Seven-core trench-assisted-graded-index ring fiber for orbital angular momentum modes

Abstract To meet the ever-increasing demand for data rate and bandwidth, mode-division-multiplexing technology utilizing orbital angular momentum (OAM) emerges as a promising solution to substantially expand the transmission capacity of optical fiber communication systems. In this work, we propose and numerically investigate a refractive index profile for multi-core fiber, termed the seven-core trench-assisted-graded-index ring fiber. This designed fiber introduces a graded-index profile to enhance OAM mode purity and increase the number of supported modes within a multi-ring core structure. A trench-assisted design is implemented to further suppress the crosstalk between multiple ring cores. To our knowledge, the seven-core trench-assisted-graded-index ring fiber is adopted for the first time to simultaneously maximize OAM purity and minimize the inter-core crosstalk. Through systematic numerical analysis using the finite element method, the optimized fiber can support 266 distinct OAM modes within the C-band. The purity of these OAM modes, synthesized from intrinsic EH modes, exceeds 99.27%. The effective mode area ( A eff ) for most supported OAM modes reaches 190 µ m 2 , significantly surpassing that of step-index ring core fiber. Through careful optimization of structural parameters, the crosstalk between adjacent cores can be maintained below −90 dB after a 100 km transmission distance for the majority of modes.