Waveguide structure

A waveguide structure, comprising:

• a first waveguide on a substrate;
• a second waveguide on the substrate;
• and
• a third waveguide on the substrate,
• the first waveguide, the second waveguide, and the third waveguide each being substantially parallel to the substrate,
• the first waveguide being at a different height than the second waveguide,
• the first waveguide and the second waveguide being configured to cause light to couple between the first waveguide and the second waveguide,
• the second waveguide and the third waveguide being configured to cause light to couple between the second waveguide and the third waveguide,
• the first waveguide being composed of a first material having a first index of refraction,
• the second waveguide being composed of a second material having a second index of refraction,
• the third waveguide being composed of a third material having a third index of refraction, the third material including silicon and nitrogen, and
• the second index of refraction being greater than the first index of refraction, and less than the third index of refraction,
• wherein the first waveguide is a waveguide in a crystalline silicon layer on a buried oxide layer, the crystalline silicon layer having a thickness of more than 1 micron,
• wherein the waveguide structure further comprises a fourth waveguide and a fifth waveguide,
• wherein the fourth waveguide is at a same height as the first waveguide,
• wherein the fourth waveguide is in the crystalline silicon layer above the buried oxide layer and laterally spaced apart from the first waveguide by a layer of silicon dioxide,
• wherein the second waveguide is above, and overlaps in a plan view with, the first waveguide in a region of overlap,
• wherein the fifth waveguide is above, and overlaps in the plan view with, the fourth waveguide,
• wherein the fifth waveguide comprises the second material,
• wherein the third waveguide is above, and overlaps in the plan view with, each of the second waveguide, the layer of silicon dioxide, and the fifth waveguide,
• wherein the third waveguide and the fifth waveguide are configured to cause light to couple between the third waveguide and the fifth waveguide, and
• wherein the fifth waveguide and the fourth waveguide are configured to cause light to couple between the fifth waveguide and the fourth waveguide.