Fiber Loss Calculator

Light dims as it travels through fibre, connectors and splices. An optical link works when enough power arrives to be above the receiver’s sensitivity, with a safety margin on top. The page adds up the losses along a fibre run and compares them with what the transmitter and receiver can tolerate.

Where the loss comes from

Fibre itself loses a fixed number of decibels per kilometre. The values built into the page are the maximum figures from TIA-568: 3.5 dB/km for multimode at 850 nm, 1.5 dB/km for multimode at 1300 nm, 1 dB/km for single-mode inside plant at 1310 or 1550 nm and 0.5 dB/km for single-mode outside plant. You can override the value if your cable’s data sheet gives a better one. Each mated connector pair loses a fraction of a decibel (0.75 dB is the TIA maximum), and each splice a few tenths (0.3 dB is the TIA maximum); enter your own figures if you have test results.

The power budget

The transmitter’s output power minus the receiver’s sensitivity, both taken from the transceiver data sheet, is the power budget: the total loss the link can afford. Subtract the actual losses and a safety margin (the preset uses 3 dB) to get the margin left. A positive margin means the link should work; a negative one means it will not, or will work only on a good day. The expected received power is the transmit power minus the total loss, which lets you check against both the sensitivity and the maximum input level of the receiver.

Using the page

Choose the fibre type and wavelength, enter the length in kilometres, the number of connector pairs and splices and their losses, the transmit power and receive sensitivity, and the safety margin. Presets fill in a 2 km single-mode link and a 300 m OM4 multimode link. The answer lists the fibre loss, connector loss, splice loss, total loss, power budget, margin and expected received power, and says whether the link fits.

A worked example

The first preset is a 2 km single-mode link at 1 dB/km. Fibre loss is 2.00 dB, connector loss 1.50 dB, splice loss 0.30 dB, so the total is 3.80 dB. With a transmit power of −3 dBm and a receiver sensitivity of −14 dBm, the power budget is 11.0 dB, the margin left after the safety margin is 4.20 dB, the expected received power is −6.8 dBm, and the page reports that the link fits the power budget. Replacing the fibre with the multimode preset shows how much more a few hundred metres can cost at 850 nm.

Limits

This is a loss budget only. It ignores chromatic and modal dispersion, which limit the bandwidth of long or fast links independently of power, and it does not check receiver overload. The loss values are standards maximums or your own entries; a certified test with a light source and power meter remains the way to prove an installed link. For radio links, the Link Budget Calculator works in the same way.

Related tools

• Link Budget Calculator does the radio equivalent of this sum.
• Network Cable Chart lists distance limits for fibre and copper types.
• PoE Power Calculator covers power over copper links.
• SLA Uptime Calculator turns availability goals into downtime for the link you design.