9.2 Fibre Cable Types, Signal Degradation and Optical Components
Key Takeaways
- Multimode avionics fibre typically has a 50 µm or 62.5 µm core in a 125 µm cladding and is driven by an 850 nm LED or VCSEL; single-mode fibre has a core of about 8–10 µm and needs a 1310 nm or 1550 nm laser.
- Graded-index multimode fibre reduces modal dispersion relative to step-index multimode fibre because high-order modes travel in lower-index glass and catch up with the axial mode.
- Attenuation is loss of optical power in decibels (3 dB halves power); dispersion is pulse spreading that can close the eye even when received power still looks adequate.
- A fibre-optic data bus uses couplers so that a control terminal can exchange light with remote terminals; a star coupler equalises port power better than a chain of T-couplers, which accumulate tee loss.
- A 40 m jumper at 3.0 dB/km contributes only 0.12 dB, so connector, coupler and contamination losses usually dominate an aircraft loss budget.
9.2 Fibre Cable Types, Signal Degradation and Optical Components
Topic 5.10 at B2 level 2 (B1 level 1) does not stop at total internal reflection. The pre-12 June 2024 detailed Appendix I description named fibre terms, the fibre-optic data bus, couplers, and control and remote terminals; the current consolidated Appendix I retains only the Fibre optics heading and levels. This section groups those terms around two engineering questions: which fibre you are holding, and how light is shared among boxes on a bus rather than on a single jumper.
Multimode and single-mode fibre
A mode is a distinct electromagnetic field pattern that the waveguide can support. Multimode (MM) fibre has a relatively large core — 50/125 µm or 62.5/125 µm are the usual core/cladding diameters — so many modes propagate. An LED or VCSEL at about 850 nm can launch into that large core, which is why multimode jumpers dominate short aircraft runs in the cabin, in-flight entertainment and rack-to-rack looms. Single-mode (SM) fibre has a core of about 8–10 µm (cladding still 125 µm). Only the fundamental mode propagates at 1310 nm or 1550 nm, so a laser and tighter connector geometry are required. Single-mode wins on distance and bandwidth; it loses on coupling tolerance and contamination sensitivity because the light is confined to a core comparable with a dust particle.
Do not confuse cladding diameter with core diameter. Both MM and SM avionics glass commonly use a 125 µm cladding; the difference is the core. A 62.5 µm core is multimode, not “thick single-mode”. Mixing an SM laser jumper into an MM plant, or the reverse, produces a coupling mismatch that looks like a failed LRU.
Step-index and graded-index profiles
Step-index fibre has an almost abrupt index drop from core to cladding. In multimode step-index fibre, high-order modes take a longer zigzag path than the axial mode, so a narrow pulse arrives as a spread packet. That modal dispersion limits the bit-rate × length product and was the reason early optical links needed either short runs or low data rates.
Graded-index multimode fibre has a refractive index that is highest on the axis and falls, often approximately parabolically, toward the cladding. High-order modes travel farther but through lower-index glass, so they travel faster; the delay spread shrinks. Graded-index 50/125 or 62.5/125 is the usual multimode construction in aircraft data and IFE harnesses. Single-mode fibre is essentially step-index (or a slightly depressed cladding) because there is only one mode; chromatic and waveguide dispersion then dominate, not modal dispersion. Chromatic dispersion is the wavelength dependence of group velocity: a laser that is not perfectly monochromatic still has a spectrum, and the edges of a pulse walk apart along a long fibre.
| Type | Core / cladding (typical) | Source | Dispersion that usually limits | Typical aircraft use |
|---|---|---|---|---|
| Step-index MM | 50 or 62.5 / 125 µm | LED | Modal (large) | Older or very short links |
| Graded-index MM | 50 or 62.5 / 125 µm | LED / VCSEL 850 nm | Modal (reduced) | Cabin, IFE, short avionics |
| Single-mode | about 9 / 125 µm | Laser 1310 / 1550 nm | Chromatic / waveguide | Long or high-rate backbones |
Attenuation versus dispersion
Attenuation is loss of optical power. It is quoted in decibels:
A(dB) = 10 log10 (P_in / P_out)
A 3 dB loss halves the power; a 10 dB loss leaves one tenth. Mechanisms on the aircraft are absorption in the glass and in residual OH⁻, Rayleigh scattering (stronger at shorter wavelength, which is why 850 nm multimode is lossier per kilometre than 1310 nm single-mode), macrobend and microbend leakage, and connector, splice and coupling loss at every interface.
Cable attenuation for avionics lengths is often small compared with connector and coupler loss. A 20 m jumper at 3 dB/km contributes only 0.06 dB, while two connectors at 0.3 dB each contribute 0.6 dB. That arithmetic is why a technician who “tests the cable” with a copper continuity meter learns nothing, and why a dirty terminus can fail a system whose fibre is intact.
Dispersion is pulse spreading in time. Received power may still sit above the photodiode’s minimum, yet overlapping pulses make bits unreadable (a closed eye). Modal dispersion dominates multimode step-index; graded-index reduces it; chromatic dispersion dominates single-mode. Multimode bandwidth is often quoted as a MHz·km product: a 200 MHz·km fibre used over 0.1 km still offers about 2 GHz of analogue bandwidth before other limits intervene, which is why short aircraft MM runs can carry high-rate digital video.
Worked example — loss budget
A control terminal launches 100 µW into a graded-index jumper. The run is 40 m of fibre specified at 3.0 dB/km, plus two connectors at 0.35 dB each, plus a star-coupler drop of 13 dB. A 16-port star is a typical teaching order of magnitude: an even split of 1/16 is 12 dB, plus about 1 dB of excess loss in the mixing region.
- Fibre: 0.040 km × 3.0 dB/km = 0.12 dB
- Connectors: 2 × 0.35 = 0.70 dB
- Coupler: 13 dB
- Total: 13.82 dB
P_out = 100 µW × 10^(−13.82/10) ≈ 100 × 0.0415 ≈ 4.15 µW
If the remote-terminal photodiode needs 5 µW for the specified bit-error rate, this drop is already out of budget. The first suspects are a dirty connector or a bent jumper adding extra decibels locally, not “the glass went cloudy over 40 m”, because 0.12 dB of cable is negligible beside 13 dB of splitting.
Connectors
An avionics fibre connector is a ferrule that holds the cleaved fibre so that two cores face each other within a fraction of a micrometre. Physical-contact (PC) and ultra-physical-contact (UPC) polishes reduce the air gap and therefore Fresnel loss and back-reflection into the laser. Angled physical contact (APC, typically an 8° face) dumps residual reflection away from the core; it is more common on single-mode than on short multimode runs. Contact ferrules (including civil LC/SC/ST styles and avionics termini such as MIL-PRF-29504 pins or ARINC 801) demand cleanliness. Expanded-beam connectors use lenses so that the beam is wide at the interface; they tolerate more contamination in a dirty bay at the cost of higher insertion loss. Unused ports still need dust caps: a wide beam is not a licence to leave the terminus open.
Couplers, control terminals and remote terminals
A fibre-optic data bus must let more than two boxes share light. A simple point-to-point jumper is a link, not a bus.
A T-coupler (tee) taps a fraction of the through power to a drop. Each tee takes a share and adds insertion loss, so the last terminal on a long daisy-chain can be starved of photons, and each tap is a reflection point. A star coupler (transmissive or reflective) brings fibres to a mixing region so that power from any input is divided among the outputs. Ports are more nearly equal, which is why star architecture is the usual teaching picture for an optical bus.
The control terminal (CT) is the bus-master analogue: it issues commands or frames and owns timing. Remote terminals (RTs) are the LRUs that respond. A textbook optical mapping of command/response copper buses in the MIL-STD-1553 family is MIL-STD-1773, which keeps the protocol and replaces the twisted pair with fibre and electro-optic converters. Do not claim that every aircraft optical network is 1773. Civil types also use point-to-point fibre Ethernet, optical versions of AFDX/ARINC 664-style networks, fibre-channel video (including ARINC 818 optical physical layers on some installations), and proprietary IFE backbones. This historical study scope emphasises the roles — coupler, CT, RT, bus versus link — not a single type certificate.
[!WARNING] Optical power at a star-coupler unused port still exists. Fit dust caps. Do not inspect a live fibre with the unaided eye or with an unfiltered microscope; use the inspection adapters specified for the terminus.
B2 candidates should be able to read a loss budget, name why graded-index MM is preferred to step-index MM on a short bus, and sketch CT–coupler–RT. B1 candidates should distinguish MM from SM and attenuation from dispersion, and recognise that a fibre data bus is more than a single jumper.
How do typical avionics multimode and single-mode optical fibres differ?
Why is graded-index multimode fibre preferred to step-index multimode fibre on a short high-rate aircraft link?
On a fibre-optic data bus, what roles do couplers, the control terminal and remote terminals play?
A 40 m graded-index jumper is specified at 3.0 dB/km. Two connectors add 0.35 dB each and a star coupler adds 13 dB. Which statement correctly separates attenuation from dispersion and reads this budget?