光学接收器的灵敏度英文.docx

光学接收器的灵敏度英文.docx

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光学接收器的灵敏度英文

Optical-ReceiverSensitivity

Inadditiontotheeffectivedeliveryofthesignaltothedetector,theperformanceoftheopticallinkalsodependsonthereceiversensitivity（measuredintermsofreceivedphotonsperbit）.Becauseofthehighcostassociatedwithincreasingthetransmitpowerandsystemaperture,improvingthereceiversensitivityisanimportantfactorinthedeep-spacelasercomsystemdesign.

Eitheracoherentreceiveroradirect-detectionreceivercanbeusedtodetecttheopticalsignal.Inacoherentopticalreceiver,theincomingsignalismixedwiththeoutputofastronglocaloscillator（LO）beam,andtheinterferencebetweenthesignalandLOinthecombinedfieldisdetectedusingapairofphotodetectors.Figure2-4showsaconceptualblockdiagramofacoherentreceiver.

ThemixingoftheweaksignalfieldandthestrongLOfieldatthefrontendofacoherentreceiverprovideslinearamplificationanddown-convertstheopticalsignalintoanelectricaloutputattheintermediatefrequency（IF）withgain（usuallytensofdecibels）.WithasufficientlystrongLOfield,thisraisesthesignallevelwellabovethenoiselevelofsubsequentelectronics.Thesensitivityofthecoherentreceiveristhuslimitedbytheselfnoise（i.e.,signalshotnoise）oftheincidentsignal.Furthermore,becauseofthespatialmixingprocess,thecoherentreceiverissensitiveonlytosignalandbackgroundnoisethatfallswithinthesamespatial-temporalmodeoftheLO.Acoherentreceivercan,inprinciple,operatewithaverystrongbackground（e.g.,withtheSuninthefieldofview）withoutsignificantperformancedegradation.

Whereλsistherateofdetectedsignalphotons,andthelastapproximationwasmadeinthelimitoflargesignalbandwidthB.Equation（2.2-4）statesthatthelimitingcapacityofaheterodyneopticalchannelis-1.44bitsperdetectedphoton.

Eventhoughthecoherentreceivercaninprincipalprovidenear-quantumlimitedreceiversensitivity,suchperformanceisachievedonlythroughnearperfectspatial-modematchingbetweentheincomingsignalandtheLO.Theaddedcomplexitytoaccomplishthespatialwavefrontmatchingcanbeverydifficulttoachieveforaground-basedreceiver.Thisisbecausetheatmosphereeffectivelybreaksuptheincidentwavefrontintoanumberofcoherentcellsofsizesapproximatelythecoherencelengthoftheatmosphere.Thesizeof,undertypicaloperatingcondition,isontheorderof5-30cm.Althoughadaptiveopticstechniqueshavebeendevelopedtopartiallycompensateforthewavefrontdistortion,effectivewavefrontcorrectionoverthelargeaperture

LinkandSystemDesign

diameterenvisionedforthedeep-spacereceiverwillrequireanactivemirrorwithalargenumberofactuators.Becauseofthecomplexityofsuchasystem,andbecausethesimplerdirect-detectionreceivershavemanagedtoachievesimilar,ifnotbetterperformance,coherentreceiversarenotbeingconsideredforaground-basedreceiver.Instead,thebulkofthedevelopmenthasbeenfocusedonthedirect-detectionreceiver.

Inadirect-detectionreceiver,thereceivedopticalintensityisdetectedwithoutextensivefront-endopticalprocessing.Figure2-5showsaconceptualblockdiagramofadirect-detectionreceiver.Theincidentsignaliscollectedbythereceivetelescope.Apolarizationfilterfollowedbyanarrowbandfilter,andafieldstopeffectivelyreducestheamountofbackgroundnoiseincidentontothedetector.

Thecapacityofadirect-detectionopticallinkhasbeenstudiedextensively.Whenthereceiveriscapableofdetectingindividualphotons,Pierce[2]firstshowedthatthecapacityoftheopticalchannelcanbeimprovedbyusingamodulationformatwithveryhigh-bandwidthexpansionratios.SubsequentworkbyWyner[3]showedthatthecapacityofadirectdetentionopticalchannelinthepresenceofbackgroundcanbewrittenas:

whereλsistherateofarrivalforthedetectedsignalphoton（measuredinphotordsec）,ρ=λs/ABisthe（detected）peaksignaltobackgroundpowerratioandMisthepeak-to-averagepowerratioofthesignal.

Figure2-6showsaplotofthechannelcapacityversusthepeak-to-averagesignalratioforseveralvaluesoftheaveragesignal-to-backgroundnoiseratios.Itispossibletotransmitmorethan1bit/photonatasufficientlyhighpeak-to-averagepowerratio[12,13].Inotherwords,aphoton-countingdirect-detectionreceivercanachieveahigherchannelcapacitythanacoherentreceiverbyusingmodulationformatsthatexhibithighpeak-to-averagepowerratios.showsthatthecapacityofadirectdetectionopticallinkusingidealphoton-countingdetectorcanbeimprovedby

1）Improvingλs,orequivalently,increasingthephotondetectionefficiencyforagivenreceiveopticalpowerlevel,

2）IncreasingM,thepeaktoaveragepowerratio:

theperformanceofthedirectdetectionopticalchannelcanbeimprovedbyselectingamodulationformatthatmaintainsahighpeaktoaveragepowerratio,

3）Improvingρ,thesignaltonoisepowerratiobylimitingtheamountofbackgroundopticalpowerdetectedbythephotodetector.EventhoughEq.（2.2-5）wasderivedfromanidealphoton-countingreceivermodel,thegeneralbehaviorofthechannelcapacityremainsvalidforawiderangeofreceiverddetectorsthatareshot-noiselimited.Thatis,theperformanceofthedirect-detectionlinkcanbeimprovedbyincreasingthedetectorsensitivity,selectingamodulationformatwithhighpeaktoaveragepowerratio,andreducingtheamountofbackgroundlightdetected.Eachofthesefactorsisbrieflydescribedbelow.

1.PhotonDetectionSensitivity

Improvingthephotondetectionefficiencyisanobviousmethodofimprovingthechannelperformance.Foradirect-detectionreceiver,thisisgenerallyaccomplishedbyusingdetectorswithinternalamplifications,suchasavalanchephotodiodesandphotomultipliertubes.

Inthelimitofaveryhighamplificationgain,thereceiver’snoisecontributioncanbeignored,andthereceiveriscapableofdiscriminatingtheindividualphotonarrivaleventsandcountingphotons.Ifthedetectorcontributenegligibleamountofdarkcounts,suchareceiveriscapableofachievingthechannelcapacityshownin.Foramoregeneralclassofopticalreceiverthatisnotcapableofdiscriminatingindividualphotonarrivals,thechannelcapacitywilldependonthenoiseaddedbythereceiver,includingthenoiseintroducedbytheamplificationprocessandthethermalnoisefromthecircuitelements.Evenifthereceiverisnotphoton-counting,improvingthereceiversensitivitycanstillresultinacorrespondingincreaseinthechannelcapacity.Thisisaccomplishedbyincreasingthedetectoramplificationwhilecontrollingthenoiseintroducedbytheamplificationprocess（e.g.,excessnoise）andthethermal/leakagecurrentnoise.RefertoSection6.2formoredetaileddiscussionofthephotondetection.

2.ModulationFormat.

Onepracticalmodulationformattoachievehighpeak-to-average-powerratioistheM-arypulse-positionmodulation（PPM）.InanM-aryPPMmodulationscheme,eachchannelsymbolperiodisdividedintoMtimeslots,andtheinformationisconveyedthroughthechannelbythetimewindowinwhichthesignalpulseispresent.AnillustrationofthePPMmodulationforasimplecaseofM=8isshowninFig..Whenthetransmitlaserexhibitsasufficientmodulationextinctionratio,thepeak-to-averagepowerratioofanM-aryPPMchannelisequaltoM,andthecapacityoftheM-aryPPMchannelcloselyapproximatestheidealPoissonchannelcapacitystatedin.Additionally,whenM=2k,eachPPMchannelsymbolcanbemappeddirectlytoak-bitssequence,thussimplifyingthebit-to-symbolmappingproblem.Forthesereasons,exceptwhenthetransmitterispeak-powerlimitedorwhenthesystemismodulation-bandwidthlimited,mostdeep-spaceopticallinksanalyzedtodatehadassumedM-aryPPMmodulations.

.3.BackgroundNoiseControl

Thediscussionfollowingshowsthattheperformanceofthedirectdetectionchannelcanbeimprovedbyreducingtheamountofbackgroundnoisedetectedbythereceiver.Foratypicalgroundbasedreceiver,thesourcesofbackgroundnoiseinclude:

1）Diffused（extended）backgroundfromtheatmosphere,ThebackgroundirradiancefromtheextendedbackgroundcanbewrittenaswhereLλ（θ）representsskyradiance,whichisafunctionofwavelengthandsolarilluminationgeometry,Aristheeffectivereceiverarea,Ωisthesolidanglefieldofviewinsteradians,Aristheopticalbandpass,andηRistheefficiencyoftheopticalreceivingsystem.

2）Planetaryorstellarbackgroundobjectswithinthereceiverfieldofview.Forapointsource（e.g.,astar）inthereceiverfieldofview,theamountofbackgroundpowercollectedbythereceiveriswrittenas

whereH（λ）isthespectralirradianceofthebackgroundsource,withunitsofwattspermetersquared.micron.

3）Inadditiontothepointsourcesandextendedbackgroundsources,anothermajor

sourceofbackgroundphotonsisthescatteredlightcollectedbythereceiveoptics.Astrongbackgroundsourcenearthefieldofviewofthereceivercanleadtosignificantscatteringintothereceiverfieldofview.Foranopticalreceiverdesignwithopticsunderdirectexposuretosunlight,thescatteringcontributionisoneofthemajorbackgroundnoisesources

TheamountofscatteredsunlightcollectedbythereceivercanbewrittenaswhereT（λ）representstheatmosphericattenuationandIλrepresentstheexo-atmosphericsolarconstant（0.074W/cm*pm）andBSDF（θ）isthebidirectionalscatterdistributionfunctionasafunctionofincidentangle.TheBSDFvaluesdependon.thesurfacemicroroughnessandcontaminationlevelsand,ingeneral,theyexhibitapow