<p>An optical waveguide capable of having various characteristics and a method of manufacture thereof as well as a method of manufacturing a crystal film are provided. An optical functional material KTaxNb1-xO3 is used as an optical waveguide. The input optical signal (3) is transmitted to the KTaxNb1-xO3 film (2). The KTaxNb1-xO3 film (2) undergoes changes in optical property when an external voltage signal is applied to the electrode (4). Therefore, as it passes through the KTaxNb1-xO3 film (2), the input optical signal is modulated by the characteristic change. The modulated optical signal is taken out as an output optical signal (5). Alternatively the optical functional material can be K1-vLivTaxNb1-xO3. <IMAGE></p>