Page 472 - Introduction to Information Optics

8.5. Holographic Optical Storage 4bl

when the hologram was recorded will reconstruct the object. This characteristic
provides multiplexing capability. A number of holograms can be recorded
successively with reference beams having different incident angles on the same
holographic plate. A specific angular reference beam would reconstruct only
the object that was recorded with it at a certain position. Note that other
objects are reconstructed at shifted positions. The multiplexing hologram can
also be produced with reference beams having specific wavefronts [44]. The
wavefront is generated by passing a plane wave through a phase-only spatial-
light modulator. In fact, this phase modulator can also generate a wavefront
similar to that of an oblique plane-wave reference beam.
Since the page memory must be displayed on a spatial-light modulator, and
the reconstructed image of the page memory must be read by an array detector,
the size of the page memory is restricted by the state of the art of the spatial-
light modulator and the array detector. The size of the page memory is also
restricted by the size of lenses and other optical elements used in the system.
Based on today's technology, it should be realistic to construct a page memory
that is 1000 x 1000 in size. Assuming the wavelength of light, A, is approxi-
2
mately 1 /irn, a page memory in principle could be stored in an area of 1 mm
either in a bit pattern or holographically.
6
The capacity of plane holographic storage might be larger than 10 bits,
6
while the string of bits arranged in a page memory has only 10 bits.
Consequently, many page memories can be stored in a plane hologram. As
mentioned previously, one may apply angular multiplexing technique to
superimpose a number of holograms on the same plate. However, larger optics
and higher laser power are required to cover the whole holographic plate at
one time. It is more practical to record each page memory in a tiny
subhologram. Subholograms form an array on the holographic plate. A laser
beam deflector is able to select a specific subhologram. In other words, a page
memory is retrieved by addressing the read beam to a selected subhologram
using the deflector. Figure 8.7 depicts the schematic diagram for plane
holographic storage. Needless to say, the object beam in the recording is also
deflected accordingly to form the subhologram array on the plate.
The experiment using an erasable magneto-optic MnBi thin film to record
a 8 x 8 bit page memory was demonstrated by Rajchman in 1970 [45]. The
MnBi thin film modulates the polarization of the read beam based on the
Faraday effect and the Kerr effect in the transmissive and reflective modes,
respectively. Note that the polarization modulation is actually phase modula-
tion in two polarization directions. Thus, a MnBi thin film acts as a phase
hologram and no analyzer is required in the reconstruction.
Instead of using beam deflectors, subholograms may be recorded on a
moving media [46]. In fact, holographic disks consisting of 1-D subholograms
have been experimentally demonstrated. The prototypes of a WORM holo-
graphic disk using a photographic plate and an erasable holographic disk using