Thompson and Vladan Vuletic We investigate experimentally the in64258uence of the grating re64258ectivity grating resolution and diode facet antire64258ection AR coating on the intrinsic linewidth of an externalcavity diode laser built with a diffra ID: 25941
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metricbecauseofthelinearincreaseinlaseroutputpowerwiththewavelength.Toremovethislineardependence,weaverageovermeasurementsonbothslopesoftheatomicline.Thenoiseasmeasuredbytheavalanchephoto-diodeinFig.2containsbothfrequencyandamplitudenoises.WemeasuretheamplitudenoisebyremovingtheatomiccellandinsertinggrayÞlterstoattenuatethebeampowertoitspreviousvalueonthephoto-diode.WeÞndthattheamplitudenoisescalesap-proximatelyasthesquarerootofthepowerincidentonthephotodiode,anindicationthattheamplitudenoiseatthefrequenciesofinterestisdominatedbyphotonshotnoise,inagreementwithacalculationusingtheknownphotodiodegain.Figure3showsboththetotalnoiseandtheamplitudenoisecontri-butionsforatypicalmeasurement.Thefrequencynoiseisobtainedbysubtractingtheuncorrelatedam-plitudenoisefromthetotalnoiseinquadrature.Thenoisebelow5kHzgenerallyreßectsmechanicalvi-brationsofthelasermount,whereasthenoiseabove5kHzbecomesapproximatelyindependentoffre-quency,indicatingaLorentzianlineshape.ApplyingEq.(1)tothewhite-noiseportion,wemea-suretypicallinewidthsofthe780and852nmlaserstobeinthe250Ð600kHzrange(seeTables1and2),whichisexpectedfordiodesoflength1mmandanindexofrefractionof3.5,setinanexternalcavityoflength3.3cm.Wealsodetermineasimilarlinewidthof250kHzfora399nmNichiaNDHV310APClaserdiode(non-ARcoated)assembledwitha2400mmgratingofForcomparison,themethodofconvertingfre-quencyintoamplitudenoiseviatheatomicabsorp-tionlinehasallowedustomeasurealinewidthasnarrowas30kHzforadistributedBraggreßectorlaserdiode,whoseintrinsiclinewidthwasnarrowedbyopticalfeedbackfromalowÞnesseopticalcavityusingasetupsimilartothatdescribedbyDahmanietal3.EffectsofGratingReßectivity,GratingResolution,andAntireßectionCoatingontheLaserLinewidthTable1showsthelinewidthsmeasuredforvariousgratings(EdmundOptics43222,43753,and43773)assembledwiththesameAR-coatedlaserdiodechip(SacherLasertechnikSAL-850-50,backfacetreßec-0.85,frontfacetreßectivitym,estimatedintrinsiclinewidth35MHz)andcollimator(ThorlabsC390TM-B, Fig.3.(Coloronline)Total(opencircles)andamplitude(Þlledtriangles)spectralnoisedensitiesofan852nmAR-coatedlaserassembledwitha1200mmgratingofreßectivity0.21.Forthefrequencynoise,1V Hzcorrespondsto156MHz Hz.Fortheamplitudenoise,1V Hzcorrespondstoafractionalnoiseof Hz. Fig.4.(Coloronline)CalculatedproÞlesof1200and1800mmgratings,overlappedwithdiode-chipmodesandexternal-cavitymodesspacedbyfrequencyintervalsdeterminedfromFig.1.Thepassiveexternal-cavitymodespectrumiscalculatedforagratingofreßectivity0.61anddiodebackfacetreßectivity0.85.Thediodechipmodespectrumiscalculatedforandiodewitharegenerativegainparameter-izedbythematerialabsorptioncoefÞcient45cm(Ref.29).Table1.Linewidthsof852nmLasersforDifferentGratingsintheLittrowConÞguration 120042000.210.67260,120042000.610.19260,180084000.160.78260,LinewidthsofAR-coated852nmlasersbuiltwithgratings43773,43753,43222fromEdmundOpticsforlines1Ð3.ForaLittrowgratinglaser,theÞrstdiffractionorderwithpowerreßec-isreßectedbackintothelaserforopticalfeedback,whilethezerothorderwithpowerreßectivityisusedasthelaseroutput.Thegratingresolutioniscomputedforabeamdiam-eterof3mmfromthegroovedensityandtheLittrowangle.Thefree-runninglaserlinewidthisestimatedtobe35MHz(Ref.listsdifferenttheoreticalpredictionsforthegivenlaserparameters,andisthelinewidthmeasuredusingEq.(1).,wherearecalculatedfromEq.(26)ofKazarinovandHenry(Ref.12).CalculatedfromEq.(8)ofthepaperofSunetal.(Ref.10).20December2006Vol.45,No.36APPLIEDOPTICS9193 effectivefocallength2.75mm).Differenttheoret-icalpredictionsforthereducedlinewidthinthestrongfeedbackregimearealsogiveninTable1.Asexpectedfromtheory,themeasuredline-widthnarrowssomewhatwithhighergratingre-foraÞxedgratingresolution,i.e.,alinewidthdecreaseby20isobservedasthere-ßectivityistripledfrom0.2to0.6.Sur-prisingly,amuchlargereffectisobservedwhenthegratingresolutionischanged:thelinewidthde-creasesby40astheresolutionisdoubledevenwhenthegratingreßectivityislowered.Theeffectofthegratingresolutionshowsthatthesimplemirror4Ð10isnotfullyadequate,andstronglysug-geststhatfordescribingthelinewidthofexternal-cavitylasers,thecompetitionbetweendiodechipmodesmustbetakenintoaccount.Table1alsoshowsthatthepreferredmethodofdecreasingthelinewidthistoincreasethegratingresolutionratherthanthegratingÕsÞrst-orderreßectivity,sincetheformerdoesnotdecreasetheavailablelaseroutputFigure4givesaphysicalpictureaccountingforbotheffectsofgratingreßectivityandresolution.Theexternalcavity,formedbetweenthebackfacetofthelaserdiodechipandthereßectivegratingsurface,hasaÞnessedeterminedbytheÞrst-orderreßectivityofthegrating.Thegratingresolution,ontheotherhand,setsthewidthofthegratingproÞleinFig.4.Highergratingresolution,achievedwithagratingofhighergroovedensityoracollimatorthatproduceslargerbeamsize,suppressesneighboringdiodechipmodesbetter,leadingtolessmodecompetitionandhencetolessfrequencynoise.Infact,thegreaterinßuenceofthegratingresolutiononthelinewidthindicatesthatthesuppressionofotherlaserdiodechipmodesismoreimportantthantheÞnesseoftheexternalcavity.TheresultsinTable1implythatanarrowerline-widthcanbeobtainedwithoutsacriÞcinglaserout-putpowerbysimplyusingagratingofhighergroovedensity.Inaddition,wehaveattemptedtoincreasethegratingresolutionbyusingacollimatorthatpro-ducedalargerbeamsize(ThorlabsC240TM-B,effec-tivefocallength8.0mm).Wefoundthatevenwhentheopticalfeedbackwasonlynearoptimized,wecouldalreadyachievealinewidthof425kHzusingthe1800mmgrating.Ontheotherhand,thecom-binationofthelargerbeamsizeandthe1800mmgratingalsomeansthattheopticalfeedbackismuchmoresensitivetothegratingangle.Atsuchhighsensitivity,slightthermaldriftsofthealuminumla-sermountmadeitextremelydifÞcultforthegratingtoremainatitsoptimalangle.Asaresult,wewereunabletomaintainreliableoperationinthemechan-icalsetupofFig.2forthiscollimator-gratingcombi-WealsostudytheeffectofthediodeARcoatingonthelinewidth.Table2showsbothmeasuredandcal-culatedlinewidthsofAR-coatedlaserdiodesassembledwiththesamegrating(1200mmHenryÕsmodelpredictsthatthebroaderdiodechiplinewidthofanAR-coatedlasercompensatesforitslargerlinewidthreductionfactorwhensetinanexternalcavity,yieldingalinewidthonthesameor-derasanon-AR-coateddiodelaserusedwiththesamegrating.The130kHzdifferencebetweenthecalculatedlinewidthsinTable2isnotnecessarilysigniÞcant,becausethetheoryemployeddoesnotac-countforotherdiodechipmodes,whichwouldhavebeenbettersuppressedintheAR-coatedlaser.Table2showsaslightlynarrowerlinewidthfortheAR-coatedlaser,althoughthedifferenceislessthantheestimatederrorof80kHzassociatedwitheachmeasurement.WeconcludethatalthoughtheARcoatingeasestheprocedureforoptimizinggratingalignmentaswellasenhanceswavelengthtunability(Fig.1),itseffectonthelinewidthisinsigniÞcant. Fig.5.Plotoflinewidthversuscurrentinjectedintothediodelaser.Thelaserjumpstoadifferentlongitudinalcavitymodenear52mA.Table2.LinewidthsofBothNon-AR-andAR-Coated780nmLasers DiodeModelSanyoDL7140-201S8400.850.154900,SAL-780-408900.858Linewidthsof780nmlaserswithoutandwithanARcoatingonthediodefrontfacet,assembledwiththesamegrating(EdmundOpticsisthediodechiplength,whilearethebackandfrontfacetpowerreßectivities,respectively.areasdeÞnedinTable1.Reference28.,wherearecalculatedfromEq.(26)ofKazarinovandHenryÕspaper(Ref.12).CalculatedfromEq.(8)ofthepaperofSunetal.(Ref.10).ThecalculationsofSunetal.donotapplyfor1(Ref.10).9194APPLIEDOPTICSVol.45,No.3620December2006 ThisresultalsoagreeswiththeÞndingsofBinderetal.,i.e.,laserswithdifferentfrontfacetreßectivi-tiesexhibitsimilarlinewidthsaslongastheyareinsingle-modeoperation.Thelinewidtherrorof80kHzisestimatedfromthefactthattheamountofcurrentinjectedintothediodelaserinßuencestheextenttowhichthelaseroperatesinasinglemode,whichinturnaffectsthelinewidth(Fig.5).Whenthelaserisabouttojumptoadifferentmode,itbecomesslightlymulti-mode,andtheline-widthincreasesbyanorderofmagnitudeduetomodecompetition.AlthoughwehaveveriÞedthatthelaseroperatedinasinglemodeduringourfrequencynoisemeasurements,Fig.5showsthatthereisstillacurrent-dependentlinewidthvariationof80kHzor20%inthesingle-moderegime.Table3summarizesthesmallestlinewidthsmea-suredforthenear-IRdiodelasers(780,852nm).Forcomparison,thelinewidthofthenear-UVlaser399nmisincluded.4.MechanicalStabilityofLaserMountsFigure6showsanewlasermount(mountB)thatisdesignedtoreducemechanicaloscillationsathigheracousticfrequencies.Fortheregularmount(mountA,seeFig.2),thegratingandlaserdiodearesepa-ratelyattachedtoathirdaluminumpiecethatservesasthebaseofthemount.Theverticalandhorizontalgratinganglesareadjustedbyturningthescrewsofthemirrormountholdingthegrating.Conversely,formountB,thealuminumblockcontainingthegratingisattacheddirectlytotheblockcontainingthelaserdiodeviafournylonpullscrews.Threestainlesssteelscrews,whichpushontheblockcontainingthegrat-ingandapiezostacksandwichedbetweenthetwoblocks,respectively,allowtheverticalandhorizontalgratinganglestobeadjusted.Infact,theverticalangleonlyneedstobeadjustedslightly,becausetheopticalfeedbackisalreadynearoptimalwhenthegratingsitsßushinthemachinedpocket.MountBalsoincludesamirror,whichcouplesthebeamoutofthemountinaÞxeddirectionregardlessofthegrat-ingÕshorizontalangle.ThemechanicalpropertiesofmountBarecharac-terizedintermsofitsfrequencynoisepowerspectraldensityandrmsjitteratlowFourierfrequencies(upto5kHz)andareplottedinFigs.7and8,respec-tively.Forcomparison,Figs.7and8alsodisplaythedataforalaserassembledwithmountA.AtÞrstglance,mountAappearstobemorestablethanTable3.BestAchievedLinewidths Cs852Yes180084000.16320Rb780Yes120041000.27450Yb399No240082000.602508Narrowestlinewidthsachievedwith852and780nmlasersandtheircorrespondingdiodeandgratingparametersasdeÞnedinTable1. Fig.6.(Coloronline)SchematicofmountB.Thefournylonpullscrewsthatattachthetwoaluminumblockstoeachotherarenot Fig.7.(Coloronline)FrequencynoisepowerspectraldensitiesformountB(opencircles)andmountA(Þlledtriangles)lasers.MountBonlyhasastrongmechanicalresonanceat500Hz,whereasmountAhasmechanicalresonancesat2kHz. Fig.8.(Coloronline)RmsjittersoflasersassembledbothwithmountB(opencirclesforunlockedandsolidcurveforlocked)andmountA(Þlledtrianglesforunlockedanddashedcurveforlocked)versusintegrationbandwidth.Thermsjitterisgivenby20December2006Vol.45,No.36APPLIEDOPTICS9195 narrowspectrallinewidthforhigh-resolutionspectroscopy,ÓJpn.J.Appl.Phys.,Part15890Ð5895(1996).26.K.L.Corwin,Z.-T.Lu,C.F.Hand,R.J.Epstein,andC.E.Wieman,ÒFrequency-stabilizeddiodelaserwiththeZeemanshiftinanatomicvapor,ÓAppl.Opt.27.B.Dahmani,L.Hollberg,andR.Drullinger,ÒFrequencysta-bilizationofsemiconductorlasersbyresonantopticalfeed-back,ÓOpt.Lett.876Ð878(1987).28.C.H.Henry,ÒTheoryofthelinewidthofsemiconductorlasers,ÓIEEEJ.QuantumElectron.259Ð264(1982).29.M.W.FlemingandA.Mooradian,ÒFundamentallinebroad-eningofsingle-mode(GaAl)Asdiodelasers,ÓAppl.Phys.Lett.511Ð513(1981).30.C.J.Hawthorn,K.P.Weber,andR.E.Scholten,ÒLittrowÕconÞgurationtunableexternalcavitydiodelaserwithÞxeddirectionoutputbeam,ÓRev.Sci.Instrum.4477Ð447920December2006Vol.45,No.36APPLIEDOPTICS9197