Coherent optical mixer circuit

A coherent optical mixer circuit including a 4-input-and-4-output multimode interference circuit having a silicon waveguide in which the multimode interference circuit has two odd-numbered or even-numbered inputs that are connected to an input mechanism of externally-inputted signal light and reference light via input waveguides and the four outputs of the multimode interference circuit are all connected to an output mechanism to the exterior via output waveguides, wherein:

• the other two inputs of the multimode interference circuit are connected to two monitor waveguides,
• one of the monitor waveguide is longer than the other of the monitor waveguide to configure a light delay circuit,
• the monitor waveguides constituting the light delay circuit are connected to respective outputs of a 2-branched light splitter, and
• the 2-branched light splitter has an input connected to a monitor light input mechanism from the exterior via a monitor input waveguide;
• wherein:
• at least a part of the monitor waveguide is formed by a silicon waveguide,
• the remaining portion of the monitor waveguide and the 2-branched light splitter connected to the monitor waveguide and the monitor input waveguide, and the monitor light input mechanism connected to the monitor input waveguide are formed by a light waveguide formed in another layer different from that of the silicon waveguide, and
• a part of the monitor waveguide formed by the silicon waveguide and the light waveguide of another layer different from that of the silicon waveguide are interlayer-coupling, and
• the interlayer-coupling is a structure comprising the part of the monitor waveguide formed by the silicon waveguide and the part of the light waveguide formed in another layer different from that of the silicon waveguide, wherein
• each tips of the waveguides are formed in a tapered shape,
• and the tips of the tapered shape are formed in the direction to face each other,
• and the tips of the tapered shape are overlapping seen from a vertical direction to substrate.