1. ~ 2 μm laser crystals doped with Tm³⁺ or Ho³⁺

Tm³⁺ has a strong absorption near ~790 nm and a large absorption cross-section, so the ~790 nm commercial LD can be directly used as a pump source. Another feature and advantage of this band is that there is a cross-relaxation (CR) energy transfer process among Tm³⁺ ions:

Tm³⁺: ³H₄ + Tm³⁺: ³H₆ → Tm³⁺: ³F₄ + Tm³⁺: ³F₄

In Figure 3, a particle in the ³H₆ ground state absorbs a ~790 nm photon, is excited to the ³H₄ energy level, and then transitions to the ³F₄ energy level through a non-radiative transition, and the energy released at the same time causes another particle in the ³H₆ ground state to be excited to the the ³F₄ energy level and remits light. In this way, the theoretical maximum quantum efficiency of Tm³⁺ can reach 200%, and the continuous laser slope efficiency can exceed 40% (determined by quantum defects).

Regardless of weak field or strong field, whether it is low phonon energy or phonon energy, whether it is ordered or disordered lattice, whether it is single component or multi-component, from fluoride to a series of host crystals such as oxides have achieved Tm³⁺ laser output at ~2 μm. The typical examples are YLiF₄ (YLF), LiLuF₄ (LLF), CaF₂, SrF₂, Y₃Al₅O₁₂ (YAG), YAlO₃ (YAP), KY(WO₄) ₂ (KYW), CaYAlO₄ (CYA), CaGdAlO₄ (CGA), Lu₂O₃ (single crystal and ceramics), etc.

Based on different crystal field environments and Stark energy level splitting, CW laser output with different wavelengths in the range of 1.817~2.14 μm has been acquired, from weak field to strong field, by using rod, disk or slab of laser crystals. The Lawrence Livermore National Laboratory (LLNL) of the United States proposed the concept of developing a high repetition rate PW laser device with an average output power of 300 kW based on Tm:YLF with diameter of 4 inches. In 2010, Ehrenreich et al. used 12 channels of 790 nm LD to pump Tm³⁺ doped glass fiber, and achieved a 2.045 μm continuous laser output with a slope efficiency of 53% and an output power of more than 1 kW. In 2018, Su Liangbi, Liu Jie, etc. used LD pumping Tm, La: CaF₂ crystals to achieve a laser output of 1.92 μm with the highest slope efficiency of 67.8% which is the highest value reported worldwide so far for LD pumped Tm³⁺ ~2 μm all-solid-state lasers.