光纖通信技術-英文版-chapter4

Chapter5Singlemodefibers--basicsFiberOpticCommunications5.1HowasinglemodefiberworksTherearemanydifferencesbetweenmultimodeandsinglemodefibers.Thesedifferencesstartwiththeprincipleofactionandrunthroughallthefibercharacteristics.Theprincipleofaction

MakingafiberthatsupportsonlyonemodeManufacturersuseallthesemeanstoachievetheirgoal.Thecorediameterofasinglemodefiberisaround10umandless,therangeofoperatingwavelengthstypicallystartsat1300nm,andtherelativeindexislessthan0.4%.HowasinglemodefiberconductsonebeamGaussianbeamHowcanwecomparedifferencebeams?Wehavetointroducesomemeasurebyconvention,knownasthemode-fielddiameter(MFD),isequalto2w0.Fig.2-23:Mode-fielddiameterMFDisthecross-sectionaldimension2w0,wherethebeam’sintensitydropsto1/e2=1.135Fig.2-23:Mode-fielddiameterCore,cladding,andmode-fielddiameterHerewediscussthestep-indexfiber.Manufacturersusethemode-fielddiameter,MFD,ratherthanthecorediameterasaparameterthatdescribessinglemodefibers.Thisisincontrasttomultimodefiber,wherethegeometricsizesofboththecoreandthecladdingaregiveninthedatasheets

Inmultimodefibers,weneedtoknowthecorediameterbecauseallthelightisconfinedwithinthecore.Insinglemodefibers,however,thisstatementisnottrue,asfig5.3,theessentialportionofthelight(typically,about20%)iscarriedbythecladding.Somanufacturersdonotevenspecifythecorediameteratall.Effectiverefractiveindex,neffisthecombinationofthecoreandcladdingrefractiveindexes,thisisanothersignificantcharacteristicofasinglemodefiber.TheMFDandOperatingWavelengththeshorterthewavelength,thelesstheMFDtypicalgraphsofMFDasafunctionofwavelengthareshowninfigure5.4.TheMFDisimportantcharacteristicofasinglemodefiber.Whenyouneedtoconnecttwofibers,ifthesetwofibershavedifferentmode-fielddiameters,youwillhaveextrainsertionloss.Thisinsertionlosscanbecalculatedbythefollowingformula:

CutoffwavelengthFromtheformulaswecanseethecutoffwavelengthInpractice,thefibersaredesignedspecificallyforeithermultimodeorsinglemodeoperation;forthelatter,theoperatingwavelengthisalwayslongerthanλcIntheotherwords,thesameopticalfibercanbesingle-modeormultimode,operatingwavelength.5.2attenuationBendinglossesmacrobendinglossmicrobendinglossAbsorptionScatteringmacrobendinglossPowerlossinacurvedfiberThelongertailshouldkeeppacewiththebeam’spropagation.Sinceithastocoveramuchlongerdistance,thetailmusttravelatamuchhighervelocitythantherestofthebeam.Thefatherthetailisfromthefiber’scenterline,thehigheritsvelocityshouldbe.Eventually,thisvelocitymustexceedthespeedoflight,whichmeansthetailwillbelostTosumup,thenmacro-bendinglossinasingle-modefiberincreasesasoperatingwavelengthincreasesandbendradiusdecreases.Micro-bendinglossAbsorption(吸收）Thesegraphshowthesimilaritiesanddifferencesbetweensinglemodeandmultimodefibers.ThekeydifferenceisinthelowerlevelofattenuationintroducedbysinglemodefiberSinglemodefiberattenuationMultimodefiber例題CalculatethelimitationintransmissionlengthcausedbyfiberlossifA=0.2dB/km,Pin=0.029mW,andPout=0.001mW,whereAisfiberattenuation,Pinislightpowerlaunchedintothefiber,andPoutispowercoupledtoaphotodiode.1)Howmanymodescanbesupportedforagraded－indexfiberifdis50um,NAis0.225,andtheoperatingwavelengthis1310nm?(2%)2)Whatisthenumberofmodesforastep-indexopticalfiberwhosed=8.3um,n1core=1.4514,n2clad=1.4468,andtheoperationwavelengthis1550nm?(2%)3)Forafixedguidedmode,theoperatingwavelengthshouldbelargerorshorterthanthecutoffwavelength?Why?(2%)4)Toguaranteeasinglemodeguiding,theoperatingwavelengthshouldbelargerorshorterthanthecutoffwavelength?Why?(2%)5.3DispersionandbandwidthThemajormechanismthatcausesdispersioninasinglemodefiberischromaticdispersion.Anotherformofdispersionispolarization-modedispersion.(偏振模色散）chromaticdispersion:materialdispersionandwaveguidedispersion1.Materialdispersion(材料色散）PulsespreadingcausedbymaterialdispersionPulsefractiondeliveredbyλ0

componentPulsefractiondeliveredby

λ0--νλ/2Pulsefractiondeliveredbyoutputpulseλ0+νλ/2Keepclearlyinmindthatmaterialdispersionequalszeroataspecificwavelength—around1300nm.Thisphenomenonstemsfromthedispersivepropertiesofsilica.Anotherpointtostressisthatforlamd<1300nm,D(lamd)isnegative.Thismeansthatlightatawavelength,say,of1000nmtravelsmoreslowlythanlightatawavelengthof1100nm,aswediscussedabove.Laserdiodeswithaspectralwidthof1nmandnarrowershouldbeusedasalightsource.Thusthelightsourcepreferredinsingle-modefibercommunicationssystemsisalaserdiode.Thisisanotherreasonwhypulsespreadinsingle-modefibersismuchlessthaninmultimodefibers.2.Waveguidedispersion（波導色散）Waveguidedispersioniscausedbythefactthatlightisguidedbystructure—here,anopticalfiber.多模光纖也有波導色散，由于與模色散相比所引起的脈沖展寬可以忽略不計。解釋：進入單模光纖后，攜帶信息的光脈沖在纖芯和包層間分布；因為纖芯和包層的折射率不同，兩個部分以不同的速度傳播；脈沖會擴展。所以即使光纖材料沒有色散特性，波導色散都會發(fā)生，這一點比較重要。所以純波導色散因將光限制在一個特定的結構中傳輸。波導色散也與波長有關，依賴于MFD(b)Claddingfractionofthepulse;(c)totalpulse.Calculatepulsespreadcausedbymaterialandwaveguidedispersionsatthe1550nmoperatingwavelengthif=1nmandL=1kmChromaticdispersioninasinglemodefiberCalculatechromaticdispersioninasinglemodefiberatthe1550-nmoperatingwavelengthwith1nmandL=1km.Conventional,dispersion-shifted,anddispersion-flattenedfibersFromtheabovediscussion,wesawthatthezero-dispersionregionisaround1310nm.Butthespectralattenuationcurveshowsthatminimumattenuationoccursaround1550nm.Therefore,toobtainasingle-modefiberwiththebestproperties,itisnecessaryeithertomovetheminimumattenuationpointto1310nmortoshifttheminimumdispersionpointto1550nm.Changethewaveguide-dispersionvaluebyredesigningtherefractive-indexprofileofthefibercoreRefractive-indexprofilesofaconventionalfiber:

(a)Matched-claddingdesign;(b)depressed-claddingdesign;

(c)exampleoftopographicprofileofdepressed-claddingdesignDispersion-shiftedfiberRefractive-indexprofilesofadispersion-shiftedfiber:(a)refractive-indexprofiles;(b)exampleoftopographicprofileDispersion-flattenedfiberRefractive-indexprofilesofadispersion-flattenedfiber:(a)refractive-indexprofiles;(b)exampleoftopographicprofile.In1988,PhillipsCorporationreportedthefirstcommerciallymanufactureddispersion-flattenedsinglemodefiber.Nowmanufacturersfabricatesuchfibersinawiderangeofprofiles,someofwhichareshowninFigureOncemanufacturerslearnedhowtocontrolwaveguidedispersionbychangingtherefractive-indexprofile,fabricatingfiberswithspecificcharacteristicsbecamepossible.Chromatic-dispersionparametersforconventional,dispersion-shifted,anddispersion-flattenedfibersPolarizationModeDispersion(PMD)Linear-polarizedOtherformsoflinear-polarizedCircular-polarizedEllipticallypolarizedEventhoughwecallthefiber“singlemode,”itactuallycarriestwomodesunderonename.thesemodesarelinear-polarizedwavesthatpropagatewithinafiberintwoorthogonalplanes,.Ideally,eachofthemodescarrieshalfofthetotallightpower.Ifthefiberisperfect,bothofthempropagateatthesamevelocityandarriveatthefiberendsimultaneously.Thus,signaltravelalongthefiberremainsundisturbedandthepresenceofthepolarizedmodesgoesunnoticed.Beawareinparticularoftwokeyfactsregardingpolarization-modedispersion:(1)DPMD

doesnotdependonwavelengthand,(2)itdoesdependonthesquarerootofthefiberlength.WhatphysicalreasonsbringaboutPMD?Tokeepthestateofpolarizationundisturbed,afibermusthaveidealsymmetriccross-sectionalproperties;inotherwords,therefractiveindexsalongthefiber’sxandyaxis—nxandny,respectively–mustbeequal.Obviously,thereissomeasymmetryineveryfabricatedfiber,butthemostlikelytimesofseriousasymmetrytooccurareduringthefiber-cablingand–splicingprocesses.Bychoosinganoperatingwavelengthequaltothezero-dispersionwavelength,chromaticdispersioncanbe—theoretically—eliminated.ButthereisnosuchmeanstoexcludePMD.Thisiswhyforlong-haullinks,PMDbecomesthebandwidth-limitingfactor.BandwidthofasinglemodefiberInfiber-opticcommunicationstechnology,thehighestbandwidthcanbeachievedusingsingle-modefibers.Forthisreason—togetherwiththeirattenuationrate,whichisthelowestattainable—single-modefibersarethelinksofchoiceinlong-distanceterrestrialandunderwatertelecommunicationssystems.小結Single-modefiberHowsingle-modefiberworks,d=9um,dat