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Silicon photonics

What is the main role of the buried oxide (BOX) layer in the SOI wafers used in silicon photonics?

  1. AIt acts as the absorption layer of a photodetector beneath the waveguide, absorbing telecom-wavelength light and converting it into current
  2. BIt acts as a light-emitting layer that generates light in the top silicon layer
  3. CIt serves as the lower cladding of the waveguide, preventing light from leaking into the silicon substrate
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Answer: C. It serves as the lower cladding of the waveguide, preventing light from leaking into the silicon substrate

An SOI (Silicon on Insulator) wafer consists of a silicon substrate topped with a buried oxide (BOX) and then a thin single-crystal silicon layer. Waveguides are formed by patterning the top silicon layer (220 nm thick is common), and the low-index BOX serves as the lower cladding. If the BOX is too thin, light extending outside the waveguide (the evanescent field) leaks into the high-index silicon substrate, so a thick BOX of about 2–3 µm is commonly used. Oxide has a large band gap and neither absorbs nor emits light.

Silicon waveguide cross-sections (strip and rib) SiO₂ (n ≈ 1.45) Si (n ≈ 3.5) Optical mode SiO₂ cladding Buried oxide (BOX) Si substrate Width ~500 nm t = 220 nm SiO₂ cladding Buried oxide (BOX) Si substrate Width ~500 nm Slab Strip waveguide Rib waveguide Strong confinement, tight bends Slab connects to electrodes
Silicon waveguide cross-sections (strip and rib) · View in diagrams
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More Silicon photonics questions

  1. 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?
  2. Which is a correct advantage of a grating coupler for connecting optical fiber to a silicon photonic circuit?
  3. Which correctly describes an edge coupler, which couples to optical fiber at the chip facet, compared with a grating coupler?
  4. Why is an inverse taper, in which the silicon waveguide narrows toward the chip edge, used in edge couplers?
  5. When a silicon photonic circuit receives light from standard single-mode fiber, which is a correct reason polarization becomes an issue?