Focal splitting and optical vortex structure induced by stress birefringence

A variety of interesting polarization effects can be observed using a parallel-face window placed under symmetric stress of order m = 3 and illuminated with polarized light. Such windows, when placed under sufficient stress, can produce rings of alternating vortex and non-vortex fields. When light is brought to a focus, one component of circular polarization forms two nearly diffraction limited focal spots with axial separation larger than the usual depth of focus. We analyze and experimentally test these phenomena using interferometric methods as well as a Strehl ratio model and conclude by discussing applications to optical imaging.