Rational design of Yb-doped fluorophosphate fiber for narrow-linewidth single-frequency fiber laser at 1013 nm

The development of Yb³⁺-doped fiber lasers operating at short wavelength (<1030 nm) is crucial for applications in quantum science and precision metrology, but is hindered by the low gain and severe reabsorption of conventional host materials in this spectral region. Exploring high-gain fibers tailored for this band offers a material-based solution. Here, we propose a rational design strategy for developing a multi-component fluorophosphate (FP) glass fiber, aiming at addressing the challenges of devitrification and spectral property modulation. The methodology begins with selecting a highly stable host matrix from the glass-forming region, followed by engineering the rare-earth local environment using modifier cations, guided by molecular dynamics simulations and Raman spectroscopy. The custom-designed Yb³⁺-doped FP glass exhibits a blueshifted emission peak at 1013 nm, a broad effective linewidth of 86.1 nm, a prolonged fluorescence lifetime of 2.38 ms, and a large Stark splitting of 812 cm^-1, which results in high gain (6.56 dB/cm at 1064 nm and 9.09 dB/cm at 1013 nm). To validate its performance, a single-frequency fiber laser (SFFL) was constructed using only a 9 mm segment of this active fiber, achieving single-longitudinal-mode operation at 1013.4 nm with a narrow linewidth of 4.5 kHz, a low pump threshold of 8.3 mW, and exceptional stability (RMS instability<0.8% over 1.5 hours). This work presents a high-gain medium for narrow-linewidth, short-wavelength SFFLs and demonstrates a generalizable design-to-device pipeline for other high-gain fibers targeting specific operational wavelengths.