Theoretical Analysis of Second Harmonic Generation Considering Laser Absorption with Repetitive Irradiation of Focused Beam

In the frequency conversion, phase matching is essential to obtain the most effective efficiency. However the crystal temperature deviation causes undesirable conversion because the phase matching condition depends on the temperature. Frequency conversion efficiency is higher as the input intensity is higher. Thus the focused beam is usually used as input in order to obtain the higher efficiency, but the focusing also yields high temperature-rise and large temperature gradient near the focal spot; conversion efficiency decrease in the case that the degree of focusing is too much because of its large effect of thermal phase mismatching. Most of conventional theoreticalstudies exclude effects of laser absorption and treated the problem as steady state although practical frequency conversion is non steady state problem with repetitive irradiation. We analyzed theoretically second harmonic generation considering laser absorption with repetitive irradiation of focused beam, and discussed the contribution of both the focused beam divergence and temperature distribution induced by repetitive irradiation to conversion efficiency. Based on these analysis results, we estimate the several thermal controls that utilize the internal temperature predicted. As a result we obtained larger enhancement in the control considering the internal crystal temperature.