Instructions

930XC-20C • 930XC-20M • 930XC-30F • 930XC-30P
Tempo Communications Inc. 1390 Aspen Way • Vista, CA 92081 USA • 800-642-2155
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Section 3. Basic OTDR Information
Principle of OTDRs
An OTDR (Optical Time Domain Reectometer) is a measurement instrument for identifying optical ber transmission features.
The OTDR is used to measure the overall attenuation of a ber optic link and to provide details relating to the position of
each event in that link. Events include splices, connectors, bends, and optical components. Its non-destructive, single-
ended connection and rapid measurement have made the OTDR an indispensable tool for manufacturing, construction, and
maintenance of optical ber links.
The faults and heterogeneity of optical ber itself cause Rayleigh scattering of light transmitted in optical ber. Part of the light
pulse is scattered in the reverse direction, and this is called Rayleigh backscattering, which actually provides attenuation details
relating to ber length.
Information relating to distance is obtained through time information (thus “time domain” in the name OTDR). Fresnel reection
occurs at the boundary between two media of different IOR (for example, connections of faults, connectors, or optical ber end).
This reection is used to locate the discontinuous points on optical ber. The magnitude of reection depends on the difference
between IOR and the smoothness of boundary.
An OTDR sends out a light pulse into the optical ber and receives reections of events and backscattering power of the pulse in
time. Locations will be displayed on the LCD. The y-axis is the dB value of backscattering power, and the x-axis is the distance.
Basic Denition and Classication of Events
Events refer to any abnormal points causing attenuation or sudden change of scattering power besides the normal scattering of
optical ber, which include losses such as bending, connections, and ruptures.
Event points displayed on the LCD are abnormal points that cause traces to deviate from a straight line.
Events can be classied as reection events or non-reection events.
Reection Events
When some pulse energy is scattered, reection events occur. When reection events occur, a peak is displayed on the trace
(Figure 3-1).
Non-reection Events
Figure 3-2. Non reection Event
Figure 3-1. Reection Event