Monitoring surface gravity wave variability with State of Polarization sensing on a subsea telecommunication cable

Abstract. State of polarization (SoP) sensing is a fibre-optic interrogation technique that recovers cumulative polarization variations of probe lasers transmitted between opposite ends of optical fibres. These changes can be triggered by various external stimuli interfering with the guided light in the fibre, including surface gravity wave-induced motions, as evidenced by prior studies. In this study, we advance this knowledge by quantitatively assessing the capacity of SoP sensing to continuously capture surface gravity wave height and frequency variability in the ocean along a subsea telecommunication cable via comparison with reference in situ and validated model data. Our SoP dataset consists of nearly 11 months of data collected at the landing site of a ∼240 km back-looped fibre link across the Norwegian trench over the North sea, between the city of Egersund in the Southwestern coast of Norway and an offshore bottom-fixed platform. Spectral and correlation analyses demonstrate the capacity of SoP to detect both, wind waves and swells acting simultaneously over different regions of the cable in the 0.03–0.6 Hz range. During the observation period, wind waves affected mostly the offshore end of the cable, while swell signatures dominated in the coastal region, close to the landing site. Correlation coefficients of up to nearly 70 % are obtained between SoP data and the wave analysis model for the full observation period. We discuss various physical aspects related to the SoP measurements and outline some of their limitations for sea state monitoring, chiefly in terms of interpretation capacity and sensitivity. Our results also evidence that SoP sensing systems can be a practical way to obtain dynamic, first-order estimates of the sea state variability where cables are sufficiently shallow. A non-negligible portion of the extensive infrastructure of submarine telecommunication cable networks has thus the potential to be turned into arrays of remotely-operated sensors of opportunity that are complementary to well-established oceanographic instruments.