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