Holographic system for the storage of audio, video and computer data

Abstract

A rotating disc-shaped carrier upon which have been recorded audio, video or computer data in holographic form, a method for forming such a carrier, and an apparatus for reading such a carrier is disclosed. A beam of coherent radiation is spatially modulated in a prescribed way, this beam then illuminating one radial of a rotating carrier upon which has been recorded a series of holograms in the form of a spiral track or series of concentric tracks. The holograms are elongated in a direction perpendicular to movement of the rotating carrier and overlap in directions both parallel and perpendicular to such movement, the amount of overlap in the perpendicular direction being inversely proportional to the amount of overlap in the parallel direction such that data storage density remains constant across the surface of the rotating carrier. The sequentially illuminated holograms generate a series of signal images which vary in amplitude, width, spatial frequency, azimuth or position relative to a centerline. Detection of the signal images and extraction of uncorrelated noise resulting from a coded reference beam multiplexing procedure are accomplished by means of a differential photodetection system. Toleration of lateral misalignment between signal images and the photodetection means is accomplished by elongation of signal images in a direction substantially perpendicular to the signal image path, or by employment of point-source-type signal images which are detected by a transversely-positioned array of differentially-operating pairs of photosensors.