How do you read optical power in dBm, and how would you calculate a fiber link budget?
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Say this first
dBm is power relative to 1 milliwatt on a log scale: 0 dBm is 1 mW, every -3 dB halves the power and every -10 dB divides it by ten. The budget is the minimum transmit power minus the receiver sensitivity. Subtract the loss of the fiber, connectors and splices, and keep at least 3 dB of margin for aging, dirt and repairs. The receive power should sit inside the optic's range: too low gives errors, too high can saturate the receiver.
Go deeper
Conversions to know
| dBm | mW |
|---|---|
| +3 | 2 |
| 0 | 1 |
| -3 | 0.5 |
| -10 | 0.1 |
| -20 | 0.01 |
| -30 | 0.001 |
Worked example
A single-mode optic's datasheet gives minimum Tx power -6 dBm and Rx sensitivity -14 dBm, so the power budget is 8 dB. The link is 12 km with 4 connector pairs and 2 fusion splices.
| Item | Planning loss | Total |
|---|---|---|
| Fiber at 1310 nm | 0.35 dB per km x 12 | 4.2 dB |
| Connector pairs | 0.5 dB x 4 | 2.0 dB |
| Fusion splices | 0.1 dB x 2 | 0.2 dB |
| Link loss | 6.4 dB |
Margin is 8 - 6.4 = 1.6 dB. It will probably link up today, but it is below a 3 dB design margin. I would reduce connector count or choose an optic with a larger budget.
Reading it on a switch
show interfaces transceiver lists Tx and Rx power per port with alarm and warning thresholds. Compare the
Tx of one end with the Rx of the other: the difference is the real link loss.
Too much light
A long-reach optic on a short patch can overload the receiver. Use an inline attenuator (for example 5 or 10 dB) or a shorter-reach optic.
They will ask next
One end shows Rx -2.5 dBm and the other end shows Rx -18 dBm on a short link. What do you suspect?
One direction is fine and the other has lost about 15 dB more. Look at that direction: a dirty or damaged connector, a bad patch cord strand, or a tight bend. Clean and inspect both ends of that strand first, then test with a light meter.
Why do vendors use dBm instead of milliwatts?
Losses multiply along a link, and logs turn multiplication into subtraction. You can add up dB losses for each component and subtract them from the dBm launch power.
Traps
- Treating dB and dBm as the same. dBm is an absolute power; dB is a ratio or a loss.