Laser

Laser“Laserlight”redirectshere.Forthesong,seeLaserLight.Forotheruses,seeLaser(disambiguation).AlaserisadevicethatemitslightthroughaprocessUnitedStatesAirForcelaserexperimentRed(660&635nm),green(532&520nm)andblue-violet(445&405nm)lasersofopticalamplificationbasedonthestimulatedemissionofelectromagneticradiation.Theterm“laser”originatedasanacronymforlightamplificationbystimulatedemissionofradiation.[1][2]Alaserdiffersfromothersourcesoflightinthatitemitslightcoherently.Spatialcoherenceallowsalasertobefocusedtoatightspot,en-ablingapplicationssuchaslasercuttingandlithography.Spatialcoherencealsoallowsalaserbeamtostaynar-rowovergreatdistances(collimation),enablingapplica-tionssuchaslaserpointers.Laserscanalsohavehightemporalcoherence,whichallowsthemtoemitlightwithaverynarrowspectrum,i.e.,theycanemitasinglecoloroflight.Temporalcoherencecanbeusedtoproducepulsesoflightasshortasafemtosecond.Amongsttheirmanyapplications,lasersareusedinopticaldiskdrives,laserprinters,andbarcodescanners;fiber-opticandfree-spaceopticalcommunication;lasersurgeryandskintreatments;cuttingandweldingmate-rials;militaryandlawenforcementdevicesformarkingtargetsandmeasuringrangeandspeed;andlaserlightingdisplaysinentertainment.1FundamentalsLasersaredistinguishedfromotherlightsourcesbytheircoherence.Spatialcoherenceistypicallyexpressedthroughtheoutputbeinganarrowbeam,whichisdiffraction-limited.Laserbeamscanbefocusedtoverytinyspots,achievingaveryhighirradiance,ortheycanhaveverylowdivergenceinordertoconcentratetheirpoweratagreatdistance.Temporal(orlongitudinal)coherenceimpliesapolarizedwaveatasinglefrequencywhosephaseiscorrelatedoverarelativelygreatdistance(thecoherencelength)alongthebeam.[3]Abeamproducedbyathermalorotherincoher-entlightsourcehasaninstantaneousamplitudeandphasethatvaryrandomlywithrespecttotimeandposition,thushavingashortcoherencelength.Lasersarecharacterizedaccordingtotheirwavelengthinavacuum.Most“singlewavelength”lasersactuallypro-duceradiationinseveralmodeshavingslightlydifferingfrequencies(wavelengths),oftennotinasinglepolariza-tion.Althoughtemporalcoherenceimpliesmonochro-maticity,therearelasersthatemitabroadspectrumoflightoremitdifferentwavelengthsoflightsimultane-ously.Therearesomelasersthatarenotsinglespa-tialmodeandconsequentlyhavelightbeamsthatdivergemorethanisrequiredbythediffractionlimit.How-ever,allsuchdevicesareclassifiedas“lasers”basedontheirmethodofproducinglight,i.e.,stimulatedemission.Lasersareemployedinapplicationswherelightoftherequiredspatialortemporalcoherencecouldnotbepro-ducedusingsimplertechnologies.1.1TerminologyThewordlaserstartedasanacronymfor“lightamplifi-cationbystimulatedemissionofradiation”.Inmodernusage,theterm“light”includeselectromagneticradia-tionofanyfrequency,notonlyvisiblelight,hencethetermsinfraredlaser,ultravioletlaser,X-raylaser,and123LASERPHYSICSLaserbeamsinfog,reflectedonacarwindshieldsoon.Becausethemicrowavepredecessorofthelaser,themaser,wasdevelopedfirst,devicesofthissortop-eratingatmicrowaveandradiofrequenciesarereferredtoas“masers”ratherthan“microwavelasers”or“radiolasers”.Intheearlytechnicalliterature,especiallyatBellTelephoneLaboratories,thelaserwascalledanopticalmaser;thistermisnowobsolete.[4]Alaserthatproduceslightbyitselfistechnicallyanop-ticaloscillatorratherthananopticalamplifierassug-gestedbytheacronym.IthasbeenhumorouslynotedthattheacronymLOSER,for“lightoscillationbystimulatedemissionofradiation”,wouldhavebeenmorecorrect.[5]Withthewidespreaduseoftheoriginalacronymasacommonnoun,opticalamplifiershavecometobere-ferredtoas“laseramplifiers”,notwithstandingtheappar-entredundancyinthatdesignation.Theback-formedverbtolaseisfrequentlyusedinthefield,meaning“toproducelaserlight,”[6]especiallyinreferencetothegainmediumofalaser;whenalaserisoperatingitissaidtobe“lasing.”Furtheruseofthewordslaserandmaserinanextendedsense,notreferringtolasertechnologyordevices,canbeseeninusagessuchasastrophysicalmaserandatomlaser.2DesignMainarticle:LaserconstructionAlaserconsistsofagainmedium,amechanismtoener-gizeit,andsomethingtoprovideopticalfeedback.[7]Thegainmediumisamaterialwithpropertiesthatallowittoamplifylightbywayofstimulatedemission.Lightofaspecificwavelengththatpassesthroughthegainmediumisamplified(increasesinpower).Forthegainmediumtoamplifylight,itneedstobesup-pliedwithenergyinaprocesscalledpumping.Theen-ergyistypicallysuppliedasanelectricalcurrentoraslightatadifferentwavelength.Pumplightmaybeprovidedby12345Componentsofatypicallaser:1.Gainmedium2.Laserpumpingenergy3.Highreflector4.Outputcoupler5.LaserbeamAnimationexplainingthestimulatedemissionandthelaserprin-cipleaflashlamporbyanotherlaser.Themostcommontypeoflaserusesfeedbackfromanopticalcavity—apairofmirrorsoneitherendofthegainmedium.Lightbouncesbackandforthbetweenthemirrors,passingthroughthegainmediumandbeingam-plifiedeachtime.Typicallyoneofthetwomirrors,theoutputcoupler,ispartiallytransparent.Someofthelightescapesthroughthismirror.Dependingonthedesignofthecavity(whetherthemirrorsareflatorcurved),thelightcomingoutofthelasermayspreadoutorformanarrowbeam.Inanalogytoelec