All-Fiber Broadband Degenerate Mode Rotator for Mode-Division Multiplexing Systems

An all-fiber broadband degenerate mode rotator for the rotation between arbitrary degenerate modes is proposed and investigated numerically. Furthermore, the rotator combined with a mode selective coupler (MSC) can be used to realize mode multiplexing. The mode multiplexing can be implemented in one physical dimension, which is considered easier than 3-D cascading MSCs. It is found that the rotator for any two degenerate modes rotation possesses a high rotating efficiency from 1450 to 1650 nm, and the insertion loss at the interconnection of a fiber-based rotator and a normal fiber is lower than 3 dB for any modes. Moreover, the rotators in performance are not sensitive to the four geometrical parameters of design. Finally, a six-mode multiplexer using the proposed rotator combined with MSC is modeled and analyzed. It is shown that multiplexing efficiencies of higher than 70% and mode extinction ratios of more than 20 dB for mode LP01, LP11a/LP11b, LP21a/LP21b, and LP02 are achieved in the C-band.