Page 475 - Introduction to Information Optics

460 8. Information Storage with Optics

once again, to return the light polarization to 0°. Therefore, the reconstructed
light is transmitted through the rest of layers unaffected.

8.5,4. VOLUME HOLOGRAPHIC 3-D OPTICAL STORAGE

We have discussed a hologram that records the wavefront from a flower
using a window analogy. Through the window we can see not only a flower
but many other objects as well. The window can transmit wavefronts from an
infinite number of objects, since photons represented by wavefronts do not
interact with each other. However, only a limited number of wavefronts can be
recorded on a hologram because the wavefronts must be recorded in interfer-
ence patterns, and only a limited number of patterns can be recorded
depending on the resolution and dynamic range of the hologram. Obviously,
the larger the hologram is, the larger the storage capacity will be.
In contrast to conventional imaging that has to store the light pattern on a
plane, the holographic technique transforms a 2-D pattern into a 3-D interfer-
ence pattern for recording [5,6, 52, 53]. The recorded 3-D interference pattern,
which is known as a volume hologram, can be transformed back to the original
2-D object pattern in the reconstruction process. Consequently, the storage
capacity can be increased significantly by recording the interference pattern in
a 3-D medium. Since a volume hologram acquires the properties of a 3-D
diffraction grating, the reconstruction is subject to Bragg's condition. The
reconstruction beam with the same wavelength must be at the same angle as
the reference beam in the recording. Contrary to a plane hologram that can be
reconstructed from almost any angle, the reconstruction beam of a volume
hologram will fail to produce the object wavefront at any angle except the
recording angle.
Holographic storage techniques usually apply the same beam for writing
and reading, or different wavelengths of light for writing and reading provided
that Bragg's condition is satisfied, regardless of whether it is a plane or volume
hologram. Therefore, there is almost no difference in the architectures of plane
or volume holographic storage.
It is interesting to note that in ordinary applications, a plane hologram is
used to produce the 3-D display of an object. On the other hand, in digital
optical storage applications, a volume hologram is used to record and display
a 2-D page memory.
The first prototype of 3-D holographic optical storage with an ingenious
engineering design was demonstrated by D'Auria et al. in 1974 [54]. It
was based on the architecture shown in Fig. 8.7. However, each tiny
plane subhologram was replaced by a volume subhologram employing
Fe- doped lithium niobate. With an additional deflector (not shown in
Fig. 8.7), their ingenious design enabled the reference beam (which was