A report on the design and performance of a coaxial laser system with a stable resonator with internal axicon is given. Efficient diffusion cooling is obtained by using an annular transverse high-frequency excited discharge. Experimental results are compared with the results of a loaded resonator model that takes tilted mirrors into account. The agreement between theory and experiment for extraction efficiency, alignment sensitivity, polarization and mode structure is very good.

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A diffraction formula for annular beam propagation is suggested. Significant computational savings are obtained without restriction to low azimuthal mode orders. Azimuthal mode discrimination is shown to exist in stable annular resonators. High-order azimuthal modes can suffer low diffraction losses with certain mirror parameters. These high-order modes are identified with azimuthal revolving rays that satisfy known geometric relations for multipass resonators.

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An output power of 2kW is attained with a compact coaxial slow-flow CO2 laser. The resonator consists of two toric copper mirrors, which are tilted to a small amount. These mirrors form an unstable resonator in azimuthal direction, from which two laser beams are extracted with high efficiency through a coupling aperture in one of the mirrors. The beam divergence of a single beam is nearly diffraction limited in radial and azimuthal direction.