An optical switch, an optical modulator, and a wavelength variable filter
each have a simple configuration, which requires only a low driving
voltage, which is independent of polarization, and which can operate at
high speed. An optical switch includes a 3-dB coupler placed on an
output, a 3-dB coupler placed on an output, and two optical waveguides
connecting the input-side 3-dB coupler and the output-side 3-dB coupler
together. The optical switch also includes a phase modulating section
that applies electric fields to one or both of the two optical
waveguides. At least two optical waveguides are a crystal material
including KTa.sub.xNb.sub.1-xO.sub.3 (0