21 spatial mode erbium-doped fiber amplifier for mode division multiplexing transmission

The differential modal gain is dramatically reduced by using the double cladding erbium doped fiber (DC-EDF) with a refractive index trench structure, which helps to tightly confine all the 21 spatial modes in the core and to mitigate the bending loss. Our experimental results show the differential modal gain is about 3 dB when average signal modal gain for all 12 mode groups is up to 15 dB across the C band.

Our method of using DC-EDF with a properly designed trench structure can be used to develop gain equalized few mode amplifiers supporting more spatial modes.