: Ehrlichmann, D.; Habich, U.; Plum, H.-D.; Herziger, G.

Abstract
Stable-unstable resonators have proved suitable for the extraction of a high tow substantially different transverse dimensions and the resonators are stable in the small dimension while unstable in the larger one. Using off-axis unstable resonators avoids a central beam obscuration and improves beam quality. The adaption of stable-unstable resonators to annular gain areas is described in this paper. The resulting resonators are stable in radial direction and unstable in azimuthal direction. Different unstable resonators, wound to match the annular geometry, are presented. The resonator modes are calculated numerically using a 3D-diffraction code that considers gain and misalignment. Resonator disign parameters are obtained from a geometrical description of the resonator. Experimental results from a diffusion-cooled CO2 laser confirm theoretical predictions and show that the resonators are capable of extracting beams which are nearly diffraction-limited with high efficiency from an annu lar gain medium. Output powers of 2 kW have been obtained from a gain length of 1.8 m.