Optimization of a double-pass scheme for thermal de-phasing compensation in high-power second-harmonic generation

The performance of a double-pass scheme is theoretically investigated for the efficiency enhancement of a second-harmonic (SH) beam generated by using a high-power fundamental laser beam. Based on a modified version of coupled equations that include the possible effects leading to the thermal de-phasing, the Collins integral is used for optimizing the focusing optics to obtain maximum efficiency of SH conversion. We found through simulation that at a fundamental power of 40 W, when the focusing regime is set for the loose condition, a conversion efficiency of 74% can be reached. It is possible if the focusing points of the first and second passes are designed to be located at 19 mm and 6 mm away from the input face of the crystal, respectively.