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Laser-Based Velocimetry and Velocimetry Systems for Velocity Measurements

Laser-based velocimetry systems for velocity measurements are safe, lightweight and easily scaleable to high energy per pulse or continuous wave operation. Utilizing telecom lasers with superior single mode spectrum stability and frequency stability provides the optimal measurement solution.

A velocimeter measures object or wind velocity by collecting Doppler-shifted signals from scattered light in its path and cross-correlating them with signals received from non-Doppler shifted reference beams. Velocities along multiple noncolinear axes can be determined simultaneously.

Optical system

Laser Doppler Velocimetry (LDV) is an nonintrusive technique for measuring fluid flow velocities. LDV uses Doppler effect scatter light to detect tracer particles moving with the flow, making accurate measurement independent from flow conditions such as anemometry which requires calibration instruments; LDV is independent from flow conditions so measurements may be conducted without calibration – an advantage over anemometry which requires calibrated instruments if accuracy is desired; typical applications of laser Doppler Velocimetry include measuring speed of conveyor belts or mechanical devices where attaching rotary encoders is impractical;

An optical system must be used to generate and collect scattered light, consisting of a laser source, transmission optics (Bragg cell, lenses, mirrors, beam expanders etc.), receiver, and light detector such as photomultiplier. To obtain accurate measurements with this type of setup it must produce sufficient time delay between transmitted and received signals as well as be capable of transmitting through suitable volumes such as flow chambers or optical path length analyzers.

LDV is just one method for measuring the velocity of flowing liquids; there are other techniques as well, including laser-induced thermal acoustic imaging (LITA), which uses a continuous wave probe beam to excite soundwaves which are then detected by a broadband sensor; or heterodyne non-resonant laser-induced acoustics, which uses interference between two beams to produce counterpropagating soundwaves; both approaches are simple enough for application even with liquids with high levels of turbulence.

There are various systems available for measuring the velocities of fluids and solids, such as laser Doppler velocimetry (LDV). These systems can be classified by whether they measure only one component or multiple components simultaneously and by their spatial resolution; to accurately quantify complex three-dimensional fluid flows it is preferable that LDV measure all three orthogonal components at once.

TSI provides several preconfigured one-, two- and three-component (1D, 2D or 3D) laser Doppler velocimetry systems using our PowerSight solid state laser. Our systems contain everything required for immediate deployment: solid state laser module with transmission/receiving optics as well as control electronics – including individual modules which may be integrated with existing equipment or used together as one complete package.


Laser surface velocimeters are non-contact velocity measurement devices that utilize Doppler effect to evaluate light scattered from moving objects or surfaces, such as planes. This device relies on laser light and reflective spectrometer, and offers highly accurate, reliable, and versatile analysis for many different fluid systems – global wind measurements, pollution assessments, V/STOL flow rates and true airspeed measurements; detection of aircraft wake vortices, dust devils or water spouts may all be measured using this device.

Three-dimensional laser Doppler velocimetry (LDV) configurations are highly sought after features in LDV technology as they measure all three orthogonal velocity components of turbulent flows directly. Fuel efficiency and environmental issues have contributed to the creation of new three-dimensional LDV configurations suitable for turbomachinery applications.

A typical 3D LDV configuration begins by seeding the system with tracer particles that act as optical inhomogeneities, followed by optimal flow conditions where they move without slip and are detected by interference patterns formed at the intersection of two crossing coherent laser beams. Doppler frequency of these interference patterns gives us particle velocity which can then be used to calculate three orthogonal velocity components.

The spectrometer features mirrors and an attenuator to filter out reflected laser signal, while permitting transmitted laser signal through. After passing through these components, the transmitted laser signal enters an avalanche diode through beam splitter polarizer filter conversion which converts it to an electronic signal which then gets amplified by power amplifier before being sent directly to transceiver modules.

The output signal’s amplitude relates directly to the velocity of tracer particles, as determined by multiplying their Doppler frequency and fringe spacing. It can be likened to that produced by hot wire anemometers used for measuring on-axis turbulent flows’ turbulent power measurements; its accuracy depends on how many sensors measure its intensity.


Laser-based velocimetry is a technique for measuring the velocity of fluid or solid objects. The basic principle behind it is directing a beam of light at the flow, and then monitoring any scattered or reflected light that returns through an optical fiber to a photomultiplier tube (PMT) output amplitude which directly corresponds with intensity of scattered or reflected light detected; frequency is related to Doppler effect caused by movement within the flow.

Conventional LDVs use Doppler frequency shift to measure an object’s velocity. For this, small particles with neutral buoyancy must be mixed into its flow so as to scatter light back out into its path and cause Doppler frequency shifts that are then measured by LDVs.

Heterodyne non-resonant laser-induced thermal acoustics (LITA) is a seedless equivalent of LDV which measures velocity along a single component. No molecular or particulate seeds are added into this system – rather, two beams from a short-pulse pump laser are crossed together, producing interference that produces counterpropagating sound waves which are then detected with a spectrometer.

Laser velocimeters use optical fibers to transmit and receive all optical signals, both transmission and reception. An amplifier amplifies a source laser before sending it onward to one or more transceiver modules, each designed as both transmission device and receiving system for receiving radiation from target regions; their power amplifiers are placed away from lenses to reduce nonlinear optic effects on signal transmission and reception.

Laser-based velocimeters use an optical system consisting of a lens group and beam conversion filter to transmit and convert signals from their original forms into analog or digital forms before being processed by digitizer and processor to measure and control flow rates.

Laser velocimetry systems offering accurate and precise measurements even at lower velocities combine high-speed digitizing with heterodyne-based interferometry for accurate and precise measurements, even at low velocities. Thanks to technological advancements, modern velocimeters now boast smaller designs while simultaneously increasing accuracy, data handling capability and sensitivity across a range of industrial applications.


Laser Doppler Velocimetry (LDV) is an optical velocity measurement technique that utilizes light waves to detect movement of fluid or solid objects. LDV can be applied for various applications including flow control and automation, industrial process measurement, biomedical engineering and more. The system relies on Doppler scattering light from particles within a flow. LDV offers relatively inexpensive noninvasive data with a high resolution but low noise rating.

LDV technology works by passing two beams of collimated monochromatic laser light through a measurement volume, separated by an optical beam splitter to allow observation of their paths. When they intersect at some point in the measurement volume they create an interference pattern consisting of straight lines or fringes which reflect tracer particles moving through it and scatter laser light modulated with their Doppler frequency, which allows us to calculate their speed by measuring time intervals between successive fringes.

Laser Doppler velocimeters offer several key advantages over conventional measurements: none require adding particles or foreign matter into the fluid being measured, making it particularly suitable for viscous, dense or toxic liquids. Their use is relatively straightforward: their system comprises a laser source and detector linked to a digital signal processor which then outputs its results onto computer screens as images or videos for analysis or stored as files for future processing or analysis.

There are various kinds of laser Doppler velocimeters. One common variety is known as a backscatter laser Doppler lidar (BLDL). This sensor measures backscatter of moving objects within fluid to detect their velocity and direction, making them a more economical option than an interferometer while still offering spatial resolution.

Planar Doppler Velocimetry (PDV), using the Doppler effect, measures instantaneous three-dimensional velocity vectors within its field of view using laser light sheets scattered with light scattering aerosol. Suitable for facilities of any size; provided it can easily inject light scattering aerosol into flow. TSI offers preconfigured one, two and three component (2D or 3D) laser Doppler velocimetry systems using their Powersight solid state lasers.
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