Microsphere-lensed chalcogenide fibers for highly efficient and alignment-tolerant mid-infrared coupling

Efficient and robust coupling between mid-infrared (mid-IR) optical fibers remains a critical challenge due to the lack of integrated micro-optical components for beam shaping and alignment. We propose and experimentally demonstrate a microsphere-lensed chalcogenide fiber fabricated via direct fusion melting of an arsenic sulfide (As2S3) fiber end, providing both enhanced input coupling and output beam focusing through the optical-field transformation introduced by the microsphere interface. Flat-end-fiber exhibits severe performance degradation under misalignment, reaching a 3 dB coupling loss at an axial offset of 900 μm, a radial offset of 80 μm, and an angular offset of 25°. In contrast, the proposed microsphere-lensed fiber significantly improves both coupling efficiency and alignment tolerance across multiple spectral regimes, including the 1.55 μm telecommunication band and the representative 4.7 μm mid-IR band. While the 3 dB tolerance is extended to 1200 μm (axial), 120 μm (radial), and 40° (angular). In addition, the microsphere structure provides intrinsic beam-focusing capability, enabling significantly reduced output spot size. Unlike conventional heterogeneous fiber fusion splicing, the proposed approach relies on direct fiber-end reshaping, thereby eliminating splicing-induced insertion loss and substantially simplifying fabrication. These results establish a practical and scalable strategy for high-performance, alignment-relaxed mid-IR fiber coupling, offering significant potential for integrated all-fiber systems in sensing, spectroscopy, and high-power laser applications.