Periodically Poled Lithium Niobate (PPLN) is a nonlinear crystal used for highly efficient wavelength conversion. It features a broad transmission range covering the near- and mid-infrared spectral regions, enabling efficient frequency conversion processes—such as second-harmonic generation (SHG), sum-frequency generation (SFG), and optical parametric oscillation (OPO)—from visible to mid-infrared wavelengths. By designing its periodic structure, PPLN can generate output at any wavelength within its transmission range, meeting the growing demand for diverse laser wavelengths in modern optics.

Periodically Poled Lithium Niobate (PPLN) crystals have been widely used in various fields, including laser displays, environmental monitoring, mid-infrared spectroscopy, all-optical wavelength conversion, and optical sensing.
By incorporating magnesium oxide (MgO) doping, the crystal's optical damage threshold and photorefractive threshold can be significantly enhanced while maintaining a high nonlinear coefficient. Compared to stoichiometric PPLN, MgO-doped PPLN (MgO:PPLN) crystals exhibit more stable performance at lower temperatures and within the visible wavelength range.