Recent advances in Er3+:YAG solid-state heat-capacity technology

Abstract

The Er3+:YAG Solid State Heat-Capacity Laser (SSHCL) as a source for medium and high energy laser systems in the “eye-safe” range is currently under investigation at ISL. The aim is to obtain a robust laser source with low complexity, high beam quality (M2 < 3) and scalable to 100 kW and beyond. In a SSHCL the laser medium is cooled only after the laser action has ended, resulting in low temperature gradients in the laser medium itself during operation. Previous investigations demonstrated the scalability of the SSHCL and up to 4.65 kW and 440 J in less than 800 ms have been achieved. Optical-to-optical efficiencies of more than 41% and slope efficiencies of over 51% has been obtained. The residual thermal gradients, due to non perfect pumping homogeneity, negatively affect the performance in terms of laser pulse energy, duration and beam quality. Using an intra-cavity adaptive optics system, beam aberrations were limited to less than 1/10 of the wavelength for each of the considered Zernike polynomials, and the shot duration lengthened by about 50%. In this paper we investigate how to further increase the SSHCL pulse duration. The influence of the crystal geometry on the pump distribution homogeneity and the pulse duration are analysed. We consider the use of a mechanical crystal changer for extending the laser pulse duration. By using a revolver with several crystals, we demonstrated that crystals can be correctly positioned in less than 100 ms, allowing a quasi-cw operation that can largely exceed the time constraints imposed by the heating of the crystal. Finally, we address the problem of measuring the laser beam quality. Since the current standard techniques are suitable only for stable cw lasers, they cannot be used for the SSHCL. A new kind of device, capable of measuring the M2 at intervals of less than 100 ms, is presented.