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1. 3 2 ~ 3 μm laser crystals doped with Cr2+ The mid-infrared luminescence of transition metal ions (Ni2+, Co2+, Cr2+, Fe2+, etc.) is based on 3d→3d transitions. According to the different types of sites occupied by transition metal ions in the host material, they can be divided into two categories: occupying octahedral sites with inversion symmetry (such as: Ni2+, Co2+ doped halides); Symmetric tetrahedral sites (such as: Ni2+, Co2+, Cr2+, Fe2+ doped II-VI compounds).

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.

1.5  ~ 4 μm laser crystals doped with Fe2+ Compared with Cr:ZnSe, Fe:ZnSe has a smaller band gap and is prone to produce thermally induced multi-phonon quenching, so both laser power and efficiency are low. In 1999, Adams et al. realized the tunable wavelength of 3.98-4.54 μm at low temperature for the first time in Fe:ZnSe, and obtained laser output with slope efficiency of 8.2%. Pumped by Er3+ doped or Cr:ZnSe @ 2.7 μm laser, 4.0 μm wavelength and 1 W level continuous laser output have been obtained at room temperature. In 2020, Pushkin et al.

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.

1. 4  ~ 3 μm laser crystals doped with Er2+, U4+, Ho3+, Dy3+  As an active ion, Ho3+ has achieved laser output in the ~3 μm band (5I6→5I7). In 1976, researchers first realized 2.9 μm laser output in Ho:YAP crystal. In 1990, Bowman et al. obtained 2.85 μm and 2.92 μm laser outputs in Ho:YAP crystals, and obtained 2.92 μm band-tuned laser outputs in Ho:YAP crystals in the following year. In 2017, Nie et al. pumped Ho, Pr: LiLuF4 crystals with a 1 150 nm Raman fiber laser, achieving 2.95 μm watt-level laser output for the first time. In 2018, Zhang et al.

1. Research status and future development trend of mid-infrared (2-5 μm) laser crystalsAccording to the order of laser wavelength from short to long, the main material that have achieved laser output (including some optical fibers and transparent ceramics for comparison) are listed in Table 1. Among them, the highest continuous laser output power of laser crystals corresponding to different wave bands is shown in Figure 2. The laser output power of activated ions shows an obvious attenuation trend as the wavelength expands to the mid-infrared direction.

04 Theoretical study of thermal properties The above experiment shows that the BBO crystal (www.wisoptic.com) generates serious heat in the process of frequency quadrupling. It is known that the energy band gap of the BBO crystal is 6.56 eV, while the single photon energy of 266 nm and 532 nm lasers is 4.66 eV and 2.33 eV respectively. Theoretically, the crystal does not have single photon absorption of 266 nm and 532 nm lasers.

3. Experimental EquipmentThe overall device diagram of the frequency doubling experiment is shown in Figure 3(a). The 1064nm continuous light passes through a half-wave plate and is directly focused into the CPPLN crystal by a lens. The generated frequency doubling light passes through a 532nm transparent filter and is received and detected by a power meter. The self-built LD-pumped Nd:YVO4 continuous laser used in the experiment can reach a maximum output power of 22.53W.

2. Theoretical analysis2.1 Temperature robustnessTemperature robustness refers to the stability of the frequency-doubled crystal with respect to temperature. Specifically, when the temperature fluctuates, the power of the frequency-doubled light will not be greatly affected. The influence of temperature on the frequency doubling process mainly comes from the influence on the phase mismatch.

2.3 Lithium tantalate single crystal filmAfter the 1980s, thin film preparation technology has developed rapidly. Currently, the commonly used preparation technologies of lithium tantalate single crystal (www.wisoptic.com) thin film mainly include chemical vapor deposition, physical vapor deposition, magnetron sputtering and sol-gel method.The chemical vapor deposition method synthesizes a thin film on a substrate through a chemical reaction and accurately controls the chemical composition of the product. It has the characteristics of low stress and good quality.