Laser Doppler velocimetry (LDV, also known as laser Doppler anemometry, or LDA) is a technique for measuring the direction and speed of fluids like air and water. In its simplest form, LDV crosses two beams of collimated, monochromatic, and coherent laser light in the flow of the fluid being measured. The two beams are usually obtained by splitting a single beam, thus ensuring coherency between the two. The two beams are made to intersect at their waists (the focal point of a laser beam), where they interfere and generate a set of straight fringes. The sensor is then aligned to the flow such that the fringes are perpendicular to the flow direction. As particles pass through the fringes, they reflect light (only from the regions of constructive interference) into a photodetector (typically an avalanche photodiode), and since the fringe spacing $d$ is known (from calibration), the velocity can be calculated to be

$u\; =\; f\; imes\; d$

where $f$ is the frequency of the signal received at the detector. A diagram of the method is in the second page of this description.pdf" target="_blank">brochure.

Another form of flow sensor that is typically referred to as laser Doppler velocimeter has a completely different approach akin to an interferometer. A beam of monochromatic laser light is sent into the flow, and particles will reflect light with a _Doppler shift corresponding to their velocities. The shift can be measured by interfering the reflected beam with the original beam, which will form beats according to the frequency difference. This is the type of sensor depicted doppler/laser_doppler_intro.cfm" target="_blank">here.

LDV may be unreliable near non-specular solid surfaces, where stray reflections corrupt the signal. NASA is working on a variation that uses two pairs of beams so that polarization can be used to improve noise rejection. (citation?)

LDV is chosen over other forms of fluid velocity measurement such as _Pitot tube measurements because the measurement equipment can be outside of the flow being measured and therefore have no effect on the flow.