Robustness of phase-modulated holographic vibrometry in the presence of parasitic vibrations

Holographic vibrometry is a family of interferometric methods for contactless, full-field vibration measurements. Harmonic surface vibrations with high frequencies (hertz to upper megahertz regime) and low displacements (sub-nanometer to micrometer) as found in micro-electromechanical systems can be characterized. Using frequency shifting, it is possible to obtain static interference or low-frequency beats between the reference arm and one of the sidebands generated by the vibration in the object arm. This enables the use of regular video-rate camera detectors, which offers high lateral spatial resolutions at short measurement times compared to scanning methods. The key to achieving high signal-to-noise ratios is a narrow-band lock-in evaluation at the beat frequency captured by a sequence of holograms. However, in many scenarios, the signal of interest is superimposed by parasitic low-frequency vibrations from the environment. The beat signal is then Doppler-broadened, and narrow-band filtering is no longer possible. In phase-modulated holographic vibrometry, the lock-in signal is given by a beat between the object vibration and an artificial reference vibration. In this work, we exploit that acoustic signals are less susceptible to Doppler broadening, as their absolute frequencies are orders of magnitude smaller compared to optical signals. This leads to a stable narrow-band beat frequency, even in the presence of perturbations. The superior robustness is demonstrated experimentally by successfully recovering displacements below 10 nm in the face of perturbations up to three orders of magnitude larger. We discuss limiting factors and show that cutting exposure times allows one to tolerate even larger perturbations.