Glass-on-Glass Fabrication of Bottle-Shaped Tunable Micro-Lasers and Their Applications

4.6 MB·

by  Jonathan M. Ward

Glass-on-Glass Fabrication of Bottle-Shaped
Tunable Micro-Lasers and Their Applications
Jonathan M. Ward1,*, Yong Yang1,2, and S´ıle Nic Chormaic1
1Light-MatterInteractionsUnit,OkinawaInstituteofScienceandTechnologyGraduateUniversity,Onna-son,
Okinawa904-0495,Japan.
2NationalEngineeringLaboratoryforFiberOpticsSensingTechnology,WuhanUniversityofTechnology,Wuhan,
430070,China.
6 *[email protected]
1
0
2 ABSTRACT

n
a We describe a novel method for making microbottle-shaped lasers by using a CO laser to melt Er:Yb glass onto silica
2
J
microcapillariesorfibres. Thisisrealisedbythefactthatthetwoglasseshavedifferentmeltingpoints. TheCO laserpower
2
1 iscontrolledtoflowthedopedglassaroundthesilicacylinder. Inthecaseofacapillary,theresultinggeometryisahollow,
2
microbottle-shapedresonator. ThisisasimplemethodforfabricatinganumberofglassWGMlaserswithawiderangeofsizes

onasingle,micron-scalestructure. TheEr:Ybdopedglassouterlayerispumpedat980nmviaataperedopticalfibreand
]
s whisperinggallerymode(WGM)lasingisrecordedaround1535nm. Thisstructurefacilitatesanewwaytothermo-optically
c tunethemicrolasermodesbypassinggasthroughthecapillary. ThecoolingeffectofthegasflowshiftstheWGMstowards
i
t shorterwavelengths,thusthermaltuningofthelasingmodesover70GHzisachieved. Resultsarefittedusingthetheoryof
p
hotwireanemometry,allowingtheflowratetobecalibratedwithaflowsensitivityashighas100GHz/sccm. Straintuningof
o
themicrolasermodesbyupto50GHzisalsodemonstrated.
.
s
c
i
s
y Introduction
h
p Thefabricationofglassopticalwhisperinggallerymicrolaserscanbeachievedinalimitednumberofways. Onemethod
[ involvesmakingasinglecavitybydrawingoutaglasswirefromapieceofdopedglassandmeltingthetipofthewiretoform
asphericalresonator.1 Alternatively,manysphericalresonatorscanbemadesimultaneouslybypassingparticlesofdoped
1
glassthroughafurnace.2,3 Bothofthesemethodsaretedious; inthefirstcase, onlyoneresonatorcanbemadeatatime.
v
3 Inthesecondcase,individualspheresmustbeselectedandgluedtosomeotherstructure,e.g. thetipofafibreforeaseof
9 manipulation. Forbothmethods,onlyasingleresonatorisselectedandbroughttoanevanescentwaveguidecouplerforoptical
4 excitation. On-chipfabricationofalargenumberofactiveglassresonatorsispossiblebydopingthechipbeforefabrication
5 eitherwithionimplantation4orbySolgelcoating.5,6 Forpassivedevices,individualresonators-suchasamicrosphere-can
0
alsobeactivatedbycoatingusingtheSolgelprocess.7,8Theon-chipmethodisobviouslymorecomplicatedandrequiresmuch
.
1
moreequipmentthanthesimpleheatingmethods,butithastheadvantagethatthecouplingbetweentheexcitationwaveguide
0
andanycavitiesonthechipcanbeeasilyachievedbymovingthechiprelativetothewaveguide. Coatingataperedopticalfiber
6
1 tipwithalayeroferbiumdopedphosphateglassandthenmeltingthetipintoaspherehasalsobeendemonstratedtoproduce
: lasingmicrospheres.9,10
v
Here,wepresentasimpleheatingmethodforfabricatinganumberofglasswhisperinggallerymode(WGM)laserswitha
i
X rangeofsizesonasinglemicron-scalestructurethatcanbeeasilymanipulatedrelativetotheexcitationwaveguideandcan,in
r principle,bepackagedontoamillimetrescalechip. Weexperimentallydemonstratethecoatingoftaperedopticalfibresand
a
microcapillarieswithalayerofEr:Ybdopedphosphatelaserglass. Thisisachievedbythesimplefactthatthetwoglasseshave
verydifferentmeltingtemperatures,around1,500◦Cforsilicaand500◦Cforthephosphateglass. TheEr:Ybdopedglassouter
layerispumpedat980nmandlasingisobservedat1535nm. Microlaserswithdiametersrangingfrom22µmto232µmare
madeonthesamestructure.
Adesiredfeatureofanylaseristheabilitytotunethefrequencyofthelaseroutput.Tuningamicron-scalewhisperinggallery
resonatorinafashionthatdoesnotaddtothefootprintofthedeviceorrequirecomplicatedfabricationisanontrivialtask. The
mainapproachessofarincludetheuseofexternalheaters,11,12applicationofstress/strainviaanexternalclamp,13–15pressure
tuning,16–20 electricfieldtuning,,21 chemicaletching,22,23 on-chipresistanceheating,24,25 andthermo-optictuning.26–28 Each
ofthesemethodshasitsowndistinctadvantagesanddisadvantagesandthechoiceofmethoddependsultimatelyonthefinal
application. Weshowthatthemicrolasersfabricatedusingtheglass-on-glasstechniquecanbeeasilytunedovertensofGHzby
oneoftwomethods. Thefirstmethodisuniquetoourdevicesandisapplicabletothecapillarystructure;itreliesonthermal
tuningofthelasingmodebyflowinggasthroughthecavity. Fromthismethodwedemonstratetheideaofgasflowsensing
usingtheconceptofa“hotcavity”anemometer. Measurementsandcharacterisationofthesystemasagasflowsensorare
presented. Thesecondmethodreliesonstraintuningandcanbeappliedtoboththetaperedfibreandthecapillarylasers.
Methods
Thismethodofmicrolaserfabricationcanbeappliedtoanopticalfibreorcapillaryandisthesameinbothcases. First,we
taperedamicrocapillarywithanouterdiameter(OD)of350µmandaninnerdiameter(ID)of250µm,usingaCO laser,toa
2
uniformwaistwithanODof∼80µm. Next,aglasswirewasdrawnfromabulkpieceofdopedphosphateglass(Kigre).
Thediameterofthedopedglasswirewastypicallyafewtensofmicrons. Thedopedglasswirewasthenplacedontopof
andincontactwiththemicrocapillaryandtheCO laserbeamswereappliedagain. TheCO laserpowerwasincreaseduntil
2 2
thedopedglassflowedontothecapillary,asshowninFig. 1(a). AtthispointthedopedglasswirewasremovedandtheCO
2
laserpowerwascontrolledtoallowtheremainingdopedglasstoflowaroundthecapillary. Whenthedopedglasscompletely
coveredthecapillarytheCO laserwasturnedoff. Theresultinggeometrywasahollowmicrobottle-shapedresonator. The
2
diameterofthedopedglassbottlewas170µmandthethicknessofthedopedglassattheequatorwas40µm. Thisthicklayer
meansthatanyopticalmodeinthebottlecannotinteractwiththesilicaoranymaterialinsidethecapillary.
Figure1. (a)Fabricationstepsformakingadopedglassmicrobottlelaseronasilicawire. Thecapillarydiameteris80µm
andthefinaldiameterofthedopedglassresonatoris170µm. (b)ImageoftheresonatorshowingaWGMhighlightedby
greenupconversionfluorescence. (c)LasingspectrumfromanEr:Ybdopedbottleshapedresonatoronasilicacapillary. (d)
Lasingthresholdmeasurement. (e)Lasingspectraforthreedifferentmicrobottles(notthatshowninparts(a)to(d))onasingle
capillarywithadiameterof42µm. Thediametersofcav1,cav2andcav3are120µm,170µmand155µm,respectively
ThecapillarywasgluedontoaU-shapedholderandbroughtintocontactwithataperedopticalfibreforopticalpumping.
Thetaperedfibrehadadiameterof∼1µmandwasconnectedtoa980nmdiodelaser. Whisperinggallerymodeswerevisible
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duetogreenupconversionfluorescencefromtheerbiumions,seeFig. 1(b). Theoutputendofthetaperedfibrewasconnected
toanopticalspectrumanalyser(OSA).AtypicallasingspectrumisshowninFig. 1(c),withlasingpeaksappearingbetween
1532nmand1540nm. Foragivenmicrobottlethespectrumcanbesinglemodeormultimodedependingonmanyfactors
suchasthepositionofthetaperedfibrealongthecapillaryorthepositionofthecapillaryalongthetaper.7 Toverifythelasing
behaviour, athresholdmeasurementwastaken, seeFig. 1(d). Forthismeasurementthepumplaserwasnottunedtoany
particularWGM.The980nmpumplaserusedhadaratedlinewidthof2nmsoitwasassumedthatanumberofWGMswere
simultaneouslyexcitedwithnocontroloverthecouplingefficiency. Assuch,thepumppowerlabledinFig. 1(d)represents
thepumppowerlaunchedintothetaperedfibreanddoesnotrepresentthetruepumppowerlasingthreshold. Fig. 1(d)does,
however,showaclearthresholdforthepeakoutputpower.
Byrepeatingthefabricationprocessatdifferentpointsalongacapillaryitispossibletomakeastringofresonatorsinrow.
Asademonstration,threeresonatorswithdiametersof120µm,170µmand155µmweremadeonthesamecapillary,which
hadanODof42µm. Lasingwasexcitedineachresonatorinturnbysimplymovingthetaperedfibrealongthecapillarytothe
nextresonator. Lasingwascollectedfromeachresonatoratthesamecouplingpointonthetaperedfibreandforthesamepump
power,seeFig. 1(e).
Thesamefabricationstepswererepeatedforanopticalfibre. Inthiscase,astandard780nmsinglemodeopticalfibre
wastaperedusingaCO lasertoadiameterof20µm. Thesameerbium-dopedglasswasmeltedonthetaperedfibreatfive
2
differentpointscreatingfiveseparatecavitieswithdiametersrangingfrom42µmto232µm. Fig. 2(a)-(c)showstheWGM
lasingspectratakenforthreeoftheseresonatorscollectedusingthesamepumppowerandpositiononthetaperedfibre;note
thattheothertwoonlyshowedfluorescenceundertheseconditions.
Apartfrommakingabottle-shapedorsphericalresonatoritwasalsopossibletomakeathincoatingofthedopedglass.
Thiswasachievedbyheatingthesphereofdopedglassafterithadbeentransferredtothefibre(orcapillary). Thisadditional
heatingcausedthespheretomovealongthefibreleavingbehindathinlayersurroundingthefibre. Thethicknessofthislayer
isnotconstantsomicrocavitiesareformedbythesmallvariationsinthediameter,similartoSNAPstructures.29 Figure2(d)
showssuchathinlayerspreadoutbetweentwomicrolasers. Thepositionoftheexcitationtaperedfibrewasmovedalongthis
thinly-coatedregion. Thethicknessofthedopedlayerwasmeasuredusinganopticalmicroscopeandwasdeterminedtobe
around1-2µm. AteachpositionaWGMspectrumwasobservedand,atsomepositions,lasingwasachieved. Inthefuture,a
moreaccuratemeasurementofthediametercouldbemadeformthevariationsintheWGMspectraateachposition
Figure2. (a-c)Lasingspectrafromthreeoffiveresonatorsmadeonthesame20µmfibre. Thespectracorrespondtobottle
resonatorswithdiametersof42µm(top),60µm(middle)and160µm(bottom). (d)Theexcitationofacontinuousseriesof
microresonatorsfromthicknessvariationsinathincoating. Thethincoatingwasformedbetweenthe42µmand60µm
microbottles,seenatthetopandbottomofeachimagewithcorrespondinglasingspectrashownin(a)and(b).
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Results
Thermo-opticaltuningandgasflowsensing
Hollow whispering gallery resonators, such as microcapillaries30 and microbubbles,14,31,32 have the unique feature that a
materialcanfillorflowthroughtheirinnervolumeasalreadydemonstratedfordye-filledmicrobubblelasers,33capillaries34
andon-chipmicrofluidicchannels.35 ThelighttravelingintheWGMispartiallyabsorbedbytheglassandlocallyincreases
thetemperatureoftheresonator’swall. Whenfluidflowsthroughtheresonatoritremovessomeoftheheatandreducesthe
temperature. ThiscausesablueshiftinthefrequencyoftheWGM.InthiswaytheWGMmodescanbetunedandtheshiftcan
becalibratedtorepresenttheflowrateofthefluid,withlargerflowratesgivinglargerblueshifts,seeFig. 3.
Figure3. SchematicofaWGM”hotcavity”anemometer. TheexcitationoftheWGMisrepresentedbytheredarrowsinthe
capillarywall. Withsufficientabsorbtion,thelightintheWGMcanlocallyheatthecapillary. Fluidflowingthroughthe
capillaryisrepresentedbythegreenarrow. TheflowingfluidremovestheheatandshiftstheWGMstohigherfrequencies,
representedbythemovementofthetransmissiondipontheleftofthefigure.
Thisissimilartotheconceptofall-optical,hotwireanemometry,amethodwhichhasbeeninuseforsometime. Mostof
thereporteddevicesarebasedonaSiO opticalwaveguide, whichistreatedasawirethatabsorbslighttherebycreating
2
heat.36–40 The wire is placed into, or in contact with, a fluid flow channel. The flow of the fluid cools the wire and this
changestherefractiveindexorthelengthofthewire. SuchchangescanbereadoutopticallyusingafibreBragggrating
(FBG),forexample.36–40 BecauseSiO isgenerallynotastrongabsorber,asignificantamountofopticalpowerisrequiredto
2
generateheat. Theprocesscanbeaidedbytheadditionofametal.40 Mostofthesystemsdiscussedabovearenotcapable
ofmeasuringflowinarbitrarilysmallchannelssuchasonewouldfindinmicrofluidicsystems,thoughrecentlyaFBGhot
wireanemometerwasusedtomeasureflowinsuchasystem.40 Althoughthesensorshowedgoodsensitivityinthelowflow
rateregime,thedevicerequiredhundredsofmWofpumppower,theresolutionwaslow,andgoodthermalcontactneededto
bemaintainedbetweentheopticalabsorberandtheflowchannel. Ahotcavityanemometerthatcanbeincorporatedontoa
microcapillaryautomaticallyprovidesgoodthermaloverlapbetweenthesensorandtheflowchannel. ForanEr:Ybdoped
resonator,significantheatisgeneratedbypumpabsorptionat980nmandanarrowlinewidthlasingmodeat1535nmisusedto
measurethethermo-opticalshiftofthecavitymodesinducedbytheflowoffluidthroughthecapillary. Thisdeviceoffershigh
resolution(byusingthelasingmodes)andhighsensitivity(duetothehightemperatures)withlowpumppowers.
Itiswellknownthatthereareanumberoflargenon-radiativeenergytransitionsinerbium-dopedglasswhichcanbe
accessed by pumping at 980 nm.41–43 These phonon transitions generate a significant amount of heat in the glass even
for low pump powers; in fact up to 40% of the optical pumper power can be converted to heat. By flowing a gas/fluid
throughthecapillarythisheatispartiallyremovedandtheWGMsshiftataratedeterminedbythethermo-opticalbehaviour
(dn/dT =−21×10−7K−1)andthethermalcontraction(dr/dT =114×10−7K−1)oftheglass. Basedonthecoefficients
givenbythemanufacturer44thethermalshiftrateoftheWGMsshouldbearound0.0145nm/K(or1.9GHz/K)at1535nm,
wheretheshiftisdefinedas
∆λ =(α+β)∆T, (1)
and∆T isthechangeincavitytemperature. ThecapillarywiththemicrolasershowninFigs. 1(a)and1(b)wasconnectedtoa
sourceofpressurisedairviaapressureregulator. Theoutputofthecapillarywasconnectedtoamassflowmeter. Theshiftof
thelasingpeakswasrecordedasthepumppowerwasincreased. Thiswasdonewithnoairflowingthroughthecapillaryfor
increasingflowrates,withtheresultsplottedinFig. 4(a). Forthisparticularcavity,theshiftrateoftheWGMsgoesfrom-16.2
GHz/mWto-4.1GHz/mWforzeroflowand15sccm,respectively. UsingEq. 1thechangeintemperatureofthecavityforthe
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maximumshiftineachcasecanbeestimatedandisplottedinFig. 4(b). Withnogasflowthetemperatureincreasesby66◦C,
whereaswithamaximumflowrateof15sccmthemaximumtemperatureincreasewasreducedto13◦C.Next,theinputpump
powerwasfixedandtheinputpressuretothecapillarywasincreasedwhilethepositionsoftheWGMsinthetransmittedsignal
wererecordedusingtheopticalspectrumanalyser. AstheflowrateincreasedtheWGMswereobservedtoshifttowardsshorter
wavelengthsduetothecoolingeffectoftheairflowthroughthecapillary,asdiscussedabove. Thisprocedurewasrepeatedfor
increasingpumppowersandtheresultsareplottedinFig. 4(c).
Figure4. (a)Theshiftrateofthe1535nmlasingWGMasfunctionofinputpumppowerfordifferentgasflowrates. (b)The
calculatedincreaseincavitytemperatureandcorrespondingWGMshifts. (c)Theshiftofthe1535nmlasingWGMasfunction
ofthemeasuredflowratefordifferentpumppowers. (d)ThesensitivityoftheWGMshiftrateasfunctionofmeasuredflow
rate,calculatedfromthederivativeofthefitsin(c).
Fromthetheoryofopticalhotwireanemometry,36,40theheatlost,H,fromahotwireisrelatedtotheflowrate,ν,by
H=(A+Bνn)∆T (2)
a
whereAandBareempiricalconstantsandnisafittingparameter(usually0.5forasimplehotwire). Certainamountofpump
powerisusedtogeneratethelasingsignalsothecavityisalreadyheated,thustheinitialtemperatureofthecavityisnotknown.
Evenformodestpumpingtheinitialcavitytemperaturecanbeeasilygreaterthan100◦C.45Therefore,wedefine∆T =[T (ν)-
a a
T (ν=0)],i.e. thedifferencebetweenthecavitytemperatureatzeroflowandthecavitytemperatureatsomeflowrate,ν. Based
a
onthelawofenergyconservationtheheatlostmustequaltheheatputin,therefore,inthecaseoftheWGMresonator
H=Iη(Q/Q ), (3)
abs
whereIistheinputpower,η isthecouplingefficiency,Qisthequalityfactorofthecavity,andQ istheabsorption-limited
abs
cavityqualityfactorgivenby
2πn
eff
Q = , (4)
abs
σλ
whereσ istheabsorptionlossofthematerial,λ isthewavelengthandn istheeffectiverefractiveindex. σ wasestimated
eff
fromtheglassmaterialpropertiesprovidedbythemanufacturer44usinganabsorbingionconcentrationof3×1021ions/cm3
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andanabsorptioncross-sectionof1.7×1020/cm2. ThecalculatedQ =1.2×106andtheloadedcavityQwasassumedtobe
abs
100timeslessthanthis. Todetermine∆T ,thechangeintemperatureforeachpumppowerwasdeterminedfromthemaximum
a
shiftandEq. 1. TheWGMshiftintermsofthegasflowratecanthenbewrittenas36,40
δλ =λ(α+β)[H/(A+Bνn)−∆T ]. (5)
a
TherecordedWGMshiftswerefittedusingEq. 5,seeFig. 4(c). The980nmpumplaserusedintheexperimenthadarated
linewidthof2nm,thereforethecouplingefficiencyofthepumptoaspecificWGMwasimpossibletoquantify. Forfittingwe
assumedacouplingefficiencyof20%. Thisvalueisjustifiedbythefactthatweobserveabouta20-30%dipintransmitted
power whenthetaperandmicrobottle makecontact. Thefittingparameter n wasset to0.84. Usingtheseparametersthe
correspondingfitsagreewiththeobservedshift. ThesensitivitiesoftheWGMshiftratesaredeterminedbydifferentiatingthe
fitsinFig. 4(c)andareplottedinFig. 4(d).
Straintuning
Straintuningofmicroresonatorshasbeenreportedpreviously;itisaneffective,fastandstabletuningmethod. Straintuningof
anerbium-dopedmicrobottlelaserwasreportedbyPo¨llingeretal.46 However,etchingusinghydrofluoricacidwasneeded
toaccessthecoreofanerbium-dopedfibrewhichwassubsequentlymeltedbyaCO lasertoformthebottleshape. Inour
2
work,noetchingisrequiredandalargenumberofmicrolaserswithawiderangeofsizescanbemadequicklyandeasily. The
20 µmdiameterfibrewiththefivemicrolasers,asdescribedearlier,washeldonastagethatallowedtheU-shapedmount
holdingthefibretobeextended,therebyputtingstrainonthefibre. Thestagewasfittedwithapiezostackthatprovided17µm
displacement. The42µmdiametermicrolaserwascoupledtothetaperedopticalfibreanditslasingoutputwasmonitored
whileavoltagewasappliedtothepiezostack. Thesameprocedurewasappliedtothe42µmcapillarysupportingthethree
microlasers;inthiscasethe120µmdiametermicrolaserwasselectedfortuning. Thetuningcurvesforbothmicrolasersare
showninFig. 5.
Figure5. Straintuningofa42µmdiametermicrolaseronanopticalfibrewithadiameterof20µmandstraintuningofa
120µmdiametermicrolaseronacapillarywithadiameterof42µm. Redcurves: increasingpiezovoltage,blackcurves:
decreasingpiezovoltage
Inbothcasesthetuningcurvesexperienceahysteresisduetotheintrinsicbehaviourofthepiezoactuator. Thetaperedfibre
lasershowedalargertuningofaround60GHz,mostlikelyduetoitssmallerdiameter. However,anumberoffactorscould
influencethefinaltuningrange,e.g. thediameterofthetaperandtheinitialtensiononthetaper. Forthecapillary,thediameter
andthewallthicknessmayalsoplayarole. Thetaperorcapillaryandthedopedglassmicrobottlesarenotasinglestructureso
thismayacttofurtherreducethefinaltuningrange. Nevertheless,asignificantlyusabletuningrangewasachieved.
Discussion
Wehavepresentedamethodforcreatingglass-on-glassstructures,whereoneglassismeltedandallowedtoflowontoanother
tightlycurvedglassstructure.Thisisachievedbythefactthatthetwoglasshavesignificantlydifferentmeltingpoints;therefore,
thismethodshouldalsobeapplicabletoothersoftglasses. Forexample,highindexglassessuchasleadsilicateortellurite
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couldbeformedintosmallcavitiesonataperedfibrewithdiametersaround20µm. Thenumberofresonatorsonasinglefibre
dependsonthesizeoftheresonatorandlengthofthefibre. Sofarwecanplaceeachcavityapproximately100µmapart. One
couldenvisionanumberoffibresmountedonachipcontainingwaveguidesaddressingeachresonator.
Theabilitytomakehollow,microbottle-shaped,doped-glassmicrolasersallowsustoinvestigateanewmethodofthermo-
opticaltuningwherethelasingmodescanbetunedbysimplyflowingairthroughthecavity. Thethicknessofthecapillary
wallandathick,doped-glasslayernegatesanyredshiftoftheopticalmodeinducedbyincreasesininternalpressure. We
demonstratedthatthemodeshiftscanbecalibratedtorepresentthegasflowrate,thuscreatinganintegrated,all-optical,flow
sensor with low power and high resolution. The measurement of liquid flow is also possible with this setup and we have
seenawaterflowratesensitivityof1GHz/(nL/sec)inacapillarywithanIDof100 µm. Earlytestsofstraintuningalso
showpromisingresultsandfurtherstudiesshouldrevealthedependenceofthetuningrangeonthefibrediameter,sizeofthe
resonatorandwallthicknessofthemicrocapillary. Infutureworkweplantoexplorethismethodfurtherusingvarioussoft
glassesandstructurestostudythepossibilityofcreatingnewandinterestingglass-on-glassdevices. Forexample,itmaybe
possibletoflowmeltedglassintochannelswhichhavebeenpre-etchedinsilicaglass.
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Acknowledgments
ThisworkwasfundedbytheOkinawaInstituteofScienceandTechnologyGraduateUniversity.
Author contributions statement
J.M.W.conceivedtheexperimentanddidthedataanalysis;J.M.W.andY.Y.conductedthemeasurements;S.N.C.supervisedthe
project;allauthorscontributedtodiscussionsontheprojectandmanuscriptpreparation. Allauthorsreviewedthemanuscript.
Additional information
Competingfinancialinterests: Theauthorsdeclarenocompetingfinancialinterests.
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