Design and analysis of a combined-ring core weakly coupled few-mode fiber with six linearly polarized modes and an ultra-large effective index difference.

The theory and properties of weakly coupled few-mode fiber (FMF) have been investigated by means of mode power distribution. According to that, a novel weakly coupled FMF with a combined-ring core structure has been proposed. To the best of our knowledge, it is the first time a weakly coupled FMF has realized the goal of supporting six modes and ultra-large minimum effective index difference (min Δneff) of 1.83*10-3 between all neighboring modes. The combined-ring core structure evidently relieves the pressure of modifying neff via inserting a single low-index area and a single high-index ring in step-index FMFs. Due to this structure, a weakly coupled FMF design can be realized by the smaller low-index central area (central area radius d=0.82  μm) and lightly doped high-index ring (the refractive index difference between the ring and the core Δn2=0.11%) compared with former designs. The effective area (Aeff) and differential mode delay characteristics of the designed FMF have been optimized. Furthermore, we calculated the bending loss and fiber properties at the C and L bands to prove that our proposed FMF satisfies the requirement of large-capacity mode-division multiplexing optical networking.