In-line Few-Mode Optical Amplifier with Erbium Profile Tuned to Support LP01, LP11, and LP21 Mode Groups

We report and characterize an optical amplifier capable of five-mode amplification, with erbium concentration profile adjusted for low gain excursion across the modes. The cross-talk between modes is characterized in the middle of the C-band. Introduction Mode division multiplexing (MDM) is a way to increase fibre capacity, which consists in sending several data streams along different spatial fibre modes, generally inside specificallydesigned few-mode fibres (FMF). The first experimental demonstrations of this concept were reported in 2011, based on the use of 1,2 3 4 two , three or even five modes , but over limited transmission distances. To go beyond, optical amplifiers capable of handling all the modes at once would need to be inserted periodically. In early reports, few-mode optical amplifiers are described, based either on erbium 5,6 amplification or distributed Raman 7 amplification , but they are only used in the terminals or without transmission fibre, none as in-line repeaters. Arguably, the key challenges in few-mode optical amplifiers are reducing modal crosstalk and providing similar gains across the various optical modes. In both 5,7 references , gain equalization was addressed by pre-converting the pump power distribution into a higher order mode (LP21) before feeding it into the amplifying fibre or by injecting the 6 fundamental mode with an offset . Here, we 6 pursue another approach already suggested in for our few mode erbium doped fibre amplifier (FM-EDFA).