Adaptive Pulse Period Method for Low-Frequency Vibration Sensing With Intensity-Based Phase-Sensitive OTDR Systems

The use of intensity-based phase-sensitive optical time-domain reflectometry (φ-OTDR) system to measure low-frequency vibration is the aim of this paper. It is well-known that the laser frequency drift (LFD) and other system noise act as internal vibration that challenge φ-OTDR system to capture external low-frequency vibration. We proposed theoretically and demonstrated experimentally that by adapting the probe pulse repetition rate/pulse period to the frequency of vibration to be measured, consistent intensity change should be always realized in the disturbance region compared to the induced phase-noise change from pulse to pulse. In particular, two different pulse periods were used to demonstrate the effectiveness of the adaptive pulse period method (APP) for detection of low-frequency vibration. Experimentally, the sensing of external vibrations of 5; 1; 0.5 and 0.1 Hz frequencies with intensity-based φ-OTDR system was achieved with good signal-to-noise ratio. Moreover, an approach suitable for measuring low-frequency vibration induced by a PZT through direct acquisition of the Rayleigh intensity in the disturbance region was established. Various driving voltages applied to the PZT confirmed the consistency of the method.