Integrating polarization diversity into 3 × 3 coupler passive demodulation for interferometric fiber-optic sensors

In this paper, polarization diversity is integrated into a 3 × 3 coupler-based passive demodulation system for the first time to mitigate polarization-induced fading (PIF) in interferometric fiber-optic sensors (IFOSs). This study investigates the impact of single-mode fiber at the receiving end of a polarization diversity receiver (PDR) on signal demodulation. This impact causes variations in the fixed phase difference introduced by the 3 × 3 coupler in passive systems, leading to demodulation failure. To overcome this limitation, what we believe is a new method is proposed, including an automatic selection algorithm, an ellipse fitting algorithm for phase difference, and an improved arctangent algorithm to demodulate disturbance signals correctly. In theory, the approach introduced in this paper can also be effectively utilized in other IFOS systems to mitigate polarization fading. Experimental results demonstrate that the proposed system reduces the failure probability due to PIF to 6/55 of that in the traditional system and achieves a maximum localization error of less than 17 m over a 40 km sensing range. This advancement significantly enhances the reliability and applicability of passive demodulation systems in scenarios requiring long-term stable detection, such as pipeline integrity monitoring and perimeter security.