Six-Mode 19-Core Fiber With 114 Spatial Modes for Weakly-Coupled Mode-Division-Multiplexed Transmission

This paper describes a six-mode 19-core fiber with 114 spatial modes, which was designed and fabricated for weakly-coupled mode-division-multiplexed (MDM) transmission over uncoupled cores. The 19 identical graded-index cores were packed on a hexagonal lattice with a 62-μm pitch in a ~318-μm-diameter cladding. The cladding diameter is large, but still mechanically reliable by assuming a 2% proof test and 40-mm minimum fiber bend radius. The inter-core crosstalk is negligibly suppressed and the calculation indicated that a 50-μm core pitch (corresponding to the 266-μm-diameter cladding) is achievable without significant crosstalk degradation. Though the fiber was intended for the weakly-coupled MDM transmission, the differential mode delay (DMD) within a degenerated mode group of LP21 and LP02 modes was not well suppressed. Thus, further DMD suppression within each mode group or splitting the degeneracy of the modes is required to realize multiple-input multiple-output (MIMO) digital signal processing with reduced complexity. Additionally, a numerical investigation revealed that the intra-core inter-modal crosstalk at a butt coupling was induced not only by the misalignment, but also by the refractive index profile mismatch between the butt-coupled cores. Therefore, the tight tolerance on the refractive index profile is important both for DMD control and modal crosstalk suppression.