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Which correctly describes wavelength division multiplexing (WDM)?

  1. ALight of a single wavelength is split into multiple fibers to send the same signal over many fibers simultaneously
  2. BSeparate signals are placed on multiple lights of different wavelengths and sent together over a single fiber
  3. CMultiple signals are sent in turn over a single fiber by dividing time into fine slots
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Answer: B. Separate signals are placed on multiple lights of different wavelengths and sent together over a single fiber

WDM (Wavelength Division Multiplexing) exploits the fact that light of different wavelengths travels through the same fiber with almost no mutual interference, placing a separate signal on each wavelength. For example, sending 100 Gb/s signals on four wavelengths delivers 400 Gb/s over a single fiber. A multiplexer is used on the transmit side and a demultiplexer on the receive side; in silicon photonics they are implemented with ring resonators, arrayed waveguide gratings (AWGs), Mach-Zehnder filters and the like. Sending signals in divided time slots is called time-division multiplexing.

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More Silicon photonics questions

  1. 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?
  2. 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)
  3. 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?
  4. Which correctly describes the extinction ratio, a measure of optical transmit signal quality?
  5. 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?