《電子信息類專業(yè)英語 》課件-第12章

Unit12OpticalFiberCommunicationPassageATheGeneralSystemPassageBAdvantagesofOpticalFiberCommunicationPassageCFiber-opticsCommunications

PassageATheGeneralSystem

Communicationmaybebroadlydefinedasthetransferofinformationfromonepointtoanother.Whentheinformationistobeconveyedoveranydistance,acommunicationsystemisusuallyrequired.Withinacommunicationsystemtheinformationtransferisfrequentlyachievedbysuperimposingormodulatingtheinformationontoanelectromagneticwavewhichactsasacarrierfortheinformationsignal.

Thismodulatedcarrieristhentransmittedtotherequireddestinationwhereitisreceivedandtheoriginalinformationsignalisobtainedbydemodulation.Sophisticatedtechniqueshavebeendevelopedforthisprocessbyusingelectromagneticcarrierwavesoperatingatradiofrequenciesaswellasmicrowaveandmillimeterwavefrequencies.However,“communication”mayalsobeachievedbyusinganelectromagneticcarrierwhichisselectedfromtheopticalrangeoffrequencies.

Anopticalfibercommunicationsystemissimilarinbasicconcepttoanytypeofcommunicationsystem.AblockschematicofageneralcommunicationsystemisshowninFigure1.1(a),thefunctionofwhichistoconveythesignalfromtheinformationsourceoverthetransmissionmediumtothedestination.Thecommunicationsystemthereforeconsistsofatransmitterormodulatorlinkedtotheinformationsource,thetransmissionmedium,andareceiverordemodulatoratthedestinationpoint.Inelectricalcommunicationstheinformationsourceprovidesanelectricalsignal,usuallyderivedfromamessagesignalwhichisnotelectrical(e.g.sound),

toatransmittercomprisingelectricaland

electroniccomponentswhichconvertsthesignalintoasuitableformforpropagationoverthetransmissionmedium.Thisisoftenachievedbymodulatingacarrier,which,asmentionedpreviously,maybeanelectromagneticwave.Thetransmissionmediumcanconsistofapairofwires,acoaxialcableoraradiolinkthroughfreespacedownwhichthesignalistransmittedtothereceiver,whereitistransformedintotheoriginalelectricalinformationsignal(demodulated)beforebeingpassedtothedestination.

However,itmustbenotedthatinanytransmissionmediumthesignalisattenuated,orsuffersloss,andissubjecttodegradationsduetocommunicationbyrandomsignalsandnoise,aswellaspossibledistortionsimposedbymechanismswithinthemediumitself.Therefore,inanycommunicationsystemthereisamaximumpermitteddistancebetweenthetransmitterandthereceiverbeyondwhichthesystemeffectivelyceasestogiveintelligiblecommunication.Forlong-haulapplicationsthesefactorsnecessitatetheinstallationofrepeatersorlineamplifiersatintervals,bothtoremovesignaldistortionandtoincreasesignallevelbeforetransmissioniscontinueddownthelink.

ForopticalfibercommunicationssystemshowninFigure12.1(a)maybeconsideredinslightlygreaterdetail,asgiveninFigure12.1(b).Inthiscasetheinformationsourceprovidesanelectricalsignaltoatransmittercomprisinganelectricalstagewhichdrivesanopticalsourcetogivemodulationofthelightwavecarrier.Theopticalsourcewhichprovidestheelectrical-opticalconversionmaybeeitherasemiconductorlaserorlightemittingdiode(LED).

Thetransmissionmediumconsistsofanopticalfibercableandthereceiverconsistsofanopticaldetectorwhichdrivesafurtherelectricalstageandhenceprovidesdemodulationoftheopticalcarrier.Photodiodes(p-n,p-i-noravalanche)and,insomeinstances,phototransistorsandphotoconductorsareutilizedforthedetectionoftheopticalsignalandtheoptical-electricalconversion.Thusthereisarequirementforelectricalinterfacingateitherendoftheopticallinkandatpresentthesignalprocessingisusuallyperformedelectrically.

Figure12.1TheGeneralCommunicationSystemandTheOpticalFiberCommunicationSystem

Theopticalcarriermaybemodulatedusingeitherananalogordigitalinformationsignal.InthesystemshowninFigure12.1(b)analogmodulationinvolvesthevariationofthelightemittedfromtheopticalsourceinacontinuousmanner.Withdigitalmodulation,however,discretechangesinthelightintensityareobtained?(i.e.on-offpulses).Althoughoftensimplertoimplement,analogmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseratioatthereceiverthandigitalmodulation.[1]

Also,thelinearityneededforanalogmodulationisnotalwaysprovidedbysemiconductoropticalsources,especiallyathighmodulationfrequencies.Forthesereasons,analogopticalfibercommunicationlinksaregenerallylimitedtoshorterdistancesandlowerbandwidthsthandigitallinks.

Figure12.2showsablockschematicofatypicaldigitalopticalfiberlink.Initially,theinputdigitalsignalfromtheinformationsourceissuitablyencodedforopticaltransmission.Thelaserdrivecircuitdirectlymodulatestheintensityofthesemiconductorlaserwiththeencodeddigitalsignal.Henceadigitalopticalsignalislaunchedintotheopticalfibercable.Theavalanchephotodiode(APD)detectorisfollowedbyafront-endamplifierandequalizerorfiltertoprovidegainaswellaslinearsignalprocessingandnoisebandwidthreduction.[2]

Finally,thesignalobtainedisdecodedtogivetheoriginaldigitalinformation.However,atthisstageitisinstructivetoconsidertheadvantagesprovidedbylightwavecommunicationviaopticalfibersincomparisonwithotherformsoflineandradiocommunicationwhichhavebroughtabouttheintroductionofsuchsystemsinmanyareasthroughouttheworld.

Figure12.2ATypicalDigitalOpticalFiberLinkBlock

NOTES

[1]?Althoughoftensimplertoimplement,analogmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseratioatthereceiverthandigitalmodulation.

雖然具有光纖系統(tǒng)的模擬調(diào)制易于實(shí)現(xiàn)，但其效率較低，且要求在接收端有比數(shù)字調(diào)制高得多的信噪比。

·signaltonoiseratio為信噪比。

[2]?Theavalanchephotodiode(APD)detectorisfollowedbyafront-endamplifierandequalizerorfiltertoprovidegainaswellaslinearsignalprocessingandnoisebandwidthreduction.

前置放大器與均衡器或?yàn)V波器跟在雪崩二極管檢測器的后面以獲得增益、線性信號處理及噪聲帶寬的抑制。

EXERCISES

1.?Fillintheblanks.

(1)Withinacommunicationsystemtheinformationtransferisfrequentlyachievedby

or

theinformationontoanelectromagneticwave.

(2)Thegeneralcommunicationsystemconsistsofa

or

modulator,thetransmissionmedium,andareceiveror

atthedestinationpoint.

(3)Theopticalsourcewhichprovidesthe

conversionmaybeeitherasemiconductor

orLED.

(4)Thelaserdrivecircuitdirectlymodulatesthe

ofthesemiconductor

laserwiththeencodeddigitalsignal.

(5)TheAPDdetectorisfollowedbya

amplifiertoprovidegain.

(6)Muchmoreenhancedbandwidthutilizationforanopticalfibercanbeachievedbytransmittingseveralopticalsignals,eachatdifferent

,inparallelonthesamefiber.

(7)Opticalfibershaveverysmalldiameterswhichareoften

thanthe

diameterofahumanhair.

(8)Thelightfromopticalfibersdoesnotradiatesignificantlyandthereforethey

providea

degreeofsignalsecurity.

2.?True/False.

(1)?Whentheinformationistobeconveyedoveranydistance,acommunicationsystemisusuallyrequired.()

(2)?Inanycommunicationsystemthereisamaximumpermitteddistancebetweenthetransmitterandthereceiverbeyondwhichthesystemeffectivelyceasestogiveintelligiblecommunication.()

(3)??Digitalmodulationwithanopticalfibercommunicationsystemislessefficient,requiringafarhighersignaltonoiseradioatthereceiverthananalogmodulation.()

(4)??Digitalopticalfibercommunicationlinksaregenerallylimitedtoshorterdistancesandlowerbandwidthsthananaloglinks.()

．

(5)??Theinformation-carryingcapacityofopticalfibersystemshasprovedfarsuperiortothebestcoppercablesystems.()

(6)??TheWDM(wavelengthdivisionmultiplexed)operationoffersthepotentialforafiberinformationcarryingcapacitywhichismayordersofmagnitudeinexcessofthatobtainedusingcoppercablesoraradiosystem.()

(7)??Likethesituationwithcoppercables,atransmittedopticalsignalcanbeobtainedfromafiberinanoninvasivemanner.()

(8)??Takingthesizeandweightadvantageintoaccount,theseopticalfibercablesaregenerallysuperiorintermsofstorage,transportation,handlingandinstallationtocorrespondingcoppercables,whilstexhibitingatleastcomparablestrengthanddurability.()

(9)??Hencewithfewerrepeaters,systemreliabilityisgenerallyenhancedincomparisonwithconventionalelectricalconductorsystem.()

(10)??Althoughtheuseofthelaserforfreespaceopticalcommunicationprovedsomewhat?limited,theinventionforthelaserinstigatedatremendousresearcheffortintothestudyofopticalcomponenttoachievereliableinformationtransferusingalightwave?carrier.()

(11)??Intheory,thegreaterthecarrierfrequency,thesmallertheavailabletransmission?bandwidthandthustheinformation-carryingcapacityofthecommunicationsystem.()

3.?Fillintheblankswiththebestchoice.

(1)Sophisticatedtechniqueshavebeendevelopedfortheprocessofmodulation,demodulationandtransmissionusingelectromagneticcarrierwavesoperatingatradiofrequenciesaswellas

.

a.microwavefrequencyb.millimeterwavefrequency

c.aandb

d.aorb

(2)?Theelectricaltransmissionmediumcanconsistof

.

a.apairofwires b.acoaxialcable

c.aradiolinkthroughfreespacedown d.allofabove

(3)?

arenotutilizedforthedetectionofthesignalandtheoptical-electricalconversion.

a.Lightemittingdiodes b.Phototransistors

c.Photoconductors d.Photodiodes

(4)?Thepropertyofelectricalisolationmakesopticalfibertransmissionideallysuitedforcommunicationinelectricallyhazardousenvironmentsasthefiberscreateno

.

a.arcing b.sparkhazardatabrasions

c.shortcircuits d.anyofabove

(5)?Opticalfibersformadielectricwaveguidebutnotfreefrom

.

a.electromagneticinterferenceb.radiofrequencyinterference

c.mechanicaldamage d.electromagneticpulse

(6)?Fibershavebeenfabricatedwithlossesaslowas

andthisfeaturehasbecomeamajoradvantageofopticalfibercommunication.

a.5dB/km b.0.2dB/km

c.1dB/km d.0.5?dB/km

(7)?Toobtainboththelowlossandlowdispersionatthesameoperatingwavelength,whichkindofnewadvancedsingle-modestructurehasbeenrealized:

.

a.namely b.dispersionshifted

c.dispersionflattened d.allofabove

PassageBAdvantagesofOpticalFiberCommunication

Communicationusinganopticalcarrierwaveguidedalongaglassfiberhasanumberofextremelyattractivefeatures,severalofwhichwereapparentwhenthetechniquewasoriginallyconceived.Furthermore,theadvancesinthetechnologytodatehavesurpassedeventhemostoptimisticpredictions,creatingadditionaladvantages.

Henceitisusefultoconsiderthemeritsandspecialfeaturesofferedbyopticalfibercommunicationsovermoreconventionalelectricalcommunications.Inthiscontextwecommencewiththeoriginallyforeseenadvantagesandthenconsideradditionalfeatureswhichhavebecomeapparentasthetechnologyhasbeendeveloped.

(a)Enormouspotentialbandwidth.Theopticalcarrierfrequencyintherange1013to1016Hz(generallyinthenearinfraredaround1014?Hzor105?GHz)yieldsafargreaterpotentialtransmissionbandwidththanmetalliccablesystems?(i.e.coaxialcablebandwidthuptoaround500?MHz)orevenmillimeterwaveradiosystems?(i.e.systemscurrentlyoperatingwithmodulationbandwidthsof700MHz).Atpresent,thebandwidthavailabletofibersystemsisnotfullyutilizedbutmodulationatseveralgigahertzoverahundredkilometersandhundredsofmegahertzoverthreehundredkilometerswithoutinterveningelectronics?(repeaters)ispossible.

Therefore,theinformation-carryingcapacityofopticalfibersystemshasprovedfarsuperiortothebestcoppercablesystems.Bycomparisonthelossesinwidebandcoaxialcablesystemsrestrictthetransmissiondistancetoonlyafewkilometersatbandwidthsoveronehundredmegahertz.

Althoughtheusablefiberbandwidthwillbeextendedfurthertowardstheopticalcarrierfrequency,itisclearthatthisparameterislimitedbytheuseofasingleopticalcarriersignal.Hencemuchenhancedbandwidthutilizationforanopticalfibercanbeachievedbytransmittingseveralopticalsignals,eachatdifferentcenterwavelengths,inparallelonthesamefiber.Thiswavelengthdivisionmultiplexedoperation,particularlywithdensepackingoftheopticalwavelengths(or,essentially,finefrequencyspacing),offersthepotentialforafiberinformation-carryingcapacitywhichismanyordersofmagnitudeinexcessofthatobtainedusingcoppercablesorawidebandradiosystem.

(b)Smallsizeandweight.Opticalfibershaveverysmalldiameterswhichareoftennogreaterthanthediameterofahumanhair.Hence,evenwhensuchfibersarecoveredwithprotectivecoatingstheyarefarsmallerandmuchlighterthancorrespondingcoppercables.Thisistremendousboontowardsthealleviationofductcongestionincities,aswellasallowingforanexpansionofsignaltransmissionwithinmobilessuchasaircraft,satellitesandevenships.

(c)Electricalisolation.Opticalfiberswhicharefabricatedfromglass,orsometimesaplasticpolymer,areelectricalinsulatorsandtherefore,unliketheirmetalliccounterparts,theydonotexhibitearthloopandinterfaceproblems.Furthermore,thispropertymakesopticalfibertransmissionideallysuitedforcommunicationinelectricallyhazardousenvironmentsasthefiberscreatenoarcingorsparkhazardatabrasionsorshortcircuits.

(d)Immunitytointerferenceandcross-talk.Opticalfibersformadielectricwave-guideandarethereforefreefromelectromagneticinterference(EMI),radiofrequencyinterference(RFI),orswitchingtransientsgivingelectromagneticpulse(EMP).HencetheoperationofanopticalfibercommunicationsystemisunaffectedbytransmissionthroughanelectricallynoisyenvironmentandthefibercablerequiresnoshieldingfromEMI.Thefibercableisalsonotsusceptibletolightningstrikesifusedoverheadratherthanunderground.Moreover,itisfairlyeasytoensurethatthereisnoopticalinterferencebetweenfibersandhence,unlikecommunicationusingelectricalconductors,cross-talkisnegligible,evenwhenmanyfibersarecabledtogether.

(e)Signalsecurity.Thelightfromopticalfibersdoesnotradiatesignificantlyandthereforetheyprovideahighdegreeofsignalsecurity.Unlikethesituationwithcoppercables,atransmittedopticalsignalcannotbeobtainedfromafiberinanoninvasivemanner(i.e.withoutdrawingopticalpowerfromthefiber).Therefore,intheory,anyattempttoacquireamessagesignaltransmittedopticallymaybedetected.Thisfeatureisobviouslyattractiveformilitary,bankingandgeneraldatatransmission(i.e.computernetwork)applications.

(f)Lowtransmissionloss.Thedevelopmentofopticalfibersoverthelasttwentyyearshasresultedintheproductionofopticalfibercableswhichexhibitverylowattenuationortransmissionlossincomparisonwiththebestcopperconductors.Fibershavebeenfabricatedwithlossesaslowas0.2dB/kmandthisfeaturehasbecomeamajoradvantageofopticalfibercommunications.Itfacilitatestheimplementationofcommunicationlinkswithextremelywiderepeaterspacing(longtransmissiondistanceswithoutintermediateelectronics),thusreducingbothsystemcostandcomplexity.Togetherwiththealreadyprovenmodulationbandwidthcapabilityoffibercablethispropertyprovidesatotallycompellingcasefortheadoptionofopticalfibercommunicationinthemajorityoflong-haultelecommunicationapplications.

(g)Ruggednessandflexibility.Althoughprotectivecoatingsareessential,opticalfibersmaybemanufacturedwithveryhightensilestrengths.Perhapssurprisinglyforaglassysubstance,thefibersmayalsobebenttoquitesmallradiiortwistedwithoutdamage.Furthermore,cablestructureshavebeendeveloped,whichhaveprovedflexible,compactandextremelyrugged.Takingthesizeandweightadvantageintoaccount,theseopticalfibercablesaregenerallysuperiorintermsofstorage,transportation,handlingandinstallationtocorrespondingcoppercables,whilstexhibitingatleastcomparablestrengthanddurability.[3]

(h)Systemreliabilityandeaseofmaintenance.Thesefeaturesprimarilystemfromthelowlosspropertyofopticalfibercableswhichreducestherequirementforintermediaterepeatersorlineamplifierstoboostthetransmittedsignalstrength.Hencewithfewerrepeaters,systemreliabilityisgenerallyenhancedincomparisonwithconventionalelectricalconductorsystems.Furthermore,thereliabilityoftheopticalcomponentsisnolongeraproblemwithpredictedlifetimesof20to30yearsnowquitecommon.Boththesefactorsalsotendtoreducemaintenancetimeandcosts.

(i)Potentiallowcost.Theglasswhichgenerallyprovidestheopticalfibertransmissionmediumismadefromsand—notascarceresource.So,incomparisonwithcopperconductors,opticalfibersofferthepotentialforlowcostlinecommunication.Althoughoverrecentyearsthispotentialhaslargelybeenrealizedinthecostsoftheopticalfibertransmissionmediumwhichforbulkpurchasesisnowbecomingcompetitivewithcopperwires(i.e.twistedpairs),ithasnotyetbeenachievedinalltheothercomponentareasassociatedwithopticalfibercommunications.Forexample,thecostsofhighperformancesemiconductorlasersanddetectorphotodiodesarestillrelativelyhigh,aswellassomeofthoseconcernedwiththeconnectiontechnology(demountableconnectors,couplers,etc).

Overallsystemcostswhenutilizingopticalfibercommunicationonlong-haullinks,however,aresubstantiallylessthanthoseforequivalentelectricallinesystemsbecauseofthelowlossandwidebandpropertiesoftheopticaltransmissionmedium.Asindicatedin(f),therequirementforintermediaterepeatersandtheassociatedelectronicsisreduced,givingasubstantialcostadvantage.

Althoughthiscostbenefitgivesanetgainforlong-haullinksitisnotalwaysthecaseinshort-haulapplicationswheretheadditionalcostincurred,duetotheelectrical-opticalconversion(andviceversa),maybeadecidingfactor.Nevertheless,thereareotherpossiblecostadvantagesinrelationtoshipping,handing,installationandmaintenance,aswellasthefeaturesindicatedin(c)and(d)whichmayprovesignificantinthesystemchoice.

Thereducingcostsofopticalfibercommunicationshasnotonlyprovidedstrongcompetitionwithelectricallinetransmissionsystems,butalsoformicrowaveandmillimeterwaveradiotransmissionsystems.Althoughthesesystemsarereasonablywidebandtherelativelyshortspan“l(fā)ineofsight”transmissionnecessitatesexpensiveaerialtowersatintervalsnogreaterthanafewtensofkilometers.Henceopticalfiberisfastbecomingthedominanttransmissionmediumwithinthemajorindustrializedsocieties.

Manyadvantagesarethereforeprovidedbytheuseofalight-wavecarrierwithinatransmissionmediumconsistingofanopticalfiber.Thefundamentalprinciplesgivingrisetotheseenhancedperformancecharacteristics,togetherwiththeirpracticalrealization,aredescribedinthefollowingchapters.However,ageneralunderstandingofthebasicnatureandpropertiesoflightisassumed.

NOTES

[1]?Atpresent,thebandwidthavailabletofibersystemsisnotfullyutilizedbutmodulationatseveralgigahertzoverahundredkilometersandhundredsofmegahertzoverthreehundredkilometerswithoutinterveningelectronics(repeaters)ispossible.

目前，傳輸100km的幾GHz的調(diào)制信號和傳輸300km的幾百M(fèi)Hz的調(diào)制信號都是可能的，因此，光纖系統(tǒng)的可用帶寬并沒有得到充分利用。

·availabletofibersystems為形容詞短語作“thebandwidth”的后置定語。

·but引導(dǎo)并列從句“modulation…ispossible”。

·and連接介詞“at”的并列賓語，構(gòu)成介詞短語修飾名詞“modulation”。

·“atseveralgigahertzoverahundredkilometers”中的“overahundredkilometers”為介詞短語，修飾介詞短語“atseveralgigahertz”。

[2]Thiswavelengthdivisionmultiplexedoperation,particularlywithdensepackingoftheopticalwavelengths(or,essentially,finefrequencyspacing),offersthepotentialforafiberinformation-carryingcapacitywhichismanyordersofmagnitudeinexcessofthatobtainedusingcoppercablesorawidebandradiosystem.

應(yīng)用波分復(fù)用(技術(shù))，尤其是密集波分復(fù)用(或者說，實(shí)質(zhì)上的精細(xì)頻分復(fù)用)，使光纖的信息載容量能超過電纜或?qū)拵o線系統(tǒng)好多個數(shù)量級。

·“thiswavelengthdivisionmultiplexedoperation”直譯為“這種波長分割復(fù)用(波分復(fù)用)的運(yùn)(操)作”，按中文習(xí)慣可意譯為“應(yīng)用波分復(fù)用(技術(shù))”。

·“particularlywithdensepackingoftheopticalwavelengths(or,essentially,finefrequencyspacing)”為介詞短語作狀語，副詞particularly作介詞短語的狀語，括號“or”內(nèi)引導(dǎo)“with”的與“packing”并列的賓語“spacing”。這部分直譯為：尤其光波長的密集填塞(或者說，實(shí)質(zhì)上的精細(xì)頻率分隔)。這里“光波長的密集填塞”意為在光纖低損耗波長范圍里(或者說在光纖低損耗窗口)密集地“填塞”光載波波長；波長λ與頻率v由式子v=C/λ(C為光速)相聯(lián)系，因此，波分復(fù)用實(shí)質(zhì)上也是頻分復(fù)用。當(dāng)各光載波之間間隔比較大時，用波長描述其間隔，稱波分復(fù)用；當(dāng)各光載波之間間隔比較小時，用頻率描述其間隔，稱頻分復(fù)用。

·“inexcessof?”為介詞固定搭配，意為“超過”。

·“whichismanyordersofmagnitudeinexcessofthatobtainedusingcoppercablesofawidebandradiosystem”為which引導(dǎo)的“fiberinformation-carryingcapacity”的定語從句。句中that為代詞作介詞of的賓語，代替“information-carryingcapacity”,以避免重復(fù)；“obtained”為過去分詞,作that的定語；“usingcoppercablesorawidebandradiosystem”為現(xiàn)在分詞,作“obtained”的狀語。

[3]??Takingthesizeandweightadvantageintoaccount,theseopticalfibercablesaregenerallyintermsofstorage,transportation,handlingandinstallationtocorrespondingcoppercables,whilstexhibitingatleastcomparablestrengthanddurability.

考慮到光纖尺寸和重量上的優(yōu)勢，一般來說，光纜在貯存、運(yùn)輸、操作和敷設(shè)方面都優(yōu)于相應(yīng)的銅纜，而其強(qiáng)度和耐用性至少與銅纜相當(dāng)。

·“takingthesizeandweightadvantageintoaccount”為現(xiàn)在分詞短語作狀語，“takeintoaccount”為習(xí)慣用法，意為“考慮”，其賓語可放在take后,也可放在account后。

·intermsof為固定搭配，意為“依據(jù)”、“關(guān)于”。

·superiorto意為“優(yōu)于”，這里狀語“intermsof…”插在了superior與to之間。

·“whilstexhibitingatleastcomparablestrengthanddurability”中的“whilst”為連接詞，意同“while”，它引導(dǎo)時間狀語從句，但句中的主語“theseopticalfibercables”和助動詞“are”省略掉了，避免累贅。

EXERCISES

1.?PleasetranslatethefollowingphrasesintoEnglish.

(1)光纖通信 (2)巨大的潛在帶寬

(3)小尺寸和輕重量 (4)電絕緣

(5)抗干擾和無串話 (6)信號保密

(7)低傳輸損耗 (8)結(jié)實(shí)而柔韌

(9)系統(tǒng)可靠和維護(hù)方便 (10)潛在的低成本

2.?PleasetranslatethefollowingphrasesintoChinese.

(1)?opticalcarrierfrequency

(2)?coaxialcable

(3)?millimeterwave

(4)?fiberinformation-carryingcapacity

(5)?protectivecoatings

(6)?thealleviationofductcongestionincities

(7)?dielectricwave-guide

(8)?electromagneticinterference(EMI)

(9)?radiofrequencyinterference(RFI)

(10)?long-haullink

3.?Fillintheblankswiththebestchoice.

(1)?Inthiscontextwecommence

theoriginallyforeseenadvantagesandthenconsideradditionalfeatureswhichhavebecomeapparent

thetechnologyhasbeendeveloped.

a.with,as b.to,as

c.with,with d.with,dueto

(2)Thisdensewavelengthdivisionmultiplexedoperationoffersthepotentialforafiberinformation-carryingcapacitywhichismanyordersofmagnitude

thatobtainedusingcoppercablesorawidebandradiosystem.

a.of b.to

c.inexcessof d.with

(3)?Opticalfibersarefabricated

,orsometimes

.

a.fromglasses,aplasticpolymer

b.fromglass,aplasticpolymer

c.fromaplasticpolymer,glass

d.fromaglass,aplasticpolymer

(4)?Thedevelopmentofopticalfibersoverthelasttwentyyearshasresultedintheproductionofopticalfibercables

exhibitverylowattenuationortransmissionloss

thebestcopperconductors.

a.which,compared b.which,comparing

c.what,incomparisonwith d.which,incomparisonwith

(5)?Opticalfiberform

andaretherefore

electromagneticinterference(EMI),radiofrequencyinterference(RFI),orswitchingtransientsgivingelectromagneticpluse(EMP).

a.conductor,immunefrom

b.adielectricwave-guide,freefrom

c.aconductor,immunefrom

d.adielectricwave-guide,freeof

(6)?Theglass

generallyprovidestheopticalfibertransmissionmediumismadefrom

.

a.what,sand b.which,soil

c.which,sand d.what,soil

4.?True/False.

(1)?Atpresent,thebandwidthavailabletofibersystemsisnotfullyutilizedyet.()

(2)?Evenwhenopticalfibersareconvenedwithprotectivecoatingstheyarefarsmallerbutmuchheavierthancorrespondingcoppercables.()

(3)?Opticalfibershaveverysmalldiameterwhichareoftenmuchgreaterthanthediameterofahumanhair.()

(4)?Thiswavelengthdivisionmultiplexingtechniquemakesafiberinformationcarryingcapacitymuchgreater.()

(5)Opticalfibertransmissionsareideallysuitabletocommunicationsinelectricallyhazardousenvironmentsasthefiberscreatearcingorsparkhazardatabrasionsorshortcircuits.()

(6)Opticalfibersarefreefromelectromagneticinterferenceastheyformadielectricwave-guide.()

(7)?Intheory,anyattempttoacquireamessagesignaltransmittedthroughfibersmaybedetected.()

(8)?Verylowtransmissionlossoffibersfacilitatesthe