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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?

  1. AAbout 0.2 mW (−7 dBm)
  2. BAbout 20 mW (+13 dBm)
  3. CAbout 0.05 mW (−13 dBm)
Show answer and explanation

Answer: A. About 0.2 mW (−7 dBm)

dBm is an absolute power unit referenced to 1 mW: 0 dBm is 1 mW, +3 dBm is about 2 mW, and −10 dBm is 0.1 mW. Losses can simply be subtracted in dB: 3 − 10 = −7 dBm, and 10^(−0.7) ≈ 0.2 mW. This calculation—subtracting losses from the transmit power and checking that the result exceeds the minimum power the photodetector needs (receiver sensitivity)—is called a link budget (power budget). +13 dBm is the result of adding the loss, and −13 dBm results from getting the sign of the transmit power wrong.

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