Optical Power meter

measures how much light is coming out of a fiber optic cable; it can be used to determine the amount of light being generated by an optical source, or the amount of light being coupled into an optical receiver.
Optical power is usually measured in dBm, or decibels referenced to 1mW.

Most power meters used to best communication networks are designed to work at 850nm, 1300nm, and 1550nm wavelength ranges and in the power range of –15dBm to –35dBm for multimode links, or 0-40 dBm for single mode links.