Results and Discussions The physical properties of the highly Tm³⁺-doped silica fiber are measured, which shows the element distribution and the refractive index distribution at the end face of the optical fiber (Fig. 4). The all-fiber short cavity laser system (Fig. 6) is independently built. The 808 nm laser is used to pump the highly Tm³⁺-doped silica fibers with different lengths of 2.3 cm, 4.6 cm, and 6.5 cm, and the highest slope efficiency of 14.1% is obtained when the fiber length is 4.6 cm. When the pump power is 250 mW, the optical signal-to-noise ratio (SNR) can be about 70 dB (Fig. 7). The silica fiber with a length of 2.3 cm is selected to test the gain performance of the Tm³⁺-doped silica fiber. When the seed source power is -9.34 dBm, the net small-signal gain coefficient is 0.48 dB/cm (Fig. 9). Meanwhile, the loss coefficient is 1.22 dB/cm at 1310 nm, and the absorption coefficient of Tm³⁺-doped silica fibers at 808 nm is 2.56 dB/cm. The data indicate that the input signal can be effectively amplified.

The 2.0 μm-band single-frequency laser has the advantages of narrow linewidth, low noise, and good monochromaticity, which is widely used in many fields, such as precision measurement, spaceborne lidar, and high-resolution spectroscopy. Compared with multi-component glass fibers, the rare-earth-doped silica fiber is the core gain medium of fiber lasers, which boasts stable physical and chemical properties, high mechanical strength, and easy system integration. However, it is difficult to achieve the high-concentration doping of rare earth ions by traditional fabrication processes. There is still a gap in the doping concentration between the reported multi-component glass and the silica glass prepared by mature modified chemical vapor deposition (MCVD) combined with the liquid-phase doping process. Used in the short gain fiber for single-frequency lasers based on a distributed Bragg reflection (DBR) structure, the highly Tm³⁺-doped technique ensures that the fiber has higher effective absorption to the pump source and a lower laser output threshold, which is more conducive to improving the laser performance of the system. For the high gain medium of 2.0 μm-band single-frequency lasers, how to further improve the concentration of Tm³⁺ in silica glass becomes the focus of this paper.

We use Tetracthoxysilane (TEOS) as the silicon source, Al₂O₃ as the network-forming body, and La₂O₃ as the dispersant of silica glass to prepare highly Tm³⁺-doped silica sol. Firstly, the high silica glass with the Tm³⁺ doping concentration of 8.29×10²⁰ cm^-3 is prepared by the sol-gel method and high-temperature sintering technology, which has good optical quality, and its spectral properties are characterized. Secondly, the sol-gel coating and melting taper drawing methods are combined innovatively to coat the inner wall of the silica capillary tube. After the film is heat-treated and tapered step by step, the silica fiber with a core diameter of about 4 μm and a cladding diameter of 125 μm is prepared, and the doping concentration of Tm³⁺ can reach as high as 8.29×10²⁰ cm^-3 in the silica fiber. This highly Tm³⁺-doped silica fiber could be easily fusion-spliced with commercial passive silica fibers. Finally, an all-optical fiber laser system with a DBR structure is built to test the laser performance.

In this paper, we fabricate highly Tm³⁺-doped high silica glass with a concentration of 8.29×10²⁰ cm^-3 by the sol-gel method and high-temperature sintering process. The highly Tm³⁺-doped silica fiber with a core diameter of about 4 μm and a cladding diameter of 125 μm is also prepared by the sol-gel coating and double melting taper drawing methods. For better laser performance of the highly Tm³⁺-doped high silica fiber prepared by this innovative process, the follow-up work will be carried out from the following two aspects: the composition control of the core glass and the optimization of the coating process. In terms of composition, the glass with the best fluorescence can be selected through different components. In terms of coating technology, the film thickness can be designed and adjusted, and the core size is adjusted to achieve better NA and mode-field matching when the silica fiber is fused with passive optical fibers. Meanwhile, a 789 nm source can be selected to further study the performance of fiber lasers. To sum up, this fiber preparation method has the potential to realize highly Tm³⁺-doped silica fibers, which is expected to be applied in 2.0 μm single-frequency fiber lasers and passively mode-locked fiber lasers with a high fundamental repetition rate.