胶体中布朗粒子简谐束缚下准直高斯光束的动态光散射

在将散射粒子间的相互作用看作简谐力、入射光为准直高斯光束、散射光为理想平面光波的模型下研究了胶体溶液中布朗粒子的统计特征并求解了动态光散射的散射光电场强度的自相关函数；     

Use of low-coherence speckled speckles for bioflow measurements

Formation of speckled speckles in the case of biflow perfusion by partially coherent light was considered. Dependencies of statistical characteristics of low-coherence biospeckles with a small number of scatterers on the scattering properties of the flow and on the coherence length of incident light were analyzed. It was shown that the value of the Doppler bandwidth in the scattered light essentially depends on the ratio between the coherence length and the average size of the flow's inhomogeneities. A procedure for reconstructing velocity distribution in a single blood vessel was suggested.     

Spatial correlations between chromophores in thin films by femtosecond hyper-Rayleigh scattering

Femtosecond hyper-Rayleigh scattering has been used to probe the spatial orientational fluctuations between nonlinear optical chromophores as dopants in spincoated polymer films. The fluctuation in the second-order incoherently scattered light intensity upon translating the sample is indicative of the degree of spatial correlation between the chromophores. The decay of the autocorrelation function of this fluctuating signal is characterized by a spatial correlation length. Electric-field poling of chromophores is shown to increase this length. The temporal characteristics of this correlation length have been studied and compared with relaxation times, obtained with coherent second-harmonic generation. This correlation length decays much faster than the second-harmonic intensity. The difference in relaxation time between the incoherent and the coherent process is explained by translational diffusion of free volume over the wavelength and over the coherence length, respectively.     

Sizing of irregular particles using a near backscattered laser Doppler system

A near backscattered laser Doppler system was presented to carry out velocity and size distribution measurements for irregular particles in two-phase flows. The technique uses amplitudes of particles Doppler signals to estimate the particle size distribution in a statistical manner. Holve's numerical inversion scheme is employed to unfold the dependence of the scattered signals on both particle trajectory and orientation through the measurement volume. The performance and error level of the technique were simulated, and several parameters including the number of particle samples, the fluctuation of irregular particle response function, inversion algorithms, and types of particle size distribution were extensively investigated. The results show that the size distributions for those irregular particles even with strong fluctuations in response function can be successfully reconstructed with an acceptable error level using a Phillips-Twomey-non-negative least-squares algorithm instead of a non-negative least-squares one. The measurement system was then further experimentally verified with irregular quartz sands. Using inversion matrix obtained from the calibration experiment, the average measurement error for the mixing quartz sands with a size range of 200-560 microm are found to be about 23.3%, which shows the reliability of the technique and the potential for it to be applied to industrial measurement.     

Correlation-based speckle velocimeter with self-mixing interference in a semiconductor laser diode

We propose a novel self-mixing laser diode speckle velocimeter based on autocorrelation. The self-mixing laser diode launches and receives reflected light from a moving surface that has a random reflection profile. Some portion of the scattered light backcouples into the laser cavity and causes random intensity variations in the form of speckle signals. These speckle signals obtained from a self-mixing laser diode are processed by use of autocorrelation. The linear relation between the velocity and the reciprocal of the correlation time of the speckle intensity fluctuations allows us to determine the velocity easily if proper calibration is performed. The range of the investigated velocity is 20-450 mm/s. For an aluminum target that moves at a velocity of 350 mm/s, the measurement error is less than 2%.     

基于FPGA的多通道动态光散射自相关器的研制

为提高多角度动态光散射(MDLS)实验的测试效率与测量精度,开发了一种基于现场可编程门阵列(FPGA)的多通道动态光散射自相关器。该自相关器利用FPGA同时采集4个散射角度的光子脉冲信号,实现高频脉冲计数和自相关计算;设计双计数器模块保证高频无丢失计数,结合片内环形寄存区与片外DDR3芯片实现动态存储数据;提出multi-tau相关器结构设计自相关器的相关计算模块,通过USB通信模块传输自相关计算结果。该自相关器利用有限的FPGA内部硬件资源,实现了多通道自相关运算,解决了大量计数数据的存储问题,提高了MDLS实验的测试效率。利用自相关器对50～200 nm的聚苯乙烯颗粒进行MDLS实验,实验结果与样品的标称粒径表现出良好的一致性,表明该设计能有效地表征颗粒粒径信息。     

Application of dynamic light scattering to the study of small marine particles

Small particles ranging from approximately 0.1 μm to several micrometers in size, which include detrital material, bacteria, and other planktonic microorganisms, make a significant contribution to light scattering in the upper ocean. The scattering properties of these particles are strongly dependent on their size, which is difficult to measure in the submicrometer range with commonly used electronic resistive counters and microscopic techniques. We examined the size of small marine particles by application of the dynamic light scattering (DLS) method. In this method the time-dependent autocorrelation function of scattered intensity by particles undergoing Brownian motion provides information about the size of particles. The samples were collected in clear oceanic waters off the coast of Southern California. The mean hydrodynamic diameter of particles, determined from the DLS measurements at a scattering angle of 45°, was 0.54μ m. This indicates that the major contribution to scattering at this angle comes rom submicrometer particles. We also described an inverse method for estimating the general slope of the size distribution of small marine particles from the mean hydrodynamic diameter. This method is based on calculations of the size distribution weighted by distribution from Mie theory and assumes that a power-law approximation represents the actual particle scattered intensity. These calculations suggested that particulate assemblage in our seawater samples was best characterized by a differential size distribution with a slope of -4.35. This estimation was supported by independent measurements of particle size distribution and the spectral beam attenuation coefficient taken from the same samples as those used for the DLS measurements. We also demonstrated that multiangle DLS measurements may be used to determine the representative value of the refractive index of particles.     

基于FPGA高速信号采集的多角度动态光散射法纳米粒径测量

根据动态光散射装置测量纳米粒径原理,开发了一套基于现场可编程门阵列(FPGA)的纳米粒径测量系统。该系统通过光电倍增管(PMT)输出光子脉冲信号,利用FPGA实现高速脉冲采集及自相关运算,采用双脉冲计数器实现高精度可控的连续计数,并实现DDR3异步存储以及USB通信交互等接口功能。自研板卡既可实现自相关函数实时采集运算,又可无丢失地保存海量原始数据信号。采用该系统对200nm聚苯乙烯颗粒进行了测量,分析了不同采样时间及延迟时间等参数对粒径测量结果的影响。实验结果表明:自研FPGA采集板卡测量重复性为1.2%,具有很好的稳定性和重复性。     

Exploration of the retinal nerve fiber layer thickness by measurement of the linear dichroism

An electro-optic device mounted on a slit lamp to assess the degree of polarization of a light beam that has double passed through the retina about the optic-nerve head in the living human eye is described. The asymmetric structure of the retinal nerve's fiber layer possesses a linear-form dichroism and will partially polarize an unpolarized light beam that is scattered at the fundus of the eye and has double passed the ocular media (cornea, lens, retina). This partial polarization is a function of the retinal nerve's fiber layer thickness, and its measurement may be used for exploring glaucoma and other retinal neuropathies. Experimental conditions allow us to neglect corneal dichroism. The first clinical measurements show a different degree of polarization between normal and glaucomatous eyes and a good correlation with the results obtained by optical coherence tomography.     

Using light scattering to measure the temporal coherence of optical sources

We demonstrate a simple and robust method for characterizing the temporal coherence of statistically stationary optical sources by using dynamic light scattering. Measurement of the contrast of the fluctuating speckle pattern produced by two counterpropagating beams incident on a scattering medium is used to evaluate their mutual coherence. Important features of this method are high statistical accuracy, the ability to compensate for imperfect spatial coherence, and the possibility of characterizing milliwatt-level optical beams with a wide range of spectral widths. As an example, the squared magnitude of the field autocorrelation function for light emitted by a broadband argon-ion laser is obtained.     

Ultrashort pulse propagation through a strongly scattering medium: simulation and experiments

Forward light scattering of femtosecond pulses through strongly scattering media is investigated experimentally and numerically. Computations are based on a semi-Monte Carlo method including polarization effects when experiments depend on a Ti:sapphire regenerative amplifier (100 fs, 1 kHz, 1 mJ@ 800 nm). The temporal separation between ballistic light and scattered light is exhibited and used to perform optical depth measurements up to 22 (transmission factor of approximately 10(-10)). Quantitative comparisons between experiments and Monte Carlo simulations show a good agreement. Temporal forward scattered light evolutions with concentration and particle size are presented. Numerical results show that the early scattered light contains information on particle size, opening the way to particle sizing in strongly scattering media.     

Spatial coherence of forward-scattered light in a turbid medium

We study spatially coherent forward-scattered light propagating in a turbid medium of moderate optical depth (0-9 mean free paths). Coherent detection was achieved by using a tilted heterodyne geometry, which desensitizes coherent detection of the attenuated incident light. We show that the degree of spatial coherence is significantly higher for light scattered only once in comparison with that for multiply scattered light and that it approaches a small constant value for large numbers of scattering events.     

Light-scattering studies of packed stationary phases for capillary electrochromatography

Multiply scattered light through turbid media, packed particles, or compressed powders will inherently have a significantly longer optical path length than that of light which is not scattered. The concept of using the multiply scattered light potentially generated in the packed stationary phase of a capillary electrochromatography (CEC) column for enhanced detection as a result of its increased optical path length was examined. Ultraviolet (UV) light at 365 nm or laser light at 635 nm was focused to a small spot onto the packed section of a 3 microns spherisorb ODS1 CEC column (100 microns i.d.). The light was transported inside the capillary, and an image of the multiply scattered light several millimeters along the capillary was collected using a charged-couple device detector. Even if the spot size was less than 100 microns in diameter, evidence of light scattering was observed at a detection spatial off-set distance of 1-2 mm from the illumination point. When the calcium channel blocking drug felodipine was flushed through the column, the light intensity value dropped (increase in absorbance) to a greater degree at a spatial off-set (1.5 mm) than at the illumination point. The greater absorbance values at the spatial off-set were examined experimentally when felodipine was eluted from the column in the CEC mode in 6 min using MeCN/50 mM TRIS (pH 8.0) (80:20, v/v) at an applied voltage of 300 V/cm and an injection time of 2 s at 10 kV. A factor of 8.5 increase in absorbance was observed at a spatial off-set of 1 mm compared to the value obtained at the illumination point. An efficiency value of approximately 234,000 plates m-1 was obtained for this higher felodipine peak. Higher noise values, however, were also observed with this increase in absorbance. Using a spectrophotometer or an open capillary to obtain reference values for optical length, it was possible to estimate the average optical path length of light traveled through the packed stationary phase when transmitted at a spatial off-set. It was concluded that, although an increase in absorbance of 8.5 was observed at a spatial off-set, this most likely arises from the light being "redirected" and scattered in a straightforward fashion along the capillary. It was expected that if substantial multiple scattering did occur inside the packed stationary phase, a significantly larger absorbance increase would be attained. A number of proposals are thus given to explain the relatively low degree of multiple scattering in this stationary phase and suggestions offered on means to attain even higher absorbance increases at a spatial off-set. Additional potential applications are also discussed.     

Speckle size of light scattered from slightly rough cylindrical surfaces

This research is an extension of the optical method of quality control presented in a previous paper [Appl. Opt. 39, 5811 (2000)] to the case of slightly rough cylindrical surfaces. Applying the Kirchhoff scalar diffraction theory yields an analytical expression of the autocorrelation function of the intensity scattered from slightly rough cylindrical surfaces. This function, which is related to speckle size and shape, is shown to depend on the surface correlation length, unlike for plane surfaces for which the speckle depends on the illuminated area only. The theoretical expression is compared with that for the speckle produced by the light scattered from a cylindrical bearing and from various high-quality wires, showing that the method allows the correlation lengths of high-quality cylindrical surfaces to be determined.     

Degree of polarization as an objective method of estimating scattering

A new method of determining objectively the amount of scattered light in an optical system has been developed. It is based on measuring the degree of polarization of the light in images formed after a double pass through the system. A dual apparatus composed of a modified double-pass imaging polarimeter and a wave-front sensor was used to measure polarization properties and aberrations of the system under test. We studied the accuracy of the procedure in a system that included a lanthanum-modified lead zirconate titanate (PLZT) ceramic plate able to generate variable amounts of scattered light as a function of the applied voltage. Changes in the voltage applied to the ceramics plate modified significantly the scattering contribution while hardly altering the wave-front aberration. The degree of polarization was well correlated with the level of scattering in the system as determined by direct-intensity measurements at the tails of the double-pass images. This indicates that this polarimetric parameter provides accurate relative estimates of the amount of scattering generated in a system. The technique can be used in a number of applications, for example, to determine objectively the amount of scattered light in the human eye.     

Coherence solution for bidirectional reflectance distributions of surfaces with wavelength-scale statistics

The scalar bidirectional reflectance distribution function (BRDF) due to a perfectly conducting surface with roughness and autocorrelation width comparable with the illumination wavelength is derived from coherence theory on the assumption of a random reflective phase screen and an expansion valid for large effective roughness. A general quadratic expansion of the two-dimensional isotropic surface autocorrelation function near the origin yields representative Cauchy and Gaussian BRDF solutions and an intermediate general solution as the sum of an incoherent component and a nonspecular coherent component proportional to an integral of the plasma dispersion function in the complex plane. Plots illustrate agreement of the derived general solution with original bistatic BRDF data due to a machined aluminum surface, and comparisons are drawn with previously published data in the examination of variations with incident angle, roughness, illumination wavelength, and autocorrelation coefficients in the bistatic and monostatic geometries. The general quadratic autocorrelation expansion provides a BRDF solution that smoothly interpolates between the well-known results of the linear and parabolic approximations.     

基于图像光散射颗粒粒度测量方法研究

研究了基于图像的光散射颗粒粒度测量系统。结合图像处理技术,提出一种最小二乘原理圆拟合光环定中方法,设计CCD探测器光环尺寸与颗粒粒级划分数目。搭建了颗粒光散射图像测量硬件系统,利用非独立模式差分进化算法反演出颗粒粒度。通过8种聚苯乙烯乳胶球标准颗粒实验对测量系统准确性进行验证,测量误差在3%之内;对2种不同体积分数的大豆蛋白乳液进行了测量,与图像粒度仪对比偏差在5%之内。实验结果表明:采用最小二乘原理圆拟合方法定中提取到的散射光能与理论分布更加吻合,35环的光环划分方式有利于获得相对良态特性的光能系数矩阵,该测量方法为颗粒粒度测量提供了一种光散射法有效改进方式。     

Magnetophoretic velocity modulation mass analysis of a single microparticle in an atmosphere

A new principle of the magnetophoretic velocity modulation mass analysis of microparticles, which can determine simultaneously the mass and magnetic susceptibility of a single microparticle, has been proposed, and the measurement system was constructed by applying a magnetophoretic force on a falling microparticle through a magnetic field gradient in an atmosphere. A polystyrene microparticle as a test particle adsorbed on a glass plate was selectively knocked off by a pulsed Nd:YAG laser impact into a narrow gap of pole pieces of permanent magnets having a magnetic field gradient with a maximum intensity of 850 T2 m(-1). The falling particle was irradiated by a He-Ne laser, and the scattered light was detected through a slit array mask as a function of time. A bundle of spiked signals of scattered light intensity was analyzed to obtain velocities, which gave acceleration and deceleration of the falling particle. On the basis of the equation of motion under the magnetic field gradient, the mass and magnetic susceptibility of the test particle were reasonably determined.     

Investigation of hydrodynamics of gas-solid fluidized beds using cross recurrence quantification analysis

Hydrodynamics of gas-solid fluidized bed was investigated by analyzing its pressure fluctuations using cross recurrence quantification analysis (CRQA). Pressure fluctuations were measured in a lab scale fluidized bed of various particle sizes at different gas velocities. First, the CRQA was applied to a number of well-known dynamic systems and the results demonstrated that it is a powerful method to detect similarities between nonlinear signals. Then, it was shown that graphical structures within the cross recurrence plot of pressure fluctuations of a fluidized bed vary with both superficial gas velocity and particle size. It was found that determinism and cross recurrence rate of non-normalized data initially decrease and then increase with increasing the gas velocity. When the signal is initially normalized, determinism and entropy do not change with the superficial gas velocity while cross recurrence rate is sensible to changes in the superficial gas velocity. It was concluded that entropy can be used for detecting changes of particle size and if a proper reference state is chosen, entropy can be a powerful index for detecting changes in the size of particles in a fluidized bed.     

Study of DNA internal dynamics by quasi-elastic light scattering

We have studied the large-scale internal fluctuations in DNA coils by using quasi-elastic light-scattering spectroscopy. We have measured the angular dependence of the first cumulant of the scattered-light autocorrelation function. Within the q (3) domain of this dependence, we observed the transition from the asymptotic behavior predicted for good solvents to the asymptotic behavior predicted for ? conditions. This allowed us to determine the screening length of volume interactions in DNA chains. Analysis of the autocorrelation functions by a regularization procedure allowed us to reconstruct the mode composition of the scattered light and to determine the relaxation time of the fluctuations in the coil size.