Editing Brewster's angle (section)

===Brewster windows=== <!--Brewster window redirects here.-->
[[Image:Brewster window.svg|thumb|right|A Brewster window]]
[[Gas laser]]s using an external [[Optical cavity|cavity]] (reflection by one or both mirrors ''outside'' the [[Active laser medium|gain medium]]) generally seal the tube using windows tilted at Brewster's angle. This prevents light in the intended polarization from being lost through reflection (and reducing the round-trip gain of the laser) which is critical in lasers having a low round-trip gain. On the other hand, it ''does'' remove ''s'' polarized light, increasing the round trip loss for that polarization, and ensuring the laser only oscillates in one linear polarization, as is usually desired. And many sealed-tube lasers (which do not even need windows) have a glass plate inserted within the tube at the Brewster angle, simply for the purpose of allowing lasing in only one polarization.<ref name="Hecht">''Optics'', 3rd edition, Hecht, {{ISBN|0-201-30425-2}}</ref>