Optical semiconductor device

An optical semiconductor device comprising:

• a semiconductor substrate;
• at least one semiconductor laser provided on the semiconductor substrate;
• an optical multiplexer/demultiplexer circuit provided on the semiconductor substrate, the optical multiplexer/demultiplexer circuit being configured to multiplex or demultiplex first output light of the semiconductor laser and being configured to output second output light and third output light;
• a first waveguide portion provided on the semiconductor substrate, the first waveguide portion being configured to output the second output light from an end face of the semiconductor substrate;
• and
• a second waveguide portion including:
• an optical amplifier configured to amplify the third output light, and a reflecting portion, the second waveguide portion being provided on the semiconductor substrate,
• wherein the reflecting portion includes a diffraction grating that is configured to reflect the third output light amplified by the optical amplifier to feed back to the semiconductor laser via the optical amplifier and the optical multiplexer/demultiplexer circuit, and
• the semiconductor laser and the reflecting portion are configured to form a resonator.