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

Abstract

An off-axis configuration of the negative-branch confocal unstable resonator is examined numerically and experimentally for a gain medium with rectangular cross-section. Due to less diffraction effects such a configuration yields lower beam divergences than the standard on-axis resonator. 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 the other profile resembles to the letter "L". The experiments are performed with a 10 kW class chemical oxygen iodine laser (COIL), which has a medium of low gain. Both scraper profiles are applied to a resonator of the same magnification. Measurements of the intensity distributions in the near field and in the far field are presented. The setup using the [-shaped scraper yields a higher output coupling and therefore a lower output power and a lower beam divergence, whereas the setup using the L-shaped scraper makes use of the complete gain medium. Furthermore, the L-shaped scraper is reusable for different resonator magnifications.