High-gain fluoro-sulfo-phosphate-based active fiber for compact low-threshold kilohertz-linewidth single-frequency fiber laser

Single-frequency fiber lasers (SFFLs) are of pivotal importance across optical communication , scientific research and national defense due to their excellent beam quality, long coherent length and low noise. However, the development of high-gain rare-earth-doped glass fibers, which are the key medium used in the SFFLs, remains a critical scientific challenge. To address this issue, we design and fabricate a single-mode fluoro-sulfo-phosphate-based Er 3 + ${}^{3+}$ /Yb 3 + ${}^{3+}$ -codoped glass fiber (denoted as FSP-EYDF). The gain properties and laser performance of the FSP-EYDF have been investigated comprehensively. The results show that the small signal gain coefficient of the home-made FSP-EYDF can reach 4.7 dB/cm at 1535 nm, which is three times higher than that of the silica glass fiber. A short piece of FSP-EYDF with a length of only 1.6 cm has been utilized as a gain medium to support an SFFL operating at 1534.4 nm, which has a threshold of ∼ $\sim$ 18 mW and a linewidth of 3.15 kHz. This study indicates that high-gain FSP fibers are promising candidates for compact low-threshold narrow-linewidth SFFLs.