190 Chapter 8
Measurement Theory
Temperature Change and Strain Coefficients
The temperature change and strain curves are generated by converting the spectral
shift curve from values in GHz to degrees C or microstrain. This conversion is done
using a 4
th
order polynomial fit. The default coefficients for this fit have been set
for standard SMF 28 fiber. The user can customize these coefficients for other fiber
by
recording
the
Frequency
Shift of that fiber, while
measuring temperature
or strain
on that fiber externally. (See “Temperature Change and Strain Coefficients” on
page 60.)
Spectral Shift
In SWI the backscatter optical power U( ν ) is collected in the spectral frequency
domain: detectors collect light backscattered from the FUT (fiber under test) as the
laser spectral frequency is tuned. The data is processed with a Fourier Transform
to
generate
the
backscatter
optical power U'( τ ) as a
function
of time delay τ , which
can be scaled to show the return loss as a function of FUT length.
The spectral shift option for the lower graph divides the time domain data highlighted
in the upper graph into segments and computes the backscatter spectrum for each
segment. Although the backscatter spectrum appears random for a segment of fiber,
it is actually determined by random fluctuations in the core index of refraction and
is stable and repeatable. For a specific fiber segment j, the reflection spectrum is
denoted as
U
j
( ν ) . If the fiber is exposed to a change in strain or temperature, the
reflection spectrum will experience a shift in optical frequency
∆ν
j
and can be
denoted
U
j
( ν – ∆ν
j
) . The shift in the reflection spectrum ∆ν
j
is computed by
performing a cross correlation operation
U
j
( ν ) on and U
j
( ν – ∆ν
j
) . As shown in
the equation on page 188, this spectral shift can than be related to a temperature or
strain change.
Spectral Shift Quality
The Spectral Shift Quality is a measure of the strength of the correlation between
the measurement and reference reflected spectra. This value is calculated as:
Spectral Shift Quality
=
MAXIMUM
( U (
v
) U ( v – ∆v ) )
--------------------------------------------- -----------------------------
,
∑
U
j
(
v
)
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