Design and Analysis of Annulus Core Few Mode EDFA for Modal Gain Equalization

A few mode (FM) fiber amplifier is widely under study to overcome the issue of Internet traffic in optical communication. This letter proposes annulus core FM erbium-doped fiber (EDF) with annulus or extra annulus doping for the amplification of the LP01, LP11, LP21, and LP31signal mode groups with low differential modal gain (DMG). Our simulations confirm that extra annulus doping helps in reducing DMG of higher order mode groups. We have achieved less than 2.2-dB DMG over C-band for four-mode groups using extra annulus doping. The proposed EDF would be useful for the space division multiplexing (SDM)-based optical fiber communication system.