Modeling of a ring-core trench-assisted few-mode BDFA for seven-mode signal gain equalization

In this paper, a ring-core trench-assisted few-mode bismuth-doped fiber amplifier (BDFA) is simulated on the basis of the three-energy level. The fiber is designed to support four modes of signal group transmission for practical considerations, including LP01, LP11, LP21, and LP31. The results suggest that (1) it is possible to obtain gain equalization of the three signal groups by using the LP21 mode pump independently, where the maximum difference in modal gain (MAX DMG) is about 0.9 dB, except for the LP31 mode signal; (2) by combining the LP01 and LP31 mode pumps, the average gain of the groups increases by 14%, and the MAX DMG decreases by nearly 60% (3.8 to 1.5 dB) compared to the LP01 pump alone; and (3) with the same combination of mode pumps, the ring-core BDFA (1.5 dB) achieves better gain equalization than the single-core BDFA (2.8 dB). The analysis is informative for the future development of a multimode BDFA.