A new method to measure the ultrashort thermal focal length of high-power solid-state laser

Thermal lens effect is a very important factor in designing stable resonators for high-power solid-state laser pumped by laser diodes. In a symmetric plane-parallel resonator, if the thermal lens of the laser crystal is close to a thin lens, according to the theory of transfer matrix we can get that there is a critical hollow point where the resonator changes from stable region to unstable state, then back to stable region. So we can measure the ultrashort thermal focal length of high-power solid-state laser based on this fact. We used this method to measure the ultrashort thermal focal length of a 100W magnitude Nd:YAG laser. The experimental results have shown this is a simple and effective technique.