COHERENT ERASURE AND UPDATING OF HOLOGRAMS IN LINBO3

The effects of a phase shift in the reference beam between the recording of consecutive volume holograms in LiNbO3 are studied. In particular a second exposure of a hologram using a phase shift of pi in the reference beam results in its erasure, named coherent erasure, which is several times faster than the erasure with homogeneous illumination, or incoherent erasure. Using this coherent erasure a part of a hologram is selectivity removed with almost no degradation of the rest of the image. This is achieved by pi-phase shifted recording of the part of the image to be erased. At the same time, the process can be used to write other parts of the hologram. This work shows that this procedure, if used as gradual erasure of part of an image, can be applied to image processing. If total erasure and writing of parts of holograms are performed, the effect can also be used for performing logical operations. It is shown also that these effects combined with the angular multiplexing capability of volume holograms form a high capacity random access optical memory with some interesting particular features.