Upconversion-induced heat generation and thermal lensing in Nd : YLF and Nd : YAG

We investigate the influence of interionic upconversion between neighboring ions in the upper laser level of Nd:YLF and Nd:YAG on population dynamics, heat generation, and thermal lensing under lasing and nonlasing conditions . It is shown that cascaded multiphonon relaxations following each upconver sion process generate significant extra heat dissipation in the crystal und er nonlasing compared to lasing conditions. Owing to the unfavorable temper ature dependence of thermal and thermo-optical parameters, this leads, firs t, to a significant temperature increase in the rod, second, to strong ther mal lensing with pronounced spherical aberrations and, ultimately, to rod f racture in a high-power end-pumped system. In a three-dimensional finite-el ement calculation, excitation densities, upconversion rates, heat generatio n, temperature profiles, and thermal lensing are calculated. Differences in thermal lens power between nonlasing and lasing conditions up to a factor of 6 in Nd:YLF and up to a factor of ? in Nd:YAG are experimentally observe d and explained by the calculation. This results in a strong deterioration in performance when operating these systems in a Q-switchcd regime, as an a mplifier, or on a low-gain transition. Methods to decrease the influence of interionic upconversion are discussed. It is shown that tuning of the pump wavelength can significantly alter the rod temperature. [S0163-1829(98)047 47-X].