It is a measuring device which operates by sending pulses of light into a media (single fiber) and measuring light reflected back (on the same fiber).
Comes out that there is loads of information about the media coming back with reflected photons.
One can learn:
1) Breaks, microbends, overall continuity
2) Fiber length
3) Length of individual fiber segments building entire fiber span
4) Optical loss of each segment, splice, connection point, and fiber overall
5) Location of each event (splice, connector, etc) from the beginning of the fiber
6) Reflectance of any part of the fiber (how much light bounces back from any splice, connection point)
7) Such measurement collection creates fiber signature which can be compared over the time to see changes
Setup: Wavelenght, Refractive Index, Pulse width, Fiber range, Averaging, Backscatter Coefficient, Units, Scale.
How to read OTDR output. A must to understand before starting the test: OTDR setup.