Power scaling of high brightness multi-kilowatt coaxial CO2 lasers
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The objectives of the work described in this thesis have been first, to develop a high brightness diffusion-cooled CO2 laser based on the annular discharge geometry and secondly, to scale up the output power to provide multi-kilowatt operation. A stable-unstable-hybrid resonator (helix-axicon resonator) serves as the starting point for the development. In order to address the brightness, polarization and temporal stability issues of this resonator several new features and concepts are introduced. High brightness is achieved by ensuring fundamental mode operation without truncating the mode in a free-space configuration. This is achieved by the introduction of shaped electrodes. The temporal stability problems are addressed by introducing a new resonator configuration in the unstable direction, by moving the unstable direction away from the boundary of stability in the ‘classical’ resonator stability diagram. The polarization issues are addressed by introducing a new beam shaping telescope that includes polarization correction based on the use of a dielectric coating, which transforms the azimuthal polarization emitted by the resonator into linear polarization. The overall outcome is the experimental demonstration of a laser with an output power of 2 kW, and a beam with M2 values < 1.1, exhibiting constant linear polarization and temporally stable beam characteristics. In the second part of the thesis, the power scaling laws for diffusion-cooled lasers with annular geometry and free-space propagation are derived and the scaling limits are investigated. A scaled-up version is realized by extending the discharge length and the introduction of distributed inductances to ensure a homogenous discharge distribution. A characterization of the laser confirms the conservation of all beam characteristics at higher output power levels. A maximum laser power level of about 4 kW was reached.