In an embodiment, an optical inspection circuit includes: an optical modulator comprising an optical waveguide on a substrate, the optical waveguide having a core comprising a semiconductor; a first input waveguide optically connected to the optical modulator, the first input waveguide having a core comprising the semiconductor; an output waveguide optically connected to the optical modulator, the output waveguide having a core comprising the semiconductor; a photodiode on the substrate in a vicinity of the optical modulator; a wire electrically connecting the optical modulator and the photodiode; and a second input waveguide optically connected to the photodiode, the second input waveguide having a core comprising the semiconductor.