Mode composition and beam divergence of a gasdynamic CO2 laser with a stable wide-aperture resonator

The spatial structure of the output beam of a gasdynamic laser operating at 10.6 microns using a mixture of 10 percent CO2, 45 percent N2, and 45 percent He at Mach 4.9, unsaturated gain 1/m, output power density 150 W/sq cm, and pulse duration 1 sec is investigated experimentally. The apparatus has resonator length 70 cm and comprises a 15 x 5-cm 10-m-curvature-radius totally reflecting mirror, a planar 15 x 5-cm 20-percent-transmittance output mirror, opaque baffles at 10 cm to select a portion of the beam for study, a conical calorimeter to measure the power of light passing the baffles, and a plywood screen at 25 or 400 cm to visualize the field intensity distribution. The anomalous beam structure observed is attributed to the excitation of multiple-pass modes, as discussed theoretically by Herriott et al. (1964), Ramsay and Degnan (1970), and Kiselev (1971), in the resonator. Patterns approximating a 12-pass mode are shown, and the stabilizing effect of the multipass modes on the energy parameters of the beam (without increasing its divergence) is found to permit uniform extraction of energy from large volumes of the active medium.