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Silicon photonics

Among methods of incorporating lasers into silicon photonic circuits, which correctly describes heterogeneous integration?

  1. ASilicon waveguides are heavily doped to turn silicon itself into a direct-gap material that emits light
  2. BInP-based compound semiconductor crystal layers are bonded to an SOI wafer and then processed, and the laser light is transferred into silicon waveguides
  3. CInP crystal is epitaxially grown directly on silicon, with no concern about defects from lattice-constant mismatch
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Answer: B. InP-based compound semiconductor crystal layers are bonded to an SOI wafer and then processed, and the laser light is transferred into silicon waveguides

Silicon is an indirect-gap material and cannot emit light efficiently, so compound semiconductors are used for lasers. In heterogeneous integration, crystals (wafers or dies) with InP-based light-emitting layers are bonded to an SOI wafer, the InP substrate is removed, and lasers are formed with semiconductor processes. Because lithography aligns them with the silicon waveguides, many lasers can be integrated with higher accuracy than placing components one by one, and the light is transferred into the silicon waveguides through tapers or similar structures. Doping does not change the indirect-gap nature, and InP and Si differ in lattice constant by about 8%, so defects are a problem in direct growth.

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More Silicon photonics questions

  1. Which is a correct reason it is difficult to integrate an optical isolator together with a laser on a silicon photonic circuit?
  2. Which correctly describes wavelength division multiplexing (WDM)?
  3. In CWDM (coarse wavelength division multiplexing) for data center optical transceivers, wavelengths are spaced 20 nm apart, for example 1271, 1291, 1311 and 1331 nm. What is the main reason for the wide spacing?
  4. For WDM in the C-band (around 1.55 µm), a frequency spacing of 100 GHz corresponds to roughly what wavelength spacing? (Δλ ≈ λ² × Δf / c, c = 3×10⁸ m/s)
  5. For optical transceivers, which correctly compares the parallel scheme, which sends one wavelength on each of four fibers, with the CWDM scheme, which combines four wavelengths onto one fiber?