Multi-plane light conversion-based pump-signal combiner for ring-core fiber amplifiers

Aiming to address the challenge of simultaneously combining multi-wavelength pump and signal beams into specific spatial modes of ring-core erbium-doped fiber amplifiers (RC-EDFAs), we propose and experimentally demonstrate a compact pump-signal combiner based on multi-plane light conversion (MPLC). The proposed combiner, optimized via a weighted wavefront matching algorithm, is fabricated using maskless grayscale lithography with 128-level phase quantization. It utilizes four cascaded phase plates to simultaneously convert a 980 nm pump beam (LP₀₁ mode) and three 1550 nm signal beams (LP_11a, LP_11b, LP₂₁ modes) into the eigenmodes of a ring-core fiber. By integrating this combiner into an RC-EDFA system, we achieved small-signal gains exceeding 16 dB for all signal spatial modes, specifically with a maximum gain of 20.7 dB for the LP₂₁ mode. In the saturation regime, distinct gain equalization is observed, reducing the differential mode gain (DMG) to less than 0.5 dB. Furthermore, broadband amplification is demonstrated across the C-band (1530-1565 nm), maintaining a DMG of approximately 3 dB. These results validate MPLC technology as a viable and high-fidelity solution for pump-signal multiplexing in ring-core fiber amplifiers.