Spectroscopy, modelling and laser operation of holmium doped laser crystals

Transcription

1 Spectroscopy, modelling and laser operation of holmium doped laser crystals Y. Kalisky, S. Rotman, G. Boulon, C. Pedrini, A. Brenier To cite this version: Y. Kalisky, S. Rotman, G. Boulon, C. Pedrini, A. Brenier. Spectroscopy, modelling and laser operation of holmium doped laser crystals. Journal de Physique IV Colloque, 1994, 04 (C4), pp.c4-573-c /jp4: jpa HAL Id: jpa Submitted on 1 Jan 1994 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE IV Colloque C4, supplcment au Journal de Physique 111, Volume 4, avril1994 Spectroscopy, modelling and laser operation of holmium doped laser crystals Y. KALISKY*>(~), S.R. ROTMAW*, G. BOULON***, C. PEDRINI*" and A. BRENIER*** * Laser Department, Nuclear Research Centre-Negey PO. Box 9001, Beer-Sheva 84190, Israel ** Ben Gurion University of the Negev, Department of Electrical and Computer Engineering, Beer-Sheva 84105, Israel *** Laboratoire de Physico-Chimie des Matkriaux Luminescents, Universitc? Lyon I, 43 Bd du 11 Novembre 1918, Klleurbanne cedex, France ABSTRACT Spectroscopic studies using laser induced fluorescence and numerical modelling of energy transfer and back transfer mechanism are reported in Er:Tm:Ho:YLF, Cr:Tm:Ho:YAG and Cr:Tm:YAG laser crystals at various temperatures (10 K-300 K). Direct energy transfer from ~ m excited ~ + states to HO~+ 'I~ emitting level was observed and analyzed both in YAG and YLF. Further analysis of cr3+ and ~m3+ time dependent emission curves indicate strong correlation of chromium-thulium pairs. Pulsed operation of holmium laser at high temperature will be presented. 1. INTRODUCTION Solid state laser crystals doped by HO~+, ~m~+, and ~ r have ~ + recently attracted attention for their medical application, eye-safe rangefinders, light detection and ranging (LIDAR) applications(l), wind velocity measurementd2) and atmospheric measurements to monitor the health of our planet(3). HO~+ laser emitts at -2.1 pm resulting from the 5~7 -+ 5~8 electronic transition. This emission has a relatively high stimulated -emission cross-section, and a long lifetime of the emitting level (7-14 msec) which makes it a good candidate for Q-switched operation. (') Present address: Innovative Lasers Corp., 3280 South Hemisphere Loop, Suite 120, Tucson, AZ 85706, U.S.A. Article published online by EDP Sciences and available at

3 C4-574 JOURNAL DE PHYSIQUE IV HO~+ is a quasi-three level laser system and requires cooling for an efficient erati ti on.(^?^) Codoping HO~+ with Tm3+, ~ r or ~ cr3+ + leads to an efficient laser operation at -2.1 pm due to energy transfer from ~ r3+ or ~ r3+ (4T2 and 4T1 levels) to 3~ level of Tm3+, and then by a double cross relaxation to 517 electronic level of Ho 4+. When higher repetition rates are required, a cooling system has to be used for eliminating thermal effects in the laser crystal and maintaining good beam quality. In these cases the use of YLF crystal as a host is favorable due to its better optical properties such as a negative change of refraction index with temperature, low phonon energies (which results low non-radiative rates) and low UV absorption which minimizes the formation of color centers. Therefore, we have initiated an investigation towards operation and performance of a pulsed apho:ylf laser using fieon as a coolant liquid in a closed cycle cooling system in temperatures of -looc and -50 c. Moreover, in order to obtain an efficient flashlamp pumped or diode pumped holmium laser it is necessary to understand and improve energy transfer and cross-relaxation processes from chromium and thulium ions to 517 level of holmium and also controlling the back energy transfer from 5 ~7 emitting level of holmium to the thulium 3 ~ level. 4 Especially, the exsitence of several different types of sites for the thulium, chromium and holmium iond6) with strong coupling between energy donors and acceptors can lead to a wavelength dependent pumping scheme tailored to the most coupled sites. Therefore we have also studied the spectroscopic properties and multisite dynamics of energy transfer in a doped YAG system. 2. EXPERIMENTAL Pulsed laser operation of apho:ylf and the experimental set-up which includes laser head, resonator, pum ing sources and a closed cycle cooling system were the same as previously described& Experimental set-up for low temperature absorption spectra and fluorescence decays were described in details el~ewhere.(~.~) Details of the theoretical model and the mathematical technique were described in Ref 7, and will be discussed in the following sections. The nominal concentrations (in %at.) of the samples were: ~r3+ (30%), Tm3+ (lo%), HO~+ (0.3%): YLF, supplied by Litton Airtron and used for spectroscopy and laser operation. For energy transfer studies we used cr3+(1%), Trn3+(5.76%), HO~+(O.~~%):YAG and ~r3+(1%), Trn3+(2.9%):y~G supplied by Dr. L. Esterowitz of the US Naval Research Laboratory. 3. RESULTS 3.1 Absorption spectra Absorption spectra as well as spectroscopic assignment of electronic transitions for apho:ylf at 10 K in 380 nm - 2 pm has been performed and reported. From the absorption spectra we were able to excite selectively the desired levels of rare earth ions participating in the energy transfer process. To study energy transfer from thulium to holmium (and the back transfer) we excited into the following levels of Tm3+:

4 C H 6 -> 3 H 4 (789 nm), 3 H 6 -> 3 H 5 (1.171 jam), and 3 H 6 -> 3 F 4 (1.675 urn). We also excited Cr 3+ ( 4 A 2 -> 4 T2) transition by 532 nm or nm. We have followed the temporal behavior of ^1-j level of holmium and 3 F 4 level of thulium as we be discussed in the following sections. 3.2Er:Tm:Ho:YLF The lifetime of 5 l7 level of Ho 3+ doped YLF excited via energy transfer fro 3 F 4 (Tm 3+ ) exhibited exponential decay with a lifetime of T= msec. The built-up of holmium ^17 level upon thulium 3 F 4 excitation was also observed with a built-up duration, defined by the time interval from 10% to 90% of the maximum amplitude, of 44 usee. This risetime became larger, up to 72 usee when excited into higher energy levels of Tm 3+, such as 3 H 4 level. On the time scale of msec, Tm 3+ ( 3 F 4 ) level decayed exponentially with a lifetime x=6.4 msec. This level also showed a relatively fast built-up (as defined above) with values of 1.8 usee and 1.6 (isec for YAG and YLF, respectively, when excited into higher levels of thulium. 3.3 Cr:Tm:Ho:YAG Holmium emission originating from ^Ij level was obtained by excitation to higher levels of Tm 3+ ( 3 H 6 -> 3 H 4 or 3 H 6 -> 3 H 5 ) as well as into Cr 3 + ( 4 T 2 level), peaking at nm. For the spectroscopic scheme of Cr:Tm:Ho:YAG. In all cases energy was transferred from the upper excited levels of thulium to the ^Ij level of holmium. The energy in this level was built-up in an average time of usee, and decayed exponentially with T = 7.8 msec. Thulium 3 F 4 level in the Cr:Tm:Ho:YAG exhibited an exponential decay on a msec time scale, with a lifetime of t = 7.2 msec and a relatively fast built-up (1.8 usee). It is worthwhile to mention that a direct energy transfer from 3 F 4 level of Tm 3+ to ^7 level of Ho 3+ was also observed. When excited at um ( 3 Hg -» 3 F 4 transition), we could observe a built-up of 5 l7 level (~50 usee, 10%-90% of maximum amplitude), and a decay of ~7.8 msec. 3.4 Energy transfer vs. back transfer A close inspection at higher temporal resolution of Tm 3+ ( 3 F 4 -> 3 H6) and Ho 3+ (^7 > ^ig) decay curves shows that Tm 3+ decay is a combination of "fast" (~100 usee) and slow (7-8 msec) components. The Ho 3+ decay, on the other hand, is a single exponential (~8 msec) with a built-up duration of the same order of magnitude as the Tm 3+ "fast" component. We conclude that the faster component of thulium decay is responsible for the energy transfer to holmium ^lj level, which is reflected by the duration of the built-up, while the longer component of thulium decay (with a lifetime comprable to

5 C4-576 JOURNAL DE PHYSIQUE IV holmium decay) is a result of a back transfer process. The same picture is observed in apho: YLF. 3.5 Theoretical model The theoretical model developed by Rotman et al. (6>7) assumes that there is a correlation between the presence of energy donors and the spatial distribution of energy acceptors. It was found that there is a correlation between cr3+ and ~ m sites ~ in + Cr:Tm:Ho:YAG and that these sites exhibit different spectral properties.(8) As an example, ~ m (paired) ~ + decaying at nrn which was excited into nm of paired chromium exhibits a strong correlation placement while in the case of ~m3+ unpaired pumped via cr3+ unpaired no positional correlation is observed and the decay fits the standard Forster-Dexter model. 3.6 Laser Performance Laser output energy versus lamp electrical input energy for apho:ylf at -100~ and -50 c was performed using various temporal pulse lengths. A maximum output energy of 1.5 Jlpulse (slope efficiency of -1.3%) for 1.0 msec pulse length was reported at -100~. An increase in laser output to 2.5 Jlpulse and in the total efficiency was obtained by lowering the crystal temperature to -50 c. The effects of the current density passing across the flashtube on laser performance were also studied at these temperatures. In both cases, a sharp decrease in laser efficiency was observed as the current density decreased below 2000~/cm2. REFERENCES 1. S. W. Henderson, C.P. Hale, J.R. Magee, M.J. Kavaya, and A.V. Huffaker, " Eye safe coherent laser radar system at 2.1 pm using Tm, Ho:YAG lasers", Opt. Lett. 16, , S.E. Moody, "Evaluation of laser technologies for an aircraft wind shear detection", in Laser Radar 11, R.J. Brecherer and R.C. Harney, eds., Proc. Soc. Phoyo-Opt. Instrum. Eng.783, , N.P. Barnes, "Solid state lasers for monitoring the health of planet earth", in Laser M2P fmateriaux Medecine et Biologie Phvsico-Chimiel Paper VCP1, Lyon, December 8-10, K.H. Kim, Y.S. Choi, N.P. Barnes, R.V. Hess, C.H. Bair, and P. Brockman, "Investigations of 2.1 pm lasing properties of Ho:Tm:Cr:YAG crystals under flash-lamp pumping at various operating conditions", Appl. Opt. 32(2), , 1993.

6 5. Y. Kalisky, J. Kagan, D. Sagie, A. Brenier, C. Pedrini, and G. Boulon, "Spectroscopic properties, energy transfer, and laser operation of pulsed holmium lasers", J. Appl. Phys., 70(8), , S.R. Rotman, A. Eyal, Y. Kalisky, A. Brenier, C. Pedrini, and G. Boulon, " Modelling unusually fast non-radiative energy transfer in crystals", in Second French-Israeli Meeting on Solid-State Lasers, Lyon, December 6-7, S.R. Rotman, Y. Kalisky, A. Brenier, C. Pedrini, and G. Boulon, "~m3+ decay in chromium-thulium-holmium-doped yttrium aluminum garnet at liquid helium temperatures", J. Appl. Phys. 72(1), , W. Nie, Y. Kalisky, C. Pedrini, A. Monteil, and G. Boulon, "Energy transfer from Cr3+ multisites to Tm3+ multisites in yttrium aluminum garnet", Opt. and Quant. Electron. 22, S123-S13 1, 1990.