All decibel specifications always represent the logarithmized ratio of two electrical quantities( voltage, current, power). This is also the case with the decibel milliwatt (dBm), which represents a power level in decibels, referred to 1mW. The decibel milliwatt can refer to the generation of signals or to the sensitivity of electronic components, but also to attenuation or amplification.
Decibel milliwatts is therefore equally used for characteristic values of components that generate, detect or amplify RF or optical signals. 0 dBm corresponds to a power of 1 mW, larger power values have positive, smaller negative dBm values. For example, 10 dBm corresponds to 10 mW, 20 dBm corresponds to 100 mW, and 30 dBm corresponds to 1,000 mW and 1 W, respectively, which in turn corresponds to 0 decibel watts( dBW). Negative dBm values are assigned to power values that are less than 1 mW. Thus, 100 µW corresponds to -10 dBm, 10 µW to -20 dBm, and 1 µW to -30 dBm.
As a rule of thumb, 30 dB corresponds to a 1,000-fold increase in power, 20 dB corresponds to a hundred-fold increase, 10 dB corresponds to a ten-fold increase, and 3 dB corresponds to a two-fold increase. -3 dB corresponds to a power reduction of 50 %, 10 dB to 90 % and -20 dB to 99 %.
In optical systems, 1 dBm is used as a reference for the absolute reference power (P0) of 1 mW, expressed in dBm. The following relationship exists between the power (P) in milliwatts (mW) and the dBm value: `p=10*log(P/P_0)` in dBm.
The dBm values can be used to determine power and attenuation budgets in transmission and FOC systems in the simplest way. For example, the power of 1 microwatt (µW) corresponds to -30 dBm, of 1 nanowatt (nW) to -60 dBm, and of one picowatt (pW) to -90 dBm. For example, if an optical light source such as a laser diode generates a light signal with a power of +10 dBm, and this signal is attenuated by 4 dBm during transmission, the resulting signal level is +6 dBm.