Demonstration of two-point velocity measurement using diffraction grating elements for integrated multi-point differential laser Doppler velocimeter

We have proposed a configuration of an integrated multi-point differential laser Doppler velocimeter (LDV) using arrayed waveguide gratings (AWGs) as grating elements. This paper demonstrates two-point velocity measurement using the proposed configuration with diffraction grating elements. An experiment was conducted using a free-space optical setup with bulk diffraction gratings instead of AWGs. The experimental results indicate that velocities at different positions can be measured using the proposed configuration. The measured separation of the two measurement positions was about 20.5 mm, about 11% of the working distance.     

频移分离型三维激光多普勒测速仪研究

研究一种采用频移分离方法的新型三维激光多普勒测速仪（３ＤＬＤＶ）。该仪器以单色Ｈｅ－Ｎｅ激光器和单只光电探测器并配合简单的光学发射和接收系统即实现了三维速度分量的同时测量。三个分量的速度信号由声光调制器引入的三种光学频移区分，通过电子滤波器完成信号的相互分离。在光学发射系统设计中采用独特的单轴四光束结构，通过光束组合形成三双光束差动模式的激光多普勒测速仪光。本文中还给出了该仪器的系统设计参数并分析     

Measurement of acoustic particle velocities in enclosed sound field: Assessment of two Laser Doppler Velocimetry measuring systems

The performances of two Laser Doppler Velocimetry (LDV) systems adapted for measuring the acoustic particle velocities are assessed in enclosed sound field. This assessment is performed by comparing the acoustic velocities measured by means of LDV to reference acoustic velocities estimated from sound pressure measurements. The two LDV systems are based on a single optical bench which delivers an optical signal called Doppler signal. The Doppler signal, which is frequency modulated, is analyzed by means of two signal processing systems, the BSA (Burst Spectrum Analyser from Dantec) on the one hand, and a system specifically developed for the estimation of the acoustic velocity on the other hand. Once the experimental setup has been optimized for minimizing the errors made on the reference velocities, the assessment is performed and shows that both systems can measure the acoustic velocity in enclosed field in two the frequency ranges [0-4 kHz] and [0-2 kHz] respectively for acoustic velocity amplitudes of 10 mm/s and 1 mm/s.     

Relativistic effects of laser doppler velocimeters using one light beam with two frequencies

Relativistic effects of laser Doppler velocimeters (LDV's) are discussed and novel LDV systems are proposed. If the direction of the scattered light makes a right angle with the flow direction, relativistic effects completely disappear no matter how high the velocity of a moving particle becomes. The proposed LDV's involve that the velocity can be measured from one scattered light beam with two different single frequencies. It is also predicted that the usual optical heterodyne-detection techniques can be made applicable to measure even ultra-high velocities up to the region where relativistic effects should be taken into account.     

Research on laser Doppler velocimeter for vehicle self-contained inertial navigation system

An idea of using laser Doppler velocimeter (LDV) to measure the velocity for the vehicle inertial navigation system was put forward. The principle of measuring its own velocity with laser Doppler technique was elaborated and reference-beam LDV was designed. Then Doppler signal was processed by tracking filter, frequency spectrum refinement and frequency spectrum correction algorithm. The result of theory and experiment showed that the reference-beam LDV solved the problem that dual-beam LDV cannot be used for measuring when the system was out of focus. Doppler signal was tracked so that signal-to-noise ratio was improved, and the accuracy of the system was enhanced by the technology of frequency spectrum refinement and correction. The measurement mean error was less than 1.5% in velocity range of 0-30 m/s.     

An international review of laser Doppler vibrometry: Making light work of vibration measurement

In 1964, just a few years after the invention of the laser, a fluid velocity measurement based on the frequency shift of scattered light was made and the laser Doppler technique was born. This comprehensive review paper charts advances in the development and applications of laser Doppler vibrometry (LDV) since those first pioneering experiments. Consideration is first given to the challenges that continue to be posed by laser speckle. Scanning LDV is introduced and its significant influence in the field of experimental modal analysis described. Applications in structural health monitoring and MEMS serve to demonstrate LDV's applicability on structures of all sizes. Rotor vibrations and hearing are explored as examples of the classic applications. Applications in acoustics recognise the versatility of LDV as demonstrated by visualisation of sound fields. The paper concludes with thoughts on future developments, using examples of new multi-component and multi-channel instruments.     

固体沿光轴方向的极缓慢速度和微小位移的激光多普勒测量

为了实现目标极缓慢速度和微小位移的实时、准确监测,消除各种干扰对微弱多普勒信号的影响,设计了一套激光多普勒测速(LDV)系统。该系统采用迈克尔逊干涉光路和调幅解调、相敏检测技术,具有较强的抗干扰能力。实验结果表明,该系统可实现固体目标沿光轴方向μm/s级速度和μm级位移的实时、准确测量。该系统光路结构简单,对目标的表面状况无特殊要求,可用于滑坡、地陷等地质灾害的监测,及桥梁、大坝等工程结构变形状况的检测,该方案对相关科学实验和工件精密测量也有重要意义。     

Directional discrimination for fiber-optic non-mechanical scanning laser Doppler velocimeter using single transmission path

We propose a fiber-optic non-mechanical scanning laser Doppler velocimeter (LDV) with directional discrimination of the velocity component using a single transmission path. The LDV uses a polarization maintaining fiber (PMF) as the transmission path between the main body and probe. Two beams with different frequencies induced by acousto-optic modulators (AOMs) are transmitted to the probe as two orthogonal polarization modes of the PMF. The measurement position is axially scanned using gratings in the probe and the change in wavelength. The functions of the scan and directional discrimination are demonstrated experimentally.     

Solid state and fibre optic laser doppler velocimeters

Laser Doppler velocimetry (LDV) has become established as an important technique for the measurement of velocities of macroscopic objects and fluids: the dynamic range is large (～10^-6-10⁵ ms^-1) and the measurement is absolute and non-invasive. However, the size and cost of LDV has restricted its use in some areas. This paper presents two separate approaches to reduce these problems: we describe a compact LDV system incorporating a solid state laser diode and also an investigation of the feasibility of a fibre optic LDV system in which the conventional optical components are replaced by fibre optics.The experimental arrangement used for the solid state LDV system was of the Doppler difference type; i.e. a system of parallel interference fringes is focused in the measurement volume, so that a particle passing through this volume produces a scattered light signal which is intensity modulated. In its simplest form, the technique cannot determine the direction of motion of the particle, but this difficulty may be overcome by causing the fringes to ‘move’ within the measurement volume with known velocity. In the present experiments, the laser output frequency was modulated by modulating its drive current; since the path lengths of the two beams interfering in the measurement volume were unequal, fringe motion was achieved.The fibre optic LDV experiment was also of the Doppler difference type, and it was demonstrated that the necessary stabilised interference fringe system could be projected using a fibre optic system. An electronic servo was devised to compensate for the random differential thermal drifts in the fibres which would otherwise have produced unacceptable drifts in the fringe pattern.     

A comparative study of the PIV and LDV measurements on a self-induced sloshing flow

Particle Imaging Velocimetry (PIV) and Laser Doppler Velocimetry (LDV) measurements on a self-induced sloshing flow in a rectangular tank had been conducted in the present study. The PIV measurement result was compared with LDV measurement result quantitatively in order to evaluate the accuracy level of the PIV measurement. The comparison results show that the PIV and LDV measurement results agree with each other well in general for both mean velocity and fluctuations of the velocity components. The average disagreement level of the mean velocity between PIV and LDV measurement results was found to be within 3% of the target velocity for the PIV system parameter selection. Bigger disagreements between the PIV and LDV measurement results were found to concentrate at high shear regions. The spatial resolution and temporal resolution differences of the PIV and LDV measurements and the limited frames of the PIV instantaneous results were suggested to be the main reasons for the disagreement.     

油中悬浮粒子激光多普勒测速瞬时速度的分析

二维频移激光多普勒测量油中悬浮粒子速度时,根据悬浮粒子通过针孔光阑时存在的四种不同情况,对原有激光多普勒测量悬浮粒子瞬时速度的方法进行了改进,提出一种悬浮粒子瞬时速度分析处理方法,利用该方法,对水平方管内的油中悬浮粒子的瞬时速度进行了测量分析。结果表明:改进方法获取的悬浮粒子瞬时速度的稳定性较好,能够表征在一组粒子通过激光多普勒的针孔光阑的时间段内粒子组随着时间的变化趋势,提高了频移激光多普勒采样数据的利用效率,有利于进一步准确表征悬浮粒子在油液中二维瞬时速度的分布特征和运动轨迹,为激光多普勒实验测量中悬浮粒子瞬时速度的表征提供理论支撑。     

Laser Doppler velocimeter for vehicle application with improved signal-to-noise ratio

Laser Doppler velocimeter (LDV) with improved signal-to-noise ratio (SNR) is presented to measure the vehicle velocity. A novel optical structure named split-reuse type configuration is developed to improve the SNR of LDV. Reference measurements were provided by measuring the outer surface of a precision single-axis turntable. Under these conditions, the measurement uncertainty of the instrument is better than 0.56 % with 95 % confidence interval. The split-reuse type LDV has been tested with an automobile on the open road, and the measurement results are comparing with the results of a precision spatial filter velocimeter, which verify the feasibility of split-reuse type LDV to measure the self-velocity of vehicle.     

The development of an optical Doppler technique for measuring flow velocities

For a generalized scheme of LDV, the expressions describing the light intensity distribution in a photo-receiver plane with regard to the size of the scattering particles have been calculated using Fourier optics.The LDV signal should be considered as a narrow-band stochastic process representing the sum of random phase radio frequency pulses arising from each light scattering particle. Then the expected error of the mean velocity measurement is equal to half the reciprocal of the number of interference fringes squared and the input signal-to-noise ratio is equal to the square root of the product of Doppler frequency, fringe number and averaging time.The device developed by the authors includes d.c. optical signal compensation. The device is described and the results of its use are given.     

Combined dual-beam and reference beam LDV for vehicle inertial navigation system

In order to adapt to the complicated environment of ground for the vehicle, a combined dual-beam and reference beam LDV is presented. It is made up of a single dual-beam subsystem and a single reference beam subsystem. Each optical head detects its own backscattering, and then the signal processing unit chooses one of the signals from the two detectors according to the validity of signal to extract the Doppler frequency and calculate the vehicle's velocity. Measurement results of combined LDV show good agreement with data obtained using DZL-1 electronic speedometer. The relative measurement error is less than 1.5%, which shows the reliability of the technique and potential for it to be applied to inertial navigation system.     

Photopolymer diffractive optical elements in electronic speckle pattern shearing interferometry

In this paper we present an electronic speckle pattern shearing interferometer using a photopolymer diffractive optical element in the form of a holographic grating, in combination with a ground glass to shear the images. The sheared images on the ground glass are further imaged onto a CCD camera. The distance between the grating and the ground glass can be used to control the shear and to vary the sensitivity of the system. The direction of sensitivity is easily controlled by rotation of the diffraction grating around its normal.Introducing photopolymer holographic gratings in ESPSI gives the advantage of using high aperture optical elements at relatively low cost. The fact that the diffractive optical element is a photopolymer layer on glass substrate with thickness of 2 mm makes for a compact optical system.The system was successfully used for detection of the resonant frequencies of a vibrating object.Most of the published work on vibration analysis is analytical. Very few experimental results are available in the literature. The well known laser Doppler vibrometers (LDV) and accelerometers used for modal analysis are pointwise measurement techniques, although multipoint LDV is available at significant cost.Electronic speckle pattern techniques suitable for experimental detection of the resonant frequencies of vibrating objects are very promising for vibration analysis because they are whole field and non-contact.A finite element model is developed for prediction of the vibration modes of the object under test. Detection of vibrational modes of aluminium diaphragm is demonstrated and compared with the theoretical model. The results obtained are very promising for future application of ESPSI systems with HOEs, for modal analysis. A significant advantage of shearography over electronic speckle pattern interferometry is that ESPSI is relatively insensitive to external disturbances. Another advantage of the proposed system is that it could be easily converted to a phase-shifting electronic speckle shearing interferometer.     

Cross-correlation Doppler global velocimetry (CC-DGV)

A flow velocimetry method, cross-correlation Doppler global velocimetry (CC-DGV), is presented as a robust, simplified, and high dynamic range implementation of the Doppler global/planar Doppler velocimetry technique. A sweep of several gigahertz of the vapor absorption spectrum is used for each velocity sample, with signals acquired from both Doppler-shifted scattered light within the flow and a non-Doppler shifted reference beam. Cross-correlation of these signals yields the Doppler shift between them, averaged over the duration of the scan. With presently available equipment, velocities from 0 ms⁻¹ to over 3000 ms⁻¹ can notionally be measured simultaneously, making the technique ideal for high speed flows. The processing routine is shown to be robust against large changes in the vapor pressure of the iodine cell, benefiting performance of the system in facilities where ambient conditions cannot be easily regulated. Validation of the system was performed with measurements of a model wind turbine blade boundary layer made in a 1.83 m by 1.83 m subsonic wind tunnel for which laser Doppler velocimetry (LDV) measurements were acquired alongside the CC-DGV results. CC-DGV uncertainties of ±1.30 ms⁻¹, ±0.64 ms⁻¹, and ±1.11 ms⁻¹ were determined for the orthogonal stream-wise, transverse-horizontal, and transverse-vertical velocity components, and root-mean-square deviations of 2.77 ms⁻¹ and 1.34 ms⁻¹ from the LDV validation results were observed for Reynolds numbers of 1.5 million and 2 million, respectively. Volumetric mean velocity measurements are also presented for a supersonic jet, with velocity uncertainties of ±4.48 ms⁻¹, ±16.93 ms⁻¹, and ±0.50 ms⁻¹ for the orthogonal components, and self-validation done by collapsing the data with a physical scaling.     

Two-axis-scanning laser Doppler vibrometer for precision engineering

Laser Doppler vibrometer (LVD) has been the most favorite instrument for precision dynamics measurement due to its non-contact, high accuracy and high resolution. However, LDV can only give the dynamic data of a particular location on the entire feature. In order to get the whole field data, a laser beam-scanning mechanism has to be implemented. Currently, motor-driven scanning mirror is used to move the measurement probe from one point to another. The mechanical vibrations of the scanning mirror will reduce the measurement accuracy. This paper introduces a novel scanning LDV optical system embodied in an acousto-optic deflector scanning mechanism. It can improve the measurement accuracy since there is no mechanical motion involved. One main advantage of this system is that it generates a laser scanning beam in parallel that is different from the beam scanning in the conventional scanning laser Doppler vibrometer (SLDV). The new system has a board scanning range. The measurement target size ranges from few tens of millimeters down to 10 μm. We have demonstrated the capability of the novel system on scanning measurements of features as big as ultra-precision cutting tool to features as tiny as AFM cantilever. We believe that the novel SLDV will find profound potential applications in the precision engineering field.     

分体式原向反射法水下武器速度测试技术研究

水下动态参数的测试是特种武器、两栖武器、水下专用武器性能考核的必备环节，而水下运动体的速度信息是评价水下武器性能的重要指标之一。针对现有的水下高速目标参数测试系统中存在的成本高、安装调试复杂、设备体积庞大等问题，提出一种以激光光幕为有效区域水上、水下分体式，实时、非接触的测速方法。通过分析Lambert-Beer定律和体散射函数等数学原理，确定了水下光谱传输规律综合考虑性价比获得最佳峰值波长；将1m的圆柱体作为散射体模拟光在水中的散射情况，追迹空间区域内的光线总数为1×105，获得位于传播方向上1，3，5和7 m处的接收面上辐照度的光能量分布，从而获取系统激光光源的最佳峰值功率。以此为依据，采用定距测时原理和一维原向反射技术，由峰值波长为532 nm的半导体光纤耦合绿光激光器、光纤耦合式鲍威尔棱镜防水扩束器、一维原向反射器等构建光学系统。激光光源、光电转换部分和信号调理部分位于水上，激光光幕和原向反射器位于水下，通过光纤束完成两路光信号的发射和反射光的回收。发射端光纤一端与光源耦合，另外一端与鲍威尔棱镜耦合置于水下形成扇形光幕。接收端光纤一端均布于鲍威尔棱镜出口，另一端与PIN型光电传感器耦合。设计齿形一维原向反射器并完成加工制造，光线将沿着入射光方向原向返回，另外一维方向则仍为镜面反射，将接收系统置于发射点垂直光面内附近即可接收大部分光能量，解决了现有原向发射器因水介质折射率不同于空气而导致原向反射特性消失的问题。实验采用波长为(532±5) nm绿光激光器，功率稳定性<1%，光学噪声< 0.5%，准直后耦合至长度为2 m的单模光纤再经过鲍威尔棱镜展宽为60°扇形一字线光幕，扩束模块封装采用尼龙防水材料，接收光纤均布于光源周围形成环形光纤束，光纤另外一端均匀排列与PIN光敏二极管直接耦合。光敏二极管前加中心波长为532 nm的光学滤光片，FWHM=(3±1) nm，透过率为70%。PIN型光敏二极管有效尺寸为5.0 mm×5.0 mm。采用多档可调的光电信号调理电路以适应不同尺寸的测试对象。该系统进行了不同目标速度参数测试实验，以钢弩为发射装置，信号经过光纤回收、信号调理，采集至计算机处理获得波形及区间内平均速度，两激光光幕之间的距离为定值300 mm，波形峰值作为计时时刻。成功获取了较高信噪比的波形信号和目标速度值。利用水下运动体模型与模拟结果进行比较得到其绝对误差。实验结果表明： 本方法结构简单、重复性好，可实现有效区域达到1 m×1 m，最小可测目标尺寸为5 mm，理论测速上限可达1 000 m·s-1，实验数据通过与理论经验公式结果比对表明，系统测试精度可达0.2%。     

基于爆炸火光光谱分析的大当量爆炸场破片速度测试方法研究

为了解决战斗部爆炸过程中,因爆炸物当量较大造成爆燃火球持续时间长,覆盖面积大,近场位置破片速度参数难于获取的问题,提出一种以激光光幕为有效传感区域的光电收发一体的测试方法。通过分析三种不同类型战斗部爆炸火光特征光谱分布可知,在0.3~1.0 μm波段内火光相对光强度较低。以此为依据,采用定距测时原理和原向反射技术,由固体激光器、菲涅尔透镜、窄带滤光器、高速光电传感器等关键光学元件构建破片速度参数获取的光学系统。系统光路收发一体,结构紧凑,窄带滤光片与激光光源配合使用避开火光光谱,有效抑制背景光的干扰。采用该系统进行了不同型号、当量的战斗部静爆破片速度参数测试现场实验,通过美国NI数据采集系统记录数据并对信号进行去噪和识别,成功获取了较高信噪比的波形信号。实验结果表明：本方案可完成爆心10~15 m附近破片速度的准确测试,最小可测破片尺寸为4 mm,获取破片速度可达1 200 m·s-1,与靶板测试结果对比可知捕获率优于95%。由于采用菲涅尔透镜形成矩形光幕,光幕上下的光强分布一致,水平方向光强均匀度达到80%以上,因此系统还可初步区分预制破片速度与尺寸的对应关系。