Differential Modal Gain Reduction of L-band 5-Mode EDFA Using EDF With Center Depressed Core Index

We report a 5-mode erbium-doped fiber (EDF) amplifier operating in the L-band (1570-1605 nm) designed to extend the bandwidth of mode division multiplexed transmission. We design an EDF with a center depressed core index and a ring-doped erbium concentration profile to obtain a low differential modal gain (DMG) and a low splice loss with a graded-index fiber as a transmission line. Then, we fabricate a center depressed core EDF and realize a gain of 20 dB for all the modes and a low DMG with a deviation of less than 3 dB in the 1570-1605 nm range with a 48-m-long erbium-doped fiber when the pump mode is LP 01 mode.