Improving Modal Gain Equalization for Few-mode EDFA Enabled by Mode Routing Optimization

A few-mode erbium-doped fiber amplifier with two few-mode EDFs of uniformly doped step-index profile is proposed and experimentally demonstrated, employing two optical circulators to control the routings of different modes. In order to minimize the differential modal gain (DMG), amplifier parameters are discussed, including the lengths of two few-mode EDFs, core pump modes, and power ratio. In simulations, the gain performance of the proposed FM-EDFA is investigated, combining the conditions of the EDFs' lengths and pump configurations. Additionally, we construct an experimental setup of a four-mode EDFA and measured results indicate a DMG of 0.2 dB at 1550 nm, with less than 2.57 dB DMG and an average gain of 19.56 dB across the C-band.