A novel approach to reduce the influence of external conditions on the laser Doppler vibrometer signal acting in the nanometer range

In this work, the operation of laser Doppler vibrometer (LDV) studies in the presence of a third coherent light wave that interferes in detector in addition to the two waves of classical LDV. If amplitudes of vibration are small, presence of the third wave may give a practical advantage. Specifically, such "tree-wave interference" (TWI) vibrometer with the probing wave and additional waves propagating along the same path through the media with unstable refractive index (such as fiber or turbulent air) would have a higher resistance to the external influence on the propagating light in comparison to the classical LDV. The challenge here is to provide the optimal relations between phases and amplitudes of the interfering waves. We propose a simple approach to check optimal phase difference (Δφ = π ) by monitoring the high frequency component of LDV photo-detector signal, particularly spectrum of its amplitude. Our experiments have demonstrated that LDV modified by the additional coherent wave have increased signal-to-noise ratio (SNR) from ldB to 18 dB in the presence of wideband noise. Therefore, TWI vibrometry can be used in noisy environmental conditions where classical LDV shows poor SNR. (paper)