Comparison of cw laser performance of Nd:KGW, Nd:YAG, Nd:BEL, and Nd:YVO 4 under laser diode pumping

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1 Appl. Phys. B 67, (1998) Applied Physics B Lasers and Optics Springer-Verlag 1998 Comparison of cw laser performance of Nd:KGW, Nd:YAG, Nd:BEL, and Nd:YVO 4 under laser diode pumping A.A. Demidovich 1, A.P. Shkadarevich 1, M.B. Danailov 2,P.Apai 2,T.Gasmi 2, V.P. Gribkovskii 3,A.N.Kuzmin 3, G.I. Ryabtsev 3,, L.E. Batay 3 1 Inst. of Molec. & Atom. Physics, F. Skaryna Ave. 70, Minsk, Belarus (Fax: / ) 2 ICTP Laser Lab, Trieste, Italy (Fax: / ) 3 Stepanov Institute of Physics, F. Skaryna Ave. 70, Minsk, Belarus (Fax: / ) Received: 4 August 1997 Abstract. We have compared the cw laser performance of Nd:KGW, Nd:YAG, Nd:BEL, and Nd:YVO 4 crystals under low-power laser diode end-pumping. Output power dependencies on the pump power and the pump wavelength of these diode-pumped solid state lasers were investigated. The high Nd 3+ concentration of the Nd:KGW samples used in our measurements as well as up-conversion and exited-state absorption processes in Nd:KGW cause the reduced laser output power dependence on the pump wavelength which was experimentally observed. At pump levels up to 270 mw a slope efficiency of η sl 46% was reached for the Nd:KGW laser. Nd:KGW microchip laser operation with a slope efficiency of η sl 50% was demonstrated. Thermal lensing in Nd:KGW at pump powers up to 3Wwas measured. PACS: Rz Compact diode-pumped solid-state lasers radiating in the IR and green spectral ranges can be created on the basis of laser crystals with a high concentration of Nd 3+ ions and, as a consequence, with high absorption of pump radiation in a small crystal region. Among such crystals neodymiumdoped potassium gadolinium tungstate (Nd:KGd(WO 4 ) 2 or Nd:KGW) [1, 4] and lanthanum beryllate (Nd:La 2 Be 2 O 5 or Nd:BEL) [5 8] are known as effective laser media. An attractive feature of these crystals with anisotropic structure is polarized laser radiation, which is an advantage when a nonlinear crystal for second-harmonic generation is used [9]. A theoretical comparison of Nd:YAG and Nd:BEL efficiencies was carried out by Barnes et al. [7]. In our communication which compares cw low-power laser properties of different active media we experimentally investigated the output powers of Nd:YAG, Nd:KGW, Nd:BEL, and Nd:YVO 4 lasers versus laser diode pump power and pump wavelength under the same cavity configuration and pumping conditions. We demonstrate here the possibility of applying Nd:KGW to microchip-type lasers also. Because the thermal lensing effect IEEE Member plays an important role in microchip lasers the thermal lens was measured for Nd:KGW at pump powers up to 3W. A multimode laser diode (LD) with about 320 mw output power mounted on a thermoelectric cooler (TEC) was used for pumping the crystals. The LD wavelength tuning range provided by the TEC was nm. The optical system for pump beam focusing into the laser crystal consisted of a triplet collimator (NA = 0.5), 4 cylindrical telescope, and focusing lens ( f = 10 mm). The LD radiation of 280 mw power behind the optical system was focused in a pump spot with a diameter of about 80 µm. The cavity configuration was nearly hemispherical and included an HR dielectric mirror on the input crystal facet and an external spherical mirror as output coupler. The cavity length l and output coupler curvature radius r were 25 mm and 30 mm, respectively. Input facets of the pumped crystals were AR-coated at 810 nm. For pump radiation suppression the infrared filter IKS-22 was placed behind the output coupler. This filter served as a laser beam splitter directing laser radiation to the power meter and to the spectral multichannel analyser simultaneously. The main characteristics of the tested crystals are presented in Table 1. Figure 1 shows the output power P out of Nd:YAG, Nd:KGW, Nd:BEL, and Nd:YVO 4 lasers versus the pump power P pump for output couplers with different reflectivities. In all experiments the beam profile was TEM 00. As can be seen, the optimised outcoupling mirrors have the reflectivities R = 0.98 for Nd:KGW and Nd:BEL, R = 0.94 for Nd:YAG, and R = 0.90 for Nd:YVO 4 lasers. For better analysis of the data obtained we have evaluated the material/coating loss value L (see Table 1) by fitting slope efficiencies η sl measured for output couplers with reflectivities R = 0.90, 0.94, 0.98, and 0.99 to the relation η sl = η 0 T/(L + T),whereT=1 R. The highest slope efficiency η sl = 46% with optimised output coupler was reached for the Nd:KGW laser. The low pump power and small Nd 3+ concentration in the Nd:YAG and Nd:YVO 4 crystals used in our experiments (see Table 1) explain the low slope efficiency for these media in comparison with the results of other authors. Dependencies of the output power on the pump wavelength for different crystals under the same cavity config-

2 12 Table 1. Characteristics of the tested crystals Nd:YAG Nd:KGW Nd:BEL Nd:YVO 4 Formula Nd:Y 3 Al 5 O 12 Nd:KGd(WO 4 ) 2 Nd:La 2 Be 2 O 5 Nd:YVO 4 Orientation [100] along b along a along a Length /mm Nd concentration /at.% Fluorescence lifetime /µs Material/coating loss Transmittance at 809 nm/% Emission wavelength /nm Peak emission crosssection /cm [7] 3.8 [2] 0.5 [5] 1.5 [5] 15.6 [13] Fig. 1a d. Output power P out of Nd:YAG (a), Nd:KGW (b), Nd:BEL (c), and Nd:YVO 4 (d) lasers vs absorbed pump power P pump for output couplers with R = 0.98, R = 0.94, and R = 0.90 uration are shown in Fig. 2. The maximum output power for all lasers coincides with the maximum of material absorption: 808.5nmfor Nd:YAG, 810.5nmfor Nd:KGW and Nd:BEL, and 809 nm for Nd:YVO 4. Only for the Nd:KGW laser with output couplers of R = 0.94 and R = 0.90 did we observe a decreasing output power at the maximum crystal absorption. Simultaneously, intensive yellow fluorescence in the Nd:KGW crystal under operation with R = 0.94 and R = 0.90 couplers took place. Yellow fluorescence in KGW under LD pumping was also seen by Flood et al. [3]. This phenomenon as evidence of up-conversion was associated with two possible processes energy transfer up-conversion and excited-state absorption (ESA). These processes cause a reduction of the upper laser level population and therefore a decrease in the laser efficiency. On the other hand, when intensive up-conversion and ESA take place in a crystal, a decreased output power dependency on the pump wavelength is observed (R = 0.94 and R = 0.90 curves in Fig. 2b). We evaluated relative differences between the maximum and the minimum of the normalized laser output P out and LD absorption P abs for Nd:YAG and Nd:KGW lasers in a wavelength range of ±1nmat the crystal absorption peak (Fig. 3). Dependencies of the absorbed LD power on the pump wavelength were calculated from the overlap of the LD spectra and crystal absorption. All laser power measurements were made for cavity conditions with optimised output cou-

3 13 Fig. 2a d. Output power P out of Nd:YAG (a), Nd:KGW (b), Nd:BEL (c), and Nd:YVO 4 (d) lasers vs pump wavelength λ at constant incident pump power for output couplers with R = 0.98, R = 0.94, and R = 0.90 pler and TEM 00 mode operation. The results are presented in Table 2. For the comparison of the output power sensitivity on pump wavelength we used the ratio P out / P abs.inthe case when P out / P abs = 1 only the crystal absorption spectrum is responsible for the P out (λ pump ) dependence. For the Nd:YAG laser we obtained P out / P abs 4.5. The fact that P out / P abs for Nd:KGW is closer to 1 than P out / P abs for Nd:YAG can be explained by considering the high Nd 3+ ion concentration in the Nd:KGW crystal used in our experi- ments. If the pump wavelength does not coincide with the maximum of the absorption spectrum for crystals with low Nd 3+ concentration, as in the case of Nd:YAG, the pump radiation is not fully absorbed in a small crystal region (l eff in Fig. 4) where effective energy exchange between pump radiation and lasing cavity modes takes place. A part of the absorbed pump beam does not support the laser mode, and the laser efficiency is decreased. Another dependence is observed in the case of crystals with high Nd 3+ concentration where Fig. 3a,b. Dependencies of normalized output P out and absorbed pump power P abs on pump wavelength λ of Nd:YAG (a) andnd:kgw (b) lasers

4 14 Fig. 4. Scheme of pump beam and TEM 00 cavity mode overlapping in the lasing crystal Table 2. Differences between maximum and minimum of normalized DPSSL output P out and LD-absorbed P abs powers for Nd:YAG and Nd:KGW lasers in the wavelength range of ±1nmat the crystal absorption peak P out /% P abs /% P out / P abs Nd:YAG Nd:KGW almost all pump radiation is absorbed in the region marked by l eff in Fig. 4 even when the pumping wavelength does not coincide with the absorption maximum. As was demonstrated above, ESA processes in Nd:KGW media can reduce P out / P abs. However, in our analysis the experimental data related to the cavity configuration where an output coupler reflectivity of R = 0.98 was used, where the yellow fluorescence was extremely weak (see Fig. 2b). High Nd 3+ concentrations in an active medium allow the use of short crystals (less than 1mm) in LD-pumped solidstate lasers. In this case the focusing system of the pump unit can be simplified or can be absent. Here we present results of direct LD end-pumping of 8% Nd:KGW without any focusing system between the LD and solid-state medium. The configuration is shown in Fig. 5. A Nd:KGW crystal with dimensions 3mm 1mm was attached to the front LD facet. The distance between the LD output mirror and the crystal facet was 15 µm, so the spot size of the pumping beam at the rear crystal facet was estimated to be 100 µm 50 µm. Figure 6 shows the output power of the miniature Nd:KGW laser with different output coupler reflectivities (output coupler radius of curvature was r = 40 mm) versus the pump power. The best result was obtained with an output coupler reflectivity R = 98%. The slope efficiency in this case reached up to 23%. We obtained microchip Nd:KGW laser operation also. In contrast to the scheme in Fig. 5 an optical system in the pump unit was used and a short cavity (l 1mm) with a plane output mirror was built. A slope efficiency of 50% at a reflectivity of R = 0.98 for the output coupler in the microchip type resonator was reached. Thermal lens effects play an important role in mediumand high-power LD-pumped solid state lasers. Since the thermal conductivity of Nd:KGW is smaller and its thermo-optic coefficients are higher than those of Nd:YAG, strong deterioration of the laser performance might be expected even at moderate pump powers. To study this effect we have performed experiments at pump powers up to 3W. The pump source was a fibre-bundle-coupled diode array (OptoPower Corp. Mod. E FCPC). The pump light from the bundle was imaged into the crystal face by a combination of a single lens and a high-numerical-aperture lens giving a smooth spot of 350 µm in diameter. A µm AR/ARcoated, wedged, 3.2%-doped KGW crystal was studied. It was placed in a cavity formed by a HR µm spherical mirror (r = 154 mm), and a plane output coupler with reflectivity of 97.5%. The crystal was wrapped in indium foil and mounted in a water-cooled aluminium holder. The transverse mode was adjusted to nearly pure TEM 00 by moving the crystal along the cavity axis and finding the position with proper ratio of the pump spot size to the cavity mode spot size. The typical slope efficiency of the laser in this configuration was 25% with a good spatial mode quality. The relatively low efficiency (for example, compared to the results presented in [2]) arises mainly from the high reflectivity of the AR coating (0.7% per face). The thermal lens was calculated by measuring the spot size at a fixed distance from the laser taking into account the fact that the beam waist is always located at the plane output mirror and using the embedded-mode [10] notation. The measurements were done both in the plane of polarization and in the perpendicular plane, since the spot was astigmatic at high pump levels. The calculated thermal lens focal distance is presented in Fig. 7 for the two planes. As can be seen, strong negative lensing is observed in the plane of polarization and the lensing in the perpendicular plane is Fig. 5. Configuration of the short-crystal Nd:KGW laser with direct end-pumping (l = 15 µm, l = 1mm)

5 15 Fig. 6. Output power P out of directly end-pumped short-crystal Nd:KGW laser vs pump power P pump weaker and positive. The measurement was verified by inserting known cylindrical lenses in the cavity at low pump levels and determining their focal length by the same method. The accuracy was better than 10%. We should note that although there is strong astigmatism in Nd:KGW, thermal lensing can be expected because of its high anisotropy (thermal conductivity and thermo-optic coefficient strongly vary with direction). Both the measured sign and magnitudes are difficult to link to the available thermo-optic coefficients. Further understanding of the matter would require more data on the relative refractive index changes and face deformation due to the thermal lensing, for example by interferometric measurements [11]. Similar thermal lens behaviour is observed in Nd:YLF in the σ plane [12] and can be well compensated by using an intracavity cylindrical lens. In our case this was not necessary, since the laser output power was not affected by the lens at the maximum pump level applied. If a higher pump power or a smaller pump spot size, however, are used in a Nd:KGW laser, an astigmatic thermal lens compensation has to be taken into account. In our experiments with Nd:BEL we have obtained simultaneous two-wavelength operation. Figure 8 shows typical lasing spectra of Nd 3+ -doped lanthanum beryllate. The Nd 3+ fluorescence maxima of the 4 F 3/2 4 I 11/2 transition for E X, E Y, and E Z are1070 nm and 1079 nm [5]. This Fig. 7. Focal distance f of thermal lens in Nd:KGW versus pump power P pump ( f v - in plane of polarization, f h -in perpendicular plane) Fig. 8. Nd:BEL lasing spectrum is in good agreement with the peak emission cross section for the crystal orientation along the a axis. The two-wavelength operation of the Nd:BEL laser may be useful for spectroscopic and calibration applications because no appreciable wavelength shift of spectral lines as a function of temperature was detected [5]. In conclusion, cw laser performances of Nd:KGW, Nd:YAG, Nd:BEL, and Nd:YVO 4 crystals with low-power laser diode end-pumping have been compared. Output power dependence on the pump power and the pump wavelength for lasers with these active media was investigated. The high Nd 3+ concentration of Nd:KGW samples used in our measurements as well as up-conversion and exited-state absorption processes in Nd:KGW cause a reduced laser output power dependence on the pump wavelength which was experimentally observed. For pump levels up to 270 mw a slope efficiency of η sl 46% was reached for Nd:KGW. Nd:KGW microchip laser operation with a slope efficiency of η sl 50% was demonstrated. Thermal lensing in Nd:KGW at pump powers up to 3Wwas measured. Acknowledgements. The work was partially supported by ISTC Project #B-082. References 1. S.L. Galkin, A.L. Zaakgeim, V.M. Makarov, V.M. Nikolaev, A.A. Pavlyuk,I.P. Petrovich,V. Yu. Petrun kin, A.P. Shkadarevich,V.D. Yarzhemkovskii: J. Appl. Spectrosc. 37, 886 (1983) 2. J.M. Esmeria, H. Ishii, M. Sato, H. Ito: Opt. Lett. 20, 1538 (1995) 3. C.J. Flood, D.R. Walker, H.M. van Driel: Appl. Phys. B 60, 309 (1995) 4. Y. Chen, L. Major, V. Kushawaha: Appl. Opt. 35, 3203 (1996) 5. H.P. Jenssen, R.P. Begley, R. Vebb, R.C. Morris: J. Appl. Phys. 47, 1496 (1976) 6. R. Scheps, E.J. Schimitschek, J. Myers, D.F. Heller: Opt. Eng. 27, 830 (1988) 7. N.P. Barnes M.E. Storm, P.I. Cross, M.W. Scolaut: IEEE J. Quantum Electron. QE-26, 558 (1990) 8. K.D. Li, J.A. Sheridan, D.M. Bloom: Opt. Lett. 16, 1505 (1991) 9. M. Oka, S. Kubota: Opt. Lett. 13, 805 (1988) 10. P.A. Belanger: Opt. Lett. 16, 196 (1991) 11. C. Pfistner, R. Weber, H.P. Weber, S. Merazzi, R. Gruber: IEEE J. Quantum Electron. QE-30, 1605 (1994) 12. G. Cerullo, S. De Silvestri, V. Magni: Opt. Commun. 93, 77 (1992) 13. D.G. Matthews, J.R. Boon, R.S. Conroy, B.D. Sinclair: J. Mod. Opt. 43, 1079 (1996)