Fiber Optic Loss Budget Calculator
Calculate total optical link loss from distance, connectors and splices, and compare it against your transceiver's power budget.
Inputs
0.35 at 1310 nm SM, 0.22 at 1550 nm SM, 3.0 at 850 nm MM.
Count every patch panel and coupler the light passes through.
From the transceiver datasheet — a negative number.
Held back for ageing, future repairs and splice degradation.
Results
- Remaining margin
- 8.30dB
- Total link loss
- 5.70dB
- Optical power budget
- 17.00dB
- Received power
- -8.70dBm
- — fibre attenuation
- 3.50dB
- — connectors
- 2.00dB
- — splices
- 0.20dB
Link closes with margin to spare.
Launch power minus receiver sensitivity.
What a loss budget answers
An optical link works if the light arriving at the receiver is still strong enough for it to detect. The loss budget compares two figures:
Power budget = launch power − receiver sensitivityLink loss = fibre + connectors + splicesMargin = power budget − link loss − safety allowance
If the margin is positive, the link closes. If it is negative, no amount of configuration will fix it — the photons are simply not arriving.
Where loss comes from
Fibre attenuation
Loss per kilometre depends on wavelength and fibre type. Longer wavelengths travel further, which is why long-haul systems use 1550 nm:
| Fibre / wavelength | Typical attenuation |
|---|---|
| Single-mode, 1310 nm | 0.35 dB/km |
| Single-mode, 1550 nm | 0.22 dB/km |
| Multimode OM3/OM4, 850 nm | 3.0 dB/km |
| Multimode OM3/OM4, 1300 nm | 1.0 dB/km |
Multimode attenuation looks alarming until you remember multimode links are measured in hundreds of metres, not tens of kilometres. Over 100 m, 3 dB/km is only 0.3 dB.
Connectors and splices
- Mated connector pair: 0.3-0.75 dB. Standard practice allows 0.75 dB; good quality factory-terminated connectors achieve 0.3 dB or better. Count every patch panel — a link crossing three panels has three pairs, plus one at each end.
- Fusion splice: around 0.1 dB. Effectively negligible individually, but they accumulate on long routes with many joints.
- Mechanical splice: 0.3 dB or worse, and less stable over time. Use fusion where you can.
On short data center links, connectors dominate. On a 50 m OM4 run with four connector pairs, the fibre contributes 0.15 dB and the connectors contribute 2 dB. Structured cabling designs that chain many patch panels together can fail on connector count alone.
Why hold a safety margin
A link that closes with 0.2 dB to spare is a link that will fail. Hold back 3 dB, conventionally, to cover:
- Repairs. A cut cable gets a splice, and each repair adds loss permanently.
- Ageing. Connectors degrade, and transmitters lose output power over their service life.
- Contamination. Dust on an end-face is the single most common cause of optical faults. Clean and inspect before every mate.
- Bends and stress. Cable moved during later work picks up macrobend loss that was not there at commissioning.
The failure mode people forget
Too much light is also a failure. Every receiver has an overload threshold as well as a sensitivity floor. Plugging a long-reach transceiver designed for 40 km into a 2 m patch lead saturates the receiver and the link either errors heavily or does not come up at all.
The fix is an inline attenuator, sized to bring received power back into the operating window. If a short link with expensive optics is behaving strangely, check received power against the overload figure before assuming a fault.
Verify with a light meter
This calculation predicts what a link should do. Commissioning should measure what it actually does, with an OLTS for insertion loss and an OTDR to locate individual events along the route. Where measurement and prediction disagree, the measurement is right.