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Which correctly describes optical links combining VCSELs at around 850 nm with multimode fiber?

  1. AThe components are inexpensive and alignment is easy, but reach is limited to short distances of tens to hundreds of m
  2. BThey use the wavelength band with the lowest fiber loss and are mainstream for long-haul communication exceeding several hundred km
  3. CSilicon transmits 850 nm light well, making them ideal for direct connection to silicon waveguides
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Answer: A. The components are inexpensive and alignment is easy, but reach is limited to short distances of tens to hundreds of m

VCSELs are surface-emitting lasers that can be tested on the wafer, mass-produced inexpensively and consume little power. Multimode fiber has a thick core, such as 50 µm in diameter, so alignment is easy. As a result, they have been widely used for short links within and between racks connecting servers and switches. On the other hand, multimode fiber suffers modal dispersion, in which the multiple propagation paths (modes) arrive at different times, so reach shortens as speed rises (about 50–100 m at 100 Gb/s per lane), and fiber loss at 850 nm is also relatively high, around 3 dB/km. Silicon absorbs 850 nm light, so it cannot be used for waveguides.

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More Silicon photonics questions

  1. 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?
  2. What is the main reason silicon photonics is said to be well suited to mass production in semiconductor fabs?
  3. Which is considered one of the main causes of propagation loss (dB/cm) in silicon waveguides?
  4. Which correctly describes the cutback method for measuring the propagation loss of silicon waveguides?
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