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Single-Beam Setup Simplifies Holography

Photonics Spectra
Nov 2001
Hank Hogan

Holographic memory storage is no longer the stuff of science fiction but the target of scientific and commercial research. Thanks to a clever setup that uses one laser beam as both the object and the reference beam, the technology may be closer to reality.

Holographic memory, storing information throughout the volume of the medium, promises greater data capacities and faster data-retrieval rates. However, current holographic storage systems are too large to be commercially viable.

One contributing factor to their bulk is the need for both an object and a reference beam that cross in the recording medium. In the past, these were created from a single source and a beamsplitter, leaving the systems vulnerable to mechanical vibration.

Researchers Hideta Mitsuhashi and Minoru Obara of Keio University in Yokohama, Japan, have constructed a holographic memory system that uses a single laser beam and an Fe-doped LiNbO3 crystal. They shine a laser beam on the edge of the 45°-cut crystal and place a mirror on the opposite side.

Passing through the crystal face, the beam bends and crosses itself. When one leg of the bent beam strikes the mirror, it travels back along its incoming optical path. As a consequence of this crossing and doubling back, the same beam can be both the object and the reference light source.

The team constructed a prototype system based on this technique using a 635-nm laser beam and a 1 x 1 x 1-cm crystal. The total system is only 10 x 4.5 x 4.0 cm, but it is theoretically capable of storing 23 Gb of data if the beam size is 5 mm. The researchers believe the system's performance approaches that needed for a commercial random access memory product.

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