OTDRs: Basics and Usage Techniques Roberto Noris Rertech
What is an OTDR?
- OTDR: optical time domain reflectometer
- Same principle as radar: it emits pulses into the fiber optic network and “listens” to the reflections along the network
- It allows you to generate a map of the entire optical network, to establish the distance, any connectors or joints present, breaks, bends in the fiber, etc.
- Its use requires a thorough understanding of the operating principle and the main setting parameters
Rayleigh backscattering operating principle
- This is a physical phenomenon of optical fiber: 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 core cross-section, the lower the backscattering
Fresnel's Reflections
In addition to backscattering, which is intrinsic to optical fiber, reflections are generally generated when there is a sudden change in the transmission index through which the light flux passes. These are called "Fresnel reflections." For example:
- In case of fiber interruption (sudden passage to air), for example fiber end or breakage
- In the case of a pair of optical connectors (fiber/air gap/fiber) or a mechanical joint
- In particular cases of small imperfections of the optical fiber core