VNU Journal o f Scicucc. M athcinutics - Physics 24 (2008) 89-96
Wavelength shift in inicrosphere lasers
T ran Thi Tam^ % D ang Q uoc T rung-, Trail A uh Vu“, Le H uu M inh^, Do N goc C hung“
I'a m llv o f H ngineenng Physics and Nano-Technolo<^\ College o f Technology, VNU Ỉ44 Xuan Thuy, Can Giay, Hanoi, Vietnam
^Institute o f M a teh a ls Science, Vielnam Academ y o f Science and Technology 18 H oan^ Quoc lĩet, Can Giay, Hanoi, Vietnam
Rccci\'cd 20 March 2008: received in revised fonn 15 M av 2008
A b stra ct. 'I'lic vvavelengih shift cfiect o f the W hispering G a llcn ’ Mode (W G M ) laser w ith I'r’^ ' co-dopcd phosphate glass microsphcre has been investigated. The experim ent was carried oiil bv h alf iibcr taper coupling lechriique. The inicrosphcrc lasers have been pumped at 9 8 0 IIUI to take full ad \’antagc o f cnerg}^ transfer cffect from ion Ytterbium to ion Erbiutn.
'riic VVGM’s w avclcnglh shift were analyzed for sphere diainclcrs o f 9 0 /itn. The 0bscr\'cd lasing lines extends from ,1532 nin to K i l l nin.
K eyw orks: M icrosplierc, W hispering G allety M odes, Lasers, Hrbium Ytlcrbium co-dopcd phos
phate glass.
1. I n tr o d u c tio n
R are carth -d o p cd íĩlass m icro sph crical lasers arc su b ject to nu m erous studies and significant pro ercss has b een ach ieved in the past decadc. In the m icrosphcrc, the m o rp h o lo g y -d ep en d en t resonance (M D R ), vSo callcd W h isp erin g Gallcr>' M ode (W G M ) - a p articu la r m ode o f m icro cav ity resonances - occurs w h en the flu o rescen t lich t travels in a dielectric m edium alo n e thin layer n ear equatorial.
A fter rep eated to ta l in ternal reflections at the curved b o u n d ary tlic electro m agn etic field can close on itself, g iv in g rise to rcsonanccs and fom ied “w h isp c rin g -sa llc ty ’* w av egu id e m odes. Inside the m icrosphcre, th e circu latio n o f the '\v h isp e rin g -e a llc ry ” m odes provides th e ncccssary p ath length for absorption, th u s m a k in g it p o ssib le to rcduce the laser thresho ld drastically. In order to couple li£>ht in or out o f th e m icro sp h crc, it is n ccc ssaty to utilize overlap ping o f tlic cv an csccn t rad iation field o f W G M s w ith th e ev an cscen t field o f a phase-m atched optical w aveg uide. T he m icro cavity W G M s w ith its u n iq u e co m b in atio n o f stro n g tem poral and spatial con finem en t o f lie h t have attracted increasing interest clue to th e ir high po ten tial fo r a large nu m b er o f ap plicatio ns in c ith e r fu n d am en tal research from q u an tu m electro d y n am ics (Q E D ) to n o n lin ear optics, as the realization o f m icro lasers |1], hiữh resolution sp cctro sco p y [2 ], or in applied pho tonics and optical co m m un ication s areas su ch as m iniature bio sensors |3 |, n arro w filters |4], optical sw itch ing [5], etc. F or th e d ielectric m edium as a m icrosphcre, m ode volum e can b e as low as a few hund red cubic w av elen g th s w ith very high finesse. T he Rare earth-doped (E r or N d ) glass are ideal su bject for realizing th ese m icro sp h erical lasers w ith v ery hich quality factors Q .
C o rresp o n d in g a u th o r E -m ail: d rtra n th itam @ g m a il.c o m 89
B .R . Jo h n so n th e o re tic a lly stu d ied th e b eh av io r o f the m orp h o lo g y -d ep en d en t resonances o f a d ielectric sphere on o r n e a r a p la n e o f in fin ite conductivity. H is resu lt show s th a t the locations and w id th s o f the reso n an ces ch an g e as th e sp h ere ap pro ach es th e surface [6]. I f the sphere is in itially located a t a d ista n ce d th a t is m o re th a n ap p ro x im ate ly 2D/2> aw ay fro m the po in t o f con tact w ith the c o n d u ctin g p la n e, th e re so n a n c e s w ill h av e th e sam e locations and w id th s as th e y do in an isolated sphere. T h en as th e sp h ere is b ro u g h t c lo se r to the surface or ev en tu ally in con tact w ith it, the locations and w id th s o f th e reso n an ces chan g e. T he lo catio ns o f the T E -m o de resonances sh ift to h ig h e r size p aram eters (i.e . B lu e -sh ift in w a v e le n g th ), th e T M -raod e reson an ces sh ift to lo w er size param eters (i.e. R ed -sh ift in w a v e le n g th ) and th e w id th s o f b o th t\p e s o f reso nance increase. M o st o f the change in lo cation and w id th o ccu rs w h e n th e sp here is qu ite close to th e co n d u ctin g plan e. A p p ro x im ately 90% o f the to tal reso n an ce sh ift o ccurs w h en th e distance from th e p o in t o f contact is less than 0.05 o f th e d ia m e te r o f th e sp h ere. T h is p re se n ts th e p o ssib ility o f tu n in g th e W G M w avelengths.
In th is p a p e r w e d e sc rib e research resu lts on laser realization u s in g a tapered fiber fo r efficient co u p lin g as w e ll as w a v e le n g th sh iftin g effect o f th e laser. O u r ex p erim en ts have b een carried out for th e I]3/2 I i 5/2 tra n s itio n at 1550 nm o f E rb iu m ions in p h o sp h ate glass m icrosphcres.
90 Tran Thi Tam eí al. / in\^u Journal o f Science, Mathematics - Physics 24 (2008) 89-96
2. E x p e r im e n t
T h e m a teria l u se d fo r fa b ric a tio n o f m icro sp h ercs w as an E r^+ /Y b^+ p h o sp h ate glass (S chott IO G -2) doped w ith 2 % w e ig h t o f EriO a and co-dopcd w ith 3% w eig h t Y b 2Ơ3 . M icrosphercs have been p ro d u ced from p h o s p h a te g la ss p o w d e r u sin g a m icrow ave p la sm a torch (o scillato r frequency o f 2.4 G H z and m ax im u m p o w e r o f 2 k W ) w ith A i^ o n is u sed as p la sm a gas and oxygen or nitrogen as sheath gas. P o w d ers arc a x ia lly injcctcd and m e lt w h en p a ssin g througli th e p la sm a flam e, superficial tension forces g iv in g th e m th e ir sp h e ric a l form . T he m icrow ave p o w e r and gas discharges can be adjusted to o b tain o p tim al c o n d itio n s to sp h ero id ize ilu o rid c or silicate glass. T h e d iam eter o f the spheres w a s v aried fro m 10 to 2 0 0 /Lim d e p e n d in g essen tially on th e p o w d e r size. Free spheres arc collected a few te n c e n tim e te rs low er. O b tain ed spheres th en are glu ed at th e tip o f optical fib er o f about 10 /im to 30 i-im in d ia m e te r w h ic h allo w to m anipu late th e m easily and to insert th e m in the setup.
T h e use o f an E i^ + /Y b ^ + co -d o p ed p h o sp h ate glass is associated w ith the 9 7 5 n m pum p in g w aveleii2th in o rd er to p o p u la te th e ^ F s/2 m etastab le level o f Y tteiiiium ions w h ich tran sfer th e ir e n ergy to th e n e ig h b o rin g E rb iu m ions b y rad iativ e and non -rad iative w ay s. To tak e fu ll advantage o f this excitation m eclian ism , w e ch o se 9 7 5 n m am o n g th e d ifferen t ap p rop riate w av elen g th s for pum pin g E r
b iu m /Y tte rb iu m co -d o p ed g la sses ( 8 1 0 n m , 9 7 5 n m and 1 4 8 0 n m ) in o u r ex perim ent. The pum p source w as fib er p ig tailed S D L O -2 5 6 4 - 120 L a se r D io de g enerating 9 7 6 .1 n ? n radiation w ith the m axim um c w p o w er o f l 2 0 m W . A lso , vve u se a h ie h d o p in g concentration g lass (1.710^° ions/cm ^ fo r Erbium and 2.510^° io ns/cm 3 fo r Y ttertiiu m ). T h e u se o f Y tterb iu m ions help s to avoid th e side uffects o f a too hig h E rtiium c o n c e n tra tio n ( s e lf p u ls in g etc).
2.Ỉ. E xcita tio n a n d re c e iv in g o f W G M s
C o u p lin g lig h t in to and o u t o f th e m icro sp h eres m u st be realized b y m eans o f optical tunnel effect th ro u e h e v a n e sc e n t field . F o r effic ien t co u p lin g light into m icro sp h eres or to get W G M s signal
Tran Thi Tam et a i / l ^ u Journal o f Science, M athem atics - Physics 24 (2008) 89-96 91
out from m icro sp h ere one m ust adju st the frcqucn cy o f the cx citatio n b eam to a W G M rcso nan cc and a lien the cx citatio n beam so th at it also has an a n g u lar m o m en tu m m atch cd th e an g u lar m om entum o f th a t m ode. T h ere arc m any d iftcrcn t techniques fo r th is p u rp o se u s in s high-indciX p rism s, tapered fibers, anelc p o lish e d fib e r couplers or w aveeuidcs. S p h eres m u s t h e set ver>' clo sc to th e p rism inside the evan escent field. In th is experim ent \vc use h alf-tap ers fo r co u p lin g lig h t in and o ut b ccau sc o f its relatively sim p le in m ak in g and m o u n t i n g tech n iq u e. T h e co u p lin g can b e ach ieved if w e p u t th e m icrosphercs vcrv' closc to the h a lf fiber tap er tip. T he d ista n c e b etw ee n m icro sp h cres and fib er ta p er tip as w ell as an angle regarding m icro sp h eres’s e q u a to r w a s co n tro lled b y m icro p o sitio n in g stages and /o r w ith p ic zo elc ctric actuators. We pro d u ced th e h a lf ta p e rs b y ch cm ical e tc h in g in H F or b y heatin g and strc tc h in g a standard telecom m unication s in a le m o de at 1.55 ịirn fib e r u n til b reak in ti it, u s- in s eith er C(3-2 la ser or fusion optical splicing system . T h e fib e r tip w a s ta p ered to ^ 2/xrn in diam eter.
2.2 . T h e e x p e r i m e n i a l s e tu p
T he e x p e rim e n t (see Figure I) w as realized w ith stan d ard fib e r-o p tic co m p o n en ts spliced or conncctcd by A PC co n n ecto rs. O ur experim ents w ere p c rfo m ic d w ith tw o d ire c t fib e r coupline: schcm e u s in s half-tap cred fiber: a) tw o separate half-tapcrs, one fo r c o u p lin e 9 8 0 n r n p u m p in ( l.a ) , the other for coupliim sig n al o u t from th e sphere (Figure l.a ), and b ) u sin g one sin g le h a lf-ta p e r to couplc both pum p em issio n in an d th e m icro spherical laser out (P iíĩure l.b ). T h e o u tp u t 1 .5 5 /im la ser radiation is coup led into th e op tical fib er and fed to Spectrum A nalyzer.
Tậ ịnĩ'
Ibị
Fig. 1. T he principal experimental setup: a) d oub le h a lf tapers; b ) sin g le h a lf taper.
A lth o u s h th e op tim u m cou p lin g conditions fo r tw o w a v e le n g th s, À ~ 9 7 5 /^ m fo r th e pum p and À ^ 1.55/x?n fo r th e la ser signal are not the sam e, w e receiv ed good resu lts e v e n in sing le half-taper schem e (See F igure 2). W e fixed th e co-doped E ĩ^ '^ /Y h ^ ^ p h o sp h a te g la ss m ic ro sp h e re s b u t m ounted h a lf tapers on X Y Z L in ear M icro translations w ith R o ta tio n Stage. T h is setup allo w s fo r establishing th e equator region o f the m icro sp h eres in the ev an escen t field s u ư o u n d in g th e h a l f ta p e r and adjusting
92 Tran Thi Tam et al. / l ^ u Journal o f Science, Mathematics - Physics 24 (2008) 89-96
th e accep tan ce angle (b etw een th e tip axis o f the h a lf ta p e r and eq u ato rial p lan e). The collcctcd laser sig nal w as analyzed w ith a O.Oónm resolution O ptical S pectrum A n aly ze r (O SA - M odel; A g ilen t 861 42 B ).
3. R esults and discussion
T h e excited m icro sphere em itted strong green upco nv ersio n flu orescence along equ atorial. O b served sp ectru m (by S 2000 S pectrom eter - O cean O ptics, U SA ) show s the ex istence o f th e red em ission aroun d 6 6 0 n m besid es th e stron g green em ission around 5 4 5 n m . In the 1 5 5 0 n m region o f th e 113/2
—^ I i 5/2 tran sitio n o f ion th e optical spectrum o f th e o u tp u t signal from the sphere b elo w the la se r th re sh o ld p resen ts th e lu m inescence in tensity w ith series o f sm all peak s. E stim ating th e m icro- sph ere d ia m e te r througli p e a k s distance in these spectrum s gave resu lt w ell m atched th e one received b y o ptical m ethod. W hen in creasin g th e pum p in tensity w e obtained laser oscillation. B ecause o f the d iffic u lty in q uan tify in g p u m p p o rtio n coupled in sphere, w e con trolled o n ly th e total ou tpu t p o w e r o f L D p u m p . T he actual p um p p o w e r at the tip w as ap pro xim ately 70% o f th a t value. F igures 2 presen ts several la se r spectra from th e m icrosphere o f 1 4 0 /im diam eter u n d e r d ifferen t to tal p u m p pow er.
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Fig. 2. The laser spectrum from microsphere o f 140 Ịim diameter, total pump: 45m W (lOOinA): a) double half tapers; b) single h alf taper.
S u ccessfu l co llectin g th e laser em ission from th e m icro sp h ere d epen ds on coupling h a lf ta p e r p aram eters. T he form o f th e h a lf ta p e r i.e. length o f tapered p a rt o f fib e r affects th e co upling efficiency as w ell. S haip angle h a lf ta p e r (length about or m ore th a n 8 00/x m i m akes it easie r to collect signal w h ile b lu n t angle h a lf ta p e r allow s easie r to select laser m ode. B y ad ju stin g th e co upling param eters (the m icro sp h ere - ta p e r g ap, th e angle b etw een the ta p e r and m icro sp h ere equator...) w e can extract laser rad iatio n in certain w av ele n g th region. T he sh ortest observed laser line w as 1532.2 n m and the lo n g est w a s 1 6 18 .9 n m . T h e W D M line w id th is lim ited b y O S A reso lu tio n (O.OGnm). In m ost cases a good co u p lin g is obtained sim u ltan eo u sly for several lines w ith d ifferen t w av elen g th s À so w e have ob seiv ed m u ltilin e laser sign al. T h e laser em ission can be extracted even w h en the h a lf tapers arc in
Tran I'hi Tam ei al. / VNU Joum al o f Science, Mathematics - Physics 24 (2008) 89-96 93
contact w ith tlic splicrc, th o u eh in tliis case \vc m ay s e t sim u ltan eo u sly series o f laser lines in b ro ad er raniic (from 1557.8 m n up to 1 6 1 1 .9 n m , see F i" 2). T he sin slc laser line can be sclcctcd b y var>’ing the anelc b etw een h a lf tap ers axis and equatorial plane o f the sphere in d ouble ta p er schcm c. Figure 3 presents an exam ple o f sclcc tin a a single laser m ode at 1534.4 nm from th ree lines by chang in g the acccptancc an^lc. D ue to h a lf ta p er non-constant diam eter and co n seq u en tly variable gap b etw een the fib er and th e m icrosphcrc, b y c h o o sin s the co upling p o in t in the h a lf ta p e r i.e. adju stin g th e distance from tip to acccptancc p o in t w c m ay also find the appropriate p o sitio n to sclect one lasing m ode.
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Fig. 3. Selecting a single Vãsoĩ mode by changing Ihc acccptancc angle: a) three lines em ission, marker at 1534.4 mn b) single line at 1534.4 nm.
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Fig. 4. The w avelength sliift in laser spectrum, microsphere o f 140 i-im diameter, pum p: 25 m W and 60 m W - sh ift right.
W h en increased th e pum p p o w er w e m ay co llect som e n ew ly em erged laser lines besid e th o se existed. Wc also observed red sh ift in th e w avelength o f W G M s. T he ty p ic a l resu lt o f th e laser spectra
94 Tran Thi Tam et al. / VNU Journal o f Science, Mathematics - Physics 24 (2008) 89-96
analyzed b )’ an O p tic al S p ectru m A n a ly z e r w ith a resolution o f O.OGnrn. is illu strated in figure 4. The tw o w av elen g th s at 1 6 0 8 .8 0 n m and 1 6 1 1 .7 4 n m w h en the to tal p u m p p o w e r in ten sity w as 2 5 m W , shifted furth er to 1 6 0 8 .9 6 n m an d 1 6 1 1 .9 0 n rn , respectively, w h en to tal pum p p o w er w as increased to 6 0 m W . S im ilar re d -sh ift b e h a v io rs h av e also b een observed fo r sphere o th er size. E xccpt several n e w lin e s em erg e d u n d e r s tr o n g p u m p , a ll e x is t W G M s s h ifte d b y 0.16ri7Ti to w a r d s lo n g e r w a v e le n g th u n d er th e to tal p o w e r d o m a in in c re a sin g from 25 to 60. T his red sh ift pheno m en on w as ex perim entally
observed in E r/Y b p h o s p h a te m icro ch ip la ser [7] and explained b y a m odel based on th e m ial effects 8]. T h e ph o n o n s in sid e activ e m icro sp h erical laser cavity associated w ith the non radiative decay betw een the m a n ifo ld s o f E rb iu m ions, and b e tw e e n th e intra-S tark levels o f th e laser m anifolds, th us create th e n n a l d e p o sitio n an d h e a t th e m icro sp h ere. A n increase o f cav ity tem peratu re results in b oth an expansion o f tlie m ic ro sp h c rc c a v ity leng th and a change o f index o f refraction. B oth changes then affect th e lasin g c o n d itio n and sh ift th e w av ele n g th o f cvcr>' W G M .
We have in v e stig ated in te ra c tio n b e tw e e n W G M lasers w ith th e un p ro tected A lum in um flat m iư o r w hich is a good ap p ro x im a tio n o f a c o n d u c tin g plane. T he m iư o r w a s driven b y m icro tran slatio n stage w ith lO u m step from b e lo w th e m icro sp h ere (Fig. 5). T h e m icro sp h cre h as a diam eter D ~ 9 0f i m. The la ser em issio n h ad b e e n ob serv ed w ith th e m iư o r at a distan ce ~ 200/xm . W hen tran slatin g the m irror to w a rd s th e sp h ere w c o bserv ed a line sh ift to w ard s th e sh o rter w av elen g th (Fig. 6 - a).
T he change o f th e m o d e in te n sity is p resen te d in Fiii. 6 - b. T h e influcncc o f the m irro r is d e a r from d ^ D . F or a lo w e r w a v e le n g th , w e have observed the sam e ’ b lu e ” sh ift b eh av io r (Fig. 7).
T he w av elen g th sh ift w a s 0 .2 n r n fo r 1 6 0 8 .7 n rn , 0 .2 2 n rn for 1 5 6 6 .9 n rn , 0.18ri7Ti to r 1548.2 8 n m and 0 .1 6 rim for 1 5 3 5 .7 5 n m lines. W e also ob served th a t som e lines do n o t sh ift b u t disappear.
Half taper
Mirror d
Fig. 5. Setup for the Sphere-Mirror interaction experiment.
C om pare to J o h n s o n ’s th e o re tic a l p re d ic tio n o f th e W G M ’s behav ior, ou r resu lt show s th at w e observed th e w a v e le n g th sh ift c o ư e s p o n d in g to T E m odes, p. F eron et al. approached th e shift o f resonances p re d ic te d b y J o h n so n fro m th e effective p o ten tial p o in t o f v iew [9], In th e ir approach, for an isolated sp h ere, th e ra d ia l eq u a tio n is ver>’ sim ilar to th e S ch ro d in g er equation w ith a pockct-like p seud o p o te n tial d u e to th e refractiv e in d e x d isco n tin u ity at the surface o f th e sphere. The m iư o r associated to a m irro r refle ctio n s>nimetr>' o p eratio n gives an even sy m m etric p o ten tial. s>'mnietric ỉ>,g and an tisy m m etric e ig e n sta te s asso ciated resp ectiv ely to b lu e -sh iftcd (s>Tiimctric) and red-shifted
Tran Thi Tam et a l / Journal o f Science, M athem atics - Physics 24 (2008) 89-96 95
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b) Fig. 6. D ependence from sphere-tnirror distance of: a) W iivelength b ) Intensity.
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Fig. 7. WGM wavelength shift o f a) 1 5 4 8 .2 7 n m ; b) 1 5 3 5 .7 6 n m .
(antisym m etric) w av elen g th s. T aking into account th e vcctor a sp e c t o f T E and T M m odes and th at th e electrical field is q u asi-tan g en tial to the sphere fo r T E m o d e s (q u a si-ra d ia l fo r T M m odes) fo r a large d iam eter {D > 2 0 A), th u s T E m odes are associated o n ly to sy m m e tric sta te s and T M to an tisym m etric states. T h e m od el ex p lain ed th e resonance shift b u t it d oes n e t ta k e in to a cc o u n t the m etallic properties o f the m iư o r and could n o t give reasonable ex p lan atio n on q u e n c h in g o f som e m odes observed. T he coupling o f th e T M m odes (electric field norm al to th e su rface) w ith th e surface w av es o f th e m etal plane m ay lead to th e ir quenching.
T h e w id th o f the laser m ode is narrow er th a n o u r e x p e rim e n t’s eq u ip m e n t reso lu tion , so w e do not investigate its behavior.
4. Conclusions
T he m icro sp h ere W G M lasers w as realized in E i^ + /Y b ^ + co -d o p ed p h o sp h ate g lass u sin g 9 7 6 n m p u m p , to tak e fu ll advantage o f energy tra n s fe r e ffe c t from ion Y tte rb iu m to ion Erbium . The co u p lin g w a s e a rn e d out b y fib er h a lf ta p er te c h n iq u e in tw o sch em e s, both gave good results.
The single laser line p o w e r m ay reach 1 5 0 n iy w ith only 2 b m W to ta l p u m p pow er, and laser range
96 Tran Thi Tam et al. / IW U Journal o f Science, Mathematics - Physics 24 (2008) 89-96
extends from Ì5 3 2 .2 n r n to 1618.972m. T he red shift ciTcct, w h ich associated w ith the th c n n al e f
fect o ccu ư cd in sid er sphere took place u n d er strong pum p. We have ex p erim en tally observed only a e m is s io n w a v e le n g th s h ift b y a b o u t 0 .2 n m to th e sh orter s id e (b lu e { s h ift) w h ile v a n i n s the d ista n ce sp h crc-m iư o r from lO O ^m ( ~ D) to I Qf nn ( ~ O.ID ). T he propo sed red sh ift have n o t been confirm ed.
A c k n o w le d g e m e n ts. T h is w o rk is supported b y a N ation al B asic R esearch P rogram K T -04. The authors th a n k C N R -IFA C , N cllo C arrara In stitute o f A pp lied P hysics, 5 0127 F irenze, Italy for sup p ly in a the E r^+ /Yh^+ co-doped p h o sp h ate elass.
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