Millijoule-level, sub-nanosecond green laser system using Nd:YAG microchip laser with polarization-selective grating mirror

Abstract

Passively Q-switched microchip (MC) laser with a compact cavity configuration, allowing a sub-nanosecond pulse generation, is an attractive source for the industrial applications including laser processing. Polarization control in such a laser system can be achieved by arranging a polarization-selective element inside of the cavity, e.g. thin film polarizer, resulting in the linearly-polarized output. However, the arrangement impacts on the cavity-length, which leading to expanded the pulse width of the laser output. In this work, we have successfully demonstrated a compact, sub-nanosecond green pulse laser, based on second harmonic generation (SHG) of the MC laser, in which the polarization-selective photonic crystal grating mirror was employed as an output coupler. This system enables to freely select the polarization direction of the linearly-polarized output by just azimuthally rotating the output coupler and thus can accomplish the second harmonic process via a nonlinear crystal without a half-wave plate. The MC laser, pumped by a fiber-coupled 808 nm quasi-continuous wave laser diode, was comprised of 4 mm long Nd3+:YAG crystal with high reflectivity at 1064 nm, Cr4+:YAG crystal as a saturable absorber, and the photonic crystal grating mirror with 50 % reflectivity for 1064 nm. The resulting millijoule-level, sub-nanosecond laser pulse with 45o polarization direction to the crystal axis of a KTiOPO4 (KTP) crystal (Type-II, 9 mm long) for the SHG was frequencyconverted to 532 nm laser output. The total length of this laser system (including the MC laser and the KTP crystal) was also around 35 mm.