A simple method of measuring vibration frequency based on φ-OTDR

Distributed vibration sensors by amplitude-demodulated phase sensitive optical time-domain reflectometers (φ-OTDR) have been widely used in many applications like safety monitoring of large-scale infrastructures and fence security. In order to locate vibrations, every two Rayleigh backscattering traces (RBTs) spaced apart by a certain interval (i.e., differential step k) are traditionally subtracted over a certain measurement time, and then all the subtractions are summed together and used to detect external vibration events. This method is denoted as amplitude-based accumulative differential method (ADM). Yet, the ADM has been demonstrated that the superimposing differential signal shows highly dependent on k which is closely related to vibration frequency (i.e., fs). Inappropriate k values would fail to locate the vibration. It is thus possible to envisage measuring fs by taking advantage of this strong dependence of the ADM on fs. In this manuscript, a simple method of estimating fs is proposed by measuring the relationship between the ADM signal and k. The validity of this method is demonstrated by our experiment, in which a vibration event induced by a sinusoidal-driven PZT is detected.