Coherent Optical Correlator Research Articles

On the use of a multi-raster input of one-dimensional signals in two-dimensional optical correlators

A mathematical description of a coherent optical correlator for the multi-raster input of long signals is given. It is shown that such an input makes it possible to reduce the value of false correlation maxima that are generally found at the output of a correlator with a single-raster input. It is shown that false maxima do not appear when processing signals with a thumbtack ambiguity function, allowing one to do without a multi-raster input. The results of the theoretical analysis are confirmed by experiments with chirp signals and M-sequence type signals.

Multipoint Refractometer Based on Combined Correlation and Frequency Multiplexing

We demonstrate a new simple multi-point refractometer using Fresnel reflection at the fiber tip with closely spaced interferometer for intensity reference. A combined technique for simultaneous interrogation and multiplexing of identical sensor heads was developed to obtain multipoint operation with a standard continuous-wave DFB diode laser as light source. The technique is based on the combination of coherent optical frequency division reflectometry and coherent correlation optical time-domain reflectometer. Feasibility of accurate operation for multipoint refractive index measurements is demonstrated in proof-of-concept experiments. The proposed refractometer could be useful for sensing in chemical and biological applications.

Coherent optical correlator based on combined halftone and position modulation of light phase

Usually only a halftone (grayscale) modulation of light is used to present the processed and reference signals in the input plane of analog coherent optical (ACO) correlators, based on spatially integrating the product of two (processed and reference) signals. The halftone modulation requires two transparencies to record two signals separately, as the desired product of two spatial signals is obtained by sequential location of two spatial halftone records along the light path. Such an optical layout leads to the need for precision mutual alignment of two separate signal recordings. This paper presents a one-stage ACO correlation method based on combined halftone and position modulation of the light phase, which is produced by joint phase recording two signals on a single transparency. The joint phase recording provides the high optical efficiency of informational light modulation, automatically supports the spatial coincidence of corresponding elements of both recorded signals, and provides the same spatial scale for both recordings. The suggested method can also be used for introducing phase weight functions in the schemes of space-time ACO processing of wave signals. Advantages of ACO signal processing methods in comparison with corresponding electronic approaches are briefly noted.

Diffractive optical elements for mode-division multiplexing of temporal signals with the aid of Laguerre–Gaussian modes

Optical aspects of space-division multiplexing with orthogonal modes of coherent light were considered in theory and experiments with the coherent optical correlator. We resorted to the mathematical tool of generating functions and technologies of diffractive optical elements to implement complex spatial filters matched to rotationally symmetrical transverse modes. Successful multiplexing and demultiplexing in free-space transmission of low-frequency temporally modulated signals through different spatial modes was demonstrated. Experimental results show low cross talk between different mode channels and feasibility of further applications in multimode fiber optical communication data links.

Coherent optical three-dimensional spectrum-correlation processing of wave signals based on space–time integration

The architectures of classical analog coherent optical (ACO) spectrum analyzers and correlators are not designed to process the wave signal as a whole, i.e., simultaneously in three dimensions. In this paper, the theory of ACO three-dimensional direct spectrum-correlation processing of spatial-temporal optical replicas (copies) of wave signals is discussed. In the single-stage and two-stage ACO systems, the spatial power spectrum and spatial correlation function of the wave signal (envelope) are obtained on the basis of space-time integration. The geometry of the final compressed signal in the output plane of either optical system allows one to evaluate the angle of wave arrival. The wave signal to be processed can theoretically have any form (due to autocorrelation properties of the systems) and an unlimited duration (due to time integration of wave energy and possibility of electronic subtraction of the intermediate bias terms of the time integration).

Analysis of coherent optical filter and correlator systems using pixellated spatial light modulators

The authors analyse the optical performance of 4f coherent optical filtering systems using pixellated spatial light modulator devices for the input and filter planes. The performance of such systems is frequently analysed using oversampling fast Fourier transform (FFT) techniques which we show to require large oversampling factors (and hence complexity) to fully take into account the optical field distribution across each individual pixel of the filter plane and thus provide improved accuracy. The authors proposed two methods by which they obtain a piecewise series expansion of the optical field across each pixel using FFT techniques and demonstrate that these methods show faster convergence with increasing complexity and require less memory storage to compute than the oversampling FFT techniques usually used. If the output plane intensity is only required at a few points, such as when optimisation techniques are being used to design the filter, the authors present a third method which also results in improved accuracy but with no penalty in speed.

Volume holograms for optical correlation

Pattern recognition of objects within highly cluttered images is a demanding task, although it has many applications in both the military and civilian sectors. The techniques are extremely computationally intensive, making real-time processing using standard computer equipment difficult if not impossible. One possible solution to overcome the computational limitations employs a coherent optical correlator, i.e., an optical processor that allows objects to be identified and tracked at the speed of light. The coherent optical correlator has had a long and successful history, starting from Vander Lugt’s original design. The basic arrangement is still in use today: an input signal or image is coherently encoded on a beam of light, it is subsequently Fourier transformed, and correlated with a holographically stored Fourier signal or filter. The correlated output signal is again Fourier transformed and a peak is produced in the output plane when the filter matches. One of the nice features of this approach is that it is shift invariant, implying that the peak position corresponds to the previously unknown position of the input object. This means that we can identify the object and track its position in a 2D plane. In modern systems, the filter is implemented on a phase-only spatial-light modulator (SLM) that can be dynamically updated, thus allowing correlation of many templates for real-time target identification. However, the overall bandwidth is limited to the data-bus speed, the frame rate of the SLM, or the frame rate of the camera used for capturing the output. This is a significant weakness. If one wanted to test 36 filter templates for every video frame, each correlation must take less than 926μs or occur at a frequency higher than 1080Hz. If the filter consisted of 1024×1024 pixels, one would need a data bus of over one gigahertz. If the number of templates were increased further, the speed would become even more problematic. Figure 1. Schematic layout of the correlator. MQW SLM: Multiplequantum-well-based spatial-light modulator.

Discrimination among similar looking noisy color patches using Margin Setting

The goal of discrimination of one color from many other similar-appearing colors even when the colored objects show substantial variation or noise is of obvious import. We show how to accomplish that using a technique called Margin Setting. It is possible not only to have very low error rates but also to have some control over the types of errors that do occur. Robust spectral filtering prior to spatial pattern recognition allows subsequent filtering processes to be based on conventional coherent optical correlation that can be done monochromatically.

Optical implementation of segmented composite filtering

We investigate possible performance improvements of coherent optical correlators by using an appropriate filter design. Multidecision strategies are often required in high-level image-processing tasks. For an optical system characterized by a given space-bandwidth product we show that the filter design plays a crucial role in satisfying both system and processing requirements, with respect to the optimization of the encoding capacity. This leads us to the definition of segmented composite filtering, which is discussed in terms of processing performance. This filtering is assessed experimentally in the case of a face-recognition problem.

Double-multiplexed computer-generated holograms

One of the detour-phase computer-generated-hologram encoding methods was improved for obtaining two objects simultaneously without increasing the plotter resolution. The encoded patterns were reconstructed along two orthogonal diffraction directions. Such a hologram could be useful for two-channel coherent or incoherent optical correlation. Laboratory experiments demonstrate the suggested technique.

Crossed Bragg cell implementation of a Fourier-plane filter for optical image correlators

A two-dimensional optical image processor is described and demonstrated in which the point-spread function is generated by crossed acousto-opticells in the Fourier plane of a coherent optical correlator. Coherence effects and generation of nonseparable point-spread functions are considered.

Optical hit–miss morphological transform

We present an optical correlator implementation of the morphological hit-miss transform. This provides improved recognition of objects in clutter compared with standard optical pattern-recognition correlator techniques. The hit-miss transform is modified to use rank-order filtering since this gives better performance in noise and clutter. By varying the correlation plane threshold, we can perform dilations, rank-order filters, and erosions on the same multifunctional coherent optical correlator system. We quantify the thresholds required for generic object part recognition and provide simulated and optical laboratory data.

Acousto-optic processing with electronic image feedback for morphological filtering

An optical morphological processor is described and demonstrated in which the structuring element is generated by means of an acousto-optic cell in the Fourier plane of a coherent optical correlator. A magneto-optic spatial light modulator is employed in the input plane. A single PC generates the drive signals for the acousto-optic cell, processes the output image, and controls the input magneto-optic spatial light modulator. Complex morphological operations of opening and closing are experimentally demonstrated by using electronic image feedback. The results of a morphological noise-removal algorithm implemented by using the optical processor are compared with computer simulations, possible sources of discrepancies are proposed, and potential remedies are discussed.

Optical morphological image processing with acoustooptic devices

An optical morphological image processor is described and demonstrated in which structuring elements are generated using an acoustooptic device in the Fourier plane of a coherent optical correlator. The size and shape of the structuring element can be modified easily and rapidly by changing the drive signal to the acoustooptic device. Space-bandwidth limitations on the system due to the use of an acoustooptic device are discussed and results of erosion and dilation experiments are presented.

Real-time coherent optical correlator for machine vision systems

The problems of using optical coherent information processing techniques for the design of machine vision systems for updatable robotic complexes are studied. The construction of holographic correlators insensitive to distortions of the input image with respect to a reference one is analyzed, and a holographic on-line correlator is presented that performs image input, holographic matched filter synthesis, and real-time image processing. The choice of the optical processors' basic components-the pulsed solid state laser, the image input device based on photoconductor-liquid crystal structure, and the reversible recording medium FTIROS based on vanadium dioxide film is validated. The problems of holographic filter synthesis are also analyzed. The correlator prototype is described and the results of its experimental study for image input by means both of the optical system and the CRT are discussed.

Phase-only and binary phase-only spatial filters for optical correlators: a survey

This article presents a survey of the literature dealing with spatial filter designs that have found application in coherent optical correlator systems. Its purpose is to assist in one's choice of a spatial filter or set of filters for a particular application. A variety of spatial filters are specified in terms derived from their Fourier transforms or related transforms. Of particular and current interest are the phase-only and binary phase-only filters. Definitions of a large number of spatial filter types are presented and briefly discussed herein. Diffraction efficiency and signal-to-noise ratio evaluation criteria are also defined. For convenience a summary table of filter designs is included.

Optical correlator performance using a phase-with-constrained-magnitude complex spatial filter

Certain kinds of spatial light modulators (SLMs) can be used in a mode where the phase transmittance at a given point has the desired value, measured modulo-2pi, and the magnitude transmittance has approximately the desired value. Computer simulations, based on a specific model of a deformable-mirror SLM, show that these phase-with-coupled-magnitude SLMs perform better for the particular cases studied, in terms of discrimination and signal-to-clutter ratio, than phase-only and binary phase-only SLMs in coherent optical pattern-recognition correlators where the input is distorted by variable illumination, rotation, and noise relative to the template pattern.

Single holographic optical element based convolver and correlator: a theoretical study

A theoretical analysis is presented of a proposed multiple function single holographic optical element (MFSHOE) based simultaneous coherent optical convolver and correlator system. The proposed system offers additional advantages (for example, compactness of the total system; display of focused outputs from both the convolution and correlation channels allowing for the ability to carry out pattern recognition and signal processing operations simultaneously, etc) while preserving the beneficial features of earlier proposed systems. It is demonstrated with the support of a simulation study that the proposed system displays reasonably acceptable aberration behaviour.

Coherent optical fiber communications system using all-optical correlation processing

We propose a point-to-point or single-ended communications technique based on single-mode optical fibers that uses all-optical pulse multiplexing at the transmitter and coherent optical correlation at the receiver. This technique shows potential for application to secure untappable communications, fiber local-area networks, and optical ranging in the presence of a strong white-light background. The system can employ a low-coherence source, is insensitive to source phase noise and polarization-state rotation in the fiber link, and uses simple electronic threshold detection. Coherent correlation and anticorrelation are demonstrated experimentally, and practical requirements are examined.

Multiple Fourier transform generation for coherent optical correlators

An interferometrically generated off-axis holographic optical element images a laser diode light source to a 3 x 5 point array through 10 cm of glass. The element also reduces the elliptical beam cross section from 3:1 to 1.5:1.