In photography, information is stored in a medium which is essentially two-dimensional. Three-dimensional optical storage is possible in semitransparent colored materials, like alkali halides with color centers. With the use of coherent light sources, like lasers, large amounts of information can be stored in the volume, and retrieved with little interference. The storage of information is accomplished by the formation of interference patterns between each two plane parallel waves. This paper develops the theory of this form of storage. It turns out that the information storage capacity is as if every little cube with sides equal to the wavelength of light acts as an independent information storage cell, and the essential noise in recovering this information is only the statistical fluctuation in the number of color centers in such a cube. The storage capacity is therefore of the order of 1012–1013 bits per cm3. The main property of this way of information storage is the appearance of a “ghost image,” partly but not completely analogous to the one described previously. This property makes three-dimensional storage very suitable for associative memories. The theory lends support to Beurle’s proposed mechanism of information storage in the brain.
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