High-power multi-pass pumped microchip Nd:GdVO4 laser

N. Pavel1,2 and T. Taira1

1Laser Research Center, Institute for Molecular Science, Okazaki 444-8585, Japan2Institute for Atomic Physics, Solid-State Quantum Electronics Laboratory, 76900 Bucharest, Romania

Phone: 81 0564 55-7346, Fax: 81 0564 53-5727e-mail: npavel@ims.ac.jp; npavel@pluto.infim.ro; taira@ims.ac.jp

Abstract A microchip Nd:GdVO4 laser with 13.9 Woutput power at 1.06 mm for an absorbed power of22 W and 0.65 slope efficiency is reportedemploying a multi-pass pumping scheme.

SummaryThe disk-like geometry uses an active medium

whose axial dimension is very small compared withthe transverse one. In this configuration the ratio ofthe cooling surface to the pumped volume is greatlyincreased, compared for example with rod or slablasers, and high output power can be obtained froma small pumped volume. Because the heat flow andaxis of the laser beam are collinear, the thermaleffects are reduced and laser operation with high-beam quality can be achieved employing simpleoptical resonators. This geometry presents, however,low absorption efficiency, the absorbing length beingtwice the crystal thickness. To increase theabsorption efficiency the pumping light has to be re-circulated in the medium: this can be achieved in amulti-pass pumping scheme [1, 2].

The multi-pass pumped thin-disk scheme hasproved to be very effective for Yb:YAG, a lasermaterial that under efficient laser operation hasthree times lower heat generation than in Nd:YAG.Excellent results, namely 647 W of continuous-wave(CW) output power at 1.03 mm with 0.51 efficiency,were demonstrated from a one-crystal thin-diskYb:YAG laser in a 16-pass pumping configuration [3].Recently, this scheme has gain interest and wasinvestigated for other laser crystals. Thus, CW high-power laser emission at 914 nm in Nd:YVO4 [4] andblue light at 457-nm based on a frequency doubledNd:YVO4 thin disk laser [5], high-power CWemission at 946 nm in Nd:YAG [6] and high-powerquasi-CW emission at 1.06 mm in Nd:YAG andNd:GdVO4 [7] were demonstrated.

In this work we report a CW high-powerNd:GdVO4 microchip laser: 13.9 W output power at1.06 mm with 0.65 slope efficiency in absorbedpower was obtained from a 250-mm thick, 0.5-at.%Nd:GdVO4 crystal in a 4-pass pumping schemeunder diode pumping at 808 nm. Laser emissionunder pumping at 879 nm, directly into the 4F3/2

emitting level, was also investigated.

The experimental set-up is shown in Fig. 1. Aslaser crystal we used Nd:GdVO4, a medium thatcombines the advantages of Nd:YVO4 and YAG.

While Nd:YVO4 presents by ~7.5 larger absorptioncross-section at 808 nm for p-polarization pumpingand by ~5 higher emission cross-section for the1064-nm emission than Nd:YAG, it has a lowthermal conductivity (~5.2 Wm-1K-1) that precludesa good dissipation of the heat generated in thecrystal by non-radiative processes. This limitation isnot encountered in Nd:GdVO4: while the 808-nmabsorption and 1063-nm emission cross-sectionsare similar to Nd:YVO4, the thermal conductivity ismuch larger, ~11 Wm-1K-1. Thus, Nd:GdVO4 seemsto be a good solution for thin-disk geometry.

Fig. 1 A sketch of the microchip Nd:GdVO4 laser in a4-pass pumping scheme.

Two Nd:GdVO4 crystals of 0.5-at.% doping leveland 400 and 250-mm thickness, fabricated by OxideCo., Japan, were used in experiments. The crystalhas one side high-reflection coated at the pumpingwavelengths of 808 and 879 nm and at the lasingwavelength of 1.06 mm: this surface was in contactwith a micro-channel cooling system and a heatconducting glue was used to decrease the thermalimpedance between these two surfaces. The otherside of the crystal was coated antireflection for1.06 mm and high-transmission for 808 and 879 nm.The optical pumping at 808 nm was made with a400-mm diameter, 0.22-NA fiber-coupled diode(HLU110F400, LIMO Co., Germany). A system offolding mirrors was designed to focus the pumpbeam in the Nd:GdVO4 to a spot diameter thatcould be varied between 0.9 and 1.5 mm; 2 or 4-passes through the medium could be thusobtained. The output performances were evaluatedusing a linear plane-concave resonator of 60-mmlength with an output mirror of 250-mm radius.

Figure 2 shows the output power at 1.06 mm forthe 400-mm thick Nd:GdVO4 crystal and a pump-spot diameter of 0.9 mm. CW maximum power(Pmax) of 11.3 W at 18.4 W absorbed power (Pabs)

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and slope efficiency in absorbed power (hsa) of 0.63was obtained for the 2-pass pumping and coolingwater temperature of 16oC. The output powerincreased to 14.2 W (Pabs= 25.7 W) when the 4-passpumping was used; 0.82 of the total pump powerwas absorbed in this scheme. The threshold Pth andslope efficiency hsa were 1.12 W and 0.58,respectively. We mention that when the pump powerwas further increased the Nd:GdVO4 microchipcracked: this was attributed to the low heat transferproperties of the glue used to contact the medium tothe cooling head.

Fig. 2 Output power at 1.06 mm versus Pabs at 808 nmfor the 400-mm thick Nd:GdVO4 crystal, output mirror with

transmission T= 0.03.

The output performances obtained with the250-mm thick Nd:GdVO4 crystal are presented in Fig.3. For 2-pass pumping at 808 nm Pmax was 9.5 Wat Pabs= 14.1 W with hsa of 0.68 (Fig. 3a). Under 4-pass pumping Pmax increased to 13.9 W (Pabs= 22W) with hsa= 0.65 and Pth= 0.56 W; the pump beamabsorption efficiency was 0.62. The beam has M2

factor of 10.3¥7.9. The temperature of the outputsurface of crystal was measured with a Chinoinfrared camera (FLIR System, Sweden). While for2-pass pumping the maximum temperature was121oC, it increased at 197oC for the 4-pass scheme.This proves a low heat transfer between the mediumand the cooling head. However, in spite of theseconditions, the laser operated tens of hours withoutnoticeable decrease of output power.

The Nd:GdVO4 crystal was also pumped at 879 nm, directly into the 4F3/2 emitting level, with afiber-coupled diode (fiber diameter of 400-mm, NA= 0.20) developed by Hamamatsu Co., Japan.The maximum output power was 2.3 W (Pabs= 3.8W) and 3.6 W (Pabs= 6.2 W) with hsa= 0.71 and 0.69for the 2- and 4-pass pumping, respectively (Fig. 3b).Compared with pumping at 808 nm the slopeefficiency increased and the medium temperaturedecreased, showing the advantages of the pumpinginto the emitting level over the classical pumping intothe highly absorbing 4F5/2 level. However, the overalloptical-to-optical efficiency was decreased, as only34% of the total pump power could be absorbed inthe crystal. Increased number of passes of themedium would be thus necessary in order to matchthe performances obtained under 808-nm pumping.

Fig. 3 Output power at 1.06 mm for the 250-mm thickNd:GdVO4 under pumping at a) 808 and b) 879 nm.

In conclusion, this work reports highly-efficientCW laser emission at 1.06 mm in a multi-passpumped Nd:GdVO4 microchip laser, under diodepumping at 808 and 879 nm. We appreciate thatwith an improved heat transfer between themedium and the cooling system, which can berealized through a metallic coating, a Nd:GdVO4

medium of lower doping and an increased numberof passes of the pump radiation, this geometrycould be a good solution for efficient laser emissionat 912 nm.

This work was partially supported by the SpecialCoordination Funds for Promoting Science andTechnology of the Ministry of Education, Culture, Sports,Science and Technology of Japan. N. Pavel would like tothank Dr. T. Dascalu of CREATE-JST (Fukui, Japan) forfruitful discussions during this research.

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