Numerical calculations using the Prony method have shown that the feedback coefficient in unstable laser resonators can be increased by factors of as much as 5 over the geometrical value using a mirror with a phase step at its center. A phase shift of close to π over an area of an equivalent Fresnel number of 0.5 leads to minimum output losses. Experiments with a TEA CO2 laser confirm the prediction. The results can be attributed to a cancellation of the output wave by destructive interference, which confines the laser beam around the resonator axis.
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