188 Chapter 8
Measurement Theory
Measurement Technique
A strain or temperature sensor is formed by first measuring and storing the Rayleigh
scatter signature of the fiber under test (FUT) at an ambient state; this data is stored
as the Shift Reference. Then the scatter profile is measured at a later time with strain
or heat applied at some point along the length of the fiber. (For more details, see
“Measurement Technique” on page 61.) The scatter profiles from the two data sets
are then cross correlated along the segment highlighted by a vertical cursor, in
increments of
∆z , which represents an individual sensing element.
It is important to note that the Gauge Length,
∆z , affects the spectral resolution
and the signal-to-noise ratio of the measurement. (The Gauge Length is set by the
user in the Data Processing area of the main screen.) There is, therefore, a
relationship between the spatial resolution of the measurement and its accuracy in
measuring the change in strain or temperature. The longer the segment used, the
better the temperature accuracy. However, if the strain or temperature varies rapidly
with position, a smaller segment size is often necessary to prevent the distortion in
the position scale from blurring the cross correlation spectra.
When the user highlights a portion of temporal domain data in the upper graph using
the vertical cursors, they can view the spectral data for the highlighted portion in
the lower graph. (For more information, see “Distributed Sensing Measurements”
on page 59.) A shift in the spectrum of this data in
response
to strain ε or temperature
T
is
analogous
to
a
shift
in
the
resonance
wavelength
∆λ
or
the
spectral
shift
∆ν
of a Bragg
grating:
8
∆λ
------
=
λ
∆ν
------
ν
=
K
T
∆T +
K
ε
ε ,
where λ and ν are the mean optical wavelength and frequency, and K
T
and K
ε
are the temperature and strain calibration constants, respectively. The default values
for these constants are set at values common for most germanosilicate core fibers:
K
T
= 6.45 x 10
-6
ºC
-1
and K
ε
= 0.780. The OBR software allows the user to set their
own values for these constants, according to their specific application. (See
“Temperature Change and Strain Coefficients” on page 60.)
The values for K
T
and K
ε
are somewhat dependent on the dopant species and
concentration in the core of the fiber, but also to a lesser extent on the composition
of the cladding and coating. Variations of 10% in
K
T
and K
ε
between standard
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