In which of the following optical fibers will the four-wave mixing phenomenon be most serious when the optical signal propagates, and will also cause the greatest transmission damage?

Question

Options

• AG.652
• BG.655
• CG.653
• DG.654

• A
  3% (1)
• B
  3% (1)
• C
  83% (25)
• D
  10% (3)

Explanation

G.653 (Dispersion-Shifted Fiber, DSF) is the worst offender for four-wave mixing (FWM) because it was engineered to move the zero-dispersion wavelength (ZDW) to ~1550 nm - the same C-band window used by WDM systems. Near-zero dispersion means co-propagating WDM channels maintain phase coherence over long distances, creating ideal conditions for FWM to continuously generate interfering spurious frequencies and cause severe crosstalk.

Why the distractors are wrong:

• G.652 (A) - Standard SMF has relatively high chromatic dispersion at 1550 nm (~17 ps/nm·km), which causes channels to "walk off" quickly, disrupting phase matching and suppressing FWM.
• G.655 (B) - Non-Zero DSF was specifically invented to fix the G.653 FWM problem; it maintains a small but deliberate non-zero dispersion (~1–6 ps/nm·km) at 1550 nm, enough to break sustained phase matching.
• G.654 (D) - Cutoff-shifted fiber (used mainly in undersea links) has a ZDW near 1300 nm and meaningful dispersion at 1550 nm, similar to G.652, so FWM is not a dominant concern.

Memory tip: Think of G.653 as the fiber that was "too perfect" - shifting dispersion to zero at the exact transmission window made it ideal for single-channel use but catastrophic for multi-channel WDM. G.655 is its successor that deliberately adds a little dispersion back to tame FWM.

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