Laser Doppler velocimetry
Encyclopedia
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.
(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 engine
s 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.
, 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.
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:
One disadvantage has been that LDV sensors are range-dependent; they have to be calibrated minutely and the distances where they measure has to precisely defined. This distance restriction has recently been at least partially overcome with a new sensor that is range independent.
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 cell
s 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 displacement.
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
in a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
beam to measure the velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
in transparent or semi-transparent fluid
Fluid
In physics, a fluid is a substance that continually deforms under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids....
flows, or the linear or vibratory motion of opaque, reflecting, surfaces.
Technology origin
With the development of the helium-neon laserHelium-neon laser
A helium–neon laser or HeNe laser, is a type of gas laser whose gain medium consists of a mixture of helium and neon inside of a small bore capillary tube, usually excited by a DC electrical discharge.- History of HeNe laser development:...
(He-Ne) at the Bell Telephone Laboratories in 1962, the optics community had available a source of continuous wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
electromagnetic radiation
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
highly concentrated at a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of 632.8 nanometers (nm), in the red portion of the visible spectrum
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
. It was soon shown fluid flow measurement could be made from the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
on a He-Ne beam scattered by very small polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
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 engine
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
s 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
Paper mill
A paper mill is a factory devoted to making paper from vegetable fibres such as wood pulp, old rags and other ingredients using a Fourdrinier machine or other type of paper machine.- History :...
and steel
Steel mill
A steel mill or steelworks is an industrial plant for the manufacture of steel.Steel is an alloy of iron and carbon. It is produced in a two-stage process. First, iron ore is reduced or smelted with coke and limestone in a blast furnace, producing molten iron which is either cast into pig iron or...
mills, to measuring vibration frequency and amplitude of surfaces.
Operating principles
In its simplest and most presently used form, LDV crosses two beams of collimatedCollimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...
, monochromatic, and coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
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
Ion laser
An ion laser is a gas laser which uses an ionized gas as its lasing medium.Like other gas lasers, ion lasers feature a sealed cavity containing the laser medium and mirrors forming a Fabry–Pérot resonator. Unlike HeNe lasers, the energy level transitions that contribute to laser action come from ions...
, or laser diode
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
), 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
Photodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
(typically an avalanche photodiode
Avalanche photodiode
An avalanche photodiode is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint,...
).
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
Optical heterodyne detection
Optical heterodyne detection is an important special case of heterodyne detection. In heterodyne detection, a signal of interest at some frequency is non-linearly mixed with a reference "local oscillator" that is set at a close-by frequency...
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
Laser Doppler Vibrometer
A laser Doppler vibrometer is a scientific instrument that is used to make non-contact vibration measurements of a surface. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam...
, also abbreviated LDV.
Applications
In the decades since the LDV was first introduced, there has been a wide variety of laser Doppler sensors developed and applied.Flow Research
Laser Doppler velocimetry is often chosen over other forms of flow measurementFlow measurement
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways.Positive-displacement flow meters acumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow...
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 precisely defined. This distance restriction has recently been at least partially overcome with a new sensor that is range independent.
Medical Applications
Laser Doppler velocimetry is used in hemodynamicsHemodynamics
Hemodynamics, meaning literally "blood movement" is the study of blood flow or the circulation.All animal cells require oxygen for the conversion of carbohydrates, fats and proteins into carbon dioxide , water and energy in a process known as aerobic respiration...
research as a technique to partially quantify blood flow
Blood flow
Blood flow is the continuous running of blood in the cardiovascular system.The human body is made up of several processes all carrying out various functions. We have the gastrointestinal system which aids the digestion and the absorption of food...
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
Laser diode
The laser diode is a laser where the active medium is a semiconductor similar to that found in a light-emitting diode. The most common type of laser diode is formed from a p-n junction and powered by injected electric current...
) penetrates the skin sufficiently to be scattered with a Doppler shift by the red blood cell
Red blood cell
Red blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
s 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
Vascular
Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....
areas.
The laser Doppler vibrometer is being used in clinical otology
Otology
Otology is a branch of biomedicine which studies normal and pathological anatomy and physiology of the ear as well as its diseases, diagnosis and treatment....
for the measurement of tympanic membrane
Eardrum
The eardrum, or tympanic membrane, is a thin membrane that separates the external ear from the middle ear in humans and other tetrapods. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles...
(eardrum), malleus
Malleus
The malleus or hammer is a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum...
(hammer), and prosthesis
Prosthesis
In medicine, a prosthesis, prosthetic, or prosthetic limb is an artificial device extension that replaces a missing body part. It is part of the field of biomechatronics, the science of using mechanical devices with human muscle, skeleton, and nervous systems to assist or enhance motor control...
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 displacement.
External links
See also
- Particle tracking velocimetryParticle tracking velocimetryParticle tracking velocimetry is a velocimetry method, i.e a technique to measure velocity of particles. The name suggests that the particles are tracked, and not only recorded as an image as it is suggested in another form, particle image velocimetry...
- Particle image velocimetryParticle image velocimetryParticle image velocimetry is an optical method of flow visualization used in education and research. It is used to obtain instantaneous velocity measurements and related properties in fluids...
- Hot-wire anemometry
- Molecular tagging velocimetryMolecular tagging velocimetryMolecular Tagging Velocimetry is a specific form of flow velocimetry, a technique for determining the velocity of currents in fluids such as air and water. In its simplest form, a single "write" laser beam is shot once through the sample space...
- Photon Doppler velocimetryPhoton Doppler velocimetryPhoton Doppler velocimetry is a one-dimensional Fourier transform analysis of a heterodyne laser interferometry. It was developed at Lawrence Livermore National Laboratory by Strand. In recent years PDV has achieved popularity in the shock physics community as an adjunct or replacement for VISAR...
- Laser surface velocimeterLaser surface velocimeterA laser surface velocimeter is a non-contact optical sensor measuring velocity and length on moving surfaces. Laser surface velocimeters use the laser Doppler principle to evaluate the laser light scattered back from a moving object...