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.
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.
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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