激光原则 english.docx

激光原则 english.docx

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激光原则english

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Basicprinciplesofmedicallasers

Author

HenriGColt,MD

SectionEditor

HilarySanfey,MD

DeputyEditor

KathrynACollins,MD,PhD,FACS

Disclosures:

HenriGColt,MDNothingtodisclose.HilarySanfey,MDNothingtodisclose.KathrynACollins,MD,PhD,FACSNothingtodisclose.

Contributordisclosuresarereviewedforconflictsofinterestbytheeditorialgroup.Whenfound,theseareaddressedbyvettingthroughamulti-levelreviewprocess,andthroughrequirementsforreferencestobeprovidedtosupportthecontent.AppropriatelyreferencedcontentisrequiredofallauthorsandmustconformtoUpToDatestandardsofevidence.

Conflictofinterestpolicy

Alltopicsareupdatedasnewevidencebecomesavailableandourpeerreviewprocessiscomplete.

Literaturereviewcurrentthrough:

Jul2015.|Thistopiclastupdated:

Jan07,2014.

INTRODUCTION — Lasersaredevicesthatemitasingle,coherentwavelengthofelectromagneticradiationthatisusedtocut,coagulateorablatetissueforavarietyofclinicalapplications.Lasersystemsproduceavarietyofwavelengthsofvaryingpulsedurationandenergylevels.Computer-basedimagingandguidancesystemsallowprocedurestobeperformedprecisely,quicklyandwithgreatercontrol.Althoughlasersarecommonlyusedsuperficiallyforcutaneousandocularapplications,smaller,efficientlaserdeliverysystemsareavailableforminimally-invasiveapplicationsincludingendoscopy,bronchoscopy,laparoscopyandendovenousablation.

Thesafeandappropriateuseoflasersrequiresatrainedclinicianwithaworkingknowledgeoflaserdeliverysystemsandlaser-tissueinteractionstoachievethedesiredclinicaleffectwhileminimizingcomplications.

Thebasicprinciplesofmedicallaserswillbereviewedhere.Theuseandeffectivenessoflasersforspecificclinicalindicationsarediscussedinseparatetopicreviews.（See'Clinicalutilityoflasers'below.）

ELECTROMAGNETICSPECTRUM — Lightiselectromagneticradiationwithintherangeofwavelengththatisvisibletothehumaneye.Medicallasersproducephotonsofelectromagneticenergythatcanbewithin,aboveorbelowthisrange（figure1）.Therangesofwavelengthforeachregionoftheelectromagneticspectrumareasfollows:

▪Gammarays:

<0.1nm

▪X-Rays:

0.1to10nm

▪Ultraviolet:

10to400nm

▪Visible:

440to760nm

▪Near-infrared:

700to1400nm

▪Mid-infrared:

1400to20,000nm

▪Far-infrared:

20,000to100,000nm

▪Microwaves:

>100,000nm

LASERPRINCIPLES — Lasersaredevicesthatrelyuponthestimulatedemissionofradiationtoproduceabeamoflight.Thewordlaserisanacronymforlightamplificationbystimulatedemissionofradiation.Lasersarecomprisedofanenergysource,aresonantchamber,andanactivemedium（figure2）.

Intheunexcitedstate,electronsorbitthenucleusattheirlowestenergylevelorgroundstateoccupyingorbitsthatareclosertothenucleus.However,absorptionofenergycausestheelectronstobecomeexcitedmovingtoahigherorbit（figure3）.Astheelectronsreturnfromtheexcitedstatebacktogroundstate,theyspontaneouslyemitphotonsofenergy（electromagneticradiation）.Inotherwords,theystimulateemissionofradiation[1].

Theradiationthatisproducedhasuniquepropertiesincludingmonochromaticity,coherenceandcollimation.

▪Monochromaticmeansthatallthephotonsinalaserbeamareofthesamewavelength.Thus,thelaserbeamachievessufficientintensitytodestroytissuethedegreetowhichisbaseduponitswavelengthandthescattering,reflection,andabsorptioncoefficientsofthetargettissue.（See'Tissueablation'below.）

▪Coherence referstothesynchronizationofthelaserbeamintimeandspace.Thephotonsofthebeamarein-phaseorcoherent.Bycomparison,thephotonsinconventionallighttravelrandomly.

▪Collimation indicatesthattheelementsofthelaserbeamarenearlyparallel.Becausethereislittledivergence,laserbeamscanbefocusedtoasmallarea.Thispropertydiffersfromconventionallightwhichdivergessubstantially.Becauseofcollimation,theenergyemittedfromalasersourcecanbecapturedanddeliveredthroughflexibleopticalfibers.

Energysource — Thesourceofexternalenergy,knownasthepumpsource,usedtoexcitetheelectronscanbeelectrical,opticalorchemical.Inmedicalapplications,theenergysourceismostcommonlyelectrical（eg,electriccurrentflowingthroughalasermedium）oroptical（eg,fromanotherlaser）.Chemicalenergyisoftenusedinindustrialapplications.Thepumppowercanbemaintainedcontinuouslyorswitchedononlyforshortintervals,whichmayhelptolimitthedamagingthermaleffectsofthelaser.（See'Modesofoperation'below.）

Resonantchamber — Theresonantchamberorcavitycontainstheactivelasermediumandreflectivemirrors.Photonsreflectbackandforthbetweenmirrors.Sinceoneofthemirrorsishighlyreflectiveandtheotherisonlypartiallyreflective,someofthelaserlightthatisproducedispermittedtoexitthedeviceandisdirectedtothetissues.（See'Laserbeamproduction'below.）

Activemedium — Theactivemediumcontainstheatomsthatproducetheelectromagneticradiation.Thetypeofactivemediumusuallygivesthelaseritsnameand,formedicalapplications,includesgas,solidcrystallinematerials,semiconductormaterials,andliquiddyesolutions.

Gas — Gaslasersapplyanelectriccurrentthroughthegas.Gaslasersusenoblegases（eg,argon,helium）andothertypesofgases（eg,carbondioxide）.Thefirstgaslaserusedhelium-neon（HeNe）toproduceacoherentinfraredbeam.

▪Carbondioxide–Thecarbondioxide（CO2）laserusescarbondioxide,nitrogen,andheliumandwasdevelopedin1964[2].Itproducesamid-infraredwavelength（10,600nm）（figure1）.TheCO2laserisexcellentasacuttinginstrumentbecausescatteringisminimal,absorptioninwaterisexcellent,softtissuevaporizationisrapidandthesurroundingtissuedamageisnegligible.TheCO2laserpermitsthecoagulationofbloodvesselssmallerthan0.5mmindiameter.Thislasertypeisprimarilyusedinotorhinolaryngology.

▪Argonion–Argoniongasproducesblue-greenlightatawavelengthof488to514nm（figure1）.Theargonionlaserisusedprimarilyforcoagulationofbloodvesselsindermatology,ophthalmologyandliversurgery.Softtissueeffectsareunpredictable.Argonlasersarealsousedtopumpdyelasersforphototherapy.

▪Excimer–Theexcimerlaserusesanactivemediumcomposedofexciteddimers,whichareacombinationofanoblegas（eg,argon,krypton,orxenon）andareactivegas（eg,fluorineorchlorine）.Theexcimerlaseremitsultravioletradiationwithenoughenergytobreakchemicalbondsbetweenmoleculesbutwithnotolittlethermaldamage.Theexcimerlaserisusedtoremovesurfacematerialwithalmostnoheatingandtheselasersareoftenreferredtoas“cool”lasers.Agoodexampleislaser-assistedinsitukeratomileusis,otherwiseknownasLASIK.

Solidstate — Solid-statelasersuseanactivemediumthatisasolid.Semiconductor-basedlasersarealsointhesolidstate,butaregenerallyconsideredasaseparateclassfromsolid-statelasers.（See'Semiconductor'below.）

Generally,theactivemediumofasolid-statelaserconsistsofaglassorcrystallinematerial（eg,sapphire,ruby）thatisdopedwithneodymium,chromium,erbium,orotherions.Neodymiumdoped:

yttriumaluminumgarnet（Nd:

YAG）lasersareperhapsthemostcommonlyusedsolidstatelaser.Themediumisarodcomposedofneodymiumionsandcrystalsofyttrium-aluminum-garnet.Nd:

YAGlasersemitlightatmid-infraredwavelengths（1320nm,1064nm）withpulsedurationsinthemillisecondrange（figure1）.ThelongerwavelengthoftheNd:

YAGlaserpenetratesdeeperintothetissueandcancausecollateralthermaldamage.

Othersolid-statelasersemittingradiationwithmillisecondpulsedurationsincludethepotassiumtitanylphosphate（KTP）,andalexandritelasers.KTPlasersemitgreenlightat532nmcorrespondingtothesecondoxyhemoglobinpeakandoverlappingtheabsorptionpeakofmelanin.Alexandritelasersemitredlightatawavelengthof755nmwhichisabsorbedbydeoxygenatedhemoglobinandmelanin.

Semiconductor — Semiconductorlasers（ie,laserdiode）useanactivemediumthatisformedbydopingathinlayeronthesurfaceofacrystalwafertoformap-njunction,whichcomprisesadiode.Semiconductorlasersareinjectionlaserdiodesascomparedwiththeoptically-pumpedlaserdiodes（solid-statelasers）describedabove.Diodelasersemitlightatwavelengthsbetween800and900nm（figure1）.

Dye — Dyelasershaveliquidactivemedium.Afluorescentorganicdyeinliquidsolutionisinjectedintoatube.Pulsedyelasers（PDLs）emityellowlightat585and595nmwhichcorrespondstothesecondoxyhemoglobinpeak（figure4）.Theselasersproducepulsedurationsinthemillisecondrangeandareusedincutaneousvascularapplications.

Laserbeamproduction — Thecompositionanddesignofthelaserdeviceproducesacoherentbeamofelectromagneticradiationasaresultofthefollowingsequenceofevents[3,4]:

Atomswithintheresonantchamberabsorbenergyfromtheenergysource.Asatomsinhighenergystatesspontaneouslyreturntogroundstate,theyreleaseenergy,someofwhichisabsorbedbyotheratomscausingthemtoachieveahighenergystate.Thisphenomenonisknownas‘pumping’.Asmoreandmoreatomsbecomeexcitedandthenreturntogroundstate,moreandmoreenergyisproduced（amplificationofstimulatedemission）.Eventually,stimulatedemissionofradiationbecomestheprimarysourceofenergywithinthechamber,andeventuallyp