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 Reectometer) 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 reection
occurs at the boundary between two media of different IOR (for example, connections of faults, connectors, or optical ber end).
This reection is used to locate the discontinuous points on optical ber. The magnitude of reection depends on the difference
between IOR and the smoothness of boundary.
An OTDR sends out a light pulse into the optical ber and receives reections 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 Denition and Classication 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 classied as reection events or non-reection events.
Reection Events
When some pulse energy is scattered, reection events occur. When reection events occur, a peak is displayed on the trace
(Figure 3-1).
Non-reection Events
Figure 3-2. Non reection Event
Figure 3-1. Reection Event