On Distributed Polarization-Dependent Loss Monitoring and Mitigation: An Optical Layer Approach Enabled by Pilot Tone Technique

Polarization-dependent loss (PDL) presents a significant challenge in modern optical fiber networks by causing variations in signal attenuation that depend on the polarization state. This variability degrades the optical signal-to-noise ratio and overall system performance. In most optical links, PDL primarily originates from wavelength selective switches (WSSs), and the variation in signal power occurs because the polarization state of light is not preserved throughout the fiber. Most existing approaches either focus on compensating aggregated PDL through advanced algorithms at the transceiver level or estimating the PDL of each WSS to optimize margin design. In this paper, we introduce a method that integrates distributed PDL monitoring and compensation using low-cost amplitude pilot tone (PT) technology. Our approach involves modulating a polarization-multiplexed signal with two different PT frequencies on each polarization. By measuring the power difference between these polarizations at any PT detection point along the link, we can determine the PDL of devices within the link. Additionally, by placing a polarization controller (PC) between two WSSs, we can adjust the overall PDL by tuning the PC. This enables effective PDL compensation based on our distributed monitoring technique. Extensive experiments have been performed, and the results demonstrate that our method accurately estimates the wavelength-dependent PDL of WSS devices with an accuracy better than $ 0.1$ dB. Moreover, it reduces the total PDL of two WSSs from approximately $ 1.5$ dB to $ 0.3$ dB over a $ 675$ km multiple-span optical link.