Diffraction effects on degenerate transverse laser modes

The influence of intracavity non-circular apertures on the frequency of degenerated transverse modes is studied and measured in a CO/sub 2/ laser. Beats were detected due to degeneracy lifting and their frequency was measured as a function of aperture's parameters. The effect of mirror tilt on the beat frequency was also measured and attributed to diffraction, since the limiting aperture is driven off-axis by the tilt. An analytical model based on Gaussian aperture was developed for the case of a symmetric slit introduced in the resonator, and experimental results mere confirmed also by numerical simulations. Cooperative frequency locking was observed in all cases, and the critical beat frequency for this effect was determined.