OTDR Basics and techniques of use Roberto Noris Rertech
What is an OTDR
- OTDR: optical reﬂectometer in the time domain
- Same principle as radar: it emits pulses into the optical fibre network and 'listens' for reﬂections along the network.
- Allows you to generate a map of the entire optical network, to determine the distance, any connectors or joints present, breaks, bends in the ﬁbre, etc.....
- Its use requires in-depth knowledge of the operating principle and the main setting parameters.
Rayleigh backscattering operating principle
- This is a physical phenomenon of the optical ﬁbra: while light propagates from A to B, a small portion (about 1 millionth, --60 dB) propagates in the opposite direction from B to A.
- The larger the cross-section of the core, the lower the backscattering
In addition to backscattering, which is intrinsic to the optical ﬁbra, reﬂections are generated in general when there is a sudden change in the transmission index through which the light ﬂow passes. These are called 'Fresnel reﬂections'. For example:
- In the event of ﬁre failure (sudden passage to air), e.g. ﬁre failure or breakage
- In the case of a pair of optical connectors (ﬁre/air gap/ﬁre) or a mechanical coupling
- In special cases of small imperfections in the optical ﬁbra core