Polarization sensing using submarine optical cables

TLDR

Polarization modulation measured at the output of a submarine link can monitor geophysical events such as sea waves and earthquakes. The study analyzes how birefringence perturbations affect output polarization in long‑haul submarine links and derives analytical expressions linking modulation to event amplitude and extent. By symmetry, in standard single‑mode fibers with random polarization coupling, strain or pressure‑induced polarization fluctuations equal relative birefringence fluctuations, which are the averaged phase fluctuations. The analysis indicates that pressure‑induced strain explains observed polarization modulations and demonstrates that operating submarine links can serve as powerful sensors for deep‑ocean geophysical events.

Abstract

Observation of polarization modulation at the output of a submarine link, extracted from a standard coherent telecom receiver, can be used to monitor geophysical events such as sea waves and earthquakes occurring along the cable. We analyze the effect of birefringence perturbations on the polarization at the output of a long-haul submarine transmission system, and provide analytical expressions instrumental to understanding the dependence of the observed polarization modulation on the amplitude and spatial extension of the observed events. By symmetry considerations, we show that in standard single mode fibers with random polarization coupling, if polarization fluctuations are caused by strain or pressure, the relative birefringence fluctuations are equal to the relative fluctuations of the polarization averaged phase. We finally show that pressure induced strain is a plausible explanation of the origin of polarization modulations observed in a long submarine link. The presented analysis paves the way for the transformation of transoceanic fiber optic links during operation into powerful sensing tools for otherwise inaccessible geophysical events occurring in the deep ocean.