Laser Doppler Vibrometer
Encyclopedia
A laser Doppler vibrometer (LDV) 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 frequency due to the motion of the surface. The output of an LDV is generally a continuous analog voltage that is directly proportional to the target velocity component along the direction of the laser beam.
Some advantages of an LDV over similar measurement devices such as an accelerometer
are that the LDV can be directed at targets that are difficult to access, or that may be too small or too hot to attach a physical transducer
. Also, the LDV makes the vibration measurement without mass-loading the target, which is especially important for MEMS
devices.
, although laser diodes, fiber lasers, and Nd:YAG lasers are also used. The test beam is directed to the target, and scattered light from the target is collected and interfered with the reference beam on a photodetector
, typically a photodiode
. Most commercial vibrometers work in a heterodyne
regime by adding a known frequency shift (typically 30–40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg cell, or acousto-optic modulator.
A schematic of a typical laser vibrometer is shown below. The beam from the laser, which has a frequency fo, is divided into a reference beam and a test beam with a beamsplitter. The test beam then passes through the Bragg cell, which adds a frequency shift fb. This frequency shifted beam then is directed to the target. The motion of the target adds a Doppler shift to the beam given by fd = 2*v(t)*cos(α)/λ, where v(t) is the velocity of the target as a function of time, α is the angle between the laser beam and the velocity vector, and λ is the wavelength of the light.
Light scatters from the target in all directions, but some portion of the light is collected by the LDV and reflected by the beamsplitter to the photodetector. This light has a frequency equal to fo + fb + fd. This scattered light is combined with the reference beam at the photo-detector. The initial frequency of the laser is very high (> 1014 Hz), which is higher than the response of the detector. The detector does respond, however, to the beat
frequency between the two beams, which is at fb + fd (typically in the tens of MHz range).
The output of the photodetector is a standard frequency modulated (FM) signal, with the Bragg cell frequency as the carrier frequency, and the Doppler shift as the modulation frequency. This signal can be demodulated to derive the velocity vs. time of the vibrating target.
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
measurements of a surface. The 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 from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler
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...
shift of the laser beam frequency due to the motion of the surface. The output of an LDV is generally a continuous analog voltage that is directly proportional to the target velocity component along the direction of the laser beam.
Some advantages of an LDV over similar measurement devices such as an accelerometer
Accelerometer
An accelerometer is a device that measures proper acceleration, also called the four-acceleration. This is not necessarily the same as the coordinate acceleration , but is rather the type of acceleration associated with the phenomenon of weight experienced by a test mass that resides in the frame...
are that the LDV can be directed at targets that are difficult to access, or that may be too small or too hot to attach a physical transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
. Also, the LDV makes the vibration measurement without mass-loading the target, which is especially important for MEMS
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
devices.
Principles of operation
A vibrometer is generally a two beam laser interferometer that measures the frequency (or phase) difference between an internal reference beam and a test beam. The most common type of laser in an LDV is 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:...
, although laser diodes, fiber lasers, and Nd:YAG lasers are also used. The test beam is directed to the target, and scattered light from the target is collected and interfered with the reference beam 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 a photodiode
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
. Most commercial vibrometers work in a heterodyne
Heterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...
regime by adding a known frequency shift (typically 30–40 MHz) to one of the beams. This frequency shift is usually generated by a Bragg cell, or acousto-optic modulator.
A schematic of a typical laser vibrometer is shown below. The beam from the laser, which has a frequency fo, is divided into a reference beam and a test beam with a beamsplitter. The test beam then passes through the Bragg cell, which adds a frequency shift fb. This frequency shifted beam then is directed to the target. The motion of the target adds a Doppler shift to the beam given by fd = 2*v(t)*cos(α)/λ, where v(t) is the velocity of the target as a function of time, α is the angle between the laser beam and the velocity vector, and λ is the wavelength of the light.
Beat (music)
The beat is the basic unit of time in music, the pulse of the mensural level . In popular use, the beat can refer to a variety of related concepts including: tempo, meter, rhythm and groove...
frequency between the two beams, which is at fb + fd (typically in the tens of MHz range).
The output of the photodetector is a standard frequency modulated (FM) signal, with the Bragg cell frequency as the carrier frequency, and the Doppler shift as the modulation frequency. This signal can be demodulated to derive the velocity vs. time of the vibrating target.
Applications
LDVs are used in a wide variety of scientific, industrial, and medical applications. Some examples are provided below:- Aerospace – LDVs are being used as tools in non-destructive inspection of aircraft components.
- Acoustic – LDVs are standard tools for speaker design, and have also been used to diagnose the performance of musical instruments.
- Architectural – LDVs are being used for bridge and structure vibration tests.
- Automotive – LDVs have been used extensively in many automotive applications, such as structural dynamics, brake diagnostics, and quantification of Noise, vibration, and harshnessNoise, Vibration, and HarshnessNoise, vibration, and harshness , also known as noise and vibration , is the study and modification of the noise and vibration characteristics of vehicles, particularly cars and trucks...
(NVH). - Biological – LDVs have been used for diverse applications such as eardrum diagnostics and insect communication.
- Calibration – Since LDVs measure motion that can be calibrated directly to the wavelength of light, they are frequently used to calibrate other types of transducers.
- Hard Disk Drive Diagnostics – LDVs have been used extensively in the analysis of hard disk drives, specifically in the area of head positioning.
- Landmine detection – LDVs have shown great promise in the detection of buried landmines. The technique uses an audio source such as a loudspeaker to excite the ground, causing the ground to vibrate a very small amount with the LDV used to measure the amplitude of the ground vibrationsGround vibrationsGround vibrations is a technical term that is being used to describe mostly man-made vibrations of the ground, in contrast to natural vibrations of the Earth studied by seismology. For example, vibrations caused by explosions, construction works, railway and road transport, etc - all belong to...
. Areas above a buried mine show an enhanced ground velocity at the resonance frequency of the mine-soil system. Mine detection with single-beam scanning LDVs, an array of LDVs, and multi-beam LDVs has been demonstrated.
Types of Laser Doppler Vibrometers
- Single-point vibrometers – This is the most common type of LDV. Vendors include Sunny Instruments Singapore, Polytec, MetroLaser, B&K, Brimrose and Piezojena.
- Scanning vibrometers – A scanning LDV adds a set of X-Y scanning mirrors, allowing the single laser beam to be moved across the surface of interest.
- 3-D vibrometers – A standard LDV measures the velocity of the target along the direction of the laser beam. To measure all three components of the target's velocity, a 3-D vibrometer measures a location with three independent beams, which strike the target from three different directions. This allows a determination of the complete in-plane and out-of-plane velocity of the target.
- Rotational vibrometers – A rotational LDV is used to measure rotational or angular velocity.
- Differential vibrometers – A differential LDV measures the out-of-plane velocity difference between two locations on the target.
- Multi-beam vibrometers – A multi-beam LDV measures the target velocity at several locations simultaneously.
- Self-mixing vibrometersSelf-mixing interferometrySelf-mixing interferometry is a measurement technique, in which a laser beam is reflected from an object, back into the laser. The reflected light interferes with the light generated inside the laser, and this causes changes in the optical and electrical properties of the laser...
– Simple LDV configuration with ultra-compact optical head. These are generally based on a laser diode with a built-in photodetector, leading to a very rugged and compact optical system. - Continuous Scan Laser Doppler Vibrometry (CSLDV)Continuous Scan Laser Doppler Vibrometry (CSLDV)Continuous Scan Laser Doppler Vibrometry is a method of using a Laser Doppler Vibrometer in which the laser beam is swept across the surface of a test subject to capture the motion of a surface at many points simultaneously...
– A modified LDV that sweeps the laser continuously across the surface of the test specimen to capture the motion of a surface at many points simultaneously