Numerical and experimental study of a Few-Mode (FM) Erbium Doped Fiber Amplifier (EDFA) suitable for mode division multiplexing (MDM) is reported. Based on numerical simulations, a Few-Mode Erbium Doped Fiber (FM-EDF) has been designed to amplify four mode groups and to equally amplify LP11 and LP21 mode groups with gains greater than 20 dB and with a differential modal gain of less than 1 dB. Experimental results confirmed the simulations with a good concordance. This modal gain equalization is obtained by tailoring the erbium spatial distribution in the fiber core with a ring-shaped profile.

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We report and characterize an amplifier which provides gain to a record six spatial modes simultaneously. Two sections of erbium-doped fibre with different mode-dependent gain characteristics are concatenated to equalize gain between modes. The overall gain excursion does not exceed 4dB across all modes. The noise figure is found lower than 7dB.