A compact acousto-optic time integrating correlator has been developed for passive surveillance applications. The correlator has a bandwidth of 100 MHz about a center frequency (IF) of 300 MHz to provide time-difference-of-arrival information with a resolution of less than 10 nsec useful in locating wideband radar jammers when signals are received at two widely separated antennas. On an airborne platform, the correlator may perform passive ranging. This correlator features the use of counterpropagating surface acoustic waves on a single lithium niobate delay line interacting with light from a laser diode. Diffracted beams are interferometrically combined on an integrating photodiode array. Integration time of 1 msec provides a time-bandwidth product of 105. A two dimensional correlator allows processing of both time-difference-of-arrival and differential Doppler, and a related processor performs both cross correlation and cross spectrum analysis.
Three generations of optical architectures for the implementation of two-dimensional correlation are presented. Emphasis is placed on the application of such architectures to triple-product correlation. Technological improvements incorporated in these designs are discussed as well as limitations of current technology which impede progress in the area of optical implementation of such processing functions.
The Wigner distribution (WD) function is a two-dimensional representation that displays the space and spatial frequency content of a one-dimensional signal. Its two-dimensional Fourier transform, the radar ambiguity function, displays the space and spatial frequency shifts of the same signal. The WD has the property that a space or spatial frequency shift of the signal leads to a corresponding shift of the Wigner distribution function. This represents a translation invariance of the WD, a property that is useful for impulse response characterization of a space-invariant system. Similarly, if one signal is shifted with respect to the other, magnitude of their cross-ambiguity function is shifted by the same amount. There are many optical systems that are space-variant. A particular space-variant system, encountered in many imaging applications, is the so-called scale-invariant system. In this paper, scale-invariant Wigner distribution and ambiguity functions are defined. These new functions represent local scale-frequency spectrum of the signal, and scale correlation in space and scale frequency. Properties of these functions are described and the differences and similarities to translation-invariant WD and ambiguity function are pointed out. Potential applications and analog optical implementations of these functions aie also discussed.
An acousto-optic filter was developed utilizing two Bragg-Cells, an integrating photo-detector array and a high frequency, non-integrating Avalanche Photo-Detector (APD). The basic architecture is that of a Mach-Zehnder interferometer. This device simultaneously provides two output signals, 1) The power energy spectrum of all signals S(t) within the one octave bandpass of the device, and 2) a down converted IF signal containing the real phase and amplitude information of any desired individual signal, S1(t), or S2(t), or S3(t), etc, etc, within S(t).
The design and performance characteristics of a compact, wideband, space-integrating, acousto-optic signal processing system which provides the unipolar convolution of pairs of 10-ns digital pulses is described.
A new class of discriminant filter functions for use in a matched filter correlator for multi-class distortion-invariant pattern recognition is described. Three variations of these optimal linear discriminant functions (OLDFs) that optimize different performance measures are described and initial performance results are presented.
Using real data from infrared imaging systems, we have designed a generalized matched filter to recognize tank images viewed from many different angles. Performance parameters of the GMF as a function of input rotation angle are presented. Amplitude and phase GMFs are progressively phase biased in order to better understand the tradeoff in performance between discrimination capability and light efficiency. An unexpected and encouraging result is that phase-only GMFs exhibit high figures in both performance categories.
Space-integrating, acoustooptic processors for adaptive, temporal filtering are examined. The basic architecture is then extended to the space-time domain for application in broadband phased array processing. An acoustooptic processor capable of such 2-dimensional, adaptive processing is described.
A modified form of a composite or multiplexed matched filter has been computer simulated and tested. The modification consists of using only the phase function and setting the amplitude function equal to unity - a so-called phase-only filter (POF). The original filter was D. Casasent's synthetic discriminant function (SDF) filter. The filter and test images were made from actual IR imagery. The results are compared in terms of efficiency, correlation peak height and width, and signal/noise ratio. A binary phase version of the SDF/POF was also tested. Its performance is between the SDF and the SDF/POF.
Computer-generated holograms of geometrical shape and synthetic discriminant function (SDF) matched filters are modeled and produced. The models include ideal correlations and Allebach-Keegan binary holograms. A distinction between Phase-Only-Information and Phase-Only-Material Filters is demonstrated. Signal-to-noise and efficiency measurements were made on the resultant correlation planes.
General expressions are derived for the degradation in signal to noise ratio (SNR) as a function of rotation and scale distortions for modified matched spatial filters (MSFs). These are numerically evaluated for an image class with Gaussian shaped spectral densities to understand the effects of training set size, input noise level and image space bandwidth product (SBWP) on the resulting SNR.
Optical matched filter image correlators (OMFICs) have usually been associated with visual sensors. This paper presents a discussion of OMFIC application to IR, SAR, and visual target data employing a validated digital simulation and/or laboratory experiments to detect, and in some cases track, targets using the three kinds of imagery. Extensive results are presented.
A new hybrid electrical/optical system for pattern classification is proposed. The optical circuit consists of c sets of analog electro-optic Bragg modulators, each capable of performing an n-dimensional inner product. This circuit is coupled with an optical threshold comparator to provide a binary representation of the class index. Pattern classification is effected through the use of a linear discriminant. The hybrid system is sufficiently rapid to make on-line classification feasible.
The application of a programmable magneto-optic spatial light modulator to white-light optical signal processing is presented. We have shown that the magneto-optic device responds to the polarized white-light, in which a wide range of color object patterns can be generated. Since the magneto-optic device is a transmitted type spatial light modulator, it is very suitable for real-time programmable spatial filter synthesis and object pattern generation for optical signal processings. Experimental demonstrations of some of the elementary spatial filter syntheses and pseudocolor encodings are provided.
This paper describes the use of Lloyd's mirror in two optical processing applications. First it is shown that if the narrow slit in the usual Lloyd's mirror set up is illuminated by polychromatic light, the interference pattern is the Fourier Cosine transform of the spectral intensity distribution of the light. Lloyd's mirror may thus be used as the transforming element in a Fourier transform spectrometer of exceedingly simple and rugged design. An experimental realization of this process is displayed. Second it is shown that if the narrow slit in the Lloyd's mirror set up is replaced by a wide slit or, more generally, by a one dimensional object illuminated by non-coherent monochromatic light, the interference pattern obtained is the Fourier transform of the intensity distribution of the object. Lloyd's mirror may thus be used to produce Fourier holograms in non-coherent light of one dimensional objects. An experimental realization of this process is displayed. If the object is two dimensional the interference pattern is the Fourier transform of the intensity distribution of the object along one direction. If the transform of the object is obtained in several orientations of the object, the object may be recovered by the same process used in CAT SCAN.
A few schemes will be described in which ultra short pulses are used to see through highly scattering specimens, as typified by biological objects. The information thus obtainable should be similar to X-ray pictures and reveal details about structure in the inside of the object by fleeting shadows on the exit skin. The schemes to be analysed are: The fast Kerr gate followed by photographic film, a holographic approach using high resolution film (the few experiments reported in this paper refer exclusively to this scheme) and various schemes involving non linear effects including four wave mixing. A few remarks will be made about peculiarities resulting from the shape of the wavefronts involved in ultra short pulse imaging and a few very preliminary experimental observations will be reported.
The generation of a phase-conjugate wavefront from a Fourier hologram via wave-mixing in Bi12SiO20 crystals(BSO) is presented. Using this technique, the disturbance of the object-wave caused by a phase-distorting medium can be cancelled out in real time. The preliminary experimental results are shown.
We have used a number of experimental techniques to identify the photorefractive species in commercial samples of BaTiO3. We find that iron impurities (in the Fe and Fe states) are the most likely sodrce of photorefractive effect, although other transition metal impurities may also contribute.