PRODUCTION OF OPTICAL WAVEGUIDE

PURPOSE: To suppress external diffusion of Li2 O from a substrate and to decrease the transmission loss due to fine dust produced from a LiNbO3 powder by disposing a dummy substrate which is same material as the substrate around the substrate and heating them at one time. CONSTITUTION: A heating furnace 6 is equipped with a heater 10 around the furnace, a gas inlet 11 on one side face, and a gas outlet 12 on the other side face. Plates 3a, 3b having fitting grooves are disposed along from the upstream side in the inside of the furnace 6 and a dummy substrate 5 comprising LiNbO3 is mounted and fitted on the fitting groove of the plate 3a. A Ti metal film is deposited on a LiNbO3 substrate 1 and etched by photolithography to form a core pattern 2 arranged on a line. This substrate 1 is mounted and fitted on the plate 3b in such a manner that the line of the core pattern 2 is parallel to an oxygen flow. Then a lid 4 having a fitting groove is placed on the plate 3b to cover. Thus, the core pattern 2 comprising Ti is made to diffuse into the substrate 1 which is heated and thereby, an optical waveguide element is obtd.