Concentric-Ring-Assisted Multimode Fiber Supports Numerous High-Order LP Beams

This study proposes and numerically investigates the design and characterization of a ring-assisted (RA) fiber supporting 11 LP mode groups and a concentric-ring-assisted (CRA) fiber supporting 13 LP mode groups. Based on the relationship between the normalized frequency and the number of LP modes, a step-index (SI) fiber capable of supporting 13 LP mode groups is first designed. By leveraging the overlap between the high-index ring-assisted structure and the LP22 mode, the effective index difference (Δneff) between the LP22 and LP03 modes is enhanced. The resulting RA 11-LP mode fiber achieves a minimum effective index difference Min|Δneff| of 0.78 × 10−3, comparable to that of a standard SI 4-mode fiber, and a minimum effective area Min|Aeff| of 164 μm2, which effectively suppresses nonlinear effects. Furthermore, by introducing a second ring structure to form a CRA design, we realize a 13-LP mode fiber. This structure selectively increases the effective index of the LP61 mode through overlap with its power distribution, while leaving the effective index of the LP13 mode unaffected. The CRA 13-LP mode fiber exhibits highly stable effective indices across the C band. It demonstrates a Min|Δneff | of 0.55 × 10−3, which ensures effective mode separation and reduced inter-mode crosstalk. The Min|Aeff| is 131 μm2—still above 100 μm2—thereby mitigating nonlinear impairments. With support for 46 spatial modes in total, this fiber significantly enhances transmission capacity.