電子信息專業(yè)英語（第二版）課件：Different Types of Lasers

DifferentTypesofLasersPassageACrystalandGlassLasers

PassageBLiquidLasers;theFree-electronLaserandtheX-rayLaser

PassageCGasLasers

PassageACrystalandGlassLasers

Whatmaterialscanbemadeintolasers?Almostany,itseems.TheodoreMaimanjokesthatrightafterscientistsheardthathehadfashionedarubyrodintothefirstlaser,nearlyeverybodywithcrystalinhisorherlabtriedputtingmirrorsontheendofittoseeifitwouldlaser.[1]Theamazingthingisthatmayofthesescientistssucceeded.

Crystals,solids,liquids,gases—many,manydifferentkindsofmaterials—arenowusedastheactiveingredientsinlasers.Laseractionhasbeenobservedonliterallythousandsoftransitions.Eachtransitionproducesadifferentwavelength,andthesewavelengthscoverabroadrange,frommicrowavestoXrays.

Thelongestwavelengths,severalcentimeters(aninchortwo)long,whichareinthemicrowaveregion,areproducedbymasers.Masers,thoughdiscoveredfirst,areactuallyregardedasasubcategoryofthelaserfamily.Astheirnameindicates,masersoperateonlyinthemicrowavespectralregion.Theshortestwavelengthsatwhichlaseramplificationhasbeenreportedis1.4nanometers;that’s1.4billionthsofameter(5.5×10-8in),atthelong-wavelengthendoftheX-rayregionofthespectrum.ThisfeatwasaccomplishedbyagroupattheLawrenceLivermoreNationalLaboratory,accordingtoreportinAviationWeekandSpaceTechnologythathasyettobeconfirmedinthescientificliterature—orbyLivermore,wheretheofficialreactionis“nocomment”becauseofpossiblemilitaryapplications.

Ifyoutakeacarefullookatthelastparagraph,you’llnotethatthefamilyoflaserdevicescoversawavelengthrangeofoveramillionfold.Visiblelightisonlyatinypartoftherangeoflaserwavelengths,justasitisonlyatinypartoftheelectromagneticspectrumasawhole,andyoushouldrealizethatmostlasersemitbeamsoflightthatarenotvisibletothehumaneye.[2]Startingwiththelongestwavelengths,lasersemitmicrowaves,millimeterwaves,sub-millimeterwaves,infraredlight,visiblelight,ultravioletlight,and,apparently,X-rays.Theselasershavemanythingsincommon,buttherearealsosomestrikingdifferences.Inthesub-millimeterregion,forexample,awiremeshmayserveasamirror,despitethefactthatitobviouslycan’tbeusedasamirrorinthevisibleregion.

Aswedescribevarioustypesoflasers,we’lltrynottooverwhelmyouwithnumbers.Butthereisonenumberthat’simportant:that’sthewavelength.Thewavelengthofalaserdefinesthekindofradiationitemitsandalsoidentifiesthetypeoflaser.Rememberthatwavelength,frequency,andphotonenergyareallinterchangeabletermsthatcanrefertothesamelight-wave.Andremember,too,thateachtypeoflaseremitslightofauniquewavelength.

Besideshavingdifferentwavelengths,therearealsobigdifferencesamonglasersinpowerandoperatingconditions.Somelasersarefeeblethings,whichatbestproducepulsesofamillionthofawattlastingonlyamillionthofasecond.Thentherearehugesystemsproducingoveramillionwattsforafewsecondsatatime.Somelasersworkonlywhencooledtoextremelylowtemperatures,200degreesbelowzeroontheCentigradescale.Othersneedtemperatureshighenoughtovaporizemetals,becausethelaserwavelengthisproducedbyatransitioninametalvapor.

Inanefforttobringorderoutofthischaos,wewillcategorizelasersbythekindofmaterialsthatproducethelight.Thefirstfourcategories—crystal(andglass),gas,semiconductor,andliquidlasers—arestraightforward.Thelasttwo—free-electronandX-raylasers—arethestrangestmembersofthelasersfamily.

CrystalandGlassLasers

TheodoreMaimanmadethatfirsthistoricallaserfromacrystalofsyntheticruby,andrubylasersarestillcommonusetodayandaretypicalofthecrystal/glassclassoflasers.

Syntheticruby,likethenaturalgem,isaluminumoxidethatcontainsasmallpercentageofchromiumasanimpurity.Thisimpuritythechromium,iswhatgivesthecrystalitsredorpinkcolorandistheactivematerialinthelaser.Thealuminumoxideservesmerelyasthe“host”crystal,thelatticeinwhichthechromiumatomsareembedded.

Theredcolorinrubycomesfromthefluorescenceproducedbythechromiumatomsaftertheyabsorblight.Tobeprecise,chromiumselectivelyabsorbslightwavesof550nanometers,or5.5×10-7m.Aswesawearlier,chromiumthendropstoametastableenergylevel,whichinturnproducesvisibleredlightat694nanometerswhenitdecaystothegroundstate.(Thelongerthewavelength,thelessenergythelightwillhave;that’swhythewavesgetlongerwhenthechromiumdropstolowerenergylevels.)

Arubylasercanbemadelargeenoughandpowerfulenoughtodrillsmallholesinthinsheetsofmetal.Earlylaserresearcherslikedtopunchholesinrazorblades,andsotheybeganmeasuringlaserpowerin“gillettes.”A3-gillettelaser,forinstance,coulddrillthroughthreerazorbladesinarow.Thistongue-in-cheekwayofmeasuringhassincesuccumbedtoamoretechnicalway.Infact,theseparateinsert-ablerazorbladehasitselfsuccumbedtomoresophisticatedshavingequipment.Onceagain,romanticismandhumorloseouttoadvancedtechnology.Pity.

Therubylaserwasthefirsttobeusedinmaterialsworking,suchasdrillingholesinthediamonddiesthroughwhichcertaintypesofwirearedrawn.Unlikemanyothertypesofearlylasers,therubylaserisstillcommonlyusedtoday,thoughitsapplicationsarerestrictedbyitslimitedpowerandthetimebetweenpulsesneededforthecrystaltocool.

TodaythemostcommoncrystallaseriscalledaYAGlaser.Anothersyntheticmaterial,YAGisamemberofthegarnetfamilyofcrystalsandiscomposedprimarilyoftheelementsyttrium,aluminum,andoxygen,towhichasmallamountofneodymiumisadded.ThenameYAGisanacronymforyttrium-aluminumgarnet.Justastherubyactsasahostforchromiumatoms,sotheYAGisthehostforneodymiumatoms,whichemitastronglaserbeamat1.06micrometers(1.06×10-6m),awavelengthintheinfraredregion,slightlylongerthanthatofthelongestvisiblelight.Likeacrystalofruby,aYAGcrystalispumpedbyaflashlamporasimilarintenselightsource.Thiscrystalconductsheatwell,soitcanemitacontinuousbeamaswellaspulses.YAGlasersareusedfordrillingholesinmetalandasmilitaryrangefindersamong,otherthings.

Neodymiumcanalsobeaddedtoglasstomakeaneodymium-glasslaser.GlassissubstitutedforYAG,becauseit’scheaperandeasiertomake,particularlywhenbigchunksoflasermaterialareneeded.ThelightproducedisalmostidenticalinwavelengthwiththatoftheYAGlaser.Itdoesdiffersomewhat,becauseoftheinteractionoftheneodymiumatomswiththehost,butbylessthanonepercent.ThedrawbackisthatglassisapoorerconductorofheatthanYAGandisthereforeimpracticalinapplicationsinwhichalasermustbepulsedrapidlyoremitacontinuousbeam.

Manyothercrystallinelasershavebeendemonstratedinthelaboratory,butonlyahandfulhaveeverbeenproducedcommercially.Themostimportantoftheseuseothercrystallinehostsforneodymiumandchemicallysimilarelements.Nonehasyetcomenearachievingtheimportanceoftheruby,neodymium-YAG?,orneodymium-glasslasers,though.Likerubyandneodymiumlasers,allarepumpedbylightfromaflashlamporotherintenselightsource(sometimesanotherlaserisevenused).EXERCISES

1.?Fillintheblanks.

(1)?Thelongestwavelengths,severalcentimeters(aninchortwo)long,whichareinthemicrowaveregion,areproducedby

.

(2)?Rememberthat

,

,and

areallinterchangeabletermsthatcanrefertothesamelight-wave.

(3)?Somelasersworkonlywhencooledtoextremelylowtemperatures,othersneedtemperatureshighenoughtovaporizemetals,becausethelaserwavelengthisproducedbyatransitionina

.

(4)?Syntheticruby,likethenaturalgem,isaluminumoxidethatcontainsasmallpercentageof

asanimpurity.

(5)?Theredcolorinrubycomesfromthe

producedbythechromiumatomsaftertheyabsorblight.

(6)?Todaythemostcommoncrystallaseriscalleda

,anditisamemberofthegarnetfamilyofcrystalsandiscomposedprimarilyoftheelements

,

,and

,towhichasmallamountof

isadded.

(7)?Neodymiumcanalsobeaddedtoglasstomakea

.

2.?True/False.

(1)?Crystals,solids,liquids,gases—many,manydifferentkindsofmaterials—arenowusedastheactiveingredientsinlasers.()

(2)Theshortestwavelengthsatwhichlaseramplificationhasbeenreportedis1.4nanometers;that’s1.4billionthsofameter(5.5×10-8in),atthelong-wavelengthendoftheX-rayregionofthespectrum.()

(3)?Youshouldrealizethatmostlasersemitbeamsoflightthatarevisibletothehumaneye.()

(4)?Besideshavingdifferentwavelengths,therearealsobigdifferencesamonglasersinpowerandoperatingconditions.()

(5)?Thelongerthewavelength,themoreenergythelightwillhave;that’swhythewavesgetlongerwhenthechromiumdropstolowerenergylevels.()

(6)?Arubylasercanbemadelargeenoughandpowerfulenoughtodrillsmallholesinthinsheetsofmetal.()

(7)?Therubylaserwasthefirsttobeusedinmaterialsworking,suchasdrillingholesinthediamonddiesthroughwhichcertaintypesofwirearedrawn,anditisnotusedtoday.()

(8)Manyothercrystallinelasershavebeendemonstratedinthelaboratory,butonlyahandfulhaveeverbeenproducedcommercially.()PassageBLiquidLasers;theFree-electronLaser

andtheX-rayLaser

LiquidLasers

Ifgasesandsolidscansustainlaseraction,whynotliquids?Indeedliquidscanbetheactivemediainlasers,andthere’salargeandimportantfamilyofliquidlasersbasedonorganicdyes.Thedyes,whichareactuallysolidsatroomtemperature,aredissolvedinaliquid(generallyanorganiccompound,suchasalcohol)toformasolution.

Whatmakesthedyelaserspecialisthenatureofitstransitions.Inalmostallotherlasers,thelasertransitionisbetweentwostatesatfixedenergylevels,whichmeansthatthelaseremissionisinafixed,verynarrow,well-definedbandofwavelengths.Evenincarbondioxidelasersandothertypeswithafamilyofcloselyspacedenergylevels,therearediscretetransitionsatdistinctwavelengths.Organicdyes,incontrast,haveenergylevelssocloselyspacedthattoallintentsandpurposestheyformacontinuum.Thislargenumberoflevelsexistsbecauseofthecomplexityofthedyemolecules,andasaresult,electronictransitionsinthedyecanproduceabroadrangeofwavelengths,mostofthemvisible.

It’spossibletodeviseanopticalsystemthatwillselectasinglewavelengthinthelaser’srange.Moreover,theseopticscanbeadjustedtotunetheoutputwavelengthcontinuouslyacrosstherangeofwavelengthspossibleforthatparticulardye.Mostdyesaretunableacrossatbest10percentofthevisibleregion,butit’spossibletoarrangeseveraldyesinsequencesoastobeabletotunetheoutputwavelengthacrosstheentirevisiblespectrum.

Whatmakesacommercialdyelaserworthitspricetagof$5,000to$50,000isthefactthatitcanbetunedprecisely—oftenwithjustatwistofadial—toemitasmallerrangeofwavelengthsthananyothersource,andthatwithinthisrange,thelightismuchmoreintense.Thiscanmakeadyelaserinvaluableforexperimentsinsuchareasaschemistryandatomicphysics.

Incidentally,oneofthestrangestlaserseverbuiltwasadyelaser.Itdoesn’tevenemitabeam,butratherahalo.Z.G?.Horvath,oftheCentralResearchInstituteforPhysicsinBudapest,Hungary,andtwocolleaguesattheLebedevPhysicsInstituteinMoscowbuiltadisk-shapedlaserthatemitslightalongall360degreesofitscircumference.Inthisdevice,thecircumferenceiscoatedwithapartiallyreflectingfilm,producingadisk-shapedlasercavity.Attheheartofthisweirdlaserisadyethat’spumpedbyanotherlaser.Horvathbelievesitshouldbepossibletoextendhistechniquetomakeasphericallaser—onethatwillemitlaserlightfromtheentiresurfaceofaball-shapeddevice.Butit’sunclearifeitheradiskorasphericallaserwilleverfindapplicationsoutsidethelaboratory.

TheFree-ElectronLaser

Arecentadditiontothelaserbestiaryisastrangecreatureknownasthefree-electronlaser.Thetermfreeelectroncomesfromthefactthattheactivemediumemittingthelightisabeamofelectrons,freefromatoms,whichispassingthroughamagneticfield.

Thislaserrequiresalargeelectronacceleratororstorageringtoprovidetheelectronbeam.Thenitneedsasetoflarge,powerfulmagnetstokeepchangingthedirectionofthebeam.ThefirstsuchlaserwasoperatedattheStanfordLinearAcceleratorCenterina1977experimentbyJohnM.J.Madey,aStanfordUniversityphysicist.Togiveyouanideaofwhatleaguewe’reinhere,theStanfordacceleratorcosthundredsofmillionsofdollarstobuild,andthat’sjustonepartofthelaser.Youcangetbywithasmallaccelerator,however,althoughit’snotyetclearhowsmall.

Whathasarousedinterestisthepredictionthattheoutputoffreeelectronlasersshouldbetunableacrossawiderangeofwavelengths:perhapsfromthemicrowaveregionallthewaytoX-rays,althoughnosingledevicewouldoperateacrossthatentirerange.[1]

Themilitaryisinterestedinthislaserforanobviousreason—itshighpower.Originally,itwasthoughtthatabeamofelectronswoulditselfmakeagoodweapon.Butitwouldbedifficulttofocusabeamofnegativelychargedelectrons,whichwouldmutuallyrepeloneanother,overlongdistances.Transformingthishigh-energy,butcumbersome;beamintoalaserbeamwouldsolvealotofproblems.Alaserbeamiseasiertofocusonadistanttarget.Aseriesofexperimentstotestfree-electronlasers,sponsoredbytheDefenseAdvancedResearchProjectsAgency,wasgettingunderwayasthisbookwasbeingwritten.

EnterTheX-rayLaser(Maybe)

There’snobetterwaytolearnhowfast-movingafieldisthantowriteabookaboutit.Wewereinthefinalstagesoffinishingthischapterwhenanewtypeoflaserapparentlyemergedonthescene:theX-raylaser.

Weusethewordapparently,becauseitisn’tyetclearexactlywhathappened.OperationofanX-raylaserat1.4nanometers(1.4×10-6mm)wasreportedinAviationWeekandSpaceTechnology,andit’sclearthatanexperimentwasperformedbyagroupattheLawrenceLivermoreNationalLaboratoryinCalifornia.However,whathappenedisnotyetcertain.Livermore’sofficialreactionis“nocomment,”andthere’smuchskepticismamongotherresearchersaboutsomeofthedetailsin“AviationLeak,”asthemagazineiscalledintheaerospacecommunity.

LaserresearcherslendtobeverycautiousaboutreportsofX-raylasers,becausethisisn’tthefirstsuchreport.Backin1972,aphysicistattheUniversityofUtahmadeabigsplashbyannouncingthathehadbuiltanX-raylaser.Hehadn’t,however.TheeffectshethoughtwerecausedbyX-rayswerereallycausedbysomethingelse.Hiseffortstopublicizethe“discovery”(whichwasreportedinNewsweekandelsewhere)beforethoroughlycheckingitoutlefthimdisgracedinthephysicscommunity.

There’sanotherreasonforskepticismtoo.Evenintheory,anX-raylaserisveryhardtobuild.Forexample,justtoproduceapopulationinversionona0.1-nanometertransitiontakestwowattsperatom.Atomsexcitedonsuchatransitiondropdowntothegroundstateinabout10-15sec.TheveryprocessofpumpinganX-raylaserwouldvaporizeit.Anditwouldn’tevenbealaseroscillator,becausenomirrorsexistforX-rays,makingitimpossibletobuildaresonator.

Whybothertrying?Becauseit’sachallengetophysicists.Ofcourse,therearesomepotentialapplicationsforX-raylasers,someofthemmilitary,afactthatphysicistsregularlypointouttothepeoplewhofundtheirresearch.Yetthepeopleworkingontheproblemgenerallyseemmoreinterestedintheintellectualchallengeoffindingawayofdoingthisverydifficulttask.“Difficult”maybetoomildaword.ItreportedlytookasmallnuclearexplosiontopumpLivermore’sX-raylaser.MuchoftheenergyfromsuchanexplosionisintheformofX-rays,andtheseX-raysexcitedatomstoproduceanX-raypopulationinversion.Thewholeprocesshappenedveryfast,probablyinaboutonepica-second(10-12sec,oronetrillionthofasecond)orpossiblyevenless,accordingtoX-raylaserresearchersoutsideLivermore.Aviationweeksaidthatduringthatultra-shortpulse,thelaserproducedaveryhighlevelofenergy—hundredsoftrillionsofwatts.Yetbecausethepulsewassoshort,thetotalenergyinthepulsewasprobablyonlyafewhundredjoules(ajouleistheamountofenergyprovidedbyapowerofonewattdeliveredforonesecond)—enoughtokeepanordinarylightbulbburningforonlyfewseconds.

WhetherornottheX-raylaserwilleverbe“practical”dependsonhowyoudefinethatword.Theneedtoenergizeitwithanuclearbomb(albeitasmallone)obviouslypresentssomeseriousproblems.Developersofmilitarysystemshavesomeideas,andwhiletheirideassoundlikesciencefictionnow,it’stooearlytobecertain.Whatnewmaterialswillbefoundtolaser?WeputthisquestiontoArthurSchawlowrecently.Hejustshookhisheadandsaidhecouldn’tanswerthequestion,thatscientistshadprovedthatsomanymaterialscouldbeusedinlasersthatitwasimpossibletopredictwhatwouldbenext.Hewouldonlysaythatlaserscientistsshouldlookfornewmaterialsandnewtransitionsthatwouldproducebetterlasersinthevisibleregionofthespectrum.Existingvisible-beamlasersaretooinefficient,hesaid.That’sasentimentyoucanhearechoedinthehallsofthePentagonandtheDepartmentofEnergy’sheadquarters,foraltogetherdifferentreasons.

IfSchawlowandmanyothersgettheirway,thenextbreakthroughsinlasertechnologywillbeoneswecanseewiththenakedeye.NOTES

[1]?Whathasarousedinterestisthepredictionthattheoutputoffreeelectronlasersshouldbetunableacrossawiderangeofwavelengths:perhapsfromthemicrowaveregionallthewaytoX-rays,althoughnosingledevicewouldoperateacrossthatentirerange.

這種激光器引起人們關(guān)注的原因是自由電子激光器的輸出波長范圍很寬，可能從微波段到X射線段，盡管還沒有一個單獨的裝置能涵蓋整個波長范圍。

·whathasarousedinterest為主語從句。

·theoutputof…wavelengths為定語從句修飾prediction。EXERCISES

1.?Fillintheblanks.

(1)?Indeedliquidscanbetheactivemediainlasers,andthere’salargeandimportantfamilyofliquidlasersbasedon

.

(2)?Mostdyesaretunableacrossatbest

percentofthevisibleregion,butit’spossibletoarrangeseveraldyesinsequencesoastobeabletotunetheoutputwavelengthacrosstheentirevisiblespectrum.

(3)?Inthisdevice,thecircumferenceiscoatedwitha

,producingadisk-shapedlasercavity.

(4)?Thefree-electronlaserrequiresalargeelectronacceleratororstorageringtoprovidethe

.Thenitneedsasetoflarge,powerful

tokeepchangingthedirectionofthebeam.

(5)?WhetherornottheX-raylaserwilleverbe“practical”dependsonhowyoudefinethatword.Theneedtoenergizeitwith

obviouslypresentssomeseriousproblems.

2.?True/False.

(1)?Inalmostallotherlasers,thelasertransitionisbetweentwostatesatfixedenergylevels,whichmeansthatthelaseremissionisinafixed,verynarrow,well-definedbandofwavelengths.()

(2)?It’simpossibletodeviseanopticalsystemthatwillselectasinglewavelengthinthelaser’srange.()

(3)?Thetermfreeelectroncomesfromthefactthattheactivemediumemittingthelightisabeamofelectrons,freefromatoms,whichispassingthroughamagneticfield.()

(4)?AnX-raylaserisveryeasytobuild.()

(5)?MuchoftheenergyfromsuchanexplosionisintheformofX-rays,andtheseX-raysexcitedatomstoproduceanX-raypopulationinversion,andhewholeprocesshappenedveryslowly.()PassageCGasLasers

ThreeoftheearliestAmericanlaserpioneers—CharlesTownes,ArthurSchawlow,andGordon—allconcentratedtheireffortsoriginallyonbuildinggaslasers.Maimanbeatthemwiththeruby,buttheinstinctsofthethreeturnedouttobegood.Todayover5,000lasertransitionsingasesareknown:about1,300inatomsandtherestinmolecules.(Someofthesetransitionsareinionizedgasescalledplasmas,whichsomephysicistsconsidertobeaseparatestateofmatter,butthat’safinedistinction,whichisn’timportanthere.)

Themostfamiliarlaserofanykindisavarietyofgaslaser—theubiquitoushelium-neonlaser,foundinhigh-schoolphysicslaboratories;inautomatedcheckoutsystemsinsupermarkets,whereit’susedtoscanthosefunnypricecodesonproducts;andonconstructionsites,whereit’susedtoalignwallsandbuildings.Althoughdollarsalesofcrystallinelasersareonlyslightlylowerthanthoseofgaslasers($105millionversus$125millionin1980,accordingtoestimatesbyLaserFocusmagazine),manymoretypesofgaslasersareincommonuse.Andbecausegaslasersaregenerallylessexpensive,therearemanymoreinusethancrystallinelasers.

Gaslasersmakeupsuchalargecategorythattheyarefurtherdividedintofamiliesinseveralways.Wewilltakethesomewhatarbitrary,butsimple,approachofdividingthemaccordingtothewaysinwhichtheiratomsormoleculesareexcited.Inotherwords,bythewaysinwhichtheyarepumped.

Themostcommonmethodofpumping,orenergizing,agaslaseristopassanelectricalcurrentthroughthegas(generallyamixtureoftwoormoregases)inthetube.Intheorytheideaissimple.Youputtwoelectrodesonoppositesidesofthegasandapplyalargeenoughvoltagetogetcurrenttoflowbetweenthem.Thisgenerallytakesacoupleofthousandvolts,whichiswhyit’simportanttobecarefularoundlaserpowersupplies.Theresultiswhat’scalledanelectricaldischargeinthegas:electronsflowthroughthegasbetweenthetwoelectrodes.Intheprocessofflowingthroughthegas,theytransfersomeoftheirenergytotheatomsinthegas.Afteralittleinter-atomicjugglingofenergy,apopulationinversionisproduced.

Thecommonestexampleofanelectric-dischargelaseristhefamiliarhelium-neonlaser.Itcanemitlightcontinuouslyformanythousandsofhours.Itdoesn’tproducemuchpower,typicallyonlyafewthousandthsofawatt,butitcandirectallofitslightintoabeamonlyabout1?mm(0.04in)indiameter.Inthehelium-neonlaserthedischargeexcitestheheliumatoms,whichtransfertheirenergytotheneonatoms,whichthenemitredlight.Similarthingshappeninaredneonlamporafluorescentlamp.Butofcoursethesedon’tproducelaserbeams.

Thereareseveralsimilartypesoflasersinwhichheliumgasisexcitedandthentransferstheexcitationenergytoanotheratom,oftenametalvapor,whichthenemitsthelaserbeam.Oneexampleisthehelium-cadmiumlaser,whichemitsabluebeam.Cadmiumisasoft,bluishwhitemetal,whichisvaporizedintoagasbybeingheatedtoseveralhundreddegreescentigrade.Someoftheselaserscanemitattwoormoredifferentwavelengths,dependingontheinternaloptics.Thefirsthelium-neonlasers,forinstance,actuallyproducedinvisiblebeamsintheinfraredregion.Onlylaterwerethered633-nanometerlasersnowincommonusedeveloped.

Othermembersofthenormallychemicallyun-reactivefamilyof“inert”or“rare”gasesbesidesheliumcanalsobeexcitedbyelectricity.Argonandkryptonarethebestexamples,bothofwhichcanemitawholefamilyofwavelengths,mostofwhicharevisible.Thetwogasescanalsobemixedtogetlaserlightatmostofthevisiblewavelengthsemittedbyeithergas.Argonlasers,whicharemorepowerful,aregenerallyusedinindustryandresearch.Kryptonlasersandlaserscontainingbothargonandkryptonhaveamoreinterestingrangeofcolorsandareusedmostlyinlightshows.

Thegaslaserswe’vedescribedsofarallproducebeamscontinuouslybutarelimitedinpower.It’simpracticaltobuildhelium-neonlasersmorepowerfulthan0.05wattorargonlasersmorepowerfulthan100watts.

Anothertypeoflaserpoweredbyanelectricaldischargeismuchmorepowerful.Thisistheglamorouscarbondioxidelaser.Itemitslightinabandofwavelengthscenterednear10micrometers,or0.01mm.Thewavelengthsofthisinvisibleinfraredlightisabouttwentytimeslongerthanthevisiblewavelengthsproducedbyhelium-neon,argon,andkryptonlasers.Carbondioxidelaserscancontinuouslyproducepowersfromlessthanawatttohundredsofthousandsofwatts.Thelatterwouldbemilitarylaserthatpresumablyoperateonlyafewsecondsatatimeandareclassified.Carbondioxidelaserscanalsobedesignedtoproduceextremelyshortpulsesofevenhigherpowers.

Thecarbondioxidelasershasbecometheworkhorseofthehigh-powerindustriallasers.Onereasonisitshighefficiency,uptoabout30percent,comparedtoabout1percentfortheneodymiumcrystallinelaser.Thishighefficiency,combinedwiththerelativeeaseofremovingexcessheatfromagas,meansthatthecoolingproblemsthatlimithowmuchacrystallineorglasslasercanbeusedareavoided.

There’sasubtle,butsignificant,differencebetweentheinternalworkingsofthecarbondioxidelaserandthoseoftheotherlaserswe’vetalkedaboutsofar.Theenergyusedtoexcitecarbondioxidedosenotraiseanelectrontoahigherorbit;instead,itcausesthea