📖 The lesson
How a fibre guides light
An optical fibre is a hair-thin strand of very pure glass (or plastic) that carries information as pulses of light instead of electrical current. It has two layers of glass with slightly different refractive indices: the central core (higher index, n₁) and the surrounding cladding (lower index, n₂).
- When light passes from a denser to a less dense medium it bends away from the normal. Beyond the critical angle (sin θc = n₂/n₁) it cannot escape at all and is totally internally reflected. Light launched into the core at a shallow enough angle therefore bounces along the fibre, trapped, even around gentle bends.
- The acceptance angle is the largest angle to the fibre axis at which entering light is still guided. Its sine is the numerical aperture (NA) = √(n₁² − n₂²), typically 0.2–0.3 for multimode glass fibre. A large NA collects light easily from an LED.
Fibre types
- Multimode step-index: a large core (50 or 62.5 µm in glass, about 1 mm in plastic optical fibre) with a sharp change of index at the cladding. Light follows many paths (modes) of different lengths, so pulses spread — modal dispersion limits bandwidth and distance. Cheap, easy to couple light into, used with LEDs for short links.
- Multimode graded-index: the core's index falls gradually from the centre outwards, so rays travelling the longer, outer paths move faster. All modes arrive almost together — much higher bandwidth than step-index.
- Single-mode (monomode): a core of about 9 µm carries only one mode, so there is no modal dispersion — the highest bandwidth and longest distances. It needs a laser source and very precise alignment at connectors and splices.
- Sources: LEDs (cheap, wide beam, multimode) and laser diodes or VCSELs (narrow, powerful, fast — single-mode and high-speed multimode). Detectors: PIN photodiodes and, for weak signals, avalanche photodiodes (APD). Aircraft fibre normally uses invisible infrared at 850 nm or 1 300 nm.
Fibre-optic terms
| Term | Meaning |
|---|---|
| Attenuation | Loss of light power along the fibre (absorption and scattering), in dB/km. 3 dB is half the power, 10 dB one tenth. |
| Dispersion | Spreading of light pulses with distance — modal (multimode) and chromatic (different wavelengths travel at different speeds). It limits bandwidth. |
| Bandwidth | The data rate (or frequency) a given length of fibre can carry, often in MHz·km. |
| Bend radius | The tightest curve allowed. Tighter bends let light escape (macrobend loss) and can crack the glass. Microbends are tiny kinks from crushing or over-tight ties. |
| Insertion loss | The loss added by a connector, splice or coupler, in dB. |
| Back reflection (return loss) | Light reflected back towards the source by a poor joint — it can upset lasers. |
| Numerical aperture | The light-gathering ability of the fibre — the sine of the acceptance angle. |
| Mode | One possible path of light through the fibre. |
Fibre compared with electrical wire
| Advantages | Disadvantages |
|---|---|
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Terminations, splices and couplers
- Terminations: the fibre is stripped, bonded (usually with epoxy) or crimped into a precision ferrule or terminus, and the end face is polished flat so that two fibres meet core-to-core with physical contact. Aircraft use rugged connectors — for example multi-way MIL-DTL-38999 shells with fibre termini, or ARINC 801 connectors — rather than office types.
- Splices join fibres permanently. A fusion splice melts the two ends together with an electric arc (lowest loss, about 0.1 dB or less). A mechanical splice aligns them in a sleeve with index-matching gel.
- Couplers split or combine light. A tee coupler taps light off a line; a star coupler mixes the light from every input and shares it among all outputs — transmissive (separate input and output fibres) or reflective (light is reflected back to all fibres).
The fibre-optic data bus
A fibre-optic data bus connects terminals through couplers instead of wires. A control terminal (bus controller) manages the traffic, and remote terminals in each system send and receive when commanded — the same idea as MIL-STD-1553B, which has a fibre version (MIL-STD-1773). Ethernet and AFDX links can also run on fibre, point-to-point between end systems and switches.
Fibre optics on aircraft
- Data networks: parts of the avionics and cabin networks on aircraft such as the Airbus A380 and Boeing 787 run on fibre, especially long runs and high-rate links.
- In-flight entertainment: the high-bandwidth video distribution to the cabin.
- Fly-by-light: flight control signals on fibre, immune to HIRF and lightning — flying on the Kawasaki P-1 maritime patrol aircraft and in research.
- Sensors: fibre-optic gyros in some attitude and heading reference units, and optical sensors in hazardous areas such as fuel tanks.
- Lighting: fibre is used to pipe light for some panel and cabin lighting effects.
Handling, cleaning and safety
- Respect the minimum bend radius (typically 10 to 20 times the cable diameter — use the value in the wiring practices manual), do not crush cables under clamps or over-tighten ties, and never pull on the fibre itself.
- Fit dust caps as soon as a connector is opened. Inspect, clean and re-inspect every end face before mating, using lint-free wipes or approved cleaning tools and solvent.
- Test with an optical light source and power meter (insertion loss) or an OTDR (optical time-domain reflectometer), which locates breaks and bad joints by timing reflected light.
- Never look into a fibre or connector — the infrared light is invisible and a laser source can damage your eyes, even when the fibre seems dark. Use a microscope with a filter, or a video probe, with the source switched off.
- Fibre off-cuts are tiny glass needles: collect them in a marked container and keep them away from skin, eyes and food.