Detection-Localization-Identification of Vibrations Over Long Distance SSMF With Coherent $\Delta \phi$-OTDR

We propose a novel interrogation technique for Distributed Acoustic Sensing (DAS) and demonstrate the detection and recognition of multiple vibration events over 50 km of SSMF. A differential-phase optical time-domain reflectometry (Δφ-OTDR) approach is used. At the transmitter side, a continuous ultra-narrow-linewidth laser is modulated with polarization-multiplexed orthogonal binary sequences and at the receiver side, the back-scattered signal is captured through a polarization-diversity coherent mixer in a self-homodyne configuration. The detection of the full optical field vector and the use of an ultra-narrow-line-width laser source enhance the sensitivity of the interrogating solution. A comparison of the experimental results to a simulation model is developed to better understand the noise limits. A low-complexity localization technique based on a multi-resolution approach is also presented. Major results include the identification of an engine noise, paving the way for numerous smart-city/industrial monitoring applications over deployed telecom fibres.