Enhancing Long-Haul 15-Mode Fiber Performance: Mode Permutation for Reduced Modal Dispersion

We explore the efficacy of mode permutation to mitigate the impact of modal dispersion in a 15-mode fiber link for long-haul space-division multiplexed transmission. By introducing strong coupling between all the fiber modes, mode permutation reduces the growth rate of the link's intensity impulse response (IIR) with transmission distance, yielding a reduction in the receiver MIMO-DSP complexity. Using a recirculating fiber-loop configuration, we experimentally compare four permutation schemes and find that they are similarly effective in reducing the increase of the IIR duration from proportional to the square-root of propagation distance. At the reach of \text{530 km} - the largest achievable with the time-domain MIMO window of 71.4 ns available in the experiment in the absence of mode permutation - the IIR duration is seen to reduce from almost \text{40 ns} to less than \text{15 ns}, while the maximum reach achieved with the use of mode permutation increases to \text{1178 km}. We also devise a simple model to simulate propagation in realistic MMF links with independent fiber spans, whose parameters can be conveniently extracted from the data. In achieving good agreement between the simulated and experimental results, the model suggests that the effectiveness of mode permutation in a realistic 15-mode fiber link, composed of independent fiber spans, is only slightly greater than in the experimental recirculating-loop configuration.