Measurement of statistical phase distortions of active medium

The trend of development in fast axial flow CO2 lasers with transversal RF excitation leads to increasing laser power with high beam quality. Since the power density of the discharge is limited due to thermal instabilities scaling the output power enforces an increase of the discharge dimensions and the mass flow for the convective cooling of the medium. Under the boundary conditions of large discharge volume and high mass flow the active medium affects the optical path of the laser beam by stationary as well as statistical phase distortions. Output power and beam quality are strongly impaired by phase distortions of the active medium. The description and diagnosis of the instationary, statistical phase distortions is an important field of activity in the development of high power CO2 lasers and an important step in optimizing the beam source. In order to estimate the influence of gas flow state and discharge conditions, a measurement technique to evaluate the statistical phase distort ions is presented. Parallel beams of coherent light being chopped at high frequency pass the medium to be investigated. Propagating through the medium the beams suffer different refractions which can be measured as a relative motion of the beams to each other. The different refractions are caused by density and temperature inhomogeneities and fluctuations in the discharge. The relationship between measured signals and characteristical quantities of the statical phase distortions is discussed. The measurement technique as well as experimental examples are presented.