Few-Mode Erbium-Doped Fiber Amplifier With High Gain and Low Differential Modal Gain for Mode-Division-Multiplexed Systems

The increasing development of information technology has led to a higher demand for communication capacity. Moreover, the mode division multiplexing (MDM) is considered one of the important technologies to expand the optical fiber transmission capacity, and the few-mode erbium-doped fiber amplifier (FM-EDFA) is a main device applied for optical fiber loss compensation under long-haul communication transmission systems. This article reports the design and characterization of a six-mode erbium-doped fiber amplifier (6M-EDFA) for MDM systems. A center-depressed optical fiber with a trench-assisted structure, is designed and the adjustment of the relevant parameters is applied to reduce the splice loss between the six-mode erbium-doped fiber (6M-EDF) and the transmission fiber. The results show the lowest splice loss is theoretically 0.293 dB. By analyzing the mode field distribution, two erbium ion doping regions are initially identified, the ring region is further layered, and the doping concentration of the three layers is optimized to achieve high gain and low differential modal gain (DMG) using a genetic algorithm (GA). In the case of core forward pumping of the LP01 mode at 1480 nm, the simulation results show that the average gain of 25.7 dB and the DMG of 0.277 dB are obtained at 1550 nm considering the mode coupling. Moreover, a 6-mode MDM transmission system is built to fully verify the performance of the designed amplifier to meet the requirements of the MDM communication system.