高斯型弱散射屏产生的像面散斑场的分布特性研究

弱散射屏产生的远场散斑由一个中央亮斑和一个分布于亮斑周围而与正态散斑类似的散斑结构, 根据弱散射屏远场的散斑图样, 人们假设弱散射屏产生的像面散斑为均匀背景与正态散斑两者相干叠加的结果, 但这种假设与实际像面散斑存在歧异, 基于上述情况, 本文利用4f高通滤波光学成像系统, 研究了高斯型弱散射屏产生的像面散斑场的统计特性, 得出只有表面均方根粗糙度与入射光波的波长相差不多时上述假设才是可行的结论. 关键词： 弱散射屏 f光学成像系统')" href="#">4f光学成像系统 像面散斑     

Characterizing individual scattering events by measuring the amplitude and phase of the electric field diffusing through a random medium

We describe observations of the amplitude and phase of an electric field diffusing through a three-dimensional random medium, using terahertz time-domain spectroscopy. These measurements are spatially resolved with a resolution smaller than the speckle spot size and temporally resolved with a resolution better than one optical cycle. By computing correlation functions between fields measured at different positions and with different temporal delays, it is possible to obtain information about individual scattering events experienced by the diffusing field. This represents a new method for characterizing a multiply scattered wave.     

Thickness-dependent coercivity of ultrathin Co films on a rough substrate: Cu-buffered Si(111)

The hysteresis loops, of Co films with thicknesses ranging from 12- to 80-monolayer-equivalent (MLE) coverages grown by thermal evaporation on a Cu-covered Si(111) surface, were measured in situ by the surface magneto-optic Kerr effect (SMOKE) technique. The hysteresis loops were measured as a function of Co coverage under an external sinusoidal magnetic field at fixed driving frequency. The coercivity H_c of the Co film versus thickness t followed a power law t⁻ⁿ with n=0.4±0.1 between 12 and 44 MLE, and stabilized after 44 MLE, up to the 80 MLE studied. The surface morphology of the 80-MLE Co film was imaged ex situ by atomic force microscopy (AFM) and scanning tunneling microscopy (STM), revealing cauliflower-like islands that were rough both in the short and long range. Analysis of the height–height correlation function for the largest image gave measurements of the effective roughness exponent (0.8), the vertical interface width w(2500 Å), and the lateral correlation length ξ(10 000 Å). We suggest that the coercivity changed in part due to changes in roughness of the Co films, deposited on a rough substrate; the spatial roughness would create an additional surface anisotropy, contributing to a fluctuation in the domain wall energy, resulting in a roughness-dependent coercivity.     

Used digital speckle correlation method to realize orientation function of optical mouse

Used digital speckle correlation method (DSCM) to realize orientation function of optical mouse is researched. The experiment is designed to obtain the dynamic speckle patterns which are shot by CCD, then the DSCM is used to process the sequential images and also the experiment is simulated. The experimental results show the DSCM can confirm orientation function of the optical mouse, and have a good agreement with the simulation results, and the resolution we obtained is higher than the resolution of mouse on the market.     

Optical and electrical properties of binary WO3–Pb3O4 glasses

A glass system of the composition xWO₃+(100−x)Pb₃O₄, with x=5, 10, 20 and 30 mol.% was prepared. The optical absorption, ac and dc conductivities are the subject of the present work. The optical absorption indicates that the electronic transition is indirect and is associated with phonon assisted transition. The exponential dependence of the absorption coefficient as a function of the incident photon energy suggests that the Urbach rule is obeyed, and indicates the formation of a band tail. On the other hand, ac conductivity measurements are performed in the frequency range 0.1–100 kHz, and in the temperature range 300–600 K. The results of the electrical conductivity are discussed on the basis of electronic glass conduction models. Correlated narrow-band limit for random sites and single polaron hopping model are found to describe the experimental results effectively. The dielectric constant was correlated to the optical band gap and a satisfactory relation was found. It was also possible to calculate the thermochromic properties from independent ac and dc measurements, and it was possible to evaluate the optical gap at 0 K by extrapolation.     

Digital Speckle Photography of Subpixel Displacements of Speckle Structures Based on Analysis of Their Spatial Spectra

We have investigated the capabilities of the method of digital speckle interferometry for determining subpixel displacements of a speckle structure formed by a displaceable or deformable object with a scattering surface. An analysis of spatial spectra of speckle structures makes it possible to perform measurements with a subpixel accuracy and to extend the lower boundary of the range of measurements of displacements of speckle structures to the range of subpixel values. The method is realized on the basis of digital recording of the images of undisplaced and displaced speckle structures, their spatial frequency analysis using numerically specified constant phase shifts, and correlation analysis of spatial spectra of speckle structures. Transformation into the frequency range makes it possible to obtain quantities to be measured with a subpixel accuracy from the shift of the interference-pattern minimum in the diffraction halo by introducing an additional phase shift into the complex spatial spectrum of the speckle structure or from the slope of the linear plot of the function of accumulated phase difference in the field of the complex spatial spectrum of the displaced speckle structure. The capabilities of the method have been investigated in natural experiment.     

Measurement of surface roughness properties using speckle patterns with non-gaussian statistics

The surface roughness properties (i.e. the rms surface roughness and the correlation length) of strong diffuse objects are investigated by using the speckle patterns which obeys the non-gaussian statistics. The intimate linear relation is found to exist between the rms surface roughness of objects and the maximum contrast obtained from the varying curves of the average image speckle contrast as a function of the point spread of an optical imaging system. The correlation length of surface roughness of objects is related to the averaged intensity distribution of speckle patterns produced at the far-field diffraction plane. It now becomes clear that the rms roughness and the correlation length of strong diffuse objects are determined, respectively, from the maximum speckle contrast at the image plane and the averaged speckle intensity distribution at the far-field diffraction plane.     

Monitoring foam coarsening using a computer optical mouse as a dynamic laser speckle measurement sensor

In this paper, we present an experimental approach to track coarsening process of foam using a computer optical mouse as a dynamic laser speckle measurement sensor. The dynamics of foam coarsening and rearrangement events cause changes in the intensity of laser speckle backscattered from the foam. A strong negative correlation between the average speed of the cursor and the evolution of bubble diameter was found. We used microscopic images to demonstrate that decrease in speed is related to increase in bubble size. The proposed set-up is not very expensive, is highly portable and can be used in laboratory measurements of dynamics in other kinds of opaque materials.     

Phase shifting speckle shearing polarization interferometer using a birefringent wedge

A digital speckle pattern shearing interferometer is presented which utilizes a single birefringent wedge. Two orthogonally polarized images of an object measured are sheared by the wedge, and the bias phase difference between two polarized images is shifted by translating the wedge. The phase shifting technique applied to speckle pattern shearing interferometry makes it possible to analyze a slope and surface strain numerically. The fringe formation in the present interferometer is analyzed with statistical theory. Experimental results are also presented in two cases: (1) measurement of a slope and normal displacement with a single illumination; (2) simultaneous and separate measurement of a slope and surface strain with two symmetrical illuminations.     

Optical mouse acting as biospeckle sensor

In this work we propose some experiments with the use of optical computer mouse, associated to low cost lasers that can be used to perform several measurements with applications in industry and in human health monitoring. The mouse was used to grab the movements produced by speckle pattern changes and to get information through the adaptation of its structure. We measured displacements in wood samples under strain, variations of the diameter of an artery due to heart beat and, through a hardware simulation, the movement of an eye, an experiment that could be of low cost help for communication to severely handicapped motor patients. Those measurements were done in spite of the fact that the CCD sensor of the mice is monolithically included into an integrated circuit so that the raw image cannot be accessed. If, as was the case with primitive optical mouse, that signal could be accessed, the quality and usefulness of the measurements could be significantly increased. As it was not possible, a webcam sensor was used for measuring the drying of paint, a standard phenomenon for testing biospeckle techniques, in order to prove the usefulness of the mouse design. The results showed that the use of the mouse associated to a laser pointer could be the way to get metrological information from many phenomena involving the whole field spatial displacement, as well as the use of the mouse as in its prime version allowed to get images of the speckle patterns and to analyze them.     

A fast optical profilometer

A simple optical profilometer is described for the measurement of surface profiles. The device is based on the use of a photodiode array that allows a fast examination of the surface and a high sensitivity. It also overcomes the randomness requirements which limit the use of optical devices working on angular diagram or speckle correlation measurements. Preliminary measurements of workpieces profiles are shown.     

Whole field surface roughness measurement by laser speckle correlation technique

This paper describes a speckle correlation technique for the determination of surface roughness, ranging from 1.6 to 50 μm. Instead of moving the laser beam, the specimen is rotated to achieve angular speckle correlation (ASC) in the far-field plane. The technique is simple and requires minimum optical alignment. The experimental results show a good agreement with the standard specimen of known roughness. An error analysis on the experiment has been carried out. Together with the theoretical curves, the roughness values can be easily related to the change of incidence angle at a particular visibility of the correlation fringes between two speckle patterns.     

Magnetic properties of UFe5Sn single crystals

Millimetre-size UFe₅Sn single crystals were grown by the top seed solution growth method and characterized by magnetization, ⁵⁷Fe Mössbauer spectroscopy and specific heat measurements in order to study the magnetic transitions detected in powder samples at 248 and 178 K. The magnetization measurements show different behaviour along the three crystallographic directions but with similar values of spontaneous magnetization along a and c. The transition at 248 K is associated with ferromagnetic ordering of iron moments along the c-axis, while the transition at lower temperature is associated with a reorientation towards b. Mössbauer data show that this reorientation is concomitant to the ordering of the Fe2 sites, which in a large proportion remain paramagnetic between the two transition temperatures. Specific heat measurements are consistent with the establishment of magnetic ordering at 248 K, followed by a spin reorientation at 178 K, yielding γ(0 K)140 mJ/(mol K²) and θ290 K for UFe₅Sn.     

Phase transitions in 1,3,4-oxadiazole crystals under high pressure

Crystalline 2,5-di(4-nitrophenyl)-1,3,4-oxadiazole (DNO) has been investigated at pressures up to 5 GPa using Raman and optical spectroscopy as well as energy dispersive X-ray techniques. At ambient pressure DNO shows an orthorhombic unit cell (a=0.5448 nm, b=1.2758 nm, c=1.9720 nm, density 1.513 g cm⁻³) with an appropriate space group Pbcn. From Raman spectroscopic investigations three phase transitions have been detected at 0.88, 1.28, and 2.2 GPa, respectively. These transitions have also been confirmed by absorption spectroscopy and X-ray measurements. Molecular modeling simulations have considerably contributed to the interpretation of the X-ray diffractograms. In general, the nearly flat structure of the oxadiazole molecule is preserved during the transitions. All subsequent structures are characterized by a stack-like arrangement of the DNO molecules. Only the mutual position of these molecular stacks changes due to the transformations so that this process may be described as a topotactical reaction. Phases II and III show a monoclinic symmetry with space group P2₁/c with cell parameters a=1.990 nm, b=0.500 nm, c=1.240 nm, β=91.7°, density 1.681 g cm⁻³ (phase II, determined at 1.1 GPa) and a=1.890 nm, b=0.510 nm, c=1.242 nm, β=89.0°, density 1.733 g cm⁻³ (phase III, determined at 2.0 GPa), respectively. The high-pressure phase IV stable at least up to 5 GPa shows again an orthorhombic structure with space group Pccn with corresponding cell parameters at 2.9 GPa: a=0.465 nm, b=1.920 nm, c=1.230 nm and density 1.857 g cm⁻³. For the first phase a blue pressure shift of the onset of absorption by about 0.032 eV GPa⁻¹ has been observed that may be explained by pressure influences on the electronic conjugation of the molecule. In the intermediate and high-pressure phases II–IV the onset of absorption shifts to increased wavelengths due to larger intermolecular interactions and enhanced excitation delocalization with decreasing intermolecular spacing.     

Holography, speckle, and computers

Surveys are presented on automatic and quantitative deformation measurements using CCDs and computers in holographic interferometry and speckle metrology for diffusely reflecting surfaces. For delivering relationships between object deformation and observed quantities, we discuss formations of fringes and signals observed in these methods in terms of correlation functions of spatially randomly varying complex amplitude of light. Dependencies of the observed patterns on object deformation and optical systems are discussed. Physical meanings of the derived relationships are explained in terms of the dynamic behaviors of speckles resulting from surface deformation. Automatic measurements are described in chronological orders. They include analysis of fringe patterns resulting from photographic recording of specklegrams, video recording and analysis of speckle patterns used in electronic speckle pattern interferometry as well as direct digital correlation techniques, and digital holography that uses both digital recording and reconstruction of holograms.     

Tantalum irradiation by high power pulsed laser at 1315 and 438 nm wavelengths

The high power pulsed laser Prague asterix laser system (PALS), operating at the fundamental (1ω) and third (3ω) harmonics (1315 and 438 nm wavelengths, respectively), is employed in a single-shot mode to irradiate tantalum targets in vacuum. The laser pulse width is 400 ps and the laser pulse energy ranges between 43 and 736 J at 1ω and between 12 and 230 J at 3ω. High ablation yields (0.1–0.6 mg per pulse) are measured as a function of the laser pulse energy at both wavelengths; at 438 nm higher etching rates are observed. The produced craters are analysed by the scanning electron microscope (SEM) and by the high sensitivity surface profiler system. They are investigated in dimension, shape and angle aperture as a function of the incident laser energy. Different possible mechanisms responsible for the different crater shapes are presented and discussed.     

数字散斑相关方法实现鼠标定位原理

在激光鼠标工作原理的基础上,利用数字散斑相关方法对激光鼠标的定位原理进行了研究。设计实验得到散斑场并用数码相机拍摄。用图像采集卡采集CCD扫描数散斑图像得到的图像,将图像数字化,分析数据得到了图像最优采样的像素单元和图像位移两个参量。基于两个参量,实验采集散斑图像在一定的轨迹运动下的散斑场,用Matlab软件模拟,并比较分析了实验和模拟结果。结果表明,用数字散斑相关方法可以实现激光鼠标的定位功能,并且分辨力可达到1μm,远高于市场上激光鼠标30μm的分辨力。     

On the determination of elastic residual strain fields in rail wheels using moiré interferometry

Moiré interferometry was used to determine the residual strain fields in a rail wheel. Hoop, radial, and shear displacement fields were determined in a whole field manner using a novel four-beam moiré interferometer. Diffraction gratings of 150 lines/mm were employed on the specimen surface. A system of data interpolation utilizing a simple control sample scheme was developed for field environments where sample loading occurs away from the optical device. Agreement was found between the moiré technique and physical measurements of wheel contraction during sectioning. Marginal agreement was found with strain gage measurements.     

Roughness determination by direct visual observation of the speckle pattern

There are mechanical and optical methods of measuring the roughness of surfaces. Mechanical methods are of a destructive type, while optical methods, although they are non-destructive, involve relatively complex systems and calculations. In this work a simple method is introduced, which allows one—through the direct observation of the speckle pattern—to make a visual correlation, comparing the first pattern with others obtained when the beam incidence angle varies. With this method it is possible to obtain results with acceptable accuracy for many industrial uses.     

Measurement of the near-tip displacement field in a fatigue damaged steel plate by digital speckle pattern interferometry

A digital speckle pattern interferometry system with automatic data analysis is used to measure the near-tip in-plane displacement field in a stainless steel specimen previously cracked by a fatigue test. In-plane displacements over a zone of 3.15 mm×3.15 mm are determined through the evaluation of the optical phase distribution. A theoretical solution developed for elasto-plastic materials is used to evaluate the obtained experimental results.