Silicon photonics
Why is an inverse taper, in which the silicon waveguide narrows toward the chip edge, used in edge couplers?
- ANarrowing the waveguide strengthens light confinement, so no light at all is reflected at the facet
- BAs the waveguide narrows, light spreads into the surroundings and the mode grows, making it easier to match the fiber mode
- CNarrowing the waveguide raises silicon's refractive index, making it easier to draw light in from the fiber
Show answer and explanation
Answer: B. As the waveguide narrows, light spreads into the surroundings and the mode grows, making it easier to match the fiber mode
The mode diameter of standard single-mode fiber is about 9–10 µm, while a silicon waveguide's mode is very small, 1 µm or less; butting them together directly causes a large loss from mode mismatch. Narrowing the waveguide to about 100–200 nm at the tip leaves silicon unable to confine the light, which spreads widely into the surrounding cladding. This enlarges the mode to a few µm, bringing it closer to the fiber mode. Narrowing actually weakens confinement, and the refractive index of the material itself does not change.