Active mirror high energy Yb:YAG amplifier

Diode-pumped lasers and amplifiers using Yb-doped materials have attracted increasing interest in the past years. This is owed to the high achievable efficiency of high energy amplifiers with high repetition rates and their potential to provide ultrafast laser pulses in the sub 100fs range. These properties allow their use in high intensity physics, laser fusion or pump sources for ultra-short Ti:Sa and OPCPA systems. Yb:YAG is one of the most important material for ns pulse generation and amplification with high efficiency also at room temperature. Here we present a joule-class Yb:YAG active mirror amplifier and the time-resolved analysis of its thermal lens. The active mirror concept is one of the most promising techniques for room temperature operation of Yb-based lasers providing efficient cooling of the laser material and energy extraction. A pulse energy of 730mJ at a repetition rate of 1Hz was obtained at an optical-to-optical conversion efficiency of 16%. Here a relay-imaging multipass cavity with 4 extraction passes and also 4 pump passes was employed. For seeding a Yb:YAG pre-amplifier with a output energy of 65mJ was used. The disk with an Yb-concentration of 5mol% comprises a thickness 2.5mm and diameter of 12.5mm. At pump energies of 4J and above the small-signal gain was drastically reduced by transverse lasing which was observed as a roll-over of the measured gain and efficiency. In order to get rid of parasitic lasing the doping concentration will be reduced while the thickness is increased such that the optical thickness of the disk remains. Furthermore, a multiple active mirror will be used in a redesigned setup allowing reduced thermal load of each single disk. The thermal lens measurements were compared with a rod type (transversally) cooled disk. At a repetition rate of 10Hz the thermal equilibrium of the disk was attained within the first pump shots while in the case of the rod type cooling serveral seconds were required.