Page 416 - Introduction to Information Optics

7.7. Pattern Recognition with Photorefractive Optics 401

0 0 0 0 0 0
1 1 1 1 1 1
2 2 2 2 2 2 P>~
3 3 3 3 3 3 LA.
4 4 4 4 4 4
5 5 5 5 5 5
(a)

Fig. 7.39. (a) Input to the JTC -NNC, where a normal Times New Roman "2" is used as the input
character, (b) Output correlation distribution.

cross-correlation peak intensity. The grids provided show the region of interest
for each character.

7.7. PATTERN RECOGNITION WITH PHOTOREFRACTIVE OPTICS

Recent advances in photorefractive (PR) materials have stimulated interest
in phase-conjugate correlators for pattern recognition. Although SLMs can be
used to display complex spatial filters, current state-of-the-art SLMs are
low-resolution and low-capacity devices. On the other hand, PR materials offer
real-time recording, high resolution, and massive storage capacity; all desirable
traits for multiplexed matched filter synthesis.

7.7.1. DETECTION BY PHASE CONJUGATION
In most optical correlators the input objects are required to be free from
any phase distortion. The input objects are usually generated by an SLM.
However, most SLMs introduce phase distortion, which severely degrade
detection performance. Other problems are that the autocorrelation distribu-
tion is too broad and the cross-correlation intensity is too high. These result
in low accuracy and reliability of detection. To improve performance, the input
target can be pre-encoded, for which the inherent nonlinearities in phase
conjunction can be used for the encoding process. Let us consider a four-wave