How to use the fiber optic link loss budget calculator
- Enter the link length in kilometers and the fiber loss per kilometer from the cable data sheet.
- Enter the number of connector pairs and the loss of each, then the number of splices and the loss of each.
- Add a safety margin in decibels if you want one for aging and repairs.
- Optionally enter the transmit power and the receiver sensitivity in dBm to see the power margin that is left.
Formula
Fiber loss = length x attenuation per km
Connector loss = number of connector pairs x loss per pair
Splice loss = number of splices x loss per splice
Link loss = fiber + connectors + splices, and with margin = link loss + safety margin
Power margin = transmit power - link loss with margin - receiver sensitivity
Losses in decibels add. A loss of 3 dB means about half the power is gone, and 10 dB means one tenth is left.
A 10 km single mode link
Ten kilometers of fiber at 0.35 dB/km gives 3.5 dB. Four connector pairs at 0.5 dB add 2.0 dB, and two splices at 0.1 dB add 0.2 dB. The link loss is 3.5 + 2.0 + 0.2 = 5.7 dB, and with no safety margin the loss with margin is also 5.7 dB.
If the transmitter sends -3 dBm and the receiver needs -20 dBm, the power available is 17 dB and the margin left after 5.7 dB is 11.3 dB. The values on the page are examples, so use the figures for your own cable, connectors and equipment.
What to put in the budget
The attenuation figure depends on the wavelength, and a cable data sheet lists a value for each. Connector and splice losses depend on the type and on how well they are made and cleaned. Some designers use the largest figures from the data sheet, and others use the average. Whichever you use, say so in your documentation.
Also allow for patch cords at both ends, optical splitters, and any bends in the route. A splitter has a loss that grows with the number of outputs, so add it as its own line if you use one.
Units in decibels
To convert between decibels and a power ratio, or between dBm and milliwatts, use the decibel calculator. For a wireless link, compare the approach in the free space path loss link budget calculator.
Once the link works, the bandwidth delay product calculator shows how much data is in flight on it.
Assumptions and limits
- Losses are given for one wavelength and add in decibels.
- Attenuation, connector and splice values are examples or the numbers you enter from data sheets.
- The page does not model dispersion, which limits the speed on long links.
- Margins are your choice. Standards and equipment makers may require specific values.
Frequently asked questions
What is a loss budget?
It is the total of all the losses between the transmitter and the receiver, compared with the loss the equipment can tolerate. If the budget is within the limit, the link should work.
How do I calculate fiber link loss?
Add the fiber loss (length times attenuation), the connector losses and the splice losses in decibels, then add any margin.
What margin should I use?
It depends on your design. Some designs include a few decibels for aging and repairs. Use the requirement of your equipment maker or your own policy.
What is receiver sensitivity?
It is the weakest signal power the receiver can decode correctly, given in dBm. The power margin is how far above that limit the signal arrives.