Cryogenic-cooled Yb:YAG amplifier for ns-pulses obtaining high optical efficiencies

Since typically low temperatures turn the quasi three level system of Yb:YAG into a four level system much higher efficiencies are expected by cryogenic cooling. Former experiments achieved efficiencies up to 30 %, which is below theoretical expectations. To increase the efficiency through a better overlap between the pumped area and the seed beam profile, a novel all-reflective relay imaging amplifier has been built. Astigmatism compensation of the setup was achieved by folding the beam pass out of plane. The Yb:YAG crystal was placed in a liquid nitrogen cooled mount inside a vacuum chamber. The amplifier design allows to switch between different numbers of passes easily. In a six pass configuration a maximum optical-to-optical efficiency of 46% with a output energy of more than 600 mJ was achieved.