MPLC-based Low-modal-crosstalk Nine-mode-group Demultiplexer for DSP-free IM/DD MDM Transmission over MMF

Weakly coupled mode-division multiplexing (MDM) transmission over legacy laid multimode fiber (MMF) has great economic efficiency and can enormously enhance the capacity of short-reach optical interconnections. In order to be compatible with cost-efficient intensity-modulation/direct-detection (IM/DD) transceivers, weakly coupled mode-group demultiplexers that can simultaneously receive each mode group of MMFs are highly desired. In this paper, we propose a scalable low-modal-crosstalk mode-group demultiplexer over MMF based on multiplane light conversion (MPLC). Multiple input Hermite-Gaussian (HG) modes of MMF are first converted to bridging modes that are composed of modes distributed as a right-angle triangle in Cartesian coordinates, and then each mode belonging to a degenerate mode group is mapped to different overlapped modes with vertical orientation for simultaneous detection. With the help of bridging modes, the MPLC-based mode-group demultiplexer can efficiently demultiplex all mode groups in standard MMFs with less than 20 phase masks. A nine-mode-group demultiplexer is further designed for demonstration, and simulation results show that the MPLC-based demultiplexer achieves low modal crosstalk of lower than -22.3 at 1550 nm and lower than -17.9 over the C-band for all the nine mode groups with only 16 phase masks.