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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?

  1. AOptical phase
  2. BPolarization orientation
  3. COptical intensity (power)
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Answer: C. Optical intensity (power)

In IM-DD (Intensity Modulation / Direct Detection), the transmitter modulates the light's intensity, and at the receiver a photodiode converts the intensity directly into current for decision. PAM4 varies the light's intensity in four levels, sending 2 bits per symbol. The photodetector can only detect intensity, but in return the setup is simple, low-cost and low-power. Putting information on the phase or polarization of light requires the coherent scheme, which detects the signal by interfering it with light from another laser at the receiver, making the circuits complex.

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  2. 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?
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