Laser Doppler Velocimeters Research Articles

Measurement range expansion of single-beam Laser Doppler velocimeter based on a focusing transmitter

The own velocity of a ground vehicle with high precision provided by the single-beam Laser Doppler velocimeters (LDV) promotes the localization accuracy of vehicles efficiently. However, the present LDVs are unable to be mounted on air vehicles for the velocity measurement of vehicles so far because of the restricted working distance. In order to expand the measuring range, a mid-distance LDV is designed in this paper. An optical focusing transmitter that transforms the Gaussian beam waist to the pre-set distance with an appropriate radius is constructed through theoretical simulations. Further experiments show that the mid-distance velocimeter can measure the velocity of moving objects within 50 m and the signal quality of measurement holds at a high level. This improved single-beam LDV with a longer working distance has great potential in acquiring the high-precision velocity of low-altitude air vehicles and many other remote velocity measurement applications.

Simultaneous range and velocity measurement using a frequency shifted feedback laser

Laser Doppler velocimeters (LDVs) are non-contact sensors for measuring velocity, and generally used for measuring length of steel products. However, the distance range of the measurement target is restricted to a certain region in principle, making it difficult to measure the velocity of steel products with large distance fluctuations. To remove this restriction, a frequency-shifted feedback laser range and velocity meter (FSFL-RVM) was developed with an FSFL whose frequency changes proportionally with time at ultra-high speeds. This new sensor can measure the velocity without this restriction and additionally measure the distance without contact. The measurement accuracy was evaluated experimentally, and its measurement principle was verified. As a result, it was confirmed that its ability to measure velocities with the same accuracy as the LDV in a distance range five times that of the LDV and simultaneously measure distances within an accuracy of 0.1 mm.

Simple and accurate calibration system for Laser Doppler Velocimeters

The calibration of Laser Doppler Velocimeters (LDVs) is of a great importance for the accurate measurement of the fluid flow rate and the speed of moving objects. In this paper, a simple, low-cost and portable calibration system for LDV is introduced. This system, which is based on a commercial optical chopper, can offer calibration uncertainty of (0.048%) (σ = 2) for velocities up to 1914 m/min and dynamic lengths of up to 15.4 km, which is better than the uncertainties reported by similar systems with a much complicated design. Therefore, this calibration system can be implemented easily by any calibration laboratory or even by national metrology institutes.

Accelerating space-charge gratings in wide-bandgap semiconductors

The excitation of the non-steady-state photoelectromotive force and two-wave mixing signals is theoretically investigated for the case of uniformly accelerated motion of the recording light pattern. Such illumination is usually created by the linear frequency modulation of the interfering light beams. For the both effects we predict the pulsed response arising at the moments, when the velocities of the interference pattern and space-charge wave coincide. The application of the effects in laser Doppler velocimeters and accelerometers is discussed.

Detection of phase- and frequency-modulated optical signals using dynamic space-charge gratings in wide-bandgap crystals

The excitation of the non-steady-state photoelectromotive force and two-wave mixing signals is theoretically investigated for the case of uniformly accelerated motion of the recording light pattern. Such illumination is usually created by the linear frequency modulation of the interfering light beams. For the both effects we predict the pulsed response arising at the moments, when the velocities of the interference pattern and space-charge wave coincide. The application of the effects in laser Doppler velocimeters and accelerometers is discussed as well.We also study the excitation of the non-steady-state photoelectromotive force in semiinsulating SiC crystals illuminated by a phase-modulated light signal. Influence of an external ac electric field and shallow traps on the signal is revealed. Photoelectric parameters of the material and sensitivity of the detectors are estimated for the light wavelength λ=532nm.

Deformation of zirconium – niobium alloy E635 in sub-microsecond shock waves

Strength characteristics of zirconium - niobium alloy E635 were measured under shock - wave loading conditions at normal and elevated temperatures and results of these measurements are presented. Measurements were taken in conditions when samples were impacted by plane shock waves with the pressure up to 13 GPa and duration from ∼0.05 μs up to 1 μs. Free-surface velocity profiles were recorded with the help of VISAR and PDV laser Doppler velocimeters having nanosecond time resolution. Evolution of elastic precursors with samples thickness varying from 0.5 up to 8 mm is also considered. Measured attenuation of the elastic precursor was used to determine plastic strain rate behind the precursor front. Temperature effect on the value of dynamic elastic limit and spall strength at normal and elevated temperatures is studied. This work is implemented with the support of the State Atomic Energy Corporation “Rosatom” under State Contract H.4x.44.90.13.1111.

Non-steady-state photoelectromotive force and two-wave mixing in photorefractive crystals under frequency modulated illumination

We report the excitation of non-steady-state photoelectromotive force and two-wave mixing signals using uniformly accelerated motion of the recording light pattern. Such illumination is created by linear frequency modulation of the interfering light beams. The pulse response is predicted theoretically and observed experimentally in GaAs and Bi12TiO20 crystals at λ=633  nm. The evolution of the pulse shape versus sweep rate is demonstrated and explained in the frames of the developed theory. The application of the effects in laser Doppler velocimeters and accelerometers is discussed as well.

Non-steady-state photo-EMF in semi-insulating GaAs under frequency-modulated illumination

We report the excitation of the non-steady-state photoelectromotive force using a uniformly accelerated motion of the recording light pattern. Such illumination is created by the linear frequency modulation of the interfering light beams. The pulsed response is observed experimentally in a semi-insulating GaAs crystal at a light wavelength of 633 nm. The evolution of the pulse shape vs. sweep rate is demonstrated. We also discuss the possible application of the effect in laser Doppler velocimeters and accelerometers.

Experimental modal analysis using a tracking interferometer

Experimental modal analysis is carried out for machine tools and in various other industries. It is used for dynamic characterization, diagnostics, condition monitoring, design optimization and as comparison data for computation results. 3D-scanning laser Doppler velocimeters and three-axial accelerometers dominate the experimental modal analysis today. This paper presents a method for modal analysis using a tracking interferometer as well for geometric measurement as for simultaneous 3D-vibration measurement with a single laser beam. A holistic measurement procedure is developed, its performance and robustness against external influences is investigated and a benchmark against accelerometers is carried out at a machining center.

(Invited) Low-Temperature Bonding Technologies for Photonics Applications

This paper focuses on low temperature bonding technology and its applications in photonics devices in use in various fields. Fabrication of a wafer bonded two-layer germanium (Ge) structure for far-infrared detector application was demonstrated by surface activated bonding (SAB) method at room temperature. Very low-resistive Ge/Ge junctions were obtained by SAB method. The feasibility of low-temperature 3-D integration of optical semiconductor chips was also demonstrated by Au-Au SAB in ambient air. Using this technique, compact and thin micro encoders and laser Doppler velocimeters (2.8 mm × 2.8 mm × 1 mm thick) were developed.

Laser Doppler Velocimetry Using Integrated Waveguide Technology

Integrated laser Doppler velocimeters (LDVs) using integrated waveguide technology are reviewed. LDVs have been widely used to measure the velocity of a fluid flow or rigid object in various research and industries. However, bulk optical systems used in conventional measurement need special care from temperature change and vibration, which become the expected sources of uncertainty in measurement, due to large optical path length. Therefore, the optical system should be more compact. The LDV can be drastically reduced by using a planar lightwave circuit (PLC). The possibility for realizing several types of novel integrated LDVs, including a wavelength-insensitive LDV, a scanning LDV and a multi-point LDV, is discussed.

Cortical blood flow assessment with frequency-domain laser Doppler microscopy

We report the assessment of cerebral blood flow (CBF) changes with a wide-field laser Doppler imager based on a CCD camera detection scheme, in vivo, in mice. The setup enables the acquisition of data in minimally invasive conditions. In contrast with conventional laser Doppler velocimeters and imagers, the Doppler signature of moving scatterers is measured in the frequency domain, by detuning a heterodyne optical detection. The quadratic mean of the measured frequency shift is used as an indicator of CBF. We observe a significant variability of this indicator in an experiment designed to induce blood flow changes.

Convection Velocity of Vortex Structures in the Near Wake of a Circular Cylinder

The convection velocity of vortex structures in the near wake of a circular cylinder was experimentally investigated over the region 1.6-2.5⩽x/D⩽12.0 for R=160-12,000. Dye injection technique of flow visualization and two completely noninvasive laser Doppler velocimeters were employed for R⩽320 and ⩾400, respectively. The convection velocity, Uc, is defined as the mean traveling velocity of vortex cores passing a streamwise separation during a mean elapsed time. For R⩽320,Uc was determined directly from the motion of dye-marked vortex cores filmed by a video camera. In the cases of R⩾400, the positions of peak vorticity and half of the half-velocity-defect width at each downstream section were first used to identify the mean path of vortex cores (i.e., the most probable trajectory of the vortex structures), along which spatial correlation measurements were then performed to determine the mean elapsed time corresponding to the maximum cross correlation. The present results show that, in laminar and transitional wakes, the ratio Uc/Uo increases from 0.53 to 0.84 over a region of 1.6⩽x/D⩽6.0 and then tends to be a constant of 0.84 for x/D⩾6.0. In a turbulent wake, Uc/Uo also increases from a certain value at a point downstream from the position of vortex formation to a mean value of about 0.86 at x/D⩾5.0-6.0, and then changes little with the increase of x/D. In addition, it is found that the dependence of Uc/Uo on R almost disappears for x/D⩾5.0.

A miniaturized dual-fibre laser Doppler sensor

The integration of laser Doppler velocimeters into small components such aswind tunnel models requires highly miniaturized sensors. This paperpresents a novel miniaturization concept, suitable for reducing thedimensions of a fibre-coupled laser Doppler sensor to a millimetre scale.Only a few low-cost, passive elements such as two fibres, a diffractiongrating and refractive optics are employed. Preliminary results of arealized sensor head, of about 7 mm diameter and about 15 mm length, arepresented. The generated measurement volume of about 0.02 mm diameter andabout 0.1 mm length allows highly spatially-resolved velocity measurements.The outstanding features of the sensor are its simplicity, ruggedness and thehigh optical power of about 0.5 W in the measurement volume.

Statistical frequency measuring error of the quadrature demodulationtechnique for noisy single-tone pulse signals

The standard deviation of the centre frequency of a signal isinvestigated for the quadrature demodulation technique (QDT). Signalfrequencies can be measured by QDT unaffected by the amount of availablesignal periods. It can be used for the measurement of nonstationary signalsgenerated by laser Doppler velocimeters. The dependence of the frequencymeasuring error on the averaging time of noisy single-tone Gaussian pulsesignals is analysed. Assuming a quantum noise process, it is shown that theminimum measuring error results for an averaging time of approximately1/ e2 of the pulse duration of the signal. Alternatively, definedweighting of the measured values leads to a monotonically decreasingmeasuring error with increasing averaging time until the Cramer-Rao lowerbound is reached. Therefore, the weighted QDT provides the lowest measuringerror of all linear unbiased frequency estimators. It allows the evaluationof small frequency changes of laser Doppler signals, e.g. frommicro-turbulent flows. The theory presented here is verified by Monte Carlosimulations and experiments.

The influence of operating conditions on the accuracy of in-plane laser Doppler velocimetry measurements

The use of laser Doppler velocimeters in the analysis of in-plane motion of a solid body is spreading both in the scientific research and in the industrial experimentation fields. The applicability and accuracy of this measurement technique depend on the conditions in which the system operates: the level of the signal available, the characteristics of the surface observed, the environmental conditions, the presence of other shifts in addition to the ones pointed out, etc. This is the subject of this analysis carried out through a comparison between an axial vibrometer in conditions of ideal operation and a tangential vibrometer operating on various surface states. The experimental results have been supported by a theoretic modelling of the interaction between this measuring system and the measurand, thereby providing a rational description of the phenomena observed. This study has made it possible to verify the interfering inputs due to the various operative conditions and the chance of extending the use of the measurement system also to cases in which the applications of the technique proves to be difficult.

Development and Uncertainty Analysis of a Laser Doppler Velocimeter Calibrator for the Flow Velocity Standard.

A calibrator of laser Doppler velocimeters (LDVs) has been developed as the measurement standard of flow velocity. This calibrator has a rotor with single fine wire and generates various velocities from the rotation. To minimize the influence of wire deformation, the orbit diameter in movement was measured with an interferometer as the distance of a rotor traversing. A measurement control volume of tested LDV was used to detect the wire orbit position. The calibration factor of tested LDV was decided as a ratio of wire velocity to the average frequency of the Doppler burst signal. In this paper, the uncertainty of LDV calibration with this calibrator was analyzed and unknown sources were quantified by the experiment. The maximum uncertainty source was pitching of the carriage table, which would be reduced by remodeling of the devices. The combined standard uncertainty of the calibration was about 100 ppm in the velocity range of 2 to 30 m/.

B244 砕波帯内の流速場の測定

There are, until now, many attempts to measure the velocity fields contained a large amount, of air bubble under any breaking waves. Unfortunately, the attempts refused because of the existing of air bubble. In latest studies, electric magnetic current meters or laser-Doppler velocimeters have been used for measurement of complex velocity fields. The new and powerful current meters are very difficult, to measure the water flow, if a lot of air bubbles are mixed in.Using a super-miniature proper current meter, the authors can disclose nott only the distribution of the mean flow velocity but the coherent structures of turbulence flow. under breaking waves.

Geometry of the fringe field formed in the intersection of two Gaussian beams

A general expression is obtained for the exact computation of the fringe field in the intersection volume of two paraxial Gaussian beams for arbitrary beam waist positions and sizes. The expression is then simplified to allow easy fringe field computation while retaining good accuracy. By relating the simplified expression to the system parameters relevant to dual-beam laser Doppler velocimeters, simple design equations are obtained. These equations permit rapid evaluation of the fringe field variation along the major and minor axes of the intersection volume and clearly identify the system parameters controlling this variation.

Method for analysis of the fundamental measuring uncertainty of laser Doppler velocimeters

A method is presented for analysis of the relationship of the velocity measuring uncertainty and the system parameters of laser Doppler velocimeters. The method permits optimization of the laser Doppler system to minimize the uncertainty of fluid velocity measurements.