《电子信息类专业英语 》课件-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.

虽然具有光纤系统的模拟调制易于实现，但其效率较低，且要求在接收端有比数字调制高得多的信噪比。

·signaltonoiseratio为信噪比。

[2] Theavalanchephotodiode(APD)detectorisfollowedbyafront-endamplifierandequalizerorfiltertoprovidegainaswellaslinearsignalprocessingandnoisebandwidthreduction.

前置放大器与均衡器或滤波器跟在雪崩二极管检测器的后面以获得增益、线性信号处理及噪声带宽的抑制。

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“lineofsight”transmissionnecessitatesexpensiveaerialtowersatintervalsnogreaterthanafewtensofkilometers.Henceopticalfiberisfastbecomingthedominanttransmissionmediumwithinthemajorindustrializedsocieties.

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

NOTES

[1] Atpresent,thebandwidthavailabletofibersystemsisnotfullyutilizedbutmodulationatseveralgigahertzoverahundredkilometersandhundredsofmegahertzoverthreehundredkilometerswithoutinterveningelectronics(repeaters)ispossible.

目前，传输100km的几GHz的调制信号和传输300km的几百MHz的调制信号都是可能的，因此，光纤系统的可用带宽并没有得到充分利用。

·availabletofibersystems为形容词短语作“thebandwidth”的后置定语。

·but引导并列从句“modulation…ispossible”。

·and连接介词“at”的并列宾语，构成介词短语修饰名词“modulation”。

·“atseveralgigahertzoverahundredkilometers”中的“overahundredkilometers”为介词短语，修饰介词短语“atseveralgigahertz”。

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

应用波分复用(技术)，尤其是密集波分复用(或者说，实质上的精细频分复用)，使光纤的信息载容量能超过电缆或宽带无线系统好多个数量级。

·“thiswavelengthdivisionmultiplexedoperation”直译为“这种波长分割复用(波分复用)的运(操)作”，按中文习惯可意译为“应用波分复用(技术)”。

·“particularlywithdensepackingoftheopticalwavelengths(or,essentially,finefrequencyspacing)”为介词短语作状语，副词particularly作介词短语的状语，括号“or”内引导“with”的与“packing”并列的宾语“spacing”。这部分直译为：尤其光波长的密集填塞(或者说，实质上的精细频率分隔)。这里“光波长的密集填塞”意为在光纤低损耗波长范围里(或者说在光纤低损耗窗口)密集地“填塞”光载波波长；波长λ与频率v由式子v=C/λ(C为光速)相联系，因此，波分复用实质上也是频分复用。当各光载波之间间隔比较大时，用波长描述其间隔，称波分复用；当各光载波之间间隔比较小时，用频率描述其间隔，称频分复用。

·“inexcessof ”为介词固定搭配，意为“超过”。

·“whichismanyordersofmagnitudeinexcessofthatobtainedusingcoppercablesofawidebandradiosystem”为which引导的“fiberinformation-carryingcapacity”的定语从句。句中that为代词作介词of的宾语，代替“information-carryingcapacity”,以避免重复；“obtained”为过去分词,作that的定语；“usingcoppercablesorawidebandradiosystem”为现在分词,作“obtained”的状语。

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

考虑到光纤尺寸和重量上的优势，一般来说，光缆在贮存、运输、操作和敷设方面都优于相应的铜缆，而其强度和耐用性至少与铜缆相当。

·“takingthesizeandweightadvantageintoaccount”为现在分词短语作状语，“takeintoaccount”为习惯用法，意为“考虑”，其宾语可放在take后,也可放在account后。

·intermsof为固定搭配，意为“依据”、“关于”。

·superiorto意为“优于”，这里状语“intermsof…”插在了superior与to之间。

·“whilstexhibitingatleastcomparablestrengthanddurability”中的“whilst”为连接词，意同“while”，它引导时间状语从句，但句中的主语“theseopticalfibercables”和助动词“are”省略掉了，避免累赘。

EXERCISES

1. PleasetranslatethefollowingphrasesintoEnglish.

(1)光纤通信 (2)巨大的潜在带宽

(3)小尺寸和轻重量 (4)电绝缘

(5)抗干扰和无串话 (6)信号保密

(7)低传输损耗 (8)结实而柔韧

(9)系统可靠和维护方便 (10)潜在的低成本

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