Both dBm (decibel-milliwatts) and mW (milliwatts) are units of optical power. They can be converted as follows: dBm = 10 x lgP. Essential tool for optical engineers and network technicians. dBm is the unit of optical power, and dB is the unit of an optical power gain or. When reviewing DPSK, DQPSK, interleaver, tunable filter, OPM and OCM specifications of fiber-optic devices, some calculations in relation to wavelength, frequency, power, etc. These calculations may include: We provide these calculators for your convenience. dBm is a logarithmic unit representing the power level relative to 1 milliwatt.