EXFO OTDR 2 - Manual (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 Light pulses
Analog-to-digital
converter (A/D)
Returned signal
Analyzed signal
Laser
diode Optical
coupler OTDR
port
Fiber