Laser surface velocimeter

Here, 2 laser beams which are each incident to the optical axis at an angle φ, are superimposed on the surface of the object.

Due to the interference of the scatter waves from the two laser beams, there are different frequency components in the superimposition.

When both incidental laser beams are at the same frequency (same wavelength), this is seen as a difference of fe2 and fe1 to: If point P moves vertically with reference to the optical axis and at the same angle of incidence φ, it can be said that: and This means the final result is: The Doppler shift is thus directly proportional to the velocity.

A graphic explanation which leads to the same result follows: Both the laser beams are superimposed in the measurement volume and in this spatial area, generate an interference pattern of bright and dark fringes.

As a result of this, a photo receiver in the sensor head generates an AC signal, the frequency fD of which is directly proportional to the velocity component of the surface in measurement direction vp and it can be said that: Laser surface velocimeters work in the so-called heterodyne mode, i.e. the frequency of one of the laser beams is shifted by an offset of 40 MHz, e.g..

This makes the fringes in the measurement volume travel with a velocity corresponding to the offset frequency fB.