How does the optical time domain reflectometer use "backscattered" light to make measurements?

Question

Options

• AIt uses pulses to locate and test for sheath faults, damaged conductors, loose connectors,
• BIt performs wiremapping to measure that all pins are correctly connected.
• CIt correlates the returned light pulses with a location in the fiber.
• DIt measures light-pulse width to measure that all pins are correctly connected.

• B
  6% (2)
• C
  91% (32)
• D
  3% (1)

Explanation

Option C is correct because an OTDR works by firing light pulses into a fiber and measuring the tiny fraction of light that scatters back toward the source (Rayleigh backscattering). It uses the time delay between sending the pulse and receiving the returned scatter to calculate the precise distance to any event (splice, connector, break, or bend) along the fiber - distance = (speed of light × time) / 2.

Why the distractors are wrong:

• A describes a cable fault locator/TDR for copper cables, not a fiber OTDR - OTDRs don't test sheaths or conductors.
• B describes a wiremap tester, a completely different tool used for verifying copper Ethernet pin assignments.
• D is nonsensical for fiber - pulse width affects measurement resolution, but OTDRs don't measure pin connections at all.

Memory tip: Think of an OTDR like a fiber radar - just as radar measures object distance by timing a reflected radio wave, an OTDR times the returning backscattered light to pinpoint exactly where in the fiber something changed. Time = Location. That's the core of option C.

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