Wikipedia
An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber An OTDR is the optical equivalent of an electronic time domain reflectometer It injects a series of optical pulses into the fiber under test and extracts, from the same end of the fiber, light that is scattered (Rayleigh backscatter) or reflected back from points along the fiber The scattered or reflected light that is gathered back is used to characterize the optical fiber This is equivalent to the way that an electronic time-domain meter measures reflections caused by changes in the impedance of the cable under test
Optical time-domain reflectometer - Wikipedia, the free encyclopedia