Abstract

A ring-core few-mode erbium-doped fiber (FM-EDF) with step refractive index is designed and proposed to realize the modal gain equalization of the amplifier. The layered-doping method is used to reduce the mode gain difference (DMG), and the doping radius and doping concentration are adjusted to obtain the best FM-EDF structure. When it is applied to the few-mode erbium-doped fiber amplifier (FM-EDFA), the DMG at 1550 nm is less than 0.44 dB. The gains of each mode in the entire C-band are greater than 16 dB. To further improve the gain equalization characteristics of the designed fiber, the DMG can be reduced to 0.03 dB by adding trenches at appropriate positions on the basis of the same core structure. The tolerances in the optical fiber manufacturing process are considered to verify the reliability of the FM-EDFA performance. The doping radius and doping concentration are changed by $\pm {15}\%$ based on the precise value to consider the tolerances in the fiber manufacturing process to verify the reliability of the FM-EDFA performance. Generally speaking, doping radius changes have a greater impact on DMG, so higher production accuracy is required. To explore the experimental structure and procedures of FM-EDFA, a preliminary experiment is carried out. The experimental results obtained have great reference significance for subsequent experiments. The characteristics of the FM-EDFA, such as all-fiber and low DMG, are difficult to implement by other methods.

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