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6 Introduction to OTDR
6.1 Measuring Purpose of OTDR
OTDR displays the power of the returned signal related to distance. The information can be
used to confirm the transmission quality of a fiber optic link.
6.1.1 Measuring Contents of OTDR
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Position (distance) of event, result or fractured position of fiber optic link;
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Attenuation coefficient of optical fiber in the optical fiber link;
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Loss of a single event (for example, a optical connector or bending), or total losses from
end to end on the fiber optic link;
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Reflection amplitude (or reflection level) of an event, such as connector
6.1.2 OTDR Curve Analysis
OTDR analyzes the curve automatically. Location of the curve:
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Reflection event generated from connection and mechanical connector;
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Non-reflection event (usually be fusion splice);
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Optical fiber bundling: through scanning the first loss event that larger than the bundling
threshold, OTDR detects the bundling of optical fiber;
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Events list: Category of events, loss, reflection, and distance are listed through
calculation.
6.2 Basic Principle of OTDR
Optical Time Domain Reflectometer is the full name of OTDR. OTDR is accurate
optoelectronic integrated instrument made according to back scattering that generated from
Rayleigh scattering and Fresnel reflection when lights are transmitted in the optical fiber. It is
widely used in the maintenance, construction and monitoring of cable line. It can measure the
optical fiber length, transmission attenuation of optical fiber, attenuation of splice, and failure
location, etc.
When the pulse being transmitted downwards along with the optical fiber, and some small
changes (such as changes of refractivity and discontinuity) in the material make the lights
scatter to different directions, Rayleigh scattering occurs. Partial lights are scattered back along
with the direction that opposite to pulse, therefore, it is called Rayleigh back scattering. Back
scattering light shows attenuation details that related to the length. Information related to length
is gained through time (i.e.: origin of domain of Optical Time Domain Reflectometer). These
back scattering signal indicates the attenuation (loss/distance) degree caused by optical fiber.
The curve formed is a downwards curve, which reflects the transmission characteristics of the
optical fiber.
When lights transmitted downwards along with the optical fiber encountering sudden
change of material density, Fresnel reflection occurs. Material density change may occur at the
connection or fracture parts where air gap existed. This phenomenon is used by OTDR to
accurately confirm the position of discontinuity point along with the length of optical fiber.
Compared to Rayleigh scattering, Fresnel reflection will reflect quite a lot of lights. The power of
Fresnel reflection is tens of thousands times of that of back scattering. The reflection strength is
ascertained according to the change degree of refractivity.
