Achromatic 90° polarization rotating prisms.

Half wave plates are often used to rotate the plane of polarization by 90° at one wavelength. To rotate the plane of polarization over a range of wavelengths, Fresnel rhombs may be used. Fresnel rhombs, however, exhibit significant deviation from precisely 180° retardation over any broad wavelength range. This note describes two prism (or mirror) designs which rotate the incident polarization exactly 90° at all wavelengths. In one case the incident beam direction is deviated by 90°, while in the other case the incident beam is transmitted undeviated.