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Theoretical and experimental study of hybrid unstable-guided resonator for diffusion-cooled CO2 laser

Diffusion cooled carbon-dioxide laser sources allow nowadays to produce more than two kilowatt output power with weights, dimensions and costs reduced with respect to the traditional fast axial flow sources of the same power level. In particular, they can be easily integrated in traditional as well as in robot-laser workstations. These kinds of sources are characterized by two large area water cooled coaxial or planar electrodes with small spacing (few millimeters) in order to guarantee a good mixture cooling. Consequently the discharge geometry results 'optically hard:' a thin ring or a thin rectangle. In this communication we present theoretical and experimental results concerning the design and development of the optical resonator and of the external optical chain for a carbon-dioxide laser prototype. The work was done in the frame of a national project. The prototype is a diffusion cooled, rf excited slab source with 1 kW output power. The surface of each electrode is 110 by 700 mm and their spacing is 2 mm. For this geometry a hybrid unstable-guided resonator has been adopted. The main problem of this configuration is that the extracted beam is elliptical and astigmatic, and therefore needs to be manipulated before the working point where high optical quality is required for material processing. A particular computer code has been developed to calculate the resonator modes and the calculated profiles of the beam have been compared with the measured ones with and without external optical chain. The good agreement of the results confirms that the criteria adopted for the simulation are correct and that the code developed can be successfully employed in the design stage. This is particularly significant for this class of sources because an extensive experimental study of different optical combinations of the resonator mirrors can become expensive. In fact the mirrors have large dimensions (about 110 by 40 mm) and non standard curvature radii. Moreover they need an optimized high reflectance coating for the high degree of pollution due to the particular discharge conditions. For these reasons a good simulation 'tool' can be useful to save time and money.