Stable resonators with variable reflectivity mirrors for multi-kW Nd:YAG lasers

Abstract

A new method for obtaining high-power laser beams with drastically improved beam quality for fiber coupling is presented. The central idea is to use variable reflectivity mirrors (VRM) as outcoupling mirrors for high-power solid- state lasers with stable resonators, operated near the maximum power. The reflectivity profile of the VRM, properly chosen, acts similar to a mode aperture. Without any further modifications of the laser, the beam parameter products were substantially reduced: the maximum values at intermediate pumping powers were reduced to the beam parameter product occurring at the highest input power. The laser power, however, was not reduced within the principal working interval. Different Nd:YAG laser systems with 350 W, 1.9 kW, and 3 kW maximum output power were equipped with VRM. The 1.9 kW system was coupled without any loss of power into a 400 micrometer fiber instead of the 600 micrometer fiber normally used. The influence of the VRM spot size on laser power and beam parameter product (BPP) was investigated. Furthermore, it was shown that better beam quality is achieved by adding further laser cavities inside the resonator instead of using them as external amplifiers.