光纤通信.docx

光纤通信.docx

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光纤通信

Fiber-opticcommunication

光纤通信

Fiber-opticcommunicationisamethodoftransmittinginformationfrom

one

placetoanotherbysendinglightthroughanopticalfiber.Thelightformsan

electromagneticcarrierwavethatismodulatedtocarryinformation.Firstdeveloped

inthe1970s,fiber-opticcommunicationsystemshaverevolutionizedthe

telecommunicationsindustryandplayedamajorroleintheadventoftheInformation

Age.Becauseofitsadvantagesoverelectricaltransmission,theuseofopticalfiber

haslargelyreplacedcopperwirecommunicationsincorenetworksinthedeveloped

world.

Theprocessofcommunicatingusingfiber-opticsinvolvesthefollowingbasic

steps:

Creatingtheopticalsignalusingatransmitter,relayingthesignalalongthe

fiber,ensuringthatthesignaldoesnotbecometoodistortedorweak,andreceiving

theopticalsignalandconvertingitintoanelectricalsignal.

Applications

Opticalfiberisusedbymanytelecommunicationscompaniestotransmit

telephonesignals,Internetcommunication,andcabletelevisionsignals.Duetomuch

lowerattenuationandinterference,opticalfiberhaslargeadvantagesoverexisting

copperwireinlong-distanceandhigh-demandapplications.However,infrastructure

developmentwithincitieswasrelativelydifficultandtime-consuming,andfiber-optic

systemswerecomplexandexpensivetoinstallandoperate.Duetothesedifficulties,

fiber-opticcommunicationsystemshaveprimarilybeeninstalledinlong-distance

applications,wheretheycanbeusedtotheirfulltransmissioncapacity,offsettingthe

increasedcost.Sincetheyear2000,thepricesforfiber-opticcommunicationshave

droppedconsiderably.Thepriceforrollingoutfibertothehomehascurrently

becomemorecost-effectivethanthatofrollingoutacopperbasednetwork.Prices

havedroppedto$850persubscriberintheUSandlowerincountrieslikeThe

Netherlands,wherediggingcostsarelow.

Since1990,whenoptical-amplificationsystemsbecamecommerciallyavailable,

thetelecommunicationsindustryhaslaidavastnetworkofintercityandtransoceanic

fibercommunicationlines.By2002,anintercontinentalnetworkof250,000kmof

submarinecommunicationscablewithacapacityof2.56Tb/swascompleted,and

althoughspecificnetworkcapacitiesareprivilegedinformation,telecommunications

investmentreportsindicatethatnetworkcapacityhasincreaseddramaticallysince

2002.

History

Theneedforreliablelong-distancecommunicationsystemshasexistedsince

antiquity.Overtime,thesophisticationofthesesystemshasgraduallyimproved,from

smokesignalstotelegraphsandfinallytothefirstcoaxialcable,putintoservicein

1940.Asthesecommunicationsystemsimproved,certainfundamentallimitations

presentedthemselves.Electricalsystemswerelimitedbytheirsmallrepeaterspacing

（thedistanceasignalcanpropagatebeforeattenuationrequiresthesignaltobe

2

amplified）,andthebitrateofmicrowavesystemswaslimitedbytheircarrier

frequency.Inthesecondhalfofthetwentiethcentury,itwasrealizedthatanoptical

carrierofinformationwouldhaveasignificantadvantageovertheexistingelectrical

andmicrowavecarriersignals.

In1966KaoandHockhamproposedopticalfibresatSTCLaboratories（STL）,

Harlow,whentheyshowedthatthelossesof1000db/kminexistingglass（compared

to5-10db/kmincoaxialcable）wasduetocontaminants,whichcouldpotentiallybe

removed.[1]

Thedevelopmentoflasersinthe1960ssolvedthefirstproblemofalightsource;

furtherdevelopmentofhigh-qualityopticalfiberwasneededasasolutiontothe

second.Opticalfiberwasfinallydevelopedin1970byCorningGlassWorkswith

attenuationlowenoughforcommunicationpurposes（about20dB/km）,andatthe

sametimeGaAssemiconductorlasersweredevelopedthatwerecompactand

thereforesuitableforfiber-opticcommunicationsystems.

Afteraperiodofintensiveresearchfrom1975to1980,thefirstcommercial

fiber-opticcommunicationsystemwasdeveloped,whichoperatedatawavelength

around0.8μmandusedGaAssemiconductorlasers.Thisfirstgenerationsystem

operatedatabitrateof45Mbit/swithrepeaterspacingofupto10km.

On22April,1977,GeneralTelephoneandElectronicssentthefirstlivetelephone

trafficthroughfiberoptics,at6Mbit/s,inLongBeach,California.

Thesecondgenerationoffiber-opticcommunicationwasdevelopedfor

commercialuseintheearly1980s,operatedat1.3μm,andusedInGaAsP

semiconductorlasers.Althoughthesesystemswereinitiallylimitedbydispersion,in

1981thesingle-modefiberwasrevealedtogreatlyimprovesystemperformance.By

1987,thesesystemswereoperatingatbitratesofupto1.7Gb/swithrepeaterspacing

upto50km.

ThefirsttransatlantictelephonecabletouseopticalfiberwasTAT-8,basedon

Desurvireoptimizedlaseramplificationtechnology.Itwentintooperationin1988.

TAT-8wasdevelopedasthefirstunderseafiberopticlinkbetweentheUnitedStates

andEurope.TAT-8ismorethan3,000nauticalmiles（5,600km）inlengthandwas

thefirsttransatlanticcabletouseopticalfibers.Itwasdesignedtohandleamixof

information.Wheninaugurated,ithadanestimatedlifetimeinexcessof20years.

TAT-8wasthefirstofanewclassofcables,eventhoughithadalreadybeenusedin

long-distancelandandshort-distanceunderseaoperations.Itsinstallationwas

precededbyextensivedeep-waterexperimentsandtrialsconductedintheearly1980s

todemonstratetheproject'sfeasibility.

Third-generationfiber-opticsystemsoperatedat1.55μmandhadlossofabout

0.2dB/km.Theyachievedthisdespiteearlierdifficultieswithpulse-spreadingatthat

wavelengthusingconventionalInGaAsPsemiconductorlasers.Scientistsovercame

thisdifficultybyusingdispersion-shiftedfibersdesignedtohaveminimaldispersion

at1.55μmorbylimitingthelaserspectrumtoasinglelongitudinalmode.These

developmentseventuallyallowed3rdgenerationsystemstooperatecommerciallyat

2.5Gbit/swithrepeaterspacinginexcessof100km.

3

Thefourthgenerationoffiber-opticcommunicationsystemsusedoptical

amplificationtoreducetheneedforrepeatersandwavelength-divisionmultiplexing

toincreasefibercapacity.Thesetwoimprovementscausedarevolutionthatresulted

inthedoublingofsystemcapacityevery6monthsstartingin1992untilabitrateof

10Tb/swasreachedby2001.Recently,bit-ratesofupto14Tbit/shavebeenreached

overasingle160kmlineusingopticalamplifiers.

Thefocusofdevelopmentforthefifthgenerationoffiber-opticcommunications

isonextendingthewavelengthrangeoverwhichaWDMsystemcanoperate.The

conventionalwavelengthwindow,knownastheCband,coversthewavelengthrange

1.53-1.57μm,andthenewdryfiberhasalow-losswindowpromisinganextensionof

thatrangeto1.30to1.65μm.Otherdevelopmentsincludetheconceptof"optical

solitons,”pulsesthatpreservetheirshapebycounteractingtheeffectsofdispersion

withthenonlineareffectsofthefiberbyusingpulsesofaspecificshape.

Technology

Modernfiber-opticcommunicationsystemsgenerallyincludeanoptical

transmittertoconvertanelectricalsignalintoanopticalsignaltosendintotheoptical

fiber,acablecontainingbundlesofmultipleopticalfibersthatisroutedthrough

undergroundconduitsandbuildings,multiplekindsofamplifiers,andanoptical

receivertorecoverthesignalasanelectricalsignal.Theinformationtransmittedis

typicallydigitalinformationgeneratedbycomputers,telephonesystems,andcable

televisioncompanies.

Transmitters

Themostcommonly-usedopticaltransmittersaresemiconductordevicessuchas

light-emittingdiodes（LEDs）andlaserdiodes.ThedifferencebetweenLEDsand

laserdiodesisthatLEDsproduceincoherentlight,whilelaserdiodesproduce

coherentlight.Foruseinopticalcommunications,semiconductoropticaltransmitters

mustbedesignedtobecompact,efficient,andreliable,whileoperatinginanoptimal

wavelengthrange,anddirectlymodulatedathighfrequencies.

Initssimplestform,anLEDisaforward-biasedp-njunction,emittinglight

throughspontaneousemission,aphenomenonreferredtoaselectroluminescence.The

emittedlightisincoherentwitharelativelywidespectralwidthof30-60nm.LED

lighttransmissionisalsoinefficient,withonlyabout1%ofinputpower,orabout100

microwatts,eventuallyconvertedinto«launchedpower»whichhasbeencoupledinto

theopticalfiber.However,duetotheirrelativelysimpledesign,LEDsareveryuseful

forlow-costapplications.

CommunicationsLEDsaremostcommonlymadefromgalliumarsenide

phosphide（GaAsP）orgalliumarsenide（GaAs）.BecauseGaAsPLEDsoperateata

longerwavelengththanGaAsLEDs（1.3micrometersvs.0.81-0.87micrometers）,

theiroutputspectrumiswiderbyafactorofabout1.7.Thelargespectrumwidthof

LEDscauseshigherfiberdispersion,considerablylimitingtheirbitrate-distance

product（acommonmeasureofusefulness）.LEDsaresuitableprimarilyfor

local-area-networkapplicationswithbitratesof10-100Mbit/sandtransmission

4

distancesofafewkilometers.LEDshavealsobeendevelopedthatuseseveral

quantumwellstoemitlightatdifferentwavelengthsoverabroadspectrum,andare

currentlyinuseforlocal-areaWDMnetworks.

Asemiconductorlaseremitslightthroughstimulatedemissionratherthan

spontaneousemission,whichresultsinhighoutputpower（~100mW）aswellasother

benefitsrelatedtothenatureofcoherentlight.Theoutputofalaserisrelatively

directional,allowinghighcouplingefficiency（~50%）intosingle-modefiber.The

narrowspectra