We report tidal-induced latency variations on a transpacific subsea cable. Week-long recordings with a precision phase meter suggest length changes in the sub-meter range caused by the Poisson effect. The described method adds to the toolbox for the new field ‘optical oceanic seismology’.

1. INTRODUCTION

A novel method based on utilizing the architecture of subsea cables for oceanic seismology has recently been explored experimentally[1],[2]. It is conceptually distinguishable from previous techniques for remote sensing in which fiber serves as the transport medium between underwater sensor arrays[3,4] or is analysed using distributed acoustic sensing (DAS) to monitor the cable performance[5] .

\ In its most mature form, the method can sense large geographic areas since it uses the entire cable length as an aperture and is compatible with simultaneous operation of commercial traffic on the same fiber [2]. This feature makes it universally deployable on existing routes for low-cost and long-term surveillance purposes.

\ The two implementations of this method, which were tested on subsea earthquakes localization,share the same basic principle of detecting optical phase distortions caused by mechanical stresses and strain on the cable. One version interferometrically resolves phase distortions induced by cable motion and tension[1]. The second version analyses the state of polarization (SOP) fluctuations of data channels logged by modern coherent transponders[2]. While the exact optomechanical coupling between a vibrating sea floor and cabled fiber is still being investigated, it is known empirically that the SOP of light in SSMF is sensitive to birefringence changes caused by fiber micro-bending or motion.

\ Here we report, for the first time, on cable length variations in the sub-meter range that depend on water pressure generated by tidal variations. In contrast to the two aforementioned implementations, the cable is not impacted by any kind of abrupt sea floor motions. However, changes in local water pressure cause cable length changes that are detectable with an ultra-stable phase meter. In contrast to commonly held views, subsea cables diverge from the ‘loose tube’ model that suggests a forceless bearing on the fibers. Our observations indicate a strong coupling between the cable jacket and the sheathed fiber.

:::info This paper is available on arxiv under CC BY 4.0 DEED license.

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