Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping and a simple LP₀₁ pump configuration.

We experimentally validate a numerical model to study multimode erbium-doped fiber amplifiers (MM-EDFAs). Using this model, we demonstrate the improved performance achievable in a step index MM-EDFA incorporating a localized erbium doped ring and its potential for Space Division Multiplexed (SDM) transmission. Using a pure LP₀₁ pump beam, which greatly simplifies amplifier construction, accurate modal gain control can be achieved by carefully tuning the thickness of the ring-doped layer in the active fiber and the pump power. In particular, by optimizing the erbium-ring-doped structure and the length of active fiber used, over 20dB gain for both LP₀₁ and LP₁₁ signals with a maximum gain difference of around 2 dB across the C band are predicted for a pure LP₀₁ pump beam delivering 250 mW power at 980 nm.