Influence of laser-target interaction of the polarization of a CO2-laser

Laser materials processing shows a special pecularity compared to other customary techniques: the (generally reflecting) target introduces optical feedback into the system. This feedback changes the mode properties of the laser radiation according to the targets dynamics. We report on one of these aspects of laser-target interaction resulting in the change of the polarization of the incident light. Based on rate equations, a theoretical model is presented that allows the calculation of this change with respect of the target properties, yielding a simple relation for the two orthogonal planes of polarization of a laser mode. This relation turns out to be linearly dependent of a function Psi (t) which describes the optical feedback. The relation holds for target reflexions of up to 10% and for times larger than Tau 1 x Tau 2/Tau 1 - Tau 2 (where Tau 1, Tau 2 are the time constants of the passive resonator for the two orthogonal planes of polarization). The model offers a method for the m odulation of laser radiation without change of frequency or intensity. It might also be of interest for high-power CO2 laser cutting and welding of metals.