How to calculate fiber loss
The total loss of a fiber link is the fiber's attenuation over its length plus the loss at every connector pair and splice. Designers add a safety margin for aging, repairs and dirty connectors. The link works when the transceiver's power budget (Tx power minus Rx sensitivity) is larger than the total loss.
Example: 10 km single-mode at 1310 nm
10 × 0.35 = 3.5 dB. 2 connector pairs × 0.75 = 1.5 dB. 4 splices × 0.3 = 1.2 dB. Plus 3 dB margin = 9.2 dB. A 10 km LR optic with −3 dBm Tx and −20 dBm Rx sensitivity has a 17 dB budget, so the link passes with 7.8 dB to spare.
TIA-568 maximum values
| Component | Max loss | Typical |
|---|---|---|
| Single-mode, 1310 nm | 0.5 dB/km (outside plant) | 0.35 dB/km |
| Single-mode, 1550 nm | 0.5 dB/km | 0.22–0.25 dB/km |
| Multimode OM3/OM4, 850 nm | 3.5 dB/km | 2.5–3.0 dB/km |
| Mated connector pair | 0.75 dB | 0.2–0.5 dB |
| Fusion splice | 0.3 dB | 0.05–0.1 dB |
Frequently asked questions
What is an acceptable fiber loss?
Any loss below your transceiver's power budget minus about 3 dB margin. Compare OTDR or power-meter results with the calculated budget.
How much loss does a fiber connector add?
TIA-568 allows up to 0.75 dB per mated pair; clean, quality connectors usually measure 0.2–0.5 dB.
Why is 1550 nm used for long distances?
Single-mode fiber has its lowest attenuation near 1550 nm, about 0.2–0.25 dB/km.