1. 2  ~ 2.3 μm laser crystals doped with Tm³⁺ 

Compared with the 2 μm band (³F4 → ³H₆) of Tm³⁺, the 2.3 μm laser operation based on the ³H₄ → ³H₅ transition of the Tm³⁺ doped laser medium has the following advantages:

(1) ~790 nm LD is directly pumped to the upper energy level of the laser. Tm³⁺ has a strong absorption around 790 nm (directly corresponding to the ³H₄ → ³H₆ transition), which can match the emission wavelength of the current mature commercial AlGaAs LD, so as to realize high-performance LD pumping all-solid-state high-efficiency 2.3 μm laser operation.

(2) Four-level laser operation at room temperature. The energy level of ³H₅ locates at ~ 9 000 cm^-1 above the ground state ³H₆, which is a four-level operation without reabsorption. It has weak temperature effect, and can be operated at room temperature.

(3) Compared with the 2 μm band, 2.3 μm is in the weak absorption zone of water, and the laser operation is less affected by the surrounding air humidity.

(4) Excellent spectral characteristics. Tm³⁺ has a large stimulated emission cross-section in the 2.3 μm band and a wide gain bandwidth, which can support fs-level mode-locked pulse generation. The Tm:YLF fluorescence spectrum can cover the 2.2-2.45 μm band, and the stimulated emission cross section at the peak is 3.5×10^-20 cm² @ 2300 nm. The stimulated emission cross-section at the Tm:YAG peak is 4.4×10^-20 cm² @ 2300 nm, and the bandwidth reaches 50 nm. In 2019, LD-pumped Tm:YLF crystals have achieved 1.15 W continuous laser output in the 2.3 μm band, but the slope efficiency is low (≤19%). In 2020, Muti et al. used 780 nm Ti-sapphire pumped  and graphene mode-locked Tm: KY₃F₁₀, to generate 739 fs ultrafast laser output in the 2.3 μm band.

At present the research on Tm³⁺ doped laser is mainly focused on obtaining high-efficiency high-power 2 μm laser output (³F₄ → ³H₆) . There are less research on 2.3 μm laser (³H₄ → ³H₅) , and the progress is relatively slow. The quality of the 2.3 μm output is far behind the 2 μm band, and the highest continuous output power of the single-doped 2.3 μm system is only 2.6 W, so there is a lot of room for improvement. Using LD to directly pump Tm³⁺ single-doped solid gain medium to generate high-power and high-efficiency 2.3 μm laser is an important research direction.