High sensitivity micro-displacement homodyne interferometry北大核心CSCD

Interferometry is widely used in nano-scale micro-topography measurement. In order to improve its accuracy and sensitivity, a high-sensitivity homodyne interferometry based on white light interference and laser secondary interference was proposed. A high-sensitivity homodyne interferometry system was designed, and the zero point of the laser secondary interference was used to locate the dark striation of white light interference, so that it could reach the maximum slope when optical path difference was zero. The signals of white light and laser were analyzed by using the wave principle and intensity formula of interference fringes, and a sensitivity calculation method based on the combination of white light and laser interference signal was proposed. The system and its sensitivity were simulated. Finally, the optical path was built, and the white light interference fringes were adjusted to the dark striations position, so as to locate the zero position of laser secondary interference and carry out the data acquisition. It is showed that the sensitivity of the measurement method is at least 1 832 times higher than that of the laser secondary interference, and the corresponding measurement uncertainty is only ±0.288 7 mV. The measurement system can effectively solve the problem of large amount of calculation in traditional interferometry, and has high sensitivity, stability and reliability. Copyright ©2022 Journal of Applied Optics. All rights reserved.     

Microstructure and properties of plastic deformed martensite induced by laser shock processing

Firstly, 45# steel was quenched by the NEL-2500A rapidly axial flow CO2 laser. The experimental parameters were the laser power of 750 W, the laser beam diameter of 4 mm, the scanning velocity of 7 mm/s. The thickness of coating layer was 0.1 mm and the width was 8 mm. Secondly, the martensite induced by laser quench was shocked by Nd:YAG laser. The parameters of laser shock processing were the wavelength of 1.06 μm, the pulse duration of 23 ns, and the output energy of 16-20 J. The laser was focused on a spot of φ7 mm. K9 optical glass was used as confinement. The sample was coated with black paint 86-1 (the thickness is about 0.025 mm). By testing and analysis of samples which were treated by laser quench and laser quench+shock with transmission electron microscope (TEM), it was discovered that the surface layer of martensite was deformed plastically by laser shock processing. In the secondary hardened zones, there were a lot of slender secondary twin crystal martensites, dislocation tangles, and cel     

Removing paint from a metal substrate using a flattened top laser

In this paper,we investigate laser cleaning using a flattened top laser to remove paint coating from a metal substrate.Under the irradiation of a flattened top laser,the coating paint of the metal substrate can be removed efficiently by laser induced ablation,stress,and displacement force.The temperature distribution,stress,and displacement are calculated in the coating layer and substrate using finite element analysis.The effects of a Gaussian laser and a flattened top laser and the results of different diameters of laser spot are compared.The investigation shows that the flattened top laser can reduce the substrate damage and enhance the cleaning efficiency.This method meets the need of large area industrial cleaning applications by optimizing the flattened top laser parameters.     

Effects of annealing time on the structure,morphology, and stress of gold–chromium bilayer film

In this work, a 200-nm-thick gold film with a 10-nm-thick chromium layer used as an adhesive layer is fabricated on fused silica by the electron beam evaporation method. The effects of annealing time at 300℃ on the structure, morphology and stress of the film are studied. We find that chromium could diffuse to the surface of the film by formatting a solid solution with gold during annealing. Meanwhile, chromium is oxidized on the surface and diffused downward along the grain grooves in the gold film. The various operant mechanisms that change the residual stresses of gold films for different annealing times are discussed.     

Close-range optical measurement of aircraft's 3D attitude and accuracy evaluation

A new screen-spot imaging method based on optical measurement is proposed,which is applicable to the close-range measurement of aircraft's three-dimensional (3D) attitude parameters.Laser tracker is used to finish the global calibrations of the high-speed cameras and the fixed screens on test site,as well as to establish media-coordinate-frames among various coordinate systems.The laser cooperation object mounted on the aircraft surface projects laser beams on the screens and the high-speed cameras syn- chronously record the light-spots' position changing with aircraft attitude.The recorded image sequences are used to compute the aircraft attitude parameters.Based on the matrix analysis,the error sources of the measurement accuracy are analyzed,and the maximum relative error of this mathematical model is estimated.The experimental result shows that this method effectively makes the change of aircraft position distinguishable,and the error of this method is no more than 3′while the rotation angles of three axes are within a certain range.     

Light Irradiation through Small Particles and Its Applications for Surface Nanostructuring in Near Field

We investigate the light scattering through small particles and its applications in nanostructuring, such as nanobumping, nanopatterning and dry laser cleaning. The theoretical calculation based on Mie theory provides an exact solution for sphere cavity resonance and plasmon resonance, which are two mechanisms for dielectric and metallic particles assisted surface nanostructuring in near field. The experimental results indicate that nanobumps on glass surface and subwavelength holes array on silicon surface can be formed without cracks with the self-assembly of 1 μm silica particle mask under laser irradiation. It is also found that the scattering wave by 40 nm gold particles can propagate 200 times away in terms of particle radius as recorded by photoresist under the UV light irradiation. Meanwhile, dry laser cleaning of 40nm gold particle on silicon wafer is demonstrated at plasmonic resonance frequency. The total cleaning efficiency is estimated to be 80%.     

A variational principle analysis of surface acoustic waves propagating under periodic metal grating with finite thickness

A theoretical method is presented,which analyzes properties of surface acoustic waves propagating on metallic gratings with finite thickness by combining finite element method with variational principle on surface acoustic waves propagating on periodic metal gratings. Based on D.P.Chen and Haus theory,a finite element method is used to investigate the effects of metallic gratings upon the propagation of surface acoustic waves.The coupling-of-modes parameters contributed by mechanical loading are expressed by the matrix derived from the finite element method.Consequently D.P.Chen and Haus theory can also be applied to analyze the properties of surface acoustic waves propagating on metallic gratings with finite thickness and arbitrary shape.Finally,the characteristics of surface acoustic waves propagating under gold and aluminum or silver gratings on a few piezoelectric crystals are studied.Numerical results of the coupling-of-modes parameters of the surface acoustic waves are obtained.     

Mixed noise removal algorithm of EBAPS image under low illumination condition based on FPGA北大核心CSCD

In order to solve the problem that single median filtering and gaussian filtering algorithm is not effective in suppressing impulse noise and poisson noise simultaneously in low illumination image, and the edge detail protection is insufficient, an open and close mix-median-gaussian (OCMMG) filtering algorithm based on field programmable gate array (FPGA) was proposed. Firstly, the minimum four-direction difference was used to detect the anomaly degree of each pixel point, the weight was allocated according to the threshold of pulse noise discrimination, and the first step was filtering. Then, the four-direction edge detection algorithm was used to extract image edges, and the second step was filtered according to the set edge confidence characterization value. Finally, the images collected by electron bombarded active pixel sensor (EBAPS) under the condition of 1×10−3 lx illumination were processed by FPGA in real time. The experimental results show that the FPGA processing results are consistent with the software simulation processing results. Compared with the median filtering and gaussian filtering algorithm, the peak signal-to-noise ratio (PSNR) of the algorithm is improved by 3.23% and 16.34%, the structural similarity is improved by 14.66% and 33.86%, and the edge retention index is improved by 0.49% and 4.21%, respectively, which can effectively remove the mixed noise of EBAPS image and meet the real-time requirements. © 2022 Editorial office of Journal of Applied Optics. All rights reserved.     

Development of afterglow time test system for nanosecond fluorescent screen of low-level-light image intensifier北大核心CSCD

The time characteristics of fluorescent screen is one of the important parameters to evaluate the performance of image intensifier. At present, there is no measurement method for the afterglow time of nanosecond fluorescent screen of low-level-light image intensifier. Based on the traditional test scheme of image intensifier afterglow time, a afterglow time test system for nanosecond fluorescent screen was developed. This system used a high-speed signal generator with the sampling rate of 250 MHz to complete the excitation of the laser diode light pulse, and a photomultiplier tube was used with the descending time of 0.57 ns to complete the photoelectric conversion of the fluorescent screen light signal. The weak photocurrent signal of μA magnitude was amplified and converted to a single-terminal differential circuit to complete the AD conversion in AD9684. Then the digital luminance information of the fluorescent screen was stored in the double data rate SDRAM (DDR) unit after field programmable gate array (FPGA), and the host computer sent instructions to read the DDR memory. The USB3.0 high-speed transmission protocol was used to transmit data to the host computer. In the data processing, the Kalman filtering and fast finding falling edge algorithm were used to realize the accurate measurement of noise filtering from collected data and afterglow time. The test results show that the proposed afterglow time test system for nanosecond fluorescent screen can effectively test the image intensifier with ultrafast optical characteristics. The afterglow test results of P47 phosphor reaches 118.094 4 ns, and the repeatability reaches 2.08%. © 2022 Editorial office of Journal of Applied Optics. All rights reserved.     

Research of photolithography technology based on surface plasmon

This paper demonstrates a new process of the photolithography technology,used to fabricate simply fine patterns,by employing surface plasmon character.The sub-wavelength periodic silica structures with uniform silver film are used as the exposure mask.According to the traditional semiconductor process,the grating structures are fabricated at exposing wavelength of 436 nm.At the same time,it provides additional and quantitative support of this technique based on the finite-difference time-domain method.The results of the research show that surface plasmon characteristics of metals can be used to increase the optical field energy distribution differences through the silica structures with silver film,which directly impact on the exposure of following photosensitive layer in different regions.     

基于LIBS光谱与成分分析的飞机蒙皮激光除漆可控性研究

激光除漆作为激光清洗技术的分支,有望替代传统打磨及化学除漆工艺,实现飞机蒙皮表面漆层的可控清除,但除漆过程及质量的可控性依赖于有效的原位、在线监测技术。针对飞机铝合金蒙皮表面多漆层结构,采用LIBS技术对不同漆层、不同厚度时漆层特征元素进行光谱与成分分析,在信号解译基础上建立漆层去除层数、去除厚度与LIBS光谱变化的内在关联,实现除漆过程质量的实时监测与反馈控制。结果表明,分层除漆过程中面漆、底漆完全清除后,漆层特征元素（Fe,Ti）的光谱峰消失。LIBS监测到面漆的特征元素Fe在501.494 1和521.517 9 nm处Fe Ⅰ 的光谱特征峰消失时,判定面漆完全清除。监测到底漆特征元素Ti在498.173 0,499.107 0和521.039 0 nm处Ti Ⅰ 的光谱特征峰消失时,判定底漆完全清除。厚度除漆时,随漆层厚度降低或激光脉冲作用次数增加,漆层特征元素（Ca）的光谱峰强相应降低,至漆层厚度为0时（完全去除）,漆层特征元素光谱峰消失,同时基体特征元素（Al）光谱峰出现。LIBS监测616.217 0,643.907 0和422.673 0 nm处Ca Ⅰ 的光谱信号强度变化能够监测激光除漆时剩余漆层厚度,实现对激光除漆厚度的可控清除监测。另外,结合EDS与SEM测试分析,验证了LIBS用于飞机蒙皮激光除漆过程与效果监测、分层与厚度控制的可行性,表明在不损伤基体氧化层的前提下,通过监测对应波长位置的面漆、底漆特征元素光谱与成分变化规律能够实现激光分层可控、厚度可控除漆。     

用激光清洗金膜表面硅油污染物

采用CO2激光对镀金K9玻璃表面的二甲基硅油污染物进行清洗,在激光器单点作用模式下,分别研究了激光功率和作用时间对清洗效果的影响;并研究了连续扫描工作模式下的激光清洗效果。采用光学显微镜和傅里叶变换红外光谱仪表征激光清洗效果,研究结果表明:通过良好的控制激光参数,采用CO2激光清洗二甲基硅油具有明显的效果;此种非接触式清洗方式可确保K9玻璃表面的金膜完好无损。采用有限元分析软件模拟计算了激光功率和作用时间对清洗过程中温度的影响,计算结果与实验结果规律一致。     

高功率脉冲TEA CO2激光除漆的研究

选用高功率脉冲TEA CO2激光器,对不同颜色不同种类的油漆进行了清洗实验,运用数码照片分析程序计算出清洁率,找出了完全清洗阈值和损伤阈值.实验结果较为清晰地给出了激光输出能量和重复频率对清洗效果的影响.对红色醇酸漆来说,完全清洗阈值为10.37J/cm2,而损伤阈值为11.43 J/cm2;红色金属喷漆的完全清洗阈值为9.66 J/cm2,其损伤阈值达10.37 J/cm2;黄色金属喷漆的完全清洗阈值为10.71 J/cm2,损伤阈值则为11.07 J/cm2.输出能量和重复频率未达指定参数时,激光清洗清洁率则低于100%.     

Microstructure and properties of plastic deformed martensite induced by laser shock processing

Firstly, 45# steel was quenched by the NEL-2500A rapidly axial flow CO2 laser. The experimental parameters were the laser power of 750 W, the laser beam diameter of 4 mm, the scanning velocity of 7 mm/s.The thickness of coating layer was 0.1 mm and the width was 8 mm. Secondly, the martensite induced by laser quench was shocked by Nd:YAG laser. The parameters of laser shock processing were the wavelength of 1.06 μm, the pulse duration of 23 ns, and the output energy of 16-20 J. The laser was focused on a spot of φ7 mm. K9 optical glass was used as confinement. The sample was coated with black paint 86-1 (the thickness is about 0.025 mm). By testing and analysis of samples which were treated by laser quench and laser quench+shock with transmission electron microscope (TEM), it was discovered that the surface layer of martensite was deformed plastically by laser shock processing. In the secondary hardened zones,there were a lot of slender secondary twin crystal martensites, dislocation tangles, and cellular dislocations.Compared with that of the hardened zones through laser quench only, the residual stress and mechanical properties of the secondary hardened zones were improved and increased through laser compound method.     

Study of the laser cleaning on plaster sculptures. The effect of laser irradiation on the surfaces

The focus of this paper is to study the effects caused by the laser irradiation on nineteenth and twentieth century plaster sculptures. Before applying the laser cleaning on the sculptures, it was tested on samples prepared in laboratory according to the results of the scientific investigation carried out on the selected works of art. The characterization of the surface finishing materials of the sculptures was performed by Fourier Transform Infrared spectrometry (FTIR), X-ray Fluorescence spectroscopy (XRF), UV fluorescence photography, and internal micro stratigraphic analysis. Regarding the finishing materials, shellac, zinc white, siccative oil and proteins were found on the surfaces. The results of the scientific investigation, together with the examination of the ancient technical manuals, were used to create the laboratory samples to carry out the irradiation tests with laser. The laser irradiation and cleaning tests were carried out with a Q-switched Nd:YAG system. The irradiated surfaces were analyzed before and after the laser tests with the aid of a video microscope and a reflectance spectrophotometer, in order to evaluate the color changes of the surfaces. The possible morphological modifications caused by laser irradiation were also investigated by Scanning Electron Microscopy (SEM) together with ancillary Energy Dispersive Spectroscopy (EDS) elemental analysis. Concerning the laser cleaning test on the samples, in general little color changes were observed both with the 532 and 1064 nm wavelength. Total color changes, expressed as ΔE*, are always small apart from the samples made of shellac and zinc white in linseed oil, as finishing layer. As regards these samples the surface irradiated with the laser greyed lightly, corresponding to a decrease of L* parameter (lightness). SEM imaging of the treated and not-treated samples, both at low and high magnification, does not show evidence of significant morphological differences due to the laser beam irradiation. Starting from these results, some laser cleaning tests were carried out on the original sculptures, putting them in comparison with an aqueous cleaning method. It was found that the laser cleaning removes effectively the dirt layer preserving the original finishing on the sculpures’ surface, whereas the aqueous cleaning system removes also the finishing.     

The potential use of plume imaging for real-time monitoring of laser ablation cleaning of stonework

In this work, the potential of plume imaging was investigated as a real-time monitoring technique for the control of the laser cleaning process in cultural heritage objects with emphasis on encrusted stonework. Optical pulses generated by Nd:YAG laser were used to remove a high absorbing layer of crust from the surface of a lower absorbing marble substrate. Two regimes of operation have been employed for laser cleaning test, Q-switched ns mode, and short free running μs mode. Laser induced breakdown spectroscopy (LIBS) was also employed to study the stratigraphy of the cleaned surfaces, and thus to confirm the validity of the suggested monitoring technique. The tests proved the prospective of introducing plume imaging techniques as a straightforward and inexpensive tool for the real-time monitoring of laser ablation and the determination of the critical point for a safe surface cleaning intervention.     

约束靶面黑漆涂层对激光冲击波的影响

 介绍了约束靶表面的黑漆涂层对激光冲击波的影响。激光功率密度 10⁹W/cm²量级，脉宽33ns，波长1.06μm。靶材选用两种不同厚度的铜和铝，靶表面用有机玻璃约束。通过对冲击波压力的直接测量发现，涂层不仅可以增加激光冲击波压力，而且还影响冲击波的演化过程。比较有无涂层的靶面SEM照片发现，黑漆涂层能有效地保护激光辐照表面，使之不受激光烧蚀。     

Forensic examination of multilayer white paint by lateral scanning Raman spectroscopy

Multilayer samples of white architectural paint potentially have very high evidential value in forensic casework, because the probability that two unrelated samples will have the same sequence of layers is extremely low. However, discrimination between the different layers using optical microscopy is often difficult or impossible. Here, lateral scanning Raman spectroscopy has been used to chemically map the cross‐sections of multilayer white paint chips. It was found that the spectra did allow the different layers to be delineated on the basis of their spectral features. The boundaries between different layers were not as sharp as expected, with transitions occurring over length scales of > 20 µm, even with laser spot diameters < 4 µm. However, the blurring of the boundaries was not so large as to prevent recording and identification of spectra from each of the layers in the samples. This method clearly provides excellent discrimination between different multilayer white paint samples and can readily be incorporated into existing procedures for examination of paint transfer evidence. © 2011 Crown copyright.     

扩展目标光学特性的理论分析与研究

利用双向反射分布函数经验公式推导出激光主动照明条件下的目标反射截面表达式,给出了目标反射截面的一种近似计算方法,基于该方法计算了圆锥体目标反射截面。利用固体激光器和CMOS相机搭建实验系统,实际测量了目标反射截面随入射角度和不同目标体等参数变化时的变化情况,实验系统及数据同提出的方法计算得到的结果基本一致。结果表明,目标形状相同,入射角小于45时,目标反射截面随着无涂层、白色油漆、灰色油漆逐渐变小,大于45时白色油漆涂层目标反射截面最大;在目标表面特性相同的时候,目标反射截面随圆板、圆柱、圆锥渐次减小。