Optical modulator comprising a mode filter

摘要

An optical modulator is disclosed which suppresses a bias shift which is a deviation of a phase relationship between output signal light and monitoring light. The optical modulator includes a substrate (91) having an electro-optic effect, a modulating optical waveguide (7) formed on a surface layer portion of the substrate (91) and forming an interference optical modulator for modulating input light, and an output optical waveguide (5) and a monitoring optical waveguide (6) each formed on the surface layer portion of the substrate (91) and branched from and connected to a downstream side portion of the modulating optical waveguide (7). The monitoring optical waveguide guides light for monitoring optical modulation operation of the modulating optical waveguide (7). The monitoring optical waveguide (6) has a reduced width region which has a reduced waveguide width.