## 2 Mid-Infrared Optical Parametric Oscillation Laser (OPO)
### AgGaSe\({}_{2}\) and AgGaS\({}_{2}\) Optical Parametric Oscillator  
AgGaSe\({}_{2}\) and AgGaS\({}_{2}\) are semiconductor chalcopyrite symmetry crystals. Both crystals are transparent in infrared, and they have been used for a long time in the mid-infrared band. The descriptions of two crystals are shown in Figure 4.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Crystal** & **Year** & **Research** & **Crystal** & **Pump Source** & **Mid-Infrared Output** & **References** \\ \hline \multirow{8}{*}{KTP} & \multirow{2}{*}{2003} & Harbin Institute of Technology & \multirow{2}{*}{\(7\times 7\times 25\) mm\({}^{3}\)} & \multirow{2}{*}{1.06 \(\upmu\)} & Output wavelength 3.29 \(\upmu\) & \\  & & & & Nd: YAG & Average power 2 mW & \\  & & & & Nd: YAG & Repetition rate 1 Hz & \\  & & & & Optical efficiency 5\% & \\ \cline{3-5}  & \multirow{2}{*}{2016} & The Czech Academy of Sciences & \multirow{2}{*}{16.5 mm length} & 0.976 \(\upmu\)m & Output wavelength 3.225 \(\upmu\)m & \\  & & & & Yb\({}^{3+}\) fiber & Repetition rate 100 MHz & \\  & & & & Yb: & Average power 780 mW & \\  & & & & KGd(WO\({}_{4}\))\({}_{2}\) & & \\  & & & & & Optical efficiency 12\% & \\  & & & & & Output wavelength 3.17 \(\upmu\)m & \\  & & & & & & Optical efficiency 14\% & \\  & & & & & & Output wavelength 3.467 \(\upmu\)m & \\  & & & & & & Average power 84 mW & \\  & & & & & Nd: YAG & Repetition rate 100 Hz & \\  & & & & & Optical efficiency 14\% & \\  & & & & & Output wavelength 3.475 \(\upmu\)m & \\  & & & & & & Optical efficiency 14\% & \\  & & & & & Optical efficiency 14\% & \\  & & & & & Output wavelength 3.467 \(\upmu\)m & \\  & & & & & Output wavelength 3.467 \(\upmu\)m & \\  & & & & & & Average power 84 mW & \\  & & & & Nd: YAG & Repetition rate 100 Hz & \\  & & & & & Optical efficiency 14\% & \\  & & & & & Output wavelength 3.475 \(\upmu\)m & \\  & & & & & & Average power 2.125 W & \\  & & & & Nd: YAG & Repetition rate 25 Hz & \\  & & & & & Optical efficiency 14.3\% & \\  & & & & & Output wavelength 3.440 \(\upmu\)m & \\  & & & & & Output wavelength 3.467 \(\upmu\)m & \\  & & & & & & Average power 0.335 W & \\  & & & & & Optical efficiency 5.6\% & \\  & & & & & Output wavelength 3.75 \(\upmu\)m & \\  & & & & & Output wavelength 3.75 \(\upmu\)m & \\  & & & & & Average power 600 mW & \\  & & & & & Repetition rate 10 Hz & \\  & & & & & Optical efficiency 7.54\% & \\  & & & & & & Output wavelength 3.27 \(\upmu\)m & \\ KTA & 2016 & Mechanics, the Chinese & \multirow{2}{*}{3 \(3\times 2.5\times\)} & \multirow{2}{*}{0.8 \(\upmu\)m} & Average power 82 mW & \\  & & & & & & Optical efficiency 14.6\% & \\  & & & & & & Optical efficiency 14.6\% & \\  & & & & & & \\  & & & & & & & Optical efficiency 14.6\% \\  & &  
For AgGaSe\({}_{2}\) and AgGaS\({}_{2}\) crystals, the biggest defect is that the damage interpretation value is generally low, which cannot meet the needs of high repetition rates and maximum energy output. In the early stage, the research on AgGaSe\({}_{2}\) and AgGaS\({}_{2}\) crystals was also extensive; the research and development status are shown in Table 4.  
It can be seen from the existing reports that the output efficiency based on these two crystals to realize mid-infrared laser is low, and the maximum energy that can be obtained