Abstract

Slab-waveguide molecular CO₂ lasers with transverse RF discharge excitation are attracting attention through their compatibility with diffusion cooling and capability for high average power under scaled off conditions [1]. The slab waveguide resonator modes can be adequately described by simple rectangular EH_ml modes when the lateral Fresnel number of the cavity considerably exceeds unity. Very poor beam quality of the laser output (parameter M² > 20) is the multimode mixture of a few different lateral modes with lateral index m of the resonator inserted between 1 and 50 [2]. Effective single-mode operation of a such laser can be obtained by so called Talbot selection of a lateral waveguide mode of the laser resonator [3]. It was accomplished via the insertion of a grid mode selector into the laser cavity to promote oscillation in a single high order waveguide mode. The spacing d of the grid wires is chosen to fulfil the Talbot imaging condition within the given rectangular waveguide structure according to Talbot condition L = (nd²/2λ), where L. is the cavity length, d is the wire spacing, λ is the wavelength, n is a small integer that represents the number of imaging planes on each round trip through the laser cavity. It was shown that the Talbot selection reduces output power to about 60-70% relative to multimode operation.

© 1996 IEEE