Introducing speckle noise maps for Laser Vibrometry

When coherent light scatters from a surface, which is rough on the scale of the wavelength of the light, a speckle pattern is produced. The Laser Vibrometer measures target vibration velocity in the direction of the incident laser beam and typically samples a region of a speckle pattern on its photodetector. Target motions can cause the speckle pattern to change on the photodetector surface, particularly when target motions are non-normal to the direction of the laser beam. This speckle motion modulates the Doppler signal and adds noise to the demodulated output signal. Periodic target motions can cause the speckle noise to become pseudo-random and produce harmonic peaks, with the same fundamental frequency as the genuine target vibrations, which can be indistinguishable from the genuine target vibrations. Typical speckle noise levels are generally considered to be low-level, but they have not so far been adequately quantified. This paper reports preliminary results quantifying speckle noise levels using controlled experimental configurations incorporating periodic in-plane and tilt target motions. Working with commercial Laser Vibrometers, various target surface finishes and treatments are considered and speckle noise maps are produced for each configuration. For a tilting surface, speckle noise has been quantified at approximately 1 μm s^?1/deg s^?1 while, for surfaces with in-plane motion, the sensitivity to speckle noise has been estimated pessimistically at 0.1% of the in-plane velocity. Ultimately, these speckle noise maps will form a valuable practical resource for the Laser Vibrometer user.     

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.     

Effect of speckle pattern on laser Doppler velocimeters

The effect of speckle pattern on laser Doppler velocimeters is studied theoretically and experimentally. We have found that dynamic speckle patterns can cause error in velocity measurement. Increasing sampling time and using proper signal processing circuits can eliminate this bad effect caused by speckle pattern.The accuracy of velocity measurement is better than 2.1％ in a wide velocity range(4.66-468.86 mm/s)when speckle effects are taken into account.     

旋转粗糙目标微运动特征识别技术

基于动态散斑技术的微运动特性识别在目标探测领域具有重要的研究价值.以粗糙面散射理论为基础,研究了旋转粗糙凸体目标的动态散斑时间相关函数,给出了旋转圆锥体目标动态散斑时间相关函数.将数值结果与相同条件下实验结果进行比较,验证了圆锥体目标时间相关长度的有效性.利用粒子群算法,反演出旋转圆锥体的旋转角速度和视线角.结果表明,该方法可识别旋转圆锥体范围为20°—90°之间的视线角值和范围为0.5—6 r/min的角速度值,为微运动特征识别提供理论和实验依据.     

Angular dependence of screen speckle and fiber speckle of coupled output of nine high-power blue laser diodes through a multi-mode fiber

Angular dependence of speckle contrast of speckle pattern projected out of a multi-mode fiber connected to a high-power blue laser module is investigated. The laser module has nine high-power InGaN/GaN blue laser diodes arranged in a three-by-three array. Each of the arrayed laser diodes have slightly different incident angle to the fiber. We have successfully extracted the fine screen speckle pattern from the projected pattern mixed up with the coarse fiber speckle pattern by processing the measured data. It is found that the speckle contrast of the both screen and fiber speckles are larger around the center area where the guided-light component closest to the paraxial direction is projected. This is because the output of the center laser in the array is likely to couple into the paraxial mode more than the rest. Speckle contrast behaviors when applying the speckle reduction methods, fiber vibration, diffuser, and spinning diffuser are also investigated.     

Comparison of laser Doppler and laser speckle contrast imaging using a concurrent processing system

Full field laser Doppler imaging (LDI) and single exposure laser speckle contrast imaging (LSCI) are directly compared using a novel instrument which can concurrently image blood flow using both LDI and LSCI signal processing. Incorporating a commercial CMOS camera chip and a field programmable gate array (FPGA) the flow images of LDI and the contrast maps of LSCI are simultaneously processed by utilizing the same detected optical signals. The comparison was carried out by imaging a rotating diffuser. LDI has a linear response to the velocity. In contrast, LSCI is exposure time dependent and does not provide a linear response in the presence of static speckle. It is also demonstrated that the relationship between LDI and LSCI can be related through a power law which depends on the exposure time of LSCI.     

A Modified Monte Carlo Model of Speckle Tracking of Shear Wave Induced by Acoustic Radiation Force for Acousto-Optic Elasticity Imaging

A modified Monte Carlo model of speckle tracking of shear wave propagation in scattering media is proposed.The established Monte Carlo model mainly concerns the variations of optical electric field and speckle.The twodimensional intensity distribution and the time evolution of speckles in different probe locations are obtained.The fluctuation of speckle intensity tracks the acoustic-radiation-force shear wave propagation,and especially the reduction of speckle intensity implies attenuation of shear wave.Then,the shear wave velocity is estimated quantitatively on the basis of the time-to-peak algorithm and linear regression processing.The results reveal that a smaller sampling interval yields higher estimation precision and the shear wave velocity is estimated more efficiently by using speckle intensity difference than by using speckle contrast difference according to the estimation error.Hence,the shear wave velocity is estimated to be 2.25 m/s with relatively high accuracy for the estimation error reaches the minimum(0.071).     

Techniques for phase correction in coherent ultrasound imaging systems

The problem of phase aberrations modeled as a near field thin phase screen is considered. Here three new techniques for phase aberration correction are presented. In the "central element" correction technique the echo-signal at each array element is aligned with the signal at the central array element. In the "variable speckle look-back" correction technique the signal at each array element is aligned with the sum of signals from all previously corrected elements. In the "variable central speckle look-back" correction technique the signal at each array element is aligned with the sum of signals from the central group of all previously corrected elements. The performance of new techniques for phase aberration correction is tested by means of simulating the point-spread function for both point and speckle targets. The results of simulation show that the new techniques have the better accuracy of phase correction than the classical "neighbor element" technique.     

Velocity measurement using the time-space cross-correlation of speckle patterns

The vector velocity measurement of the in-plane object motion based on the dynamic properties of speckle patterns is conducted. The time-space cross-correlation function of speckle intensity variations is used to determine the magnitude and direction of the in-plane object velocity. Theoretical and experimental studies of the speckle correlation technique for the determination of the in-plane object velocity are discussed and the vector velocities of the diffuse object are displayed as a two-dimensional image.     

成像型任意反射面速度干涉仪的散斑抑制

成像型任意反射面速度干涉仪（VISAR）是激光驱动聚变实验中诊断冲击波速度的重要设备。由于采用了激光照明靶面的方式,所获得的速度条纹图中不可避免有激光散斑的干扰,严重影响动态条纹的质量。介绍了该系统的激光散斑形成原因和散斑对速度分析的影响,提出了一种频谱面滤波的方式去除高频散斑噪声的方法,并通过搭建散斑光路、合理设置滤波孔位置和大小,对该方法进行了验证。结果表明,该方法对影响条纹图的高频散斑噪声具有抑制作用,适合应用于成像型VISAR系统。     

成像型任意反射面速度干涉仪的散斑抑制

成像型任意反射面速度干涉仪(VISAR)是激光驱动聚变实验中诊断冲击波速度的重要设备。由于采用了激光照明靶面的方式,所获得的速度条纹图中不可避免有激光散斑的干扰,严重影响动态条纹的质量。介绍了该系统的激光散斑形成原因和散斑对速度分析的影响,提出了一种频谱面滤波的方式去除高频散斑噪声的方法,并通过搭建散斑光路、合理设置滤波孔位置和大小,对该方法进行了验证。结果表明,该方法对影响条纹图的高频散斑噪声具有抑制作用,适合应用于成像型VISAR系统。     

Spacetime correlation functions of dynamic speckles produced by a series of moving phase screens

Spacetime correlation functions of the time-varying speckle intensity have been evaluated for the case where speckle patterns are produced in the Fresnel diffraction region by light propagating through a series of moving random-phase screens. The correlation functions are given by a set of recursion formulae which are useful for numerical computations. Three statistical quantities relating to the motion of speckle patterns are derived to examine the relationship between the velocity distribution of the phase screens and the temporal behaviour of resultant speckle patterns. It is found that the velocity differences between the phase screens have a strong effect on both the decorrelation of speckle patterns and the fluctuating speed of the speckle intensity.     

利用散斑空一时相关性测散射体的慢运动

本文研究了利用远场动态散斑的空-时相关性测量散射体运动速度矢量的理论和实验技术.作者在推导出远场动态散斑空-时相关函数的基础上,提出并建立了一种新的微机辅助检测系统.利用它从实验上证明了在单高斯光照明、散射体平动情况下运场动态散斑的空-时相关性,以及空-时相关的差分与运动速度之间的线性关系.通过测量动态散斑的空-时相关性获取散射体的运动速度矢量.实验结果与理论分析吻合.     

Phase aberration correction in medical ultrasound using speckle brightness as a quality factor

Medical ultrasonic images are degraded by tissues with inhomogeneous acoustic velocities. The resulting phase aberration raises the off-peak response of the imaging system's point spread function (PSF), decreasing dynamic range. In extreme cases, multiple images of a single target are displayed. Phase aberration may become a limiting factor to image quality as ultrasonic frequency and aperture size are increased in order to improve spatial resolution. A method is proposed to correct for unknown phase aberration, which uses speckle brightness as a quality factor. The phase delays of a phased array transducer are modified, element by element, to maximize mean speckle brightness in a region of interest. The technique proposed is analogous to the correction technique used by Muller and Buffington [J. Opt. Soc. Am. 64 (9), 1200-1209 (1974)] to adaptively focus incoherent optical telescopes. The method is demonstrated using a computer model with several different simulated aberration profiles. With this model, mean speckle brightness is calculated using the two-dimensional PSF. Experiments have also been conducted in which speckle brightness is shown to increase as the phase delays of an ultrasonic scanner are modified in order to compensate for a rippled aberrating layer made of silicone rubber. The characteristics of the proposed method, and the possibility of employing it clinically to correct for unknown inhomogeneities in acoustic velocity, are discussed.     

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.     

Spacetime correlation functions of dynamic speckles produced by a series of moving phase screens

Abstract

Spacetime correlation functions of the time-varying speckle intensity have been evaluated for the case where speckle patterns are produced in the Fresnel diffraction region by light propagating through a series of moving random-phase screens. The correlation functions are given by a set of recursion formulae which are useful for numerical computations. Three statistical quantities relating to the motion of speckle patterns are derived to examine the relationship between the velocity distribution of the phase screens and the temporal behaviour of resultant speckle patterns. It is found that the velocity differences between the phase screens have a strong effect on both the decorrelation of speckle patterns and the fluctuating speed of the speckle intensity.     

Extending the bandwidth of speckle interferometry

In speckle interferometry, the scale of the speckle array is wavelength dependent, and this limits the spectral bandwidth that can be used. A simple optical system is described to correct this effect, over a wide spectral range, by introducing a chromatic difference of magnification of opposite sign to the speckle dispersion, and variable in magnitude.     

Improvement of the sensitivity in velocity sensing using dynamic speckles

Dynamic speckle patterns can be used for imaging of relative velocity of moving objects in fields of biomedical and mechanical measurements. In spite of the widespread use of this method, the effect of speckle size on velocity sensing has not fully been estimated so far. In addition, effects of speckle contrast and random noises on the sensitivity of velocity sensing have not been investigated yet. In the present study, we estimated condition of image processing of speckle patterns for reducing effects of random noises with relation to linearity and sensitivity in velocity sensing. We further introduced binarization of the speckle pattern to improve the sensitivity in velocity sensing. Experiments were conducted for sample models using a diffusive plate and fluid flows to confirm the feasibility of the proposed method.     

Autocorrelation of self-mixing speckle in an EDFR laser and velocity measurement

Autocorrelation analysis for a velocity measurement based self-mixing speckle in an erbium-doped fiber ring (EDFR) laser is presented. The expression for the velocity measurement is deduced. Theoretical analysis reveals that the velocity of a random surface is inversely proportional to the autocorrelation time of the self-mixing signal produced in the EDFR laser. In experiments, a linear fit between the velocity and the reciprocal of autocorrelation time (autocorrelation frequency) is obtained for velocity measurement. In a range of velocities from 100 mm/s to 350 mm/s, the measurement errors are evaluated to be less than 5%. The signal length, the measurement distance, and the pump power are experimentally studied for gaining knowledge of the influence on the velocity measurement.     

Velocity contours by speckle photography

We describe a method of generating velocity contours of an object using speckle photography. The object is illuminated by a series of laser pulses following each other at equal intervals. A speckle record of the object is observed in a spatial filtering setup with a small aperture in the frequency plane. A fringe pattern representing a map of velocity contours is obtained in the output plane.