Two laser beams intersect each other, generating an overlapping region, which is known as the measurement volume. A pattern of interference fringes is created in the measurement volume, where the distance between the interference fringes is defined by the wavelength of the laser light and the angle between the two laser beams.

As a particle moves within the measurement volume, it passes through alternating bright fringes. Consequently, the particle scatters light with a frequency that depends on its velocity.

This back scattered light is transformed into a voltage signal by a photodetector, then filtered and amplified in the LDV controller. The Doppler frequency contained in this signal is determined by FFT processing. The velocity component perpendicular to the optical axis of the optic can be calculated by multiplying the fringe distance with the Doppler frequency.

Fig. 1: Interference fringe pattern inside the measurement volume