A dual-wavelength waveplate is a special type of multi-stage waveplate. It is made of materials with birefringence and can achieve the required phase delay for two wavelengths simultaneously. It is widely used in solid-state frequency doubling lasers to improve conversion efficiency, and can also be used in optical systems where phase control for two specific wavelengths is required, such as some dual-wavelength laser processing and dual-wavelength optical communication fields.