Diode-pumped Yb:CaF 2 multipass amplifier producing 50 mJ with dynamic analysis for high repetition rate operation

The dynamic thermal issues of the Yb:CaF2 crystals within a multi-tens-mJ-energy multipass amplifier operating in the 20–100 Hz repetition rate range and pumped in quasi-cw regime have been studied at different timescales. Thermal response times of the system have been precisely investigated and analyzed, for the first time to our best knowledge in such amplifiers. This study includes a dual timescale analysis: in the long-time-scale (second) with direct thermography mapping and in the millisecond range with thermal lensing in a pump-probe configuration. Very atypical positive lens behavior with fluorites will also be presented and discussed. This complete analysis is used to demonstrate the capability of Yb:CaF2 multipass amplifier systems for operating the amplifier at 20 Hz with 57 mJ and 100 Hz with 32-mJ stable regime. Indeed, high repetition rate multipass amplifier has been realized for the first time with Yb:CaF2 and for this energy. The results have been analyzed precisely to take into account the thermal issues and excellent beam quality, with a M2 of 1.1. The pointing stability of 20 lrad has been measured documenting the reliability of the high repetition rate mJ amplifier.