Re-Routing a Fibre Link: Whose Margin Is It Anyway
An 850 nm multimode fibre link runs from a forward galley access point to the avionics bay. Measured data: launch power into the fibre minus 10.0 dBm; receiver minimum sensitivity minus 28.0 dBm; fibre attenuation 3.5 dB/km at 850 nm. The installed route is 1.6 km with two connectors at 1.0 dB each and one splice at 0.2 dB. The company design rule requires at least 6.0 dB margin above receiver sensitivity at end of life, and the end-of-life calculation must include an extra 1.5 dB for ageing, contamination and connector wear. A cabin re-route is proposed that adds 0.9 km of identical fibre and two additional connectors. The design authority suggests that switching both ends to a 1300 nm pair (launch minus 6.0 dBm, sensitivity minus 30.0 dBm, fibre attenuation 1.0 dB/km) would settle the matter.
1. Calculate the optical budget, the end-of-life loss and the margin for the current route and for the proposed re-route at 850 nm. State compliance with the 6.0 dB rule.
Read the worked answer
Budget = launch power - sensitivity = -10.0 - (-28.0) = 18.0 dB.
Current route (1.6 km)
- Fibre: 1.6 x 3.5 = 5.60 dB
- Connectors: 2 x 1.0 = 2.00 dB
- Splice: 0.20 dB
- Subtotal 7.80 dB; plus ageing 1.50 dB = 9.30 dB
- Margin: 18.0 - 9.3 = 8.7 dB -> compliant (>= 6.0 dB)
Proposed re-route (2.5 km)
- Fibre: 2.5 x 3.5 = 8.75 dB
- Connectors: 4 x 1.0 = 4.00 dB
- Splice: 0.20 dB
- Subtotal 12.95 dB; plus ageing 1.50 dB = 14.45 dB
- Margin: 18.0 - 14.45 = 3.55 dB -> not compliant
End-of-life received power would be -10.0 - 14.45 = -24.45 dBm, only 3.55 dB above the -28.0 dBm floor. The re-route as proposed cannot be approved at 850 nm.
2. Evaluate the 1300 nm proposal with numbers, and list the conditions that must be satisfied before it can be accepted as a solution.
Read the worked answer
1300 nm budget = -6.0 - (-30.0) = 24.0 dB.
Re-route at 1300 nm
- Fibre: 2.5 x 1.0 = 2.50 dB
- Connectors: 4 x 1.0 = 4.00 dB
- Splice: 0.20 dB
- Subtotal 6.70 dB; plus ageing 1.50 dB = 8.20 dB
- Margin: 24.0 - 8.2 = 15.8 dB -> compliant with large reserve
Conditions to verify before acceptance
- The installed fibre really is multimode with the stated 1300 nm attenuation; do not take a catalogue figure for a different grade.
- Both ends must be changed; a 1300 nm transmitter into an 850 nm receiver (or vice versa) is not a valid mix.
- Source type matters: LED versus laser launch into multimode changes the modal distribution and can produce modal noise and connector-dependent loss.
- Connectors, adapters and splices must be rated for 1300 nm, and the loss figures used must be for that wavelength.
- The route still has to be physically installable within bend radius limits; the optical calculation does not prove mechanical acceptability.
3. The design authority asks whether replacing the two extra connectors with splices would rescue the 850 nm option on its own. Answer with numbers, and comment on whether the 1.5 dB ageing allowance is being double counted against the 6.0 dB margin.
Read the worked answer
Splice substitution at 850 nm. Two 1.0 dB connectors replaced by two 0.2 dB fusion splices saves 2 x 0.8 = 1.6 dB, giving 3.55 + 1.6 = 5.15 dB. Still below the 6.0 dB rule, so this measure alone is not sufficient - it improves the link but does not make it compliant. It also trades connector loss for a permanent joint that must be re-made if the route is ever disturbed.
On double counting. The two figures serve different purposes: the 1.5 dB is a known end-of-life degradation allowance and belongs in the loss column; the 6.0 dB is a design guard band for unknowns. Applying both is conservative, not wrong. What is not acceptable is mixing conventions between proposals, so ask the design authority to state explicitly whether its 6.0 dB is defined before or after the 1.5 dB allowance.
- If 6.0 dB already includes ageing: the 850 nm re-route gives 3.55 + 1.5 = 5.05 dB against 6.0 dB and still fails.
- Either way the 1300 nm option or a shorter route is required. Record the convention used.