An optical power meter (OPM) is a device used to measure the energy in an optical signal.

An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber. An OTDR injects a series of optical pulses into the fiber under test. It also extracts, from the same end of the fiber, light that is scattered and reflected back from points in the fiber where the index of refraction changes.