Micro-electro-mechanical system-based digitally controlled optical beam profiler

An optical beam profiler is introduced that uses a two-dimensional (2-D) small-tilt micromirror device. Its key features include fast speed, digital control, low polarization sensitivity, and wavelength independence. The use of this 2-D multipixel device opens up the important possibility of realizing several beam profile measurement concepts, such as a moving knife edge, a scanning slit, a moving pinhole, a variable aperture, and a 2-D photodiode array. The experimental proof of the optical beam profiler concept using a 2-D digital micromirror device to simulate the 2-D moving knife edge indicates a small measurement error of 0.19% compared with the expected number based on a Gaussian beam-propagation analysis. Other 2-D pixel arrays such as a liquid-crystal-based 90 degrees polarization rotator sandwiched between crossed polarizers can also be exploited for the optical beam whose polarization direction is known.     

Calibration standard for laser beam profilers: method for absolute accuracy measurement with a Fresnel diffraction test pattern

The absolute accuracy of a clip-level laser beam profiler is measured to the 0.3% level, by comparison of the profiler's reading to the known width of a Fresnel diffraction test pattern. A pair of opposed knife edges, illuminated by a quasi-uniform and quasi-plane wave, generates the pattern whose width is determined by the 50% cut points in translating the edge pair across a tightly focused beam. The convolution of the scanning aperture with the diffraction fringe pattern is modeled to remove the effect of the aperture size from the accuracy test and to give a means of measuring the aperture width. Discussions of the experimental aspects of this test method show it to be an acceptable calibration standard for optical profilers, of use to those working on the International Standards Organization draft standard for laser beam parameter measurements.     

A simple method for studying surface films on solids using transmission electron microscopy

A simple and rapid method for extracting and identifying a small amount of a foreign substance on the surface of a solid using a transmission electron microscope is developed. In this method surface species, such as contaminants, oxides or segregated surface films, are obtained by scraping a solid surface with a small piece of knife edge. The knife edge on which surface films adhere is then directly mounted on a microscope specimen holder for transmission electron microscopy observation. Pieces of surface films accumulated on the knife edge are found to be sufficient in quantity for phase identification using an electron diffraction method. Examples of this simple technique are taken from our recent structural study of gold electrodeposits.     

Vertical dynamic responses of a simply supported bridge subjected to a moving train with two‐wheelset vehicles using modal analysis method

The vertical dynamic responses of a simply supported bridge subjected to a moving train are investigated by means of the modal analysis method. Each vehicle of train is modelled as a four‐degree‐of‐freedom mass–spring–damper multi‐rigid body system with a car body and two wheelsets. The bridge, together with track, is modelled as a simply supported Bernoulli–Euler beam. The deflection of the beam is described by superimposing modes. The train and the beam are regarded as an entire dynamic system, in which the contact forces between wheelset and beam are considered as internal forces. The equations of vertical motion in matrix form with time‐dependent coefficients for this system are directly derived from the Hamilton's principle. The equations of motion are solved by Wilson‐θ method to obtain the dynamic responses for both the support beam and the moving train. Compared with the results previous reported, good agreement between the proposed method and the finite element method is obtained. Finally, the effects of beam mode number, vehicle number, beam top surface, and train velocity on the dynamic responses of the entire train and bridge coupling system are studied, and the dynamic responses of beam are given under the train moving with resonant velocity. Copyright © 2005 John Wiley & Sons, Ltd.     

一种无衍射激光图像亚像素边缘检测方法

为进一步提高无衍射激光图像的定中精度,提出了一种亮环亚像素级边缘点检测方法.首先,采用局域亮度最大方法,提取亮环像素级屋脊边缘点;然后,利用线性插值算法对每一像素级边缘点进行亚像素处理进而获得亚像素边缘点;最后,用最小二乘方法,将亚像素边缘点拟合定中.分别对距离光源40 m、50 m、60 m处,各连续拍摄的50幅图像(图像分辨率为640×480,光靶分辨率为0.057 2 mm/pixel)进行了处理,各位置处的亚像素级边缘点x方向定中标准差分别为0.021 4 nm、0.037 2 mm和0.042 3mm,y方向定中标准差分别为0.022 8 mm、0.035 1 mm和0.043 7 mm,明显优于像素级边缘点,实验结果表明,该方法切实可行.     

Quality improvements in extruded meshes using topologically adaptive generalized elements

This paper describes a method to extrude near‐body volume meshes that exploits topologically adaptive generalized elements to improve local mesh quality. Specifically, an advancing layer algorithm for extruding volume meshes from surface meshes of arbitrary topology, appropriate for viscous fluid flows, is discussed. First, a two‐layer reference mesh is generated from the layer initial surface mesh by extruding along the local surface normals. The reference mesh is then smoothed using a Poisson equation. Local quality improvement operations such as edge collapse, face refinement, and local reconnection are performed in each layer to drive the mesh toward isotropy and improve the transition from the extruded mesh to a void‐filling tetrahedral mesh. A few example meshes along with quality plots are presented to demonstrate the efficacy of this approach. Copyright © 2004 John Wiley & Sons, Ltd.     

用激光和CCD在线测量大外径

本文介绍了一种实用化的大型工件外径测量方法。用准直激光分别瞄准吸队于工件两端的磁性定位块，由四象限光电池接收判断。轻质粗糙导轨造成误差由ＣＣＤ器件探测并加以补偿，用光辉干涉仪完成测长。     

球面扫描的数学模型及其计算机仿真

在检测球体表面时,需要用激光束对其进行扫描。研究了当用固定不动的激光束扫描球心固定的旋转球体时扫描点在球面上的移动模式。用微分方程组建立了扫描点与角速度矢量变化规律的数学模型,并对此模型及其控制曲面分别用仿真软件Simulink和三维计算机图形工具OpenGL进行了成功的计算机仿真。由于在此模型中激光束是固定不动的,因此,可使检测装置大大简化。     

Folding mirror schlieren diffraction interferometer

We demonstrate the use of a mirror as a viewing diaphragm to generate a built-in diffracted reference beam in schlieren diffraction interferometry (SDI). The use of a mirror edge as a diffracting element instead of a conventional knife edge considerably enhances the contrast of the schlieren pattern, and it is shown to be equal to that of a phase knife edge. This increase in contrast is due to the fact that the otherwise unutilized diffracted beam in SDI is recombined in the described folding mirror geometry.     

Suppressing local particle oscillations in the Hamiltonian particle method for elasticity

The governing equation of elasticity is discretized into motion equations of the particles in a Hamiltonian system. A weighted least‐square method is adopted to evaluate the Green–Lagrange strain. Using a symplectic scheme for the Hamiltonian system, we obtain the property of energy conservation in the discretized calculations. However, local particle oscillations occur, and they excessively decrease low frequency motion. In this study, we propose the use of an artificial potential force to suppress the local oscillations. The accuracy of the model with and without the inclusion of the artificial force is examined by analyzing a cantilever beam and wave propagation. With the inclusion of the artificial force, the local oscillations are reduced while energy conservation is maintained. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparison of Shearography to Scanning Laser Vibrometry as Methods for Local Stiffness Identification of Beams

Local stiffness of Euler–Bernoulli beams can be identified by dividing the bending moment of a deformed beam by the local curvature. Curvature and moment distributions can be derived from the modal shape of a beam vibrating at resonance. In this article, the modal shape of test beams is measured by both scanning laser vibrometry (SLV) and shearography. Shearography is an interferometric optical method that produces full‐field displacement gradients of the inspected surface. Curvature can be obtained by two steps of derivation of the modal amplitude (in the case of SLV) or one step of derivation of the modal shape slope (in the case of shearography). Three specially prepared aluminium beams with a known stiffness distribution are used for the validation of both techniques. The uncertainty of the identified stiffness distributions with both techniques is compared and related to their signal‐to‐noise ratios. A strength and weakness overview at the end of the article reveals that the shearography is the technique that shows the most advantages.     

Vibrating knife-edge technique for measuring the focal length of a microlens

A vibrating knife-edge technique is proposed for measuring the focal length of a microlens. The technique is based on the propagation properties of Gaussian beams. A laser beam with a Gaussian intensity profile is focused in front of the microlens under test. After being transmitted through the microlens, the beam propagates toward a detector, which consists of a photodiode that is half blocked by a knife-edge. The photodiode integrates approximately half the intensity of the transmitted beam. The knife-edge vibrates sinusoidally with small amplitude in a plane normal to the direction of propagation. Our analysis shows that the output signal at the photodiode consists of a dc component plus a temporal sinusoidal signal whose amplitude is proportional to the focal length of the microlens. After system calibration, the focal length is measured with an envelope detector or a lock-in amplifier.     

基于小波分析的激光扫描回波信号处理

基于直接探测技术,建立了一套实验系统.试验采用激光直接探测方式,振镜在X、Y方向分别以不同的频率振动,当激光经过振镜反射后,就以X、Y方向对物体表面进行二维扫描,使用光电二极管(PIN)来探测从目标返回的能量,探测的信号经过滤波器后,按照摆动频率分开,根据反射回来的激光能量的变化,进而判断扫描物体的边缘轮廓.利用小波变换检测回波信号,可以确定回波信号的奇异点,进而确定被探测目标的边缘.     

Parallel efficient mesh motion using radial basis functions with application to multi‐bladed rotors

Radial basis functions are used to provide a solution to the problem of mesh motion for unsteady aerodynamic simulation. The method is independent of connectivity and produces high‐quality meshes, but is expensive for large meshes in its full form. Hence, the efficiency of the technique has been greatly improved here by reducing the number of surface points used to define deformations of the surface, and the minor error in position that this implies at other surface points is corrected with a simple decaying perturbation, thus splitting the method into a primary basis function method and a secondary local correction method. This means that the exact surface is retained, but the mesh motion is significantly faster, while splitting the motion into two stages allows both the methods to work on appropriate problems given their relative strengths. An example deformation for a 5×10⁶ cell helicopter rotor mesh with an exaggerated cyclic pitch motion shows excellent mesh quality, thus validating a scheme that is also simple, robust and readily parallelized. Copyright © 2009 John Wiley & Sons, Ltd.     

Handedness and azimuthal energy flow of optical vortex beams

Abstract

The propagation of optical vortex beams obstructed by a knife edge is investigated. The intensity profile shows a characteristic rotation around the beam axis which can be used to determine the absolute handedness of the vortex. Evolution of the intensity profile directly visualizes the rotation of the beam's local Poynting vector.     

FFT‐based spectral element analysis for the linear continuum dynamic systems subjected to arbitrary initial conditions by using the pseudo‐force method

In this paper, a fast Fourier transform (FFT)‐based spectral element method (SEM) is developed for the linear continuum dynamic systems subjected to arbitrary, non‐null initial conditions. In the FFT‐based SEM, the original equations of motion subjected to arbitrary initial conditions are transformed into a new set of equations of motion subjected to completely null initial conditions by using the pseudo‐force method so that the conventional spectral element analysis can be applied to obtain desired dynamic responses. A simply supported beam and a cantilevered beam are considered as the illustrative problems to evaluate the FFT‐based SEM. The dynamic responses obtained by using the FFT‐based SEM are shown to be in good agreement with the analytical solutions obtained by using the mode superposition method. Copyright © 2007 John Wiley & Sons, Ltd.     

High-sensitivity laser absorption measurements of broadband absorbers in the near-infrared spectral region

We describe the development and characterization of a near-infrared diode-laser-based sensor to measure the vapor from trace gases having unstructured absorption spectra. The technique uses two equal amplitude-modulated laser beams, with the modulation of the two lasers differing in phase by 180 deg. One of the laser beams is at a wavelength absorbed by the gas [for these experiments, vapor is from pyridine (C(5)H(5)N)], and the second laser beam is at a wavelength at which no absorption occurs. The two laser beams are launched onto near-coincident paths by graded-index lens-tipped optical fibers. The mixed laser beam signal is detected by use of a single photodiode and is demodulated with standard phase-sensitive detection. Data are presented for the detection and measurement of vapor from pyridine (C(5)H(5)N) by use of the mixed laser technique. The discussion focuses on experimental determination of whether a compound exhibits unstructured absorption spectra (referred to here as a broadband absorber) and methods used to maximize sensitivity.     

Null detection of ballistic photons for locating objects buried in turbid media

We describe an optical system in which a vibration induced to a laser probe beam combined with a half-blocked photodiode allows determination of the position and width of objects buried in turbid media. Our system is based on the detection of an AC signal which drastically decreases under the presence of an obstructing buried object. We describe the technique and include experimental results showing that the system is capable of detecting 2?mm wide objects buried at depths up to 3?cm from the front surface of a sample simulating scattering properties of soft tissue.     

Using Laser Scattering for Detection of Cracks on a Microsolderball Surface

In this paper, a scanning laser probe has been developed to detect and measure cracks on a microsolderball surface. The technique, based on a light-scattering method that employs a low-power He-Ne laser and conventional optical components, enables the laser beam to be focused on the test surface. In the relative scanning between the test surface and laser probe, the specularly reflected light intensity acquired by a photodiode would exhibit the change of the scattered-light intensity, which can be used to determine the size of cracks on the solderball. Proof-of-concept experiments have been performed with measurement of cracks of different sizes (from 4 m to 60 m) on the microsolderball.     

Dynamic high order numerical manifold method based on weighted residual method

High Order Numerical Manifold Method (HONMM) is a powerful method to solve static problems. A development of HONMM to achieve a dynamic solution with high accuracy and less computational cost is addressed in the current paper. In the developed method, the global approximation is obtained through increasing the order of local approximation functions without any Linear Dependence (LD) of the unknowns. The weighted residual formulations are modified to be used in dynamic high order simulation. Moreover, a modified Newmark method formulation is adjusted for time integration of high order equations. The superiority of the proposed method over the conventional NMM is demonstrated through a special beam example. The dynamic free fall block example is used to exhibit the removal of mass matrix singularity. As cases of dynamic analysis, beam free and forced vibrations are illustrated which include a moving load. Finally, a non‐uniform cross‐section beam under dynamic variable loads with accelerated motion is solved while demonstrating the capability of the new method such as simplicity, accuracy and time efficiency for simulation of complex dynamic problems. Copyright © 2014 John Wiley & Sons, Ltd.