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4.1 Basic Principles of the OTDR Module
An OTDR (Optical Time Domain Reflectometer) is a measurement device for identifying optical fiber transmission
problems. The PRO TP-P6 is mainly used to measure the attenuation of a whole optical fiber span, and to provide
attenuation details relating to the length, location and characteristics of any event within the fiber optic span. An
event can refer to faults caused by splicing, contamination, faulty connectors and/or bending in the fiber. The
TP-P6 is non-destructive, and allows for one-end connection and rapid measurement during fiber manufacturing,
construction, repair or general maintenance.
The faults and heterogeneity of optical fiber itself cause “Rayleigh Scattering” of light pulses transmitted in optical
fiber. Part of the light pulse is scattered in the reverse direction of transmission, and is therefore called “Rayleigh
Backward Scattering,” which actually provides attenuation details directly relating to the optical fiber length.
Information relating to the distance is obtained by measuring the time it takes for the light to return to the OTDR.
That’s why it’s called an Optical TIME DOMAIN Reflectometer (OTDR). “Fresnel Reflection” occurs at the
boundary between two media at a different Index of Refraction (IOR), which can occur due to macro bends, faulty
connectors, dirty connectors or even by the other end (or “end-face”) of the fiber span. The reflection is used
to locate any discontinuous points along optical fiber. The magnitude of reflection (strength) depends upon the
difference between the IOR and the known characteristics of the transmitted light.
The OTDR sends a light pulse through the connected optical fiber, then identifies events by calculating the
reflection, backward scattering and magnitude that occur within a specific time frame. The events are then displayed
onscreen. The Y-axis is a decibel (dB value) of backward scattering power, and the X-axis is the distance from the
user.
4.2 Measurement Application of OTDR
The OTDR displays power relating to distance of returning signals. This information can be used to
identify the main properties of an optical fiber chain.
4.2.1 Measurement Capabilities of the OTDR
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Event location (Distance from user) or rupture of optical fiber span
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Attenuation coefficient of a fiber span
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Loss of a single event (e.g. one optic tie-in), or total loss
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Range of a single event, like reflection of connectors (or grade of reflection)
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Auto measurement of cumulative loss of a single event
4.2.2 Trace Analysis of OTDR
The trace analysis of OTDR is fully automatic. The trace locates:
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Reflection events of connections and mechanic tie-ins
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Non-reflection events (usually at spliced tie-ins)
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End-face of optical fiber
Note:
By scanning the first loss event that is larger than the end threshold, the distance of the end of
the optical fiber can be accurately calculated.
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Information provided: event type, loss, reflection and distance
4 Basic Information of OTDR Module