Comparison of multi-pass and regenerative strategies for energetic high-gain amplifiers based on Yb:CaF2

Yb-doped materials, due to their high saturation fluence and consequently their low gain, represent a challenging choice for high-energy amplifiers. In this Letter, we study two original amplifier designs adapted to a large number of passes capable of operating in the 100 mJ energy range at repetition rates up to 100 Hz using Yb:CaF₂ crystals as active media. Amplification geometries based on double-head active-mirror configurations are presented. We confront two alternative strategies suitable for amplification of large beams: regenerative and geometrical multi-pass amplifiers. This Letter consists of finding the pivot point, allowing us to discriminate the specific interest of each strategy. We present compensation methods of the thermal lens adapted to each amplifier configuration with and without cavity, and we demonstrate that despite similar laser heads and pumping conditions, the thermal lens impacts differently the optimal performance for multi-pass or regenerative strategy. We perform amplification up to 66 mJ pulses at 10 Hz with the regenerative amplifier and 52 mJ at 100 Hz with the multi-pass amplifier.