通信工程专业英语论文

姓名:高炜班级：通信工程2班学号：10021340208AtmosphericLaserCommunicationKeywords:AtmosphericlasercommunicationModulationTransmitReceiveApplicationAbstract：AtmosphereLasercommunicationisoneofnewcommunicationtechnologies，asaresultofmanyadvantagessuchaslargemessagecapacitances，thebestconfidentialityandsimpleequipmentandsoon，whichwouldtriggerarevolutionofnewcommunicationtechnology.Thepurposeofthispaperistoexploretheelementsoftheatmosphericlasercommunication，advantageanddisadvantageoflasercommunication.Maintext：Therehasbeentremendoustechnicaladvancementofavailablecomponentssuchaslaser/LEDtransmitter,highsensitivityopticalreceiverofferingextremelyhighbandwidth,efficientmodulationtechniques,improvementinlowpowerconsumption,weight,andsize.Inspiteofmanysuchtechnologicaldevelopments,themajorlimitationoffree-spacelasercommunication(lasercom)performanceistheatmosphere.Atmosphericconditionultimatelydeterminesthelasercommunicationssystemsperformancenotonlyofterrestrialapplicationsbutalsoforuplink-downlink(e.g.,betweengroundandsatellite,aircraftorUAVterminals),becauseaportionoftheatmosphericpathalwaysincludesturbulenceandmultiplescatteringeffects.Atmosphericlasercommunicationtechnologyisanewtechnologyappearedrecently.Theelementofthenewtechnologyisthatopticalsignalcarriertransmitsthesignalpointtopointorpointtopointsbyatmosphere.Thistechnologyusesemiconductorlaserastheopticalsource，whichformsacommunicationsystemnameddigitalradiocommunicationsystem.Lasercommunicationsystemincludetwoparts：TransmittalandReception.TransmittalcontainsLasers,opticalmodulatorandopticalemittingantenna.Andreceptioncontainstheopticalreceivingantenna,opticalfiletandopticaldetector.Themessagesweneedtotransmitconnecttolasersinopticalmodulatorandopticalmodulatormodulatesthemessagestolaserandtransmittedbytheopticalemittingantenna.Atthereceivingend,theopticalreceivingantennareceiveslasersignalandsendthesignaltoopticaldetector.Theopticaldetectortransformsthelasersignaltoelectricalsignalanddemodulateselectricalsignaltooriginalmessagesafteramplification.Theprimaryfactorscharacterizinganatmosphericcommunicationchannelincludeatmosphericattenuation(frombothscatteringandabsorption)andscintillation.Attenuationoftheopticalwave(i.e.,reductionofitspower)byatmospherecanbecausedinseveralways,includingabsorptionoflightbygasmoleculesandRayleighorMiescatteringbygasmoleculesoraerosolparticlessuspendedintheair.OpticaltransmissionspectrumoftheU.S.StandardAtmosphereoverthevariouscommunicationwindowsisavailableintheliterature.[1]Theindividualabsorptionlinesarecausedbythevibration-rotationalabsorptionlinesofwatervapor,CO2,CH4,andothergasesofminorconstituentsintheatmosphere.Scatteringfromaerosolsandparticulatessuchasinfog,clouds,smoke,anddustalsocontributetothetotalattenuationOftheopticalbeam.ThecomponentofatmosphericlasercommunicationsystemAtmosphericlasercommunicationsystemconsistsoftwocommunicationmechanism，whichtransmitmodulatedlaserpulsesignal（voiceordata）reciprocally，receivinganddemodulatingthesignalemittedbyeachother，achievingduplexcommunication.OpticalmaserOpticalmasergenerateslasersignalandformsbeamintospace.Thequalityofopticalmaserdirectlyinfluencesthecommunicationqualityandcommunicationdistance.Itcancausegreateffecttosystemperformance.ModulatorandModulationmethodModulationisthataddingsignalintosignalcarrier.Modulationisakindofphotoelectricconverter,whichisaprocessthatoneparameter(intensity,frequency,phaseposition,polarization.etc)ofoutputbeamchangeswithelectricalsignal，finishingmodulationoflaser.Themodulationmethodshaveinternalmodulationandexternalmodulation.Theinternalmodulationisthataddingtheelectricalsignalmodulatedbyinformationsignalintolaser，makinglasertransmitslightsignalchangedwithinformationsignal.Theexternalmodulationisthatputtingthemodulationunitsoutofthelightsourceandaddingtheelectricalsignalmodulatedbyinformationsignalintomodulationcrystal.Whenlightbeamthroughthecrystal,oneparameter(intensity,frequency,phaseposition,polarization.etc)changeswiththeelectricalsignalchanging.Thenitbecomeslightsignalcontainedinformation.Regardlessofinternalmodulationorexternalmodulation,everymodulationmethodhasitsowndifferentmodulationformssuchasPulseamplitudemodulation,pulsewidthmodulationandpulsefrequencymodulation.Inaddition,directlymodulationalsocontainspulsecodemodulationandexternalmodulationcontainsAmplitudemodulation,frequencymodulation,pulsecodemodulation,polarizationmodulationandsoon.2、TransmitterandReceiverSystemThereareseveralatmospherictransmissionwindowswithinthe700–10,000-nmwavelengthrangethathasattenuationof<0.2dB/kmandsoarenearlytransparent.Themajorityoffree-spacelasercomsystemsaredesignedtooperateinthewindowsof780–850and1520–1600nm.Around850nm,reliable,inexpensive;high-performancetransmitteranddetectorcomponentsarereadilyavailable.Theseareoftenusedinnetworkandtransmissionequipment.Highlysensitivesilicon(Si)avalanchephotodiode(APD)andanadvancedvertical-cavitysurface-emittinglaser(VCSEL)areavailableforoperationnear850nm.Forthewavelengthrangeof1520–1600nm,highqualitytransmitteranddetectorcomponentsarealsoreadilyavailable.InGaAsisthemostcommonlyuseddetectormaterialforthislongerwavelength;itexhibitshighbandwidthcapabilitycombinedwithahighspectralresponse.Thismaterialisextremelysensitivebecauseofaninternalamplification(avalanche)process.Thesewavelengthsarealsosuitablewitherbium-dopedfiberamplifiertechnologyforhigh-powergeneration(>500mW)andhigh-datarate(>2.5Gbit/s).Thiswavelengthregionisalsoimportantfromalasereyesafetypointofview.TheAmericanNationalStandardforEyeSafeUseofLasersInstitute(ANSI)standardsforthemaximumpermissibleexposurelevel(MPE)fora10-secexposureisabout1mW/cm2foran800-nmwavelengthandabout100mW/cm2fora1550-nmwavelength.Thelongerwavelengthsystemisthusreadilydesignedtobeeyesafe.Otherbenefitsofthe1550-nmwavelengthregionincludeareducedsolarbackgroundandreducedscatteringattenuation,becauseoftheRayleighdependenceofinversewavelengthtothefourthpower.OpticalreceivingsystemOpticalreceivingisaprocessthatcollectinglightsignalmodulatedfromthedistancebyopticallens,aprocessthatfilterfiltratewave,aprocessthatphotoelectricdetectortakesphotoelectricconversion.Therearetwoapproachestoreceive.Oneisdirectdetectionreceptionandanotherisheterodynedetectionreception.DirectdetectionreceptionisaprocessthatmakinglightsignaltransferintoelectricalsignalbyOpticalsystemandopticaldetector.It’sakindofawaysimpleandpractical.AsGaAslasercommunicationisthedirectdetectionreceiver,whichhasdisadvantageisthelowsensitivity,signal-to-noiseratio.TheprincipleofheterodynedetectionreceptionissimilartoheterodynedetectionofRadiowave.AsshowninFig1,opticalsystemreceivedthelasersignalfrequencyisfc,throughthefilterandselectivelyreflectingmirrortotheopticalmixerphotosensitivesurface,atthesametimethevibrationgeneratedbythelaserfrequencyisF0ofthelaserthroughthemirrorisreflectedintothemixerphotosensitivesurface.Themixerisaphotoelectricdetector,itisontwosuperimposedwavedetectingandmixing,outputdifferencefrequencyFM=F0-FCIntermediatefrequencysignal,theFMcenterfrequencyband-passfilterintoelectricsignal.ThiskindofreceptionhasqualitiesofHighsensitivity,signal-to-noiseratio,buttheequipmentiscomplexandtechnicalisdifficult.Fig.1HeterodynedetectionschematicdiagramAtmospherictransmissiontechnologyClimatesconditionshavegreatinfluenceinlasertransmission,lightdecayveryfast.Thisismainlyduetoatmosphericgasesandaerosolmolecular"scattering"caused"absorption".ThemaininfluentfactorsaffectedthenatureoflightareCO2,oxygensmoke,dust,waterdropletsandborneol.Intheloweratmospheremostofthewatercombineswithinformofwater,representingatmosphericvolume4%.Atthesametimethepropagationoflightalsoisinfluencedbytheweather,whichmakestheatmosphericvisibilityvariation.LinkAnalysisTheoverallsystemperformanceofalasercomisquantifiedusingalinkbudgetderivedfromtherangeequation,whichcombinesattenuationandgeometricalaspectstocalculatethereceivedpower.Theprocessoffindingthelinkmarginthroughthesystemlinkcalculationisthetopicofthissection.Thepurposeofthissectionistodeveloptheparametersnecessarytocalculatetheperformanceofanopticalcommunicationlink.Weshallconsiderthesituationofopticalpropagationbetweenpointsinfree-space.ConsideralasertransmitterantennawithgainGTtransmittingatotalpowerPTatthewavelength.Thesignalpowerreceivedatthecommunicationsdetectorcanbeexpressed(fromtherangeequation)asReceivedSignal,PREC=PTGTτTτATMSGRτR,Anotherimportantparameterinopticalcommunicationslinkanalysisis“LinkMargin”,whichistheratioofavailablereceivedpowertothereceiverpowerrequiredtoachieveaspecifiedBERatagivendatarate.Notethatthe“required”poweratthereceiverPREQ(watts)toachieveagivendatarate,R(bits/sec),andreceiversensitivity,Nb(#photons/bit),isrelatedbyPREQ=NbRhν=NbRhc/λ,whereνisthefrequencyofthelaserlightofwavelength(h=Planck’sconstant,c=velocityoflight).TheparameterNbisusuallydefinedasreceiversensitivityandcanbeexpressedas#photons/bit,orindBm(takingthelogarithmtothebase10andmultipliedby10withbaseas1mW).Finally,wecandefinethelinkmarginMasM=ReceivedPower/RequiredPower.CombiningtheaboveequationsyieldstheexpressionM=[PT/(NbRhν)](D2/θ2TL2)τT10(−αL/10)τR.4、TheadvantageanddisadvantageofatmosphericlasercommunicationtechnologyTheadvantageofatmosphericcommunicationThemainadvantageincommunicating,i.e.,transferringinformationfromonepointtoanother,withopticalfrequenciesisthepotentialincreaseininformationtransferrate.Thisrateisdirectlyrelatedtothebandwidthofthemodulatedcarrier,whichisgenerallylimitedtoafixedfractionofthecarrierfrequencyitself.Therefore,increasingcarrierfrequencyfromthatof,say,RFormicrowavestothatofopticalwavesincreasestheinformationcapacityofacommunicationsystembymanyordersofmagnitude.Ausablebandwidthatanopticalfrequencyof∼200THzwillbeabout105timesthatofacarrierintheRFrange.Asecondadvantageofusingopticalfrequenciesisthestationcompactnessitallows.A6-inch-diameterlens“antenna”forusewithanopticalfrequencyof6×1014Hzhasaneffectivegainof122dB.AtanRFfrequencyof1GHztogenerateabout60dBgain,itwouldrequireanantennawithadiameterofover200feet.Thisisduetothefactthatthebeamdivergenceisroughlyproportionaltoλ/D,whereλisthecarrierwavelengthandDistheaperturediameter,andvariesinverselywithantennagain.Thus,alongerwavelengthnecessitatesaproportionatelylargerantennatoachievethesamegainFirst,lasercommunicationhaslargemessagescapacitances,intheory,lasercommunicationcantransmit10000000TVprogramsand10000000000telephones.Second,lasercommunicationhasthestrangestconfidentiality,Notonlythedirectionofthelaserisespeciallystrong,butalsocanbeusedwithoutvisiblelight,soitisnoteasytobeinterceptedbytheenemy,ithasgoodsecrecyperformance.Third,theequipmentofatmosphericlasercommunicationislightandinexpensive,Becauseofthedivergenceangleoflaserbeamissmall,andgooddirectivity,lasercommunicationrequiredfortransmittingantennaandthereceivingantennacanbeverysmall,generalantennadiametersisafewcentimetersandafewkilogramsofweight,meanwhiletheweightofsimilaritymicrowaveantennaisseveraltons,morethan10tons.Becauseoftheseadvantages,opticalcarrierfrequenciescanaccommodateveryhighdataratesof100GHzormore.Withtheadventofthenewtechnologyofwavelengthdivisionmultiplexing(WDM),wecanfurtherincreasethedatarateperopticalbeam.Asopticaltechnologymaturestheadvantagesofpracticalfree-spacelasercommunicationssystemscanberealizedwithmucharchitecture.ThedisadvantageofatmosphericlasercommunicationFirst,atmosphericattenuationisveryserious.Atmosphericandclimatehavegreatinfluenceinlasertransmission.cloud,rainanddustwillpreventthepropagationoflaser.Theseelementsaffectthecommunicationdistanceseriously.Second,it‘shardtoaimingatreceptionequipment.Laserbeamhashighlydirectivity;ittakesalotofdifficulttoaimbetweentransmissionpointandreceptionpoint.Notonlydemandthehighequipmentstabilityandaccuracyoftheproposed,butalsothecomplexoperation.TheapplicationofatmosphericlasercommunicationinmilitaryIn1989,theUnitedStatesofAmericasuccessfullydevelopedashortdistance,concealedintheatmosphericlasercommunicationsystem.Andin1990,anothersuccessoftheatmosphericlasercommunication,theultravioletlightwavecommunication,thatusinginspecialwarfareandlowintensitywarfarewasfinishedbyresearchers.Atthebeginningofthe1990s,withitshighpowersemiconductorlaserdevicedeveloped,Russianalsostartedtheatmosphericlasercommunicationsystemstudyonappliedtechnology.Subsequently,helaunchedanewtypeofsemiconductorlaseratmospherecommunicationsystem,an