1,168,687. Optical waveguides. NORTH AMERICAN AVIATION Inc. Sept. 22, 1967 [Dec.7, 1966], No.43299/67. Heading G2J. An optical waveguide device comprises a waveguide 43, Fig.6 of dielectric material having a width no greater than one half wavelength of the light to be guided and a thickness no greater than its width supported contiguously with a layer 42 of material having an index of refraction less than that of dielectric 43. As shown a substrate 41 supports the layers 42, 43, and since metals may act as dielectrics at ultraviolet wavelengths the layer 42 may be of metal. An additional metal layer 44 may be on top of dielectric 43 and overhang it as shown or the upper layer may completely surround the dielectric waveguide. Alternatively the dielectric waveguide may be supported on a ridge of the substrate or be supported solely at its ends so that the layer supported continguously with the dielectric is of air. The waveguide may be supported on a substrate which also supports, or has formed therein, a light source and photosensitive device. Materials specified for the dielectric waveguide are Al 2 O 3 , CaF 2 , Ce 2 O 3 , LaF 3 , La 2 O 3 , MgF 2 , MgO, SiO, SiO 2 , Si 3 N 4 , YF 3 and Y 2 O 3 .