- Wave equations describing optical phase conjugation in photorefractive crystals are solved using a technique that transforms a boundary-value problem into an initial-value problem. Boundary conditions are satisfied using an iterative map constructed in the parameter space. Within the iterative procedure unstable situations arise, leading to a chaotic response of the crystal. A strange attractor is discovered in the reflectivity of the crystal, existing in the multiparameter space of the single-interaction-region four-wave-mixing process. 1991 The American Physical Society.