpath.Foralarge-radiusbend,fibersguidelightwithnegligibleloss.Thereissomelossataverytightbend,however.Whenafiberisprotected—forexample,byencasingitinaplasticsheath—itisdifficulttobendthecableintoaradiussmallenoughtobreakthefiber.Fibersembeddedincablesdonotbreakeasily.Theadditionofaplasticsheathincreasesthetensilestrengthofafibertransmissionline.Steelrodscanbeplacedinsidetheplasticcabletoaddfurtherstrength,ifneeded.AnotherstrengtheningmaterialisKevlar,asyntheticpolymerfiberwithgreattensilestrength.Despitetheapparentlyfragilenatureofglass,opticfibercablesareveryruggedandserviceable.Techniqueshavebeendevelopedfortheproductionoffiberswithverylowtransmissionlosses.Manyfiberdesignsexist,butanattenuationof4dB/kmistypicalofcommercialglassfiberswhenoperatedatawavelengtharound0.82µm.Thisrepresentsatransmissionefficiencyof40%fora1kmlength.Thisdegreeoftransparencycouldnotbeachievedbefore1970.Now,fiberswithlossesofonlyafewtenthsofadB/kmareavailableforusearound1.3µmand1.55µm.Verylongcommunicationslinkscanbeconstructedbecauseoftheavailabilityoflow-lossfibers.Amplifiers,neededtoamplifyweaksignals,canbelocatedatlargeintervals.Thelossesofwiretransmissionlinesincreaserapidlywithfrequency.Athighfrequencies,linklengthsandamplifierspacingswouldbesignificantlysmallerforwiresystemsthanforfibersystems.Oneofthemostimportantadvantagesoffibersistheirabilitytocarrylargeamountsofinformationandtodosoineitherdigitaloranalogform.Forexample,asinglefiberofthetypedevelopedfortelephoneservicecanpropagatedataattheT3rate,44.7Mbps.Thisfibertransmits672voicechannels.Fiberswithevengreatercapacitiesareavailable.Althoughpulsespreadinglimitsthemaximumrate,fibercapabilitiesmeettherequirementsofmostdata-handlingsystemsandexceedthecapabilitiesofconductingcables.Intheanalogformat,modulationratesofhundredsofmegahertz,ormore,canpropagatealongfibers.Aswiththedigitalsystems,therateislimitedbydistortionoftheopticsignal.Adramaticcomparisoncanbemadebetweenastandardwiretelephonecableandafibercable.Themetalcablecontains900twisted-wirepairs,anditsdiameteris70mm.Eachpaircarries24voicechannels(T1rate),sothecablecapacityis21600calls.Onefibercabledevelopedfortelephoneapplicationshasa12.7mmdiameterandcontains144fibers,eachoperatingattheT3rate(672channels).Thiscablehasatotalcapacityof96768calls.Thefibercablehasnearly4.5timesasmuchcapacityasthewirecableandhasacross-sectionalareathatis30timesless5.Stillmoreremarkablecomparisonscanbemadeifoperationatahigherrate(say2.5Gbpsor10Gb/s)isassumed.Opticfibers,glassorplastic,areinsulators.Noelectriccurrentsflowthroughthem,eitherowingtothetransmittedsignalorowingtoexternalradiationstrikingthefiber.Inaddition,theopticwavewithinthefiberistrapped,sononeleaksoutduringtransmissiontointerferewithsignalsinotherfibers.Conversely,lightcannotcoupleintothefiberfromitsside.Weconcludethatafiberiswellprotectedfrominterferenceandcouplingwithothercommunicationschannels,whethertheyareelectricaloroptic.Forthosereasonsnoted,fibershaveexcellentrejectionofradio-frequencyinterference(RFI)andofelectromagneticinterference(EMI).RFIreferstointerferencecausedbyradioandtelevisionstations,radar,andothersignalsoriginatinginelectronicequipment.EMIincludesbothsuchsourcesofradiationandthosecausedbynaturalphenomena(suchaslightning)orcausedunintentionally(suchassparking).Ifnotrejected,thentheseundesiredsignalscouldincreasethesystemnoiselevelbeyondacceptablelimits.Afiberexcelsatrejectingexternallycausedbackgroundnoise.Theabilityofafibertoisolateitselffromitsenvironmentallowsustopacknumerousfiberstogetherinacabletotransmitmanychannelsofinformationalongasinglepath.Nocrosstalkoccurs.Becausefibersareinsulators,theywillnotpickuporpropagateelectromagneticpulses(EMP)causedbynuclearexplosions,whichcaninducemillionsofvoltsinaconductingtransmissionline.Thevoltagepulsecantravelmanymilesalongthewireandeventually(becauseofitsstrength)destroytheelectronicsattheendofthepath.Theinsulatingnatureofafiberhasseveralotherpracticalconsequences.Inanenvironmentinwhichhigh-voltagelinesarepresent,awirecommunicationslinkcouldpossiblyshort-circuitthelinesbyfallingacrossthem,causingconsiderabledamage.Sparkingoccurringinthisprocesscouldignitecombustiblegasesinthearea.Thisproblemdisappearswithfibers.Anotheradvantageisthatopticcouplingeliminatestheneedforacommongroundbetweenafibertransmitterandreceiver.Troublesomegroundloopsarenotformed.Additionally,itispossibletorepairthefiberwhilethesystemisonwithoutthepossibilityofshort-circuitingtheelectronicsatthetransmitterorreceiver.Thisproblemmightoccurwhenrepairingametalliccable.Fibersofferadegreeofsecurityandprivacy.Becausefibersdonotradiatetheenergywithinthem,itisdifficultforanintrudertodetectthesignalbeingtransmitted.Thefiberwouldhavetobephysicallyviolatedtoobtainthesignal.Breakingthefiber,orfusinganewfibertothetransmittingfiber,wouldprovideaccesstotheopticbeam.Duringsuchmodificationofthelink,thepowerreachingthereceiver,woulddrop.Asensitivereceivercanmeasurethisloss,providingknowledgethatanintrusionisoccurring.Toimprovethesuccessofdetection,thesystemwouldhavetobemonitoredcontinuously.Electroniccommunicationssystemsincludeprocessingofinformationbeforeitisdeliveredtotheinformationchannelandafteritreachesthereceivingterminal.Opticsystemsincludeprocessingthatisverysimilar.Thissimilarityallowsincorporationoffibersin