Mode-division multiplexing is a key technology to break through the capacity limit of single-mode fiber systems. In the few-mode fiber links, the pulse broadening caused by intermodal dispersion increases the complexity of multiple-input-multiple-output algorithms in the receiving terminal, which is one of the important reasons for limiting the implementation of long-haul transmission in mode-division multiplexing systems. The application of mode permutation methods can effectively compress pulse broadening and reduce mode-dependent loss, making it a mainstream technological approach for achieving long-distance, high-capacity mode-division multiplexing system transmission. However, there are numerous mode-permutation strategies with varying performance, and exploring mode-permutation strategies that are more suitable for long-distance, high-capacity mode-division multiplexing systems has become an urgent problem to be solved.

In this paper, we propose that the mode permutation strategy should match the mode group delay distribution of the fiber. Besides, we design a Mirror-Flipped Mode Permutation (MFMP) strategy based on few-mode fibers with symmetric mode delay. By modeling on a simulation platform, the transmission performance of mode-division multiplexing system based on the proposed MFMP is analyzed and verified through 6-mode multiplexing transmission experiments.

In this paper，based on the proposed MFMP strategy, 822-km 6-mode transmission over a mode-delay-symmetric few-mode fiber is experimentally demonstrated, carrying quadrature phase shift keying signals at 28 GBaud. At 616 km, compared to Cyclic Mode Permutation (CMP) and Cyclic Mode-Group Permutation (CMGP) methods, the signal pulse broadening is compressed by 37% and 6%, and the Q-factor is increased by 3.0 and 0.4 dB, respectively.

The proposed MFMP strategy can effectively compress pulse broadening, and improve the Q-factor of the system, while the advantages of this method over traditional two types of mode permutation strategies are verified via simulation and experiment. It should be noted that our work provides the theoretical basis and experimental guidance for achieving long-haul and high-capacity mode-division multiplexing transmissions based on mode permutation strategies.