Principle of operation

The measurement task is to identify and locate imperfections inside the fibre. They can be as an example in extreme cases, due to broken fibre, defective fibre connector or unreliable fibre bending. The OTDR not only permits us to detect back reflections but also to measure transmission losses of the fibre. Microstructures which are more or less distributed homogeneously, exist in every fibre and are a result of the manufacturing process. Light which impinges on these microstructures disperses in such a way that the scattered light also reaches back to the entrance of the fibre.

Measurement of the back scattered light
An additional starting signal which is generated with a photo detector for more precise interpretation of the signal. Both the signals are represented on an oscilloscope, where the starting signal serves as a trigger. When one triggers on the falling edge of the starting signal (laser off) then one gets a picture as shown in the figure right. During the phase of the switched on radiation, the fibre is filled with photons which produce the scattering light at the Rayleigh scatter centre which is directed towards the fibre entrance. The switching off of the light pulse, results in a rapid reduction of the back scattered light. After this follows the exponential decay of the intensity of the scattered light, which again after a particular time abruptly becomes 0. This point corresponds to the fibre end surface.

Interpretation of the recorded data
The intensity of the back scattered light depends on the distance between the fibre entrance and the location of its original inside the fibre mainly through absorption. Therefore, the exponential part of the curve contains the information about the fibre loss. For the determination of this important parameter, we redraw the record of the figure logarithmically in a new diagram. Using the relation: ln I = ln I0 - α . t
the absorption is derived from the a slope of the linearised part of the curve of the back scattered light intensity.