COIL emission of a modified negative branch confocal unstable resonator

Abstract

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 approximately 90% 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 100 kW class COIL devices, superior to the conventional negative branch confocal unstable resonator.

Analysis of lasing in COILs with positive and negative branch unstable resonators using a simple geometrical-optics model

“…To improve the beam quality, COILs with different kinds of unstable resonators were recently tested. 1,2 High optical extraction efficiency and small beam divergence close to the diffraction limit in the flow direction were obtained. To determine the intensity and phase distribution in the far and near fields as well as the beam divergence rather complex procedure for calculating the three-dimensional mode pattern was applied by different authors.…”

“…To determine the intensity and phase distribution in the far and near fields as well as the beam divergence rather complex procedure for calculating the three-dimensional mode pattern was applied by different authors. [1][2][3][4] This procedure included solving of the paraxial wave equation (or the equivalent integral equation of the Fresnel-Kirchhoff formulation of Huygens' principle 1,2 ) taking into account a nonuniform, transversely flowing, saturable gain medium. However if one is interested only in the energy parameters of the laser such as the laser power, optical extraction efficiency η ext and spatial distribution of the intracavity intensity I, a much simpler geometrical optics approach can be used.…”

Negative-branch unstable resonator in off-axis configuration for rectangular cross-sections

“…In order to enhance the beam quality extracted from unstable resonators, a Modified Negative Branch Unstable Resonator (MNBUR) was introduced 6,7 . The optical axis of the negative-branch resonator is shifted apart from the center and the output coupling is performed by a scraper.…”

“…The optical axis of the negative-branch resonator is shifted apart from the center and the output coupling is performed by a scraper. Numerical studies have been presented using spherical geometry 6 and numerical as well as experimental results have been shown for rectangular cross-sections 7 . The adaptation to the rectangular cross-section is only done by the shape and position of the scraper.…”

“…The setup with the [-shaped scraper and a resonator magnification M of 1.04 yields an output power of 6.3 kW and an output coupling of 5.4 %. But due to the very small magnification the far field is clearly structured7 . In order to get more power in the main lobe of the far field a larger resonator magnification is used here.…”