Abstract

High repetition rate femtosecond lasers able to deliver pulse energies ranging from few hundred microjoules to the millijoule level is a very active and competitive field in laser research. In laser bulk technology, major improvements toward high average power were (i) the reduction of the pump heat load through Yb-doped crystals development and (ii) the availability of low cost and bright diode-laser pump sources. Nodaways, Yb:YAG laser sources provide few hundreds of watts at multi-kHz repetition rates in thin disk [1] or slab [2] amplifier configurations with pulse duration close to the picosecond level due to the restricted gain bandwidth of Yb:YAG. Shortening the pulse duration of high average power laser, without the help of self-phase modulation process inside or outside the amplifier, requires Yb-doped crystal hosts associating broad gain bandwidth and good thermal conductivity. So far, Yb-doped regenerative amplifiers delivering multi-watt at multi-kHz repetition rate provide pulse duration close to 300 fs in Yb:CALGO [3] and 200 fs in Yb:CaF₂ at cryogenic temperature [4]. In this work, we report on the first 130 fs room temperature Yb:CaF₂ chirped pulse regenerative amplifier delivering an average power exceeding 4.3 watts when the laser system operates from 5 kHz to 50 kHz. By means of an Yb-doped fiber pump source, a long Yb:CaF₂ crystal with low doping concentration has been used to increase the heat load volume and therefore reduce the amplifier temperature while maintaining high optical efficiency and broad effective bandwidth emission.

© 2017 IEEE