Optical storage of digital information

Abstract

Optical data storage provides a solution for the requirements of large read-write mass memories (capacity greater than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> bits) with small random access time (typically several μs) with high speed data transfer rates (0.1-1 Gbit/s). The well-known properties characteristic of holographic storage, including lensless stigmatic imaging capabilities and redundancy, make the holographic storage technique attractive. When using a two-dimensional storage medium, theoretical studies indicate that an information density on the order of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> bit/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> are obtainable, or a wavelength of 0.5 μm. Even if one used optical components (lenses, etc.) having transverse dimensions as large as 80 cm, such a system would be limited to a capacity of about 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> bits. To obtain larger capacities it will be necessary to use a 3-dimensional storage medium allowing superposition of the stored information. This could be obtained by varying the angle of the holographic reference beam. A practical system for realizing such a memory is presented here. The organization of the memory is compatible with the use of a reversible storage material. The read-write functions are realized using an acoustooptic deflector, which eliminates any mechanically moving parts. The organization permits the use of a second deflector to vary the reference angle and allows superposition of holograms [1]. Possible recording materials which could be used in such memories are ferroelectric crystals or ceramics. Experiments in our laboratory have demonstrated the possibility of electrically controlling the fixation and erasure of holograms in these materials [2].