## 2 Mid-Infrared Optical Parametric Oscillation Laser (OPO)
### LiNbO\({}_{3}\), PPLN, MgO-Doped PPLN Optical Parametric Oscillator  
The optical parametric oscillators of lithium niobate crystals can be divided into pure lithium niobate (LiNiO\({}_{3}\)), periodically poled lithium niobate (PPLN) and periodically polarized lithium niobate doped with MgO (MgO-doped PPLN) optical parametric oscillators based on different crystals. The specific evolution process is shown in Figure 2. In order to improve the damage threshold and stability of the crystal, PPLN is used instead of the traditional LiNiO\({}_{3}\) crystal. While in order to further improve the damage threshold of the PPLN crystal, MgO-doped PPLN crystal was born. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **Technology** & **Classification** & **Advantage** & **Disadvantage** \\ \hline \multirow{4}{*}{Indirect conversion} & \multirow{4}{*}{Optical Parametric Oscillator} & LiNbO\({}_{3}\), PPLN, MgO: & \multirow{4}{*}{high energy and efficiency and excellent \(\upmu\)m} & \multirow{4}{*}{system stability and beam quality should be improved \(\upmu\)m} \\  & & PPLN, KTP, KTA, & & \\  & Oscillator & ZnGeP\({}_{2}\), AgGaSe\({}_{2}\), & & \\  & & AgGaS\({}_{2}\) & & \\  & & Quantum Cascade & InAs, AISb & wider transmission & low output power and poor \\  & & Laser & & & \\ \cline{2-5}  & \multirow{4}{*}{Chemical laser} & HF, COIL & good beam quality & high prices, toxic products \\  & & Gas laser & CO, CO, & high power and long & high temperature explosive \\  & & & service life & large volume and high cost \\ \cline{2-5}  & \multirow{4}{*}{solid-state laser} & \multirow{4}{*}{Fe: ZnSe, Cr: ZnSe} & absorption bandwidth, & \multirow{4}{*}{limited by temperature} \\  & & & wide tuning range and & & \\  & & & good beam quality & \\ \cline{2-5}  & & & & \\ \cline{2-5}  & Fiber Laser & \(\upmu\)m, \(  
From Figure 2, we can see that LiNbO\({}_{3}\), PPLN and MgO-doped PPLN all have their own advantages and disadvantages. The technology of periodically polarized crystals has been gradually developed and perfected with the increasing research of scholars. The research statuses of LiNiO\({}_{3}\), PPLN and MgO-doped PPLN optical parametric oscillation lasers are shown in Table 2. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Crystal**} & \multirow{2}{*}{**Year**} & **Research** & **Crystal Parameter** & **Pump Source** & \multicolumn{2}{c}{**Mid-Infrared Output**} & \multirow{2}{*}{**Reference**} \\  & & **Establishment** & & & & **Parameter** & \\ \hline \multirow{8}{*}{LiNbO\({}_{3}\)} & \multirow{3}{*}{2003} & North China & & & Output wavelength 3.76 μm \\  & & Institute of & & 1.06 μm & Repetition rate 5 Hz \\  & & Optoelectronic & & Nd: YAG & Average power 35 mW \\  & & Technology & & & Optical efficiency 6\% \\  & & & & Output wavelength 3.41 μm \\  & & & & 1.06 μm & Repetition rate 10 Hz \\  & & & Nd: YAG & Average power 12 mW \\  & & & & Optical efficiency 4.5\% \\  & & & & Output wavelength 3.06 μm \\  & & & 1.06 μm & Repetition rate 1 Hz \\  & & & & Output wavelength 3.06 μm \\  & & & 1.06 μm & Repetition rate 1 Hz \\  & & & Nd: YAG & Average power 15 mW \\  & & & & Optical efficiency 10\% \\  & & & & Output wavelength 4.5 μm \\  & & & & Output wavelength & \\  & & & Nd: YVO\({}_{4}\) & Average power 1.1 W \\  & & & & Optical efficiency 7.5\% \\  & & & & Optical efficiency 7.5\% \\  & & & & Output wavelength 3.66 μm \\  & & & 1.06 μm & Average power 1.54 W \\  & & & Nd: YVO\({}_{4}\) & Optical efficiency 7\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Research and development status. Figure 2: Evolution process of LiNiO\({}_{3}\) crystal. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Crystal** & **Year** & \ \begin{tabular}{c} **Research** \\ **Establishment** \\ \end{tabular}  & **Crystal Parameter** & **Pump Source** & \begin{tabular}{c} **Mid-Infrared Output** \\ **Parameter** \\ \end{tabular}