Experiments were performed on a CO2 laser with an annular resonator. The objective of these experiments was to control the polarization in an annular laser and to verify computer code predictions. A Mylar pellicle was placed in the annular leg, and its angular orientation with respect to the optical axis was varied. The pellicle had different transmittances for horizontal and vertical polarizations and also had stress-induced birefringence. At small angles of incidence in the pellicle, the birefringence dominated, and the output had a complicated nature. However, at large angles of incidence, the transmittance difference forced the laser into a well-defined mode for which the near field was horizontally polarized at the top and bottom and the two sides and vertically polarized at intermediate locations. The experimental results were in good agreement with the computer code calculations.
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