Fiber Loss Budget
Calculate total optical fiber loss budget from distance, splices and connectors.
Inputs
Minimum power the receiver needs; more negative = more sensitive
Single-mode ~0.35dB/km @1310nm, ~0.25dB/km @1550nm
Link Margin
15.80dB
Total Loss
8.20dB
Available Power Budget
24.00dB
Fiber Attenuation Loss
7.00dB
Connector Loss
1.00dB
Splice Loss
0.20dB
Step by step
Values used
Transmitter Power = 0 dBm; Receiver Sensitivity = -24 dBm; Fiber Length = 20 km; Fiber Attenuation = 0.3500 dB/km; Number of Connectors = 2; Loss per Connector = 0.5000 dB; Number of Splices = 2; Loss per Splice = 0.1000 dB
Total link loss
total_loss = (fiber_length × loss_per_km) + (connectors × connector_loss) + (splices × splice_loss)
Link margin
margin = (tx_power − rx_sensitivity) − total_loss
Link Margin
= 15.80 dB
Total Loss
= 8.20 dB
Available Power Budget
= 24.00 dB
Fiber Attenuation Loss
= 7.00 dB
Connector Loss
= 1.00 dB
Splice Loss
= 0.20 dB
How it works
An optical link budget compares how much power the transmitter provides (relative to the receiver's minimum usable power) against how much power is actually lost along the path. Total loss sums fiber attenuation (length × dB/km), connector loss (a fixed loss per connector pair, typically 0.3-0.75 dB), and splice loss (typically 0.05-0.3 dB per fusion splice). The link margin — available power budget minus total loss — must be positive, and ideally has a few dB of safety margin, for the link to work reliably over its lifetime accounting for aging and minor future changes.
Formulas
Total link loss
total_loss = (fiber_length × loss_per_km) + (connectors × connector_loss) + (splices × splice_loss)
- d
- Fiber length in km
- \alpha
- Fiber attenuation in dB/km
- n_c, L_c
- Connector count and loss each
- n_s, L_s
- Splice count and loss each
Link margin
margin = (tx_power − rx_sensitivity) − total_loss
Frequently Asked Questions
What link margin is considered safe?
A margin of at least 3 dB is a common minimum design target, providing headroom for fiber aging, additional splices added later, temperature effects, and component tolerances. Critical long-haul links often target 6 dB or more.
Why does 1550nm fiber have lower attenuation than 1310nm?
Silica fiber has a natural attenuation minimum around the 1550nm wavelength window (roughly 0.2-0.25 dB/km) compared to about 0.35 dB/km at 1310nm, due to the physics of Rayleigh scattering and absorption in glass — which is why long-haul systems favor 1550nm.
Why do splices lose less power than connectors?
A fusion splice permanently melts two fiber ends together into a continuous glass path with minimal reflection or misalignment, typically losing only 0.05-0.1 dB. A connector involves a mechanical, reconnectable interface with inherent small air gaps or alignment imperfections, typically losing 0.3-0.75 dB.
What happens if the link margin is negative?
The receiver won't get enough optical power to reliably detect the signal, causing high bit error rates, intermittent link drops, or complete link failure. You'd need to shorten the fiber run, reduce splices/connectors, use lower-loss components, boost transmitter power, or use a more sensitive receiver.