The effect of three-wave interference in laser Doppler vibrometry

Abstract

In a laser Doppler interferometer, besides the probe and reference beams, a third coherent light wave, caused by spurious reflection, may hit the detector. Interference effects between regular and unwanted waves, which are dependent on the actual intensity ratios, contribute to the generation of the electrical output signal. A mathematical analysis predicts the appearance of ripple and spikes riding on the demodulated signal due to distorted modulation of the Doppler signal. These effects are verified by experiments, demonstrating the dependence of distortions on given optical conditions.