利用椭圆投影测量锥束CT系统投影坐标原点新方法

In order to determine the projected coordinate origin in the cone-beam CT scanning system with respect to the Feldkamp-Davis-Kress （FDK） algorithm, we propose a simple yet feasible method to accurately measure the projected coordinate origin. This method was established on the basis of the theory that the projection of a spherical object in the cone-beam field is an ellipse. We first utilized image processing and the least square estimation method to get each major axis of the elliptical Digital Radiography （DR） projections of a group of spherical objects. Then we determined the intersection point of the group of major axis by solving an over-determined equation set that was composed by the major axis equations of all the elliptical projections. Based on the experimental results, this new method was proved to be easy to implement in practical scanning systems with high accuracy and anti-noise capability.     

Material parameter equation for rotating elliptical spherical cloaks

By using the coordinate transformation method, we have deduced the material parameter equation for rotating elliptical spherical cloaks and carried out simulation as well. The results indicate that the rotating elliptical spherical cloaking shell, which is made of meta-materials whose permittivity and permeability are governed by the equation deduced in this paper, can achieve perfect invisibility by excluding electromagnetic fields from the internal region without disturbing any external field.     

Three-dimensional orientation of microscopic objects using combined elliptical and point optical tweezers

We report the development of a simple approach for controlled three-dimensional orientation of microscopic objects. The approach makes use of the fact that whereas in an elliptical trap, an object lacking spherical symmetry orients with its long axis along the major axis of the elliptical tweezers, in point tweezers it orients with its long axis along the trapping beam. Therefore, a combined use of point and elliptical tweezers and a control over the relative power of the two trapping beams can be used for three-dimensional orientation of the object. The use of this approach for three-dimensional orientation of biological objects varying in size from 2 to 20 μm is discussed. PACS 07.60.-j; 87.80.Cc     

椭圆高斯光束产生近似无衍射Bessel-Gauss光

研究了轴棱锥聚焦像散椭圆高斯光束的光场分布特性,根据菲涅耳衍射积分理论导出了椭圆高斯光束经轴棱锥衍射后的光场分布,通过数值积分给出椭圆高斯光束经轴棱锥聚焦后的近轴光场强度分布情况,将其与圆高斯光束产生的近似Bessel-Gauss场进行比较,发现椭圆高斯光束经轴棱锥聚焦后的光束在一定的传播距离内也具有无衍射特性,且轴上光强分布与圆高斯光束产生的Bessel-Gauss光束的轴上光强分布具有相似的形式,而这种无衍射光场的强度在垂直于光轴的平面上不再是柱对称分布。根据近轴球面波产生近似Bessel光束的最大无衍射距离公式计算了椭圆Bessel-Gauss光束在子午面和弧矢面上的最大无衍射距离,整个光束的无衍射距离由入射到轴棱锥上的椭圆光斑短轴方向的尺寸决定。     

A study of Ni-based WC composite coatings by laser induction hybrid rapid cladding with elliptical spot

Ni-based WC composite coatings by laser induction hybrid rapid cladding (LIHRC) with elliptical spot were investigated. Results indicate that the efficiency using the elliptical spot of 6 mm × 4 mm (the major and minor axis of laser beam are 6 mm and 4 mm, respectively, the major axis is parallel to the direction of laser scanning) is higher than that using the elliptical spot of 4 mm × 6 mm (the major axis is perpendicular to the direction of laser scanning). The precipitated carbides with the blocky and bar-like shape indicate that WC particles suffer from the heat damage of “the disintegration pattern + the growth pattern”, whichever elliptical spot is used at low laser scanning speed. However, at high laser scanning speed, the blocky carbides are only formed if the elliptical spot of 6 mm × 4 mm is adopted, showing that WC particles present the heat damage of “the disintegration pattern”, whereas the fine carbides are precipitated when the elliptical spot of 4 mm × 6 mm is used, showing that WC particles take on the heat damage of “the radiation pattern”. Especially, the efficiency of LIHRC is increased much four times higher than that of the general laser cladding and crack-free ceramic-metal coatings can be obtained.     

Analysis of the fringes visibility generated by a lateral cyclic shear interferometer in the retrieval of the three-dimensional surface information of an object

We report the evaluation of the topography of an object with projected fringes generated with a lateral cyclic shear interferometer (CSI) and we then compare the topography recovery obtained with the proposed method with the one obtained from a coordinate measuring machine (CMM). We also study how the fringes visibility along the z axis affects the retrieval. Finally, we discuss the advantages and drawbacks of this profilometry system.     

极坐标激光直写系统原点定位的实验研究

分析了旋转线性光栅法对准光轴与旋转机械轴中心的理论准确度,实验研究了极坐标激光直写系统的原点定位.在极坐标激光直写系统下进行了圆环线条写入实验,得出了原点定位的准确度,原点定位准确度优于0.5 μm,实验与理论定位准确度基本一致.     

涂覆石墨烯的椭圆形电介质纳米线光波导的模式特性分析

设计了一种涂覆石墨烯的椭圆形电介质纳米线光波导.采用分离变量法,在椭圆柱坐标系中,借助Mathieu函数,得到了色散方程.通过数值求解色散方程,可以得到模式的有效折射率和场分布,从而得到模式的传播长度.研究了工作波长、结构参数以及石墨烯的费米能对模式特性的影响,并给出了前五个模式的品质因数.计算表明,当波长从4.3μm增加到8.8μm,这5个模式的有效折射率的实部减小,基模和一阶模的传播长度增大,二阶模的传播长度先增大后减小.当改变纳米线结构参数半长轴和半短轴时,对基模和一阶模的模式特性影响较小,对二阶模的模式特性影响较大.当石墨烯的费米能从0.45 eV增加到0.72 eV时,有效折射率的实部减小,传播长度可以达到2μm左右.分离变量法得到的结果与有限元方法得到的结果完全一致.本文工作可以为基于涂覆石墨烯的电介质纳米线的光波导的设计、制作和应用提供理论基础.     

Natural mode analysis of an acoustic cavity with multiple elliptical boundaries by using the collocation multipole method

This paper presents a semi-analytical approach to solve the eigenproblem of an acoustic cavity with multiple elliptical boundaries. To satisfy the Helmholtz equation in the elliptical coordinate system, the multipole expansion for the acoustic pressure is formulated in terms of angular and radial Mathieu functions. The boundary conditions are satisfied by uniformly collocating points on the boundaries. The acoustic pressure at each point is directly calculated in each elliptical coordinate system. In different coordinate systems, the normal derivative of the acoustic pressure is calculated by using the appropriate directional derivative, an alternative to the addition theorem. By truncating the multipole expansion, a finite linear algebraic system is derived. The direct searching approach is employed to determine the natural frequencies by using the singular value decomposition (SVD). Numerical results are widely discussed for several examples including an elliptical cavity, a confocal elliptical annulus cavity and an elliptical cavity with two elliptical cylinders. The accuracy and numerical convergence of the presented method is validated by comparison with available results from the analytical method and the commercial finite-element code ABAQUS. No spurious eigensolutions are found in the proposed formulation. Excellent accuracy and fast rate of convergence are the key features of the present method thanks to its semi-analytical feature.     

椭球体转动惯量的计算

用分步计算和广义球面坐标变换两种方法对椭球体绕对称轴旋转的转动惯量进行了计算，计算表明，后一种方法简单、实用，在工程实际中有应用价值．     

Tuning of plasmonic behaviours in coupled metallic nanotube arrays

We theoretically investigate the influence of the shape of nanoholes on plasmonic behaviours in coupled elliptical metallic nanotube arrays by the finite-difference time-domain (FDTD) method.We study the structure in two cases:one for the array aligned along the minor axis and the other for the array aligned along the major axis.It is found that the optical properties and plasmonic effects can be tuned by the effective surface charges as a result of the variation in the minor axis length.Based on the localized nature of electric field distributions,we also clearly show that the presence of localized plasmon resonant modes originates from multipolar plasmon polaritons and a large magnitude of opposing surface charges build up in the gap between adjacent nanotubes.     

半覆盖锥束CT中扁平物体的高效反投影滤波重建

全覆盖圆轨迹扫描的成像视野受探测器宽度限制, 对于大物体的成像效率较低.半覆盖扫描可以将成像视野扩展近1倍, 图像重建首推使用反投影滤波型算法. 反投影滤波型算法按PI线重建, 各PI线积分区间的不一致性导致通信和计算消耗大, 影响重建效率. 针对半覆盖成像中扁平形状物体的重建问题, 提出了一种改进的反投影滤波型算法, 且证明了当扁平物体的厚度小于2Rsin(2π/N_p) (R为扫描半径, N_p 为圆扫描一周均匀采集的投影数量)时, PI线积分区间的不一致性在数值计算过程中的误差是可以忽略的. 改进后的算法相比原半覆盖反投影滤波算法具有两个明显的优势: 一是数值计算过程中角度循环移至PI线循环之外, 算法的通信需求显著降低; 二是投影数据求导、反投影和沿PI线滤波三个步骤均能够并行计算, 算法的并行性得到增强.数值仿真与实际数据的实验结果表明, 本文算法与原半覆盖反投影滤波算法的重建精度相当, 但计算效率提高了4.6倍. 关键词： 锥束CT 半覆盖成像 反投影滤波 PI线     

Linear method for evaluation of radius of spherical surface from discrete set of data points

Our work deals with the problem of evaluation of the spherical surface parameters from the discrete set of measured data points. We propose a new linear method for determination of the center and the radius of curvature of the measured spherical surface. Such method can be used for evaluation of the measurements obtained from both contact and noncontact 3D coordinate measuring machines. We perform an analysis of the proposed sphere fitting method and we verify that the obtained results are comparable to results calculated using existing techniques. Finally, the proposed algorithm is implemented for evaluation of real measurements of spherical optical surfaces using the designed scanning chromatic confocal system.     

Automatic calibration method of voxel size for cone-beam 3D-CT scanning system

For a cone-beam three-dimensional computed tomography (3D-CT) scanning system, voxel size is an important indicator to guarantee the accuracy of data analysis and feature measurement based on 3D-CT images. Meanwhile, the voxel size changes with the movement of the rotary stage along X-ray direction. In order to realize the automatic calibration of the voxel size, a new and easily-implemented method is proposed. According to this method, several projections of a spherical phantom are captured at different imaging positions and the corresponding voxel size values are calculated by non-linear least-square fitting. Through these interpolation values, a linear equation is obtained that reflects the relationship between the voxel size and the rotary stage translation distance from its nominal zero position. Finally, the linear equation is imported into the calibration module of the 3D-CT scanning system. When the rotary stage is moving along X-ray direction, the accurate value of the voxel size is dynamically exported. The experimental results prove that this method meets the requirements of the actual CT scanning system, and has virtues of easy implementation and high accuracy.     

Study on maximum band gap of two-dimensional photonic crystal with elliptical holes

In this paper, the maximum photonic band gap (PBG) of two-dimensional (2D) photonic crystal (PC) with elliptical air holes was studied by the finite-difference time-domain (FDTD) method based on changing the ratio (semi-major axis length of elliptical hole to the filling ratio) and azimuth angle of elliptical holes, respectively. It is shown that the PBG exhibits a peak value when the ratio of semi-major axis length to the filling ratio is equal to 0.86 approximately by increasing the filling ratio, and central frequency and the low boundary frequency of PBG decrease linearly with the increasing of semi-major axis length. In the aspect of the influence of azimuth angle from 0 to 90°, the PBG presents a minimum value, and central frequency and the low boundary frequency of PBG become high non-linearly by the increasing of azimuth angle to any filling ratio.     

利用Stokes矢量和Muller矩阵确定椭圆偏振光的方位角和长短轴之比

本文利用Stokes矢量和Muller矩阵法分析椭圆偏振光的方位角和长短轴之比的测定，并给出获得椭圆偏振光特定方位角和长短轴之比的方法。     

球面镜的散射理论

本文用三个球坐标面将球面镜包围,同时把空间切割为三个区域,分别求得通解;再利用边界条件得到了系数方程组。用它来计算球面天线的辐射场,利用会聚波和发散波的概念,得到了严格的解析解。 关键词：     

Asymmetrical Abel Inversion of MHD Generator Discharges

The method of Yasutomo et al. for asymmetrical Abel inversion is extended. The original solution consists of a radial part and a weight function, describing the degree of asymmetry. In this work, the radial part is generalized to an elliptical part by employing a coordinate transformation. A factor ? is introduced defining the ellipticity. The extended method is applied to continuum intensity measurements of a discharge in the argon-cesium plasma of a shock-tunnel MHD generator. Two axes of observation have been used, the first one to obtain the radial part and the weight function, the second one, at an angle of 30° with the first axis, to obtain the value of ? needed in the transformation. The reconstructed spatial distribution of radiation intensity is nearly elliptical but with asymmetry in flow direction. The distribution is stretched out along the magetic field lines and fairly well reproduces the emitted intensity profiles.     

A flexible new three-dimensional measurement technique by projected fringe pattern

Aimed at the problems of inferior precision and bad maneuverability for three-dimensional (3D) measurement by projected fringe pattern, a flexible new 3D technique for performing system calibration and measuring was proposed. First, we analyzed the principle of conventional 3D measurement with projected fringe pattern, and pointed out the shortcoming of measurement system. Then, the CCD camera calibration technique is analyzed and we set up the perspective projection model which transforms the computer image coordinate to 3D world coordinate, and we get the coordinate of the CCD camera image lens. Third, the position of projection lens optical center can be obtained using the above model. At last, some experiment results presented show that this technique is more simple and robust in engineering than conventional measurement method.