The formulas
dBm = 10 × log10(mW) · mW = 10^(dBm ÷ 10)
dBm is power measured against one milliwatt: 0 dBm is exactly 1 mW. Because the scale is logarithmic, every 3 dB halves or doubles the power and every 10 dB is a factor of ten. That is why fiber people work in dB: losses along a link simply add and subtract.
Worked example
Your meter reads −7.5 dBm at the far end. 10^(−7.5 ÷ 10) = 10^−0.75 = 0.178 mW, or 178 µW. If the source launched −3.2 dBm and the meter now reads −11.9 dBm, the link loss is −3.2 − (−11.9) = 8.7 dB.
dBm to mW reference table
| dBm | mW | µW |
|---|---|---|
| +10 | 10.000 | 10000.0 |
| +6 | 3.981 | 3981.1 |
| +3 | 1.995 | 1995.3 |
| 0 | 1.000 | 1000.0 |
| -3 | 0.501 | 501.2 |
| -6 | 0.251 | 251.2 |
| -8 | 0.158 | 158.5 |
| -10 | 0.100 | 100.0 |
| -13 | 0.0501 | 50.119 |
| -15 | 0.0316 | 31.623 |
| -20 | 0.0100 | 10.000 |
| -25 | 0.00316 | 3.162 |
| -27 | 0.00200 | 1.995 |
| -30 | 0.00100 | 1.000 |
| -40 | 1.00e-4 | 0.100 |
dB versus dBm
dBm is an absolute power level, what a meter shows. dB is a ratio, the difference between two levels. Subtracting two dBm readings gives a loss in dB. Adding dBm values together does not mean anything; to combine two signals, convert to mW, add, and convert back.
What readings to expect
- Single-mode optics typically launch somewhere between about −9 and +5 dBm depending on reach; the datasheet gives the range for the exact part.
- A GPON ONT usually needs to see somewhere between about −8 and −27 dBm downstream, depending on the optic class. Too hot can be as bad as too weak.
- A reading near the bottom of your meter's range (often −50 to −70 dBm) usually means no light, a break, or the wrong wavelength selected on the meter.