Off-axis negative branch unstable resonator in rectangular geometry

Abstract

The application of an off-axis negative branch unstable resonator to an active medium of rectangular geometry is examined. The presented unstable resonator consists of spherical mirrors and a scraper mirror. The adaptation to the rectangular cross-section is performed by the scraper, which takes two different shapes. One shape resembles to a rectangular bracket "[" and an other to the letter "L". The [- and L-configurations correspond to a shift of the optical axis away from the center of the cross-section, towards one of the edges or towards one of the corners, respectively. Both scraper setups are examined numerically and experimentally. Experiments are performed with a multi kW class chemical oxygen iodine laser. The active medium is characterized by a low amplification coefficient. Measured results of the intensity distribution in the far field and of the phase distribution in the near field are shown for both resonator configurations. Using the same resonator magnification, the setup with the L-shaped scraper has a lower output coupling and therefore a higher output power and a slightly higher beam divergence. The L-shaped scraper configuration is able to cover the gain medium completely.