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Chapter1Prologue1.1Introduction...................................................41.2HistoricalBackgroundofLight...................................51.2.1Electromagneticspectrum..................................61.3OpticalWaveModels...........................................81.3.1Diffraction..............................................81.4AtmosphericEffects............................................91.4.1Atmosphericstructurewithaltitude..........................101.4.2Absorptionandscattering..................................111.4.3Meteorologicalphenomena.................................131.4.4Opticalturbulence........................................141.5ApplicationAreas..............................................151.5.1Freespaceoptics.........................................151.5.2Lasersatellitecommunicationsystems........................171.5.3Laserradarsystems.......................................201.5.4Otherapplicationareas....................................211.6ABriefReviewofCommunicationSystems.........................221.6.1Directdetectionsystems...................................231.6.2Coherentdetectionsystem.................................241.6.3Channelmodels..........................................251.7SummaryandOverviewoftheBook...............................26References....................................................32Overview:Inthisfirstchapterwepresentanintroduction/overviewofmaterialthatisrelatedtothepropagationoflaserbeamsthroughrandommedialiketheatmosphere.Theintenthereistoprovidethereaderwithabroadviewofthesubjectwithoutthedistractionofmathematicaldetailthatisrequiredinotherchaptersofthetext.Thewavelengthsofinterestthroughoutthetextarethevisibleandinfrared(IR)portionsoftheelectromagneticspectrum,althoughsomeresultscanreadilybeappliedtootherwavelengthslikemillimeterwavesand,undersomeconditions,microwaves.3Webeginbybrieflyintroducingsomeofthestandardopticalwavemodelsliketheplanewave,sphericalwave,andlowest-orderGaussian-beamwave.Next,wedescribetheoriginofcertainatmosphericeffects(includingmeteorologicalphenomena)associatedwithpropagatingopticalwaves.Someofthetraditionalapplicationareasarediscussedforlaserbeampropa-gation—freespaceopticalcommunications(FSO),laserradar,imaging,andremotesensing—followedbyashorthistoricalsummaryofdevelopmentalprogramsforlasersatellitecommunicationsystems(lasersatcom)intheUnitedStates,Europe,andJapan.Inthelastsectionofthischapterwepresentanoverviewofthematerialcontainedintheremainingchapters,delineatingtheprimarytopicstobetreatedineachindividualchapter.1.1IntroductionThefirstworkingLASER,anacronymstandingforLightAmplificationbyStimu-latedEmissionofRadiation,wasintroducedin1960andfromthatpointintimethescientificcommunityconcentratedagreatdealofattentiononitspossibleapplications.Inparticular,itwassuggestedthatlasersbeusedtoextendradio-frequency(RF)atmosphericcommunicationandradartechniquestotheoptical-frequencyband.Otherareasofinterestforlaserapplicationsincludeweap-onry,ranging,remotesensing,targetdesignation,adaptiveoptics,andmedicaluses,amongothers.However,allsystemsthatutilizeoptical(visible)orinfrared(IR)wavesmusttakeintoaccountgeneralpropagationeffectsassociatedwiththemediuminwhichitpropagatesinadditiontoeffectsassociatedwiththewaveitself.Thepropagationmediuminmanycasesistheturbulentatmosphereforwhichsmallindex-of-refractionfluctuationsalongthepropagationpathcauseavarietyofdeleteriouseffectsonthewave.Randomfluctuationsintherefractiveindexoftheatmospherearedirectlyassociatedwithmicroscopictemperaturefluctuationscausedbyturbulentmotionoftheairduetowindsandconvection.Althoughtheserefractive-indexfluctu-ationsareonlyafewpartsin106,apropagatingopticalwavepassesthroughalargenumberofrefractive-indexinhomogeneities,sotheircumulativeeffectontheopticalwaveisquiteprofound.Forexample,refractive-indexfluctuationscausethetwinklingofstarsandlimitthe“seeing”abilityofastronomerstoresolvesmallobjectstowithinafewsecondsofarc.Thislatteratmosphericeffectmotivatestheuseofadaptiveopticstechniquesandtheplacementoflargetelescopesinspace,suchasthefamousHubbletelescope.Earlyinvestigationsconcerningthepropagationofelectromagneticradiationandotherwavesthroughrandommediainvolvedthepropagationofstarlightthroughtheatmosphere,propagationofsoundwavesthroughtheatmosphereandocean,propagationofmicrowavesthroughplanetaryatmospheres,andpropagationofradiowavesthroughtheionosphereandinterplanetaryspace.Thus,someofthetheoreticalworkconcerningthepropagationofanoptical4Chapter1waveinaturbulentmediumwasdonepriortotheintroductionofthelaser.Thepropagationoflaserlight,whichissimplyoneformofelectromagneticradiation,isasubtopicofmuchofthisearlyresearch.BothChernovandTatarskiipublishedmonographsbefore1960onthepropagationofopticalplanewavesandsphericalwavesthroughturbulence;thesemonographsweresubsequentlytranslatedintoEnglishin1960and1961,respectively[1,2].Additionalbackgroundonopticalwavepropagationinrandommedia,alongwithmanyearlyreferences,canalsobefoundinLawrenceandStrohbehn[3],Prokhorovetal.[4],Fante[5,6]