Laser Doppler vibrometer

A laser Doppler vibrometer (LDV) is a scientific instrument that is used to make non-contact vibration measurements of a surface.

Also, the LDV makes the vibration measurement without mass-loading the target, which is especially important for MEMS devices.

Most commercial vibrometers work in a heterodyne regime by adding a known frequency shift (typically 30–40 MHz) to one of the beams.

The motion of the target adds a Doppler shift to the beam given by fd = 2*v(t)*cos(α)/λ, where v(t) is the velocity of the target as a function of time, α is the angle between the laser beam and the velocity vector, and λ is the wavelength of the light.

The detector does respond, however, to the beat frequency between the two beams, which is at fb + fd (typically in the tens of MHz range).