有限元法分析空心后向反射器面形精度

为了探索空心后向反射器在工作状态下光学特性的变化 ,通过运用有限元法和在DEC/ 5 0 0 0工作站上采用MSC/NASTRAN程序 ,研究了用三种不同性能材料制成的空心后向反射器在不同几何尺寸和不同支承点位置时 ,载荷与温度梯度对空心后向反射器面形精度的影响。分别得出了反射器工作面变形与载荷大小、材料性能、支承点位置、温度梯度的关系曲线。这些曲线表明 :在确定条件下的反射器有一最佳的支承点位置 ;材料性能中的弹性模量对面形精度的影响最为显著 ;面形精度与载荷大小、温度梯度基本上成线性关系 ,但温度梯度是影响面形精度诸因素中的主要因素     

Measurement of surface figure of plane optical surfaces using Fizeau interferometer with wedge phase-shifter

A Fizeau interferometer-based set-up for measurement of surface figure of plane optical surfaces has been discussed. Phase-shifting interferometry has been applied using wedge phase shifter. Technique for elimination of system aberrations has been described. Results are shown for a plane surface.     

光学自由曲面数控化加工方法

在光学系统中，为了满足特殊的成像要求，需要使用自由曲面光学零件，但由于自由曲面是一类非常复杂的曲面，用传统的光学加工方法很难加工。本文概要介绍了光学非球面的多种加工方法，探索了这些方法加工光学自由曲面的可能性，提出了用数控化加工方法加工光学自由曲面，并利用这种方法加工了一块自由曲面透镜。     

Stability of Zernike coefficients solved by the covariance matrix method in the analysis of the wave front aberration

The deduction of Zernike coefficients is usually influenced by the finite number of sampling dots on interferogram and their inherited measurement errors. In this paper, a simplified Gram-Schmidt method for solving the Zernike polynomial with the higher fitting precision is presented and used to analyze the wave front aberrations for the circle interference fringe of the fine polished aluminum disk surface captured by a Twyman-Green interferometer system. We find the stability of the Zernike coefficients changes with changing the Zernike term, which has lead to the wrong expression for the wave front aberration. By analyzing the condition number of the coefficients matrix and the fitting precision of the method, it is indicated that the instability can be avoided when the Zernike term is lower than 14. Such an analysis will be valuable in solving the Zernike polynomial for the wave front aberration analysis in optical testing.     

Curvature measurement of optical surface using digital holography

In this paper, an optical surface curvature measurement technique based on digital holography is proposed. Unlike the previous digital holographic methods, which only involve a small part of the measured data for parameter estimation, the proposed method adopts more of them using least square method. Hence, the analyzed result is not affected by data extraction direction and less sensitive to imperfect phase compensation. The proposed method is demonstrated with a sample of reflection mirror in laser resonator, and verified by comparing the measurement result with that of the white light interferometer.     

Measurement of 3-D displacement of cambered mirror of cryogenic optical system using white light speckle method

We report a method measuring 3-D displacement of cambered mirror for cryogenic optical system using white light speckle photography with double light sources and double cameras. Data acquiring and processing based on a digital image processing system is described. The 3-D displacement and displacement vector distributions in two low-temperature ranges are measured.     

Reducing phase retrieval errors in Fourier analysis of 2-dimensional digital model interferograms

In order to measure the displacements of facets on a growing spherical Cu_2−δSe crystal with sub-nanometre resolution, we investigated the reliability and accuracy of standard method of Fourier analysis of digital laser interferograms. Guided by realistic experimental conditions (two-dimensional (2D) interferograms), starting from 2D model interferograms and using original custom designed Gaussian filtering window and multistage unwrapping procedure of the retrieved phase, we demonstrate for a considerable parameter range the non-negligible inherent phase retrieval error due to non-integer number of fringes within the digital image. Our results indicate an intermediate parameter range where the error is acceptably small. We introduce an algorithm modification that significantly reduces the error, especially for low and high fringe densities. In the experimentally most common case of diagonal fringes the reduced error is an order of magnitude smaller than for nearly one-dimensional case within almost entire parameter space.     

Measurement of surface figure of plane optical surfaces with polarization phase-shifting Fizeau interferometer

A Fizeau interferometer based set up for measurement of surface forms of plane optical surfaces has been discussed. Phase shifting interferometry has been applied using polarization phase shifter. A linearly polarized (632.8 nm) He–Ne laser has been used as the source. Light reflected from the object and the reference/master surfaces are made circularly polarized in opposite senses by means of two properly oriented quarter wave retardation plates placed at appropriate positions, one inside and other outside the interference cavity of the interferometer, and phase shifts are introduced between the object and the reference/master waves by varying angular orientation of a polarizer/analyzer. Final result is made free from any residual wave-front aberrations introduced by the (intra-cavity) wave plate by subtracting phase values obtained by PSI technique between a high optical quality master surface and the reference surface from that obtained for the test object surface with respect to the same reference surface for each point of the interference field. Results are shown for a plane surface.Advantages of the technique presented are linearity and high accuracy in phase stepping, no perturbation of the interference cavity during the phase shifting and possibility of real time or dynamic interferometry.     

基于条纹反射的光学表面疵病检测法

提出一种基于条纹反射的光学表面疵病检测方法,检测系统由液晶显示屏、CCD相机和计算机组成.检测时在显示屏上分别显示水平和垂直两个方向正交的正弦性条纹,用CCD相机记录经被测表面反射的正弦条纹像,通过相移技术得到两个方向正交的相位分布和对比度分布,通过对比度分布确定被测表面的疵病位置.被测面表面面形相关信息包含在相位分布中,其表面疵病细节信息包含在局部微观相位分布之中.对相位分布进行多项式拟合后相减得到疵病引起的微观相位分布,计算得到表面疵病的梯度分布,积分得其高度分布.实验验证了该方法的可行性.     

Intensity normalization of additive and multiplicative spatially multiplexed patterns with n encoded phases

An efficient, robust and user-free intensity normalization method for patterns with n frequency-multiplexed phases of both additive and multiplicative types is proposed. For this, the parameter estimation approach by using the least-squares method is applied. The theoretical principles are given and the good performance of the proposal is verified by computer simulation and experimental results. Because the proposed method has good features such as robustness, simplicity, fast and user-free execution, it could be implemented in a wide variety of applications. This contribution motivates future generalization in both phase demodulation algorithms and experimental setups to exploit the benefits of additive/multiplicative patterns with multiple phases.     

Effect of surface lay in the surface roughness evaluation using machine vision

This work explores the influence of orientation of surface lay pattern of the machined components, while quantifying the surface roughness using machine vision approach. The surface images are captured from milled low carbon steel specimens with different roughness values using a vision system with coaxial lighting arrangement at different angular orientations of the work pieces (0°, 30°, 45°, 60°, 90°, 120°, 135°, 150°, and 180°). The captured images are subjected to preprocessing in order to retain the frequency components that attribute to roughness using a Gaussian filter by adapting the filtering procedures specified in ISO 4288. Numerous image based parameters such as gray level average (G_a), gray level co-occurrence matrix based image quantification parameters namely contrast, correlation, energy or uniformity, maximum probability and differential box courting based fractal dimension are computed from the surface images captured at different angular positions of the work piece. The computed vision based parameters are compared and correlated with the roughness average (R_a) obtained using a stylus instrument and the results are analyzed. The results clearly indicated that it is important to consider the orientation of the work piece when the machine vision approach is used to quantify the surface texture parameters.     

Effect of surface roughness on the optical constants of bulk polycrystalline gold samples

Four bulk polycrystalline samples of gold were subjected to different polishing treatments using diamond pastes of grain size 10, 6, 3 and 1 μm. The effect of surface roughness on the optical constants n and k is studied by 45° angle-of-incidence ellipsometry at 632.8 nm. Results for n and k are extrapolated to the case of an ideal surface which we believe to be highly representative of gold. Comparison with published results for the optical constants of gold thin films is presented.     

Effect of surface roughness on the determination of tissue optical properties

The scattering phase functions of the nylon film with different surface roughnesses were measured by the goniophotometric measurement technique, and the corresponding anisotropy factors of the nylon film with different surface roughnesses were determined subsequently. Consequently, the scattering coefficients of nylon with different surface roughnesses were determined. The fluence rates of intralipid with different interface roughnesses for a board beam irradiation were measured and the penetration depths were determined. The experimental results indicate that the surface roughness obviously affects the determination of the optical properties of nylon. This study suggested that the determination of tissue optical properties should take surface roughness into account.     

Sensitivity enhancement of surface plasmon resonance sensor using continuous film metallic gratings

A surface plasmon resonance (SPR) sensor based on continuous film metallic gratings is numerically investigated for enhance sensitivity. The results calculated by rigorous coupled-wave analysis (RCWA) present that interplays between localized surface plasmons and surface plasmons polaritons contribute to sensitivity enhancement. The sensitivity enhancement factor (SEF), which represents the influence of metallic grating, increased as the grating period decreased. In addition, several reflection dips can be achieved as the period of metallic grating increased. By double-dips method, the sensitivity SPR sensor based on continuous film grating-based is improved into 153.23°/RIU, which is more sensitive than conventional thin film-based SPR sensor in the same condition. The SPR sensor based on continuous film metallic gratings exhibits good linearity.     

基于光学系统像面分割的图像恢复方法研究

实际光学系统的点扩散函数随视场的不同而变化,因此基于线性不变成像系统的图像恢复方法在处理由大视场系统得到的图像时,恢复效果不理想。若把成像系统作为线性空变系统来进行处理,根据等晕成像原理,如将降质图像分割为不同区域进行复原的话,则可以有效地提高图像的恢复效果。针对区域间边界不连续性的问题,提出了区域延拓的解决方法,通过数字仿真,验证了图像分割处理与区域延拓方法的可行性。     

Evaluation of the 1D empirical mode decomposition method to smooth digital speckle pattern interferometry fringes

The 1D empirical mode decomposition method is applied to reduce speckle noise in the correlation fringes produced in digital speckle pattern interferometry. This method is based on the decomposition of a signal in a sum of well-behaved fast and slow oscillation modes through a sifting process, which generates a fully data-driven technique. Consequently, this is an adaptive approach and the use of basis functions in the analysis process is not required. The denoised signal is given by the residue obtained after the fast oscillation modes are removed. The performance and limitations of the denoising technique are analyzed using computer simulated fringes and these results are compared with those obtained using a wavelet sub-band removal approach. An application of the EMD method to denoise experimental correlation fringes is also presented.     

Investigation of the performance of an SPR-based optical fiber sensor using finite-difference time domain

Finite-difference time domain (FDTD) method was used to investigate the performance of surface plasmon resonance (SPR)-based optical fiber sensors. The results show that the performance of the fiber sensor can be optimized by choosing a proper combination of metal layer thickness of 40–60 nm and residual cladding thickness of 400–500 nm. Furthermore, the roughness effect of the gold surface layering the fiber sensor is significant in rough surfaces when sigma rms is greater than 5 nm or correlation length is lower than 100 nm.     

Parameter estimation of optical fringes with quadratic phase using the fractional Fourier transform

Optical fringes with a quadratic phase are often encountered in optical metrology. Parameter estimation of such fringes plays an important role in interferometric measurements. A novel method is proposed for accurate and direct parameter estimation using the fractional Fourier transform (FRFT), even in the presence of noise and obstacles. We take Newton׳s rings fringe patterns and electronic speckle pattern interferometry (ESPI) interferograms as classic examples of optical fringes that have a quadratic phase and present simulation and experimental results demonstrating the performance of the proposed method.     

基于数字全息干涉术的液相扩散过程可视化

提出了利用数字全息干涉术可视化观测液相扩散过程.实验中,采用马赫-曾德干涉仪光路记录乙醇与水之间两相扩散过程的多幅数字全息图;再通过数值再现不同扩散状态的波前相位分布,获得液体中的摩尔浓度分布;最后,根据菲克定律获得两相流扩散系数.结果表明:利用数字全息干涉术可实现对液相扩散传质过程的快速、实时及高精度测量;该方法还具有可实现远程监控、拥有大量微观数据的优点;此外,采用文中全息干涉光路结合波分与角分复用技术可实现多相流扩散系数的测量,为获得溶液中非线性变化特征参量提供了有效技术手段.     

A simple method of contrast enhancement and extremum extraction for interference fringes

A method of interference fringe processing by finding the average and the extremum gray levels of image points within a fixed or varying window and then computing the modified image intensity with a simple algorithm is proposed. This method can yield a uniform brightness and high (1 or near to 1) contrast at the same time over the whole image area for an input pattern with low contrast and considerable luminance variation, when a relatively large window is used. It can also perform extremum extraction if a small window is employed. This technique is simple, effective and convenient for use, as has been verified by a series of examples of simulated and experimental fringe processing.