Optical time-domain reflectometry

An optical time-domain reflectometer (OTDR) is an optoelectronic instrument used to characterize optical fibres. It injects series of optical pulses into the fibre that is being tested and detects at the same fibre end the returning light that has been scattered or reflected back from points along the fibre. It performs an efficient, precise and wide analysis of the fibre characteristics.

A laser diode launches light into one end of the optical fibre and a photo diode measures the returning light. The backscattered signal provides relevant information about events along the fibre. The signal is influenced by both the attenuation and reflections which the injected laser pulse experiences on its way through the fibre. If a certain area within the fibre has a higher degree of attenuation or reflection, for instance caused by a bending or a fibre connector respectively, this is detected by certain differences in the backscattered signals. The velocity of the light pulse is known and the exact location of the event can therefore be determined on the basis of the time difference between the injection of the light and the return of the backscattered signal.