Waveguide

Waveguide is a strip that holds light the way a wire holds current, drawn as a ribbon whose fatness is part of the optics.

A ribbon that bends. Its width is part of the guide, not a decoration.

A 0.5 micrometer change in width is a change in how the light travels, not a little extra metal. Bends need a radius. Two guides that sit too close will couple. A 450 nanometer strip on silicon-on-insulator is a typical single-mode guide. A board-like 50 micrometer polymer guide is a different world. On the drawing the guide is a long path on a dedicated layer, often all-angle through Euler bends, with ports at the ends instead of pin boxes meant for routers.

The drawing habit comes from chip layout. The physics comes from photonics. A microfluidic channel is the fluid cousin. Manhattan staircases scatter light. People pay for smooth curves and then fracture them for the mask. A digital router that accepts 90° corners can leave a file that is legal while the light complains.

Ports need a contract: width, layer, and where the mode sits. A 500 nanometer strip may taper to 200 nanometers for a coupler, and a 2 micrometer-radius bend may be one the PDK forbids. Keep metal fill out of a keep-away around the guide. The file is still a path or a polygon. The foundry still wants a stream.

The ribbon, the centerline, and the scale are covered in waveguides are paths with feelings, paths: centerlines with width, and nano: when a wavelength is huge.