Dynamics of a composite grating in photorefractive crystals for memory application

The dynamics of a composite grating that consists of an original grating and a newly superimposed grating are investigated for memory applications by solution of coupled-wave equations and photorefractive-material equations with the following initial conditions: the original grating has an arbitrary refractive index modulation, an arbitrary phase shift exists between the original and the superimposed grating, and there is an externally applied dc electric field. The effect of beam coupling, including fringe bending on the composite grating, is investigated by numerical simulation. Simplified analytical solutions that neglect beam coupling are derived, showing good agreement with experimental results. The investigation also shows that a selective erasure process in the presence of an external electric field is made possible by provision of an appropriate constant phase shift. Analytical solutions of the composite-grating dynamics are used to analyze both scheduled and incremental recording dynamics with an external electric field. It is shown that scheduled recording results in a nonuniform relative phase distribution among multiplexed gratings. In contrast, the incremental recording converges to a uniform relative phase distribution and indicates that selective erasure may be easily implemented for fast memory update.