Special Report: Optical Patterns
Joint transform correlator [56,57]: The joint transform correlator
has become popular these days because it is simple to implement and real-time
operation is possible with currently available devices (SLM's and 2-D CCD's)
[58]. The operation of the joint transform correlator can be considered
a two-step Fourier spectrum (modulus square of a Fourier transform) generation.
At first, both an input and a reference image are located in the input plane
(spatial domain) side by side. The Fourier spectrum of the two inputs is
recorded in a high-resolution recording material. The Fourier spectrum of
the two inputs is Fourier transformed again by the second Fourier spectrum
generator. The first order diffraction outputs from the second Fourier spectrum
generator are the correlations of the input and the reference images. Optics
can perform a real-time Fourier spectrum generation quite well in a simple
and low-cost setup consisting of a laser, an SLM, a 2-D image sensor, such
as a CCD, and a spherical lens. The advantages of the system are that it
allows easy real-time implementation of a correlator with currently available
devices and it does not require accurate filter positioning. Also, as in
a VanderLugt correlator, various spatial filtering operations such as bipolar
phase-only filtering can be incorporated [59-61]. Moreover, one can use
different wavelengths for hologram recording and readout without requiring
careful alignment. The disadvantages are that it requires a fast and high
resolution holographic recording material and the correlator is not all-optical,
requiring intermediate photon-to-electron conversion processes. The recent
photorefractive semi-insulating multiple-quantum-well SLM developed by A.
Partovi et al. [62] has many desirable features suitable for such correlators.
It has high speed (a several microseconds for 280 mW/cm2 at 600nm - 850
nm), high sensitivity (0.8 mJ/cm2 to allow the use of a low power laser
diode), high efficiency (3%), low operational voltage (20 V), a large index
change (0.06), and a reasonable resolution (50 lines/mm). A high-speed joint
transform correlator that uses this device has been successfully demonstrated
[63]. The system uses a low power laser diode (3 mW) and is capable of 3
X 105 correlations/s. When the system is incorporated with a fast ferroelectric
liquid crystal SLM and a fast detector, several thousand correlations can
be achieved within a second. This speed is about two orders of magnitude
faster than that of digital computers. Although the above correlators are
described in one-dimensional terms, many combinations of the above architectures
are possible [64,65]. The classical holographic pattern recognition system
in (5) has had limited application because it is bulky and is too sensitive
to misalignment, input recording materials, and filters. Such problems can
be greatly alleviated with a SELDA. Two recently developed correlators based
on microlasers are introduced below.
12.3.3 Multichannel optical correlator based on a mutually incoherent microlaser
array
Yang and Gregory [66] demonstrated a multichannel optical correlator by
using the mutually incoherent property of a microlaser array to improve
the performance of an optical correlator with respect to immunity to coherent
noise and a high SNR as well as high light throughput. As shown in Fig.
12.9, an optical correlator is incoherently duplicated by an array of light
sources to increase the SNR.
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