Multi-point vibration positioning method for long-distance forward transmission distributed vibration sensing.

Observation of intensity, phase, or polarization properties of light propagating through telecom submarine cables can enable widespread monitoring of geological and undersea events, such as earthquakes, tsunamis, and shipping lane traffic. We conducted a comparative analysis of external physical perturbations acting on submarine optical cables and unprotected optical fibers; introduced both intensity and phase demodulation-based sensing systems for long-distance vibration sensing; presented an extension to the phase-spectrum time delay method for forward-transmission distributed sensing (same as optical communications) to distinguish and quantify multiple simultaneous vibration events; and overcame the previous spatial resolution fundamental lower limit set by the time-domain sampling rate. We experimentally demonstrated multi-vibration positioning over 202.3 km single-span sensing distance, with a positioning accuracy as small as 17.9 m for sinewave vibrations, and a spatial resolution of 1.25 m. Other key sensor parameters include phase sensitivity of 40.6 mrad/µε @ 80 Hz, a corresponding limit of detection (LoD) of 101.7 pε/Hz, intensity sensitivity of 7.1%/µε @ 80 Hz, and a corresponding LoD of 20.1 pε/Hz. The tested frequency range was 0.01-100 Hz. No signal averaging was performed during signal processing to allow faster real-time processing, which would otherwise further improve the results. This forward transmission approach has the potential to upgrade the existing submerged global internet fiber-optic network into a vast ocean-spanning observation network while allowing telecom operations to operate normally without sacrificing bandwidth.