Adaptive Modal Gain Equalization Techniques in Multi-Mode Erbium-Doped Fiber Amplifiers

We demonstrate two adaptive methods to equalize mode-dependent gain (MDG) in multi-mode erbium-doped fiber amplifiers (MM-EDFAs). The first method uses a spatial light modulator (SLM) in line with the amplifier pump beam to control the modal powers in the pump. The second method uses an SLM immediately after the MM-EDFA to directly control the modal powers in the signal. We compare the performance of the two methods applied to a MM-EDFA with a uniform erbium doping profile, supporting 12 signal modes in two polarizations. We show that root-mean-squared MDGs lower than 0.5 and 1 dB can be achieved in systems having frequency diversity orders of 1 and 100, respectively, while causing less than a 2.6-dB loss of mode-averaged gain.