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

Abstract

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 (A eff ) 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.