Off-axis negative-branch unstable resonator with rectangular cross-section

Abstract

For laser media with large cross-section and low small signal gain neither the stable nor the unstable conventional resonator is an applicable choice. Either the large Fresnel number leads to a multimode operation or the low output coupling ring yields large diffraction effects and therefore a heavily structured far field. In order to reduce these diffraction effects a modified negative-branch unstable resonator (MNBUR) was introduced. In rotational symmetry the off-axis setup provides an output coupling in the shape of a half ring and is accomplished with the help of a scraper. The shape of this scraper can also be modified to adapt the MNBUR to a rectangular symmetry, whereas the spherical resonator mirrors are kept unchanged. The scraper either takes the shape of a rectangular bracket "[" or of the letter "L". The performance of both scrapers is tested using a chemical oxygen iodine laser (COIL) of the 10 kW class. The results are compared to numerical calculations.