A novel Jones matrix analysis applied on polarization data acquired from a Mediterranean sea underwater fiber telecommunication cable

Abstract

In recent years, the possibility to use underwater telecommunication cables to perform scientific measurements has been investigated, either by inserting custom instrumentation in the optical repeaters [1] or by using the fibers present in the cables as a sensor [2,3]. In this work we will focus on the SOP (State Of Polarization) technique. Change in the SOP, as transmitted through telecommunication cable, originates when intrinsic or externally induced birefringence in the fiber results in a different phase velocity for different polarization states. Measurements of the change in optical properties of the cable could, in principle, give information on its mechanical perturbation through photoelastic effect [4]. However, the birefringence is affected also by many other effects [5,6] that can be applied on any part of the fiber, thus posing an issue to disentangle the different sources. SOP data are routinely acquired on commercial telecommunication cables, using dedicated hardware, to monitor and increase the quality of data transmission, without interfering with the cable’s commercial purposes, being readily available on many existing telecommunication infrastructures. Data analysed in this paper have been acquired on the TISparkle MedNautilus cables [7]; the system is composed of two underwater telecommunication cables connecting  central and eastern Mediterranean. At the ends of both cables, the SOP data and the Jones matrix for the optical system are acquired using a standard Subsea Line Terminal Equipment produced by Infinera [8].  Following an approach similar to the one used by Jones [9] for spatial derivatives of Jones matrix and Szafraniec &  Baney [10] for frequency derivatives, we developed a novel technique, based on the infinitesimal generator of temporal translations. This analysis allows to evaluate a three dimensional complex vector whose components are related to the time variation of the attenuation and the phase delay. We will show some preliminary results obtained using this technique to analyse a set of earthquakes occurred during 2023, focusing in particular on the ones that took place in Turkey on 2/6/2023. Even if the SOP technique has many potential noise sources that could limit its sensitivity and its study is in an early stage, its development is appealing because it can take advantage of the existing underwater telecommunication cables laid on the ocean floor during the last decades without interefering with their normal use for telecommunication.   [1] G. Marinaro et al "SMART Subsea Cables for Observing the Earth and Ocean, Mitigating Environmental Hazards, and Supporting the Blue Economy" https://doi.org/10.3389/feart.2021.775544 [2] G. Marra et al "Optical interferometry–based array of seafloor environmental sensors using a transoceanic submarine cable" https://doi.org/10.1126/science.abo193 [3] T. Tonegawa et al "Extraction of P Wave From Ambient Seafloor Noise Observed by Distributed Acoustic Sensing" https://doi.org/10.1029/2022GL098162 [4] A. Barlow and D. N. Payne "The Stress-Optic Effect in Optical Fibers" https://doi.org/10.1109/JQE.1983.1071934 [5] J.Kerr "A new relation between electricity and light: Dielectrified media birefringent". https://doi.org/10.1080/14786447508641302 and https ://doi.org/10.1080/14786447508641319 [6] S. Ramaseshan "Faraday effect and birefringence" https://doi.org/10.1007/BF03172266 [7] https://it.wikipedia.org/wiki/MedNautilus [8] https://www.infinera.com [9] R. C. Jones "A New Calculus for the Treatment of Optical Systems. VII. Properties of the N-Matrices" https://doi.org/10.1364/JOSA.38.000671 [10] B. Szafraniec and D.M. Baney  "Elementary Matrix-Based Vector Optical Network Analysis" https://doi.org/10.1109/JLT.2007.891456