Page 476 - Introduction to Information Optics

8.6. Near Field Optical Storage 461

also the read beam) to arrive at the tiny volume subhologram with a rotat-
able angle. Thus each tiny subhologram could be angular-multiplexed with
2
many page memories. The area of the subhologram was 5x 5 mm , and its
thickness was 3 mm. Ten 8 x 8 bit page memories were multiplexed in a
subhologram.
The cubic subholograms mentioned above can be replaced with SBN fibers
[55, 56]. The diameter of the fibers is approximately 1 mm and they are about
4 mm long. Thirty to fifty page memories can be multiplexed and stored in an
SBN fiber. Hundreds of fibers are arranged in a near-fiber-touching-fiber
configuration, much like in a conventional microchannel plate. A merit of this
approach is that a high-quality SBN crystal fiber is easier to grow than a bulk
crystal. On the other hand, with constant progress of enabling technologies,
Mok et al. successfully demonstrated the storage of 500 holograms of images
consisting of 320 x 220 pixels in a 1 cm x 1 cm x 1 cm Fe-doped lithiun niobate
crystal [57] and then 5000 holograms in a 2 cm x 1.5 cm x 1 cm Fe doped
lithium niobate crystal [58].
Recently, 3-D holographic optical storage was demonstrated using both
wavefront and angular multiplexing [59]. Wavefront multiplexing is achieved
by passing the reference beam through a phase-only spatial-light modulator
that displays a specific phase pattern. The computer-generated phase patterns
for multiplexing are orthogonal to each other. Angular multiplexing is pro-
vided by tilting the hologram. Thirty-six orthogonal phase patterns were
generated using a computer based on a specific algorithm. The hologram was
recorded at three angles. A total of 108 holograms were successfully recorded,
six of which are shown in Fig. 8.8.

8.6. NEAR FIELD OPTICAL STORAGE

The resolution of an image displayed on a computer monitor is restricted
by the size and number of pixels that constitute the display. For example, a
liquid crystal display may have about 1 million pixels (1000 x 1000); each pixel
is about 10 /im x 10 fim. Similarly, images we see in the real world are also
limited by the size and number of light-sensitive cells in our retinas. We have
about 120 million photoreceptor rod cells of 2 /on in diameter that are highly
sensitive in dim light but insensitive to colors, and about 6 million photorecep-
tor cone cells of 6 /urn in diameter sensitive to colors but insensitive at low light
levels. If the image detail cannot be resolved by photoreceptor cells in the
retina, to clearly see the image, it must be magnified optically before entering
our eyes. This is normally done using an optical microscope.
However, there is a natural limitation that a microscope cannot magnify an
image arbitrarily and infinitely. The resolution limit of a lens is determined by