Amplification of 18 OAM modes in a ring-core erbium-doped fiber with low differential modal gain.

We theoretically propose two orbital angular momentum (OAM) erbium-doped ring-core fiber (RCF) amplifiers capable of providing relatively uniform gain for 22 modes with 18 OAM ones over the C-band. Two schemes of doping profile are discussed, one with single layer erbium doping and the other with double layer erbium doping. Theoretical analyses and numerical simulations suggest that the proposed first OAM erbium-doped fiber amplifier (OAM-EDFA) can obtain a gain larger than 20 dB for all 22 modes with differential modal gain (DMG) lower than 0.71 dB. The second OAM-EDFA performs better and can provide a larger gain over 21.5 dB for all 22 modes, with a smaller DMG below 0.27 dB and the noise figure (NF) lower than 3.9 dB over the whole C band. The presented OAM-EDFA may open up new perspectives for long-distance transmission in capacity scaling fiber-optic communications using OAM modes.