Two budgets that should not be mixed up
The transceiver's optical power budget is the difference between the transmitter's minimum output power and the receiver's sensitivity. It indicates how much total loss the link can tolerate under specified conditions. The cable installation's loss budget is the sum of the expected attenuation in fibre, connectors, splices, and other passive components.
For a stable link, the calculated attenuation and chosen system margin together must be lower than the available power budget. Also check the other limit: the receiver must not be overloaded by too high optical power.
Basic formulas
Available power budget in dB = minimum transmitter power in dBm minus receiver sensitivity in dBm. Example: −3 dBm minus −14 dBm gives 11 dB.
Calculated link attenuation in dB = fibre length × attenuation per kilometre + connector losses + splice losses + losses in splitters, mux/demux, and other passive components.
Remaining margin in dB = available power budget minus calculated link attenuation. The organisation should set a design margin for ageing, temperature, dirt, repairs, and measurement uncertainty. Do not use an arbitrary standard margin without considering the environment and requirements.
Calculation example
Assume a 10 km single-mode link with two connector pairs and four fibre splices. If the project's design values are 0.35 dB/km, 0.5 dB per connector pair, and 0.1 dB per splice, the estimated loss will be 3.5 + 1.0 + 0.4 = 4.9 dB.
If the chosen optics has an 11 dB available power budget, 6.1 dB remains before the projected system margin. The values in the example are educational – always use your own component data, standard requirements, and transceiver limits in real design.
Check both weak and strong signal
Many checks stop at receiver sensitivity. On short links with long-distance optics, the received power may instead exceed the receiver's maximum level. Compare maximum transmitter power minus the lowest possible link loss with the receiver's overload limit.
If attenuators are required, they should be sized and labelled. An attenuator does not resolve incorrect wavelength, incorrect fiber class, or an incompatible transceiver.
Measurement at commissioning
A calculation is a design check; it does not replace measurement. Measure insertion loss with an appropriate light source and power meter at relevant wavelengths. OTDR can locate events and provide information about splices, connectors, and fiber lengths, but test method and reference settings must be defined.
Save both estimated budget, measurement values, reference method, and limit values. This allows differentiation between design faults, installation faults, and later degradation.
Common mistakes
Using typical transmitter power instead of specified minimum level.
Counting one connector when an adapter connection consists of two contact surfaces.
Forgetting muxes, splitters, patch panels and spare splices.
Mixing dBm, which is an absolute power level, with dB, which describes difference or loss.
Approving the link without checking the maximum receiver power.
Documentation template
- Transceiver A and B and datasheet version.
- Transmitter min/max, receiver sensitivity and overload limit.
- Wavelength, fiber class and total channel length.
- Number of contact pairs, splices and passive optical components.
- Assumed losses, system margin and calculated result.
- Measured insertion loss and received power after commissioning.
Next step
Send link length, wavelength, number of connectors and splices as well as the transceiver's datasheet. We help you check optics and power budget before ordering.