面内位移测量的激光多普勒信号研究

在利用激光多普勒效应进行远距离处固体面内位移测量时，散射光的多普勒信号强度、信噪比及信号质量是位移测量的极重要参量，它们直接影响位移测量的距离和精度。为此研究了激光多普勒差动方法、锁相放大、数字滤波等技术，设计出能获得最好结果的差动光路以及高灵敏度、高Q值的锁相放大器和能重构丢失信号的同态滤波系统等，实现了50m处固体面内位移测量，其最大相对误差可以达到1.5％。     

Measurements of the velocity gradient using a laser Doppler phenomenon

This paper reports on a method for measuring the velocity gradient using a laser Doppler phenomenon. The velocity gradient is determined from the velocity difference between two different points of the probing object and is actually obtained from the optical heterodyning of two differently Doppler-shifted scattered light fields from two points of the object having different velocities. The properties of output beat signals are theoretically investigated from their spectral broadening including the effect of the detecting aperture. The preliminary experiment was performed by using a rotating glass disk whose velocity is different from its center to the outside. The experimental results shows the usefulness of the present method for measurements of the velocity gradient.     

Studies of the dynamic behaviour of charged dust particles in high electric fields using laser Doppler velocimetry

As a preliminary study for the collection of dust particles in an electrostatic precipitator, laser Doppler velocimetry has been used to measure the velocity of small dust particles charged in the high electric field of a model electrostatic precipitator. A differential optical system is adopted for the velocimeter. The period-measuring system is used to analyse Doppler beat signals and to obtain velocity data from them. By means of the laser Doppler velocimeter, the dynamic behaviour of the velocity and direction of moving dust particles is fully revealed as a function of the applied electric voltage in the entire collecting space of the model electrostatic precipitator, and is shown in vectorial flow diagrams of particle velocity.     

全光纤速度干涉仪设计及应用

针对冲击波物理与爆轰物理等研究领域中对高速运动物体进行连续速度测量的需求,设计了一种全光纤速度干涉仪.该干涉仪采用单模光纤作为光传输和延迟元件,对t和t-τ两个时刻由于速度变化而引起的多普勒差拍信号进行检测.由于两个时刻的两束光信号对应的待测物体速度变化不大,因而两者几乎有相等的频移量,从而大大降低了差拍信号频率.并且,通过光纤长度的改变,灵活调节条纹常量（τ值）,使差拍信号频率不超过记录系统的带宽,从原理上解决记录系统响应带宽受限问题,拓展测速的上限.单模光纤的采用,对漫反射光起到了较好的选模作用,使干涉仪实现了对漫反射靶的测量.实验设计了1.5 m·s-1和150 m·s-1两种条纹常量,对低速过程的霍普金森杆实验和高速过程的激光驱动实验分别进行了测试,取得较好结果,证明了该干涉测试技术的有效性.     

Extrinsic fibre-optic sensors for remote measurement: Part two

The impact of incorporating fibre optic components into well established optical techniques such as laser Doppler velocimetry, non-contact vibration measurement, holography and light scattering, is discussed in the two sections of this paper. The possibilities range from simply including a fibre ‘optic link’ in a conventional system to facilitate measurements in difficult or remote areas, to the realization of entirely new optical configurations in which virtually all of the conventional optical components have been replaced by their fibre-optic equivalents. In part two laser velocimetry, vibration measurement, holography and scattering are discussed.     

光栅位移激光多普勒遥测技术的研究

研究了利用运动光栅的多普勒效应进行切向位移的遥测技术。提出了测量光路，采用声光调制器作为分光和频移器件并用于鉴向，实现了５０ｍ的远距离光栅位移遥测。达到测量分辨率０．８μｍ，光栅位移４０ｍｍ，最大累积误差优于３４μｍ。并用理论分析和实验，证明了远距离目标的焦深及其计算方法，导出了计算公式。     

Its optical and signal-analysing systems and some experimental results of flow velocity

A laser Doppler microscope, consisting of optical and signal-analysing systems, has been developed. Flow velocity can be measured with an extremely high spatial resolution over an area of up to 10μm in diameter. The optical system consists of a microscope having two types of penetrative and reflective systems. On the basis of the characteristic analysis of Doppler beat signals from the laser Doppler microscope, the signal-analysing system, which uses a tracking filter together with a digital-hold element and a time-to-pulse height converter, has been found to be the most suitable for analysing beat signals. Some experimental results obtained using the laser Dopller microscope for measuring flow velocities, both in a small square cell in a venule, and in a capillary over the web of a frog's foot are presented.     

Particle imaging sizing: GLMT simulations

Defocused off-axis images of particles are computed in the framework of the generalized Lorenz-Mie theory. Two exemplifying cases are studied: interferometric sizing at large off-axis angles and imaging in near forward directions (shadow Doppler velocimetry)     

Generalized Theory for the Simultaneous Measurement of Particle Size and Velocity using laser doppler and laser two-focus methods

Essentially the laser two-focus (L2F) and laser Doppler anemometer (LDA) are time-of-flight anemometers. The velocity of particles in the micrometre range is determined within the limits of an optically fixed measurement distance. For this only one optoelectronic receiver for the detection of the scattered light is required [1, 2]. With an arrangement of two receiving optics, positioned under an off-axis angle φ and an elevation angle ± ψ to the optical axis of the measurement system, a resolution of three-dimensional structures can be achieved and with regard to spherical particles it is possible to determine the dimensions by means of the temporal or phase shift of the signals in the receiving optics. A particle size-dependent distance inside the measurement volume can be fixed, which has to be passed in order to change the signal from one receiving optics to the other. An LDA with an arrangement of two receiving optics for particle sizing is known as a phase Doppler anemometer (PDA); an L2F designed with two receiving optics can be termed a pulse-displacement two-focus anemometer (P2F). The physical analysis of the two methods with respect to a temporal signal displacement in the receivers yielded new results.     

水洞流场PIV显示与分析系统离轴测试校正

粒子图像测速技术(PIV)通过测量被测流场截面上每一位置点的速度,获得整个被测流场的信息.在PIV一般应用中所使用的照明激光片光与成像CCD装置的拍摄方向是垂直的,在某些应用场合受测试条件的限制,需要采用离轴方式进行测量,此时CCD成像方向与照明的激光片光不垂直,而是有一定夹角.离轴测试方式将对PIV系统的光学成像系统、示踪粒子选择和粒子图像处理带来影响.实验采用Scheimpflug离轴聚焦的方法对表面镀银高反射率的示踪粒子进行成像,通过调整成像透镜与CCD像面的夹角可获得清晰的粒子成像,并利用网格校正板和软件计算处理等方法有效校正了由于离轴测试带来的影响.     

Developments in laser Doppler accelerometry (LDAc) and comparison with laser Doppler velocimetry

This paper outlines the principles and early development of an interferometric technique for remote measurement of vibration acceleration — laser Doppler accelerometry (LDAc). One of the key advantages of LDAc over laser Doppler velocimetry (LDV) is its ability to measure extremely high vibration accelerations and shocks, effectively without limit, and this point is expanded upon in the paper. Early LDAc development showed how unwanted, velocity-dependent optical beats could occur on the photodetector but novel use of a frequency shifting device, whose primary purpose was for direction discrimination, was successful in isolating the required acceleration-dependent beat. A problem remained in the rate at which the velocity-dependent and acceleration-dependent beats broadened during target motion. In a further development, it was possible to 'select’ a back reflection to produce a velocity-dependent beat that was NOT modulated in the presence of target motion. The acceleration-dependent beat could then be demodulated and preliminary results are given to demonstrate this outcome.     

Zero-crossing probability density of Doppler beat signals in the wave-period measuring system

As an effect of shot noise on the analysis of Doppler beat signals in the wave-period measuring system, the zero-crossing probability density of beat signals is studied emphasizing two parameters, namely the bandwidth of a pre-filter and the signal-to-noise ratio.     

Generation of longitudinal modes in an active Fabry-Perot fiber interferometer pumped by a low-power single-frequency semiconductor laser

The results of experiments on taking beat spectra at the exit of a stationary (modulator-free) fiber Mach-Zender interferometer are presented. The interferometer is excited by a single-frequency semicon-ductor laser with an output power varying from 0.65 to 2.00 mV and a line-width of less than 20 kHz. The observed line beat spectrum is explained based on the fact that the extended (several kilometers long) arm of the interferometer serves as an active fiber Fabry-Perot interferometer. In this case, the effects of enhancement of stimulated Mandel’shtam-Brillouin scattering in the fiber and the Fresnel reflection of light from the end faces of the fiber play a decisive role. Remarkably, these effects are observed at a low (less than 0.3 mW) power deposited into the fiber. The features of beat signals are considered, and the feasibility of applying this effect in high-sensitive field and displacement optical sensors is indicated.     

Optical correlation techniques in fluid dynamics

Three flow measurement techniques make use of fast digital correlators. The most widely spread is photon correlation velocimetry using crossed laser beams, and detecting Doppler shifted light scattered by small particles in the flow. Depending on the processing of the photon correlation output, this technique yields mean velocity, turbulence level, and even the detailed probability distribution of one velocity component. An improved data processing scheme is demonstrated on laminar vortex flow in a curved channel. In the second method, rate correlation based upon threshold crossings of a high pass filtered laser Doppler signal can be used to obtain velocity correlation functions. The most powerful set-up developed in our laboratory uses a phase locked loop type tracker and a multibit correlator to analyze time-dependent Taylor vortex flow. With two optical systems and trackers, cross-correlation functions reveal phase relations between different vortices. The last method makes use of refractive index fluctuations (eg in two phase flows) instead of scattering particles. Interferometry with bidirectional counting, and digital correlation and probability analysis, constitutes a new quantitative technique related to classical Schlieren methods. Measurements on a mixing flow of heated and cold air contribute new ideas to the theory of turbulent random phase screens.     

Doppler velocimetry in a supersonic jet by use of frequency-modulated filtered light scattering

A new nonintrusive velocimetry diagnostic that combines the sensitivity of frequency-modulated (FM) absorption spectroscopy techniques and the spectral discrimination afforded by atomic-vapor absorption filters is presented. Doppler-shifted light from a FM Ti:sapphire laser scattered from a supersonic flow is imaged through a potassium-vapor cell and is detected by FM spectroscopy. The difference between the potassium resonance frequency and the laser frequency when the scattered light is in resonance is the flow-induced Doppler shift.     

Doppler velocimetry using self-mixing effect in a short Er–Yb-doped phosphate glass fiber laser

Accurate and highly sensitive speed measurements have been successfully demonstrated by the optical feedback velocimetry technique using the self-mixing modulation effect in a double-clad Er–Yb-doped fiber laser. The sensitivity to back-scattered light has been investigated regarding the Doppler frequency shift or the target distance, and it has been shown that the velocimeter is still sensitive to a target located at 20 m and for speeds as high as 13 m s^–1.     

基于裸光纤探头的双点光子多普勒测速系统

全光纤光子多普勒速度测量(PDV)系统是一种新型的激光测速系统,可广泛用于冲击波、爆轰波以及其他短时高速运动物体的速度测量。多点测量可以获得靶面不同位置的速度,以测量靶面的形变。为提高测量的空间分辨率,提出使用裸光纤束为PDV系统的探头,并在实验上实现了空间分辨率为375μm的双点速度测量。裸光纤探头的间距较小,一个探头的测量结果可能受到另一个探头反射光的干扰。理论和实验的研究结果表明,当靶面各点速度相同时,测量结果不受干扰光的影响;当各点速度不同时,其测速误差不但与两被测点的速度差有关,还与传感光和干扰光的光强和相位有关。     

基于迈克耳孙干涉仪的激光多普勒实验仪及其实验

通过对迈克耳孙干涉仪动镜的改装和机械加工,设计了激光多普勒效应实验装置.使用该装置可以监测弱振动,测量谐振动的振幅、扬声器的灵敏性能常量,也可以测量低速运动物体的运动速度.通过示波器观察光拍现象,可以帮助学生更好地理解多普勒效应,了解激光多普勒效应的应用.     

Intensification and location of relativistic laser pulses against pointing fluctuations with micro-tube plasma lenses

We analyze, via an off-axis incident model and three-dimensional particle-in-cell simulations, the influence of beam pointing fluctuations (BPFs) on the propagation properties of relativistic laser pulses in micro-tubes. It is observed that the in-tube laser intensity can be further amplified in the BPFs-induced off-axis incident case. But the intensification factor exhibits strong polarization-dependence. When the laser pulse is linearly polarized in the off-axis incident plane (p-polarization), more electrons may be dragged out from about a half of the tube inner surface in each across section than in the on-axis incident case, enhancing the effects of relativistic nonlinearity and channel focusing. The area for generating more electrons is reduced by one half in the s-polarization case, resulting in less efficient light intensification. The BPFs-induced off-axis incident also leads to pulse shortening. Moreover, light confinement in the tube core is evident and the laser pulse tends to be coaxial with the tube.     

Adaptive beamforming for photoacoustic imaging

An adaptive photoacoustic image reconstruction technique that combines coherence factor (CF) weighting and the minimum variance (MV) method is introduced. The backprojection method is widely used to reconstruct photoacoustic tomography images. Owing to the scattering of light, the quality of the photoacoustic imaging can be degraded. CF, an adaptive weighting technique, is known to improve the lateral resolution of photoacoustic images. In addition, an MV adaptive beamforming method can further improve the image quality by suppressing signals from off-axis directions. Experimental studies are performed to quantify the spatial resolution and contrast of the adaptive photoacoustic beamforming methods.