High capacity storage systems with fast data access are essential prerequisites for future multimedia services. For many applications, optical memories such as Compact Discs (CD) or Digital Veratile Discs (DVD) are preferred to magnetic high density systems due to the low production costs and high portability. However, optical disc systems store the data two dimensionally and a further increase in the storage capacities is only expected when blue laser diodes become commercially available and dual layer systems are developed. on the other hand, three dimensional storage systems have been proposed on the basis of holographic recording techniques. These systems use either photorefractive crystal cubes or layers of photopolymers to record complete digital data pages. Experiments demonstrate that holographic methods allow ultra high storage capacities but they need complex write and read-out devices, a fact which has up to now impeded the design of commercial products.
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