PDL in Optical Links: A Model Analysis and a Demonstration of a PDL-Resilient Modulation

Optical communications networks include basic components such as EDFAs and WSSs that present a polarization gain imbalance. The gain imbalance at the component output depends on the incident state-of-polarization. This well-known phenomenon called Polarization Dependent Loss (PDL) is revisited in this article and systematically analyzed within the framework of communications theory and practical achievable information rates. The rate impairment induced by PDL is discussed on an optical channel with distributed PDL and on a simplified equivalent lumped PDL channel. We have previously designed the New Spatially Balanced (NSB) signaling, an optimal 4D modulation based on this simplified model. We explore in this article its performance through numerical simulations and experimentally over a transmission in a recirculating loop with in-line PDL. Experimental measurements confirm an accurate channel modeling and significant information rate gains brought by NSB that results in SNR gains of the order of 0.4 dB at a practical transmission outage.