A modified negative branch confocal unstable resonator (MNBUR) was coupled to the chemical oxygen-iodine laser (COIL) device of the German Aerospace Center. It consists of two spherical mirrors and a rectangular scraper for power extraction. Experimentally measured distributions of the near- and far-field intensities and the near-field phase were found in close agreement to numerical calculations. The extracted power came up to of the power as expected for a stable resonator coupled to the same volume of the active medium. The output power revealed a considerable insensitivity towards tilts of the resonator mirrors and the ideal arrangement of the scraper was found to be straightforward by monitoring the near-field distributions of intensity and phase. The beam quality achieved with the MNBUR of an extremely low magnification of only 1.04 was rather poor but nevertheless in accordance with theory. The demonstrated consistency between theory and experiment makes the MNBUR an attractive candidate for lasers that allow for higher magnification. In particular, it promises high brilliance in application to class COIL devices, superior to the conventional negative branch confocal unstable resonator.
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