Study of some pumping schemes for laser diode pumped solid state lasers

Transcription

1 Study of some pumping schemes for lser diode pumped solid stte lsers L. Cbret, Gbriel Girrd To cite this version L. Cbret, Gbriel Girrd. Study of some pumping schemes for lser diode pumped solid stte lsers. Journl de Physique III, EDP Sciences, 1992, 2 (9), pp < /jp >. <jp > HAL Id jp https//hl.rchives-ouvertes.fr/jp Submitted on 1 Jn 1992 HAL is multi-disciplinry open ccess rchive for the deposit nd dissemintion of scientific reserch documents, whether they re published or not. The documents my come from teching nd reserch institutions in Frnce or brod, or from public or privte reserch centers. L rchive ouverte pluridisciplinire HAL, est destinée u dépôt et à l diffusion de documents scientifiques de niveu recherche, publiés ou non, émnnt des étblissements d enseignement et de recherche frnçis ou étrngers, des lbortoires publics ou privés.

2 of some pumping schemes for lser diode pumped solid Study lsers stte une Emission TEMOO h 1,064 m vec un bon rendement. Dns le cs du pompge obtenir l stbilitd de l puissnce mise h 1,064 m est lide ux fluctutions en temp rture trnsversl, pr lo brrettes lsers qusi continues. end-pumped nd side-pumped configurtions hve been investigted for the study Longitudinlly the prmeters which hve to be tken into ccount in lser diode pumped systems in of longitudinlly end-pumped system it is necessry to find the right opticl design to focuse the bem into the YAG rod in order to crry out m TEMOO emission with good pump In side pumping, the 1,064 m power stbility depends on the diode lser temperture efficiency. 1987, reserch in diode pumped solid-stte lsers hve been supported t Lserdot by Since fundings, by the Direction des Recherches Etudes et Techniques (DRET) nd the inteml Section d'etudes et Fbrictions des Tdldcounictions (SEFT). At tht time, we begn s J. Phi-s. III Fin<.e 2 (1992) SEPTEMBER 1992, PAGE 1703 Clssifiction Phvsi<.s Absn.cts 4? D L. Cbret nd G. Girrd Lserdot, Groupe rosptile, Route de Nozy, Mrcoussis, Frnce (Received 20 December I99I, ccepted 20 Mrch J992) Rdslmk. Nous d crivons dns cet rticle les lsers h Nd + YAG pomp s pr diodes lsers tels sont dtudids h Lserdot. Des configurtions de pompge longitudinles et trnsversles ont qu'ils exp riment es pour tudier notent les prmbtres dont il fut tenir compte dns [es td de pompge pr diodes lsers dns le cs du pompge longitudinl, il fudr expdriences choisir le mode de foclistion du fisceu de pompge dns le bneu YAG pour soigneusement des diodes lsers. Nous l'vons relev e sur une mquette dns lquelle le pompge tit rdlis Abstrct. In this pper, we describe the diode-pumped Nd + YAG lsers studied t Lserdot. fluctutions. We mde mesurement with lser system pumped by lo qusi-cw diode lser brs. Introduction. Since 1983 high power lser diodes hve been developed in the USA nd lter in Jpn nd Recently, interest in diode-pumped-solid-stte lsers hs incresed becuse of their Europe. over flshlmp pumped systems. This subject ws studied in Europe only few dvntges go, becuse lser diode cost ws dissusive nd most of the lbortories could not yers such devices. fford other lbortories [1, 2], by longitudinlly end pumped rod systems for low power output in

3 1704 JOURNAL DE PHYSIQUE III N 9 Our device is depicted in figure I. A ten stripe diode rry ( Inn m wide) is collimted with numericl perture short focl length lens. Collimting the output of the diode rry high bem with n ellipticl cross-section. Bem shping is ccomplished using n yields 807 nm nmorphic prism pir P " 1064 nm operting t 300 mw cw output is focused with the opticl system depicted in figure I rry I cm long x 0.5 cm dimeter I fb doped Nd YAG smple. The resontor is plno into Generlly the efficiency limits of these end pumped devices re unsufficient numericl perture of the focusing ssembly, I) residul Fresnel losses, 2) non totl trnsmission of the dichroic Nd 3) YAG coting, bd overlp between the pumped volume nd the Nd YAG TEMOO mode, 4) intrcvity losses, 5) ultimte physicl losses due to the efficiency with which pump photons re converted to 6) lsing photons nd to the threshold energy. the fundmentl mode. Excellent overll efficiency ws obtined in the fundmentl mode Limittion ppers in systems excited with bd mode mtching of pump nd lser bem. An of this limittion is described below. But scling possibilities of this configurtion evlution limited becuse lser diode br geometry is more dequte for side pumping [3-5]. We re recently side pumped Nd YAG lser rod. The experiment includes developed of the energy deposited into the rod nd mesure of the lser diode visulistion temperture criticity. However for intermedite output energy (I to 10mJ) in the fundmentl mode, end rod systems my be still used [6-8]. We designed Nd YAG lser longitudinlly pumped pumped by five br stck. The stck output is focused into the rod with end longitudinlly four lens ssembly ; output bem chrcteristion is presented below. Low power. Longitudinl pumping. prism Finlly sphericl lens focuses the bem into the Nd + YAG rod. nmorphic pir. is nerly circulr ; its men dimeter is pproximtively 180 m which is suitble to spot The obtin n efficient pumping. NdYAG rod DL Fig. I. Schemtic of the lser diode focusing optics nd Nd YAG cvity. An integrted system ws constructed 3 yers go (Fig. 2). A Thomson-Hybride lser diode concve with the pumped end of the Nd YAG rod being coted for high reflection t 1064 nm nd high trnsmission t 807 nm. TEMOO opertion with 60 mw output hs been chieved. Sme results hve been obtined by J. Berger t n equivlent pump power [9].

4 6[ l Fig. 2. Integrted system view of 60 mw TEmoo lser diode end pumped Nd YAG lser. designing lser source, it is importnt to estimte these losses ccurtely. Losses (1), In (3) nd (5) cn be evluted from opticl mnufcturer's dt or esily mesured. An (2), mode is overlp esy if the pump is circulr. Mode overlp is the rtio between the useful lunched energy in the size spot pump energy is deposited only in the TEMOO mode. In the second prt of the rod, perfect f', the two volumes no more overlp. A prt of the pump energy escpes from the' length spreding. dx is the contribution of the bsorption. So, t point x the intensity is tg + Wo S (x ) " ' " N 9 LASER DIODE PUMPED SOLID STATE LASERS 1705 ' description of the mplifying medium nd cvity prmeters renders n ccount of dequte (6). Losses (4) re the only losses which re not ieditely known. In the cse of the losses low power end pumping the evlution of the so clled mode nd the totl lunched energy in the rod. To optimize the energy collection in the cvity mode, the pumping spot rdius, must be smller thn wo, the TEMOO mode wist of the cvity Nd YAG resontor. Generlly, short focl length lens is required to obtin such smll But, consequently pump bem divergence is high, much lrger thn the TEMOO mode spots. The optimum pump spot position in the rod is depicted in figure 3. Due to its low divergence. the cvity mode is supposed to hve constnt rdius wo. In the first prt of the divergence length 2 f, the overlp between the pumped volume nd the TEMOO mode volume is rod, TEmoo mode. At point x intensity decrese cn be expressed by dl I (- ds/s (x) dx) = where S(x) is the bem section nd the bsorption coefficient. ds/s (x) is due to the bem So " / 2 is the input intensity. I

5 1706 JOURNAL DE PHYSIQUE III N 9 Nd;yAG rod monomode volume 21 _1_ 1' _j (, Fig. 3. Optimum pump focusing spot position in n end pumped lser. The output pumping power inside the cvity mode, of section So, is given by ' Finlly, the efficient prt of the bsorbed power is the rtio j ' stck pump source, even if severl drwbcks prevented priori to n esy implementtion high power 2-D stcked rry dimensions re not well suited for focusing into Circulr the high bsorption coefficient genertes smll pumped volume nd my cuse n bem pump _q / j = l tg W0 When this prmeter is introduced in the clcultions, there is good greement with experiments. Medium power longitudinl pumping. The high efficiency of end-pumped lsers is due to the higher bsorption, the excellent between pump volume nd lser mode nd the higher pump densities. In order to overlp these dvntges, we hve developed n end-pumped Nd keep YAG lser using 5-br spot, overlp between pump volume nd TEMOO mode volume is more difficult consequently with smll rry, thn excessive locl therml loding.

6 experiments including fiber coupled lser diode brs s pump source hve been Mny minly in the cw regime. Fiber coupling which is not too difficult to implement designed, single br, becomes n lmost impossible tsk with stck, becuse of the required using ccurcy positionning. Thus, longitudinl pumping hs been limited to low energy high developed given figure 4. The output from the stck (SDL 3230-TZD) is imged hve is in of the Nd YAG rod with four lens ssembly two 19 focl length end the into As mny rys re incident t lrge ngle, optics hve been coted with single components. AR coting. The experimentl trnsmission of the lens ssembly is 75 fb which is lower lyer For multimode opertion 200 rdius concve coupling-mirrors re used. Up to 12.5 mj, 1064 nm pulses re obtined in gin-switch regime. In Q-switch regime with electro-opticl switch, thin film Brewster polrizer nd AM plte inserted in the LiNbO 7.5 mj pulses re emitted. These results re obtined t pulse-repetition rte of cvity, /j lens f=19 Fig. 4. Schemtic of 5 br stck end pumped Nd YAG lser. N 9 LASER DIODE PUMPED SOLID STATE LASERS 1707 devices. We report, in this chpter, the design nd opertion of 5-br stck longitudinlly end Nd YAG lser. The gol ws to focus the emitting re of lo x 1.6 into the pumped dimeter Nd YAG rod with the minimum losses. The schemtic of the lser tht we 3 lenses, 30 focl length sphericl lens, nd 12 focl length spheric lens. cylindricl trcings in the two orthogonl plnes show tht, in spite of the strong divergence nd Ry emitting re of the diode stck good del of the bem is focused in spot smller thn lrge in dimeter (Fig. 5). The focusing device ws constructed from stndrd opticl 2.5 thn the theoreticl trnsmission (85-92 fb) clculted from typicl trnsmission coefficients to 2 fb for single lyer coting). This is due to noticeble vignetting by the stndrd (I lens. Only 71fb of the power emerging from the lens ssembly is trnsmitted sphericl 2.5 dimeter diphrgm plced in the device focl plne. In fct finest 3D through showed tht some skew rys were missing the min spot. simultion The NdYAG dichroic coting trnsmission being 85fb, the overll trnsmission of the coupling is 45 fb into 2.5 dimeter Nd efficiency YAG rod nd 52 fb into dimeter one. The experiments were performed with 3 dimeter rod. 3 Hz. For TEMOO opertion plne-plne cvity ws used. In fct bove pulse-repetition 50 of 25 Hz, the therml loding in the rod is sufficient to stbilize the cvity therrnl rte clcultion shows tht the bem-wist wo of the mode is respectively I nd focusing t pulse-repetition rtes of 25 nd loo Hz, Theses vlues (bout 2/3 of the rod 0.75 " ), stck polrizer NdYAG?P fll E.O. spheric ' ' cylindricl

7 % '". "' ".- /, _/ 3.5, y i,' ' I i jj Fig. 5. Ry trcing of the 5 br stck focusing in the stndrd opticl component device. the cvity output mirror, indictes nerly Gussin mode with bem wist wo = 0.85 chieved using clibrted holes in the focus plne of 0.5 m focl length lens. Results re plotted in figure 6. The theoreticl curve corresponds to truncted Gussin mode. There is 0,2 0,4 0,6 0,8 1,0 U,0 Diphrgm 1708 JOURNAL DE PHYSIQUE III N 9 I J ;/ '-j,r lp,16' o,s ' It /i re dequte for generting little loss on the fundmentl TEmoo mode while rdius) ll higher order modes. A precise mesurement of the ctul mode size, close to suppressing t pulse-repetition rte of 25 Hz. An estimtion of the fr field energy distribution ws good greement between the experimentl points nd the theoreticl curve corresponding dimeter 6. Fr field energy distribution in the focus plne of 0.5 m focl length lens for the 5 br stck Fig. pumped Nd YAG lser. Comprison with truncted Gussin mode theoreticl curve. end

8 r = to the poor fill fctor of the cvity mode, the mximum output energy is 3.5 mj per Due t pulse repetition rte of 25 Hz nd 3 mj t 100 Hz the pulse durtion is 20 ns. We pulse hve performed stndrd Frntz-Nodvick simultion introducing ctul vlues for intrcvi- losses nd mode overlp. The theoreticl nd experimentl curves re in good greement ty nd slope) if one estimtes the storge efficiency (807 nm bsorbed energylemitted (threshold rgue s previously for the pump bsorption becuse the pump spot dimension is now of the improvements re in progress to increse the output energy. First, the focusing Severl will be coted with more efficient ntireflection coting ; the vignetting will be optics experiment presented in this section ws implemented to pprecite the temperture The cceptnce of diode pumped lser hed. This is quite importnt prmeter for lser drift systems which suffer lrge temperture vritions s in militry nd spce pplictions. the dependence of the diode wvelength is pproximtively 0.3 nm/ C, nd the temperture of the Nd YAG min bsorption pek round 807 nm is bout 6 nm. These vlues FWHM in mind tht the vrition of the output power versus the diode junction temperture ber be high. In fct, severl fctors ply n importnt role, s the spectrl width of the should emission nd the length of the bsorbing medium. The globl informtion cn redily diode obtined from sclble diode pumped lser hed. So we hve built side pumped be diode brs re regulrly rrnged round the rod in 5 groups of 2 brs ech. Ech group pump coupled by thermoelectronic cooler to het sink with liquid coolnt. Thus ech group is serie do not emit the sme energy for the sme injected current. In lrger systems more Free-running experiments were performed. The lser resontor consisted of I m rdius of N 9 LASER DIODE PUMPED SOLID STATE LASERS 1709 to Gussin mode with ner field wist wo=0.85 nd truncture rdius 1.5. This proves the high mode qulity obtined. Stck energy) equl to 45 fb. It is slightly lower thn the expected vlue of 52 fb. We cn't sme order of mgnitude s the rod dimeter. The rys re diffused by the grounded side of the rod nd due to the highly divergent pump bem some rys escpe the rod before being well bsorbed. nd the focused spot dimension will be decresed by employing non stndrd suppressed Then, the rod cylinder will be Ag coted to trp ll the pump light inside the rod. lenses. Extrction efficiency could be improved by using n unstble cvity design with Gussin output mirror, but this geometry is not esy to set up with such little rod reflectivity Nevertheless, we expect up to 5 mj per pulse in dimension. TEMOO Q-switch mode. Medium power side pumping Side pumping is expected to produce non homogeneous intensity distribution cross the section of rod. Diode brs re discrete elements nd the fluorescence intensity figure will retrce the symetry of the pumping rrngement. In fct, if gret number of diodes re distributed long rod, one cn obtin n lmost homogeneous intensity figure. Nd YAG lser. The Nd YAG rod 4 in dimeter nd 75 long is enclosed in wter jcket. The 10 my be finely djust. The brs were ssocited in tking into ccount their temperture wvelength. The overll dimensions of brs, thermoelectronic cooler nd het sink emission close coupling. Consequently the output of the 2 brs of group is imged into the prevent YAG rod with 4 qurtz rod lens set 0.4 in front of the diode fcets. Nd 7 nd 8 show the pump density distribution through fluorescence mesurements Figures 7 for single br pumping, figure 8 for 10 brs pumping. The distribution hs centrl figure intensity but n intense zone is seen on the left bottom of the rod. This is more pek on 3D representtion (Fig. 9). This is due to the fct tht the brs connected in noticeble uniform integrted distribution will be obtined.

9 Fig. 7. Fig. 8. Fig Single br pump power distribution within the NdYAG rod Fig. fluorescence output mesurement. Fig, br pump power distribution within the Nd YAG rod mesured fluorescence output Fig. 9, 3D distribution of the mesured fluorescence output distribution of figure 8, curvture concve high reflector nd flt 80 fb reflector. Mirrors re seprted by 31 cm. nd the wter jcket tube. At pulse-repetition rte of 100 Hz, Nd-YAG output power ws for free running stble multiple trnsverse mode oscilltion. A mximum output mesured energy of 27.5 mj ws generted with totl pump energy of 110 mj by cooling the pulse 1710 JOURNAL DE PHYSIQUE III N 9 mesurement. The coupling between lser diodes nd Nd YAG rod ws evluted t 55fb (rtio of bsorbed power to emitted power). It cn be incresed by AR coting the cylindricl lenses 10 brs to 2.5 C, to shift the center wvelength of the emission to 807 nm. An individul fine tuning of ech group temperture doesn't increse the lser performnce. This my be due to the overlp of the br emission spectrums which fll within 3 nm, compred to their men FWHM 2.5 nm. If we consider the lser diode efficiency equls to 35 fb, our Nd-YAG lser overll efficiency is bout 9fb. This is typicl vlue. Figure 10 shows the

10 N 9 LASER DIODE PUMPED SOLID STATE LASERS 1711 # 85A _ h I YAG lser men output power (100 Hz) versus diode current nd diode temperture Nd tht the output coupling ws not optimized in this experiment). Only one side of the (note curve hs been explored becuse of the wter deposition risk t low temperture. bsorption this rther smll structure we hve shown possible ± 5 C temperture vrition if 10 fb In geometry, the temperture stbiliztion of the lser diodes is not criticl, even with pumping Nd YAG this nrrow» bsorption line. «y j 10 il ii Fig, it- Criticity of the temperture stbiliztion in lo br pumped Nd YAG lser. lser sources. Replcing flsh lmps by lser diodes llows higher repetition rtes, pumped energy storge or better bem qulity due to weker therml loding (bout three times lrger JOURNAL DE PHYSIQUE III -T 2, N'9. SEPTEMBER " 65A fiko 55A g# T( C) Fig, 10, Evolution of the 10 br pumped Nd YAG output power versus I (stck) nd T (stck). lser decrese is llowed (Fig. I I). This vlue is lrger thn previously expected even output vlue hve been reched for lrger bsorption pth. We conclude tht in such higher ' I(br) 85 A = Ql bo O fi Ql g 5 C T(OC) Conclusion. The experiments described in this letter were performed with the objective of scling diode less thn with flsh pumping). Severl criticl prmeters hve been studied in rod configurtion evlution of the coupling losses, distribution of the pumping intensity,

11 high power lser diodes re still expensive, pplictions where efficiency, Although nd bem qulity re of prime interest my require lser diodes rther thn flsh relibility ZHou B. et l., Opt. Lent. 10 (1985) 62. [Ii BERGER J. et l., APL 51(1987) [2] HANSON F, et HADDOCK D., Appl. Opt. 27 (1988) 80. [3] GERSTENBERGER D. C., Opt. Lent. 15 (1990) 124. [4] KOECHNER W, et l., SATE Opt. Spce Coun. II 1522 (199I) 169. [5] BERGER J. et l., Opt. Lent. 13 (1988) 306. [6] FAN T. Y., SANCHEZ A., DEMO W. E., Opt. Left. 14 (1989) [7] LYNN J. G., STONEMAN R. C. et EsTERow1Tz L., I99I Advnced Solid Stte Lsers Conference, [8] 1712 JOURNAL DE PHYSIQUE III N 9 criticity. Two pumping geometries, longitudinl nd trnsverse, hve been temperture ; longitudinl pumping leds to good results in the Q-switched TEMOO mode, but depicted seems difficult to scle to more thn few millijoules per output pulse. With side pumping, energy deposition hs been obtined centrl pek intensity should privilege TEMOO good nd better distribution will be relized using lrge number of brs high output mode is possible. Future studies include lso diode pumping of high verge power slb energy [10]. lsers In mny cses (industril, militry or even scientific pplictions) the use of lser lmps. should decrese considerbly the mintennce costs. diodes References Hilton Hed, pper WB-2. BERGER J, et l., Electron. Lett. 23 (1987) 669. [9] LONG L, et l., 1989 Advnced Solid Stte Lsers Conference, Slt Lke City, postdedline pper [10] MA7PD.