Dynamic phase-encoding storage of 64 images in a BaTiO3 photorefractive crystal.

We present a photorefractive (PR) memory setup that permits the storage of N = 64 images of 128 X 128 binary pixels in a BaTiO3 crystal by the deterministic phase-encoding technique.`-4 Because of the absence of any moving mechanical parts this multiplexing method permits a reliable and very fast random access to the images. This memory could be used in digital computing or for pattern recognition.' This technique consists of recording the N images by interference of the image beam with all N reference beams. These beams are angularly spaced by more than the Bragg angle so that each of them reconstructs the holograms it has written. The same N reference beams are used to record all images. However, the set of relative phase (O or -r) between the reference beams and the image beam is changed for each image and represents the image address. We reconstruct the images by reading out the holograms with the entire set of reference beams carrying the phase code corresponding to the image address. Thus, during image retrieval, all the N reconstructed beams for each of the N images interfere. If a set of orthogonal binary (i.e., 0 and 7-) Walsh-Hadamard phase codes2 -5 is chosen, all reconstructed beams corresponding to that image interfere constructively, while all others interfere destructively. However, in practice, incomplete destructive interferences produce cross talk. We previously' found the two following sources of cross talk, which were evaluated for binary stored images with the same probability for white and black pixels: (i) The phase modulator introduces a phase shift of 0 or 7r + s on each beam. The predominant source of cross talk arises when the phase-shift error 8 is the same on all N reference beams (systematic error). This is the only source that we consider here. In that case the amplitude signal-to-noise ratio (SNR) limited by cross talk was found to be