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Laser Doppler Velocimetry



Laser Doppler Velocimetry (LDV) provides a single point measurement of one component of the gas velocity. A laser is separated into two beams that cross at point that is typically about 0.5 mm in size. Light that is scattered from this point is interpreted to determine velocity. Multiple lasers at different wavelengths can be used to measure more than one component of velocity at the same time.

Operating Principle

First lets take a simple approach. The figure shows two laser beams crossing at a small angle. A laser is coherent, which means that the light is all in phase, i.e. the peaks and troughs of the waves are aligned, and the peaks of the wavefronts are shown as the thin lines in each beam. When the two beams cross in the geometry shown we see that there is an interference pattern that exists with the wavefront peaks and troughs of the two beams aligning in the horizontal direction (the peaks are shown by the red lines). This pattern is stable in space even though both waves are propagating. The spacing of the fringes is given by dfringe.

Consider now a particle so small that its inertia is negligible and it travels with the same velocity as the gas, V. When the particle traverses the intersection region of the two beams (called the probe volume) it scatters light from the stationary fringes. The y-component of velocity causes the particle to cross the fringes giving rise to an oscillating signal in time. The time between peaks in the scattered light corresponds to the time that it takes a particle to travel dfringe. Very simply then, the velocity is dfringe divide by the period of the scattered light oscillation.

The full picture is more complicated, but can be briefly described as follows. The light scattered from a stationary particle occurs at the laser frequency. For green light (500 nm) this occurs at νlaser=6x1014 Hz which is far too fast to detect.

However, if the detector simultaneously sees two frequencies one at νlaser and the other at νlaser+ν, the light will interfere (or beat) on the detector, and as a result we will see the difference between the two frequencies, or v in this example, on the output of the detector provided that v is within the bandwidth of the detector. This detection strategy is what enables LDV.

When a particle is moving the light it scatters is Doppler shifted (think of a moving train that has its pitch vary from when it approaches to when it is moving away from you). First think about a positive x-component of the particle velocity – it causes both beam 1 and beam 2 to have a negative Doppler shift because the particle is moving away from the source. In fact the Doppler shift is the same for both cases due to the symmetry. In this case the beams do not beat at the detector and we are not sensitive to this particle motion.

A positive y-component of particle velocity, however, causes the Doppler shift from beam 1 to be positive +νDop but the Doppler shift from beam 2 is negative -νDop. The two scattered light signals will then beat at the detector at a frequency of 2νDop, and this can be related to the y-component of particle velocity.