K. Terry Stalker

Bandwidth considerations for binary phase-only filters

A binary phase-only filter (BPOF) bandwidth and, correspondingly, the performance with respect to stochastic noise are introduced as filter design parameters. A BPOF figure of merit is defined which references the matched filter. Analytical bounds on the BPOF signal-to-noise ratio are derived. The noise performance is illustrated with simulation results. It is demonstrated through analysis and simulation that BPOFs can be designed to perform well with respect to stochastic noise.

Acousto-optic signal processing for real-time image recognition

The experimental results from an acousto-optic image correlator are presented. The performance ofthis system, which is evaluated in terms of the peak-to-sidelobe ratio, is shown to be improved by subtracting off the signal-dependent bias terms. These bias terms can be optically generated using the same acousto-optic correlator. Structured background features in the input scene can also dramatically affect the correlation result. This paper suggests a novel implementation of the difference-squared error algorithm using a modified acousto-optic architecture that can discriminate better than conventional correlation in scenes with structured background features. Experimental results from real-time systems constructed in the laboratory are presented.

Analysis Of Binarized Hartley Phase-Only-Filter Performance With Respect To Stochastic Noise

Binary phase-only filters (BPOFs) are a viable candidate for the replacement of matched filters in real-time image processing and pattern recognition applications. The original BPOFs were binarized versions of the real or imaginary parts of the Fourier transform. Recently, a filter has been proposed that is the binarized Hartley transform. In this paper, the noise performance of the binarized Hartley phase-only filter is analyzed and compared with other BPOFs as well as with the phase-only and matched filters. It is well known that the matched filter optimizes the signal-to-noise ratio. Further, it can be shown that the matched filter phase is the optimum phase-only filter. A bound on the BPOF signal-to-noise ratio relative to that of the phase-only filter is derived. Finally, the analysis is applied to the generalized form of the binarized Hartley transform.

A high dynamic range acousto-optic image correlator for real-time pattern recognition

The architecture and experimental results for an incoherent acousto-optic image correlator suitable for real-time applications are presented. In the basic architecture, each time a line of the raster-scanned input image is fed into the acousto-optic device (AOD), all rows of a digitally stored reference image are read into the system using an array of light emitting diodes (LEDs). Thus, the required two-dimensional correlation is performed as a series of multi-channel 1-D time-integrations in x (performed in the AOD) combined with a multi-channel correlation in y (perpendicular to the AOD axis) using a modified CCD. The LED array and detector modifications which markedly increase the dynamic range are discussed as well as the correlator design. Further, a novel memory for storing the reference object is described for rapidly changing templates. Experimental results indicate the architecture is useful for applications in the areas of character recognition and target identification.

A Comparison Of Real-Time Optical Correlators For Pattern Recognition

Two types of optical correlators have been built to investigate real-time pattern recognition. The first employs one-dimensional devices to perform the two dimensional correlation in real time. This architecture uses an array of light emitting diodes (LEDs) to input an electronically stored reference image into the processor in parallel. The input scene data is introduced into the processor one line at a time using an acousto-optic device (AOD). Multichannel time integrating correlations are performed in the row direction using the AOD and in the column direction using a charge coupled device (CCD) operating in the time delay and integrate mode. A processor has been built using this technology which correlates a 64 x 44 pixel binary reference image with a 256 x 232 input scene at video rates. The second correlator is a space integrating Fourier transform based correlator. A magneto optic-device (MOD) is used at the Fourier transform plane to rapidly change filter functions. The binary nature of the MOD device necessitates using either a binary phase or binary amplitude representation of the desired complex filter function. For this reason, several types of Binary Phase-Only Filter (BPOF) representations have been analyzed and experimentally investigated. Experimental correlation results have been obtained using both the Hartley BPOF and a newly developed class of complex binary filters, called Quad-Phase-Only Filters (QPOF). The performance of the two systems will be compared on the basis of processing speed, space bandwidth product, processor size and light efficiency. The inherent differences between incoherent and coherent processing and their implications for filter design will also be discussed. Finally, estimates of future performance will be presented.

Compact real-time acousto-optic image correlator

The design and development of a compact acousto-optic image correlator capable of performing real-time correlations on grayscale imagery will be described. The system utilizes one-dimensional optical devices to perform the desired two-dimensional correlation. The two-dimensional correlation is performed as a series of multichannel time-integrating correlations between each input image line and a reference template that is stored in an electronic memory. The rows of the reference template are introduced into the processor in parallel using a one-dimensional laser diode array. The correlation in the vertical direction is performed using a modified charge-coupled device (CCD) operating in the shift-and-add mode. A laser diode array, as opposed to an LED array used in previous systems, provides more power so that full use can be made of the dynamic range capabilities of the acoustooptic device and CCD. Key features of the system will be presented, including the random access template memory, the custom laser diode array consisting of 64 individually addressed laser diodes, and the custom optical design to achieve nearly diffraction-limited image quality and compactness.

Application of normalized gray-scale correlation

Real-time gray-scale correlation in the spatial domain has been demonstrated previously using an acousto-optical (AO) correlator. This work demonstrates normalized gray-scale correlation as implemented on an AO correlator system capable of operating at real-time video rates. Motivation for using normalized gray-scale correlation is presented. The normalized correlation algorithm as implemented on the AO correlator is detailed. The entire real-time AO correlator system is described, including the electronic support hardware and the user interface. Since normalization requires a division operation, system numerical precision issues are addressed. Test results obtained in non-real time experiments are presented.

Design and testing of space-domain minimum-average correlation energy filters for 2-D acousto-optic correlators

Two-dimensional acousto-optic (AO) correlators differ from frequency plane correlators in that multiplying, shifting, and adding, rather than Fourier transforming are used to obtain the correlations. Thus, manu of the available composite filter design techniques are not aimed at designing filters for use in AO correlators because they yield frequency-domain functions. In this paper, a method is introduced for designing filter impulse responses of arbitrary extent for implementation on AO correlators. These filters are designed to yield sharp correlation peaks. Simulation results are included to illustrate the viability of the proposed approach. Also included are some initial results from the first successful use of gray-scale composite filters on an AO correlator.

Design and testing of three-level optimal correlation filters

Previously, we have designed 3-level filters(suitable for implementation on magneto-optic spatial light modulators) to maximize the output signal-to-noise ratio (SNR) and to separately maximize peak-to-correlation energy (PCE) that measures the correlation peak sharpness. In practice, we want the correlation peaks to be sharp (i.e., large PCE) as well as noise-tolerant (i.e., large SNR). In this paper, we present a new method to optimally combine these two desirable properties into a single optimization procedure. Similar methods to trade off SNR versus peak efficiency and PCE versus peak efficiency will be presented. Both simulation and experimental results will be included.

Frequency response of a TeO{sub 2} slow shear wave acousto-optic cell exposed to radiation

Radiation testing of photonic components is not new, however component level testing to date has not completely addressed quantities which are important to system behavior. One characteristic that is of particular importance for optical processing systems is the frequency response. In this paper, we present the results of the analysis of data from an experiment designed to provide a preliminary understanding of the effects of radiation on the frequency response of acousto-optic devices. The goal is to present possible physical mechanisms responsible for the radiation effects and to discuss the effects on signal processing functionality. The experiment discussed in this paper was designed by Sandia National Laboratories (SNL) and performed by SNL and Phillips Laboratory (PL) personnel at White Sands Missile Range (WSMR). In the experiment, a TeO2 slow shear-wave aocusto-optic cell was exposed to radiation from the WSMR linear accelerator. The TeO2 cell was placed in an experimental configuration which allowed swept frequency diffracted power measurements to be taken during radiation exposure and recovery. A series of exposures was performed. Each exposure consisted of between 1 to 800, 1 microsecond(s) ec radiation pulses (yielding exposures of 2.25 kRad(Si) to 913 kRad(Si), followed by recovery time. At low total and cumulative doses, the bandshape of the frequency response (i.e. diffracted power vs. frequency) remained almost identical during and after radiation. At the higher exposures, however, the amplitude and width of the frequency response changed as the radiation continued, but returned to the original shape slowly after the radiation stopped and recovery proceeded. It is interesting to note that the location of the Bragg degeneracy does not change significantly with radiation. In this paper, we discuss these effects, and we discuss the effect on the signal processing functionality.