Optical multiplexing circuit and light source

An optical multiplexing circuit comprising:

• a plurality of branching units each configured to divide light output from a corresponding one of a plurality of input waveguides;
• a multiplexing unit configured to multiplex beams each being one beam of the light divided by each of the plurality of branching units;
• an output waveguide configured to output the light multiplexed by the multiplexing unit;
• and
• a plurality of monitoring waveguides each configured to output another beam of the light divided by each of the plurality of branching units,
• wherein at least one monitoring waveguide of the plurality of monitoring waveguides includes a bent waveguide constituted by a rib-shaped waveguide,
• wherein the plurality of input waveguides are single mode embedded waveguides in which a core is embedded in a clad on a substrate, and
• wherein the plurality of branching units are embedded waveguides having two cores that are disposed in close proximity to each other while embedded in the clad on the substrate.