OPTICAL MODULATOR WITH MONITOR PHOTODETECTOR

PROBLEM TO BE SOLVED: To provide an optical modulator capable of efficiently making monitor light emitted from a substrate side surface incident to a monitor photodetector.SOLUTION: An optical modulator with a monitor photodetector comprises: an optical modulator including a substrate, an optical waveguide, and a central conductor and a ground conductor; and a monitor photodetector receiving monitor light emitted from a side surface of the substrate. The optical waveguide includes: an input optical waveguide; a branched optical waveguide branching light made incident to the input optical waveguide; an interaction optical waveguide; a multiplexing optical waveguide multiplexing light propagating through the interaction optical waveguide; an output optical waveguide receiving signal light outputted from the multiplexing optical waveguide; and an optical waveguide for outputting monitor light receiving monitor light outputted from the multiplexing optical waveguide. The output optical waveguide for monitor light is formed toward another substrate side surface different from a substrate side surface that the output optical waveguide faces, and formed by applying optical adhesive onto a region of the another substrate side surface, through which the monitor light passes, thereby suppressing the refraction angle of the monitor light outputted from the another substrate side surface.