Calculate total optical fiber loss budget from distance, splices and connectors.
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.
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
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.
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.
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.
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.