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
- 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
Fibre Optics
Fibre optics quiz
Fibre Optics, quiz
1. An optical fibre carries information as:
Pulses of lightRadio wavesElectric current2. A major advantage of fibre optics over copper wiring is:
Immunity to electromagnetic interferenceIt needs no connectorsIt conducts electricity better3. A key advantage of fibre optics over copper is:
Immunity to electromagnetic interferenceCheaper terminationsGreater physical robustness4. Light is retained within the core of an optical fibre by:
Total internal reflectionDiffractionPolarisation5. For total internal reflection to occur, the refractive index of the cladding must be:
Lower than that of the coreHigher than that of the coreEqual to that of the core6. Compared with multi-mode fibre, single-mode fibre offers:
Greater bandwidth over longer distancesA wider core for easier couplingImmunity to bending losses7. A common cause of increasing loss in an installed fibre link is:
Contaminated connector end facesAgeing of the claddingElectromagnetic pick-up8. Graded-index fibre is used to:
Reduce modal dispersionIncrease the core diameterEliminate the need for cladding