Two-Beam Coupling with Partially Coherent Light

In photorefractive two-beam coupling the interference of two optical beams creates a refractive index grating resulting in energy transfer between the two beams. In models describing this effect it is usually assumed that both beams have perfect spatial coherence. For some applications such as achromatic volume holography [1], multimode fiber gyroscopes and optical phase conjugation through turbulence, spatial coherence effects may become important in the coupling process. Reduced spatial coherence leads to lower contrast interference fringes in an interference fringe region smaller than that of the perfectly coherent case. In recent work, we have made a quantitative study of the influence of these effects on coupling under the assumption that the coherence properties of the interacting beams do not change during the coupling [2]. In the present work, we include the influence that beam coupling has on spatial coherence of the interacting beams. It is worth noting that the problem of spatial coherence in nonlinear wave mixing has been studied earlier [3-5]. However, all of these studies referred to the case of fast Kerr-medium. Photorefractive materials are much slower and cannot follow fast changes of the phases of interacting beams. Thus description of the mixing process will be different.