Optical information processing is a title given to a wide range of optical system technologies aimed at providing a solution to the ever-growing need for higher throughput in information processing. A particularly important requirement that often accompanies the general need for increased data pro cessing throughput is that such systems be constrained in their size, complexity, and power requirements. Such constraints are typical of airborne, space-based, and robot systems.
Quantum wells, alternate thin layers of two different semiconductor materials, show an exceptional electric field dependence of the optical absorption, called the quantum-confined Stark effect (QCSE), for electric fields perpendicular to the layers. This enables electrically controlled optical modulators and optically controlled self-electro-optic-effect devices that can operate at high speed and low energy density. Recent developments in these QCSE devices are summarized, including new device materials and novel device structures. The variety of sophisticated devices now demonstrated is promising for applications to information processing.
Certain smectic liquid-crystalline mesophases of chiral materials have been shown to exhibit both ferroelectric and pyroelectric properties. Consequently, these phenomena recently have 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 these ferroelectric media in these applications is primarily dependent on the configuration of the device, and on understanding and utilizing the nature of the physical properties of the material. In this paper macroscopic properties such as spontaneous polarization, helical structuring, and phase behavior are discussed in terms of the microscopic interactions of the constituent molecules of the phase. The exploitation of these properties in various device geometries is also investigated.
High-contrast, submicrosecond switching ferroelectric liquid crystal spatial light modulators have many applications to optical computing and image processing. In this paper we describe the use of these low-voltage, low-power, and bistable devices to perform a variety of functions including polarization- and intensity-based logic gates, input/output displays, optical crossbars, and spatial filtering masks.
The Sr1-xBaxNb2O6 (SBN) and Ba2-xSrxK1-yNayNb5O15 (BSKNN) tungsten bronze solid-solution systems are shown to be promising photo-refractive materials. Because of the versatility of the bronze structure, both the response time and spectral response can be controlled by altering the type of dopant and its crystallographic site preference. This paper reviews the current status of the tungsten bronze crystals SBN and BSKNN for photorefractive applications in terms of their growth, electro-optic character, and the role of cerium dopants. Ferroelectric morphotropic phase boundary (MPB) bronze materials are also discussed as potentially important for future development.
One-dimensional, optically addressed silicon/PLZT spatial light modulators (SLMs) 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 network systems, and multiplex holography.
This paper discusses the present state of development of the Hughes charge-coupled-device-addressed liquid crystal light valve (CCD LCLV). The device is based on the photoactivated silicon LCLV and is suitable for a range of applications, including adaptive optics and as a programmable mask for optical data processing systems.
A spatial light modulator (SLM) based on a deformable gel surface is presented. It has remarkable optical properties and its construction and operation are comparatively simple. It can be optically addressed through a photoconductor layer. The surface relief pattern is read out by total reflection and a schlieren optical system. The device provides good wavefront quality (X/10 over the whole aperture of 30 x 50 mm2) and has a spatial resolution of 10 line pairs/mm. Contrast ratios for modulation up to 40:1 were measured. The input sensitivity is typically 0.3 mW/cm2. The rise and decay times are both approximately 20 ms. Besides its primary application as a light valve in large screen TV projection, it can be used in optical information processing systems, e.g., as an incoherent-to-coherent transducer. Combined with a CRT, the SLM can be addressed electronically.
The operation of the double phase conjugate mirror (DPCM) is described. It can be regarded as a bidirectional spatial light modulator and controllable filter. Two independent image-bearing beams that may be derived from different lasers exchange their spatial information as they are coupled into each other in a photorefractive crystal. The,DPCM is also shown to be an optical thresholder and spatial filtering device, displaying edge enhancement. We propose the use of a resonator with two facing DPCMs to implement iterative image processing algorithms.
An experimental demonstration of a holographic associative memory is presented. The system utilizes an array of classic VanderLugt correlators to implement in parallel the inner product between an input and a set of stored reference images. Each inner product is used to read out an associated image. Theoretical analysis of the system is given, and experimental results are shown.
Optical resonators having holographic elements are potential candidates for storing information that can be accessed through content-addressable or associative recall. Closely related to the resonator memory is the optical novelty filter, which can detect the differences between a test object and a set of reference objects. We discuss implementations of these devices using continuous optical media such as photorefractive ma-terials. The discussion is framed in the context of neural network models. There are both formal and qualitative similarities between the resonator memory and optical novelty filter and network models. Mode competition arises in the theory of the resonator memory, much as it does in some network models. We show that the role of the phenomena of "daydream-ing" in the real-time programmable optical resonator is very much akin to the role of "unlearning" in neural network memories. The theory of programming the real-time memory for a single mode is given in detail. This leads to a discussion of the optical novelty filter. Experimental results for the resonator memory, the real-time programmable memory, and the optical tracking novelty filter are reviewed. We also point to several issues that need to be addressed in order to implement more formal models of neural networks.
The utility of updatable optical image correlation in photorefractive materials for industrial applications is discussed. This allows the formation of iterative correlation products employing variable spatial frequency weighting and/or different reference images. Several aspects of the system design are discussed, including techniques for producing spatial frequency weighting and optimum laser systems to maximize data throughput and output stability.
A novel low-cost color liquid crystal television is applied in a real-time color pattern recognition system. In this system, a target is identified on the basis of both shape and spectral content. A multiple matched filtering technique is also proposed to improve the detected correlation signal. Experimental results of the bichromatic signal detection are presented.
Parabolic zone plates are constructed by the off-axis holographic method by recording on a photosensitive material the pattern produced by coherent superposition of two waves, one plane and the other cylindrical. Fraunhofer and Fresnel diffraction patterns are evaluated by the reconstruction process of parabolic zone plates to study their performance. Experimental results are presented.
The time response of one- and two-color optical pyrometry systems is evaluated theoretically and demonstrated experimentally. Optical pyrometers are found to respond much more rapidly for temperature increases and much more slowly for temperature decreases than does a linear temperature-measurement system with the same time constant. Accurate measurement of temperature decreases can require a detection system response time up to 100 times faster than the change in temperature of the object being measured.
The self-routing of optical information through a photonic switch using optically processed control is reported. Routing decisions are made on a bit-by-bit basis. Destination information is encoded in each data bit using optical spread spectrum techniques. Switching of 3.125 Mbits/s data is experimentally demonstrated. Extension of this Technique to a self-routing N x N switch is discussed.
From time to time in my editorials I have asked for feedback from readers of the journal, and I am happy to report that I have actually been getting a fairly odd response. There have been some complaints, but most of the feedback has been in the form of constructive suggestions for changes in the journal, support for changes that have already been made, or questions. In this editorial I wish to address a question that my good friend and tennis instructor, Adolf Lohmann, recently asked.
"This book contains the papers presented at a NATO Advanced Study Institute held at San Miniato, Italy, from September 2 to 13, 1985, on the latest developments in the science and technology of modifications, in particular, of metallurgical surfaces due to laser treatment."