Recent advancements in guided-wave acoustooptic Bragg modulators/deflectors1-4 and geodesic lenses have paved the way for real-time processing of wideband multichannel signals in a planar substrate. The encouraging progress obtained in semiconductor diode lasers and photodetector arrays, and in the mechanisms for butt-coupling these components to planar waveguides also points to a good prospect for hybrid integration of all components in LiNb03 and Si substrates. Fig. 1 depicts the configuration of an acoustooptic time-integrating correlator using hybrid integration in LiNb03 substrate.5 Naturally, present and future research in GaAs sub-strate may also result in monolithic integration. The resulting integrated optic modules or subsystems are expected to provide improvements in size and weight, performance, reliability, and ultimate cost.
A review is given to the integrated optical signal processing component technology. Key technical issues are outlined with emphasis on RF spectrum analyzer and related applications. A new approach to the guided wave optical components technology using photolithographically defined line patterns only is addressed. The performances of some of the key grating components are discussed.
This paper describes the integrated-optic implementation of a Bragg spectrum analyzer that employs the interaction between a coherent optical guided wave and a surface acoustic wave to determine the power spectral density of the input. The integrated-optic spectrum analyzer consists of an injection laser diode, a thin-film optical waveguide, waveguide lenses, a surface-acoustic-wave transducer, and a linear detector array with CCD readout. Design principles are given for selecting component parameters such as optical beam width, detector cell size, lens aperture and focal length, and acoustic transducer design so as to obtain specific rf resolution, spurious level, and signal-to-noise ratio. Design parameters are presented for a 750- to 1250-MHz spectrum analyzer with a resolution of 4 MHz and a 40-dB dynamic range. Also described in the paper is the development of state-of-the-art component technology for the spectrum analyzer.
Work on the development of an integrated optical data preprocessor for multispectral data is reported. The device will be capable of comparing received analog signals in 16 channels with a large set of prerecorded data sets in real time and of classification of the received data on the basis of the comparison. The operating principles of the device are discussed and details of the prototype design are provided along with a discussion of the reasons for the design choices that were made.
This paper addresses the concept of active optical processing, in which signals are processed due to a nonlinear transfer function (light output as a function of light input). The particular active device considered here is a bistable optical device (BOD), which typically has transfer curves such as those shown in Fig. 1. Devices whici have transfer functions such as these can be used fc,-- signal enhancement, optical logic, memories, and switching. The aim of developing active optical processing is to achieve high speed all-optical operation, which is not limited by electron transit time considerations. An all-optical processor should have psec response times. In addition, the all-optical nature means freedom from emi, rfi, cross-talk and security, which are also characteristics of optical fibers.
We describe the operating characteristics of the nonlinear Fabry-Perot optical logic device. A simple theory shows that this device can operate with psec switching times and pJ switching energies. Important transient effects are also described. The results of a com-puter simulation of these transient effects is also presented.
Interferometric waveguide modulators famed by Ti diffusion in LiNb03 are under development for use in a high-speed electrooptic analog-to-digital (A/D) converter operating in the 6-bit 1-GS/sec range. The operational principles of these modulators and the design considerations for the A/D converter will be reviewed and the operational characteristics of the fabricated waveguide modulators will be described. The devices have a 17-dB extinction ratio and modulation at 1.4 GHz has been demonstrated. To simulate the A/D conversion application, the modulators have also been used in conjunction with a frequency-doubled mode-locked Nd:YAG laser to sample a 69-MHz signal at 275 MS/sec.
A complete optical processor based on a demountable thermoplastic light modulator is described. A brief introduction and background on thermoplastic light modulators precedes the description of recent progress. This introduction includes the system description of the SAI optical processor.
An Elastomer Storage Device (ESD) was constructed which has potential as an adaptive Spatial Filter in optical spectrum analysers. The ESD uses a PVK-TNF photoconductor and parallel plate charging in a low pressure Argon atmosphere. Resolution greater than 100 cycles/mm and a time-bandwidth product in excess of 1. 6 x 106 have been measured. A photometric sensitivity of 5μ J/cm2 at 515.4nm was observed. The linear dynamic range in the Fourier plane was measured to be 40db. The device has a lifetime in excess of 107 cycles at a frame rate of 10 cycles/second.
The Microchannel Spatial Light Modulator (MSLM) is a relatively new, highly sensitive, optically-addressed light modulator that is being developed for low-level-light, real-time, optical information processing. This Paper presents an update of recent progress on the development of the MSLM. Vacuum-sealed and demountable devices employing electro-optic crystals of LiTaO3 and LiNbO3 respectively, are evaluated. These devices are found to have similar characteristics. A halfwave exposure sensitivity of 8.4 nJ/cm2 and a long-term optical information storage time of more than two weeks have been demonstrated with the vacuum-sealed LiTaO3 device; the demountable LiNbO3 MSLM has been cycled at 10 frames per second.
The theoretical resolution of an electrooptic spatial light modulator [such as the Pockels Readout Optical Modulator (PROM)] is a function of the electrostatic field distribution arising from stored point charges located within the active electrooptic crystal layer. The Fourier transform of the voltage distribution (which can be directly related to the modulation transfer function) is expressed as a function of the charge location within the electrooptic crystal. In addition, the resultant analytic expression contains the dielectric constants of the blocking layers and electrooptic crystal, and the thicknesses of the three layers. This formulation allows the effects of charge trapping within the bulk of the electrooptic crystal to be modeled. In particular, the low spatial frequency response decreases linearly and the high spatial frequency response decreases exponentially with the distance of the point charge from the dielectric blocking layer/electrooptic crystal interface. Thus the overall sensitivity and resolution are degraded strongly by charge storage in the bulk away from the interface. Utilizing superposition, this formulation can be readily extended to accommodate arbitrary charge distributions arising from different exposure parameters. The spatial frequency response of the PROM is calculated for both analytic (exponential hole/gaussian electron) and iterative (exposure-induced charge transport) continuous charge distributions. The limiting form of the high spatial frequency response is shown to be independent of the particular distribution of volume charge. The implications of these results for device design and operation are discussed.
In the variable grating mode (VGM) operation of a liquid crystal device, a phase grating is formed whose period depends upon the voltage placed across the cell. Typical spatial frequency variation is from 100 to 600 cycles/mm. By adding a photoconductive layer to the cell, the grating period can be optically controlled. Thus each input intensity level in an optical signal will generate a local grating structure at a different spatial frequency. If the VGM device is placed in the input plane of a coherent optical processor, each point in the Fourier transform domain will correspond to a different grating frequency, and thus to a different input signal level. By varying the attenuation at each point in the Fourier plane, any desired transformation of input intensity to output intensity can be achieved. In particular, level slicing can be achieved by placing a slit in the filter plane so that only a narrow range of spatial frequencies is transmitted and thus a narrow range of input intensities is passed. Several experimental VGM real-time devices have been constructed and the results of a level slicing experiment are presented. This device has the potential to perform a wide variety of real-time, parallel, optical processes.
A first order theoretical model of a thin nematic liquid crystal layer mated to a CdS/CdTe heterojunction photosensor is presented. Uniform parallel alignment of the liquid crystal molecules with the light valve substrates is assumed. The response of the liquid crystals is predicted through minimization of the distortion free energy density and the operation of the photosensor is obtained by assuming it to be a back biased light activated charge storage diode. Measurements of an actual liquid crystal light valve response are presented to verify the model.
The sophisticated requirements of modern-day communications, radar, and electronic warfare (EW) systems have strained the capabilities of currently available signal-processing hardware. Specifically, the require-ment for real-time, large-bandwidth analysis tends to favor analog processors and eliminate digital processors from consideration except in the low-frequency regime. An added, often complicating factor, is the necessity for operation in a very dense signal environment. This requirement can tend to favor one analog approach over another. In this paper, the newly emerging technology of acousto-optic signal processing is discussed with respect to its potential for solving the difficult signal-processing problems which will be facing both the military and civilian system designer. Specifically, by combining desirable features of acoustics and optics and in some cases charge-coupled devices (CCD's), acousto-optic technology offers great promise of fulfilling diverse requirements. This paper overviews some of the uses of acousto-optic technology, especially those using surface-acoustic-wave (SAW) devices in the areas of radar and communication signal processing. Two areas are covered: (1) Fourier transformation, including spectrum analysis, and (2) convolution and correlation.
A method is presented to realize real time nonlinear operations on two dimensional images through the use of multiple liquid crystal light valves. The technique allows the use of either coherent or incoherent illumination and provides for real time control and alteration of the specific nonlinear operation being performed. Experimental results are given for level slice, logarithm, exponentiation, and A/D conversion.
A hybrid optical-digital approach is being investigated for a real time Kalman, 2-D, nonlinear phase estimator. The 2-D convolution is calculated using incoherent optics with an LED array input and a CCD array output. Interfacing problems and the accuracy of the optical convolution are discussed.
Coherent optical processing can be effectively used to detect moving objects against a background of fixed objects and noise. If done digitally, the task can require a large amount of processing power. The optical approach is to do pattern recognition in the frequency plane of the time-integrated sky image. A spatial filter, matched to the spectral energy distribution of the moving objects, rotates in the frequency plane. Earlier experiments established the validity of the approach'. The processor has since been refined, simplified and implemented in an optical/digital configuration. A small digital computer (PDP 11/03) provides simulated input data, controls the operation of the optical processor, and displays the results.
An optical-digital processor for generalized image enhancement and filtering has been designed and is now under construction. The optical subsystem is a two PROM Fourier filter processor. Input imagery is isolated, scaled, and imaged onto the first PROM. This input plane acts like a liquid gate and serves as an incoherent to coherent converter. The image is transformed onto a second PROM which also serves as a filter medium. Filters are written onto the second PROM with a laser scan-ner in real time. A solid state CCTV camera records the filtered image which is then digitized and stored in a digital image processor. The operator can then manipulate the filtered image using the gray scale and color remapping capabilities of the video processor as well as the digital processing capabilities of the mini-computer. The operator can then try new optical filters and iteratively develop optimum methods of detecting patterns. The goal of this research is to develop automated feature extraction algorithms which will minimize the need for human intervention. This system is currently being assembled at ETL, Fort Belvoir, VA.
Incoherent optical information processing is known to be far less sensitive to noise than coherent processing. However, if positive and negative quantities have to be implemented, as is normally the case in image restoration, incoherent convolution can be used only in a three-step process involving two convolutions and one subtraction. This communication describes the difficulties which appear in that case and which in part offset the noise advantage of incoherent illumination; alignment and normalization of the two images to be subtracted are critical issues. If the subtraction is performed optically, the dynamic range of this operation is an additional problem which is discussed here on a specific example.
A digital simulation of an optical matched filter is described. Two important aspects of the optical device are investigated using the digital techniques. The first study involves the preprocessing of the image prior to performing the analog matched filtering operation and the second the sensitivity of the device to rotation of the tar-get. These studies are carried out using a military target in common scenes such as villages, woodlands, and roads.
The usefulness of PROMS for high resolution electron microscopy is outlined. A theoretical model for direct electron beam image storage in PROMS is given, and computed results discussed. Wear-atomic resolution electron images of the Bi12Si020 crystal structure are shown and discussed.
Spectrum analysis at audio frequencies can be achieved using a strobe type optical system. A demonstrator has been fabricated with sensitivity ranging from 0.1 Hz to 10 KHz. The system examines fifty-four frequency bands simultaneously over a two octave region selectable anywhere in the sensitivity range. Resolution of 1/27th octave, in approximately 2.5% frequency steps, is obtained. First applications address diagnostics of mechanical systems for main-tenance purposes.
A hybrid optical/digital system is described for analyzing images based on textural information. A partially coherent white light optical processor is used to obtain the texture related information which is then processed digitally. Specifically the optical system generates a pseudocolor encoded image where the image color is a function of the local spatial frequency content of the image. Cluster analysis is then used to identify different color/texture regions. Experimental results are shown where the system has been applied to the problem of identifying distinct cloud types from satellite photographs.