An experimental study of unstable confocal CO2resonators

The results of an experimental study to determine the spatial properties of unstable confocal resonator modes and their influence upon both the fractional output coupling and the far-field angular divergence are presented. The radial intensity distributions of the lowest order circulating mode within CO
2
unstable resonators and the corresponding near- and far-field patterns have been measured. The radial profiles exhibit a significantly nonuniform intensity distribution consistent with the results of recent theoretical analysis. The far-field intensity distribution as well as the fractional output coupling have been investigated for a number of unstable confocal resonator geometries. For modes contained within the uniform-gain region of the laser tube, the experimental results are in excellent agreement with Siegman's unstable resonator analysis.