Highly efficient cryogenic Yb:YLF regenerative amplifier with 250 W average power

We report an efficient diode-pumped high-power cryogenic regenerative amplifier operating at 1019 nm employing the c axis of Yb:YLF. Compared to usually selected 1017 transition a axis, c-axis line has three-fold higher emission cross section and still possesses full-width half-maximum (FWHM) 6.5 125 K. The chirped-pulse system is seeded by fiber front-end with energy 30 nJ stretched pulse width 2 ns. In amplification studies, using advantage gain in we have achieved record average powers up 370 W extraction efficiency 78% 50 kHz repetition rate. output pulses were centered on 1019.15 FWHM bandwidth 1.25 nm, which supports sub-1.5 ps durations. beam maintained T mathvariant="normal">E mathvariant="normal">M 00 profile power levels below 250 2 1.2. Above this level, thermally induced lensing Yb:YLF created multimode beam. thermal lens was rather dynamic deteriorated stability above level.