Page 465 - Introduction to Information Optics

450 8. Information Storage with Optics

between the two layers, the laser beam diameter on the adjacent layer may
significantly increase and the intensity may drop accordingly. This can be
easily applied to an optical disk, because both writing and reading are based
on a sequential mechanism. There is only one laser spot at a time. When a page
memory is written or read in parallel, there are many bright laser spots at the
same time. On the adjacent layers, although an individual laser spot is blurred,
there is a possibility that those blurred laser spots overlap and the super-
imposed intensity is as high as a single focused spot.
In order to be able to select a page, a switch is required to activate only a
layer at a time. The switch could be a photon-gating process in which the
writing and reading of the optical memory requires two photons at the same
site and the same time. By using a laser beam (first photon) to image the page
memory onto the selected layer and providing another laser beam (second
photon) to the selected layer only, we can selectively write or read a layer.
This scheme is best suitable for two-photon-absorption materials
[22,24,25]. The principle of this architecture is schematically shown in Fig. 8.3.
Note that the 3-D storage is a bulk transparent media made from a two-
photon-absorption material lacking a delineated layered structure. The acti-
vated layer is formed optically by illuminating the bulk medium from the side
with a sheet of light. The sheet of light is formed by focusing a collimated light
using a cylindrical lens. An experimental demonstration with one activated
layer in the bulk medium was performed recently by Hunter et al. [24].
Alternatively, the stored data can be read sequentially spot by spot instead of
a parallel reading of the whole layer. This will eliminate the difficulty of
forming a uniform sheet of light. The sequential reading architecture can also

Activating
Light

Cylindrical
Lens

Write-in Page
Beam Memory

Imaging Lens Two-Photon Imaging Lens
Absorption
Material
Fig. 8.3. Architecture of 3-D optical storage using two-photon-absorption materials.