The MgO:PPLN-based QPM OPO is one of the most important method to generate mid-infrared laser. In this paper we
attempt to briefly overview the historic development of LiNbO3 crystal and introduce the theoretical foundation of
MgO:PPLN-based QPM OPO. We subsequently give a analysis of different kinds of MgO:PPLN-based QPM OPO.
The wavelength region of 3-5 μm (belonging to the mid-infrared) is important atmospheric window in the
optical spectrum. The mid-infrared lasers have drawn enormous interest and obtained a variety of applications,
such as, air pollution detection, optical fiber communication, military countermeasures, and so on. In recent years,
along with the emergence of lots of nonlinear materials (such as AgGaS<sub>2</sub> and AgGaSe<sub>2,</sub> KTP and KTA, ZnGeP<sub>2</sub>,
LiIO<sub>3,</sub> LiNbO<sub>3</sub>), it’s become easier to obtain mid-infrared lasers. Because of those new nonlinear materials can
satisfy the follow conditions, large nonlinearity, transparency in operating wavelength range and high damage
resistance. Among those nonlinear materials, the LiNbO<sub>3 </sub>(LN) crystal is one of the most suitable materials to be
used to obtain mid-infrared laser.