Page 133 - Introduction to Information Optics

1 18 2. Signal Processing with Optics

Several semi-insulating semiconductors, such as gallium arsenide (GaAs),
gallium phosphite (GaP), and idium phosphite (InP), have demonstrated PR
effect and have been used in optical processing systems. A prominent feature
of these semiconductor crystals is their fast response to optical fields; i.e., the
small value of time constant T at low light intensity. Typically, a submillisecond
response time can be achieved with GaAs at a modest laser power intensity of
2
100 mW/cm , which is one or two orders of magnitude faster than the more
conventional materials such as BaTiO 3 under the same conditions. The
spectral response range of these materials is in the near infrared wavelength.
This can be an advantage or a disadvantage, depending upon the applications.
One of the problems of these semiconductor PR materials is the disparity
among between different samples. The PR effect of the same material varies
considerably among suppliers as well as from one ingot to another.
Photopolymer is a new type of PR material. Its PR effect was first
discovered in the early 1990s. Recently a photopolymer based on photocon-
ductor poly (N-vinylcarbazalo) doped with a nonlinear optical chromophore
has been developed; it exhibits better performance than most inorganic PR
crystals. Due to its relatively small thickness (typically tens of micrometers),
only a limited number of images can be multiplexed. However, the photo-
polymer is more suitable for the implementation of 3-D optical disks, which
can store a huge amount of data.

2.7.2. WAVE MIXING AND MULTIPLEXING

There are two generic configurations for optical pattern recognition systems
using photorefractive materials; namely, two-wave mixing and four-wave mix-
ing. Depending upon the coherence among the read and write beams, four-
wave mixing configurations can be further classified into degenerated four-
wave mixing and nondegenerated four-wave mixing.
In a two-wave mixing configuration, shown in Fig. 2.36, two coherent beams
intersect in a PR medium and create an index grating. The Bragg scattering
involved in two-wave mixing is very similar to the readout process in
holography. If one lets beam A 2 be the reference beam, the variation of the PR
index can be written as

T
yK r
AH(.X) oc: A^A 2e~^ + /M*< ' . (2.101 )
where

K = k 2 -k^

and k, and k 2 are wave vectors of the two beams, respectively. When such a