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In long-haul optical fiber communication, what is the main reason wavelengths around 1.55 µm (C-band) are widely used?

  1. AIt is the band where loss in silica optical fiber is lowest
  2. BIt is the band where silicon absorbs light most strongly, making photodetectors easy to build
  3. CIt is the band where chromatic dispersion is zero, so pulses do not spread at all
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Answer: A. It is the band where loss in silica optical fiber is lowest

Loss in silica (SiO2) fiber increases from Rayleigh scattering at short wavelengths and from infrared absorption at long wavelengths, so it is lowest in between, around 1.55 µm (about 0.2 dB/km). Even after 100 km the loss is only about 20 dB, and the ability to amplify the light directly in the optical domain with erbium-doped fiber amplifiers (EDFAs) is another reason the C-band is used for long-haul communication. In standard single-mode fiber, chromatic dispersion is nearly zero around 1.31 µm (O-band); dispersion remains at 1.55 µm. Silicon barely absorbs this wavelength.

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More Silicon photonics questions

  1. For relatively short-reach optical links inside data centers, which is a correct reason wavelengths around 1.31 µm (O-band) are often used?
  2. Which correctly pairs the photon energy of 1.55 µm light with its behavior in a silicon waveguide? (E [eV] ≈ 1.24 / λ [µm])
  3. Which correctly describes a silicon waveguide (refractive index about 3.5) surrounded by oxide (SiO2, refractive index about 1.45)?
  4. When light travels from silicon (refractive index 3.48) toward oxide (refractive index 1.45), which is closest to the critical angle for total internal reflection (measured from the normal to the interface)?
  5. When light travels 5 mm through a silicon waveguide with a propagation loss of 2 dB/cm, which is closest to the fraction of optical power remaining at the output?