Sampled-speckle photography for measurement of deformation

Speckles usually are of two kinds: laser speckle and white-light speckle. An additional kind, termed a sampled speckle, is proposed. Whereas laser speckles arise from multiple interference of light scattered from an object illuminated by a coherent laser beam and white-light speckles are a physically generated speckle pattern on the surface of the object, sampled speckles are generated as a result of the sampling of a digital image. The generation of these speckles and their application to displacement measurement are demonstrated.     

Statistics of dynamic speckles in application to distance measurements

We present an analysis of statistical properties of dynamic speckles to estimate the limiting accuracy of measurements achievable in a distance sensor using spatially filtered dynamic speckles. The main reason for inaccurate measurements using dynamic speckles is their stochastic nature. It is shown that the average lifetime of dynamic speckles is the key factor defining the measurement accuracy. Main conclusions of the theoretical analysis were confirmed in an experiment carried out with a fast moving rough surface. Special attention is paid to a recently proposed range sensor using dynamic speckles generated by a fast-deflecting laser beam. It is shown that this sensor possesses the best combination of accuracy and response time.     

用于超快束匀滑的动态波前调控新方案

提出了利用光克尔效应实现激光束波前动态调控,进而实现焦斑超快束匀滑(ps量级)的方案,其原理是利用抽运光动态改变光克尔介质的折射率分布,以对透射主激光束附加时空耦合的动态波前,进而使激光束在靶面的焦斑散斑产生更加快速、多样的变化,最终实现焦斑的超快束匀滑.当抽运光时间波形为高斯脉冲序列,且以小角度倾斜入射至光克尔介质时,由于抽运光和光克尔介质对主激光附加随时间横向移动的周期性球面相位,且球面相位的幅值随时间不断变化,因而可以同步实现激光束焦面散斑的横向和径向超快速扫动,从而更为有效地改善靶面辐照均匀性.     

Design and verification of diffractive optical elements for speckle generation of 3-D range sensors

The optical projection using speckles is one of the structured light methods that have been applied to three-dimensional (3-D) range sensors. This paper investigates the design and fabrication of diffractive optical elements (DOEs) for generating the light field with uniformly distributed speckles. Based on the principles of computer generated holograms, the iterative Fourier transform algorithm was adopted for the DOE design. It was used to calculate the phase map for diffracting the incident laser beam into a goal pattern with distributed speckles. Four patterns were designed in the study. Their phase maps were first examined by a spatial light modulator and then fabricated on glass substrates by microfabrication processes. Finally, the diffraction characteristics of the fabricated devices were verified. The experimental results show that the proposed methods are applicable to the DOE design of 3-D range sensors. Furthermore, any expected diffraction area and speckle density could be possibly achieved according to the relations presented in the paper.     

散斑位移量与照明激光束特性关系的实验研究

激光照射到粗糙物体的表面会形成激光散斑，散斑随物体的运动而运动，特别是物体在自身平面内运动时散斑的同步运动比较显著，因此可以用这一现象测物体的微小位移，散斑运动规律与照明光束的特性有关，本文通过实验研究了不同光束照明下散斑位移的规律，验证了散斑位移理论，并指出散斑位移测量中应注意的问题。     

Analysis of mechanical vibrations through speckle interferometry: A phase-space approach

The goal of this paper is to determine hidden periodicities (spatial frequencies) into speckles patterns obtained in a diffuser after reflection of an unexpanded laser beam over an aluminum membrane by means of the so-called Wigner Distribution Function (WDF). These spatial frequencies are used to find the temporal frequencies at which the mechanic system is vibrating.     

Spatial structure of the degree of coherence for broadband dispersed laser beams

1D and 2D spatial distributions of a complex degree of coherence are obtained using numerical simulation for the broadband dispersed laser beams. The configuration of the spatial inhomogeneities (speck-les) of the degree of coherence is studied at various spectral widths and dispersions of the beam. It is shown that the speckle size increases with increasing distance from the initial plane where the light field is assumed to be spatially coherent. The small- and large-scale structures of the degree of coherence are shown for the transverse cross section of the beam. The results are compared with the analytically calculated correlation functions of the spatial coherence. The features of the speckle structure are interpreted using the concept of partial speckles.     

Velocity sensitivity of laser-speckle velocimeters

The method of increasing sensitivity in laser speckle velocimetry is studied theoretically and experimentally. The sensitivity of the velocimeters is shown to have a fairly strong dependence on the optical configurations, such as the diameter of the incident laser beam-waist, or its position relative to the moving diffuse object to be measured. It is pointed out that the minimal sensitivity appears in the configuration where the converging incident light beam produces its beam waist at the position of a detecting pinhole. In some cases when using image speckles to obtain finite spatial resolution in the object plane, an optimum condition exists which gives the maximal sensitivity. The sensitivity of a laser speckle velocimeter using a spatial filter and two point cross-correlation technique is also discussed.     

基于涡旋光束的超快速角向集束匀滑方案

针对惯性约束聚变装置对激光集束辐照均匀性的需求,提出了一种基于涡旋光束的超快速角向匀滑方案,即利用螺旋相位板使2×2集束中的两子束由超高斯光束变换为涡旋光束,而其余两子束不变,进而通过对子束偏振态和中心波长的调控,使集束中的涡旋光束和超高斯光束在靶面两两相干叠加.相干叠加后的焦斑以皮秒量级为周期超快速旋转,从而在极短时间内快速抹平焦斑强度调制,改善靶面辐照均匀性.通过建立基于螺旋相位板的激光超快速角向集束匀滑方案的物理模型,分析了其角向匀滑特性,并与光谱角色散技术和径向匀滑技术进行了比较分析.结果表明,这一新型激光集束匀滑方案能实现对焦斑的超快速角向匀滑,且能在数皮秒时间内达到最佳辐照均匀性.     

用零相关相位板匀滑散斑的理论研究

对零相关相位板匀滑散斑的特性进行了理论分析和散值计算,结果表明尽管零相关相位板是针对入射场为平面波面而设计的,但在入射场的振幅和相位有一定起伏的情况下,其远场光斑的散斑对比度明显低于采用随机相位板的情形.因而用零盯关相位板匀滑散斑是一种更为有效的方法.     

Spatial and temporal coherence characterization of a smoothed laser beam

The measurement of the coherence characteristics of the speckles generated by an optically smoothed laser source is investigated. We present a new method that can be used for every kind of smoothing technique. A modified Mach-Zehnder interferometer allows us to measure both the lifetime and the spatial transverse size of the hot spots generated by a broadband and transverse multimode source. Experimental results agree well with theoretical predictions.     

Correlation analysis of the beam angle dependence for elastography

Signal decorrelation is a major source of error in the displacements estimated using correlation techniques for elastographic imaging. Previous papers have addressed the variation in the correlation coefficient as a function of the applied compression for a finite window size and an insonification angle of zero degrees. The recent use of angular beam-steered radio-frequency echo signals for spatial angular compounding and shear strain estimation have demonstrated the need for understanding signal decorrelation artifacts for data acquired at different beam angles. In this paper, we provide both numerical and closed form theoretical solutions of the correlation between pre- and post-compression radio-frequency echo signals acquired at a specified beam angle. The expression for the correlation coefficient obtained is a function of the beam angle and the applied compression for a finite duration window. Accuracy of the theoretical results is verified using tissue-mimicking phantom experiments on a uniformly elastic phantom using beam-steered data acquisitions on a linear array transducer. The theory predicts a faster decorrelation with changes in the beam or insonification angle for longer radio-frequency echo signal segments and at deeper locations in the medium. Theoretical results provide useful information for improving angular compounding and shear strain estimation techniques for elastography.     

Specific features of speckle structure formed by dispersed laser beams

The geometry of statistical average of intensity speckles formed by a broadband dispersed laser beam spatially coherent in initial plane z = 0 is studied theoretically. A computer simulation was used to obtain two-dimensional distributions of random intensity that provide a clear picture of the transformations experienced by the speckle structure upon laser beam propagation. Correlation functions and power spectra of random intensity are calculated in order to determine the characteristic size and shape of speckles as a function of longitudinal coordinate z, the width and shape of the frequency spectrum, and the degree of dispersion of the light beam. Analytical expressions that describe an increase in the speckle size along the beam axis and in the direction of beam dispersion as a function of distance from the initial plane are obtained. The ultimate (at z → ∞) width of speckles in the direction of beam dispersion is calculated.     

Toward Defeating Diffraction and Randomness for Laser Beam Propagation in Turbulent Atmosphere

A large distance propagation in turbulent atmosphere results in disintegration of laser beam into speckles. We find that the most intense speckle approximately preserves both the Gaussian shape and the diameter of the initial collimated beam while loosing energy during propagation. One per 1000 of atmospheric realizations produces at 7 km distance an intense speckle above 28% of the initial power. Such optimal realizations create effective extended lenses focusing the intense speckle beyond the diffraction limit of vacuum propagation. Atmospheric realizations change every several milliseconds. We propose to use intense speckles to greatly increase the time-averaged power delivery to the target plane by triggering the pulsed laser operations only at times of optimal realizations. Resulting power delivery and laser irradiance at the intense speckles well exceeds both intensity of diffraction-limited beam and intensity averaged over typical realizations.     

Effects of red blood cell aggregates dissociation on the estimation of ultrasound speckle image velocimetry

Ultrasound speckle image of blood is mainly attributed by red blood cells (RBCs) which tend to form RBC aggregates. RBC aggregates are separated into individual cells when the shear force is over a certain value. The dissociation of RBC aggregates has an influence on the performance of ultrasound speckle image velocimetry (SIV) technique in which a cross-correlation algorithm is applied to the speckle images to get the velocity field information. The present study aims to investigate the effect of the dissociation of RBC aggregates on the estimation quality of SIV technique. Ultrasound B-mode images were captured from the porcine blood circulating in a mock-up flow loop with varying flow rate. To verify the measurement performance of SIV technique, the centerline velocity measured by the SIV technique was compared with that measured by Doppler spectrograms. The dissociation of RBC aggregates was estimated by using decorrelation of speckle patterns in which the subsequent window was shifted as much as the speckle displacement to compensate decorrelation caused by in-plane loss of speckle patterns. The decorrelation of speckles is considerably increased according to shear rate. Its variations are different along the radial direction. Because the dissociation of RBC aggregates changes ultrasound speckles, the estimation quality of SIV technique is significantly correlated with the decorrelation of speckles. This degradation of measurement quality may be improved by increasing the data acquisition rate. This study would be useful for simultaneous measurement of hemodynamic and hemorheological information of blood flows using only speckle images.     

运动界面上反射超声散斑空间运动的研究

在Kirchhoff 衍射理论的基础上，应用随机信号相关原理，推导了作刚体运动的界面上反射超声散斑在空间运动时，保持其复振幅相关的必要条件.由此得到超声散斑的空间运动公式.界面、超声散斑和超声接收探头之间的相对运动，导致界面孔径的改变.分析表明，在保持界面孔径相关的条件下，界面所允许的最大平移量和转角值，与声源位置、接收用聚焦探头的数值孔径、超声入射角以及观察角有关.应用数字相关技术和三维扫描信号采集系统，根据散斑场子集相关系数的单峰性质，对超声散斑空间运动公式进行了实验验证.实验结果表明，理论分析所得的 关键词： 超声散斑 相关原理 空间运动     

Speckle decorrelation influence on measurements quality in vortex metrology

We study speckle decorrelation effects in connection with conventional vortex metrology techniques. Our proposal is based on processing speckled images recorded by using two different experimental set-ups. In both schemes two laterally displaced patterns are generated: one scheme allows for obtaining undecorrelated speckle distributions and the other for decorrelated ones. Vortex networks associated with speckle patterns are analyzed by employing the usual tools developed for vortex metrology. For each recorded image, a 2D pseudo-phase map is generated on the basis of the Reisz transform. Then the vortices are located, and parameterized in terms of their topological charge, eccentricity, vorticity and angles between the zero crossing lines from the real and the imaginary parts of the analytical signal. After tracking the homologous vortices onto the maps, the histograms corresponding to the coordinate displacements are analyzed. We show that histograms interpretation is prone to failure due to its high sensitivity to decorrelation. Experimental evidences are presented to support the restrictions imposed by decorrelation of actual speckles due to uniform in-plane displacements.     

散斑位移法测量激光高斯光束的空间分布

 提出一种测量激光光束高斯空间分布特征的新方法——散斑位移法。该方法利用光传播的菲涅尔衍射公式和光场的统计规律,给出散斑场光强在散射体位移前后的高斯光束相关函数的表达式。测出相关函数在光路上某一位置的数值,并由此同时求出高斯光束的束腰位置和大小。     

旋转圆柱目标的动态散斑统计特性(英)

高斯光束照射以固定角速度旋转的粗糙圆柱体表面,反射空间形成随着被测表面运动变化的动态散斑。研究了照射面积大于圆柱曲面条件下在远场衍射区形成的动态散斑统计特性,得到了不同入射条件及不同圆柱时动态散斑强度起伏空间-时间归一化相关函数。结果表明:随着圆柱半径、旋转速度以及照射光斑有效面积的增大,动态散斑强度起伏相关性快速减小;当圆柱旋转速度恒定,动态散斑光强起伏相关函数的峰值随着探测点之间距离的增大而减小,但峰值的位置随之而增大;在近似点照射情况下随着圆柱半径的增大空间相关长度基本不变,而相关时间性明显增大。     

Spatial fluctuations in intensity of dispersed laser beams with broad spectra

The statistical characteristics of spatial fluctuations of the intensity of dispersed laser beams are studied theoretically. The density of the probability distribution for the intensity is found and its transformation with a change in the longitudinal coordinate is studied. It is shown that a decrease in the intensity fluctuations and a drop in the contrast of the beam speckle structure occur with increasing distance from the initial plane in which the spatially inhomogeneous broadband field is assumed to be spatially coherent. A decrease in fluctuations is accompanied by an increase in the size of speckles in the direction of dispersion, as well as along the beam axis. An interpretation of the found regularities is given.