Exfo OTDR 2 Manuel d'utilisateur télécharger pdf (Page 16)

Introducing the OTDR

8 OTDR

OTDR Basic Principles

An OTDR uses the effects of Rayleigh scattering and Fresnel reflection to

measure the fiber’s condition, but the Fresnel reflection is tens of

thousands of times greater in power level than the backscatter.

 Rayleigh scattering occurs when a pulse travels down the fiber and

small variations in the material, such as variations and discontinuities

in the index of refraction, cause light to be scattered in all directions.

However, the phenomenon of small amounts of light being reflected

directly back toward the transmitter is called backscattering.

 Fresnel reflections occur when the light traveling down the fiber

encounters abrupt changes in material density that may occur at

connections or breaks where an air gap exists. A very large quantity of

light is reflected, as compared with the Rayleigh scattering. The

strength of the reflection depends on the degree of change in the index

of refraction.

When the full trace is displayed, each point represents an average of many

sampling points. You will have to zoom to see each point.

Microprocessor

Pulse

generator

Avalanche

photodetector (APD)

Display

Reflections come back

to the OTDR

Set of

instructions

Light pulses

Analog-to-digital

converter (A/D)

Returned signal

Analyzed signal

Laser

diode

Optical

coupler

OTDR

port

Fiber

1 2 ... 11 12 13 14 15 16 17 18 19 20 21 ... 218 219

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Exfo OTDR 2 Manuel d'utilisateur Page 16