We investigated wavefront aberrations in a cryogenically cooled Yb:YAG slab with a wavefront sensor using
a probe beam technique under non-lasing condition. To analyze the pump-induced phase aberrations created in the
crystal, the measured wavefronts were fitted with orthonormal Zernike polynomials. The Yb:YAG crystal of 2 mm
thickness, 10 mm diameter, and 3 at.% doping concentration was mounted in a copper holder in a closed-loop pulse tube
cryostat with cooling capacity of 12 W at 100 K. The gain medium was single-end pumped by a fiber-coupled laser
diode at pumping intensity of ~6.5 kW/cm2 with a maximum repetition rate of 100 Hz, pulse duration of 1 ms, and pump
spot diameter of 2.5 mm. The time resolved measurement revealed that defocus, which was the main wavefront
aberration, represents not only a thermal lensing effect but also an electronic lensing effect. The thermally induced
defocus is more dominant at high repetition rate than the electronically induced defocus.
We also measured wavefront aberrations of amplified beams in a cryogenically cooled Yb:YAG slab. A room
temperature operated thin-disk regenerative amplifier was used as a seed laser. The seed beam was amplified in the
cryogenically cooled crystal at 160 K in a double pass configuration. The wavefront measurement was conducted at
semi-saturated conditions, at three different repetition rates: 10 Hz, 20 Hz and 40 Hz, and at five different pump
intensities in the range between 6.5 kW/cm2 and 14.8 kW/cm2. Under lasing condition, only defocus aberration were
induced. Due to opposite signs of the defocus aberration of the seed beam and pumped induced in the Yb:YAG crystal,
wavefront of the amplified beam had smaller PtV (Peak to Valley) and RMS values than the seed beam.