J Hawthorn K P Weber and R E Scholten a School of Physics The University of Melbourne Parkville Victoria 3052 Australia Received 12 June 2001 accepted for publication 14 September 2001 We have developed an enhanced Littrow con731guration extended ca ID: 30179
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beamtravelsbetweenthegratingandthemirror.Foratuningrangeof1GHz,typicalfortuningthroughanatomicreso-nanceorduringlasercoolingstudies,and15mm,thedisplacementisonly80nm.Thisisinsigni®cantformostSmallshiftscanbecorrectedbytranslatingthegratingwithadcvoltageontheexistingpiezodisk,typicallyabout200Vfor80nm.Largerdisplacementscanbecorrectedbymanuallytranslatingthemirrororgrating,orusingexternalopticalelements.Forexample,theoutputbeamcouldbepassedthroughatiltedoptical¯at,rotatedappropriatelytocorrectthelateralbeamshift.Alternatively,theoutputbeamcanbere¯ectedfromanothermirror®xedtoalineartranslator.Aparticularlyelegantsolutiontotheproblemusesadihedralcon®gurationofgratingandmirror:byrotat-ingbothgratingandmirroraboutapivotpointattheinter-sectionoftheirsurfaceplanes,theoutputbeamis®xedinbothdirectionanddisplacement.Unfortunately,thiscannotbeachievedusingcommerciallyavailablemirrormountsofthetypeusedhere.Figure1alsoshowsadditionalmodi®cationstothepre-viousdesign,inparticular,theadditionofastackedpiezo-electrictransducerTokinAE0203D04.Thepiezostackdrivesthegrating-mirrorpivotarm,andhencethegratingangle,allowingelectronicwavelengthadjustmentof20GHzoverthe100Vrangeofthestack.Topreventfractureofthestackfromthehighpressureattheballendoftheadjustmentscrew,itisprotectedwithasmallbrasscapasshowninFig.2.Thepiezodiskisusedforfrequencyfeedbacklockingtoanatomictransition,butthebroadfrequencyoff-setadjust-mentprovidedbythestackreducesthevoltagerangere-quiredonthedisk,belowthe1000VusedintheoriginalIndeed,thelaserslockwellevenwhenthediskisoperatedwithinthestandardrangeoflow-voltageanalog0±15V.Thelaserfrequencynoiseisstronglyaffectedbyelectricalnoiseonthestacksupplyduetothehigh-voltagesensitivityofthestack0.2MHzpermV.Withlow-passpassive®lteringofthestackvoltage,twoidenticallaserswerelockedtoZeeman-ditheredatomicresonancesinandtherfbeatbetweenthemobservedwithafastphotodiode.Wemeasuredacombinedwidthof525kHz,averagedover100s,correspondingtoindividuallaserline-widthsof370kHz.Inadditiontothethermistorusedfortemperaturecon-trol,anLM35semiconductorsensoronthecollimationtubeprovidesindependentreadout,isolatedfromthetemperaturecontrollercircuit,withasimple10mV/ÉCcalibration.Severalfurtherimprovementsnotshownincludeala-serdiodeprotectionboardwithpassive®lteringtoremovecurrenttransientsandarelaythattripstoshortthelaserwhennotinuseorwhenthepowercableisdisconnected.Ourlasersaremountedtoaheavy5kgmetalbasetopro-videinertialandthermaldamping.ThebaseisisolatedfromtheopticalbenchbyathicklayerofinelasticpolymerSor-andenclosedwithanaluminumcover,whichisalsoisolatedfromthelaserbystripsofSorbothane.Thisshieldsthelaserfromaircurrents,improvestemperaturesta-bility,andfurtherdampensacousticvibrations.Theoutputbeamdirectionalstabilitywasdemonstratedbymonitoringthewavelengthinrealtimeasthegratinganglewasadjusted,usingaMichelsonwavemeter1.5mfromthelaser.Despitethehighsensitivityofthewavemetertoangularmisalignment,wewereabletotrackthechangeinwavelengthoverthemaximumlasertuningrangeof10.5nmwithouttheneedforanyrealignment.Thiswavelengthchangewouldnormallyshifttheoutputdirectionofthelaserbeamby1.2É,andcauseadisplacementatthewavemeterof3cm.Withtheadditionalmirror,theanglewasunchanged,andthelateralshiftwasnegligible0.4mmInconclusion,thesimplemodi®cationsdescribedherecaneasilybemadetomanyexistingLittrowcon®gurationexternalcavitydiodelasers,greatlyenhancingtheireaseofuseinmanylasercooling,spectroscopy,andotheratomicphysicsexperiments.TheauthorswouldliketothankP.J.Fox,M.R.Walk-iewicz,andL.D.Turnerfortheirassistancewiththedesignandconstructionofassociatedelectronicsandvibrationiso-lation.TheauthorsacknowledgethesupportoftheAustra-lianResearchCouncil,andtheAustralianPostgraduateRe-searchAwardsschemefortwooftheauthorsC.J.H.andK.P.W.M.W.FlemingandA.Mooradian,IEEEJ.QuantumElectron.,44R.WyattandW.J.Devlin,Electron.Lett.,110 FIG.1.Littrowcon®guredECDLwith®xedoutputbeamdirection,basedonthedesignofArnold,Wilson,andBoshierRef.6 FIG.2.Detailsofbrasscapforthepiezostackandaluminummountfortheadditionalmirror.Notethedifferentscales;dimensionsareshowninmm.4478Rev.Sci.Instrum.,Vol.72,No.12,December2001Hawthorn,Weber,andScholten 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Littrowcon®gurationtunableexternalcavitydiodelaserwith®xeddirectionoutputbeamC.J.Hawthorn,K.P.Weber,andR.E.ScholtenSchoolofPhysics,TheUniversityofMelbourne,ParkvilleVictoria3052,AustraliaReceived12June2001;acceptedforpublication14September2001WehavedevelopedanenhancedLittrowcon®gurationextendedcavitydiodelaserthatcanbetunedwithoutchangingthedirectionoftheoutputbeam.TheoutputofaconventionalLittrowECDLisre¯ectedfromaplanemirror®xedparalleltothetuningdiffractiongrating.Usingafree-spaceMichelsonwavemetertomeasurethelaserwavelength,wecantunethelaseroverarangegreaterthan10nmwithoutanyalterationofalignment. Electronicmail:r.scholten@physics.unimelb.edu.auREVIEWOFSCIENTIFICINSTRUMENTSVOLUME72,NUMBER12DECEMBER20010034-6748/2001/72(12)/4477/3/$18.002001AmericanInstituteofPhysics