Demodulation and multi-input multi-output equalization for mode division multiplexing system using a novel few-mode fiber

In order to solve the mode crosstalk in a mode division multiplexing (MDM) transmission system, we design and fabricate a novel few-mode fiber (FMF) with high mode differential group delay (MDGD) that ensure that all supporting modes can transmit through the FMF independently. Then, we propose a cascaded multi-input multi-output (MIMO) delay equalization method, in order to further suppress the mode crosstalk from the mode division multiplexer/demultiplexer (MUX/DeMUX) and improve the transmission distance and spectrum efficiency of MDM system. Our simulation results verify that the cascaded MIMO delay equalization method together with the novel FMF can effectively mitigate the mode crosstalk of MDM transmission. Compared with the traditional MIMO equalization method, our proposed equalization method can be successfully applied to the QPSK signal transmission through the FMF with large MDGD, while no obvious increase of computation complexity is observed. Our results show that for a 40 Gbps two-mode division multiplexing system, the Q factor can be improved by 1.7 dB after 40 km FMF transmission using the proposed cascaded MIMO equalization method. Therefore, all those results are very conducive to the next-generation high capacity fiber-optic transmission.