Negative-branch unstable resonator in off-axis configuration for rectangular cross-sections

Abstract

An off-axis configuration of the negative-branch unstable resonator is examined numerically and experimentally. Due to less diffraction effects such a configuration yields lower beam divergences than the standard on-axis resonator. The emphasis is placed on the application to low gain media with cross-sections of rectangular symmetry. The output coupling and the adaptation to the geometry of the gain medium are attained by a scraper. Two different scraper profiles are examined. One profile resembles to a rectangular bracket "[" and corresponds to a shift of the optical axis away from the center of symmetry towards one of the edges. The other profile resembles to the letter "L" and corresponds to a shift of the optical axis towards one of the corners. The experiments are performed with a 10 kW class chemical oxygen iodine laser (COIL). Both scraper profiles are applied to a resonator of the same magnification. Measurements of the phase of the near field and intensity distributions of the far fields are presented. The setup using the [-shaped scraper yields a higher output coupling and therefore a lower output power and a lower beam divergence. The setup using the L-shaped scraper has the advantage, that it makes use of the complete gain medium and that the scraper is reusable for different resonator magnifications. The numerical calculations coincide very well with the experimental results and are therefore the basis for the resonator design of a COIL of the 100 kW class.