ROLE OF SMALL ANISOTROPIES IN SELECTING THE POLARIZATION OF CO2-LASERS

We investigate the polarization properties of the lowest-order transve rse patterns in two different CO2 Fabry-Perot lasers. Emission on the fundamental mode TEM0,0 is always linearly polarized in a fixed direct ion. The annular intensity pattern TEM0,1 may have one of two differe nt 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 pattern s are obtained by varying the cavity detuning. The two underlying pola rization states of the annular intensity pattern can be explained usin g a simple model that includes anisotropies in both the cavity losses and the detuning for the Gauss-Hermite modes. We also provide an expla nation of the relationship between the possible polarization states an d the corresponding spatial patterns that form an annular intensity pa ttern.