Highly efficient 90μm core rod fiber amplifier delivering >300W without beam instabilities

Ultrafast ps and fs laser systems using ROD fiber amplifiers delivering megawatts of peak power and several hundreds of watts in average power have attracted significant academic and industrial interest in recent years. However, the power scaling has reached a temporary limit caused by thermal effects, which has set a limit of average power scaling of these amplifiers due to so called modal instabilities (MIs) [1]. New types of fiber designs with very large cores have been suggested to increase the average power limit [2, 3]. One way of achieving this, is using resonant structures [3], however it is often speculated that these structures are sensitive to temperature causing unstable beam quality under high thermal load (high power extraction pr unit length).