Transverse modulational instability of periodic light patterns in photorefractive strontium barium niobate crystal

Interference fringes produced by the intersection of two plane waves in photorefractive strontium barium niobate crystal break in the transverse direction if the external electric field is high enough. For a certain relation between fringe spacing and nonlinearity strength, the tendency to regular pattern formation has been observed. We present a simple theory that qualitatively explains this effect.