Polarization sensing using submarine optical cables

在上一期的Optica上找到了一篇关于偏振光遥感的文章，实现的方法很有新意，通过观察光缆信号接收端输出光的偏振态，可以实现对海底光缆沿线海浪、地震等地理事件的监测。联想到之前在网上看到过美国人在第一岛链上埋设了无数海底光纤传感器监测潜艇动向，还有专门的间谍潜艇用来监听海底光缆。设想一下未来可能都不需要这么麻烦，只需要在埋设海底光缆的时候预设一些特殊的节点，直接就可以利用海底光缆监测对方的潜艇。

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.

• https://doi.org/10.1364/OPTICA.424307

• Antonio Mecozzi, Mattia Cantono, Jorge C. Castellanos, Valey Kamalov, Rafael Muller, and Zhongwen Zhan, "Polarization sensing using submarine optical cables," Optica 8, 788-795 (2021)