Bandwidth density as a figure of merit for few-mode multi-core fibers.

We investigated the relationships between the capacity limit and various figures of merit (spatial channel density, aggregate effective area ratio to cladding area, and bandwidth density) of few-mode multi-core fibers (FM-MCFs) where the modes in each core are weakly coupled. The capacity limit was estimated based on the Gaussian noise model for nonlinear impairment of single-mode fibers by neglecting crosstalk and intermodal nonlinear effects for simplicity; therefore, the estimated capacity can be the upper bound of the weakly-coupled FM-MCF capacity. When we take account of the transmission wavelength band of the FM-MCFs and the wavelength band where efficient amplification is available, the bandwidth density had a good correlation to the estimated capacity, but the spatial channel density and the aggregate effective area ratio often overestimate the FM-MCF capacity. Thus, we propose the bandwidth density for the figure of merit of FM-MCFs rather than the spatial channel density or the aggregate effective area ratio. We also investigated the relationship between the bandwidth density and fiber design, and found that the supporting transmission bandwidth and the core count are the dominant factors for the bandwidth density, and the mode count per core has a small impact for the bandwidth density, when the core Δ is fixed.