Compensation of Thermal Phase Mismatch Effects in The High Power Second Harmonic Generation Using a Chirped Periodically Poled Non-linear Crystal

Thermal effects strongly influence the process of second harmonic generation, especially at high fundamental powers. The generation of non-uniform temperature distribution in the crystal due to the absorption of fundamental and second harmonic waves leads to significant degradation of the efficiency of the second harmonic generation process. This absorption acts as a heat source in the crystal and prevents the crystal from remaining at the phase matching temperature, resulting in a phenomenon called thermal phase mismatch in the crystal. In this paper, first, a theoretical model to simulate the thermal effects in the second harmonic generation is introduced. Then, by considering a periodically poled crystal as a nonlinear crystal and adding a chirp to its periodicity, a solution to compensate for the thermal phase mismatch is presented. Simulation results show that the chirp of a periodically poled crystal compensates for the thermal effects on the nonlinear crystal and largely offsets the efficiency degradation in the second harmonic generation. For example, at the fundamental power of 20 W, the thermal effects reduce the second-harmonic conversion efficiency from 74% to 18% and using the chirped periodically poled crystal increases the efficiency to 50%.