Cryogenic Yb:YAG total-reflection active-mirror lasers

A regenerative amplifier is developed and an additional joule-class amplifier system at 100 Hz is under construction. The regenerative amplifier is demonstrated with a cryogenic total-reflection active-mirror (TRAM) as a laser material, the bottom of the prism. The bottom surface is directly cooled by liquid nitrogen. The cavity length is 1.7 m, corresponding to 11.3 ns round trip duration. A MOW fibre-coupled laser diode is used as a pump source. The typical pump duration is 700 μs with a repletion rate of 100 Hz. A seed pulse is a chirped pulse and the centre wavelength is 1029.4 nm with a 0.37 run spectral width. A 3.5 mJ output pulse energy is obtained at the 44th round trip with a M²-factor less than 1.1. The spatial centre fluence is high at 2.6 J/cm². The spectral width is gain-narrowed at 0.17 nm and the pulse duration is shortened at 0.46 ns. The calculated pulse duration is 9.6 ps for the transform-limited pulse after temporal compression. The pump source for each TRAM is a 2.5 kW fibre coupled laser diode. Our numerical calculation shows that 1 J will be obtained after 10 passes.