High power tungstate-crystal Raman laser operating in the strong thermal lensing regime

Abstract

We report an investigation into a double metal tungstate Raman laser when pumped at elevated average powers. Potassium gadolinium tungstate (KGW) was placed in an external cavity configured for second-Stokes output and pumped at pulse repetition rate of 38 kHz with up to 46 W of average power. For output powers above 3 W, we observe preferential excitation of Hermite-Gaussian transverse modes whose order in the X(1)(') principal direction of the thermal expansion tensor scales linearly with Raman power. We deduce that strong astigmatic thermal lensing is induced in the Raman crystal with a negative component in the X(1)(') direction. At maximum pump power, 8.3 W of output power was obtained at a conversion efficiency of 18%.

Thermal lens evolution and compensation in a high power KGW Raman laser

“…Output increases linearly with pump power with a discontinuity in the slope that occurs at the 3-4 W level. The transition coincides with a marked change in the spatial properties of the output beam, from circular near TEM 00 mode to preferential excitation of a H-G mode above the transition at the 3-4 W level [19]. The maximum slope efficiency and absolute conversion efficiencies was 40% and 15%.…”

“…The system under investigation, which was similar to that previously described in [19], consisted of a Q-switched pump laser of output power 50 W focused into an external cavity KGW Raman laser. The pump laser was Q-switched at 38.6 kHz and produced pulses of duration approximately 25 ns at full width half maximum.…”

“…We have previously proposed [19] that the preferential excitation of high order modes is the result of a strong index gradient producing a defocusing (negative) cylindrical lens with its principal axis closely aligned to the X 1 ′ principal direction of the thermal expansion tensor. In order to confirm this and gain an approximate measure of the lens strength, we investigated output power and beam profiles as a function of mirror curvatures.…”

“…In Fig. 5, the H-G mode order (in terms of beam propagation factor using the relation M y 2 ~2m + 1 where m is the number of nodes in the beam profile [19,20]) is plotted as a function of Raman power for three different end-mirror curvatures. The highest Raman laser power and best beam quality (see Point C in Fig.…”

“…These anisotropies make analysis highly complex but may be potentially exploited for producing characteristic laser behavior or as an interesting method for reducing thermal effects (see, for example, athermal studies of [17,18]). Recently, we found that significant astigmatic thermal lensing in a high power KGW Raman laser was responsible for reduced efficiency and highly elongated output beams [19]. Using a 46 W pump laser, 8.3 W of second-Stokes output power was obtained in an output beam primarily consisting of a 7th-order Hermite-Gaussian (H-G) mode in the X 1 ′ principal direction of the thermal expansion tensor.…”