Few-mode erbium-doped fiber with trench and weak coupling for mode gain equalization based on layered-doping technology.

Abstract

A few-mode erbium-doped fiber (FM-EDF) with a step refractive index and trench structure is designed and proposed to realize the modal gain equalization of a few-mode erbium-doped fiber amplifier (FM-EDFA). The layered-doping technology is used to reduce the mode gain difference (DMG). The doping radius and doping concentration are adjusted to obtain the optimum FM-EDF structure. When the designed FM-EDF is applied to the FM-EDFA, the DMG of the whole C-band is less than 0.15dB and the DMG is less than 0.12dB at 1550nm. The minimum refractive index difference (Δ n eff) between modes can be calculated according to the refractive index and radius of the fiber core; i.e.,1.35×10-3, which will greatly reduce the coupling between modes in a practical application. Tolerances in the fiber manufacturing process are also considered for reliable FM-EDFA performance. When the doping radius and concentration of each doping layer fluctuate by ±15% based on the precise value, the maximum DMG increases to 1.8dB. In general, DMG can maintain a small value, which is beneficial for application in optical communications systems.