Multi-mode interference multiplexer/demultiplexer and optical element using the same

A multi-mode interference multiplexer/demultiplexer comprising:

• a multi-mode waveguide having a first end, a second end, a first side end, and a second side end, the second end being an end on an opposite side to the first end, and the first side end and the second side end opposing each other;
• a first single-mode waveguide connected to the first end of the multi-mode waveguide;
• a second single-mode waveguide connected to the second end of the multi-mode waveguide at a position opposing the first single-mode waveguide;
• a third single-mode waveguide connected to the second end of the multi-mode waveguide at a position closer to the first side end than the position at which the second single-mode waveguide is connected;
• a reflecting surface arranged at a position opposing the third single-mode waveguide in the multi-mode waveguide;
• and
• a fourth single-mode waveguide connected to the first side end or the second side end,
• wherein a direction connecting the first end and the second end is a first direction,
• a direction intersecting with the first direction is a second direction,
• the first side end and the second side end oppose each other in the second direction,
• light entering from the second single-mode waveguide or the third single-mode waveguide is reflected off the reflecting surface and forms an image at a first connection of the fourth single-mode waveguide, and
• the first connection is a connection at the first side end or a connection at the second side end.