Laser Doppler vibrometer

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Basic components of a laser Doppler vibrometer LDV Schematic.png
Basic components of a laser Doppler vibrometer

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 reflected 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.

Contents

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.

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 laser, 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. [1]

A schematic of a typical laser vibrometer is shown above. 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.

Applications

LDVs are used in a wide variety of scientific, industrial, and medical applications. Some examples are provided below:

Types

holographic vibrometry of the cantilevers of a musical box by frequency-division multiplexing CantileversLaserDopplerHolography.jpg
holographic vibrometry of the cantilevers of a musical box by frequency-division multiplexing

See also

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<span class="mw-page-title-main">Doppler radar</span> Type of radar equipment

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<span class="mw-page-title-main">Homodyne detection</span> Sensor implementation technique

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<span class="mw-page-title-main">Continuous-scan laser Doppler vibrometry</span> Method of measuring vibration across a surface

Continuous-scan laser Doppler vibrometry (CSLDV) is a method of using a laser Doppler vibrometer (LDV) 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. This is different from scanning laser vibrometry (SLDV) in which the laser beam is kept at a fixed point during each measurement and quickly moved to a new position before acquiring the next measurement.

A laser surface velocimeter (LSV) is a non-contact optical speed 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. They are widely used for process and quality control in industrial production processes.

<span class="mw-page-title-main">Laser Doppler imaging</span>

Laser Doppler imaging (LDI) is an imaging method that uses a laser beam to scan live tissue. When the laser light reaches the tissue, the moving blood cells generate doppler components in the reflected (backscattered) light. The light that comes back is detected using a photodiode that converts it into an electrical signal. Then the signal is processed to calculate a signal that is proportional to the tissue perfusion in the scanned area. When the process is completed, the signal is processed to generate an image that shows the perfusion on a screen.

<span class="mw-page-title-main">Laser scanning vibrometry</span> Device for measurement and imaging of vibration

The scanning laser vibrometer or scanning laser Doppler vibrometer, was first developed by the British loudspeaker company, Celestion, around 1979, further developed in the 1980s, and commercially introduced by Ometron, Ltd around 1986. It is an instrument for rapid non-contact measurement and imaging of vibration.

References

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  11. ↑ Xiang, Ning; Sabatier, James M. (2000). "<title>Land mine detection measurements using acoustic-to-seismic coupling</title>". In Dubey, Abinash C; Harvey, James F; Broach, J. Thomas; et al. (eds.). Detection and Remediation Technologies for Mines and Minelike Targets V. Detection and Remediation Technologies for Mines and Minelike Targets V. Vol. 4038. p. 645. doi:10.1117/12.396292. S2CID   12131129.
  12. ↑ Burgett, Richard D.; Bradley, Marshall R.; Duncan, Michael; Melton, Jason; Lal, Amit K.; Aranchuk, Vyacheslav; Hess, Cecil F.; Sabatier, James M.; Xiang, Ning (2003). "Mobile mounted laser Doppler vibrometer array for acoustic landmine detection". In Harmon, Russell S; Holloway, Jr, John H; Broach, J. T (eds.). Detection and Remediation Technologies for Mines and Minelike Targets VIII. Detection and Remediation Technologies for Mines and Minelike Targets VIII. Vol. 5089. p. 665. doi:10.1117/12.487186. S2CID   62559102.
  13. ↑ Lal, Amit; Aranchuk, Slava; Doushkina, Valentina; Hurtado, Ernesto; Hess, Cecil; Kilpatrick, Jim; l'Esperance, Drew; Luo, Nan; Markov, Vladimir (2006). "<title>Advanced LDV instruments for buried landmine detection</title>". In Broach, J. Thomas; Harmon, Russell S; Holloway, Jr, John H (eds.). Detection and Remediation Technologies for Mines and Minelike Targets XI. Detection and Remediation Technologies for Mines and Minelike Targets XI. Vol. 6217. p. 621715. doi:10.1117/12.668927. S2CID   62566351.
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  23. ↑ Heterodyned self-mixing laser diode vibrometer – US Patent 5838439 Archived 2011-06-12 at the Wayback Machine . Issued on November 17, 1998. Patentstorm.us. Retrieved on 2013-06-17.
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