Don A. Gregory

Adaptive real-time pattern recognition using a liquid crystal TV based joint transform correlator

Recently, many applications of the liquid crystal television (LCTV) to real-time signal processing have been reported. A basic description of the application of the LCTV to realtime pattern recognition was first reported by Liu et al. Gregory later also showed a successful space-invariant correlation using a LCTV as a spatial light modulator. Both methods were based on the use of a complex matched (VanderLugt) spatial filter. The joint transform architecture is an alternative approach to optical pattern recognition. The joint transform method has proved to be suitable for adaptive programmable correlation because no matched spatial filter is required. The reference pattern may simply be generated by a computer and input to a low space-bandwidth product (SBP), electronically addressed, spatial light modulator. Conversely, it is very difficult to generate a dynamic matched spatial filter in the Fourier plane. A microcomputer-based programmable optical correlator using a magnetooptical device (MOD) and a liquid crystal light valve (LCLV) was proposed recently by Yu and Ludman. In this Letter, a new implementation of the real-time joint transform correlator architecture using inexpensive LCTVs will be discussed. Preliminary experimental results are presented to verify the usefulness of the technique. There are three major objections to using commercially available liquid crystal TVs for optical processing applications: (1) low contrast ratio, (2) phase nonuniformity, and (3) low resolution and low SBP. These problems must be minimized if a LCTV is to be used as a spatial light modulator in a coherent optical system. The contrast uniformity

Self-organizing optical neural network for unsupervised learning

One of the features in neural computing must be the ability to adapt to a changeable environment and to recognize unknown objects. This paper deals with an adaptive optical neural network using Kohonens self-organizing feature map algorithm for unsupervised learning. A compact optical neural network of 64 neurons using liquid crystal televisions is used for this study. To test the performance of the self-organizing neural network, experimental demonstrations and computer simulations are provided. Effects due to unsupervised learning parameters are analyzed. We show that the optical neural network is capable of performing both unsupervised learning and pattern recognition operations simultaneously, by setting two matching scores in the learning algorithm. By using a slower learning rate, the construction of the memory matrix becomes more organized topologically. Moreover, the introduction of forbidden regions in the memory space enables the neural network to learn new patterns without erasing the old ones.

Modulation characteristics of the Epson liquid crystal television

The amplitude- and phase-modulating properties of liquid crystal televisions (LCTVs) are becoming increasingly well known. The Epson Crystal Image video projector is a relatively new TV and uses three liquid crystal panels to encode the red, green, and blue components of the video signal onto the projector light. These panels can be removed for use in optical systems. We present the results from measurements of the phase- and amplitude-modulation properties of one of these LCTV panels.

Real-time pattern recognition using a modified liquid crystal television in a coherent optical correlator

La television a cristal liquide est utilisee comme modulateur spatial dans un correlateur optique

Effects of fringe binarization of multiobject joint transform correlation

A study of the effect of fringe binarization on the joint transform correlator has shown that the interference fringes for multiple targets could produce false alarms and misses.

Full complex modulation using liquid-crystal televisions

An optical architecture is proposed that uses two modified liquid-crystal televisions (LCTVs) to control amplitude and phase modulation independently. Two applications in pattern recognition are discussed.

Terahertz digital holography using angular spectrum and dual wavelength reconstruction methods

Terahertz digital off-axis holography is demonstrated using a Mach-Zehnder interferometer with a highly coherent, frequency tunable, continuous wave terahertz source emitting around 0.7 THz and a single, spatially-scanned Schottky diode detector. The reconstruction of amplitude and phase objects is performed digitally using the angular spectrum method in conjunction with Fourier space filtering to reduce noise from the twin image and DC term. Phase unwrapping is achieved using the dual wavelength method, which offers an automated approach to overcome the 2π phase ambiguity. Potential applications for nondestructive test and evaluation of visually opaque dielectric and composite objects are discussed.

Reconfigurable interconnections using photorefractive holograms

Using coupled wave theory and the law of refraction, diffraction properties of volume holograms are discussed. Reconfigurable interconnections by either wavelength tuning or spatial division techniques are proposed. Reflection type volume holograms can be used for a large number of reconfigurable interconnections in terms of finite wavelength tunability. Transmission volume holograms encoded in pinhole holograms can be easily reconfigured by spatial light modulator. Experimental demonstrations obtained by using these methods are presented.

Experiments on nonlinear joint transform correlator using an optically addressed spatial light modulator in the Fourier plane

Correlation experiments for the images used in the proposed setup indicate that nonlinear compression of the joint power spectrum may be necessary to produce good correlation performance and a peak-to-sidelobe ratio of larger than unity.

SELF-ORGANIZATION OF SCATTERING IN PHOTOREFRACTIVE KNBO3 INTO A RECONFIGURABLE HEXAGONAL SPOT ARRAY

The scattering cone (~2 degrees ) from an Ar laser beam (514 nm) in photorefractive KNbO(3):Fe has been observed to transform into a reconfigurable hexagonal spot array. This phenomenon can be explained through the creation of laser-induced transmission and reflection grating arrays.