Low-frequency vibration measurements in harsh environments using a frequency-modulated interferometer

Abstract

Low-frequency vibration measurements in harsh environments are considerably challenging owing to strong background noise. In this study, a simple, high-dynamic-range, and high-precision vibration-measuring system using a frequency-modulated interferometer was proposed and validated. Harmonics with perfectly orthogonal phases were extracted directly from the interference signal, and noise with random frequencies was filtered using a synchronous detection method. The modulation index of the interferometer was controlled to remove the effect of Bessel functions; hence, a full-circle Lissajous diagram was obtained. The ratio of the two harmonics was used to determine the vibration; hence, the effects of intensity fluctuation and background noise can be neglected. The vibration measurement bandwidth was well controlled by controlling the modulation and cutoff frequencies of the bandpass filters. The best noise level of 1 nm/√Hz under harsh measuring conditions can be archived in the low-frequency range.