Ultra-low-loss all-fiber 976/1550 nm few-mode pumping/signal multiplexer for four-mode Erbium-doped fiber amplifier

An all-fiber few-mode pumping/signal multiplexer (FM-PSM) for four-mode Erbium-doped fiber amplifier is proposed and fabricated by fused biconical taper (FBT) technology of bonding a few-mode fiber (FMF) and a pre-pulled single-mode fiber (SMF). Based on the adiabatic coupling condition of pump light and the lossless transmission condition of four signal modes at wavelength of 1550 nm, the LP₃₁ mode is selected as the pump mode at the wavelength of 976 nm. Detailed theoretical analysis has demonstrated that the fused biconical coupler is capable of achieving a wideband, low loss pumping/signal multiplexing function. Through precise simulation design and manufacturing parameter control, a FM-PSM is fabricated, achieving high pump efficiency up to 90 % and low insertion loss of less than 0.2 dB for all 4-LP signal modes across the C-band. The proposed FM-PSM can ensure 4-LP modes amplification, facilitating the use of all-fiber optical amplifier in high-capacity modal-division multiplexing fiber communication systems.