Spatial separation of circularly polarized laser beams in sodium vapor

We observed experimentally that a superposition of two co-propagating left and right hand circularly polarized laser beams is spatially unstable during propagation through a vapor of sodium atoms containing argon buffer gas. A small spatial asymmetry between the two input beams leads to their separation into two pure circularly polarized output beams. However a single linearly polarized input beam was found to be spatially stable. Our theoretical results from Maxwell-Bloch propagation calculations using both modal expansion and direct partial differential equation solution methods are in good agreement with the experiments. The splitting mechanism can be explained in simple terms as consisting of an absorption and dispersion dominated initial part and a diffraction dominated final part.