Gain equalization for a six-mode-group Er-doped fiber amplifier

We theoretically present a method for equalizing modal gain in a six-LP-mode erbium-doped fiber amplifier by combing two sections of erbium-doped fibers (EDFs) with different doping profiles. In the two EDFs, the dopant distributions are designed to be spatially mismatched to mainly support the optical amplification of different spatial modes. The optimum fiber designs have been found by employing genetic algorithm optimization when pumped by LP 11 mode, resulting in less than 1.0 dB DMG over 25 nm bandwidth in the C-band for the six-LP signal modes. In addition, tolerance study of various fiber parameters has been carried out theoretically to assess fabrication challenge in realizing a desirable amplification performance.