High-power 980 nm fiber lasers, serving as ideal pump sources for erbium-doped fiber amplifiers (EDFA), face a critical challenge in efficiently coupling light from large-mode-area (LMA) fibers (typically 20/130 μm) to standard single-mode fibers (SMF). The significant mismatch in mode field diameter (MFD) between these fibers results in severe coupling losses, often exceeding 0.7 dB for commercial mode field adapters (MFA) operating in the 980 nm band. To address this issue, we propose and demonstrate a novel MFA based on the synergistic optimization of a multi-period graded-index optical fiber (GIOF) and thermally expanded core (TEC) technology.
The proposed MFA employs a precisely designed GIOF to achieve deterministic periodic beam focusing, enabling mode field transformation from large to small dimensions within a fixed physical length. The optimal length of the GIOF is determined by the formula L=nP+0.5P(n≥1), where P is the self-imaging period. To compensate for residual mode field mismatch caused by fabrication tolerances, the receiving 9/125 μm SMF is subjected to thermal core expansion treatment using a CO₂ laser, which controllably enlarges its MFD to better match the output field of the GIOF.
The optimized MFA is fabricated and integrated into a home-built 980 nm fiber laser system for validation. The bare laser system exhibits a slope efficiency of 15.15% and an optical-to-optical efficiency of 13.65% at an output power of 9.35 W. With the proposed MFA inserted, the system maintains stable operation with a slope efficiency of 14.18% and an optical-to-optical efficiency of 12.63%, corresponding to a minimum MFA insertion loss of 0.34 dB. For comparison, based on a typical commercial MFA insertion loss of 0.76 dB in the same band, the system efficiency would theoretically drop to approximately 13.2% (slope efficiency) and 11.43% (optical-to-optical efficiency). The proposed MFA reduces the insertion loss by over 50% compared to commercial counterparts and significantly mitigates the system efficiency penalty.
This work provides an effective and reliable device solution for high-brightness 980 nm fiber laser pumping, demonstrating the potential of combining GIOF and TEC technologies for high-performance mode field management in high-power fiber laser systems.