Sub-kHz linewidth, low noise single-frequency fiber laser based on Yb³⁺-doped fluoro-sulfo-phosphate fiber and...

Single-frequency fiber lasers (SFFLs) at 1.06 μm with sub-kHz linewidth and low noise are pivotal for applications ranging from gravitational wave detection to coherent optical communication. In this work, we demonstrate a sub-kHz linewidth SFFL utilizing a custom-developed Yb³⁺-doped fluoro-sulfo-phosphate (FSP) fiber, distinguished by optimized spectral characteristics, superior stability, and a high gain coefficient of 5.6 dB/cm. To suppress frequency noise without relying on complex electronic feedback, a self-injection locking technique was theoretically modeled and experimentally implemented. Furthermore, to circumvent the resolution limitations of traditional delayed self-heterodyne interferometry for sub-kHz sources, a precise linewidth determination method based on the coherent envelope of the signal was employed. Consequently, the linewidth of the Yb³⁺-doped FSP fiber SFFL was narrowed from 4 kHz to 512 Hz. The relative intensity noise was significantly suppressed to -155 dB/Hz, representing a 35 dB reduction compared to the free-running configuration while maintaining moderate output power and slope efficiency. These results demonstrate that combining high-gain FSP fiber with self-injection locking constitutes a robust, high-performance solution for precision laser sources.