A multicore fiber (MCF)-based mode multiplexer/demultiplexer (MUX/DEMUX) that can overcome the alignment issue of the fiber-based mode MUX/DEMUX is proposed. Design concept and fabrication results of the MCF-based mode MUX/DEMUX for two-spatial-mode operation (LP01 and LP11) (2M-MUX/DEMUX) and for three-spatial-mode operation (LP01, LP11a, and LP11b) (3M-MUX/DEMUX) are presented. The fabricated 2M-MUX/DEMUXes for C-band or L-band, using the same MCF with different elongation ratios demonstrate a coupling efficiency of greater than 90% over each band. Finally, a 3M-MUX/DEMUX with a fan-in/fan-out device is presented. The selective excitation of LP01, LP11a, and LP11b modes depending on input ports is experimentally demonstrated.
The characteristics of a multicore fiber with one-ring structure are reviewed. The one-ring structure, which has no center
core, can overcome issues on the hexagonal close-pack structure that is the most popular multicore structure. The onering
structure has flexibility in the number of cores and is unrelated to the core pitch limitation due to cutoff wavelength
lengthening thank to no center core structure. The one-ring structure is effective to suppress the worst case crosstalk that
is crosstalk assuming all cores carry equal signal power. In the case of hexagonal close-pack structure, the worst case
crosstalk of an inner core is 7.8 dB larger than that between two cores. The different worst crosstalk is observed
depending on the number of nearest neighbor cores. The one-ring structure can limit the degradation to 3.0 dB for all
cores. Fabricated 12-core fiber with the one-ring structure based on the simulation realized effective core area of 80 m2
and very low crosstalk less than -40 dB after 100-km propagation.