Bi-Direction Testing with an OTDR

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Bi-Direction Testing with an OTDR-Seymour
Goldstein
www.flukenetworks.com 2006-2017 Fluke
Corporation
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STANDARD OR APPLICATION NOTE 
Have you ever wondered how a Standard and an
Application Note differ? No, the answer is not
only that one is provided free by vendors. Both
contain guidance and useful technical information
so what is the difference? Well, a Standard is
developed by many experts using consensus to
provide accurate technical information and
guidance while an Application Note, while
technically accurate, is developed by one vendor
to position their brand. One case in point is
bi-directional testing using an OTDR. Many
application notes have been written about
bi-directional testing with an OTDR but what do
the Standards say?
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WHAT ARE WE TRYING TO MEASURE
First, lets examine what is typically being
measured. The attenuation measurement of an
optical fiber link requires the measurement of
the cabling under test as well as the two
connections, A and B, on both ends of the
link (see Figure 1). Notice that test cords are
attached to both ends of the cabling under test.
Otherwise, the attenuation (loss) at each
connector could not be measured. The importance
of measuring the end connectors is amplified when
short fiber runs have fiber attenuation that is
much lower than those of the connectors.
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WHAT IS A PROPER BI-DIRECTIONAL TEST
Launch and tail cords are used with OTDRs when
the attenuation of the Cabling under Test,
connector A, and connector B must to be
measured (see Figure 2). A proper bi-directional
test requires that only the OTDR be moved to the
far end of the tail cord (see Figure 3). The
launch cord and tail cord should never be removed
from the cabling under test. Only the OTDR is
moved. Doing otherwise would defeat the purpose
of the test and waste time. To reiterate,
a bi-directional test consists of two
measurements on the same optical fiber, made by
launching light into opposite ends of that fiber,
then averaging the attenuation at
connectors without disconnecting the launch and
tail cord from the cabling under test.
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WHAT IS A PROPER BI-DIRECTIONAL TEST
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According to IEC 61280-4-1, multimode testing
In order to properly measure the first and last
connection for bi-directional averaging, the
launch and tail cords must remain in their
initial measurement positions. Only the OTDR is
moved. This will ensure that identical optical
fibers are mated to the cabling under test so
that the effects of mode field mismatch between
the test cords and cabling can be averaged
out. If the launch and tail cord have identical
backscattering characteristics and only the total
attenuation of the link is required, it may be
sufficient to carry out an OTDR test in one
direction. However, it is rare that the launch
and tail cord will have identical backscatter
characteristics.
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According to IEC 61280-4-2, single-mode testing
OTDR testing is carried out from both ends of
the cabling when it contains splices or
connectors (according to Figure 1, that is the
case). Any differences in the component
attenuation due to variations in backscattering
coefficients can be cancelled out by averaging
the component attenuation measurements taken from
both ends of the system. To accurately measure
the first and last connection of the link using
bi-directional averaging, the launch and tail
cords remain in their initial measurement
positions. This will ensure that identical
optical fibers are mated to the cabling under
test so that the effects of mode field mismatch
between the test cords and cabling can be
averaged out. An individual attenuation is
defined as the half sum of the attenuation
recorded from each end.
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WHY DO IT
When an OTDR sends a pulse of light from its
optical port into the fiber under test, reflected
light is received on the same port. The reflected
light can be Rayleigh backscattering which
defines the fiber attenuation of an OTDR trace
and/or Fresnel reflections which define, for
example, connector return loss. Rayleigh
backscattering is fundamental to OTDR operation,
and is the method OTDRs use to measure end to end
loss, and discrete losses at splices and
connectors. The backscatter coefficient is a
measure of the amount of backscatter when an OTDR
emits a 1 nano second pulse. It is defined by all
fiber vendors for their fiber specification. To
make accurate measurements, the OTDR backscatter
coefficient (e.g. -79 dB) must be set in the
OTDR.
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WHY DO IT
Now for the bad news backscatter coefficients
vary along a fiber and from one supplier to the
next. The effect - differences between test cords
and cabling may cause variations in the
attenuation or gain of individual connections.
For example, when an optical fiber with a low
backscatter coefficient is connected to a fiber
with a higher backscatter coefficient, the OTDR
will receive more optical power from the fiber
with the higher backscatter coefficient. This can
be interpreted as a reduction in the apparent
attenuation and may even appear as a gain
(negative attenuation). The effect is known as a
gainer. Making an OTDR measurement from each end
of the fiber then averaging the results cancels
out the component attenuation variance.
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WHAT DO THE STANDARDS SAY ABOUT THE TAIL CORD
In IEC 61280-4-2, attenuation measurements of
installed single-mode optical fiber, provides
guidance on the use of a tail cord. Considered
a non-recommended practice, measurement without
a tail cord, the Standard goes on to explain
that with no tail cord, the attenuation of the
connector at the end of the cabling is not taken
into account. In other words, an improper test is
performed wasting time and money. Further, if the
cabling is short relative to the deadzone, a
measurement is not possible. The only time
testing without a tail cord is acceptable,
according to the Standard, is for qualification
of a repair of the cabling that has been tested
before the damage.
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SUMMARY

• DO
• Use a launch and tail cord when measuring the
  cabling under test
• Never disconnect the launch and tail cord from
  the cabling under test when testing in multiple
  directions
• Move only the OTDR by disconnecting from the
  launch cord and connecting to the tail cord
• Average the results from each measurement
  direction
• Pay attention to Standards
• DONT
• Move the OTDR with the launch cord
• Neglect to use a tail cord
• Waste your time with an improper measurement

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