Both dBm (decibel-milliwatts) and mW (milliwatts) are units of optical power. They can be converted as follows: dBm = 10 x lgP. Optical loss is measured in “dB” which is a relative measurement, while absolute optical power is measured in “dBm,” which is dB relative to 1mw optical power Loss is a negative number (like –3. 2 dB) while power measurements can be either positive (greater than the reference) or negative (less than. This document focuses on decibels (dB), decibels per milliwatt (dBm), attenuation and measurements, and provides an introduction to optical fibers. There are no specific requirements for this document. For example, 0 dBm corresponds to 1 mW, 10 dBm to 10 mW and 20 dBm to 100 mW.