In photorefractive holographic storages, the superimposed holograms make the refractive index of the media irregular. The readout beam is scattered by the media and the noise is added onto the reproduced messages. In this paper, we introduced a simple way to suppress the noise caused by the superimposition. In our method, holograms were recorded by convergent reference beam. We made the convergent point of the beam overlap the curvature center of a concave mirror. The reflected beam from the concave mirror was used to readout the holograms. By this method, we suppressed the noise caused by superimposition and realized noise-free readout in holographic storage.
Angular selectivity of volume grating limits the capacity of holographic 3-dimensional storage. In this work, we investigated experimentally the influence of wavefront shapes of the writing beams on the angular selectivity of thick grating in a LiNbO3 crystal. The experimental results showed that longer wavefront curvature results in more critical Bragg angular selectivity.