電子與通信工程專業(yè)英語Unit 6 Optical Fiber Communication

電子與通信工程專業(yè)英語Unit6OpticalFiberCommunicationOpticalfibersguidelightrayswithinthefibermaterial.Theycandothisbecauselightraysbendorchangedirectionwhentheypassfromonemediumtoanother.Theybendbecausethespeedofpropagationoflightineachmediumisdifferent.Thisphenomenoniscalledrefraction.Onecommonexampleofrefractionoccurswhenyoustandattheedgeofapoolandlookatanobjectatthebottomofthepool.Unlessyouaredirectlyovertheobject,itappearstobefartherawaythanitreallyis.Thisaffectoccursbecausethespeedoflightrayfromtheobjectincreasesasthelightraypassesfromthewatertotheair.Thiscausesthemtobend,changingtheangleatwhichyouperceivetheobject.YoucanobtainanappreciationforthemannerbywhichlightflowsbyfocusinguponSnell’sLaw.

HowopticalfibersworkcanbeexplainedbySnell’sLaw,whichstatesthattheratioofthesineoftheangleofincidencetothesineoftheangleofrefractionisequaltotheratioofthepropagationvelocitiesofthewaveinthetworespectivemedia[2].Thisisequaltoaconstantthatistheratiooftherefractiveindexofthesecondmediumtothatofthefirst.Writtenasanequation,Snell’sLawlookslikethis:

(1)

Inthisequation,A1andA2aretheanglesofincidenceandrefraction,respectivelyV1andV2arethevelocitiesofpropagationofthewaveinthetwomedia,andn1,n2aretheindicesofrefractionofthetwomedia.

TheparametersaredemonstratedgraphicallyinFig6.1.Ineachcase,A1istheangleofincidence,andA2istheangleofrefraction.Theindexofrefractionofmaterial1,n1,isgreaterthantheindexofrefractionofmaterial2,n2.Thismeansthatthevelocityofpropagationoflightisgreaterinmaterial2thaninmaterial1.

Fig6.1Theindexofrefraction(n1>n2)

Fig6.1(a)demonstrateshowalightraypassingfrommaterial1tomaterial2isretractedinmaterial2whenA1islessthanthecriticalangle.Fig6.1(b)demonstratestheconditionthatexistswhenA1isatthecriticalangleandangleA2isat90[dg][3].Thelightrayisdirectedalongtheboundarybetweenthetwomaterials.

AsshowninFig6.1(c),anylightraysthatareincidentatanglesgreaterthanA1ofFig6.1(b)willbereflectedbackintomaterial1withangleA2equaltoA1.ThisconditioninFig6.1(c)istheoneofparticularinterestforopticalfibers.Fiberopticcablesarenotsusceptibletoelectronicnoiseandsohavemuchlowererrorratesthannormaltelephonewireandcable.Inaddition,theirpotentialspeedfordatacommunicationsisupto10,000timesthanthatofmicrowaveandsatellitesystems.Fiberopticscommunicationsarealsoveryresistanttoillegaldatatheft,becausetapsintoittolistentoorchangethedatabeingtransmittedcanbeeasilydetected[4].Infact,itiscurrentlybeingusedbytheU.S.centralIntelligenceAgency.Fiberlookslikeacommonglasscylinderconsistingofcoreandcladdingregions.Nowadays,therearethreetypesoffiberopticcablecommonlyused:singlemode,multimodeandplasticopticalfiber(POF).

Singlemodecableismadeupofoneoranumberofquartzfiberswithadiameterof8.3mto10

mthathasonemodeoftransmission.Singlemodefiberwitharelativelynarrowdiameter,willpropagatetypically1310

nmor1550

nm,carrieshigherbandwidththanmultimodefiber,butrequiresalightsourcewithanarrowspectralwidth.Singlemodefiber,asisshowninFig6.2,givesyouahighertransmissionrateandupto50timesmoredistancethanmultimode,butitalsocostsmore.Singlemodefiberhasamuchsmallercorethanmultimode.Thesmallcoreandsinglelight-wavevirtuallyeliminateanydistortionthatcouldresultfromoverlappinglightpulses,providingtheleastsignalattenuationandthehighesttransmissionspeedsofanyfibercabletype[5].

Fig6.2Singlemodefiber

Bycontrast,multimodefiberhasacorediameterthatismuchlargerthanthewavelengthoflighttransmitted(Themostcommonsizeis62.5

m).Lightwavesaredispersedintonumberouspaths,ormodes,astheytravelthroughthecable’scoretypically850

nmor1300

nm.However,inlongcableruns(greaterthan3,000

m),multiplepathsoflightcancausesignaldistortionatthereceivingend,resultinginanunclearandincompletedatatransmission[6].Multimodefibercanbedividedintotwotypes:stepindexmultimodefiberandgradedindexmultimodefiber.Stepindexmultimodewasthefirstfiberdesignbutistooslowformostuses,duetothedispersioncausedbythedifferentpathlengthsofthevariousmodes.Stepindexfiberisrare—onlyPOFusesastepindexdesigntoday.Gradedindexmultimodefiber,asthenameimplies,therefractiveindexofthisfibergraduallydecreasesfromthecoreoutthroughthecladdingtocompensateforthedifferentpathlengthsofthemodes[7].Itoffershundredsoftimesmorebandwidththanstepindexfiber—uptoabout2gigahertz.POFisanewerplastic-basedcablewhichpromisedperformancesimilartoglasscableonveryshortruns,butatalowercost.

ThefirstideaofPOFgoesbacktothe1960sbeforeCorningdemonstratedsilicaopticalfiberswithattenuationlowerthan20dB/km.In1966,DuPontinventedthefirstPOFnamed“Crofon”thatwasofSI-typecomposedofPMMAcoresurroundedbyapartiallyfluorinated-polymercladding.Becauseoftherapidprogressmadeinsilicaopticalfibertechnologies,fiberopticshavebecomethebackbonesoflong-distancetelephonenetworksaroundtheworld.Ontheotherhand,POFshaveprovedmorepracticalforcommunicationsinvery-shortreach(VSR)networksorforlightguideandilluminationapplications,becauseoftheiradvantagessuchaslargediameterwithgreatmechanicalflexibilityandhighnumericalaperture(NA).Particularly,thepotentialofpolymermaterialssuchasgreatmechanicalflexibilityandeasyhandlingcanreducenotonlythecostofthefiberitselfbutalsothecostoffiberinstallation.Therefore,greatinteresthasbeenfocusedonsuchPOFapplicationsasthetransmissionmediainVSRnetworks.In1975,MitsubishiRayoncommercializedthefirstSIPOFwhosetradenamewas“Eska”.Then,AsahiChemicalandToraysoonfollowed.ThePOFmarketwasoriginallydominatedbythesethree-majorJapanesecompanieswhohavebeenmanufacturingSI-typePOFscomposedofPMMAcore.ExperimentalanalysesofthelossreductioninPMMA-coreSIPOFwereconductedmainlyinthe1980s.Kainoetal.reportedin1984thatverylow-lossSIPOFwasexperimentallyobtainedbyemployingperdeuteratedPMMA.Animpressiveanalysiswasmadeattheendof1980s.Grohtheoreticallycalculatedtheovertoneabsorptionlossduetocarbon–hydrogenstretchingvibrationinPMMAandotherpolymersbyintroducingMorse’spotentialenergytheory.Actually,thecalculatedpeakpositionsandtheattenuationoftheovertoneabsorptionspectrumofPMMAagreedwellwiththatexperimentallyobtained.Thus,thecalculationprocessoftheattenuationlimitofPOFswasdeveloped.ThesereportssetoffacompetitiontomakebetterPOFsamongthethreemajorJapanesecompaniesaforementioned,andalmostthelowestleveloftheattenuationwasachievedevenbythecommercial-basedSIPOFsinlate1980s.ThedevelopmentintheattenuationofSIPOFsissummarizedinFig6.3comparedtothatofGIPOFs.

Fig6.3DevelopmentintheattenuationofPOF

IntermsofGI-typePOF,thefirstreportofPMMA-coreGIPOFswaspresentedfromKeioUniversityin1976.AnearparabolicrefractiveindexprofileinthefirstGIPOFwasformedbycopolymerizingmethylmethacrylate(MMA)monomer(asM1monomer)withtheotherM2monomerwitharefractiveindexhigherthanMMA.Duringthecopolymerizationprocess,thecompositionratiooftwopolymerswasgraduallyvariedintheradialdirectionutilizingthedifferenceofmonomerreactivitybetweenM1andM2monomers.Actually,theattenuationfirstmeasuredforGIPOFcomposedofMMAandvinylbenzoatewas1000dB/km,whichwasapproximatelytentimeshigherthanthatofSIPOF.However,itisrevealedinthisprocessthattheresultingcopolymercompositionismainlydividedintotwocompositions,i.e.,M1richcopolymerandM2richcopolymer,whichlargelyincreasestheinherentexcessscatteringloss.

Inordertodecreasesuchanexcessscatteringlosscausedbythedifferenceofmonomerreactivity,anewinterfacialcopolymerizationprocessbasedonrandomcopolymerizationwasdeveloped.TheattenuationofanMMA-benzylmethacrylatecopolymerGIPOFbythisrandomcopolymerizationprocessisremarkablydecreasedtoabout200from1000dB/km.However,theexcessscatteringlossofabout100

dB/kmduetoheterogeneousstructurein“copolymer”stillremained.Basedonthefundamentalresearchontherelationshipbetweenscatteringlossandheterogeneousstructureinpolymermaterials,wecouldbreakthroughthehigh-attenuationproblemmentionedabove.

Insteadofthecopolymerizationprocess,weinventedtheprocessofdopinglow-molecularweightcompound.TherefractiveindexprofileofthenewGIPOFisformedbytheradialconcentrationdistributionofthedopant.ThereweremorefreedomsinselectingthedopantmaterialscomparedtoM2monomerselection.BydesigningthedopantstructuretohaveacompatibilitywithPMMA,wecoulddecreasethesizeoftheheterogeneousstructureinthepolymer,andremarkableprogresswasmadebythenewGIPOFindecreasingtheattenuationtobeaslowasthevaluewhichhadbeenalreadyachievedbyPMMA-coreSIPOF.

NEWWORDSANDPHRASES

revolutionize vt. 使革命化

arrogationn. 冒稱；霸占

volume n. 容量，體積

laser n. 激光；激光器

insulate vt. 使絕緣，使隔熱，使隔音；隔離，使隔絕cumbersome adj. 笨重的；遲緩而缺乏效率的

susceptible(to) adj. 靈敏的；易受影響的

index n. 指數，指標

critical adj. 臨界的

resistant(to)adj. 耐……的；對……有抵御能力的

theft n. 偷竊

dispersed adj. 分散的，散開的，漫布的

multimode adj. 多種模式的，多態(tài)的coren. 盤心；磁心；地核；巖心

taps(into) n. 分接；輕打；活栓，水龍頭

quartz n. 石英

diameter n. 直徑

propagate vt. 傳播；宣傳

spectral adj. 光譜的

distortion n. 扭曲，變形，曲解，失真

attenuation n. 變薄，稀薄化，變細，衰減

dispersedadj. 被驅散的，被分散的，散布的run n. 路程，趨向

wavelength n. 波長

refractive adj. 折射的

cost vt.價值為，(使)花費(金錢，時間，勞力等)

claddingregion 包層區(qū)域

stepindexmultimodefiber 階躍折射多模光纖

gradedindexmultimodefiber 漸變折射多模光纖fluorinated adj. 氟化的

illumination n. 照明

aperture n. 光圈，孔徑

polymer n. 聚合體

perdeuterated adj. 全氘化的

overtone n. [音]泛音，暗示，折光的色彩

copolymerize vt. [化](使)共聚合

monomer n. 單體

vinyl n. [化]乙烯基

benzoate n. 苯甲酸鹽methacrylate 異丁烯酸鹽[酯]，甲基丙烯酸鹽[酯]

heterogeneous adj. 不同種類的

doping (半導體)摻雜(質)

interfacial adj. 界面的，分界面的，面間的

dopant n. 摻雜物，摻雜劑

NOTES

[1]Althoughsatellitesystemsareexpectedtobethedominantcommunicationmediumforlongdistancesduringthisdecade,fiberopticstechnologyisexpectedtorevolutionizethecommunicationsindustrybecauseofitslowcost,hightransmissionvolume,lowarrogation,andmessagesecurity.

“becauseofitslowcost,hightransmissionvolume,lowarrogation,andmessagesecurity.”作原因狀語。本句可譯為：盡管衛(wèi)星系統(tǒng)被認為是近十年來的長途通信的優(yōu)勢方法，但是光纖技術以其低成本、高傳輸容量、低占位和信息保密性而被認為是通信工業(yè)的革命(技術)。

[2]HowopticalfibersworkcanbeexplainedbySnell’sLaw,whichstatesthattheratioofthesineoftheangleofincidencetothesineoftheangleofrefractionisequaltotheratioofthepropagationvelocitiesofthewaveinthetworespectivemedia.

“which”引導非限定性定語從句；theratioofAtoB表示A與B之比。本句可譯為：光纖的工作原理可用斯涅爾定律來解釋：入射角與折射角的正弦值之比等于光波在這兩種(傳輸)媒質中的傳播速度之比。

[3]Fig6.1(b)demonstratestheconditionthatexistswhenA1isatthecriticalangleandangleA2isat90[dg].

“[dg]”表示角度單位，dg=degree(

°)。

本句可譯為：圖6.1(b)表示當A1為臨界角，A2為90°時的情況。[4]Fiberopticscommunicationsarealsoveryresistanttoillegaldatatheft,becausetapsintoittolistentoorchangethedatabeingtransmittedcanbeeasilydetected.

“resistantto”為“耐……，對……有抵抗性的”；“tapsintoit”中的it指的是fiberopticscommunications。

本句可譯為：光纖通信也可以防止非法數據盜竊，因為為了竊聽或改變數據而做的分接很容易被監(jiān)測到。

[5]Thesmallcoreandsinglelight-wavevirtuallyeliminateanydistortionthatcouldresultfromoverlappinglightpulses,providingtheleastsignalattenuationandthehighesttransmissionspeedsofanyfibercabletype.“thatcouldresultfromoverlappinglightpulses”引導后置定語從句，修飾anydistortion。

“providingtheleastsignalattenuationandthehighesttransmissionspeedsofanyfibercabletype.”為分詞短語作結果狀語，可以理解為whichcanprovidetheleastsignalattenuationandthehighesttransmissionspeedsofanyfibercabletype的縮寫。

本句可譯為：事實上，小的纖芯和單色光波可以消除由重疊光脈沖引起的任何失真，提供所有光纖類型中最小的信號衰減和最高的傳輸速率。[6]However,inlongcableruns(greaterthan3,000m),multiplepathsoflightcancausesignaldistortionatthereceivingend,resultinginanunclearandincompletedatatransmission.

本句可譯為：但是，在長距離(超過3000

m)(傳輸中)，光的多種路徑能導致接收端信號失真，(從而)導致數據傳輸的不清楚和不完整。

[7]Gradedindexmultimodefiber,asthenameimplies,therefractiveindexofthisfibergraduallydecreasesfromthecoreoutthroughthecladdingtocompensateforthedifferentpathlengthsofthemodes.本句可譯為：漸變型多模光纖正如其名，它的折射率從纖芯到包層逐漸減小，以彌補不同路徑長度造成的模式色散。

EXERCISES

I．Translatethefollowingwordsorphases.

折射率 臨界角

塑料光纖 信號失真

單模光纖 claddingindex

multimodeopticalfiber gradedindexmultimodefiber

II．TranslatethefollowingparagraphsintoChinese.

(1)

Fiberopticcablesarereplacingcopperwireasthemajorcommunicationmediuminbuildingsandcities;majorcommunicationscompaniesarecurrentlyinvestinghugesumsofmoneyinfiberopticscommunicationsnetworksthatcancarrydigitalsignals,thusincreasingcommunicationsandcapacity.

(2)

Thepulsesoflightrepresentthe“on”stateinelectronicdaterepresentationandcanoccurnearly1billiontimespersecond—nearly1billionbitscanbecommunicatedthroughafiberopticcablepersecond.(3)

Bycontrast,multimodefiberhasacorediameterthatismuchlargerthanthewavelengthoflighttransmitted(Themostcommonsizeis62.5

m).Lightwavesaredispersedintonumberouspaths,ormodes,astheytravelthroughthecable’scoretypically850

nmor1300

nm.

(4)

Singlemodecableismadeupofoneoranumberofquartzfiberswithadiameterof8.3mto10

mthathasonemodeoftransmission.Singlemodefiberwitharelativelynarrowdiameter,willpropagatetypically1310

nmor1550

nm,carrieshigherbandwidththanmultimodefiber,butrequiresalightsourcewithanarrowspectralwidth.

參考譯文

第六單元光纖通信

盡管衛(wèi)星系統(tǒng)被認為是近十年來的長途通信的優(yōu)勢方法，但是光纖技術以其低成本、高傳輸容量、低占位和信息保密性而被認為是通信工業(yè)的革命(技術)。光纖在建筑物和城市中正在替代銅線作為主要的通信介質；當前，主要的電信公司正將巨資投入到能夠承載數字信號的光纖通信網絡中，從而提高通信能力和容量。在光纖通信中，信號被轉換為光的形式，并以激光脈沖的形式通過絕緣的、非常細(1/2000英寸)的玻璃或塑料光纖發(fā)送出去。光脈沖代表電子數據表示中on的狀態(tài)，并且每秒能產生近十億個光脈沖，即通過光纜每秒能傳送近十億比特(數據)。同樣重要的是，光纜的尺寸不大：只需1/2英寸厚的光纜(絕緣光纖捆在一起)就能夠支持將近250000個聲音信號同時交談(不久后可能翻倍到500000個)?？墒牵捎跀祿且怨饷}沖的形式通信的，因此必須使用特殊的通信設備。光纖引導光線在光纖材料中傳輸。它們能做到這點是因為當光線從一種介質進入另一種介質時會發(fā)生彎曲或改變傳播方向。光線發(fā)生彎曲的原因在于光線在每種介質中的傳播速率不同，這種現象稱為折射。當你站在池塘的邊緣，觀看塘底的一個物體時，折射就發(fā)生了，這是一個常見的折射現象的例子。除非你直接處于物體的上方，否則它看起來比它實際離你遠一些。這種現象發(fā)生的原因是：當光線從水中傳播到空氣中時，它的傳播速度增加了，這導致了光線的彎曲，改變了感知物體的角度。通過斯涅爾定律可以得到關于光傳播方式的解釋。光纖的工作原理可用斯涅爾定律來解釋：入射角與折射角的正弦值之比等于光波在這兩種(傳輸)媒質中的傳播速度之比。第二種介質的折射率除以第一種介質的折射率的值是一個常數。斯涅爾定律用方程式表示出來是這樣的：

方程式中，A1和A2是入射角和折射角，V1和V2分別是光波在兩種介質中的傳播速率，n1和n2是兩種介質的折射率。圖6.1使用圖形的方式示意了各個參數。在每種情況下，A1是入射角，A2是折射角。介質1的折射率n1比介質2的折射率n2大，這意味著光線在介質2中的傳播速率大于在介質1中的傳播速率。

圖6.1(a)表明了當A1小于臨界角時，光線從介質1進入介質2時是如何在介質2中被折射的。圖6.1(b)表示當A1是臨界角，A2為90°時的情況，光線沿著兩種介質的邊界改變了方向。如圖6.1(c)所示，任何以大于A1(圖6.1(b)中的)的入射角入射的光線將會被以反射角A2反射回介質1，且A2等于A1。圖6.1(c)中的這種情況對光纖具有特殊的重要性。

圖6.1反射指數(n1>n2)

光纜不易受到電子噪聲的干擾，因此可以擁有比常規(guī)的電話電纜更低的錯誤率。此外，它們的潛在數據通信速率可以達到微波和衛(wèi)星系統(tǒng)通信速率的上萬倍。光纖通信也可以防止非法數據盜竊，因為為了竊聽或改變數據而做的分接很容易被監(jiān)測到。事實上，美國中央情報局當前正是使用光纖(防止被竊聽)。

光纖看起來就像是普通的包含纖芯和覆層的玻璃圓柱體?，F在，通常使用三種類型的光纖：單模光纖、多模光纖和塑料光纖(POF)。單模光纖是由一個或多個直徑為8.3m～10

m的、只有一種傳輸模式的石英光纖組成的。直徑相對較小的單模光纖可以傳輸典型的1310

m或1550

m的光波長，可以比多模光纖承載更高的帶寬，但是需要光源有更窄的頻譜寬度。

如圖6.2所示的單模光纖可以有更高的傳輸速率，且傳輸的距離可以達到多模光纖(傳輸距離)的50多倍，但是它的成本也更高。單模光纖的纖芯比多模光纖的纖芯更細。事實上，小的纖芯和單色光波可以消除由重疊光脈沖引起的任何失真，提供所有光纖類型中最小的信號衰減和最高的傳輸速率。

圖6.2單模光纖

相比之下，多模光纖的纖芯直徑比所傳輸的光波的波長要大得多(最普通的尺寸是62.5

m)。當光波在典型的850

nm或1300

nm纖芯中傳播時，光波被發(fā)散到很多個路徑或模式中。但是，在長距離(超過3000

m)(傳輸中)，光的多種路徑能導致接收端信號失真，(從而)導致數據傳輸的不清楚和不完整。

多模光纖可被分為兩種類型：階躍型多模光纖和漸變型多模光纖。階躍型多模光纖是人們設計的第一種光纖。但是對于大多數應用來說，它的速率太低了，這是由多種模式下不同路徑長度引起的色散造成的?，F在，階躍型光纖很少使用了——只有塑料光纖還使用階躍型設計。漸變型多模光纖正如其名，它的折射率從纖芯到包層逐漸減小，以彌補不同路徑長度造成的模式色散。它能提供比階躍型光纖高出幾百倍的帶寬——(其帶寬)可達到2

GHz。

塑料光纖是一種較新的以塑料為基礎的光纜，它在非常短的距離上能提供類似于玻璃絲電纜的性能，但是其成本比玻璃絲電纜更低。塑料光纖的第一個想法誕生于20世紀60年代，是在高寧(Corning)發(fā)現硅光纖可以獲得小于20dB/km的衰減之前(產生的)。1966年，杜邦發(fā)明了第一根名叫“Crofon”的塑料光纖，它是被部分氟化聚合體覆層包裹著的PMMA纖芯組成的硅化物。由于硅光纖技術的快速發(fā)展，光纖成為了世界上長途電話網的骨干鏈路(材料)。另一方面，塑料光纖也被發(fā)現在短程網絡(VSR)、光導和照明應用中更實用，原因來自它們的優(yōu)點，比如由大機械靈活性而來的大直徑以及高的數字孔徑。特別的是，聚合材料的潛能如大機械靈活性和易操作性不但降低了光纖本身的成本，也降低了安裝的成本。因此，塑料光纖的應用，比如作為VSR網絡的傳輸介質，已經引起了(人們)廣泛的興趣。

1975年，三菱麗陽公司(MitsubishiRayon)造出了第一根商業(yè)化的硅型塑料光纖，它的商標為Eska。不久之后，旭化成化學公司(AsahiChemical)和東麗(Toray)公司也制造出了硅型塑料光纖。塑料光纖市場最初是由這三家主要的、制造出了PMMA纖芯的硅型塑料光纖的日本公司主導的。20世紀80年代開展了對PMMA纖芯的硅塑料光纖的損耗分析實驗。Kaino等人在1984年報道，通過使用全氘化的PMMA，在實驗上可以獲得損耗非常低的硅型塑料光纖。20世紀80年代末期的一個分析令人印象深刻。Groh從理論上計算出了PMMA中由于碳-氫伸縮振動和其他聚合體引起的諧波吸收損耗，這些聚合體是由Morse的勢能理論引入的。實際上，計算得到的峰值位置和PMMA的諧波吸收頻譜與實驗得到的結果非常符合。因此，(人們)推算出了塑料光纖的衰減極限計算過程。這些報告引發(fā)了前面提到的三家主要日本公司間的制造更好塑料光纖的競爭，20世紀80年代后期甚至在商業(yè)級硅POF的衰耗上也達到了最低水平。圖6.3總結比較了硅POF與GIPOF的衰耗發(fā)展。

圖6.3塑料光纖的衰耗發(fā)展

在GI型POF方面，1976年Keio大學提出了第一個PMMA纖芯的GI型POF。一個近拋物線形的折射率輪廓是由聚合甲基丙烯酸甲酯(MMA)單體1(作為M1單體)和另一個折射率高于MMA的M2單體形成的。在聚合過程中，兩種聚合物的合成比例在利用單體M1和M2間的單體運動徑向方向上是逐漸變化的。事實上，第一個測到的由MMA和乙烯基苯甲酸鹽合成的GI型GOF的衰耗是1000dB/km，這大概是硅型POF衰耗的10倍。但是，此過程中也表明了最終結果中的共聚化合物主要可分為兩種化合物，即富有M1的共聚物和富有M2的共聚物，這種共聚物大大增加了其內在的、過度的散射衰耗。為了減少這種由于單體反應不同而引起的過度散射衰耗，(人們)開發(fā)了一種新的基于隨機共聚合作用的界面共聚合過程。通過這種隨機共聚合過程，MMA苯甲基異丁烯酸鹽共聚物GIPOF的衰耗可以顯著地從1000dB/km降低到200dB/km。然而，由共聚物中的不均一結構引起的大概100dB/km的過度散射衰耗依然存在?；趯ι⑸渌ズ暮途酆象w材料中不均一組織結構關系的基礎研究，我們就能夠突破前面提到的高衰耗難題。我們發(fā)明了摻雜低分子量化合物的過程來替代共聚合過程。新的GIPOF的折射率輪廓是由摻雜物的射線集合分布形成的。相對于M2單體的選擇，摻雜物的選擇有較大的余地。通過對摻雜物結構的設計可使其兼容PMMA，可以減小聚合體中不均一組織的體積，且新的GIPOF已經取得了顯著的進展，衰耗已經降低到了和PMMA芯的硅POF的一樣。

EXTENSIVETEXT

AdvantagesofFibers

Wearenowreadytodiscusstheadvantagesofopticfibers.Beforedoingso,letusmentionafewwordsofcaution.Fibersystemsarenotperfect.Theyhavetechnicalandeconomiclimitations.Foranydesiredsystem,therelativemeritsofguidedchannelversusunguidedchannelandmetallicconductorversusfibermustbeevaluated.Thefollowingdiscussionofdesirablefiberspropertiescanbeusefulinthatevaluation.Thebasicmaterialforglassfibersissilicondioxide,whichisplentiful.Someopticfibersaremadeoftransparentplastic,anotherreadilyavailablematerial.Costsareoftenthemostimportantconsiderationinasystem.Comparisonsbetweenfiberandmetalliccablesmustbedonewithcare.Therearemanyfibercablesavailable,someofwhicharecheaperthantheirwireequivalents.Thesavingmaybecomeparticularlyapparentwhenthecomparisonismadeonthebasisofcostperunitofinformationtransfer.Forexample,avalidcomparisonforatelephonelinkwouldbeonthebasisofcostpermeterpertelephonechannel,ratherthanjustcostpermeter.Thisconsiderationarisesbecausefibershavegreaterinformation-carryingcapacitiesthandometallicchannels.

Economiccomparisonsshouldalsoincludethecostsofinstallation,operation,andmaintenance.Somegeneralitiesabouttheseconcernsareworthpresenting.Forlongpaths,fibercablesarecheapertotransportandeasiertoinstallthanmetalcables.Thisisbecausefibersaresmallerandlighter.(Alightguidewouldhavetobelightweight,correct?)Onecabledesignhasafiber125

mindiameterenclosedinaplasticsheathof2.5

mminouterdiameter.Theweightofthiscableis6

kg/km;thelossis5dB/km.LetuscomparethiscablewiththeRG-19/Ucoaxialcable,whichhasanattenuationof22.6dB/kmwhencarryinga100

MHzsignal.Itsouterdiameteris28.4

mm,anditsweightis1110

kg/km.Smallerandlightercoaxialcablesareavailable,buttheyhavehigherlossesthantheRG-19/U.Thesignificantsizeandweightadvantagesoffibercablesareapparentfromthisexample.Therearenogreatdifferencesbetweentheoperationoffibersystemsandthatofmetallicsystems.Thecostshereshouldbethesame.Maintenanceoffibercablesdoesdiffer,however.Ifalineisbroken,asaresultofeitheranaccidentorasystemmodification,splicesmustbemadeornewconnectorsattached.Theseoperationsrequiremoretimeandskillforfibersthanforwires.Asaresult,maintenancecostsshouldbeconsideredwhendesigningasysteminwhichmanychangesarelikelytobemade.Fibersandfibercableshaveturnedouttobesurprisinglystrongandflexible.Somefibersaresoslenderthattheydonotbreakwhenwrappedaroundcurvesofonlyafewcentimetersradius.Fibersareoftenstoredandtransportedwhiletightlywrappedaroundspoolshavingthissmallcurvature.Fiberflexibilityisattractiveforinstallationscontainingmanyturnsalongthetransmissionpath.Foralarger-radiusbend,fibersguidelightwithnegligibleloss.Thereissomelossataverytightbend,however.Whenafiberisprotected—forexample,byencasingitinaplasticsheath—itisdifficulttobendthecableintoaradiussmallenoughtobreakthefiber.Fibersembeddedincablesdonotbreakeasily.Theadditionofaplasticsheathincreasesthetensilestrengthofafibertransmissionline.Steelrodscanbeplacedinsidetheplasticcabletoaddfurtherstrength,ifneeded.AnotherstrengtheningmaterialisKevlar,asyntheticpolymerfiberwithgreattensilestrength.Despitetheapparentlyfragilenatureofglass,opticfibercablesareveryruggedandserviceable.Techniqueshavebeendevelopedfortheproductionoffiberswithverylowtransmissionlosses.Manyfiberdesignsexist,butanattenuationof4dB/kmistypicalofcommercialglassfiberswhenoperatedatawavelengtharound0.82

m.Accordingtosomedocuments,thisrepresentsatransmissionefficiencyof40%fora1kmlength.Thisdegreeoftransparencycouldnotbeachievedbefore1970.Now,fiberswithlossesofonlyafewtenthsofadB/kmareavailableforusearound1.3

mand1.55

m.Verylongcommunicationslinkscanbeconstructedbecauseoftheavailabilityoflow-lossfibers.Amplifiers,neededtoamplifyweaksignals,canbelocatedatlargeintervals.Thelossesofwiretransmissionlinesincreaserapidlywithfrequency,asindicatedinFig6.4fortheRG-19/Ucoaxialcable.Athighfrequency,likelengthsandamplifierspacingswouldbesignificantlysmallerforwiresystemsthanforfibersystems.

Oneofthemostimportantadvantagesoffibersistheirabilitytocarrylargeamountsofinformationandtodosoineitherdigitaloranalogform.Forexample,asinglefiberofthetypedevelopedfortelephoneservicecanpropagatedataattheT3rate,44.7

Mb/s.Thisfibertransmitsat672voicechannels.Fiberswithevengreatercapacitiesareavailable.Althoughpulsespreadinglimitsthemaximumrate,fibercapabilitiesmeettherequirementsofmostdata-handlingsystemsandexceedthecapabilitiesofconductingcables.

Fig6.4Effectiveattenuationofa1kmlengthofcoaxiallineandglassfiber

Intheanalogformat,modulationratesofhundredsofmegahertz,ormore,canpropagatealongfibers.Aswiththedigitalsystems,therateislimitedbydistortionoftheopticsignal.ArepresentativeplotshowinghowthesignalchangeswithmodulationfrequencyappearsinFig6.4.Inthisfigure,weseea4

dBlosswhenmodulationfrequenciesarelow.At500

MHz,thelosshasincreasedby3

dB.Wesaythatthislengthoffiberhasa3

dBbandwidth(whichwewilldenotebythesymbolf3

dB)of500

MHz.Abovethisfrequencythemodulationisfurtherattenuated.Thehigh-frequencyattenuationrequiressomeexplanation.Itisnotcausedbyanyaddedpowerlosses,suchasabsorptioninthefiber.Infact,thetransmissionefficiencyofthefiberremainsat4dBregardlessofthemodulationrate.Fig6.5illustratestheproblemthatarisesathighmodulationfrequencies.Theinformationbeingtransmittediscontainedinthetimevariationoftheopticpower.Asthemodulationfrequencyincreases,thesignaldistortioncausesalossintheamplitudeofthisvariation.Thiseffectisduetospreadingoftheregionsofpeakpowerin