Coupled-Mode Theory of Multipath Interference in Quasi-Single Mode Fibers

We use the power coupled-mode theory to study the interplay between multipath interference (MPI) and differential mode attenuation (DMA) in quasi-single mode (QSM) fibers. The analytical expressions derived assuming two mode propagation in QSM fibers show that MPI scales differently as a function of the span length for low and high DMA. Furthermore, we derive analytical expressions for the performance improvement of long-haul coherent optical communication systems using QSM fibers, taking into account the impact of excess loss and MPI on system performance. From these expressions, we calculate the maximum allowable coupling coefficient for different values of the DMA. We show, for example, that a QSM fiber with an effective area of 250 μm², a coupling coefficient κ ≤ 6 × 10^-4 km^-1, and DMA equal to 4 dB/km offers a 1-dB performance advantage over a reference pure silica core single-mode fiber for spans of 100 km.