Doppler velocimeter

The Doppler effect (or Doppler shift) is the change in the frequency of a wave when the observer is moving away from or towards the wave's source.

This effect is used also in astronomy (to measure the speed at which galaxies and stars are receding from or approaching to us), in radar (to measure the velocity of detected objects), in plasma physics and astronomy for measuring temperatures (by estimating the temperature of a gas or a ion temperature in plasma, based on the emitted spectral line), in sirens, in medical imaging and blood flow measurement (in medical ultrasonography, like: obstetric ultrasonography and neurology, echocardiogram), in underwater acoustics (in military applications, for measuring the relative velocity of a submarine to a ship), in audio (e.g. an electric motor can be used to rotate an acoustic horn around a loudspeaker, this way sending its sound in a circle).

In vibration measurement a similar device called laser Doppler vibrometer (LDV) is used. The acoustic Doppler velocimeter (ADV) is used in flow measurements. The main difference between LDV and ADV is that while the LDV emits a light beam, the ADV emits an ultrasonic acoustic burst.

An LDV has a laser as its power source, which is usually a He-Ne (helium-neon) laser or an Ar (argon) ion laser. Laser Doppler's can be used for detecting the velocity of reflective particles (which are entrained within a transparent flow field) or for monitoring the cyclic Doppler shift (which is reflected from a vibrating surface). In the first case, the device is working as a flow meter or anemometer, while in the latter case is working as a vibrometer.

If we want only one component of velocity to be measured, a single laser beam is used , which is splitted into two parallel beams. These are focused to cross at the focal point of the transmitting lens. To measure the velocity, only a certain section of the scattered light is measured (while the particles which pass through the cross point are scattering light in all directions).The scattered light is collected by the receiving lens; this is focusing the light onto the photodetector. The photodetector's output is measured by the signal processor.