Analysis of second harmonic generation considering laser absorption with repetitive irradiation

摘要

Shorter pulse duration and higher power of solid-state lasers have accelerated the development of frequency conversion technique using nonlinear optical crystals for laser precision microfabrications. Conventional theoretical-studies concerning frequency conversion supposed one pulse irradiation, accordingly the influence of temperature change of crystal induced by laser absorption is not considered, though practical frequency conversions are used with repetitive irradiation. Researches and developments of harmonic generation with repetitive irradiation have been approached experimentally or empirically. On the other hand it was reported in the previous studies that temperature gradient in the crystal is the cause of undesirable conversion in the harmonic generations such as either decline or fluctuation of the efficiency and distortion of beam profile. In this study, the second harmonic generation considering laser absorption was analyzed theoretically through a coupling model composed of truncated equations includes laser absorption and heat conduction equation. Influence of temperature rise of crystal induced by laser absorption with repetitive irradiation on conversion efficiency and output beam profile in SHG was investigated. It was shown that conversion efficiency decreases or fluctuates and beam profile of output second harmonic also changes complicatedly with repetitive irradiation, depending on the input condition. High temperature region moves from the side of output surface to the center of crystal during the continuing irradiation; the temperature difference in the crystal is not so large. Therefore, we concluded that temperature gradient naturally induced by laser absorption is not an essential reason of the undesirable frequency conversion.