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Troubleshooting Your Optical Fiber Networks - Introduction to OTDR
By Colin Yao
How Does an OTDR Work?
In fiber optic networks, OTDR (Optical Time Domain Reflectometer) is an opto-electronic
instrument used to characterize an optical fiber. OTDR is both the best known and least
understood fiber optic instrument.
OTDR does not measure loss, but instead implies it by looking at the backscatter
signature of the fiber. It does not measure cable plant loss that can be correlated to
power budgets.
An OTDR injects a series of optical pulses into the fiber under test. It also extracts,
from the same end of the fiber, light that is scattered back and reflected back from
points in the fiber where the index of refraction changes. This working principle works
like a radar or sonar, sending out a pulse of light from a very powerful laser, that is
scattered by the glass in the core of the fiber. The intensity of the return pulses is
measured and integrated as a function of time, and is plotted as a function of the fiber
length.
An OTDR may be used for estimating the fiber's length and overall attenuation, including
splice and mated-connector losses. It may also be used to locate faults, such as breaks.
Physical Limitations of OTDR Testing
The OTDR suffers from several serious uncertainties in measurement and physical
limitations. The measurement uncertainties come primarily from the variations in
backscatter of the fiber. The backscatter coefficient is a function of the material
properties of the glass in the core and the diameter of the core.
Variations of the fiber materials or geometry can cause major changes in the
backscattered light, making splice or connector measurements uncertain by as much as
+/-0.4dB. This has often led to confusion by showing a virtual gain at a connector, where
the fibers involved have different backscatter coefficients. Connector or splice loss must
be measured from both directions and averaged to remove this source of error.
OTDR Design
The principle optical components in a simple standard OTDR include a laser, a receiver,
a coupler and a front-panel connector.
A laser is pigtailed to a connector on the OTDR through a 3dB optical coupler. This coupler
is typically a fused bidirectional device but may also be made of discrete optical components.
The laser fires short, intense bursts of light that are directed through the coupler
and then out through the front-panel connector and into the fiber under test.
As the pulse travels along the fiber, some of the light is lost via absorption and
Rayleigh scattering. The pulse is also attenuated at discrete locations, such as splices,
connectors, and bends, where local abrupt changes in the waveguide geometry couples light
out the core and into the cladding. When the pulse encounters discontinuities in the index
of refraction (such as those found in connectors or the cleaved end of a fiber), part of
the pulse's optical energy is reflected back toward the OTDR.
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