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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.

Volume Holograms
To make the hologram, the reference and object beams are overlapped in a photosensitive medium, such as a photopolymer or inorganic crystal. The resulting optical interference pattern creates chemical and/or physical changes in the absorption, refractive index or thickness of the storage media, preserving a replica of the illuminating interference pattern. Since this pattern contains information about both the amplitude and the phase of the two light beams, when the recording is illuminated by the readout beam, some of the light is diffracted to "reconstruct" a weak copy of the object beam .If the object beam originally came from a 3-D object, then the reconstructed hologram makes the 3-D object reappear. Since the diffracted wave front accumulates energy from throughout the thickness of the storage material, a small change in either the wavelength or angle of the readout beam generates enough destructive interference to make the hologram effectively disappear through Bragg selectivity.

As the material becomes thicker, accessing a stored volume hologram requires tight tolerances on the stability and repeatability of the wavelength and incidence angle provided by the laser and readout optics. However, destructive interference also opens up a tremendous opportunity: a small storage volume can now store multiple superimposed holograms, each one distributed throughout the entire volume. The destructive interference allows each of these stored holograms to be independently accessed with its original reference beam. To record a second, angularly multiplexed hologram, for instance, the angle of the reference beam is changed sufficiently so that the reconstruction of the first hologram effectively disappears. The new incidence angle is used to record a second hologram with a new object beam.

The two holograms can be independently accessed by changing the readout laser beam angle back and forth. For a 2-cm hologram thickness, the angular sensitivity is only 0.0015 degrees. Therefore, it becomes possible to store thousands of holograms within the allowable range of reference arm angles (typically 20-30 degrees). The maximum number of holograms stored at a single location to date7 is 10,000.

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