In fiber optic communication, sometimes use dB, and sometimes use dBm. Are the two units the same?
dB is a ratio of two different power levers, for example the loss in a fiber optic cable. When power is measured in linear units (mW, uW or nW), dB is calculated on a log scale using this formula:
power (dB) = 10 log (power1/power2)
dBm is an abbreviation for the power ratio in decibel (dB) of the measured power referenced to one milliwatt (mW). If we are measuring absolute power levels, the measurement is generally referenced to 1 milliwatt (mW), is expressed as “dBm” and the calculation becomes:
power (dBm) = 10 log (power/1 mW)
From above formula, we could know 1 mW = 0 dBm, 1 uW = -30 dBm, 1 nW = -60 dBm and two equal powers compared are 0dB (eg. power being the same, there is no loss.)
Typically, people use dB when expressing the ratio between two power values, and use dBm when expressing an absolute value of power.