Influence of the Order of Phase Differentiation and Unwrapping on Retrieval of Temporal Waveforms in Phase-Demodulated φ-OTDR

Generally, phase differentiation and unwrapping are both used to retrieve vibration events in phase-demodulated φ-OTDR. We investigate the influence of the order of these two operations on vibration retrieval, i.e., unwrapping the differential phase (UDP) and differentiating the unwrapped phase (DUP). The results demonstrate the UDP would add an extra DC value on the resulting phase and be unable to exactly restore temporal waveforms of vibration signals. Hence, it is necessary to firstly unwrap the original demodulated phase before performing differentiation to reliably restore the disturbances. Before retrieving temporal waveforms of vibration signals, a sliding rectification average (SRA) is applied to locate multiple events for better noise performance. In our experiment, the temporal waveforms of two different vibration sources at different positions are retrieved by phase changes extracted by the DUP. It is observed that the SNR is considerably improved due to reduction of background noises, and time-domain vibration signals are well recovered by the DUP-acquired phase changes.