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