FEATURE
HANDS OFF OUR CABLES! Detecting Human Activities Around Submarine Cables BY GEOFF BENNETT
OVERVIEW
As far back as September 2022, I wrote an article in Subtel Forum Magazine describing the options for seismic detection in existing and future submarine cables. In particular, differential State of Polarization (SOP) techniques using the High Loss Loopback (HLL) facility in subsea repeaters opened the door to using existing cables as seismic detectors, and these could play a part in enhancing tsunami early warning systems. In this article, I’d like to give an update on how we could use two other important techniques to solve issues such as cable tampering detection and other man-made activities, like ship anchors and fishing nets that are dragging on the seabed near the cable. With high-profile cable cuts in the Red Sea and the Baltic, and an ongoing tension for cables around Taiwan, the need for tamper detection and early warning solutions has never been greater. Moreover, unlike the HLL approach, both techniques can be applied in terrestrial as well as submarine cables. Note that the three fundamental properties operators
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can measure in most sensing fiber techniques are amplitude, polarization and phase. Approximation polarization is about 1,000 times more sensitive than amplitude, and phase is about 1,000 times more sensitive than polarization. The integrated SOP and Coherent-Optical Frequency Domain Reflectometry (C-OFDR) techniques described below use polarization and phase, respectively. Both techniques illustrate a key message: that fiber sensing is a diverse and versatile toolbox for subsea cables. Different methods offer unique strengths depending on the use case, and together they point toward a future where cable systems are not just information pipes, but intelligent, responsive infrastructures.
STATE OF POLARIZATION (SOP)
Since their introduction, coherent transponders have used digital signal processing (DSP) to correct various impairments in optical fiber communications. Coherent transponders also make use of polarization of the signal to double the number of data bits carried by each carrier wavelength, and the DSP in the receiver circuits must con-