Off-axis folded laser beam trajectories in a strip-line CO(2) laser

Using an rf-excited strip-line CO(2) laser with large lateral dimensions (a high Fresnel number) in a hemiconfocal optical cavity configuration, we observe modes that are distinctively off axial. They can be viewed as folded Gaussian beams with trajectories in the shapes of the letters M and W. Either of these modes can be isolated by suppressing the other mode through the introduction of a suitably positioned obstacle into the cavity. Computer simulations of the radiation propagation in the cavity yielded field distributions conforming to the observed pattern.