Principle of OTDR

OTDR (Optical Time Domain Reflectmeter) is a measurement instrument for identifying optic fiber transmission features. The instrument is mainly used to measure attenuation of a whole optic fiber chain and provide attenuation details relating to length, namely detect, locate and measure any event in optic fiber chain (events refer to faults caused by welding, connectors, and bending whose transmission change can be measured). Its non-destructive, one-end connection, and rapid measurement has made the OTDR an indispensable tool for manufacture, construction, and maintenance of optic fiber.

The faults and heterogeneity of optic fiber itself cause Rayleigh scattering of light pulse transmitted in optic fiber. Part of light pulse is scattered in the reverse direction, and this is called Rayleigh backward scattering, which actually provides attenuation details relating to length.

Information relating to distance is obtained through time information (that’s the reason why there is “time Domain” in the name of OTDR). Fresnel reflection occurs at the boundary between two media of different IOR (for example, connections of faults, connectors, or optic fiber end). This reflection is used to locate the discontinuous points on optic fiber. The magnitude of reflection depends on the difference between IOR and the smoothness of boundary.

OTDR sends out a light pulse into connected optic fiber, and receive reflections of events and backward scattering power of pulse in time. Locus will be displayed on LCD. The y-axis is dB value of backward scattering power, and the x-axis is the distance.