Short-cavity single-frequency fiber lasers and high-repetition-rate passively mode-locked fiber lasers based on high-gain rare-earth (RE)-doped fibers have significant applications in the manufacturing, healthcare, and military fields. Although RE-doped silica fibers have advantages such as low propagation loss and easy splicing, the low fiber unit gain coefficient caused by the low doping concentration of RE ions severely limits the performance of short-cavity lasers. In recent years, with the expansion of fundamental research on RE-doped silica glasses and significant improvements in fiber fabrication techniques, RE-doped silica fibers have been developed and successfully applied for short-cavity lasers. The performance of short-cavity lasers at various wavelengths has been significantly improved, unleashing their potential for application. Considering fiber fabrication techniques and short-cavity laser applications using different RE-doped silica fibers, this review systematically analyzes the latest developments and application progress of highly RE-doped silica fibers and predicts their future development.

Owing to the continuous improvement of fiber fabrication techniques and the further development of basic research on RE-doped silica glasses, the doping concentration of RE ions and the pump absorption coefficient of silica fibers have been significantly improved in recent years. Table 4 lists the high absorption coefficient Nd³⁺, Yb³⁺, Er³⁺, and Tm³⁺-doped silica fiber products developed by special fiber manufacturers such as Nufern, Coractive, and Liekki. The most typical is the Coractive Yb406 Yb³⁺-doped silica fiber, which has a core absorption coefficient of 2400 dB/m at 976 nm. According to estimation, the Yb doping content (mass fraction) in the core can reach 5% (or even higher), which is significantly higher than the Yb doping content (mass fraction) of conventional Yb³⁺-doped silica optical fibers (~1%). Taking these high-absorption RE-doped silica fibers as gain media, many research groups have successfully demonstrated their applications in short-cavity single-frequency and high-repetition-rate mode-locked fiber lasers with wavelengths ranging from the 0.9 to 2.0 μm bands. For instance, in the 0.9 μm wavelength band, the research team from the Shanghai Institute of Optics and Fine Mechanics successfully developed a highly Nd³⁺-doped silica single-mode fiber with a 4.1 dB/cm pump absorption coefficient. The research team demonstrated DBR single-frequency lasers at 890‒910 nm based on this fiber, extending the single-frequency laser of RE-doped silica fibers to below 900 nm (Fig. 11). Additionally, they also demonstrated an over 200 MHz high-repetition-rate passively mode-locked laser at 920 nm with an F‒P cavity structure (Fig. 12).

In the 1.0 μm wavelength band, in 2023, a research team from Tianjin University realized a high-efficiency 1064 nm single-frequency DBR fiber laser using the above mentioned Coractive Yb406 fiber. The slope efficiency reached 66.4% with only a 1.2 cm Yb³⁺-doped fiber (Fig. 13), which is the highest efficiency recorded for short-cavity single-frequency lasers using Yb³⁺-doped silica fibers. This efficiency is comparable to that of high-gain Yb³⁺-doped phosphate fiber, indicating that the fabrication technique for high-doping-concentration RE-doped silica fibers has been significantly improved.

In the 1.5 μm wavelength band, in 2021, the research team from Shandong University conducted studies on a high-repetition-rate passively mode-locked laser using a commercial highly Er³⁺-doped silica fiber (Liekki-Er110-4/125). Through in-depth analysis of SESAM (semiconductor saturable absorber mirror) parameters and experiments, they noted that an SESAM with a smaller modulation depth was better for high-repetition-rate passively mode-locked lasers. After optimization, they realized a 5 GHz fundamental-repetition-rate passively mode-locked laser at 1.5 μm using only a 2.0 cm Liekki-Er110 fiber as the gain medium (Fig. 20).

In the 2.0 μm wavelength band, in 2024, a research team from the University of Adelaide in Australia realized single DBR fiber lasers at 1908 nm, 1950 nm, 1984 nm, and 2050 nm, using a custom high-absorption 5/125 Tm³⁺-doped silica fiber from Coherent. The numerical aperture of the fiber core is 0.21, and the absorption coefficient at 1540 nm is 195 dB/m±6 dB/m. The length of the Tm³⁺-doped silica fiber is 25 mm. The slope efficiencies corresponding to the 1908 nm, 1950 nm, 1984 nm, and 2050 nm bands are 33%, 37%, 48%, and 26%, respectively (Fig. 22). Among them, the direct output power from the cavity at 1984 nm exceeds 1 W. The efficiency and power at 1984 nm are currently the highest recorded for a DBR single-frequency laser at a wavelength of 2.0 μm.

RE-doped specialty optical fibers are crucial components of fiber lasers. For short-cavity fiber lasers, the performance of RE-doped fibers directly determines the laser parameters, including laser efficiency, wavelength, and repetition rate. As indicated above, highly RE-doped silica fibers are being increasingly applied in short-cavity lasers in the 0.9‒2.0 μm bands. Although some laser parameters obtained from RE-doped silica fibers are still worse than those from RE-doped soft glass fibers, the performance gap between RE-doped silica fibers and RE-doped soft glass fibers is constantly narrowing, indicating the application feasibility of RE-doped silica fibers in short-cavity lasers. However, we note that the fabrication techniques of highly RE-doped silica fibers are mainly mastered by international manufacturers such as Liekki, Nufern, and Coractive, and in comparison, domestic research and fiber product development are lagging behind and require further breakthroughs. For the application of single-frequency narrow-linewidth and high-repetition-rate mode-locked fiber lasers, further research on highly RE-doped silica fibers can be conducted considering two aspects: further improvement of the performance of highly RE-doped silica fibers and short cavity lasers and the regulation of the fiber gain spectrum.