Investigation on helix side-pumping few-mode coupler with high pump efficiency and excellent modal gain flatness

Space division multiplexing (SDM) technology is a crucial method for enhancing the capacity of optical communication information transmission systems. In practical applications, there is an urgent need for stable and efficient SDM fiber amplifiers (SDMFAs), and helix side-pumping coupler (HSPC) serves as an effective solution. In this work, a 3D simulation model of the HSPC has been developed using the beam propagation method (BPM). The simulation results demonstrate excellent coupling efficiency and modal gain flatness between the pump fiber (PF) and the target fiber (TF). The maximum side-pumping efficiency of the double-clad fiber (DCF) is 96.20% and the differential modal gain (DMG) is less than 0.28 dB. In experiment, the maximum coupling efficiency achieved is 70.50% through tapered welding, with a DMG of less than 1.5 dB.