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信息科学与电子工程专业英语TechnicalEnglishForInformationScienceandElectronicEngineering信息科学与电子工程专业英语Unit8OpticalCommunications信息科学与电子工程专业英语PartIElectromagneticSpectrum3NewWordsperusal精读，细读gigahertz吉赫（109Hz）impress印，铭刻，加载directionality方向性resonator谐振器tTerahertz特赫（1012Hz）atwill随心所欲incur招致，蒙受，引起synchronous同步的interleave交织throughput吞吐量nonlinearity非线性femtosecond毫微微秒(1015sec)coherent相干的obviate避免，使成为不必要micron微米miniature小型的miniaturization小型化counterpart对等物，对等的角色cladding光纤包层4NewWordsproximity接近，亲近fruition结果实，成就photon光子granular颗粒状bias偏置benign良性的51VariousobservationsrelatingtothepotentialofopticaltechnologyfortransmissionofinformationcanbemadefromperusalofthefrequencylineofTable8.1.与用于信息传输的光学技术的潜力有关的仔细研究表8.1中的频率表可以看到各种用于信息传输的光学技术的潜力。61Table8.1Afrequencylinewhichgivesthewavelengths,thefrequencies,andthephotonenergypforthevariousregionsofthefrequencyspectrumNameaudioradioμwavemmwaveIROPT/UVx-rayGammaλ300km300m30cm3mm30μm0.3μm30Å0.03Åν(Hz)10310610910111013101510171020εp3μeV0.3meV30meV3eV300eV0.3MeV72Theinformationratesinwhichoneisinterestedinconventional“modern-day”communicationssystemsgenerallycorrespondtoaudioratesintelephonesystems,radioratesincommercialbroadcastsystems,ordigitaltelevisionratesinthemostadvancedvideodistributionsystems.1…的信息传播速率通常相应于…“现代”常规通信系统中人们所感兴趣的信息传输速率通常对应于电话系统中的音频，商用广播系统中的无线电频率，或是在最先进的视频节目分配系统中的数字电视信息速率。82TheseratesaregenerallybelowseveralGHz.Ifoneweretotransmitsuchinformationwithoutimpressingitonanopticalcarrierbutinsteadonaradiofrequency(RF)carrierabithigherthanthemaximumrate,thetransmissionwavelengthoftheRFcarrierwouldbecentimetersorlarger.2如果要传输这样的信息而不将它加载于光纤上，而是加载在略高于最大速率的射频载波上，则此射频载波就会是厘米波或波长再长一些的波。92Therecan,however,begreatadvantagestousingopticalcarriers.Anobviousoneisthelowlossanddirectionalityoftheopticalfiber.Clearly,thecarriermusthaveahigherratethantheinformationrate.Amajorprinciplethathasappearedincommunicationssystemshasbeenthatthehigherthefrequency,thegreaterthetechnicalcomplexity.Microwavesarehardertohandlethanareradiowaves.通信系统的一个基本原则是102Aswavelengthsdecreasetoapproachthesizeofcircuitcomponents,circuitelementsarenolongerlumped,andleadscanactasreflectivecomponentsand/orantennasandlumpedelementsaselectromagneticresonators.3随着……,电路单元就不再是集总的，导线可起到反射元件以及（或）天线的作用，集总单元则成为电磁谐振器。112Thishasgenerallymeantthatsendingmoreinformationwouldcostmoreandtherewasthereforeacostperbit/sec(bps)oftransmittedinformationinthesensethatgoingtoahigherinformationraterequiresahigherfrequency.Thus,thefirstobservationfromthefrequencylinewouldbethat,foropticalcarriers,whichhavefrequenciesinthehundredsofTHz,informationbandwidthisinsomesensefree.因此在较高的信息率要求较高的频率这层意义上，要考虑传输信息的每个bps成本问题信息的带宽在某种意义上是免费的122Thatistosay,theopticalwavelengthissosmallcomparedtomostdevicesthatthetechnologyhaschangeddrasticallyfromelectricalandmicrowave.Onceweassumethatwehavesuchtechnology,nomatterhowhighaninformationrateonemightwantitwillnotbenecessarytochangethecarrier,asthecarrierfrequencyishigherthananyrealisticinformationratecouldbecome.所用技术与电和微波有根本的不同无论信息率有多高，再也没有必要改变载波了132Bandwidthisnotcompletelyfree,though,asencodersanddecodersmustnecessarilyoperateattheinformationrate,butmuchoftherestofthesystemmustnecessarilyhandleonlythecarrierplusmodulation.而系统其余部分大都只需要处理载波和调制142Ifacomponentcanhandleafrequencyof5×1014Hz,aninformationshiftinthatfrequencyofapartinathousand(correspondingtoa500GHzinformationrate)willhavelittleornoeffectondeviceperformance.在这个频率，信息偏移千分之一（相应于500吉赫兹的信息率）对器件的性能将没有什么影响152Therefore,oncethesystemisalreadysetup,onecanupgradesystemspeedmoreorlessatwillwithoutthekindofcostsincurredbychangingtheelectromagneticcarrierinconventionalsystems.就可以随意升级系统而不会涉及常规系统中改变电磁载波所需付出的那种代价163Aconsequenceofthesizeoftheopticalbandwidthisthattheopticalcarriercanbeusedtocarrymanydifferenttelephoneconversations,televisionprograms,etc.,simultaneously.Theprocessbywhichthisisgenerallycarriedout(atleastinsynchronousformat)iscalledtimedivisionmultiplexing(TDM).通常实现这种同时传输多路信息的过程（至少以同步格式实现）称为时分复用173Theideaisthat,ifonewishestomultiplex16differentchannelseachtransmittingat1Mbps,onecouldperformthisbydividingeachbitperiodby16andtheninterleavingthebitsintoacomposite1μsecbit(1Mbpsrate)whichactuallycarries16bitsofinformationonit.4其原理是：如果要复用16个1Mbps的不同信道，可将每一比特所占时间除以16，然后将16个数据比特交织成一个持续1微秒的复合比特（即比特率为1Mbps），这一复合比特实际上带有16比特的信息。183Withtelephoneconversationsrepresentingarateof64kbps,the100sofTbpsbandwidthoftheopticalcarrierholdsgreatpromiseforTDM.Ofcourse,TDMisnottheonlymultiplexingschemeonecanimagineusing.Onecouldimagineimpressinganumberofsubcarriers,spacedbyperhapssomeGHz,ontotheopticalcarrier.使实现TDM有了极大的可能也许相隔几个吉赫兹193Eachofthesecarrierscouldthenbemodulatedataninformationrateandthenre-separatedaccordingtotheirdifferentcarrierwavelengthsattheoutput.Suchaschemeisreferredtoaswavelength-divisionmultiplexing(WDM)orsubcarriermodulation,dependingontheimplementation.在输出端按其不同的载波波长重新分离203Manyofthepresent-dayschemesforincreasinglinkthroughputwithincreasingtrafficinvolvecombiningmanyTDMsignalsontoWDMcarriers.Infact,thelimitationondensityofWDMturnsouttobenotbandwidthbutpower.Thatis,eachcha