Pattern dynamics and competition in a photorefractive feedback system

We investigate the temporal dynamics of transverse optical patterns spontaneously formed in a photorefractive single-feedback system with a virtual feedback mirror. The linear stability analysis for the system is reviewed and extended to the region of larger propagation lengths. The stationary patterns obtained experimentally are classified as a function of feedback reflectivity and feedback mirror position. Inserting masks into the feedback path permits pattern selection and control by Fourier filtering. When an asymmetry that is due to noncollinear pump beams is introduced, the otherwise stationary hexagons show several complex but periodic rotationlike motions. Furthermore, the competition of hexagonal and square patterns can be observed by the appropriate choice of feedback mirror position and coupling strength. The origin of this behavior is discussed. The temporal evolution of the patterns is illustrated by a method based on unfolding the angular distribution of the spots in the far field. (C) 1998 Optical Society of America.