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Transceiver datasheets quote dBm, power meters read µW, and amplifier specifications use watts. Type a value in any field and every other unit updates instantly.
dBm and mW. dBm is a logarithmic scale referenced to one milliwatt: 0 dBm is exactly 1 mW, +10 dBm is 10 mW, −10 dBm is 0.1 mW. Because it is logarithmic, losses simply subtract — a 3 dB loss takes you from 0 dBm to −3 dBm regardless of where you started. That is the whole reason optical engineering uses it.
The 3 dB and 10 dB rules. Three decibels is a factor of two in power, and ten decibels is a factor of ten. So −3 dBm is half a milliwatt, −10 dBm is a tenth of a milliwatt, and −13 dBm is a twentieth. Once those two numbers are in your head you can do most optical power arithmetic without a calculator.
dBW versus dBm. dBW is referenced to one watt instead of one milliwatt, so it is exactly 30 dB lower for the same power: 0 dBm equals −30 dBW. It turns up in amplifier and RF-over-fibre specifications; in datacom optics you will almost always see dBm.
Because they are below 1 mW. A typical 10G SFP+ launches around −3 dBm, which is 0.5 mW. Negative dBm is completely normal — it just means less than a milliwatt.
No. Adding two −3 dBm sources gives 0 dBm, not −6 dBm, because you have to add the linear powers and convert back. You can, however, add and subtract dB values (gains and losses) to and from a dBm value freely — that is the point of the scale.
Convert with this tool, or remember that 1 µW is −30 dBm and every factor of ten is 10 dB. 10 µW = −20 dBm, 100 µW = −10 dBm, 1000 µW (1 mW) = 0 dBm.
Turn insertion loss in decibels into the percentage of light that actually gets through
Open tool →Total channel loss, power budget and pass/fail margin for single-mode and multimode links
Open tool →nm ⇄ THz ⇄ GHz, plus photon energy and the optical band each wavelength falls in
Open tool →Transceivers, MPO assemblies, WDM and patch cords — built and tested to the figures you just calculated.