Hybrid Mode-Bases Multiplexed Few-Mode Erbium-Doped Fiber Amplifier for Mode Gain Equalization

A mode gain equalization inline few-mode erbium-doped fiber amplifier (FM-EDFA) is an important device in a long-distance mode division multiplexing fiber communication system. In this paper, we propose and demonstrate an FM-EDFA with low differential mode gain (DMG) developed from a hybrid mode-bases multiplexed amplification scheme. This strategy exploits the differential mode-bases gain (DMBG) between the linearly polarized (LP) and orbital angular momentum (OAM) modes with the same vector mode group to equalize the DMG of an FM-EDFA. Based on this scheme, the mode gain equalization of a 5-mode-group EDFA is achieved, and mode matching between the EDF and transmission fiber is attained without increasing the difficulty of EDF fabrication. The simulation results reveal that the maximum DMG among the 5 mode groups at 1550 nm can be reduced from 0.75 dB to 0.11 dB by switching the signal mode LP_3,1 to OAM_3,1 when the FM-EDFA is pumped by a fundamental mode. We fabricated an FM-EDF and characterized it by constructing a testing system that supports the multiplexing and switching of all 5 mode groups. A decrease in the DMG from 1.88 dB to 1.08 dB was experimentally demonstrated. Finally, we construct a 5-mode-group all-fiber FM-EDFA using a custom-made isolated wavelength division multiplexer, and an average mode gain above 26 dB and an average DMG of 1.79 dB over the C band are achieved.