版权说明:本文档由用户提供并上传,收益归属内容提供方,若内容存在侵权,请进行举报或认领
文档简介
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.
这种激光器引起人们关注的原因是自由电子激光器的输出波长范围很宽,可能从微波段到X射线段,尽管还没有一个单独的装置能涵盖整个波长范围。
·whathasarousedinterest为主语从句。
·theoutputof…wavelengths为定语从句修饰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
温馨提示
- 1. 本站所有资源如无特殊说明,都需要本地电脑安装OFFICE2007和PDF阅读器。图纸软件为CAD,CAXA,PROE,UG,SolidWorks等.压缩文件请下载最新的WinRAR软件解压。
- 2. 本站的文档不包含任何第三方提供的附件图纸等,如果需要附件,请联系上传者。文件的所有权益归上传用户所有。
- 3. 本站RAR压缩包中若带图纸,网页内容里面会有图纸预览,若没有图纸预览就没有图纸。
- 4. 未经权益所有人同意不得将文件中的内容挪作商业或盈利用途。
- 5. 人人文库网仅提供信息存储空间,仅对用户上传内容的表现方式做保护处理,对用户上传分享的文档内容本身不做任何修改或编辑,并不能对任何下载内容负责。
- 6. 下载文件中如有侵权或不适当内容,请与我们联系,我们立即纠正。
- 7. 本站不保证下载资源的准确性、安全性和完整性, 同时也不承担用户因使用这些下载资源对自己和他人造成任何形式的伤害或损失。
最新文档
- 二年级上册数学教案-观察物体 人教新课标
- 八年级物理5-7章教案
- 2023-2024学年一年级上学期数学1.3小猫钓鱼(学案)
- 一年级下册数学教案-7.1找规律 人教版
- 《梯形的认识》(教案)-四年级上册数学人教版
- 2024年CT技术服务购买合同
- 法律咨询服务推广方案
- 2024年合同延续协议样本
- 学校装修供货合同的法律要点
- 船舶行业阀门安装方案
- 船员劳务市场分析
- 2017版高中物理新课标解读
- 麻醉及应激反应调控
- 通过游戏培养幼儿的社交能力
- 建筑常用玻璃入门知识
- 深度神经网络全面概述
- 0-36个月儿童中医药健康管理服务规范
- 医院人事工作制度-劳务派遣人员管理办法全套
- 《学习的本质》读书会活动
- 食物过敏诊疗规范2023版
- 道路危险货物运输企业安全生产标准化评价实施细则
评论
0/150
提交评论