Birefringent Filter Plate

The birefringent filter plate is a linear retarder used intracavity to selectively tune a desired wavelength and create reflection losses at other wavelengths. In use, it is placed at Brewster’s angle, and then rotated around its axis to achieve full wave retardation at the wavelength of interest. The selected wavelength will contain only a p polarization component, and thus experience no reflection loss. Other wavelengths will undergo a different retardation, and thus have some amount of s polarization, leading to reflection losses.