Distortion Analysis of Beam Profile and Pulse Shape of Second Harmonic Due to Repetitive Irradiation of Picosecond Pulses

摘要

Frequency conversion technique using nonlinear optical crystals is indispensable to solid-state laser applications. It is well known that the conversion efficiency is very sensitive to temperature change of crystal, which is caused by laser absorption. We have been analyzing problems of second harmonic generation (SHG) by coupling the electric field to the temperature field. In this study, temporal pulse-shape dependence of SHG was investigated theoretically, and Gaussian pulse and rectangular pulse were compared. Main conclusions obtained are as follows: (1) Both conversion efficiencies with Gaussian pulses and rectangular pulses fluctuate with irradiation time, however the former fluctuation is slower and more gentle. (2) Pulse of output second harmonic fluctuates most remarkably at the center, where power density of incident fundamental is highest, in the Gaussian pulse. (3) As a result, pre-pulse and post-pulse are generated before and after the main pulse. (4) Such distortion of output pulse shape becomes more remarkable as power density increases.