Light reflected from a moving object is subject to a change in frequency which depends on the object’s velocity (Doppler effect). From the frequency shift measured with an interferometer the vibrational motion of the object is known.
A = 2π f V = 4π² f² D .
I(t) = Ic + Iv + 2 (Ic Iv)1/2 cos (2 π Δz(t)/λ) .
The variation in intensity is independent on whether the object is approaching or moving away from the vibrometer. This ambiguity is removed by heterodyning. When the frequency of the reference beam is shifted by a fixed amount fb , the interference of both beams for a non-moving probe results in a harmonic intensity variation with frequency fb . This carrier signal ∝ cos (2 π fb) is modulated by the motion at the measurement object. Depending on its direction of motion the frequency of the intensity is shifted towards larger or smaller frequencies.
The information on the motion of the object measured is obtained via demodulation from the intensities. After conversion into a digital signal, a signal processor determines displacement and velocity of the measurement object in real time. Demodulation (often also referred to as decoding) is done either for the velocity or the displacement.