Abstract
The transient thermal lens in a high-average power double metal tungstate Raman laser has been investigated. An external cavity potassium gadolinium tungstate (KGW) laser designed for second-Stokes output was burst-pumped with up to 46 W of average power at a pulse repetition rate of 38 kHz. At low duty-cycle, the laser generated up to 18 W of on-time average Raman power with a conversion efficiency of 40%. At high duty cycle, efficiency is reduced and the near-field beam profile expands in the X1' crystal direction over a period of tens of milliseconds. The evolution of the spatial beam properties occurs in response to the development of a highly astigmatic thermal lens with fast-axis susceptibility of approximately -1.7 m-1 per watt of Raman output power. We show that the likely cause for astigmatism is primarily photo-elastic in origin. Beam circularization was achieved by incorporating an intracavity convex cylindrical lens.
Highlights
The end-pumping technique for optical excitation is widely used in lasers for its advantages in providing an intense pump field that has a large volume overlap with the output beam
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]
Model input included specific heat (0.3 J g−1 K−1), density (ρ = 7.3 g cm−3), and thermal conductivity, which for KGW is anisotropic with values shown in Table 1 for a and c crystal directions. (We note that the principal directions of the thermal conductivity tensor are more likely to be aligned to the thermal expansion tensor based on results published for KLuW [23].) For the investigated power deposition values, the maximum temperature increases are sufficiently small to assume constant
Summary
The end-pumping technique for optical excitation is widely used in lasers for its advantages in providing an intense pump field that has a large volume overlap with the output beam. The temperature increase and spatial gradients lead to deleterious effects that may include birefringence, lensing, perturbations to phonon and electronic state populations, and in extreme cases lead to crystal fracture. All these effects are of major concern in all high power laser devices including inversion lasers, Raman lasers, and nonlinear frequency convertors. Most analysis of thermal induced effects has been conducted in end-pumped lasers operating under steady-state conditions (see for example [1] and references therein). It is important to investigate the transient behavior in leading to the establishment of steady-state conditions in order to better understand thermal effects and their role on laser behavior when operating in bursts or with reduced duty-cycle
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