光学相关器系统的固化设计与性能分析

小型固化光学相关器,以偏振分光棱镜和反射型傅里叶透镜作为基本部件,并采用透明固体介质填充光学系统的空隙。该系统通过各元件端面相连,简化了校准难度;使用反射型透镜,具有色像差和轴外像差小的优势。模拟分析表明,这种固化结构具有良好的系统性能,并满足识别的需要。     

Serial transform optical correlator design principles

Optical correlators such as the 4f and VanderLugt optical systems have been an active area of research for many years; we refer to these types of optical system collectively as serial transform correlators (STCs). Despite being well known, misconceptions regarding the design of STCs are not uncommon. We show, for example, that one correlator configuration reported to suffer from a phase curvature problem in fact works correctly. We present and prove a simple set of rules to follow in the selection of a STC design that does not contain unnecessary constraints and that makes it easier to identify permissible optical systems. As examples, we discuss three representative types of correlator configuration, one of which is discussed in detail to highlight its practical advantages. A novel proof of the design rules is presented that does not depend on details such as what lenses or combination of lenses are used, what their focal lengths are, or what their locations are within the optical system. We also present a conventional Fourier optics proof.     

Modified filter synthetic discriminant functions for improved optical correlator performance

By the filter modulation operator N, the modified filter synthetic discriminant function permits advantageous preprocessing of individual training-set images that are used in a linear combination to construct the filter synthetic discriminant function, which applies a modulation operator M to the synthetic discriminant function. A relaxation algorithm is used to satisfy the equal correlation peaks rule in the correlator output plane. As the filter modulation operators M and N can be given any functional form, the modified filter synthetic discriminant function design proposed is sufficiently general to be described as a unified filter modulation synthetic discriminant function design.     

新型联合变换相关器光学系统设计

以往联合变换相关器的光学系统结构是一支光路反馈,实时性差且体积大。针对其缺点,设计了一种具有双光路的联合变换相关器。其中,最主要的特点是傅里叶透镜组与准直扩束系统共用同一片正透镜。改进后的装置可以清晰地探测到相关点,并具有体积小的特点。     

Wavelet filter for improving detection performance of compression-based joint transform correlator

A new method for improving fingerprint detections by a compression-based joint transform correlator (JTC) via wavelet filter is proposed. The simulation results show that the proposed method has advantages over the conventional compression-based JTC in that detection performance can be maximized to be higher than a classical JTC by using a smaller file size for the compressed-reference images.     

Phase-only joint-transform correlator: analysis and experimental results

An analysis of the popular joint-transform optical correlator is presented for architectures employing spatial light modulators that operate only on the phase of the coherent light. Experimental results are also presented, for simple scenes that produce analytic solutions, which support the analysis.     

Compact phase-conjugating correlator: simulation and experimental analysis

A simulation and experimental investigation of a recently proposed, compact, phase-conjugating correlator is undertaken. The effects of noise and other distortions in the input image and in the correlator filter plane are considered. As with other phase-only designs, the phase-conjugating correlator is sensitive to distortion of the input image while being robust in the presence of filter-plane distortions; this robustness is enhanced by the phase-conjugating property of the design.     

Acousto-optical wideband correlator system: design, implementation, and evaluation

We describe the design and development of an acousto-optical wideband correlator that we integrated into a digital signal-processing system testbed. We report its measured performance and compare it with various theoretical performance measures, one of which compares the optical system with an equivalent efficient digital correlator. The measured performance of the optical system was 20 to 70 times that of a VAX 6410 computer using a fast-Fourier-transform correlation algorithm and a vector processor, even though the electronic interface system limited the performance of the optical system to less than 0.5% of its potential. We also compare the system with commercially available digital signal-processing boards.     

Multiresolution topology optimization using isogeometric analysis

The paper introduces a novel multiresolution scheme to topology optimization in the framework of the isogeometric analysis. A new variable parameter space is added to implement multiresolution topology optimization based on the Solid Isotropic Material with Penalization approach. Design density variables defined in the variable space are used to approximate the element analysis density by the bivariate B‐spline basis functions, which are easily obtained using k‐refinement strategy in the isogeometric analysis. While the nonuniform rational B‐spline basis functions are used to exactly describe geometric domains and approximate unknown solutions in finite element analysis. By applying a refined sensitivity filter, optimized designs include highly discrete solutions in terms of solid and void materials without using any black and white projection filters. The Method of Moving Asymptotes is used to solve the optimization problem. Various benchmark test problems including plane stress, compliant mechanism inverter, and 2‐dimensional heat conduction are examined to demonstrate the effectiveness and robustness of the present method.     

Bessel function output from an optical correlator with a phase-only encoded inverse filter

We report on a technique for producing a Bessel function correlation output for an arbitrary input pattern. The central dark spot at the center of the Bessel function correlator output is narrower than the width of the normal correlation spot and can be extremely useful for locating the center of the correlation signal. The Bessel function is produced by convolution of the extremely sharp correlation produced by an inverse filter with the Bessel function and is encoded with a single phase-only liquid-crystal spatial light modulator. To encode amplitude information on the filter, we spatially modulate the phase encoded on the filter. Amplitude modulation is obtained by modulation of the diffraction efficiency of the phase grating. Experimental results are presented.     

Nonpolarizing and polarizing filter design

We describe a nonpolarizing filter design at oblique incidence and a polarizing filter design at normal incidence that use a uniaxially anisotropic layer. The phase thicknesses and the optical admittances of the layers are compensated for by the birefringent properties of a thin film at oblique incidence. This concept can be applied to the design of nonpolarizing bandpass and edge filters at oblique incidence and of polarizing beam splitters at normal incidence. Besides, the dependence of narrow-bandpass filters on normal incidence is discussed.     

Field cross correlator for analysis of ultrafast signals

The cross-correlation function between two light fields is recorded with the help of a new device. The proposed correlator exhibits ultrashort time resolution. The optical path difference between the two interfering beams does not have to be known with interferometric precision. The experimental dynamic range proved to be as large as 10(5). The device features imaging capabilities that could be applied to the analysis of two-dimensional images with ultrashort time resolution.     

Chirped-cavity dispersion-compensation filter design

A new basic structure of a dispersive-compensation filter, called a chirped-cavity dispersion-compensator (CCDC) filter, was designed to offer the advantages of small ripples in both reflectance and group-delay dispersion (GDD). This filter provides a high dispersion compensation, like the Gires-Tournois interferometer (GTI) filter, and a wide working bandwidth, like the chirped mirror (CM). The structure of the CCDC is a cavity-type Fabry-Perot filter with a spacer layer (2 mH or 2 mL) and a chirped high reflector. The CCDC filter can provide a negative GDD of -50 fs2 over a bandwidth of 56 THz with half the optical thickness of the CM or the GTI.     

Efficient decimation filter design for lofargram analysis in passive sonar systems

A new class of low-sensitivity decimation filters based on the polyphase decomposition of infinite impulse response (IIR) transfer functions is proposed. The design approach makes use of optimized positioning of poles and zeros so that transfer functions having unequal numerator and denominator orders are obtained. Comparisons with conventional elliptic filters are shown. An illustrative example of application in passive sonar systems using real noisy data is provided. A weighted stopband filter design is presented and its performance in improving lofargram analysis in passive sonar signal processing is verified.     

Flattopped tunable wavelength-division-multiplexer filter design

We present what is to our knowledge the first design of a tunable optical filter with a flattopped passband. The filter consists of a Fabry-Perot etalon with multiple reflection gratings as dielectric mirrors. Wavelength tunability is achieved by modulation of the refractive indices of the cavity and the dielectric mirrors. Specifically, a filter with a 1-nm linewidth and a 40-nm wavelength-tuning range is designed for applications in wavelength-division-multiplexing (WDM) optical fiber communication systems. We also discuss several factors, including absorption and variations of other design parameters, that may affect the performance of the filter.     

Database filter: optoelectronic design and implementation

The results of a successful demonstration of the selection module of an optoelectronic parallel-processing database filter is presented. The module utilizes 4 x 4 arrays of AND and XOR logic gates that respectively perform the functions of reducing the data fields and determining a match between the input data and a selection argument. The logic arrays were fabricated with InGaP/GaAs heterojunction phototransistors that drive vertical-cavity surface-emitting lasers (VCSEL's). The VCSEL's provide the free-space optical interconnection between stages. The design of the system and the optical power budget are discussed.     

Fresnel transform-based correlator

We present a new technique for information processing using Fresnel transform-based correlation. The main emphasis is on the design of a correlator involving the reference object and its near-field diffraction pattern at an optimized distance. The input-scene image and its diffraction pattern constitute the input pattern of the new correlator. The new technique shows a significant increase in discrimination ability and optical efficiency. Moreover, different encoding methods, such as the phase-only filter or the matched filter, can be used in conjunction with this method. A theoretical analysis as well as examples are given.     

Log-polar transform-based wavelet-modified maximum average correlation height filter for distortion invariance in a hybrid digital-optical correlator

We discuss and implement a log-polar transform-based distortion-invariant filter for automatic target recognition applications. The log-polar transform is a known space-invariant image representation used in several image vision systems to eliminate the effects of scale and rotation in an image. For in-plane rotation invariance and scale invariance, a log-polar transform-based filter was synthesized. In cases of in-plane rotation invariance, peaks shift horizontally, and in cases of scale invariance, peaks shift vertically. To achieve out-of-plane rotation invariance, log-polar images were used to train the wavelet-modified maximum average correlation height (WaveMACH) filter. The designed filters were implemented in the hybrid digital-optical correlation scheme. It was observed that, for a certain range of rotation and scale differences, the correlation signals merge with the strong dc. To solve this problem a shift was introduced in the log-polar image of the target. The use of a chirp function for dc removal has also been discussed. Correlation peak height and peak-to-sidelobe ratio have been calculated as metrics of goodness of the log-polar transform-based WaveMACH filter. Experimental results are presented.