Flat-Top Beam Shaping for High-Power High-Energy ps Yb:YAG Lasers

Yb:YAG Joule-level picosecond kilowatt laser systems present significant scientific and technological challenges. Cryogenically-cooled disks have demonstrated 1.1 J at 1 kHz [1], while thin disks have shown 720 mJ at the same repetition rate [2]. HORIZON, a laser development program targeting 1 J-l kHz-l ps, combines thin-disk technology in a custom regenerative amplifier and rotating water-cooled disk technology for the final multi -pass power amplifier [3]. The gain of such amplifiers typically follows the flat-top spatial distribution of the high-power pumping diodes, and matching the spatial beam profile of the seeder to the spatial gain is crucial for optimizing both the compactness of the final multipass amplifier and the extracted energy. We report here the experimental validation of phase mirrors under 20 W, 20 mJ nanosecond pulses, demonstrating a highly efficient method of shaping Gaussian beams into flat-top ones at the entrance of the final Joule amplifier.