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.
Highlights
The depths of the oceans are the places on Earth most immune to anthropic perturbations
As a byproduct of this investigation, we show by symmetry considerations that the way birefringence in standard single mode fibers (SSMFs) with randomly coupled birefringence is affected by strain and pressure is not arbitrary, but is unequivocally determined by the way the polarization averaged phase changes
The observation of state of polarization (SOP) modulation at the output of a system, extracted from standard coherent telecom receivers, can be used to monitor geophysical events occurring in the ocean
Summary
The depths of the oceans are the places on Earth most immune to anthropic perturbations. We show that the random orientation of the fiber birefringence causes an incoherent addition of polarization modulation along the fiber path, and this in turn produces a linear growth of the mean square deviation of the Stokes vector with the length of the portion of the cable affected by the perturbation This linear dependence increases the dynamic range of the measurement, and explains the recent observations [4] that even earthquakes of magnitude larger than Mw = 7 did not produce on a 10,000 km long submarine link polarization modulations that covered the entire Poincaré sphere.
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