Self-induced thermal distortion in the near field for a laser beam in a moving medium

A theoretical model is developed that predicts the steady-state self-induced thermal distortion of a laser beam in an absorbing medium flowing transverse to the direction of propagation. A general perturbation solution is obtained that is appropriate when the thermal distortion effects are small. A computer iteration of this solution is performed to obtain results that are applicable over an increased range of distortion conditions. The effects of diffraction are neglected, and therefore the solution is limited to the near-field range of the laser beam or to a range where a caustic or infinite intensity is predicted as a result of the self-focusing associated with the thermal distortion effects of the flow. The thermal distortion of an initially Gaussian collimated laser beam causes a shift of the beam into the direction of the flow and, at the outset of the thermal effects, a decrease in the peak intensity. As the thermal distortion becomes more severe, the self-induced effects cause an increase in the peak intensity, i.e., a focusing to occur. Experimental results are presented for the thermal distortion of a CW CO2laser beam propagating in CS2that moves transverse to the propagation direction. The data verify the predicted beam deflection and the defocusing as well as focusing effects of the self-induced thermal distortion.