High-inversion densities in Nd:YAG-upconversion and bleaching

We report on the investigation of upconversion in Nd:YAG and its implications for intensely pumped devices. Analysis of lifetime measurements and the performance of a 1 at.% Nd-doped YAG amplifier give an Auger upconversion rate of 7/spl times/10/sup 3/ s/sup -1/. This is significantly smaller than previously reported, but modeling of the performance of Nd:YAG devices with high-inversion densities shows that even this rate can still seriously degrade the small-signal gain and significantly increase the thermal load. The variation of cross-relaxation and upconversion rates with doping level is also described. Finally, it is found that the effect of bleaching of the Nd:YAG absorption can lead to a reduced spatial overlap between the signal and inversion profiles and thus can also significantly reduce the gain.