Page 452 - Introduction to Information Optics

8.2. Upper Limit of Optical Storage Density 4 3 /

wavelength of light. In practice, one may take an approximation

Q x L (8.2)
2
For further simplification, one may consider that an area of A is required
to store a bit optically. Thus, the upper limit of storage capacity for a 2-1)
medium is

(8.3)

By considering the third dimension, one may similarly infer that a volume of
3
A is required to store a bit in a bulk optical memory, and the upper limit of
the storage capacity for 3-D medium becomes

. (8.4)

Instead of an image, we may alternatively record its Fourier-transform
hologram as the memory. The original image would appear when the holo-
gram is illuminated with the same reference beam as it was recorded. A
hologram has a total area of a x a, and the size of its smallest element is / x /,.
The image is holographically formed by a Fourier-transform lens. Recalling
Fourier analysis [2, 3, 4], the smallest element A x A will determine the size of
the image that is proportional to I/ A x I/A, and the size of the hologram a x a
will determine the size of the smallest element of the image, which is propor-
tional to I/a x I/a. Correspondingly, the storage capacity of a hologram is

SC H = -75 . (8.5)
A~

The storage capacity of the image is

l
SC- -!£-?- (86)
(8.6)
5C,- i/fl2 - _
2
The storage density of 2-D medium is the same; that is, I/A , regardless of
whether there is a direct bit pattern or a hologram. However, as we will find
in a later section, the density of near field optical storage is higher than the
limit set by the diffraction, since no diffraction occurs in near field optics.
When a bulk holographic medium is used to make a volume hologram, it
can be multiplexed by n holograms. The maximum multiplexing is

(8,7)