Basic Knowledge Of Otdr - Saluki S2106 Series User Manual

2. Basic Knowledge of OTDR

OTDR is a precise optical fiber measuring instrument based on the principle of Rayleigh scattering and
backscattering generated by Fresnel reflection when laser propagates in optical fibers. By measuring the
transmission time of optical pulses from one to the other, the distance L is calculated using the formula
L=c×t/(2n), L unit is m, C is the propagation speed of light in vacuum 2.99792 ×10
pulse from emission to return (unit is s), n is the group refractive index, designated by the optical fiber
manufacturer, divided by 2 in the formula, because the measurement is round-trip time.
When light propagates in optical fibers, Rayleigh scattering occurs due to the inhomogeneity of the doped
components or the defects of the fiber link itself. Some of the optical signals will be scattered back in the
opposite direction of the incident pulse, which is called backward Rayleigh scattering. The Rayleigh
backscattering light can be observed regularly. With the change of signal intensity, the loss distribution and
connection quality of optical fibers and cables can be measured accurately.
According to the theory of optical transmission, Fresnel reflection occurs when light encounters the
boundary of two kinds of transmission media with different refractive index (e.g. movable connector, break or
end of optical fiber). By receiving the Fresnel reflection signal regularly, the position of discontinuous points
along the length of optical fiber can be accurately located. The magnitude of reflection depends on the
5
8 m/s, t is the time of light