Laser Doppler velocimeter with variable optical frequency shift

An opto-electronic system that allows a stable accurate frequency shift, variable from 10 kHz to 50 MHz, to be introduced in a laser Doppler velocimeter (LDV) has been developed. The system combines a fixed optical-frequency shift obtained using a Bragg cell with an electronic shift which results from mixing the detector signal with a variable local oscillator frequency phase locked to the Bragg cell driver. This paper discusses the advantages of the system in both a tracker and a counter based LDV.     

The application of an optical Doppler frequency discriminator in a laser velocimeter

A laser velocimeter is described which uses an optical Doppler frequency discriminator in the form of a spherical mirror confocal Fabry-Perot interferometer (SFPI). The SFPI pass band is made to coincide with the laser frequency using a system of external control. Experimental results are given.     

Laser speckle velocimeter using a pair of virtually shifting photodiodes

A new method for measuring the magnitude and direction of the velocity of a diffuse object is proposed. The method is based on the Doppler effect using the virtual shift of a pair of photodetectors placed on a plane in the far-field of the speckle pattern.     

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.     

基于TMS320VC5402的激光多普勒测速系统

介绍了激光多普勒测量技术的原理和差动后向散射测量方式以及数字信号处理式激光多普勒测速仪的设计方案.其中包括基于TMS320VC5402的多普勒信号采集卡硬件电路和软件设计.运用该测速仪对固体表面运动速度进行测量.对采集到的多普勒信号进行高通滤波处理,以提高信噪比,运用傅里叶变换处理数据,计算出运动目标的速度并与实际速度进行了比较.结果表明该系统用于固体表面速度的测量切实可行,精度可达到2%.     

Laser beam deflection by frequency shifting

A new type of beam scanner has been developed which utilizes frequency shifting of a laser beam followed by deflection of the beam in a dispersive element. Theoretical considerations involve the maximization of the frequency shift and number of resolvable spots. An experimental deflector has been constructed which employs a mode-locked 0.633 μm He-Ne laser, lithium-niobate frequency shifter, and a Fabry-Perot dispersion element. Scans of 15 resolvable spots have been obtained with a theoretical access time of 18.5 ns.     

A fourier optical model for the laser Doppler velocimeter

The treatment is based on a fourier optical model. It is shown how the various configurations (i.e. differential mode and reference beam mode with both one and two incident beams) are incorporated in the model, and how it can be extended to three dimensions.The particles are represented by a sum of weighted delta functions of random position with respect to their influence on the scalar field.The influence of the receiver configuration is investigated: it is shown that the aperture size is roughly inversely proportional to the effective area in the measuring plane from which beams are heterodyned on the detector.The purpose of the pinhole, namely to filter the measuring volume out from the rest of space, is shown to depend on the aperture size. The diameter of the pinhole should never be smaller than the real image- found by geometrical optics- of the measuring volume (naturally), but its filtering ability vanishes as the aperture size converges towards zero.The results based on fourier optics are compared with the rough estimates obtainable by using the antenna formula for heterodyning (A_rr ²).     

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.     

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.     

Some effects of optical alignment errors in a differential laser Doppler velocimeter

A formulation has been developed to treat the effects of alignment errors of the optical system used in a differential laser Doppler velocimeter. It is then applied to analyse the properties of output beat signals obtained by the velocimeter. The effects of optical alignment errors on the spectral width and signal-to-noise ratio of the output beat signals and on the probing volume are investigated in some detail. The spectral width is not affected very much by the deviation of the angle between two beams incident on the probing volume, but depends mainly on the number of interference fringes produced in that volume. The signal-to-noise ratio is very sensitive to the displacement of two incident beams on the probing volume and is also affected by the intensity and beam width differences of these two beams. The probing volume is evaluated in the case of two incident beams having different beam widths.     

Flat optical frequency comb generation using polarization modulator and frequency shifter with double recirculating frequency shifting loops

A scheme to generate ultra-flat and stable optical frequency comb (OFC) based on multi-wavelength optical seed source and polarization-modulator-based complementary frequency shifter (PCFS) has been proposed and demonstrated. The OFC generation scheme includes two stages: 5-carrier light source is generated by utilizing a polarization modulator at the first stage, and the light source is then used as the seed light in the PCFS at the second stage. Compared with the previous flat OFC generator based on a recirculation frequency shifter, the proposed scheme can greatly reduce the recirculation times, and such a property makes the accumulated noise of the system enormously decrease. In order to achieve the frequency comb with the best flatness, the optical amplifier gain, filter stop-band attenuation and modulator drive voltage are optimized. Experiment simulations show that high-quality OFC with hundreds of frequency lines and power fluctuation within 2 dB can be achieved without direct-current bias control and wavelength-selective-switch.     

High-precision optical phase-locking based on wideband acousto-optical frequency shifting

We present a high-precision optical phase-locking based on wideband acousto-optical frequency shifting.Increasing the modulating bandwidth stabilizes the loop at a high loop gain,thus improving phase correction capability.An optical phase-locked loop with a wide control bandwidth is constructed.The closed-loop residual phase error is only 0.26°or smaller than λ/1000.The loop exhibits excellent correction capability for high-frequency noises.The correctable frequency range reaches 35 kHz when the noise amplitude is ±λ/2,and becomes even wider for smaller noise amplitudes.     

Turbulent transport measurements with a laser Doppler velocimeter

The power spectrum of phototube current from a laser Doppler velocimeter operating in the heterodyne mode has been computed. The spectrum is obtained in terms of the space time correlation function, G(r,), of the fluid. The spectral width and shape predicted by the theory is in agreement with experiment. For normal operating parameters the time average spectrum contains information only for times shorter than the Lagrangian integral time scale of the turbulence. To examine the long time behaviour one must use either extremely small scattering angles, much longer wave-length radiation or a different mode of signal analysis, e.g. F.M. detection.     

Laser frequency stabilization and shifting by using modulation transfer spectroscopy

The stabilizing and shifting of laser frequency are very important for the interaction between the laser and atoms. The modulation transfer spectroscopy for the87 Rb atom with D2 line transition F = 2 → F = 3 is used for stabilizing and shifting the frequency of the external cavity grating feedback diode laser. The resonant phase modulator with electro–optical effect is used to generate frequency sideband to lock the laser frequency. In the locking scheme, circularly polarized pump- and probe-beams are used. By optimizing the temperature of the vapor, the pump- and probe-beam intensity, the laser linewidth of 280 kHz is obtained. Furthermore, the magnetic field generated by a solenoid is added into the system. Therefore the system can achieve the frequency locking at any point in a range of hundreds of megahertz frequency shifting with very low power loss.     

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.     

激光多普勒测速仪中散射光特性的研究

后向散射光特性与多普勒信号的质量密切相关。为了设计高性能的激光多普勒测速仪,运用散斑理论详细分析了激光多普勒信号的强度与散射光斑大小的关系,结合泛函理论给出了计算多普勒电流的散斑表达式,并通过实验的方法研究了回波信号的偏振特性及其强度分布。理论分析与实验结果表明,激光多普勒信号的强度与接收器件光敏面的直径成正比,与光斑的直径成反比;选用光斑较小的圆偏振激光束,并用光敏面尺寸较小的探测器在镜面反射方向上接收信号光,可以大大提高多普勒信号的信噪比,增强系统的探测能力,为提高系统的测量精度创造有利的条件。     

A Bessel beam laser Doppler velocimeter

We present a laser Doppler velocimeter that is based on the use of two intersecting Bessel beams instead of the conventionally used Gaussian beams. Due to the quasi non-divergent nature of the Bessel beams, a short effective measurement volume of about 40 μm length could be realized with a low fringe spacing variation of 0.3%. Both of these advantageous values were experimentally verified in flow measurements.     

Dual-polarization fiber grating laser-based laser Doppler velocimeter

A dual-frequency laser Doppler velocimeter implemented by a dual-polarization fiber grating laser is proposed,with the two laser frequencies produced by the two orthogonally polarized laser outputs of the fiber grating laser.The reflected laser outputs from a moving target experience the Doppler frequency shift,which is shown to be linearly related to the velocity and the beat note frequency difference between the laser outputs and the reflected light. With a digital frequency demodulation scheme,a high sensitivity of 0.64 MHz/(m/s) and a velocity resolution of less than 0.5% of the velocity for velocity measurement are demonstrated,which shows that the proposed laser Doppler velocimeter is capable of measurement of wide range of velocity.     

Theoretical evaluation of a shadow Doppler velocimeter

The shadow Doppler velocimeter (SDV) allows one to measure the velocity and size of non-spherical and optically non-homogeneous particles with high temporal and spatial resolution. The technique is based on coherent, near forward off-axis imaging of the particles. This paper is devoted to the rigorous evaluation of the image formation of a spherical particle illuminated by two continuous laser beams and to define the theoretical limits and the sizing accuracy of the SDV approach.     

参考光束型激光多普勒测速仪的误差分析

阐述了参考光束型激光多普勒测速仪的基本原理,从理论上分析了原理公式近似、高斯光束干涉、激光线宽、探测器孔径、有限渡越时间、信号处理算法、空气折射率变化及角度测量等诸多因素对激光多普勒测速仪测量结果造成的误差,研究并给出了每一种影响因素所引起误差的理论公式,在每一种影响因素中具体讨论了各种光学参数对测量误差的影响并对部分参数进行实验研究。理论分析和实验表明：在实验中需要选择线宽较窄的激光器及孔径较小的光电探测器,而在选择光束光斑半径和信号光中心光线的方向与粒子运动方向的夹角时,既要考虑激光的质量和多普勒信号的强度,同时也需要考虑原理公式近似、探测器孔径的尺寸、有限渡越时间及高斯光束干涉等多种因素对测量结果的影响。