Fibre Optics
Advantages and disadvantages of fibre optic over electrical wire propagation; the fibre optic bus; related terms, terminations, couplers, control terminals and remote terminals.
Fibre optics — summary notes
Free- An optical fibre guides light along a glass or plastic CORE surrounded by CLADDING of lower refractive index. Light striking the boundary beyond the critical angle is totally internally reflected and cannot escape.
- Step-index fibre has an abrupt core/cladding index change; graded-index fibre varies the index smoothly to reduce modal dispersion. Multi-mode fibre has a wide core and carries many light paths; single-mode has a very narrow core and carries one, giving the greatest bandwidth over the longest distance.
- Advantages: enormous bandwidth, complete immunity to electromagnetic interference and HIRF, no crosstalk, no spark hazard, light weight and no earth-loop problems.
- Disadvantages: the fibre is brittle and has a minimum bend radius, terminations and splices are difficult and demand special tooling and cleanliness, and joints introduce loss. Contamination of a connector face is a common in-service fault.
- Aircraft uses include ARINC 664/AFDX backbones, in-flight entertainment distribution, engine control data links and fibre-optic gyros.
- ⚠ Exam trap: fibre is NOT more robust than copper and its terminations are NOT cheaper — the genuine advantages are bandwidth, weight and EMI immunity.
- Total internal reflection
- occurs when the angle of incidence exceeds the critical angle
- Critical angle
- sin θ𝒸 = n₂ ÷ n₁ (n₁ = core, n₂ = cladding)
- Index rule
- n(core) > n(cladding), always
Why is a fibre-optic data link preferred for routing through an area of high electrical noise?
The signal is light, not current, so it neither radiates interference nor picks any up. No shielding, bonding or earth-loop management is needed, and HIRF or lightning-induced transients cannot corrupt the data.
An installed fibre link shows a gradual loss of signal margin with no physical damage. What is the first thing to check?
The connector end faces. Dust, grease or a fingerprint on a polished ferrule scatters light and is by far the most common cause of increasing loss — inspect and clean to the manufacturer's procedure before suspecting the fibre itself.
Fibre optics concept map
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Fibre optics quiz
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