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

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