Silicon photonics
What is the main reason rib waveguides—shallowly etched so that thin silicon slabs remain on both sides—are often used in silicon optical modulators?
- AThey confine light more tightly than fully etched strip waveguides and can be bent sharply at small radii
- BThe slabs allow electrical connection from electrodes placed at a distance to the pn junction in the waveguide
- CLight is amplified in the slabs, offsetting the loss generated in the modulator
Show answer and explanation
Answer: B. The slabs allow electrical connection from electrodes placed at a distance to the pn junction in the waveguide
In a rib waveguide, the slabs left on both sides are doped to connect the electrodes to the pn junction at the center of the waveguide. Placing metal electrodes right next to the waveguide would absorb light, so being able to place them away from where the light travels is important. In contrast, fully etched strip waveguides have a large lateral index contrast and confine light tightly, making them suited to sharp bends and miniaturization. Rib waveguides confine light weakly, and sharp bends cause light to leak into the slabs. Silicon slabs do not amplify light.