Page 377 - Introduction to Information Optics

7. Pattern Recognition with Optics

Qtfri)

Fig. 7.9. A wavelength-multiplexed RC.

storage capacity, but the shift invariant property is limited by the Bragg
diffraction condition. This can be minimized by using a thinner crystal;
however, diffraction efficiency and storage capacity is substantially reduced.
For high storage capacity, high diffraction efficiency, and a large shift invari-
ance, a reflection-type wavelength-multiplexed PR matched filter can be used.
A reflection-type matched filter correlator (RC) is depicted in Fig. 7.9, in which
a z-cut PR crystal is used for matched filter synthesis. The matched filter is
recorded by combining the Fourier spectrum of an object beam qi(xj with a
reference plane wave from the opposite direction. Correlation can be done by
x at
inserting an input object q 2( 2) plane x. The output correlation distribution
r
can then be observed at plane x 3. In order to separate the reading beam q 2( i)
from the writing beam, the reading beam can be made orthogonally polarized
to the writing beam by using a polarized beam splitter. The insertion of a
polarizer P at the front of plane x 3 is to prevent the polarized writing beams
from reaching the output plane.

7.1.4. ROBUSTNESS OF JTC
One of the major distinctions between VLC and JTC is that VLC uses a
Fourier-domain filter and JTC uses a spatial-domain filter. We note that the
VLC filter is input signal independent, while JTPS is input signal dependent,
which produces poor diffraction efficiency under intense bright background or
a multitarget input scene. Nevertheless, these shortcomings can be easily
mitigated by the removal of the zero-order diffraction by means of computer
intervention, as will be discussed later. The major advantages of the JTC are
the flexibility to operate and robustness to environmental perturbation.
If we assume the Fourier-domain matched filter is transversely misaligned;