When light in the visible region of the spectrum falls perpendicularly upon a transparent material such as glass or fused quartz, about 4% of the incident intensity is reflected back in the direction from which the light has come. If the light passes through a parallel-sided slab of the material, or window, a similar back reflection will occur at the second surface from which the beam exits. The fraction of the light intensity that is reflected in this way increases with the difference in refractive index between the transparent window and the medium on either side of it. Differences in refractive index between the window and the amplifying medium are particularly large in the case of gas lasers. The performance of low gain lasers can be seriously impaired by these reflection losses as the laser beam will pass backwards and forwards through the amplifying medium many times and reflection losses would occur on each transit.
In gas lasers, this problem can be overcome by attaching brewster windows to the ends of the laser tube which contains the gaseous amplifying medium. A brewster window is simply a piece of material that is transparent at the laser wavelength and that is inclined at an angle known as the brewster angle, as indicated in the diagram below.
An unpolarized light beam that is incident upon the brewster window can be considered to be made up of two polarized components. One component would be polarized in the plane of the diagram and the other polarized perpendicular to the plane of the diagram. For a window that is inclined at the brewster angle, the refracted beam travels in a direction perpendicular to the direction of the beam reflected from the first surface. Also, the transmitted beam is perpendicular to the beam that is internally reflected at the second surface. When this is the case, the reflected beams are polarized only in a direction perpendicular to the plane of the diagram. The component of the incident beam that is polarized in the plane of the diagram suffers no losses arising from reflection. The result of this polarization dependent reflection is that the component of the incident beam that is polarized perpendicular to the plane of the diagram is reduced in intensity on each passage through a brewster window leaving only the other component that suffers no reflection losses.
A laser that is equipped with brewster windows would have an output beam that is polarized. Helium-neon lasers, argon and krypton lasers and carbon dioxide lasers are usually fitted with brewster windows.