Waveguide couplers for multi-mode waveguides

An optical coupler, comprising:

• a first waveguide including a first multi-mode waveguide section;
• and
• a second waveguide that is distinct and separate from the first waveguide, the second waveguide including a second multi-mode waveguide section that is distinct and separate from the first multi-mode waveguide section, wherein:
• the first multi-mode waveguide section has a cross-section characterized by a first height and a first width that is greater than the first height;
• the second multi-mode waveguide section has a cross-section characterized by a second height and a second width that is greater than the second height;
• the first multi-mode waveguide section is positioned adjacent to the second multi-mode waveguide section at least partially above or below the second multi-mode waveguide section so that light entering the first multi-mode waveguide section is coupled from the first multi-mode waveguide section to the second multi-mode waveguide section;
• and
• the first multi-mode waveguide section and the second multi-mode waveguide section are separated by a first edge-to-edge distance so that light entering the first multi-mode waveguide section in a fundamental mode is coupled from the first multi-mode waveguide section to the second multi-mode waveguide section while remaining in the fundamental mode.