High-power fast-axial-flow CO2 laser with a variable-reflectivity output coupler

We report what is to our knowledge the first cw, dc-excited, fast-axial-flow CO(2) laser using a variable-reflectivity output coupler. A super-Gaussian reflectivity profile of order 8 with 96% central reflectivity has been achieved by depositing a low-absorption coating onto a ZnSe substrate. The unstable resonator oscillated in the lowest-order mode to produce a diffraction-limited beam of 300 W of power and 0.35-mrad divergence angle (FWHM). The measured near field and far field are in good agreement with the mathematical model. The technique lends itself to extrapolation to the kilowatt level.