Tm–Sn Codoped Fiber Fabrication and Optical Properties via Vapor–Liquid Doping

2 μm laser systems offer superior eye safety and wide‐ranging applications in remote sensing, biomedical fields, and industrial applications. The increasing demand for high‐power 2 μm lasers faces challenges from thermal instability at fiber splice points during high‐power operation. To address this, a novel Tm–Sn codoped fiber combining photosensitivity and gain properties through modified chemical vapor deposition and liquid‐phase doping is developed. The fabricated fiber features a core/cladding diameter of 9.03/125.50 μm with 0.54 μm concentricity, demonstrating a refractive index fluctuation of 2.591 × 10 − 3 and Tm 2 O 3 concentration of 2.70 wt%. Characterization by Raman and fluorescence spectroscopy confirms successful Tm–Sn codoping. The fiber shows exceptional compatibility with standard single‐mode fibers (0.04 dB splice loss) and enhanced photosensitivity, achieving a refractive index change of 6.75 × 10 − 5 under 248 nm UV exposure—surpassing Ge‐doped fibers by 3 × 10 − 5 . This innovative design enables integrated gain medium‐resonator structures without fusion splices, providing a robust solution for high‐power 2 μm laser development. The fiber's unique combination of optical properties addresses critical thermal stability issues while maintaining performance efficiency.