Agree with Abdallah, SDM (and especially MDM) is of high interest. Can refer to following papers:
“Wavelength-Selective Switches for Mode-Division Multiplexing: Scaling and Performance Analysis”
“Design of a Compact Two-Mode Multi/Demultiplexer Consisting of Multimode Interference Waveguides and a Wavelength-Insensitive Phase Shifter for Mode-Division Multiplexing Transmission”
“Mode-Division Multiplexing Transmission System With DMD-Independent Low Complexity MIMO Processing”