fiberoptical.docx
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fiberoptical.docx
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fiberoptical
INTRODUCTION
Theuseoffiberopticsinlocalareanetworks(LANs),suchasEthernet,hasincreasedduetotheinherentadvantagesofusingfiber.Highdataratescanbemaintainedwithoutelectromagneticorradiofrequencyinterference(EMI/RFI).Longerdistancescanbeachievedoverthatofcopperwiring.Fortheindustrial/commercialuser,fiberoffershigh-voltageisolation,intrinsicsafetyandeliminationofgroundloopsingeographicallylargeinstallations.Ethernetwillfunctionwithnodifficultyoverfiberopticsaslongassomesimplerulesarefollowed.
CABLINGBASICS
Opticalfiberconsistsofthreebasicelements:
core,claddingandthecoating.Thecoreconstructedofeitherglassorplasticprovidesthebasicmeansfortransmittingthelightenergydownthecable.Thecladdingpreventsthelightfromexitingthecoreandbeingabsorbedbythecableitself.Thecoatingprovidesprotectiontothefibercorewhileprovidingstrength.Finalprotectionisprovidedbyanoveralljacketthatmayconsistofotherstrengthandprotectiveelements.
Figure1—Asinglefiberconsistsofthreebasicelements.
FiberSize
Opticalfibersareclassifiedbytheirdiameterinmicrons(1micron=one-millionthofameter).Frequentlythecore,claddingandcoatingarespecifiedusingslashes(/)toseparatethevalues.Forexample,50/125/250meansthecoreis50µm,thecladdingis125µmandthecoatingis250µm.Thesedimensionsallpertaintotheconcentricdiametersofthevariouselements.Ashortformwayofspecifyingthefiberistoonlylistthecoreandcladdingsizes.Intheaboveexample,thisfiberwouldbeclassifiedas50/125.Coresizesrangefromassmallas5µmtoashighas1000µm.Dependinguponthecoresize,eitheroneortwomodesoflighttransmissionwillbeexperienced.Thetwomodesarecalledsingle-modeandmultimode.
Figure2—Fiberopticcableisavailableaspairedcable(duplexcable)
withanappearancesimilarto"zipcord."
Single-ModeOperation
Withverysmalldiameterfibersintherangeof5to10µm,alllightrayshaveatendencytopropagatealongtheaxisofthefiber.Sincethereisonlyonepathforthelighttotake,thelightistermedtobeexperiencingasingle-modeofoperation.Asthecorediameterincreases,thelightrayshavetheoptionoftravelingatanangletothecoreaxiswhileattemptingtoexitthroughthecladding.Thissecondeffectiscalledmultimodeoperation.
MultimodeOperation
Withfibercoresizesof50,62.5andabove,multimodeoperationwillbeexperienced.Notonlywillthelighttransferdowntheaxisofthefiber,butitwillalsotravelawayfromtheaxisandtowardthecladding.Thecladdinghelpsreflectthelightraysbacktowardthefiberaxis.Thecladdingprovidesthiseffectbecauseithasalowerindexofrefractionthanthecore.
IndexofRefraction
Theindexofrefractionofamaterial(n)isdefinedastheratioofthespeedoflightinavacuumcomparedtothespeedoflightinthematerial.Whenlightpassesfromonematerialtoanotherwithadifferentdensity,partofthelightwillbereflectedandtheremainderrefracted.TheangleoftherefractedraywillbedifferentfromtheincidentwaveandwillobeySnell’sLaw:
n1sinΘ1=n2sinΘ2
wheren1,andn2arethecorrespondingindexesofrefractionandthetwoanglesaremeasuredrelativetoaperpendicularaxistotheboundaryofthetwomaterials.Atsomeanglecalledthecriticalangle,Θ2becomes90°.ForallvaluesofΘ1,greaterthanthecriticalangle,totalinternalreflectionwilloccur.Thisisthefundamentalprincipleoffiberopticcommunications.Thelightenergyisconstrainedtotheinnercore.Thecladdingwithitslowerindexofrefractionprovidesthetotalinternalreflectionnecessaryforproperoperation.
MultimodeSignalDistortion
Inmultimodeoperation,lightwavestraveldowntheaxisofthefiberaswellasazigzagcoursebouncingoffofthecladding.Sincesomeofthelightraystakealongertripwhentheyexitthefarendofthecore(duetoitszigzagcourse),distortionoftheoriginalsignalwilloccurasitrecombineswiththelightraythattooktheshorterpathdowntheaxisofthecore.Thisresultsinpulsebroadeningatthereceiverend.Thisdistortioniscalledmodaldispersionbecausethepathsofthelightraysareatdifferentlengths.Tocounteractthismultimodephenomenon,graded-indexfiberwasdeveloped.
Withgraded-indexfiber,theindexofrefractionishighestalongthecenteraxisofthefiberandgraduallydecreasesfromtheaxistothecircumference.Lighttravelsslowerwithahigherindexofrefractionandfasterwithalowerindexofrefraction.
Withthisapproach,thelightthattravelsdownthecenteraxisisdeliberatelyslowedtomatchthetimerequiredforlighttotravelazigzagcoursenearerthecircumference.Theresultislessdistortionandhigherbandwidth.
BandwidthrequirementsaregenerallynotanissuewithEthernet.Multimodefibershavebandwidthspecificationsinfrequency-distanceunits(MHz-km)thatdependupontheoperatingwavelength.Doublingthedistancehalvesthesignalingrate;however,evenatminimalbandwidthspecifications(160MHz-kmorso),theattenuationlimitationsofincreasedfiberlengthwillbemetbeforethebandwidthlimitations.
Lowerbandwidthfiberexistswitha200µmcorediameter.Thisisstep-indexfibermeaningthatonlyoneindexofrefractionexistsinthecoreandanotherinthecladding.Thisfiberisintendedforshorterrunsandiseasiertoconnectandismoreresilienttophysicalabuseduetoitslargercoresize.ThisfiberisfoundinplantfloorapplicationsbutisnotrecommendedforEthernet.
Figure3—Totalinternalreflectionoccurswhentheincidentangle
exceedsthecriticalangle.
OperatingWavelengths
Fiberoptictransmittersandreceiversaregenerallyclassifiedtooperateineitherofthreefrequencies.Thesefrequencieshavebeenfoundtohavethelowestattenuationacrossabandoffrequencies.Theregionsoflowestattenuationarecalledwindows.Theparticularfrequenciestheindustryusesare850nm,1300nmand1550nm.Thetwolowerwavelengthsoffercost/performancetradeoffsthatareofinterestinEthernetapplications.The850nmtechnologyisreadilyavailableatthelowestcost.However,fiberopticcableattenuationishigherinthe850nmbandthaninthe1300nmbandandthebandwidthisless.ThisattenuationiswhatlimitsthefiberopticsegmentlengthswhenusingEthernet.The1300nmreceiversandtransmittersaremorecostlybutarerecommendedwhenlongdistancesaretobeencounteredor100Mbpsoperationisrequired.The850nmtechnologyisgenerallyusedwithmultimodeapplications,whilethe1300nmtechnologyisusedwitheithersingle-modeormultimodeoperation.Becauseofcost,the1550nmtechnologyisnotpopularwithEthernet.
FiberOpticTransmitters
Both850nmand1300nmfiberoptictransmitterscanbefoundinhubsandnetworkinterfacemodules(NIMs)andthetwotechnologiescannotbemixed.ThesetransmittersareavailablewitheitherST,SCorMICconnectors.TheSTconnectoroperatessimilartoasmallcoaxialBNCconnector.Itpreventsover-tighteningandprovidesrepeatableinsertionloss.TheSCconnectorisalow-cost,snap-inconnectorwhilethesimilarstyleMICconnectorwasoriginallyintendedforFiberDistributedDataInterface(FDDI)applications.
Infiberopticimplementations,aseparatetransmitterandreceiverareusedinsteadofatransceiver.FiberopticlinksuseaduplexcableforNIM-to-hubandhub-to-hubconnections.AtransmitteratpointAconnectstoareceiveratpointB.PointB’stransmitterattachestopointA’sreceiver.Therefore,acrossoverfunctionmustbeaccomplishedinthecabling.Transmittersandreceiversmayormaynotbecolor-codedsocaremustbeexercisedtopairatransmittertoareceiver.
Figure4—Step-indexfiberhasthelowestbandwidth,
whilesingle-modefiberoffersthehighest.
TransmitterPower
TransmittersareratedindBmwith0dBmcorrespondingto1milliwattofpower.Transmitteroutputcanvaryfromdevicetodevice,soitisimportantto100%testtransmitterstoensurethatnoneareshippedbelowtheminimumspecifiedintheEthernetstandard.Testingisusuallyaccomplishedbyapplyingasquarewavesignalandmeasuringtheaveragepowerwithanopticalpowermeter.Transmitteroutputalsodependsuponthefibersize.Moreenergyislaunchedintolargerfibersizes;therefore,apowerratingshowninaspecificationisbaseduponaparticularcoresize.
ReceiverSensitivity
ReceiversensitivityisalsoratedindBmandisbaseduponreceivingthesamesquarewavesignalgeneratedbythetransmitter.Typicallyonlyamaximumsensitivityratingisgivenwhichrepresentstheweakestsignaldiscernablebythereceivingelectronics.Separatereceiversarerequiredfor850nmand1300nmoperation.Receiversensitivityistypicallythesameoverabatchofreceiversanddoesnotexhibitthesamevariabilityastransmitters.
OpticalPowerBudget
Whenspecifyingafiberopticinstallation,attentionmustbepaidtotheavailableopticalpowerbudget.ThepowerbudgetisthedifferencebetweenthelightsourcestrengthminusreceiversensitivityexpressedindB.Thisvaluemustbecomparedtothelinklossthatisthetotalattenuationduetoopticalcableandopticalconnectors.Thelinklossmustbelessthanthepowerbudget.Thedifferenceiscalledthepowermarginthatprovidesanindicationofsystemrobustness.
LinkLoss
Todeterminethelinkloss,alllossesduetofiberlengthandcableconnectionsmustbesummed.Fiberopticcableattenuationisusuallyspecifiedbythecablemanufacturer.Usethisfiguretodeterminethe
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