OMA和消光比

AVAILABLEFunctionalDiagramsPinConfigurationsappearatendofdatasheet.FunctionalDiagramscontinuedatendofdatasheet.UCSPisatrademarkofMaximIntegratedProducts,Inc.ApplicationNote:HFAN-02.2.2Rev1;04/08OpticalModulationAmplitude(OMA)andExtinctionRatioApplicationNoteHFAN-02.2.2(Rev.1;04/08)MaximIntegratedPage2of5OpticalModulationAmplitude(OMA)andExtinctionRatio1IntroductionTheopticalmodulationamplitude(OMA)ofasignalisanimportantparameterthatisusedinspecifyingtheperformanceofopticallinksusedindigitalcommunicationsystems.TheOMAdirectlyinfluencesthesystembiterrorratio(BER).Withanappropriatepointofreference(suchasaveragepower),OMAcanbedirectlyrelatedtoextinctionratio.ThepurposeofthisapplicationnoteistodefineOMAandhowitrelatestootherparameterssuchasextinctionratioandaveragepower.Further,thisapplicationnotewillclarifythetrade-offsbetweenspecifyingOMAversusextinctionratioandexploreappropriatespecificationrangesforeach.2DefinitionsandRelationshipsForbi-levelopticalsignalingschemes,suchasnonreturn-to-zero(NRZ),onlytwodiscreteopticalpowerlevelsareused.Thehigherlevelrepresentsabinaryone,andthelowerlevelrepresentsazero.WewillusethesymbolP1torepresentthehighpowerlevelandthesymbolP0torepresentthelowpowerlevel.Usingthesesymbolswecanmathematicallydefineanumberofusefultermsandrelationships.OMAisdefinedasthedifferencebetweenthehighandlowlevels,whichcanbewrittenmathematicallyas:01PPOMA-=(1)Averagepowerissimplytheaverageofthetwopowerlevels,i.e.,201PPPAVG+=(2)Wewilluseretorepresenttheextinctionratio,whichistheratiobetweenthehighandlowpowerlevels:01PPre=(3)Throughalgebraicmanipulationofequations1,2,and3,wecanderivethefollowingrelationships:
+-=112eeAVGrrPOMA(4)10+=POMAre(5)
+=+=12211eeAVGAVGrrPOMAPP(6)
+=-=112210eAVGAVGrPOMAPP(7)3AbsoluteVersusRelativeSpecsOMAandextinctionratiobythemselvesarerelativequantities,sincetheyonlyspecifythedifferenceorratioofthepowerlevels.InordertoderiveanabsolutequantityfromtheOMAorextinctionratio,wemusthaveanadditionalpointofreference,suchasPAVG,P1,orP0.Therelationshipsofequations4-7alldependononeoftheseabsolutepointsofreference.Forexample,anOMAof100
WcancorrespondtoaninfinitenumberofpossiblevaluesforPAVG,P1,orP0:P1couldbe100
WwithP0equalto0
W,orP1couldbe150
WwithP0equalto50
W,orP1couldbe100mWwithP0equalto99.9mW,etc.,etc.Inthealternatecaseofextinctionratio,asimilarexampleusingre=10cancorrespondtoaninfinitenumberofpossiblevaluesforPAVG,P1,orP0:P1couldbe100
WwithP0equalto10
W,orP1couldbe150
WwithP0equalto15
W,orP1couldbe100mWwithP0equalto10mW,etc.,etc.ApplicationNoteHFAN-02.2.2(Rev.1;04/08)MaximIntegratedPage3of5If,inadditiontotheOMAorextinctionratio,wespecifyareferencepointofPAVG=100
W,forexample,thentheambiguityisgone.WithanOMAof100
WandPAVG=100
W,P1canonlybe150
WandP0canonlybe50
W.Iftheextinctionratiois10andPAVG=100
W,thenP1canonlybe182
WandP0canonlybe18.2
W.4OpticalAttenuationUptothispointinthediscussion,itmayseemapparentthatOMAandextinctionratioarebasicallyequivalent.Eithercanbecomputedwithknowledgeoftheotherandonereferencepoint.BothcanbequantifiedwhenthevaluesofP1andP0areknown,etc.Therearedifferences,however,andoneoftheseishowOMAandextinctionratiochangeasthesignalpropagatesthroughanopticalsystem.Assumingasystemwithlinearattenuationbetweentwopoints,theextinctionratiowillstayconstanteventhoughthesignalisattenuated,whiletheOMAwillchangebyafactorequaltotheattenuation.Forexample,over10kmofopticalfiberwithanattenuationof0.3dB/km,thetotalattenuationoverthelengthofthefiberis3dB,whichisequivalenttoafactorof2.IfwetransmitasignalthroughthefiberthatstartswithP1=1mWandP0=0.1mW,thenre=1/0.1=10andOMA=1–0.1=0.90mWatthefiberinput.Afterpassingthroughthefiberthesignallevelsarereducedbyafactorof2,soP1=0.5mWandP0=0.05mW.Therefore,atthefiberoutput,re=0.5/0.05=10(thesameasattheinputre)andOMA=0.5–0.05=0.45mW(halfoftheinputOMA).Fromthisexampleweseethatoncetheextinctionratioisknown,asimpleaveragepowermeasurementanywhereinthesystemwillyieldenoughinformationtocalculateP1,P0,andevenOMA.Ontheotherhand,ifwehaveknowledgeoftheOMAatonepointinthesystem,wecannotdetermineit’svalueafterattenuationwithoutknowingthemagnitudeoftheattenuationorelsemeasuringadditionalparameters(suchasP0,P1,orPAVG).5Power-LevelEffectsonTransmittersandReceiversIntheory,thesystembiterrorratio(BER)isdeterminedentirelybytheopticalsignal-to-noiseratio,whichiscommonlycalledtheQ-factor(seeMaximapplicationnoteHFAN-9.0.2“OpticalSignal-to-NoiseRatioandtheQ-FactorinFiber-OpticCommunicationSystems”).TheQ-factorisdefinedastheOMAdividedbythesumofthermsnoiseonthehighandlowopticallevels,i.e.,0101σσ+-=PPQ(8)Basedonequation8(andassumingthatthenoiseisafixedquantity)itisclearthatthesystemBERperformanceisdirectlycontrolledbytheOMA.Therefore,inordertooptimizeBERperformance,theOMAshouldbeaslargeaspossible.Also,equation8saysnothingaboutPAVG,implyingthatwewillgetthesameBERperformancewhetherP1andP0are100mWand1mWor200mWand101mW.Inrealsystems,therearepracticalupperandlowerpracticallimitsonPAVGandthereforeOMA.Fromtheopticalreceiverpointofview,thereisanupperlimitontheopticalpowe