Periodically poled lithium niobate (PPLN) is a highly efficient crystal for nonlinear wavelength conversion processes, which has a wide light transmission range and covers the NIR and MIR spectral regions, PPLN is used for SHG、SFG、OPO, and other nonlinear processes from Visible to MIR. In order to meet the requirements of modern optics for diverse laser wavelengths. the output of any wavelength in the transmittance range can be realized through the design of periodic structure.