Design of variable reflectivity mirrors and unstable resonators for Nd:YAG lasers with high average power

Abstract

Positive branch as well as negative-branch unstable resonators in different quadrants of the stability diagram were examined both theoretically and experimentally. Variable reflectivity mirrors (VRM) Were used as outcoupling elements. The resonators comprised up to three Nd:YAG rods (6* 3/8 inch) with a maximum input power of 12 kW each. This system provided a maximum output power of 550 W per rod using a symmetric flat resonator. A novel design of unstable resonator (near-concentric unstable resonator NCUR), showed to be quite insensitive to input power changes within the complete range: the round trip magnification only changed by a factor of 3, resulting in a nearly constant slope efficiency and a very much reduced shift of the focal spot. With one rod, 420 W maximum laser power with a beam parameter product of 2.3 mm mrad (two rods: 680 W, 7 mm mrad) was obtained. Kirchhoff-integral based numerical calculations indicate smaller beam parameter products. This deterioration is caused by spherical aberration of the thermal lens and by differences of refractive power for r- and phi -polarizations. The VRMS Were produced at FLI's coating laboratory. An industrial evaporation machine equipped with the two electron-beam heated crucibles and a planetary drive was used. Thin-film designs with only one shaped layer yield reflectivity profiles of nearly any super-Gaussian shape with peak reflectivities of up to 90%. If an additional layer is inserted, one obtains apodized apertures. The phase shift of the reflected beam is similar to that of single-layer VRM. With a geometric model, the production parameters of the fixed mask method, such as aperture diameter and distance to the substrate, can be estimated as before. The damage threshold is 10-16 3 cm-2 (15 ns).