Extended hartmann test based on the pseudoguiding property of a hartmann mask completed by a phase chessboard

We propose to add a specific phase chessboard to the classical Hartmann mask used for wave-front sensing. By doing this we obtain a pseudoguiding of the energy issuing from this mask, allowing for an increase in the sensitivity of the Hartmann test. This property is illustrated by experiment, and a comparison between classical and new Hartmanngrams is presented.     

Aberration measurement of photolithographic lenses by use of hybrid diffractive photomasks

In optical lithography the degradation of image quality due to aberrations present in the exposure tool is a serious problem. Therefore it is desirable to establish a reliable aberration measurement procedure based on the analysis of printed images in the photoresist. We present what is to our knowledge a new method for characterizing the aberrations of an exposure tool using a hybrid diffractive photomask. By utilizing each different impact on the aberrated image from each diffracted illumination, we were able to extract the aberration present in the stepper system. We experimentally verified this method with a G-line stepper and verified its spherical aberration astigmatism.     

Design of adaptive digital filters for phase extraction in complex fringe patterns obtained using the Ronchi test

A powerful technique is presented for processing complex fringe patterns with high noise levels and arbitrary distributions of spatial frequencies, which can successfully extract the phase information. Artifacts that arise from phase extraction in local filtering approaches are avoided by using a simple design and implementation strategy for the adaptive filter, based on the theory of digital filter design used in electronics, and applied to pixel rows (or columns) in the fringe-pattern. The filter designed in this manner is then applied to phase extraction in an experimental fringe pattern measured in a digital Ronchi test setup using a Carré phase-shifting procedure. The filtering strategy has a very low computational cost and allows phase extraction in noisy ronchigrams regardless their spatial frequency distribution, provided the fringes are still visible.     

Making optical low-pass spatial filters by thin-film phase grating

A modified rule was applied to design a grating optical low-pass spatial filter (GOLF) with a thin-film phase grating. This filter differed from the crystal type of optical low-pass spatial filter (OLPF). The fabrication method of the thin-film phase GOLF was also different from the conventional lithographic process. A new fabrication method utilizing a metal mesh mask and a thin-film coating process was applied to make the GOLF. The process had the advantages of being simple and cheap, and would thus be good for mass-production. The modulation transfer function (MTF) of the device was measured and the results agree well with the theoretical calculations. Finally, the GOLF was installed on a digital camera which was used to capture images of the circular zone plane (CZP), to show the Moiré pattern suppression ability.     

Analysis of quasielastic neutron scattering in condensed media by periodic spatial filters

A method is proposed for Fourier analysis of quasielastic neutron scattering in condensed media using moving spatial filters with a periodic neutron transmission function, allowing the time correlation function of the object to be measured. Pis’ma Zh. Tekh. Fiz. 25, 77–81 (February 12, 1999)     

Aberration measurement of projection optics in lithographic tools by use of an alternating phase-shifting mask

As a critical dimension shrinks, the degradation in image quality caused by wavefront aberrations of projection optics in lithographic tools becomes a serious problem. It is necessary to establish a technique for a fast and accurate in situ aberration measurement. We introduce what we believe to be a novel technique for characterizing the aberrations of projection optics by using an alternating phase-shifting mask. The even aberrations, such as spherical aberration and astigmatism, and the odd aberrations, such as coma, are extracted from focus shifts and image displacements of the phase-shifted pattern, respectively. The focus shifts and the image displacements are measured by a transmission image sensor. The simulation results show that, compared with the accuracy of the previous straightforward measurement technique, the accuracy of the coma measurement increases by more than 30% and the accuracy of the spherical-aberration measurement increases by approximately 20%.     

Object recognition in subband transform-compressed images by use of correlation filters

We introduce subband correlation filters (SCFs) as a solution to the problem of object recognition at multiple resolution levels in quantized transformed imagery. The approach synthesizes correlation filters that operate directly on subband coefficients rather than on image data. We explore two techniques to accomplish the reduced-resolution recognition: (1) training the correlation filters to incorporate downsampling tolerance and (2) adaptation of the subband decomposition filters to accommodate the reduced resolutions. For compression ratios of 20:1, SCFs demonstrate recognition performance of at least 90%, 85%, and 75%, respectively, on 2-, 4-, and 8-ft-resolution synthetic aperture radar data.     

Gabor滤波器的掌纹特征提取研究

掌纹识别是近年发展起来的一种生物识别技术,掌纹特征提取是掌纹识别核心部分之一,传统Gabor滤波器特征提取速度慢致使掌纹识别速度不能满足实时性的要求.将传统Gabor、改进Gabor和迭代Gabor应用于掌纹特征提取,对比研究了它们在掌纹识别中特征提取速度和识别精度的问题,旨在探求识别精度高且特征提取速度更快的Gabor滤波器,进而改良掌纹特征提取方法.实验表明,迭代Gabor滤波器能确保掌纹识别系统在高识别精度的前提下具有更快的识别速度.     

Two-color imaging by the use of patterned optical filters bonded to focal-plane-array detectors

Two-color imaging by the use of a patterned optical filter directly bonded to a HgCdTe focal-plane array is demonstrated. The optical filter is spatially patterned, by multilayer lithographic techniques, to provide multicolor response in the midwavelength infrared by the use of a single substrate.     

Space-variant simultaneous detection of several objects by the use of multiple anamorphic fractional-Fourier-transform filters

A fractional correlator that is based on the anamorphic fractional Fourier transform is defined. This new, to our knowledge, correlator has been extended to work with multiple filters. The novelty introduced by the suggested system is the possibility of the simultaneous detection of several objects in different parts of the input scene (when anamorphic optics are dealt with), thereby permitting an independent degree of space invariance in two perpendicular directions. Computer experiments as well as experimental optical implementation are presented.     

Realization of time gating by use of spatial filtering

A method for simulating conventional time gating in low-coherence optical imaging processes in highly scattering media is given. The method uses monochromatic instead of broadband light, and spatial filtering is substituted for time gating. The process enables the study of imaging techniques in scattering media to be carried out in an easy and highly controllable way. Experimental results are given.     

Estimating the precision characteristics of a hartmann sensor in detection of poisson signals

An analytic expression for the distribution density of a random variable at the output of a quadrant photodetector that performs sum-difference processing a, Poisson signal is obtained. The precision characteristics of a Hartmann sensor in the presence of Poisson and Gaussian noise are analyzed on the basis of the expression. Expressions for the mathematical expectation and variance of the noise-induced error variance and the correlation coefficents, of the results of reconstruction of the phase front are obtained. Translated from Izmeritel'naya Tekhnika, No. 9, pp. 38–42, September, 1999.     

Schmidt棱镜偏振像差的校正

为了校正Schmidt棱镜的偏振像差,消除双像和鬼影像现象,根据棱镜Jones矩阵B因子与衍射光强的关系,推导出B因子与屋脊面反射相移差δ之间的关系,明确了消除偏振像差,要求B=0需要屋脊面反射相移差δ=π的条件。采用在屋脊面镀制相位膜的方案,对比了单层膜与多层膜的反射相移差特性,得到了在可见光区实现宽波段消除偏振像差的膜系特性;证明了B=0是一条可以同时校正所有不同偏振状态入射光偏振像差的有效技术途径。实验镀制了屋脊面相位膜样品,进行了多种入射光偏振状态下的偏振像差检测,得到了很好的消偏振像差结果。     

Rotationally invariant pattern recognition by use of linear and nonlinear cascaded filters

We discuss the merits of using single-layer (linear and nonlinear) and multiple-layer (nonlinear) filters for rotationally invariant and noise-tolerant pattern recognition. The capability of each approach is considered with reference to a two-class, rotation-invariant, character recognition problem. The minimum average correlation energy (MACE) filter is a linear filter that is generally accepted to be optimal for detecting signals that are free from noise. Here it is found that an optimized MACE filter cannot differentiate between the characters E and F in a rotation-invariant manner. We have found, however, that this task is possible when a single optimized linear filter is used to achieve the required response when a nonlinear threshold function is included after the filter. We show that this structure can be cascaded to form a multiple-layer, cascaded filter and that the capability of such a system is enhanced by its increased noise tolerance in the character recognition problem. Finally, we show the capability of a two-layer cascade as a means to detect different species of bacteria in images obtained from a phase-contrast microscope.     

Simulation of cross-talk reduction in multimode waveguide-based micro-optical switching systems by use of single-mode filters

We propose a method to reduce cross talk by using single-mode filters, namely, extracting the fundamental mode from a multimode optical network (EFMON). The EFMON effect is evaluated for a three-step switching system consisting of cascade electro-optic (EO) waveguide prism deflector (WPD) micro-optical switches (MOSs) and a multimode waveguide network. The WPD MOS is optimized for single-mode operation in lead lanthanum zirconate titanate thin films as EO slab waveguides with a driving voltage of 12 V, a length of 310 microm, and a channel distance of 20 microm. Beam propagation method simulation reveals that mode disturbance, higher-order modes, and cross talk are accumulated by switching steps. A single-mode filter for EFMON in the output region of the switching system reduces cross talk to below -20 dB at a propagation length of 2000 microm in the single-mode filter and below -30 dB at 20000 microm.     

Complex amplitude modulation by use of liquid-crystal spatial light modulators

We propose to study the conditions for implementation of complex amplitude modulation on standard liquid-crystal spatial light modulators when illuminated by polarized light. The spatial light modulators are used in a conventional configuration, i.e., the voltage is applied parallel to the wave-front propagation direction. The most commonly used liquid-crystal materials are compared and their usefulness in some general applications discussed. Their specificities with respect to different modulation types and application requirements are briefly described. Typical characteristics such as response time, modulation range, and wavelength insensitivity are also discussed. Finally, as an illustration, a first attempt at nomenclature is proposed for the case of a linearly polarized light.     

Aberration evaluation of alignment optics in lithographic tools by use of a step-height structure highly sensitive to the asymmetry of an optical image

To satisfy the increasing demand for extremely tight overlay accuracy in semiconductor manufacturing processes, all the measurement error factors in alignment systems and overlay measurement tools need be identified and eliminated. The principle of most alignment systems is based on image processing of target marks on the wafer under bright-field illumination. Although the phenomenon that the sensitivity to the alignment error varies with the step height (SH) of the mark has been known and used for evaluating the performance of the alignment optics, no investigation has been made into the origin and the physical mechanism of the phenomenon. We propose a simplified optical model that can account for the origin of the asymmetric image and clarify its relation to the SHs. The model is validated with simulation and experimental results. The improved performance of an alignment system using marks with optimally designed SHs is demonstrated.     

Analysis of multitone holographic interference filters by use of a sparse Hill matrix method

A theory is presented for the application of Hill's matrix method to the calculation of the reflection and transmission spectra of multitone holographic interference filters in which the permittivity is modulated by a sum of repeating functions of arbitrary period. Such filters are important because they may have two or more independent reflection bands. Guidelines are presented for accurately truncating the Hill matrix, and numerical methods are described for finding the exponential coefficient and the coefficients of the Floquet-Bloch waves within the filter. The latter calculation is performed by use of a computational technique known as inverse iteration. The Hill matrix for such problems is sparse, and thus, even though the matrix can be quite large, it may be efficiently stored and processed by a desktop computer. It is shown that the results of using Hill's matrix method are in close agreement with numerical calculations based on thin-film decomposition, a transfer-matrix technique. An important result of this research is the demonstration that Hill's matrix method may, in principle, be used to analyze any multiperiodic problem, so long as the periods are known to finite precision.     

Filter multiplexing by use of spatial Code Division Multiple Access approach

The increasing popularity of optical communication has also brought a demand for a broader bandwidth. The trend, naturally, was to implement methods from traditional electronic communication. One of the most effective traditional methods is Code Division Multiple Access. In this research, we suggest the use of this approach for spatial coding applied to images. The approach is to multiplex several filters into one plane while keeping their mutual orthogonality. It is shown that if the filters are limited by their bandwidth, the output of all the filters can be sampled in the original image resolution and fully recovered through an all-optical setup. The theoretical analysis of such a setup is verified in an experimental demonstration.     

Beam shaping of focused partially coherent beams by use of the spatial coherence effect

We demonstrate that when a partially coherent beam with a Gaussian intensity distribution is focused by a lens, the desired partially coherent flat-topped intensity distribution or doughnut-shaped intensity distribution at the geometrical focus can be generated by choice of appropriate form of spectral degree of coherence. We provide a novel approach to beam shaping of a partially coherent beam and offer new schemes for their potential applications such as material processing, optical therapy, and optical tweezers.