Simply put, optical power is the "brightness" or "intensity" of light. The term usually refers to a device used for measuring the average power in fiber optic systems. For example, if a transmitter outputs +3 dBm and the. What is an optical power meter? An optical power meter (OPM) measures the power levels of light signals in devices that transmit data or power using light. In optical fiber networks, the units of optical power are often expressed in milliwatts (mw) and decibel milliwatts (dbm). The relationship is: 1mw=0dbm, that is to say, 2mw=3dbm, 10*lgmw is the dbm value.