Silicon photonics
Which is considered one of the main causes of propagation loss (dB/cm) in silicon waveguides?
- ALight scattering from fine roughness on sidewalls created by etching
- BBand-to-band absorption in which telecom-wavelength photons excite silicon electrons from the valence band to the conduction band
- CSignal distortion caused by excessive amplification of light in the oxide surrounding the waveguide
Show answer and explanation
Answer: A. Light scattering from fine roughness on sidewalls created by etching
In silicon waveguides, roughness of a few nm formed on the sidewalls by lithography and etching scatters light out of the waveguide. Because the index contrast between silicon and oxide is large and the waveguide is thin, the light intensity at the sidewalls is also high, so scattering has a strong effect. Propagation loss of a typical strip waveguide is roughly 1–3 dB/cm depending on conditions (below 1 dB/cm in low-loss processes), and it is reduced by using rib waveguides, widening the waveguide, or smoothing the sidewalls by oxidation or annealing in hydrogen. The photon energy at telecom wavelengths is smaller than silicon's band gap, so band-to-band absorption does not occur.