Mode-medium instability and its correction with a Gaussian-reflectivity mirror.

A high-power CO(2) laser beam is known to deteriorate after a few microseconds because of a mode-medium instability (MMI) that results from an intensity-dependent heating rate that is related to the vibrationalto-translational decay of the upper and lower CO(2) lasing levels. An iterative numerical technique has been developed to model the time evolution of the beam as it is affected by the MMI. The technique is used to study the MMI in an unstable CO(2) resonator with a hard-edge output mirror for different parameters, e.g., the Fresnel number and the gas density. The results show that the mode of the hard-edge unstable resonator deteriorates because of the diffraction ripples in the mode. We use a Gaussian-reflectivity mirror to correct the MMI. This mirror produces a smoother intensity profile, which significantly reduces the effects of the MMI. Quantitative results on the peak density variation and beam quality are presented.