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Optical Time-Domain Reflectometry
End users use optical time-domain reflectometry to characterize optical fiber and
optical connection properties in the field. In optical time-domain reflectometry, an OTDR
transmits an optical pulse through an installed optical fiber. The OTDR measures the
fraction of light that is reflected back due to Rayleigh scattering and Fresnel
reflection. By comparing the amount of light scattered back at different times, the OTDR
can determine fiber and connection losses. When several fibers are connected to form an
installed cable plant, the OTDR can characterize optical fiber and optical connection
properties along the entire length of the cable plant. A fiber optic cable plant
consists of optical fiber cables, connectors, splices, mounting panels, jumper cables, and
other passive components. A cable plant does not include active components such as optical
transmitters or receivers.
The OTDR displays the backscattered and reflected optical signal as a function of
length. The OTDR plots half the power in decibels (dB) versus half the distance. Plotting
half the power in dB and half the distance corrects for round trip effects. By analyzing
the OTDR plot, or trace, end users can measure fiber attenuation and transmission loss
between any two points along the cable plant. End users can also measure insertion loss
and reflectance of any optical connection. In addition, end users use the OTDR trace to
locate fiber breaks or faults.
Figure 5-13 shows an example OTDR trace of an installed cable plant. OTDR traces can
have several common characteristics. An OTDR trace begins with an initial input pulse.
This pulse is a result of Fresnel reflection occurring at the connection to the
OTDR.
Following this pulse, the OTDR trace is a gradual downsloping curve interrupted by abrupt
shifts. Periods of gradual decline in the OTDR trace result from Rayleigh scattering as
light travels along each fiber section of the cable plant. Periods of gradual decline are
interrupted by abrupt shifts called point defects. A point defect is a temporary or
permanent local deviation of the OTDR signal in the upward or downward direction. Point
defects are caused by connectors, splices, or breaks along the fiber length. Point
defects, or faults, can be reflective or nonreflective. An output pulse at the end of the
OTDR trace indicates the end of the fiber cable plant. This output pulse results from
Fresnel reflection occurring at the output fiber-end face.
Figure 5-13. - OTDR trace of an installed cable plant.
ATTENUATION. - The fiber optic test method for measuring the attenuation of an
installed optical fiber using an OTDR is EIA/TIA-455-61. The accuracy of this test method
depends on the user entering the appropriate source wavelength, pulse duration, and fiber
length (test range) into the OTDR. In addition, the effective group index of the test
fiber is required before the attenuation coefficient and accurate distances can be
recorded. The group index (N) is provided by fiber manufacturers or is found using
EIA/TIA-455-60. By entering correct test parameters, OTDR fiber attenuation values will
closely coincide with those measured by the cutback technique.
Test personnel can connect the test fiber directly to the OTDR or to a dead-zone
fiber. This dead-zone fiber is placed between the test fiber and OTDR to reduce the effect
of the initial reflection at the OTDR on the fiber measurement. The dead-zone fiber is
inserted because minimizing the reflection at a fiber joint is easier than reducing the
reflection at the OTDR connection.
Figure 5-14 illustrates the OTDR measurement points for measuring the attenuation of
the test fiber using a dead-zone fiber. Fiber attenuation between two points along the
test fiber is measured on gradual downsloping sections on the OTDR trace. There should be
no point defects present along the portion of fiber being tested.
Figure 5-14. - OTDR measurement points for measuring fiber attenuation using a
dead-zone fiber.
OTDRs are equipped with either manual or automatic cursors to locate points of interest
along the trace. In figure 5-14, a cursor is positioned at a distance zo on the
rising edge of the reflection at the end of the dead-zone fiber. Cursors are also
positioned at distances z1 and z2. The cursor positioned at z1
is just beyond the recovery from the reflection at the end of the dead-zone fiber. Since
no point defects are present in figure 5-14, the cursor positioned at z2
locates the end of the test fiber. Cursor z2 is positioned just before the
output pulse resulting from Fresnel reflection occurring at the end of the test fiber.
The attenuation of the test fiber between points z1 and z2 is (P1
- P2) dB. The attenuation coefficient (α) is
The total attenuation of the fiber including the dead zone after the joint between the
dead-zone fiber and test fiber is
If fiber attenuation is measured without a dead-zone fiber, z0 is equal to
zero (z0 = 0).
At any point along the length of fiber, attenuation values can change depending on the
amount of optical power backscattered due to Rayleigh scattering. The amount of
backscattered optical power at each point depends on the forward optical power and its backscatter
capture coefficient. The backscatter capture coefficient varies with length depending
on fiber properties. Fiber properties that may affect the backscatter coefficient include
the refractive index profile, numerical aperture (multimode), and mode-field diameter
(single mode) at the particular measurement point. The source wavelength and pulse width
may also affect the amount of backscattered power.
By performing the OTDR attenuation measurement in each direction along the test fiber,
test personnel can eliminate the effects of backscatter variations. Attenuation
measurements made in the opposite direction at the same wavelength (within 5 nm) are
averaged to reduce the effect of backscatter variations. This process is called
bidirectional averaging. Bidirectional averaging is possible only if test personnel have
access to both fiber ends. OTDR attenuation values obtained using bidirectional averaging
should compare with those measured using the cutback technique in the laboratory.
POINT DEFECTS. - Point defects are temporary or local deviations of the OTDR signal
in the upward or downward direction. A point defect, or fault, can be reflective or
nonreflective. A point defect normally exhibits a loss of optical power. However, a point
defect may exhibit an apparent power gain. In some cases, a point defect can even exhibit
no loss or gain. Refer back to figure 5-13; it illustrates a reflective fault and a
nonreflective fault, both exhibiting loss. Figure 5-15 shows a nonreflective fault with
apparent gain and a reflective fault with no apparent loss or gain.
Figure 5-15. - An OTDR trace showing a nonreflective fault with apparent gain and a
reflective fault with no apparent loss or gain.
Point defects are located and measured using EIA/TIA-455-59. Test personnel must enter
the appropriate input parameters including the source wavelength, the pulse duration, and
the fiber or cable group index into the OTDR. The nature of fiber point defects depends on
the value of each parameter entered by the end user. The pulse duration usually limits the
length of the point defect while other input parameters, such as the wavelength, can vary
its shape.
If the length of the fiber point defect changes with the pulse duration, then the OTDR
signal deviation is in fact a point defect. If the length remains the same, then the OTDR
signal deviation is a region of high fiber attenuation. Regions of high fiber attenuation
are referred to as attenuation non-uniformities.
Fiber point defects occur from factory fiber splices or bends introduced during cable
construction or installation. For shipboard applications, manufacturers are not allowed to
splice fibers during cable construction. Fiber joints are natural sources of OTDR point
defects. However, fiber breaks, cracks, or microbends introduced during cable installation
are additional sources of point defects.
Point defects that occur at fiber joints are relatively easy to identify because the
location of a fiber joint is generally known. A reflective or nonreflective fault occurs
at a distance equal to fiber joint location. In most circumstances, an optical connector
produces a reflective fault, while an optical splice produces a nonreflective fault.
Reflective and nonreflective faults occurring at distances other than fiber joint
locations identify fiber breaks, cracks, or microbends. A fiber break produces a
reflective fault because fiber breaks result in complete fiber separation. Fiber cracks
and microbends generally produce nonreflective faults.
A point defect may exhibit apparent gain because the backscatter coefficient of the
fiber present before the point defect is higher than that of the fiber present after. Test
personnel measure the signal loss or gain by positioning a pair of cursors, one on each
side of the point defect. Figure 5-16 illustrates the positioning of the cursors for a
point defect showing an apparent signal gain. The trace after the point defect is
extrapolated as shown in figure 5-16. The vertical distance between the two lines in
figure 5-16 is the apparent gain of the point defect.
Figure 5-16. - Extrapolation for a point defect showing an apparent signal gain.
Point defects exhibiting gain in one direction will exhibit an exaggerated loss in the
opposite direction. Figure 5-17 shows the apparent loss shown by the OTDR for the same
point defect shown in figure 5-16 when measured in the opposite direction. Bidirectional
measurements are conducted to cancel the effects of backscatter coefficient variations. Bidirectional
averaging combines the two values to identify the true signal loss. Bidirectional
averaging is possible only if test personnel have access to both ends of the test sample.
Figure 5-17. - The exaggerated loss obtained at point defects exhibiting gain in one
direction by conducting the OTDR measurement in the opposite direction.
OTDRs can also measure the return loss of a point defect. However, not all OTDRs are
configured to make the measurement. To measure the return loss of a point defect, the
cursors are placed in the same places as for measuring the loss of the point defect. The
return loss of the point defect is displayed when the return loss option is selected on
the OTDR. The steps for selecting the return loss option depend upon the OTDR being used.
Q.27 An OTDR measures the fraction of light that is reflected back from the fiber or
link under test. What causes light to be reflected back into the OTDR?
Q.28 List the types of fiber optic components considered part of a fiber optic cable
plant.
Q.29 What is a temporary or permanent local deviation of the OTDR signal in the upward or
downward direction called?
Q.30 Why is a dead-zone fiber placed between the test fiber and OTDR when conducting
attenuation measurements?
Q.31 The amount of backscattered optical power at each point depends on what two
properties?
Q.32 How can test personnel eliminate the effects of backscatter variations?
Q.33 If the length of the fiber point defect changes with pulse duration, is the OTDR
signal deviation a point defect or a region of high fiber attenuation?
Q.34 Give the type of fault (reflective or nonreflective) normally produced by: (a) fiber
breaks, (b) fiber cracks, and (c) fiber microbends.
Q.35 Explain how a point defect may exhibit an apparent gain.
Q.36 A point defect exhibiting an apparent gain in one direction will exhibit what, when
measured in the opposite direction?
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