Networking
Fiber Optic Loss Budget
Optical loss across a fiber link from length, connectors, and splices, compared against the transceiver budget with the margin remaining.
Calculated reference results
- Total link loss
- 4.40dB
- Margin after reserve
- 8.60dB
- Transceiver power budget
- 16.00dB
- Expected received power
- -12.40dBm
- Fiber attenuation
- 0.80dB
- Connector loss
- 3.00dB
- Splice loss
- 0.60dB
Healthy margin.
-8.0 dBm transmit minus -24.0 dBm sensitivity
2.000 km at 0.4 dB/km
4 pairs
2 splices
The arithmetic
- Fiber = 2.000 km x 0.4 dB/km = 0.80 dB
- Connectors = 4 x 0.75 dB = 3.00 dB
- Splices = 2 x 0.30 dB = 0.60 dB
- Total loss = 4.40 dB
- Power budget = -8.0 - (-24.0) = 16.00 dB
- Margin = 16.00 - 4.40 - 3.0 reserve = 8.60 dB
What this does not account for
- Measure the finished link rather than trusting a budget. An optical power meter and light source, or an OTDR, tell you what the link actually does.
- A dirty connector is the most common cause of a fiber link that worked at commissioning and fails later. Clean and inspect before replacing anything.
- Receivers also have a maximum input power. On a very short single-mode link the received power can be too high, which requires an attenuator.
- Attenuation figures here are typical values. Use the actual cable specification and the transceiver datasheet for a design.
How this is calculated
A fiber loss budget compares the transceiver power budget, which is transmit power minus receiver sensitivity, against the total link loss from fiber attenuation, connectors, and splices. What is left is the margin, and a link commissioned with little margin fails as connectors age and get dirty.
Formulas
total loss = (km x dB/km) + (connectors x dB) + (splices x dB)
margin = (Tx power - Rx sensitivity) - total loss - reserve
Assumptions and limitations built into this calculator
- Typical attenuation figures rather than a specific cable specification.
- Does not check the receiver maximum input power, which matters on very short links.
- Does not model dispersion, which limits distance on long high-speed links.
Frequently asked questions
- How much margin should a fiber link have?
- At least 3 dB beyond the repair and ageing reserve is a common target. A link commissioned with almost no margin will fail as connectors age and collect contamination.
- Can a fiber link have too much power?
- Yes. Receivers have a maximum input power as well as a sensitivity floor. On a very short single-mode run the receiver can be overdriven, and an inline attenuator is the fix.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.