光纤通信Word格式.docx
《光纤通信Word格式.docx》由会员分享,可在线阅读,更多相关《光纤通信Word格式.docx(35页珍藏版)》请在冰点文库上搜索。
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