Properties of an optical cavity for a high-power CO2 laser

Properties of a positive-branch nonconfocal unstable resonator in a one- and two-pass (folded) configuration, designed for a CO2-N2-He transverse flow laser excited by a high-power glow discharge1 have been studied numerically. Calculations were carried out with the help of a computer code2 that takes into account an idealized, saturable, homogeneously broadened gain medium model. The near- and far-field behavior of both configurations, as well as the theoretical dependence of the output power (P) on the small signal-gain (g0), saturation intensity (Is), mirror reflectivity (R i ), and the discharge length (L a ) are discussed. For g0 = 0.4/m (the average value of g0 found experimentally), l s = 0.5 kW/cm2, R j , = 0.88, and L a = 0.86m, our calculations predict for the one-pass cavity P = 1.6 kW and for the two-pass cavity P = 4.2 kW; measured values of P were ~1.7 and ~4.4 kW, respectively. The calculations also predict the rather poor quality of the output beam. In both cases ~90% of the output power is focused within a circle of angular width 2θ = 1.06 mrad, but only a small fraction of the beam energy (~40%) is contained in the centermost beam lobe (2θ = 0.3 mrad). According to the mirror tilt analysis there is no significant difference in the tolerances of both cavities to angular misalignment. With increasing mirror tilt noticeable distortions in the far- and near-field distributions are observed. (Poster paper)