Coupled Few-Mode Multicore Fiber With Low Differential Mode Delay Characteristics

We investigate a coupled few-mode multicore fiber (FM-MCF) design to realize an ultrahigh spatial density space division multiplexing (SDM) transmission line with low differential mode delay (DMD) characteristics. We describe a super-mode-based FM-MCF design that takes account of the DMD between modes in the inter-linearly polarized (LP) mode group (e.g., between LP01- and LP11-like super modes). We also show a randomly coupled FM-MCF design based on fiber bending and twisting to reduce the DMD between modes in the intra-LP mode group. We fabricated a coupled three-mode seven-core fiber and realized a low DMD value of less than 0.2 ns/km for all modes where the intercore nonuniformity induced DMD was successfully suppressed owing to distributed random mode coupling. The fabricated fiber has 21 spatial modes within a 125 μm cladding, and a relative core density of 21, which is the highest of any reported 125 μm cladding SDM fiber.