Rectangular Brewster windows are uncoated substrates used in an optical system at Brewster’s Angle, the angle at which p reflectance drops to zero. This can be calculated from:

ΘB = tan-1(n)

where ΘB is Brewster’s Angle and n is the material’s index of refraction. When used in a laser cavity, a Brewster Window causes polarized laser output.

Fixed beam polarization is often required so that optical components in the system perform consistently as designed. Since many optics and coatings are polarization sensitive, a laser with a time varying polarization state can cause fluctuations in system performance.

While virtually all of the p component of polarization is transmitted by a Brewster Window, most of the s component is reflected. For ZnSe, 50% of the incident s-polarized light is reflected per surface. Ge, with a higher index of refraction, has an approximately 87% fresnel reflection of the s-polarization component at 10.6µm.