Mode-division multiplexing (MDM) using few-mode fibers (FMFs) appears as a promising technology to increase fiber capacity by a few orders of magnitude and sustain the traffic demand for the decades to come. The potential of MDM lies in its ability to exploit multiple modes within a single optical fiber strand. Since 2011 and the first promising demonstrations, impressive progress has been made. In this article, we will show how the most recent advances in design and fabrication have improved the performance of FMFs and MDM systems, and allowed to turn research demonstrations into practical deployments.

A channel modeling method is developed and proposed to practically model the various effects of mode couplings for optical communication systems with multi-core fiber (MCF) and/or few-mode fiber (FMF). The method is similar to existing channel modeling methods in which strong coupling is exclusively considered but extends them to cover both strong and weak couplings. With the method, the entire channel is divided into K statistically independent sections where each section may represent an optical device or the span of a few-mode fiber transmission line that maintains coherence. Each section is modeled by a channel matrix model comprising full random unitary matrices and a differential mode delay (DMD) matrix. To further enhance the modeling of weak coupling, the matrix is modified by segregating it into partial block matrices and inserting correlation terms among blocks. The method is evaluated with data obtained in two representative cases: midrange (40 km) and long-haul (527 km) transmissions. Its effectiveness and practicality are validated from agreement obtained between simulation and measured results for the two cases.