Maximizing the Capacity of Graded-Index Multimode Fibers in the Linear Regime

In this article, we investigate the design of multimode fibers (MMFs) guiding over 1000 spatial modes. A trench-assisted graded-index core profile is optimized for low differential mode delay (DMD) over the C-band. The optimization results show that for a maximum DMD of 250 ps/km as much as 400 spatial modes can be supported ideally. It is shown that the optimized DMD scales with the number of modes before it reaches a plateau for a given core-cladding contrast – indicating that maximizing the core radius can allow to increase spatial cardinality without significant delay spread increase. Closed-form equations for optimal refractive-index profile parameters are introduced. The throughput supported by the optimized fibers is studied and the scaling trend with the number of guided modes is analyzed for increasing core-cladding contrast. Throughput is shown to be significantly impacted by Rayleigh scattering, macro-bend loss and coating loss, introducing a practical limit to the scaling of the number of modes – at approximately 1000 spatial modes. Here dubbed as the half-mode number, it corresponds to the number of modes beyond which the extra throughput per additional mode decays to half of that of the best mode. Finally, the impact of the fabrication margins on the DMD of the optimized profiles is analyzed.