RF-excited unstable-resonator planar CO2 laser on all-metal electrode–waveguide structure

The radiation characteristics of a planar CO₂ laser excited by a diffusion-cooled rf discharge at a frequency of 40 MHz are studied. A single-mode cw lasing power of ~50 W is achieved with an efficiency of ~10% for a nearly diffraction-limited radiation divergence of 4–7 mrad. The spatial structure, output power, stability and laser radiation quality are studied as functions of longitudinal and angular alignments of the resonator mirror for two types of hybrid unstable-waveguide resonators of the laser. It is shown that for the resonator corresponding to the negative branch of the stability diagram, a misalignment of 0.02 rad of the mirrors leads to a 50% decrease in the output laser power, while its value for the positive branch resonator is about 100 times smaller. It is found that for the resonator corresponding to the negative branch, the sensitivity to the violation of confocal arrangement of the mirrors upon an increase in the resonator length is an order of magnitude higher. The dependence of the density of input rf power on the working gas pressure is studied experimentally in the interval 50–110 Torr. Power density values of 1–4 W cm^-2 are obtained for normal discharge current density. These values are important for optimisation and scaling of the lasing characteristics of high-power planar CO₂ lasers.