Design of a large read-only holographic memory

Factors affecting the design of a read-only optical memory having a total capacity of the order of 108 bits with fast random access are discussed. Photographic information storage is used and random address selection is provided by a laser beam scanned across the photographic plate using non-mechanical beam deflection. `Page¿ organization of the memory involving parallel read-out of 104 bits from each of 104 beam addresses is used in order to achieve the large total storage capacity. the information array from each address is projected on to an array of 104 photodetectors and selection of the required bits from each `page¿ is done electronically. Information is stored at each address in the form of a small hologram, which offers the following advantages over the conventional method of storage: uncritical alignment of the photographic plate, reduced sensitivity of the plate to accidental damage by incorporating redundancy in the hologram, and the ease with which the reconstructions of each hologram may be superimposed at the output photodetectors.