Laser Doppler velocimetry
Laser Doppler velocimetry (LDV), also known as laser Doppler anemometry (LDA), is the technique of using the Doppler shift in a laser beam to measure the velocity in transparent or semi-transparent fluid flows, or the linear or vibratory motion of opaque, reflecting, surfaces. The measurement with LDA is absolute, linear with velocity and requires no pre-calibration.
With the development of the helium–neon laser (He-Ne) at the Bell Telephone Laboratories in 1962, the optics community had available a source of continuous wave electromagnetic radiation highly concentrated at a wavelength of 632.8 nanometers (nm), in the red portion of the visible spectrum. It was soon shown fluid flow measurement could be made from the Doppler effect on a He-Ne beam scattered by very small polystyrene spheres entrained in the fluid.
At the Research Laboratories of Brown Engineering Company (later Teledyne Brown Engineering), this phenomenon was used in developing the first laser Doppler flowmeter using heterodyne signal processing.
The instrument was soon called the laser Doppler velocimeter (LDV) and the technique laser Doppler velocimetry, also abbreviated LDV. Another application name is laser Doppler anemometry (LDA). Early LDV applications ranged from measuring and mapping the exhaust from rocket engines with speeds up to 1000 m/s to determining flow in a near-surface blood artery. A variety of similar instruments were developed for solid-surface monitoring, with applications ranging from measuring product speeds in production lines of paper and steel mills, to measuring vibration frequency and amplitude of surfaces.
In its simplest and most presently used form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the fluid being measured. The two beams are usually obtained by splitting a single beam, thus ensuring coherence between the two. Lasers with wavelengths in the visible spectrum (390–750 nm) are commonly used; these are typically He-Ne, Argon ion, or laser diode, allowing the beam path to be observed. A transmitting optics focuses the beams to intersect at their waists (the focal point of a laser beam), where they interfere and generate a set of straight fringes. As particles (either naturally occurring or induced) entrained in the fluid pass through the fringes, they reflect light that is then collected by a receiving optics and focused on a photodetector (typically an avalanche photodiode).
The reflected light fluctuates in intensity, the frequency of which is equivalent to the Doppler shift between the incident and scattered light, and is thus proportional to the component of particle velocity which lies in the plane of two laser beams. If the sensor is aligned to the flow such that the fringes are perpendicular to the flow direction, the electrical signal from the photodetector will then be proportional to the full particle velocity. By combining three devices (e.g.; He-Ne, Argon ion, and laser diode) with different wavelengths, all three flow velocity components can be simultaneously measured.
Another form of LDV, particularly used in early device developments, has a completely different approach akin to an interferometer. The sensor also splits the laser beam into two parts; one (the measurement beam) is focused into the flow and the second (the reference beam) passes outside the flow. A receiving optics provides a path that intersects the measurement beam, forming a small volume. Particles passing through this volume will scatter light from the measurement beam with a Doppler shift; a portion of this light is collected by the receiving optics and transferred to the photodetector. The reference beam is also sent to the photodetector where optical heterodyne detection produces an electrical signal proportional to the Doppler shift, by which the particle velocity component perpendicular to the plane of the beams can be determined.
Similar arrangements using optical heterodyning are also used in laser Doppler sensors for measuring the linear velocity of solids and for measuring vibrations of surfaces; the latter sensor is usually called a laser Doppler vibrometer, also abbreviated LDV.
In the decades since the LDV was first introduced, there has been a wide variety of laser Doppler sensors developed and applied.
Laser Doppler velocimetry is often chosen over other forms of flow measurement because the equipment can be outside of the flow being measured and therefore has no effect on the flow. Some typical applications include the following:
- Wind tunnel velocity experiments for testing aerodynamics of aircraft, missiles, cars, trucks, trains, and buildings and other structures
- Velocity measurements in water flows (research in general hydrodynamics, ship hull design, rotating machinery, pipe flows, channel flow, etc.).
- Fuel injection and spray research where there is a need to measure velocities inside engines or through nozzles
- Environmental research (combustion research, wave dynamics, coastal engineering, tidal modeling, river hydrology, etc.).
One disadvantage has been that LDV sensors are range-dependent; they have to be calibrated minutely and the distances where they measure has to be precisely defined. This distance restriction has recently been at least partially overcome with a new sensor that is range independent.
Laser Doppler velocimetry can be useful in automation, which includes the flow examples above. It can also be used to measure the speed of solid objects, like conveyor belts. This can be useful in situations where attaching a rotary encoder (or a different mechanical speed measurement device) to the conveyor belt is impossible or impractical.
Laser Doppler velocimetry is used in hemodynamics research as a technique to partially quantify blood flow in human tissues such as skin. Within the clinical environment, the technology is often referred to as laser Doppler flowmetry (LDF). The beam from a low-power laser (usually a laser diode) penetrates the skin sufficiently to be scattered with a Doppler shift by the red blood cells and return to be concentrated on a detector. These measurements are useful to monitor the effect of exercise, drug treatments, environmental, or physical manipulations on targeted micro-sized vascular areas.
The laser Doppler vibrometer is being used in clinical otology for the measurement of tympanic membrane (eardrum), malleus (hammer), and prosthesis head displacement in response to sound inputs of 80- to 100-dB sound-pressure level. It also has potential use in the operating room to perform measurements of prosthesis and stapes (stirrup) displacement.
Vibration and acoustics
Laser Doppler velocimetry is effective in measuring surface vibrations via reflection of the laser light from the vibrating surface. The technology, adapted to include a scanning capability (to provide measurement of the vibration over an array of points), has been used to measure vibration generation and propagation for ultrasonic motors and acoustic and ultrasonic microfluidics. Remarkably, it is possible to measure the deformation of capillary waves as well using a laser Doppler vibrometer.
Laser Doppler velocimetry has been employed in laser computer mouse. The advantages include low power consumption, and the ability to track on most surfaces, including featureless surface such as glass, where traditional image-sensor based optical mouse fails to estimate motion.
The Autonomous Landing Hazard Avoidance Technology used in NASA's Project Morpheus lunar lander to automatically find a safe landing place contains a lidar Doppler velocimeter that measures the vehicle's altitude and velocity. The AGM-129 ACM cruise missile uses laser doppler velocimeter for precise terminal guidance.
- Particle tracking velocimetry
- Particle image velocimetry
- Hot-wire anemometry
- Molecular tagging velocimetry
- Photon Doppler velocimetry
- Laser surface velocimeter
- Velocity Interferometer System for Any Reflector (VISAR)
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