In electronic engineering and also in photonics, power levels are frequently specified with dBm values, which are a logarithmic measure. They are defined as decibels relative to a reference power level of 1 mW. It is thus a dimensionless unit, actually specifying a power ratio rather than a power.
dBm values are convenient e.g. in the context of systems containing amplifiers, because one may simply add an amplifier gain in dB to obtain the dBm value of the amplified power level. Similarly, one can subtract dB values specifying power losses. Therefore, such specifications are quite common in optical fiber communications, where gain and losses occur in fiber amplifiers, at fiber splices and within passive optical fibers, for example. Optical spectrum analyzers and optical power monitors also often display dBm values.
In the context of electronics, even voltages and currents (e.g. of audio signals) are often specified in dBm; this is possible by assuming a certain reference value of the impedance, which may e.g. be 50 Ω or 600 Ω. With such an impedance value, one can calculate the electrical power associated with a voltage or current, and thus calculate the dBm value.
dBm in Spectral Quantities
Such logarithmic specifications can look somewhat confusing in the context of spectral quantities. For example, if a spectral flux, which would normally have to units of W/nm (or W/Hz), is specified with dBm/nm, this should not be understood as “dBm per nanometer”, but rather as “dBm in a bandwidth of 1 nm”. One should be aware that the dBm value would not double if the reference bandwidth were increased to 2 nm; instead, it would rise by 3 dBm.