An optical method and neural network for surface roughness measurement

The measurement of surface roughness using stylus equipment has several disadvantages. A non-contact optical method is needed for measuring the surface roughness of engineering metals with improved accuracy. One candidate for an optical method is the use of a laser source, where the laser light intensity reflected from the surface represents the surface roughness of the illuminated area. A relation can be developed between the reflected laser beam intensity and the surface roughness of the metal. The present study examines the measurement of the surface roughness of the stainless steel samples using a He-Ne laser beam. In the measurement a Gaussian curve parameter of a Gaussian function approximating the peak of the reflected intensity is measured with a fast response photodetector. In order to achieve this, an experimental setup is designed and built. In the experimental apparatus, fiber-optic cables are used to collect the reflected beam from the surface. The output of the fiber-optic system is fed to a back-propagation neural network to classify the resulting surface profile and predict the surface roughness value. The results obtained from the present study are then compared with the stylus measurement results. It is found that the resolution of the surface texture improves considerably in the case of optical method and the neural network developed for this purpose can classify the surface texture according to the control charts developed mathematically.     

Experimental investigation of a flowing fluid layer on metal surface roughness measurement and aberration correction using adaptive optics

The induced optical aberration of laser beam passing through a transparent flowing fluid layer on a metal specimen is experimentally and empirical formula studied. The proposed study presents an experimental investigation of metal surface roughness measurement by combining an optical probe of laser-scattering phenomena and adaptive optics (AO) for aberration correction. In the absence of the AO correction scheme, induced flow velocity of 0.278 m/s can severely degrade the residual wavefront root mean square (RMS) error to 0.58 μm and decrease the scattered laser intensity. Real-time AO correction in closed-loop at a sampling rate of 8Hz can reduce the wavefront RMS error to 0.19 μm and improve the attenuation of scattered laser intensity. The maximum relative error of the estimated roughness (R _a) is less than 7.8% compared with the stylus method. The experimental results show satisfactory correction in the presence of a flowing fluid layer using the AO system.     

Roughness measurement of metals using a modified binary speckle image and adaptive optics

This paper proposes an integrated roughness measurement system that is based on adaptive optics (AO) and binary analysis of speckle pattern images. The aim of this study was to demonstrate the necessity for AO compensation in regions containing both heat and fluid flow turbulences. A speckle image was obtained by projecting a laser beam onto the specimen surface, and the laser pattern image reflected from the surface was binarized to experimentally correlate the intensity with the surface roughness. In the absence of the AO correction scheme, induced turbulences can severely increase the residual rms error from 0.14 to 1.4 μm. After a real-time closed-loop AO correction, we can reduce the wavefront root mean square (rms) error to 0.12 μm, which not only compensates for the aberration error from induced disturbances but also improves the overall performance of the optical system. In addition, an AO system having different gains was investigated, and a threshold gain value was found to be able to steadily compensate for the wavefront errors in less than 2 s. Measurement results of five steel samples having roughness ranging from 0.2 to 3.125 μm (0.3λ and 5λ, where λ is the diode laser wavelength) demonstrate an excellent correlation between the intensity distribution of binary images and average roughness with a correlation coefficient of 0.9982. Furthermore, the proposed AO-assisted system is in good agreement with the stylus method and less than 9.73% error values can be consistently obtained.     

Experimental investigation of a modified Beckmann-Kirchhoff scattering theory for the in-process optical measurement of surface quality

An experimental investigation of a modified Beckmann-Kirchhoff scattering theory applied in an in-process optical measurement of surface quality is described. The proposed theory describes the scattered light intensity distribution from a surface with the additional layers, and can be employed to analyze the surface characteristics in in-process measurement. Based on light scattering principle and machine vision method, the surface roughness is extracted to testify the correction of the modified Beckmann-Kirchhoff scattering theory. The experimental apparatus consists of a collimated laser diode, a beam splitter, a screen, a measuring lens and a camera. Test specimens with different surface roughness are studied. The results obtained demonstrate the feasibility of in-process optical measurement of surface quality using the modified model.     

计算海面激光反射特性的几何光学方法研究

研究了计算海面激光反射的几何光学方法。首先根据JONSWAP非稳态海谱模型数值模拟出二维随机粗糙海面,再采用几何光学方法对入射激光光束在海面上的反射光进行建模,最后计算出海面激光光斑反射光强的空间分布。编制了相关的计算机程序,分析了在不同入射角度和不同风速下的激光光束反射特性。计算结果表明该方法对研究二维随机粗糙海表面激光反射特性有一定参考价值。     

Top-hat cw laser induced thermal mirror: a complete model for material characterization

A complete theoretical model is presented for the thermal mirror technique under top-hat laser excitation. Considering the attenuation of the top-hat excitation laser intensity along the thickness of a sample due to its optical absorption coefficient, we calculate the laser-induced temperature and surface deformation profiles. A simplified theoretical model for a high absorption sample is also developed. The center intensity of a probe beam reflected from the thermal mirror at a detector plane is derived. Numerical simulation shows that the thermal mirror under the top-hat laser excitation is as sensitive as that under Gaussian laser excitation. With top-hat laser excitation, the experimental results of thermo-physical properties of opaque samples are found to be well consistent with literature values, validating the theory.     

地面对激光雷达信号散射的统计研究

从理论角度研究了利用高斯光束照射远场目标时,激光散斑的统计特性.导出了散射光场的自相关函数和光强度的二阶矩的解析表达式,计算了激光散斑的面积.研究表明:在接收面上,光强的分布与目标表面高度的相关长度和均方根高度密切相关,激光散斑面积和散斑光强的相关系数只与激光束腰的尺度有关,与目标的均方根高度和相关长度无关. 关键词： 激光散斑 自相关函数 高斯分布 协方差     

粗糙海面对高斯分布激光光束的反射模型推导

研究激光光束海面反射光强的方向分布特性对海上光电对抗等领域的工程实践具有重要意义.本文采用分形方法模拟粗糙海面,并在海面基准坐标系中建立起描述粗糙海面几何特征的数学方程,然后基于蒙特卡罗方法模拟高斯光束,依据几何光学原理在基准坐标系下推导了高斯光束的海面反射模型,采用该模型可以编程计算激光光束海面反射光强的方向分布.将模拟计算结果与实验结果进行了对比分析,结果表明该模型可以较好地反映激光光束海面反射光强的分布趋势,验证了模型的有效性.     

真实加工表面后向散射特性分析及表面参数反演

无合作目标激光绝对测距中,远距离、真实表面的后向散射特性机理不明朗,严重影响测距结果,是制约测距技术发展的重要瓶颈。以立铣、平铣、平磨等三种典型实际机械加工方式下的粗糙表面为研究对象,测量了在1 550 nm红外激光照射下形成的后向散射场,探究了不同加工方式下特殊后向散射场形成的原因,深入分析了表面纹理、入射方位角、入射角度、粗糙度对后向散射场分布的影响。实验结果分析表明,加工制造表面的后向散射光谱形态分布受加工方式的影响很大,且相互入射几何关系和粗糙度对每种加工方式下的实际粗糙表面均有规律性影响。为了能够获取足够的后向散射能量,对表面参数的识别反演显得十分重要。进一步构建了一种加工表面多维参数反演模型,采用另外一种加工方式(刨床)的样块数据进行验证,加工方式能够被准确区分,入射方位角和粗糙度反演的相对误差分别达到1.21%和1.03%,反演精度较高。经实验验证,通过表面参数的反演极大拓宽了无合作目标激光绝对测距的范围,有效降低了表面纹理、入射方位角、粗糙度等对测距范围的影响。这一研究结果还对具有纹理特征加工表面的后向散射光谱的研究和在其他领域的应用均具有一定的参考价值。     

Fatigue damage detection using a speckle-contrast technique

An optical setup based on speckle-contrast measurement is proposed as an alternative to analyze fatigue damage on surfaces of metal samples. Based on the dispersion of the light, generated by a laser beam reflected by the surface, roughness and fatigue damage are analyzed after a specially designed fatigue test of metallic samples. The experimental results are obtained from an aluminum alloy type A319 modified with strontium with nodular silicon precipitates that was subjected to a fatigue procedure based in the norm ASTM E647.     

Three Gaussian beam heterodyne interferometer for surface profiling

A novel three Gaussian beam interferometric technique for profiling optical smooth surfaces is presented. The technique is based on the heterodinization of three Gaussian beams, two of them with the same temporal frequency. The first beam is used as a probe beam after being focused and reflected from the surface under test. The second beam is reflected from a reference surface. The third beam is obtained from the first diffraction order of a Bragg cell and thus, it is shifted in its temporal frequency. The three beams are coherently added at the sensitive plane of a photodetector that integrates the overall intensity of the beams. We show analytically that the electrical signal at the output of the photodetector consists of a temporal carrier whose amplitude is a sinusoidal function of the local topography. We include the measurement of the topography of a sample consisting in a blazed-reflecting grating calibrated by means of an atomic force microscope.     

Interference wedge properties relevant to laser applications: transmission and reflection of restricted light beams

The optical properties of the interference wedge are analysed for the case of restricted laser beam illumination. By further development of Brossel's method, equations describing this case are derived and used to calculate the reflected and transmitted beam profiles for typical intracavity laser beam diameters (0.2–0.6 mm) for Gaussian and uniform intensity distributions. In particular, the experimentally observed formation of bright fringes outside the beam impact area at resonant wavelengths is explained. some useful dependencies concerning the wedge reflection and transmission for laser applications are obtained and experimentally demonstrated.     

光学针描法的理论与实验研究

在光纤探针描描使表面轮廓对光产生调的基础上提出了光学针找法的粗糙度测量技术。对光学针描法的理论基础，如表面轮廓原理和光耦合区理论进行限较为深入的分析。对单模光纤探针式轮廓仪的实验系统进行了介绍，最后给出了实验结果和误差分析。     

利用4400系统构造具有光学多道分析功能的信号处理系统

基于4400系统,通过引入光电二极管列阵作为传感器,构造了具有光学多道分析功能的信号处理系统。利用该系统对实际的激光空间分布进行了描绘,给出了高斯场分布的相应参数。通过进一步观测p偏振光在K9玻璃上下表面反射光强比(γ)的角度调制曲线,获得了玻璃表面层的光学参数。实验结果表明,该系统测量精度高,可应用在激光时间信号及空间场分布的表征与测量领域。     

A novel optical diagnostic technique for analyzing the recrystallization characteristics of polycrystalline silicon thin films following frontside and backside excimer laser irradiation

Excimer laser annealing (ELA) is a widely used technique for producing polycrystalline silicon (poly-Si) thin films. An optical inspection system with simple optical arrangements for rapid measurement of recrystallization results of poly-Si thin films is developed in this study. The recrystallization results after both frontside ELA and backside ELA can be easily visible from the profile of peak power density distribution using the optical inspection system developed with an optimized moving velocity of 0.312 mm/s of the specimen. The method of backside ELA is suggested for batch production of low-temperature polycrystalline silicon thin-film transistors due to higher laser beam utilization efficiency and lower surface roughness of poly-Si films.     

Direct near-field phase measurement of laser diodes employing a single-mode fiber interferometer

A method of direct measurement of near-field phase and intensity distribution of laser diodes employing a single-mode fiber interferometer is proposed and demonstrated. The phase and intensity of the output beam of the laser in the vicinity of the output facet are measured directly via interferometry. Using a 980 nm laser diode as an example, we obtained a beam width of 0.9 and 3.6 μm at the output facet in the vertical and horizontal axes, respectively. In addition, the phase information of the output beam was also obtained by using interferometry. This technique is particularly useful for laser diodes whose near-field phases are difficult to measure directly. The measured vertical and horizontal wavefront radius of curvatures of a laser diode are in good agreement with the calculation from Gaussian beam theory. Detailed understanding and measurement of the near-field phase and intensity distributions of light sources and optical components are essential for micro-optic designs with better mode matching to minimize the insertion loss.     

Talbot effect in metallic gratings under Gaussian illumination

Metallic gratings can be found in applications such as optical metrology. Due to their fabrication process, the surface presents a certain roughness. In this work, the effect of roughness on Talbot effect has been analyzed when the grating is illuminated with a Gaussian beam. A model based on Fresnel regime is used in order to determine the intensity distribution in the near field. Contrast of the self-images is obtained and it is found that it decreases in terms of the distance between the grating and the observation plane. When the autocorrelation function of roughness presents a Gaussian behaviour, the diffracted beams are still Gaussian although some of their properties change. For example, the width of the diffracted beams increases with respect to the case of the standard chrome on glass gratings. On the other hand, the power of each diffracted beam is independent on the roughness properties of the surface.     

The improvement of the measuring level of the surface curvature based on the spot-radius of a collimated Gaussian laser beam

In this study, we propose a system for determining the curvature of a material surface by a collimated Gaussian laser beam (CGLB) as the test light source. We will demonstrate that the measuring level can be improved and the experimental procedure can be reduced. The spot-radius of the test beam is nearly a constant and it is independent of the swing of the spot intensity. The proposed system can decrease the optical path-length and has the higher sensitivity and accuracy than the test system by using a Gaussian laser beam (GLB).     

基于激光散斑成像的零件表面粗糙度建模

表面粗糙度是衡量机械表面加工水平的重要参数. 通过构建一套激光散斑成像采集系统, 获取了不同表面加工类型和不同粗糙度值的零件表面激光散斑图像. 应用Tamura纹理特征理论提取图像的纹理粗糙度、对比度、方向度特征, 并分析了这三个特征与表面粗糙度的关系. 发现了纹理粗糙度特征与表面粗糙度的单调关系, 推导出平磨、外磨、研磨三种表面加工工艺的粗糙度值与图像纹理粗糙度特征的数学函数关系, 实现了表面粗糙度的测量. 同时, 利用Tamura纹理特征与加工工艺的依赖关系, 建立了基于贝叶斯网络的工艺识别推理模型, 推理出了零件表面加工工艺. 通过为多种加工类型表面建立粗糙度测量模型, 为粗糙度测量提供了新思路. 实验证明所提的粗糙度测量模型能以较高的准确率识别出零件表面加工类型并测量出其表面粗糙度值.     

Experimental and theoretical investigation of loss of coherence on scattering of a beam with helical wavefront

Theoretical and experimental investigations of the second order intensity coherence for a beam with helical wavefront i.e. an optical vortex scattered through a rotating ground glass have been reported. The results have been compared with scattering of the Gaussian beam of a He-Ne laser. It shows that the second order intensity coherence or intensity correlation curve for the vortex decreases much faster than that of the Gaussian beam of the He-Ne laser. We attribute this behaviour to the helicity of the wavefront.