I. Introduction
Space division multiplexing has been widely investigated during the last few years as a new technique that could permit to increase the capacity of optical fibers [1] –[6]. Today, two technologies are deeply considered, namely multicore fibers (MCF) and few-mode fibers (FMF). In the case of FMF, each transverse mode can be used as an individual pathway able to carry the total data transmission capacity of a current single-mode fiber (SMF). However, so as to make this technology compatible with long-haul transmissions, periodic amplification of the signals, multiplexed on the different modes and the different wavelengths, will be necessary. In this paper, a study on few-mode erbium doped fiber amplifier (FM-EDFA) especially designed for mode division multiplexed (MDM) transmissions is reported. The role of such amplifiers is to provide high and flat gain for the different channels that are used for data transmission (meaning the same gain over wavelengths and modes). Different techniques can be used to reach such gain equalization: 1) tailoring the Er doping profile (EDP) [7]–[11], 2) tailoring the pump intensity pattern [11]– [13], and 3) concatenating different FM-EDF [14].