Role of small anisotropies in selecting the polarization of CO2 lasers

We investigate the polarization properties of the lowest-order transverse patterns in two different CO2 Fabry-Perot lasers. Emission on the fundamental mode TEM0,0 is always linearly polarized in a fixed direction. The annular intensity pattern TEM0,1* may have one of two different underlying polarization states: either linear polarization that is the same as for the fundamental mode or the superposition of two Gauss-Hermite modes with different optical frequencies and with linear and nearly orthogonal polarizations. Transitions among these three patterns are obtained by varying the cavity detuning. The two underlying polarization states of the annular intensity pattern can be explained using a simple model that includes anisotropies in both the cavity losses and the detuning for the Gauss-Hermite modes. We also provide an explanation of the relationship between the possible polarization states and the corresponding spatial patterns that form an annular intensity pattern.