Implementation of a disk-type holographic memory system using photopolymer films and its applications

Holographic data storage attracts attention because of its potential storage capacity and parallel data retrieval. In volumetric holographic memories, thick photorefractive crystals are commonly used for the data storage medium. Recently, with the development of photopolymers as holographic recording materials, disk-type holographic memory systems are also being investigated. In this case, photopolymers are coated on a disk or film and then multiple pages of digital data are stored in the material holographically by use of multiplexing techniques. Shift multiplexing seems suitable in disk-type holographic memories. However, the memory performance of the system adopting shift multiplexing may be sensitive to disk wobble that may occur in a practical situation, because the reference beam is a spherical wave. Angle multiplexing, which is widely used in volumetric holographic memories, cannot be a sufficient one, when the thickness of the photopolymer is very thin. This is because the angle selectivity which is inversely proportional to the thickness becomes low and thus the number of pages that can be multiplexed at one spot of the photopolymer is limited. In our disk-type holographic memory system we implemented both angular and rotational multiplexing methods by controlling the direction of the plane wave reference beam using a pair of identical wedge prisms. Spatial multiplexing is accomplished simply by translating the medium itself.