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Silicon photonics

Learn optical components such as silicon waveguides, optical modulators, ring resonators and Ge photodetectors, the basics of telecom wavelength bands, optical interconnects for data centers using WDM and CPO, and key points of fabrication and testing through multiple-choice questions.

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Silicon photonics: key points

Silicon photonics integrates waveguides that guide light, modulators, photodetectors and other components on a single chip using the same silicon fabrication technology as CMOS. Silicon barely absorbs light in the 1.31–1.55 µm bands used for communications, and its large refractive index contrast with the surrounding oxide confines light tightly in small circuits.

Data center traffic is surging with AI training and other workloads, and electrical wiring keeps incurring more loss and consuming more power as speeds rise. Optical interconnects such as optical transceivers and CPO, which mounts optical components right next to the switch IC, are drawing attention as the key technology for solving these problems.

All Silicon photonics questions (50)
  1. In long-haul optical fiber communication, what is the main reason wavelengths around 1.55 µm (C-band) are widely used?
  2. For relatively short-reach optical links inside data centers, which is a correct reason wavelengths around 1.31 µm (O-band) are often used?
  3. Which correctly pairs the photon energy of 1.55 µm light with its behavior in a silicon waveguide? (E [eV] ≈ 1.24 / λ [µm])
  4. Which correctly describes a silicon waveguide (refractive index about 3.5) surrounded by oxide (SiO2, refractive index about 1.45)?
  5. 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)?
  6. 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?
  7. Laser light with an output of +3 dBm reaches a photodetector after a total loss of 10 dB (fiber coupling, waveguides, modulator, etc.). Which is closest to the received power?
  8. What is the main role of the buried oxide (BOX) layer in the SOI wafers used in silicon photonics?
  9. 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?
  10. Which is a correct advantage of a grating coupler for connecting optical fiber to a silicon photonic circuit?
  11. Which correctly describes an edge coupler, which couples to optical fiber at the chip facet, compared with a grating coupler?
  12. Why is an inverse taper, in which the silicon waveguide narrows toward the chip edge, used in edge couplers?
  13. When a silicon photonic circuit receives light from standard single-mode fiber, which is a correct reason polarization becomes an issue?
  14. Which correctly describes how a Mach-Zehnder modulator converts a change in optical phase into a change in optical intensity?
  15. Why is the carrier dispersion effect (plasma dispersion effect) used to change the refractive index with an electrical signal in silicon optical modulators?
  16. Why do high-speed silicon modulators often use the depletion type, which applies reverse bias to the pn junction of the phase shifter to vary the depletion width?
  17. Which correctly describes VπL ("V-pi-L"), a figure of merit for Mach-Zehnder modulators?
  18. Which correctly explains why a silicon ring resonator responds strongly only to specific wavelengths?
  19. Why do optical circuits using ring resonators often place a heater near the ring to control its temperature?
  20. Which correctly describes what happens when the circumference of a ring resonator is shortened?
  21. Silicon photonics sometimes combines silicon nitride (SiN) waveguides with silicon waveguides. Which correctly describes SiN waveguides?
  22. Which is a correct reason germanium (Ge) is widely used for telecom-wavelength photodetectors in silicon photonic circuits?
  23. What is the main cause of the tendency toward high dark current in germanium photodetectors grown on silicon?
  24. Which is closest to the responsivity (current per 1 W of light) of a photodiode with 80% quantum efficiency at a wavelength of 1.55 µm? (R [A/W] ≈ quantum efficiency × λ [µm] / 1.24)
  25. What is the role of the transimpedance amplifier (TIA) placed right after the photodiode in an optical receiver?
  26. Among methods of incorporating lasers into silicon photonic circuits, which correctly describes heterogeneous integration?
  27. Which is a correct reason it is difficult to integrate an optical isolator together with a laser on a silicon photonic circuit?
  28. Which correctly describes wavelength division multiplexing (WDM)?
  29. 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?
  30. 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)
  31. 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?
  32. Which correctly describes the extinction ratio, a measure of optical transmit signal quality?
  33. For high-speed links of a few m or more, such as those spanning racks, what is the main reason optical fiber has an advantage over copper cables and board traces?
  34. Which is a correct advantage of pluggable optical transceivers inserted into a switch's front panel?
  35. Which is a correct challenge of CPO (Co-Packaged Optics), which mounts the optical transmit/receive components in the same package as the switch IC?
  36. In CPO, an external laser source—placing the laser outside the switch IC package and supplying only light via fiber—is often considered. What is the main reason?
  37. Which correctly describes the concept of "linear drive" (called linear pluggable optics, LPO, etc.) in optical transceivers?
  38. Which is a correct challenge of linear-drive optical transceivers without a DSP?
  39. When PAM4 is used with the intensity modulation/direct detection (IM-DD) scheme widely used for optical links within data centers, what is varied among four values?
  40. With PAM4, which is closest to the symbol rate (symbols per second) when sending a signal of about 106 Gb/s per wavelength (including error-correction coding, etc.)?
  41. Which correctly describes the coherent scheme, used for links of several tens of km or more such as between data centers, compared with IM-DD?
  42. If an 800 Gb/s optical transceiver consumes 16 W, what is the energy per bit?
  43. For a switch with a total bandwidth of 51.2 Tb/s, if all optical links operate at 5 pJ per bit, which is closest to the power consumption of the optical links?
  44. Which correctly describes optical links combining VCSELs at around 850 nm with multimode fiber?
  45. Why is active alignment—aligning while actually passing laser light through and measuring the output—often used when attaching single-mode fiber to a silicon photonic chip?
  46. What is the main reason silicon photonics is said to be well suited to mass production in semiconductor fabs?
  47. Which is considered one of the main causes of propagation loss (dB/cm) in silicon waveguides?
  48. Which correctly describes the cutback method for measuring the propagation loss of silicon waveguides?
  49. In CPO, which integrates optical components with a switch IC and others in one package, why is it especially important to test optical characteristics at the wafer level before assembly?
  50. Why is a configuration used in which the chip with optical devices (PIC) and the chip with electronic circuits such as drivers and TIAs (EIC) are fabricated separately and then stacked and connected?

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