Page 470 - Introduction to Information Optics

8.5. Holographic Optical Storage 455

original image. This section reviews techniques to store a bit pattern in a
hologram.
In the previous section, we learned that a 3-D storage in principle consists
of multilayers whether there is a delineated layered structure (ET material) or
not (two-photon absorption). When a specific layer is selected to be written or
read, other layers must be effectively deactivated. The signal light interacts with
only the selected layer and passes all other layers unaffected. This is analogous
to selecting a page while reading a book.
In holographic 3-D optical storage, the input 2-D page memory is coded
and distributed to the entire storage space. If we consider that the storage
consists of layers, the page memory is stored in every layer with different
coding. When the stored page memory is going to be read, every layer is read,
and no layer is deactivated. With proper decoding, the stored page memory
can be reconstructed. Instead of reading a book, this is analogous to imaging
a cross section of the human body by employing computer tomography.
However, no computer is needed to reconstruct a volume hologram.

8,51. PRINCIPLE OF HOLOGRAPHY

Holography was originally invented by Gabor [37] in 1948 to improve
electron microscope images. Holography is a technique to record and to
reconstruct wavefronts. Consider that we are in a room looking at an object,
such as a flower, through a window. The light from the flower must transmit
through the window to form an image on our retinas. If we could record the
light at the window plane, then by reproducing the recorded light we would be
able to see the flower, although there is no window. This cannot be done by
traditional photographic techniques, because the recording medium is sensitive
only to energy or light intensity and is not sensitive to phase. The light from
the flower at the window plane is represented by its wavefront that has both
amplitude and phase. The artificial window can only be made by recording and
reconstructing the wavefront at the window plane. The wavefront can be
recorded and later reconstructed by recording the interference pattern of the
object wavefront and a reference wavefront [37]. Holography has been
improved and made practical by Leith and Upatnieks [38], who introduced
the concept of carrier frequency in 1962.
The principle of holography can be described briefly as follows. The
recorded intensity on a hologram is

2 2 2
\0 + R\ = \0\ + \R\ + OR* + 0*R, (8.9)
where O and R are complex amplitudes of object and reference beams,
respectively, on the hologram plane. The symbol * indicates a complex