Mode-selective coupler pair for mode-group demultiplexing over graded-index few-mode fiber

We propose a mode-selective coupler pair (MSCP) to demultiplex the degenerate mode groups in graded-index few-mode fibers (GI FMF). The MSCP comprises a GI FMF core as input and two orthogonally positioned two-mode fiber (TMF) cores as output. It can simultaneously and selectively demultiplex the power of all spatial modes belonging to the third or fourth mode group in the GI FMF core into the two output TMF cores. The principle of the MSCP was analyzed by comparing its mode coupling behavior with that of conventional three-core mode-selective coupler and degenerate mode-selective coupler (DSMC). MSCPs were designed for the third and fourth mode groups, and a complete four-mode-group demultiplexer was constructed by cascading them with a DMSC. Simulations were performed to evaluate the performance of the individual MSCP as well as the cascaded demultiplexer, including demultiplexing efficiency, modal crosstalk and bandwidth characteristics across the C-band. The proposed MSCP-based demultiplexer enables MIMO-free mode-group-division multiplexing transmission over GI FMF, eliminating the need for complex digital signal processing and ensuring high compatibility with standard commercial direct-detection optical transceivers.