We demonstrate a three-stage diode-pumped Yb:YAG single-crystal-fiber amplifier to generate femtosecond pulses at high average powers with linear or cylindrical (i.e., radial or azimuthal) polarization. At a repetition rate of 20 MHz, 750-fs pulses were obtained at an average power of 85 W in cylindrical polarization and at 100 W in linear polarization. The report includes investigations on the use of Yb:YAG single-crystal fibers with different length/doping ratio and the zero-phonon pumping at a wavelength of 969 nm in order to optimize the performance.

A two-stage master-oscillator power-amplifier (MOPA) system based on Yb:YAG single-crystal-fiber (SCF) technology and designed for high peak power is studied to significantly increase the pulse energy of a low-power picosecond laser. The first SCF amplifier has been designed for high gain. Using a gain medium optimized in terms of doping concentration and length, an optical gain of 32 dB has been demonstrated. The second amplifier stage designed for high energy using the divided pulse technique allows us to generate a recombined output pulse energy of 2 mJ at 12.5 kHz with a pulse duration of 6 ps corresponding to a peak power of 320 MW. Average powers ranging from 25 to 55 W with repetition rates varying from 12.5 to 500 kHz have been demonstrated.