eFunda: Introduction to Laser Doppler Velocimeters
engineering fundamentals Laser Doppler Velocimeter: Introduction
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Overview

The laser Doppler velocimeter sends a monochromatic laser beam toward the target and collects the reflected radiation. According to the Doppler effect, the change in wavelength of the reflected radiation is a function of the targeted object's relative velocity. Thus, the velocity of the object can be obtained by measuring the change in wavelength of the reflected laser light, which is done by forming an interference fringe pattern (i.e. superimpose the original and reflected signals).

Typical Laser Doppler Velocimeter
Further Information
A laser power source is the essential part of a laser Doppler velocimeter (LDV). Typically, a Helium-Neon (HeNe) or Argon ion laser with a power of 10 mW to 20 W is used. Lasers have many advantages over other radiation/wave sources, including excellent frequency stability, small beam diameter (high coherence), and highly-focused energy.

Laser Dopplers can be configured to act as flow meters or anemometers, by detecting the velocity of reflective particles entrained within a transparent flow field. They can also be used as a vibrometers by monitoring the cyclic Doppler shift reflected from a vibrating surface. To improve signal-to-noise ratios, a highly reflective material (e.g. tape with small reflective beads) can be attached to the vibrating target.

Pros and Cons

  • Pros:
  - Non-contacting measurement.
  - Very high frequency response.
  • Cons:
  - Sufficient transparency is required between the laser source, the target surface, and the photodetector (receiver).
  - Accuracy is highly dependent on alignment of emitted and reflected beams.
  - Expensive; fortunately, prices have dropped as commercial lasers have matured.

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