激光扩束镜原理与应用技术介绍

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ThemostcommontypeofbeamexpanderisderivedfromtheGalileantelescopewhichusuallyhasonenegativeinputlensandonepositiveoutputlens,asshowninFigure1.Theinputlenspresentsavirtualbeamfocusattheoutput.Forlowexpansionratios(1.3-20′),theGalileantelescopeismostoftenemployedduetoitssimplicity,smallpackagesize,andlowcost.Figure1iagramofabeamexpander

AsshowninFigure1,thelensM3focusesthelaserbeamontothefrontfocusplaneandthenewbeamwaistw￠0anddivergenceangleq￠canberepresentedas

(1)
and

(2)

(3)
wherew(l)istheradiusofthebeamenteringthelensM3,listhedistancebetweenthelensM3andthebeamwaistw0fromthelasergenerator,andf3isthefocallengthofthelensM3.
Sincew0￠liesonthebackfocusplaneofthelensM4withalongerfocallength,f4,theGaussianbeamwithabeamwaistw￠0willbecollimatedbythebeamexpander.ThecollimationratioofthebeamexpanderforaGaussianbeamisasfollows

(4)
whereT1=f4/f3.Thebeamwaistw20anddivergenceangleq2afterthebeamexpanderare

(5)
and

(6)
SubstitutingEquation(1)intoEquation(5),thefollowingexpressioncanbeobtained

(7)
FromEquations(4)-(7),itisconcludedthatthebeamexpansionratioandthecollimationratioforaGaussianbeamdependnotonlyonthespecificationsofthebeamexpander,butalsoonthelaserbeamparametersaswellasthepositionsoftheopticallenses.
Thefunctionofabeamexpanderistoreducethedivergenceangleoflaserbeamsandthusmakethefocusedbeamdiametersmaller.