Abstract
In this paper we report on a high energy, low repetition rate 2-micron-laser, with high conversion efficiency in terms of output energy per pump power. The laser consists of a Ho<sup>3+</sup>-doped LiYF<sub>4</sub> (YLF) crystal cooled to cryogenic temperatures in an unstable resonator, pumped by a thulium fiber laser. The cooling to 77 K makes Ho:YLF a quasi four level laser system, which greatly enhances the extraction efficiency. We achieved 356 mJ in Q-switched operation at 1 Hz PRF when pumping the laser with 58 W for 36 ms. The high beam quality from the fiber laser and the use of an unstable resonator with a graded reflectivity mirror (GRM) resulted in a high quality laser beam with a M<sup>2</sup>-value of 1.3.
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