Fibre types and connectors — Fibre Optics Practice

Sample questions

1. 01You are handed a test report for a singlemode backbone. Which pair of test wavelengths would you expect to see on it?

   • A850 nm and 1550 nm
   • B1300 nm and 1383 nm
   • C1310 nm and 1550 nm
   • D850 nm and 1300 nm

   Why: Singlemode runs with laser sources at 1310 nm and 1550 nm, where its loss is lowest. 850 nm and 1300 nm is tempting because it is a familiar pair, but it is the multimode pair. 850 nm is a multimode wavelength, and 1383 nm is the water peak band that G.652.D makes usable, not a standard test pair with 1300 nm.

2. 02Modern multimode fibre has a graded-index core. What does the grading actually do?

   • ARemoves modal dispersion so it behaves like singlemode
   • BNarrows the core along the run so fewer modes travel
   • CSpeeds up light near the edge so the modes keep pace
   • DLowers the loss so the fibre can be used at 1550 nm

   Why: A graded-index core has a lower refractive index towards its edge, so light travelling there speeds up and roughly catches up with light in the centre. The match is never perfect in real glass, so pulses still spread: grading reduces modal dispersion but does not remove it, which is why the removes-modal-dispersion answer is wrong. 1550 nm is a singlemode wavelength, and the core size does not change along the run.

3. 03A singlemode data sheet quotes a mode field diameter instead of a core size. What is that figure?

   • AThe core size of the matching multimode, 50 or 62.5 µm
   • BThe effective size of the light path, 8 to 10 µm
   • CThe tightest radius the fibre is designed to bend to
   • DThe outside diameter of the cladding, which is 125 µm

   Why: Singlemode is strictly specified by its mode field diameter, the effective size of the light path, in the 8 to 10 µm range; the core is usually called about 9 µm. The 125 µm cladding is the same on both families, so it is not what the mode field figure describes. A minimum design radius is a separate G.657 figure, and 50 or 62.5 µm are multimode cores.