Kinky beam paths inside photorefractive crystals

Abstract
Beam paths inside photorefractive crystals under conditions of beam fanning or self-pumped conjugation that uses crystal corners are often kinky-i.e., they consist of short, straight line segments as opposed to smoothly curved paths. We show that kinky paths occur naturally when the beam-coupling equations including noise source terms are solved to all Bragg orders without phase-matching approximations. Higher-order gratings arise in high-gain two-wave mixing without noise but are apparently unobservable because fanning always dominates in real crystals. In the presence of noise and fanning the residual signature of the higher-order gratings is kinky beam paths.