All-fiber few-mode Er/Yb co-doped fiber amplifier for mode division multiplexing under cladding pumping

We experimentally demonstrate an all-fiber Er/Yb co-doped fiber (EYDF) amplifier with a low differential modal gain of 1.6 dB (for LP₀₁, LP_11a and LP_11b signal modes at 1535 nm, using a 980 nm multi-mode pump). The pump power is coupled into the fiber core by side pumping, which is used a piece of double-clad EYDF and helix-wounded tapering operation. The gain of all three modes is higher than 16 dB. The all-fiber amplifier scheme eliminates the spatial lens in the traditional core-pumping configuration and shows natural ability in integration with the transmission fiber links due to the stable fiber interconnect. Thus, we believe that the all-fiber few-mode amplifier should be probably the highly practical solution in the long-haul mode-division multiplexed transmission.