Holographic Data Storage -3D Optical Memories

Mass memory systems serve computer needs in both archival and backup needs. There exist numerous applications in both the commercial and military sectors that require data storage with huge capacity, high data rates and fast access. To address such needs 3-D optical memories have been proposed. Since the data are stored in volume, they are capable of much higher storage densities than
existing 2-D memory systems. In addition this memory system has the potential for parallel access. Instead of writing or reading a sequence of bits at each time, entire 2-D data pages can be accessed at one go. With advances in the growth and preparation of various photorefractive materials, along with the advances in device technologies such as spatial light modulators(SLM), and detector arrays, the realizations of this optical system is becoming feasible.

A hologram is a recording of the optical interference pattern that forms at the intersection of two coherent optical beams. Typically, light from a single laser is split into two paths, the signal path and the reference path.. The beam that propagates along the signal path carries information, whereas the reference is designed to be simple to reproduce. A common reference beam is a plane wave: a light beam that propagates without converging or diverging. The two paths are overlapped on the holographic medium and the interference pattern between the two beams is recorded. A key property of this interferometric recording is that when it is illuminated by a readout beam, the signal beam is reproduced. In effect, some of the light is diffracted from the readout beam to “reconstruct” a weak copy of the signal beam. If the signal beam was created by reflecting light off a 3D object, then the reconstructed hologram makes the 3D object appear behind the holographic medium. When the hologram is recorded in a thin material, the readout beam can differ from the reference beam used for
recording and the scene will still appear.

Optical and electronic computers will soon require memories with capacities beyond those of magnetic or electronic systems. 3-D storage and parallel access are needed for densities greater than 1TBit/cm3, and I/O bandwidth in excess of 1 GBit/s. large 3-D memories offer a promise to meet such requirements. With state of the art yet off-the-shelf optoelectronic devices, (SLM’s, CCD array detectors, optical array generators etc.) thousands of data pages can be stored in the same volume of photorefractive crystal with phase code and rotation multiplexing. This approaches the theoretical limitation of a 3-D storage density.

A fast, compact, rugged, low-cost, and very high density 3-D holographic memory. If developed will have a significant impact on today’s data storage community and will have wide applications in areas of supercomputing, information highway, virtual reality, artificial intelligence, fingerprint analysis, medical diagnostics, large scale databases, etc.