OTDR (Optical Time-domain Reflectometer) is a device that tests and characterizes a fiber optic cable. This device can do many things, including finding a break or fault in a Fiber optic line.
An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize an optical fiber.
It is the optical equivalent of an electronic time domain reflectometer which measures the impedance of the cable or transmission line under test.
An OTDR 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. The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of length of the fiber.