Amplification of laser radiation in disk Yb:YAG crystals cooled down to the liquid nitrogen temperature

The parameters of a diode-pumped Yb:YAG amplifier are calculated. The derived expressions allow one to determine the optimal doping of the YAG crystal with Yb ions, which is ~10% for a 600-μm-thick crystal. The calculations take into account the absorption line profile and the characteristics of the pump spectrum (the shape, width, central wavelength). When the 10% Yb-doped YAG crystal is cooled down from 296 K to 80 K, the shift of the gain profile centre (~0.3 nm) and the increase in the stimulated emission cross section (by four times) as well as the depletion of the lower working level are observed. The small-signal gain (the maximum value is 1.7, which corresponds to the gain 4.4 cm^-1) is measured as a function of the absorbed pump power and well agrees with the results of theoretical calculations.