Stark splitting and concentration quenching of Yb³⁺ in a mixed-ion phosphate fiber for kHz single-frequency lasers across 1.0∼1.1 µm

Yb³⁺-doped fibers are extensively utilized in single-frequency fiber lasers (SFFLs) for applications such as deep-space exploration, atomic physics, and high-resolution spectroscopy, owing to the simple energy scheme, strong absorption band, and minimal quantum defect. However, a small Stark splitting of Yb³⁺ tends to induce a quasi-three-level configuration, leading to difficult population inversion and significant thermal blocking effects. In this study, we propose a mixed-ion strategy that exploits the high rare-earth solubility of phosphate-based systems, coupled with enhanced Stark splitting, enabling Yb³⁺ to exhibit quasi-four-level radiative behavior. A combined molecular dynamics simulation and spectroscopic investigation demonstrates that the mixed-ion strategy not only alleviates crystallization or quenching issues associated with higher Yb³⁺ concentration but also enhances the crystal field strength in the local environment surrounding Yb³⁺. Based on these findings, a heavily doped (up to 20 mol% Yb³⁺) mixed-ion phosphate (MIP) fiber was developed, and ∼kHz SFFLs operating at various wavelengths have been demonstrated, confirming the amplification capabilities of MIP fiber across 1.0∼1.1 µm, which provides insights into the compositional influences on Yb³⁺ clustering, Stark splitting, and laser performance.