The principle of information retrieval by association has been suggested as a basis for parallel computing and as the process by which human memory functions.1 Various associative processors have been proposed that use electronic or optical means. Optical schemes,2-7 in particular, those based on holographic principles,8'8' are well suited to associative processing because of their high parallelism and information throughput. Previous workers8 demonstrated that holographically stored images can be recalled by using relatively complicated reference images but did not utilize nonlinear feedback to reduce the large cross talk that results when multiple objects are stored and a partial or distorted input is used for retrieval. These earlier approaches were limited in their ability to reconstruct the output object faithfully from a partial input.
The PEMLM has been used to demonstrate several real-time image-processing operations: contrast reversal, edge enhancement, image addition/subtraction, and synchronous detection of time-modulated light in a scene. Experimental results will be presented, and the relationship between the device physical parameters and performance characteristics will be discussed. Future developments to improve the PEMLM performance will also be described. Demonstration of quantum-limited sensitivity, millions of resolution elements, and frame rates greater than a kilohertz is expected in the near future.
The deformation behavior of thin viscoelastic control layers for eventual use in Schlieren light valves is investigated theoretically as well as experimentally. Electrical excitation and viscoelastic properties of the control layer are incorporated in a differential equation which is solved analytically. Based on a quantitative evaluation of this solution, the time behavior and the spatial-frequency response of the control-layer deformation are described and the influence of several parameters is considered. Stability problems and alternative geometries are discussed in view of the analytical results. Interferometric and diffraction measurements of the control-layer deformation confirm the analytical results at least in principle. Maximum deformation amplitudes of approximately 0.14m and time constants on the order of a few ms are obtained in agreement with the theory. It is concluded that viscoelastic control layers are suitable for light-valve projection if the viscoelastic material, the control-layer geometry, the electrical addressing, and the optical system are optimized.
Three different optical switching processes in ferroelectric materials were examined. These processes were electro-optic switching, photorefractive switching, and ferroelectric switching. The material properties that fundamentally limit each optical switching rate were determined and examples were presented.
One-dimensional, optically addressed silicon/PLZT spatial light modulators have been fabricated and tested. A study of theoretical and measured performance, design configurations, and functional flexibility leads to the conclusion that 1-D silicon/PLZT SLMs will find application in optical linear algebra processors, neural net systems, and multiplex holography.
A new type of optical modulator has been developed, and has proved to be particularly well suited to the task of projection display. The device uses electrocapillarity (an electrochemical surface tension effect) to switch small mercury mirrors on and off. These mirror elements have the high optical efficiency required for projection light valves; they have thresholding and memory, are amenable to matrix address, and can be switched with small voltages. An 8 x 8 array has been built and operated.
Certain smectic liquid-crystalline mesophases of chiral materials have been shown to exhibit both ferroelectric and pyroelectric properties. Consequently, these phenomena have recently been evaluated for their device potential. For example, these modifications have been proposed for use in displays, heat sensors, linear arrays, and spatial light modulators. Success of this ferroelectric media in these applications is primarily dependent on understanding and utilizing the nature of the physical properties of the material. In this article the macroscopic properties such as spontaneous polarization, helical structuring, and phase behavior are discussed in terms of the microscopic interactions of the constitutent molecules of the phase.
The electro-optic display industry has developed liquid-crystal technology to the level of TV imaging. For optical processing and other special applications, a much faster response time is desirable. Techniques which optimize the response of nematic liquid crystals are reviewed. Ferroelectric liquid crystals are described, with which switching speeds well beyond nematic limitation are achieved. A photoaddressed spatial light modulator using bismuth silicon oxide as the photoconductor addressing a surface-stabilized ferroelectric liquid crystal is described. Initial experimental results are presented.
Principles for selecting or synthesizing liquid crystal mixtures with desired birefringence are analyzed. Based on these principles a liquid crystal mixture exhibiting large birefringence and low visco-elastic coefficient at room temperature was formulated. Optical and electro-optic properties of this liquid crystal were characterized at elevated temperature. Results are compared with those of some commercially available liquid crystals.
We have observed for the first time amplification of a weak probe beam by a strong pump beam via two and four wave mixing process, the effect occurs if one of the beams is frequency shifted or by moving the film.
Nematic liquid crystal devices are slow compared to solid state liquid state devices based on electronic transitions. This stems from the origin of such effects in the mechanical interaction of long range order. The following paper describes an approach to optimizing the response of such devices by limiting the lineal dimension over which these effects occur.
Ferroelectric liquid crystal (FLC) compounds are investigated for use in optical parallel processing. Spatial light modulators using the smectic C phase FLC's show better contrast ratio, faster switching speeds, and lower cost than twisted nematic liquid crystal spatial light modulators. In this paper we present measurements of characteristic parameters of FLC's, and demonstrate a new optical logic gate capable of performing fourteen Boolean logic operations using single cell FLC devices with photographic masks as inputs. The XOR and XNOR Boolean functions can be achieved by cascading together several optical logic gates. Recent results using 32 x 32 FLC matrix arrays demonstrate an OR gate which has programmable inputs.
The Hughes cadmium sulfide (CdS) liquid crystal light valve (LCLV) is a versatile spatial light modulator (SLM). It is continually being refined and the purpose of this paper is to update its performance status and to discuss some future developments.
The Hughes Aircraft Corporation (HAC) LCLV has been used in a Vander Lugt type optical correlator designed for target tracking. The important characteristics of the LCLV for this application are resolution, sensitivity, and response time. The results of measuring these characteristics for two LCLV's are reported.
This paper describes how a liquid crystal material could be used as a passive, all optical, broad-band and adjustable threshold optical power limiter. The voltage biased method has been utilized to improve the response time of the proposed optical power limiter. Infrared applications of this device are emphasized.
The transmission of guided waves through a thin-film waveguide with an oriented liquid-crystal cladding of K18 exhibits, near the liquid-crystal nematic-isotropic phase transition, a number of all-optical operations, such as switching and bistability.