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.     

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.     

Chirp heterodyne laser Doppler velocimeter using a powerful fibre-coupled green laser

The chirp heterodyne technique allows a directional discrimination of laser Doppler velocity measurements without the use of additional frequency shift elements. This paper presents, for the first time, the use of a chirp frequency-modulated solid-state laser, emitting in the green spectral range. Experimental results of a fluid flow measurement verify the function of the realised chirp heterodyne laser Doppler velocimeter.     

Simultaneous measurement of flow velocity and Doppler angle by the use of Doppler optical coherence tomography

Monitoring blood flow velocity could have great value for biomedical research and clinical diagnostics. One of current restrictions to determine flow velocity by the use of Doppler optical coherence tomography (Doppler OCT) is that the Doppler angle should be predefined. However, from a practical point of view, it is not easy to predetermine Doppler angle for a flow beneath the tissue surface. In this work, a novel method for measuring both flow velocity and Doppler angle simultaneously by the use of Doppler OCT is proposed and demonstrated. Based on Doppler spectrum analysis, this technique measures both longitudinal and transverse components of flow velocity by detecting its Doppler shift and Doppler bandwidth to determine velocity and Doppler angle simultaneously. Such a technique extends flow velocity measurement into a broadening practical use of Doppler OCT where Doppler angle would not need to be predefined, for example, blood flow beneath the tissue surface. Therefore, with this technique, Doppler OCT could be applied to more practical diagnoses of microcirculation.     

A phase-stepped grating technique for frequency shifting in laser Doppler velocimetry

We demonstrate a novel approach to the moving grating technique for removal of velocity ambiguity in a laser Doppler velocimeter. The method replaces the continuous motion of a moving grating with a grating which can be positioned or switched to three distinct phase positions. Advantages of this approach are its potential for compact implementation and high stability of the velocity offset.     

Research on heterodyne efficiency of laser Doppler velocimeter

Heterodyne efficiency is a very important factor in a laser Doppler velocimeter (LDV). Gaussian-Airy mode is put forward to analyze the heterodyne efficiency. And the calculate formulas and results of simulation of heterodyne efficiency are given. The results of numerical analysis show that heterodyne efficiency of LDV depends on the parameters—x₀, A, θ and x_a. Heterodyne efficiency can reach 81.45%, when x₀ = 3.59, A = 5.04 and θ = x_a = 0. At last, the antenna theorem of LDV is derived, and a defocused telescope system is used to adjust the waist's position and radius of the reference beam to match the signal beam.     

Laser Doppler velocimeter using a single longitudinal mode solid-state laser source

An experimental demonstration of using a single longitudinal mode solid-state laser source in laser Doppler velocimeter (LDV) is presented. The technology of frequency spectrum correction is used in processing Doppler signal. The results of the experiments show that: the magnitude and signal-to-noise ratio (SNR) of Doppler signal are both enhanced by the solid-state laser; the measurement accuracy of LDV is improved by the technology of frequency spectrum correction, and the variance of the measured Doppler frequency is larger than the Cramer-Rao low bound (CRLB) of Doppler frequency about one order of magnitude.     

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.     

Flow characterization using a laser Doppler vibrometer

This paper shows how a scanning laser Doppler vibrometer (LDV), an instrument designed to measure vibrations of structures or objects, can be used in a non-traditional fashion to measure the flow around a cylinder. In particular the von Karmann vortex street which appears in the cylinder wake will be visualized. This is achieved by measuring the changes in the optical path induced by local fluctuation of air refraction index to which the laser vibrometer is sensitive.The measurements obtained with the LDV will be compared visually to measurements done with particle image velocimetry and also with CFD computations for one test case. The specific frequency of the Von Karmann street predicted by the vibrometer, will be compared numerically to the other techniques for two different sized cylinders at three different velocities.     

Signal processing of double-beam and double-scattering laser Doppler velocimeter

The common method of laser Doppler velocimeter (LDV) signal processing is fast Fourier transform (FFT). Because of the restraint of spectral interference, fence effect and spectrum leakage, the precision of FFT is not high. In order to improve the accuracy of double-beam and double-scattering laser Doppler velocimeter signal processing, an algorithm based on spectral refinement and interpolation is put forward. First, high-pass filter is adopted to remove the baseplate of LDV signal. Second, FFT is adopted to get the estimate of Doppler frequency and then Zoom-FFT is used to refine the local range of Doppler frequency. Third, four-term fifth derivative Nuttall windowed interpolation is used to correct the refined spectrum. Then the accurate estimate of Doppler frequency can be obtained. Simulations and experiments based on the LDV system built in laboratory show that Zoom-FFT can improve the resolution of spectrum and four-term fifth derivative Nuttall windowed interpolation can further increase the precision of signal processing.     

基于FPGA的激光测速仪信号处理方案设计与实现

为满足高动态环境下的激光多普勒测速仪信号处理需要,提出了一种基于现场可编程门阵列（FPGA）的激光测速仪信号处理方案。在FPGA内部完成全部信号处理的内容,利用快速傅里叶变换（FFT）算法得到信号的频谱,利用能量重心法对离散频谱进行校正,开发采样频率自适应算法,兼顾测量准确度与测量范围的要求,最后将结果通过通用串行总线上传个人计算机显示。程序采用流水线方式设计,提高信号处理速度。经过实验验证,数据更新率达到2.4~24 kHz, 数据延迟时间为123~1230 s,测量准确度优于810-4,测量稳定度优于2.510-7。     

Laser Doppler velocimeter for remote measurement of polluted water and aerosols discharges

A continuous wave Doppler velocimeter has been developed for remote estimation of the volume of polluted water and gaseous discharges using an actively stabilized ring dye laser. Doppler shifted backscattered signals induced by fluids in motion using heterodyne detection technique has been applied for measurement of Doppler shifts. Polluted water and gaseous discharge flow rates have been measured remotely for simulation purposes. The system is capable of measuring Doppler shifts as low as 0.25 MHz corresponding to 0.1 m/s velocities of the fluids.     

相位调制激光多普勒频移测量方法的改进

本文对现有相位调制激光多普勒频移测量方法进行了改进,通过定义新的鉴频参量来同时利用相位调制信号直流和交流分量中的有用信息进行多普勒频移测量.由于相位调制信号直流分量中包含着调制信号光的Fabry-Perot干涉仪光强透过率,所以这一改进本质上是将基于Fabry-Perot干涉仪的边缘技术激光多普勒频移测量方法的优势引入到相位调制测量方法中,以提高其自身的性能.理论上证明改进后的相位调制激光多普勒频移测量方法无需对信号光的光强进行测量,所以可以进一步简化探测系统的结构和较少噪声混入的通道.另外,通过对改进前后鉴频和测量灵敏度曲线进行对比,还证明了其具有更高的测量灵敏度和动态范围.实验上对硬目标反射的频移可控信号光进行测量,不但证明了理论的正确性,而且证明了改进后的相位调制激光多普勒频移测量方法,测量动态范围提高约1倍,测量标准偏差降低约35%.     

Influence of laser irradiation on the optical and structural properties of poly(ethylene terephthalate) fibres

Laser irradiation has been previously investigated for achieving uniform heating of polyethylene terephthalate (PET) fibres in the hot-drawing stage of the production process, so as to obtain better fibre mechanical properties. The optical properties and dye uptake of PET fibres also depend on the polymer chain orientation and crystallinity within the fibre structure. This paper reports an investigation of a concept whereby laser irradiation and interferometry could be used to modify and trace a small change in the optical properties of a PET monofilament fibre, but the corresponding change in the dye uptake would not be detected visually. A copper vapour laser (550-580 nm wavelengths) was used to expose consecutive 4 mm lengths along a running length of monofilament to 39.8 W cm⁻², at a pulse rate of 9.89 kHz in order to modify, in a controlled way, the polymer crystallinity and orientation. A 3D finite element simulation, based on uncoupled heat-transfer analysis, indicated that rapid heating and cooling could be obtained with the laser to give the small changes required. Irradiated and untreated samples were analysed by interferometry and a 0.16% change was detected in the birefringence profiles, corresponding to a small reduction in the degree of orientation and crystallinity of the irradiated samples. Density measurements and wide-angle X-ray scattering (WAXS) analysis confirmed the change in crystallinity. Tests conducted for dye adsorption and tensile strength showed a small increase in the former and only a very small decrease in the latter. It was concluded that these changes in property provide the opportunity for a laser-irradiated PET monofilament fibre to be used as a subtle tracer element in brand labels for textile garments as an anti-counterfeit measure.     

Correction of quadratic phase error in two-wavelength digital holographic interferometry using laser diodes

We present the correction of a quadratic phase error in two-wavelength digital holographic interferometry using laser diodes. This phase error arises from numerical reconstructions of wavefronts from digital holograms based on the Fresnel diffraction integral. To correct the quadratic phase error, it is numerically produced by computer on the basis of the theoretical prediction and is subtracted from the phase difference map in two-wavelength digital holographic interferometry. Experimental results show that the method of correction in this paper is useful for two-wavelength digital holographic interferometry using laser diodes.     

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.     

Multi-wavelength laser sensor for intruder detection and discrimination

An intruder detection and discrimination sensor with improved optical design is developed using lasers of different wavelengths to demonstrate the concept of discrimination over a distance of 6 m. A distinctive feature of optics is used to provide additional transverse laser beam scanning. The sample objects used to demonstrate the concept of discrimination over a distance of 6 m are leaf, bark, black fabric, PVC, wood and camouflage material. A camouflage material is chosen to illustrate the discrimination capability of the sensor. The sensor utilizes a five-wavelength laser combination module, which sequentially emits identically-polarized laser light beams along one optical path. A cylindrical quasi-optical cavity with improved optical design generates multiple laser light beams for each laser. The intensities of the reflected light beams from the different spots are detected using a high speed area scan image sensor. Object discrimination and detection is based on analyzing the Gaussian profile of reflected light at the different wavelengths. The discrimination between selected objects is accomplished by calculating four different slopes from the objects' reflectance spectra at the wavelengths 473 nm, 532 nm, 635 nm, 670 nm and 785 nm. Furthermore, the camouflage material, which has complex patterns within a single sample, is also detected and discriminated over a 6 m range by scanning the laser beam spots along the transverse direction.     

Research on speckle noise of laser Doppler velocimeter for the vehicle self-contained navigation

A novel multipoint layer-type laser Doppler velocimeter (MLLDV) is designed to measure the velocity of a vehicle for the self-contained navigation system. In order to investigate the speckle's influence on the Doppler spectrum, formulas of time-lagged covariance and speckle broadening were derived for our MLLDV. Simulations and experiments are made for detailed analysis. The results show that the time-lagged covariance of photocurrent is directly proportional to the incident angle, and is inversely proportional to the elevation fluctuation of the ground together with the velocity of the vehicle. Speckle broadening is a function of the vehicle's velocity, the 1/e² Gaussian spot radius and the phase correlation length of the ground. For our MLLDV, Doppler frequency and Doppler broadening are both directly proportional to the velocity of the vehicle. Besides, the ratio between Doppler broadening and the corresponding Doppler frequency is about 0.72% when the speed of the vehicle varies from 0 to 9.6 m/s.     

Correction and correlation formation of probing field in fiber laser Doppler anemometers and vibrometers

In this work we offer a method of field formation with given spatial features at the output of multimode fiber sensors using interference phenomena. The method consists of a field two-stage correlation transformation at a fiber output. Transformation is realized by a phase modulator and a hologram. The developed method is practically realized in the fiber Doppler anemometer and vibrometer scheme. Features of sensors are discussed in this work.     

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。