## 2 Mid-Infrared Optical Parametric Oscillation Laser (OPO)
### ZnGeP\({}_{2}\) Optical Parametric Oscillator  
ZnGeP\({}_{2}\) crystal is the most important nonlinear crystal in optical parametric oscillator technology. The description of it is shown in Figure 5.  
For the ZnGeP\({}_{2}\) crystal, its good physical and chemical properties, high thermal conductivity and damage threshold have achieved its advantages when operating in a high-power environment. Therefore, it is the best nonlinear crystal for a high-power, 3-5 um mid-infrared OPO.  
The ZnGeP\({}_{2}\) crystal has been deeply studied by many scholars because of its excellent characteristics. The research development is shown in Table 5.  
According to the literature, the best results of mid-infrared laser output based on ZGP crystal are an average output power of 103 W at a frequency of 10 kHz. The optical efficiencies are 78% and 44.2% with an output wavelength of 4.6 um and 4.57 um, respectively.  
As mentioned above, several optical parametric oscillators for mid-infrared (3-5 um) output are discussed. The properties parameters of mid-infrared nonlinear optical crystals are shown in Table 6.  
The nonlinear crystals mentioned above have transmittance in the mid-infrared of 3-5 um, which are currently widely studied in the world. Compared with LiNiO\({}_{3}\) and PPLN, MgO-doped PPLN crystal owns a larger damage threshold, and now it has become a research hotspot. However, the thermal conductivity of KTP, AgGaSe\({}_{2}\) and AgGaS\({}_{2}\) are relatively small, which will induce serious thermal effect under high-power operation and even cause the damage of crystals. Therefore, the output and applications of high-power mid-infrared in the future are limited. The thermal conductivity is smallest, and the damage threshold is the highest of ZGP crystal, which may be the reason why the output power is largest among these nonlinear crystals. It has a compact laser structure, the advantages of a wide tuning range of output wavelength and so on. Therefore, it can be said that the realization of mid-infrared laser output based on ZGP crystal is mainstream through an indirect way.  
Figure 5: Description diagram of ZGP crystal.  
\begin{table}
\begin{tabular}{|c c c c c c|} \hline \hline \multirow{2}{*}{**Year**} & **Research** & **ZGP Crystal** & **Pump Source** & **Mid-Infrared Output** & \multirow{2}{*}{**References**} \\  & **Establishment** & **Parameter** & & & **Parameter** & \\ \hline  & \multicolumn{3}{c}{} & & & Output wavelength & 4.5 μm \\
2010 & \multicolumn{3}{c}{} & & & & Average power 22 W \\  & \multicolumn{3}{c}{} & & & Ho: YAG & \multicolumn{1}{c}{} & \\  & \multicolumn{3}{c}{} & & & & \\  & \multicolumn{3}{c}{} & & & Optical efficiency & 58\% \\  & \multicolumn{3}{c}{} & & & Output wavelength & 4.32 μm \\
2011 & \multicolumn{3}{c}{} & & & & Average power 5.7 W \\  & \multicolumn{3}{c}{} & & & KPOPO & \multicolumn{1}{c}{} & \\  & \multicolumn{3}{c}{} & & & Optical efficiency & 46.6\% \\  & \multicolumn{3}{c}{} & & & Output wavelength & 3.5 μm \\
2013 & \multicolumn{3}{c}{} & & & & \\  & \multicolumn{3}{c}{} & & & Ho: YAG & \multicolumn{1}{c}{} \\  & \multicolumn{3}{c}{} & & & Optical efficiency & 69\% \\  & \multicolumn{3}{c}{} & & & Output wavelength & 3.7 μm \\
2014 & \multicolumn{3}{c}{} & & & & Average power 2.8 W \\  & \multicolumn{3}{c}{} & & & & \\  & \multicolumn{3}{c}{} & & & & \\  & \multicolumn{3}{c}{} & & & & \\  & \multicolumn{3}{c}{} & & & & \\  & \multicolumn{3}{c}{} & & & Optical efficiency & 46.6\% \\  & \multicolumn{3}{c}{} & & & Output wavelength & 3.5 μm \\
2014 & \multicolumn{3}{c}{} & & & & \\  & \multicolumn{3}{c}{} & & & Ho: YAG & \multicolumn{1}{c}{} \\  & \multicolumn{3}{c}{} & & & Optical efficiency & 69\% \\  & \multicolumn{3}{c}{} & & & Output wavelength & 3.7 μm \\