Abstract: Laser apparatus operable in the range 8 to 50 microns, has an optical cavity defined by two mirrors M1 and M2, one of which is partially transmitting. The cavity contains a laser active medium AM and a reflecting polarizer RP. The reflecting polarizer comprises a substrate carrying a metallic reflecting coating covered by a dielectric layer of alumina, silicon dioxide or magnesium oxide. The wavelength of operation determines the dielectric material to be used, and the thickness of the layer determines the absorption loss introduced into one of two perpendicular planes of polarization.