LasertechnologyR.E.SlusherBellLaboratories,LucentTechnologies,MurrayHill,NewJersey07974Lasertechnologyduringthe20thcenturyisreviewedemphasizingthelasersevolutionfromsciencetotechnologyandsubsequentcontributionsoflasertechnologytoscience.Asthecenturydrawstoaclose,lasersaremakingstrongcontributionstocommunications,materialsprocessing,datastorage,imagerecording,medicine,anddefense.Examplesfromtheseareasdemonstratethestunningimpactoflaserlightonoursociety.Laseradvancesarehelpingtogeneratenewscienceasillustratedbyseveralexamplesinphysicsandbiology.Free-electronlasersusedformaterialsprocessingandlaseracceleratorsaredescribedasdevelopinglasertechnologiesforthenextcentury.S0034-6861(99)02802-01. INTRODUCTIONLighthasalwaysplayedacentralroleinthestudyofphysics,chemistry,andbiology.Lightiskeytoboththeevolutionoftheuniverseandtotheevolutionoflifeonearth.Thiscenturyanewformoflight,laserlight,hasbeendiscoveredonoursmallplanetandisalreadyfacilitatingaglobalinformationtransformationaswellasprovidingimportantcontributionstomedicine,industrialmaterialprocessing,datastorage,printing,anddefense.Thisreviewwilltracethedevelopmentsinscienceandtechnologythatledtotheinventionofthelaserandgiveafewexamplesofhowlasersarecontributingtobothtechnologicalapplicationsandprogressinbasicscience.Therearemanyotherexcellentsourcesthatcovervariousaspectsofthelasersandlasertechnologyincludingarticlesfromthe25thanniversaryofthelaser(AusubellandLangford,1987)andtextbooks(e.g.,Siegman,1986;AgrawalandDutta,1993;andReady,1997).Lightamplificationbystimulatedemissionofradiation(LASER)isachievedbyexcitingtheelectronic,vibrational,rotational,orcooperativemodesofamaterialintoanonequilibriumstatesothatphotonspropagatingthroughthesystemareamplifiedcoherentlybystimulatedemission.Excitationofthisopticalgainmediumcanbeaccomplishedbyusingopticalradiation,electricalcurrentanddischarges,orchemicalreactions.Theamplifyingmediumisplacedinanopticalresonatorstructure,forexamplebetweentwohighreflectivitymirrorsinaFabry-Perotinterferometerconfiguration.Whenthegaininphotonnumberforanopticalmodeofthecavityresonatorexceedsthecavityloss,aswellaslossfromnonradiativeandabsorptionprocesses,thecoherentstateamplitudeofthemodeincreasestoalevelwherethemeanphotonnumberinthemodeislargerthanone.Atpumplevelsabovethisthresholdcondition,thesystemislasingandstimulatedemissiondominatesspontaneousemission.Alaserbeamistypicallycoupledoutoftheresonatorbyapartiallytransmittingmirror.Thewonderfullyusefulpropertiesoflaserradiationincludespatialcoherence,narrowspectralemission,highpower,andwell-definedspatialmodessothatthebeamcanbefocusedtoadiffraction-limitedspotsizeinordertoachieveveryhighintensity.Thehighefficiencyoflaserlightgenerationisimportantinmanyapplicationsthatrequirelowpowerinputandaminimumofheatgeneration.Whenacoherentstatelaserbeamisdetectedusingphoton-countingtechniques,thephotoncountdistributionintimeisPoissonian.Forexample,anaudiooutputfromahighefficiencyphotomultiplierdetectingalaserfieldsoundslikeraininasteadydownpour.Thislasernoisecanbemodifiedinspecialcases,e.g.,byconstantcurrentpumpingofadiodelasertoobtainasqueezednumberstatewherethedetectedphotonssoundmorelikeamachinegunthanrain.Anopticalamplifierisachievedifthegainmediumisnotinaresonantcavity.Opticalamplifierscanachieveveryhighgainandlownoise.InfacttheypresentlyhavenoisefigureswithinafewdBofthe3dBquantumnoiselimitforaphase-insensitivelinearamplifier,i.e.,theyaddlittlemorethanafactoroftwotothenoisepowerofaninputsignal.Opticalparametricamplifiers(OPAs),wheresignalgainisachievedbynonlinearcouplingofapumpfieldwithsignalmodes,canbeconfiguredtoaddlessthan3dBofnoisetoaninputsignal.InanOPAthenoiseaddedtotheinputsignalcanbedominatedbypumpnoiseandthenoisecontributedbyalaserpumpbeamcanbenegligiblysmallcomparedtothelargeamplitudeofthepumpfield.2. HISTORYEinstein(1917)providedthefirstessentialideaforthelaser,stimulatedemission.Whywasntthelaserinventedearlierinthecentury?Muchoftheearlyworkonstimulatedemissionconcentratesonsystemsnearequilibrium,andthelaserisahighlynonequilibriumsystem.Inretrospectthelasercouldeasilyhavebeenconceivedanddemonstratedusingagasdischargeduringtheperiodofintensespectroscopicstudiesfrom1925to1940.However,ittookthemicrowavetechnologydevelopedduringWorldWarIItocreatetheatmosphereforthelaserconcept.CharlesTownesandhisgroupatColumbiaconceivedthemaser(microwaveamplificationbystimulatedemissionofradiation)idea,basedontheirbackgroundinmicrowavetechnologyandtheirinterestinhigh-resolutionmicrowavespectroscopy.SimilarmaserideasevolvedinMoscow(BasovandProkhorov,1954)andattheUniversityofMaryland(Weber,1953).ThefirstexperimentallydemonstratedmaseratColumbiaUniversity(Gordonetal.,1954,1955)wasbasedonanammoniamolecularbeam.Bloembergensideasforgaininthreelevelsystemsresultedinthefirstpracticalmaseramplifiersintherubysystem.ThesedeviceshavenoisefiguresveryclosetothequantumlimitandwereusedbyPenziasandWilsoninthediscoveryofthecosmicbackgroundradiation.Towneswasconfidentthatthemaserconceptcouldbeextendedto