Design of a graded-index weakly coupled large-mode-area six-mode fiber with a low-index core region

This paper proposes a graded-index few-mode fiber design featuring a low-index central region within the core, aimed at achieving weakly coupled transmission of six spatial modes. The design addresses signal degradation in mode-division multiplexing systems caused by inter-mode crosstalk and differential mode delay (DMD). Through finite-element simulation, the radius and refractive index contrast of the low-index core region are optimized, resulting in an effective index difference greater than 0.001 between the first five adjacent mode groups, which ensures effective weak coupling. At 1550 nm, all supported modes exhibit a mode-field area exceeding 200µm², effectively suppressing nonlinear effects. The dispersion characteristics across the C + L bands are analyzed, showing DMD values below 6 ps/km, indicating excellent dispersion performance. Furthermore, the introduction of a trench-assisted cladding structure significantly reduces bending loss, enhancing the fiber's suitability for long-haul transmission. The results demonstrate that the proposed fiber offers favorable transmission performance across the C + L bands, meeting the requirements of high-capacity MDM optical networks.