Increase of reliability of surface displacement field recovery by optical speckle-displacement correlation technique

The optical speckle-displacement correlation (OSDC) technique was developed to increase the reliability of surface displacement field recovery near stress concentrators. The performance of optical speckle correlators based on joint transform correlator (JTC) architecture and a joint power spectrum (JPS) nonlinear filtering (median thresholding, adaptive median thresholding, ring median thresholding) is studied by using computer models of these correlators. The design of hybrid joint transform speckle correlator is detailed. Example results of correlation signal using computer models of digital speckle correlation and OSDC techniques and created hybrid joint transform speckle correlator setup are described.     

Accurate measurement of the cutoff wavelength in a microstructured optical fiber by means of an azimuthal filtering technique

A simple self-referenced nondestructive method is proposed for measuring the cutoff wavelength of microstructured optical fibers (MOFs). It is based on the analysis of the time-dependent optical power transmitted through a bow-tie slit rotating in the far-field pattern of the fiber under test. As a first demonstration, the cutoff wavelength of a 2 m MOF sample is found to be close to that provided by numerical predictions (approximately 25 nm higher). Because of the high dynamics of the measurement, the uncertainty is limited to Dlambda= +/-10 nm.     

Accurate measurement of wall skin friction by single-pixel ensemble correlation

The present work deals with accurately estimating wall-skin friction from near-wall mean velocity by means of PIV measurement.The estimation accuracy relies on the spatial resolution and the precision of the resolved velocity profile inside the viscous sublayer,which is a big challenge for conventional window-based correlation method(K?hler C J,et al.Exp Fluids,2012,52:1641–1656).With the help of single-pixel ensemble correlation,the ensemble-averaged velocity vector can be resolved at significant spatial resolution,thus improving the measurement accuracy.To demonstrate the feasibility of this single-pixel ensemble correlation method,we first study the velocity estimation precision in a case of steady near-wall flow.Synthetic particle images are used to investigate the effect of different image parameters.It is found that the velocity RMS-uncertainty level of the single-pixel ensemble correlation method can be equivalent to the conventional window correlation method once the effective particle number used for the ensemble correlation is large enough.Furthermore,a canonical turbulent boundary layer is synthetically simulated based on velocity statistics resolved by previous Direct Numerical Simulation(DNS)work(Schlatter P,et al.J Fluid Mech,2010,659:116–126).The relative error of wall skin friction coefficient is shown to be one-order smaller than that of the window correlation method.And the optimization strategy to further minimize the measurement uncertainty is discussed in the last part.     

基于光学相关的多目标检测

运用傅里叶变换相关识别技术实现了无畸变、有平移多目标的检测。在此基础上讨论了图像边缘信息和非边缘信息对相关性能的影响。运用分数傅里叶变换相关识别技术实现了目标图像存在尺度、旋转畸变而无平移变化时的目标检测。结果表明,分数傅里叶变换的级次选择对目标图像的旋转和尺度畸变有一定的补偿作用。在分数傅里叶变换相关中,通过简单的级次调节可实现一定范围内的旋转、尺度畸变不变识别。     

An optical correlation technique for characterizing the crack velocity in concrete

In the present work, an experimental method named “the rocking spalling test” is proposed to investigate the crack-propagation velocity in concrete and rock-like materials under dynamic tensile loading. This method is based on the use of double-notched specimens loaded in spalling tests. A compressive pulse is transmitted to a rectangular specimen by means of a Hopkinson bar. It is reflected as a tensile wave on the opposite free surface of the sample. A large notch provides a rocking effect of the rear part of the specimen whereas a short notch is used to trigger a single unstable crack. This experimental configuration has been optimized through a series of numerical simulations. Finally, a series of tests have been conducted on dry and wet concrete specimens. Crack gauges and ultra-high speed camera coupled to Digital Image Correlation have been used to characterize the crack speed in dry and wet concrete samples.     

Accurate estimation of Brillouin frequency shift in Brillouin optical time domain analysis sensors using cross correlation

Current methods of estimating the Brillouin frequency shift in Brillouin optical time domain analysis sensors are based on curve-fitting techniques. These techniques apply the same weight to all portions of the curve and dutifully fit into the peak and noisy ends of the curve. This makes them very sensitive to noise, initialization of fitting parameters, symmetry, and start and stop frequencies. We introduce a method based on the cross-correlation technique to estimate the central frequency of noisy Lorentzian curves, which is more robust to noise and free from initial settings of fitting parameters.     

Accurate position detection of targets in noise image

Targets at various positions in a noise image were detected by the optical matched filtering technique. In this case the thermoplastic optical recorder was used as a matched filter recorder. High accuracy of position detection was obtained, i.e. the position error was 2% or less.     

A novel definition method of optical asphericity

In view of the fact that methods for calculating asphericity of optical aspheric surface are nonuniform, a novel definition method named least maximal error method is proposed on the basis of an analysis of the common calculating methods. It fully unifies the asphericity calculation for conicoid and high-order aspheric surface. It finds the best-fitting spherical surface, notes the maximum deviation between aspheric surface and the best-fitting spherical surface minimum, and this maximum deviation is defined as the maximal asphericity of aspheric surface. The theory of the definition method and some calculation examples are put forward. The method is suitable for computer programming and its result is accurate.     

Videomicroscopy calibration of optical tweezers by position autocorrelation function analysis

We present a simple method to calibrate optical tweezers by using only videomicroscopy to measure the position autocorrelation function of the trapped bead in the potential well of the tweezers. To accomplish this task, we use a high-speed camera with ??500 fps (frames per second), which provides a precise measurement of the relaxation time of the bead Brownian fluctuations. We also study the variation of the trap stiffness as a function of some parameters of interest such as the laser power and the distance from the bead center to the microscope coverslip, showing that the presented method returns precise results.     

高峰值光场的定点产生

用有限差分方法联立求解 Maxwell-Bloch方程时,发现2p光脉冲通过1维共振介质时的两个现象：当共振介质的偶极矩较大或共振粒子数密度较大时,2p脉冲波形会发生分裂,不再以孤子形式传播;峰面积符号相同、脉宽不同的两个2p脉冲传播时,脉宽小的脉冲将赶上脉宽大的脉冲,赶超过程中光电场大小的变化不明显,而峰面积符号相反、脉宽不同的脉冲在赶超过程中会引起光场的叠加而形成光电场峰值高、脉宽小的脉冲,因而可以通过控制两脉冲的相对脉宽或相对距离,在介质的指定位置产生高峰值的光场。     

高峰值光场的定点产生

 用有限差分方法联立求解 Maxwell-Bloch方程时,发现2p光脉冲通过1维共振介质时的两个现象：当共振介质的偶极矩较大或共振粒子数密度较大时,2p脉冲波形会发生分裂,不再以孤子形式传播;峰面积符号相同、脉宽不同的两个2p脉冲传播时,脉宽小的脉冲将赶上脉宽大的脉冲,赶超过程中光电场大小的变化不明显,而峰面积符号相反、脉宽不同的脉冲在赶超过程中会引起光场的叠加而形成光电场峰值高、脉宽小的脉冲,因而可以通过控制两脉冲的相对脉宽或相对距离,在介质的指定位置产生高峰值的光场。     

基于超表面材料的光波相位精密操控新技术

近年来,基于超表面材料的研究发现了很多新的光学现象,其中几何相位调制是最具吸引力的方向之一。笔者介绍了超表面材料用于光波相位精密操控方面的研究,包括电磁响应的各向异性、电磁共振等机理研究、以及一系列新概念光器件。研究表明,基于金纳米棒超表面材料制造的计算全息片,能够在波长为630 nm~1 050 nm的宽带范围内高效工作,且在波长825 nm处的衍射效率超过80%;基于硅材料超表面材料制造的光分束器,能够在远场形成衍射角为59°×59°的4×4个均匀点阵,且其衍射效率在波长为1 530 nm~1 565 nm的范围内超过50%;基于硅材料超表面材料制造的偏振分离器,其在纳米棒长轴方向的反射率高达98.5%,在短轴方向透过率达到94.7%,且仅需通过调节纳米棒的宽度,就可以在波长为1 460 nm~1 625 nm的宽带范围内任意选择峰值反射波长。研究结果表明,基于几何相位调制机理的超表面材料在具备连续、任意、精密、高效的相位操控等优点的同时,在制造上却仅需要简单的二台阶微纳光学工艺条件,可用于打造新一代高性能、芯片级的光电子元器件,在光纤通信、军事国防、工业及消费电子等领域得到重要应用。     

Fingerprint identification by an optical joint transform correlation system

An optical correlator has been used in fingerprint identification, and in order to test its properties in a practical application a compact joint transform correlation system was constructed. The system’s identification performance was qualitatively and statistically evaluated, based on fingerprint data gathered from 500 fellow workers. The power of parallelism and fast processing of optical processing enabled the system to verify an individual’s identity in less than 0.1 s on each try. The system performed a verification with a 4.1% false rejection rate and no false acceptance in “three-try.”     

Transverse magnetic peak type metal-clad optical waveguide sensor

Transverse magnetic (TM) waves in a four-layer slab waveguide structure are studied for optical sensing applications. The structure consists of a semi-infinite substrate, a thin metal layer, a medium with negative permittivity and permeability as a guiding layer, and a semi-infinite layer as a cover. The proposed sensor is operated in reflection mode in which the angular position of the reflectance peak is used to detect small changes in the refractive index of the cover medium. The optimal structure parameters that correspond to the sharpest and highest peak are presented. The results reveal that for aluminum metal layer, a thickness of about 9 nm represents the optimum metal thickness. Moreover, the thickness, negative permittivity, and negative permeability of the guiding layer are found to have great impacts on the performance of the proposed optical waveguide sensor.     

Accuracy evaluation of optical distortion calibration by digital image correlation

Due to its convenience of operation, the camera calibration algorithm, which is based on the plane template, is widely used in image measurement, computer vision and other fields. How to select a suitable distortion model is always a problem to be solved. Therefore, there is an urgent need for an experimental evaluation of the accuracy of camera distortion calibrations. This paper presents an experimental method for evaluating camera distortion calibration accuracy, which is easy to implement, has high precision, and is suitable for a variety of commonly used lens. First, we use the digital image correlation method to calculate the in-plane rigid body displacement field of an image displayed on a liquid crystal display before and after translation, as captured with a camera. Next, we use a calibration board to calibrate the camera to obtain calibration parameters which are used to correct calculation points of the image before and after deformation. The displacement field before and after correction is compared to analyze the distortion calibration results. Experiments were carried out to evaluate the performance of two commonly used industrial camera lenses for four commonly used distortion models.     

Accurate characterization of complex pulses by delay-controlled fringe-free interferometry of spectral high-resolution technique

We discuss the disadvantage in conventional Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER) technology in complex femtosecond pulse measurement. An improved version of conventional technology named DC-FISH is presented, where single replica of the unknown pulse upconverts synchronously with two frequency-shifted narrow-banded long pulses. The spectral phase of the unknown pulse can be directly calculated from the fringe-free spectra with the introduction of a suitable small delay between the upconverted pulses. The numerical simulation results are achieved to identify a higher efficiency and lower requirements on measurement in novel approach.     

Quality control of textile yarn by optical filtering technique

An optical filtering technique is used to improve the on-line quality characteristic of textile yarn. This enables measurements to be made of the size of the main body of the yarn and the variation in hairiness along the length of the yarn. This technique has been found to be entirely satisfactory in practice because the two parameters are parallel- recorded. A prototype system is described; it offers a wide range of benefits such as on-line recording results, easy measurements, very high operating velocity and compactness. Examples of results are presented.     

Suppression of multiplicative disturbance by coherent optical feedback technique

The transfer function of a coherent optical feedback system depends on an adjustable phase. This property is used to produce flexible filters. As an application, suppression of multiplicative noise in object information is shown.     

高精度全息干涉计量—相移技术的应用

本文介绍了一种精确的全息干涉相位测量系统。采用双曝光、双参考光束,并引入相移技术,能够同时获得全场256×256个点的相位变化值;相位测量的重复性优于2％,全场处理时间少于2分钟。