Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02629446 2014-08-21
METHOD AND SYSTEM FOR SIMULTANEOUS MEASUREMENT OF STRAIN AND
TEMPERATURE
The present invention relates to a method and system for the simultaneous
measurement of strain and temperature utilizing principles associated with
Brillouin
scattering.
BACKGROUND OF THE INVENTION
Brillouin scattering is an inelastic or nonlinear scattering of light from
acoustic
phonons in a dielectric material, such as an optical fiber. Brillouin
scattering can be
spontaneous, as when light in a fiber interacts with density variations in the
fiber, or it can
be stimulated. The Brillouin frequency is the difference between the
frequencies of the
input and scattered beams of light within the fiber. The Brillouin frequency
can be described
by the equation:
2nV,0
vB ____________________________ sin¨ (1)
2
where: Va is the sound velocity in the optical fiber;
n is the refractive index;
Ap is the wavelength of the pump laser.
The Brillouin frequency is a physical property that is related to temperature
and
strain within the optical fiber, in accordance with the following equation:
4113=1/B01-CAT ¨T0)+Ce(6-60) (2)
where CT and Ce are coefficients of temperature (T) and strain (E),
respectively. These
coefficients are determined experimentally for each fiber.
With Brillouin amplification, the scattered light is amplified. There can be
an energy
exchange between two counter-propagating laser beams, which exchange is
maximum
when v1 - v2 = vEs=
The Brillouin frequency spectrum is obtained by scanning the beat frequency of
the
fiber. It is characterized by the peak power, the shape of the frequency
curve, the center
frequency, and the linewidth, with full linewidth occurring at half-maximum
(see Figure 1).
It has been known that the principles of Brillouin scattering can be used to
measure
strain or temperature in an optical fiber. Because there is only one peak of a
Brillouin
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spectrum from a single mode fiber (eg. SMF-28) and because strain and
temperature
change simultaneously in accordance with equation 2, it is impossible to
simultaneously
extract information respecting both strain and temperature from a single peak
of the
Brillouin spectrum.
In the past, when it has been desired to measure both strain and temperature
simultaneously, it has been necessary to take special measures to achieve
these
measurements. For example, if temperature is maintained constant it is
possible to
measure strain, or if the strain is maintained constant it is possible to
measure temperature.
Another measure would be to install an additional fiber for temperature
measurement in
order to compensate for the temperature influence on the Brillouin spectrum
caused by both
temperature and strain. One then could measure both the Brillouin frequency
and the
intensity of the Brillouin spectrum. Alternatively, one can use special
fibers, such as
photonic crystal fiber (PCF), or large effective area fiber (LEAF) as the
sensing media.
Figure 2 shows simultaneous measurement of strain and temperature using PCF
and LEAF.
Figure 3 shows the effect of temperature with such measurements, where it is
seen
that the central frequencies of the peaks at a and c increased linearly with
temperature.
The temperature coefficients are 0.96 for peak a and 1.23 MHz/ C for peak c at
1320 nm.
The pulse width was 1.5 ns ¨15 cm spatial resolution.
Figure 4 shows the effect of strain with such measurements, where it is seen
that the
Brillouin frequencies of peaks a and c have a linear dependence on the strain.
The strain
coefficients are 4.78 x 10-2 for peak a and 5.5 x 10-2 MHz/pE for peak c at
1320 nm. The
pulse width was 1.5 ns ¨15 cm spatial resolution.
There are disadvantages to using PCF or LEAF for simultaneous measurement of
strain and temperature. In real-life applications, peak c is easily covered by
the noise
resulting in a low signal to noise ratio. The intensity of the peak may vary
greatly because
of tension or compression in the fiber. In order to increase the spatial
resolution, an
increased baseline for the input pulses may be required, resulting in a
complication of the
Brillouin spectrum, and increased difficulties in identifying peak c.
There is therefore a need to devise a method and a system for the simultaneous
measurement of strain and temperature in an optical fiber, and which does not
suffer from
the drawbacks associated with present methods and systems.
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SUMMARY OF THE INVENTION
The present provides a method and a system which meets the above requirements.
The present invention utilizes a pair of fibers connected or installed
together, with one of
the fibers having a refractive index that differs from that of the other
fiber. For example, a
first fiber uses pure silica as the cladding and pure silica doped with Ge as
the core, and the
second fiber uses pure silica doped with F as the cladding and pure silica as
the core.
Another example has a first fiber using pure silica as the cladding and pure
silica doped with
Ge as the core, and a second fiber using pure silica as the cladding and pure
silica doped
with a different dose of Ge as the core, such as SMF-28 and LEAF. Preferred
fibers for this
invention are single mode fibers (SMF), because they are cheaper and more
conventional.
The first and second fibers can be connected to a splitter at each end
thereof, or they
can be spliced together at one end only. In the first case, the splitters
would be used as
input/output or inputs of probe and pump lasers. In the second case the non-
spliced ends
of the fibers would be used for input/output of a single laser or as inputs of
probe and pump
lasers.
Broadly speaking, therefore, the present invention can be considered as
providing a
method of simultaneously determining strain and temperature characteristics of
an object
comprising the steps of: providing first and second optical fibers having
different refractive
indices; determining coefficients of strain and temperature for each of the
fibers;
connecting the fibers together at at least one end thereof; securing the
fibers to the object
along a length thereof; inputting laser light into at least one of the fibers
at the other ends
thereof; measuring the Brillouin frequency for each of the fibers; and
calculating strain and
temperature characteristics based on the coefficients of strain and
temperature and the
measured Brillouin frequencies for the fibers.
The present invention also contemplates a system for simultaneously
determining
strain and temperature characteristics of an object comprising: first and
second optical
fibers having different refractive indices; means connecting the first and
second fibers
together at at least one end thereof; means securing the fibers to the object
to be
monitored; laser means for inputting laser light into at least one of the
fibers at the other
ends thereof; means for measuring the Brillouin frequency for each of the
fibers; and
means for calculating strain and temperature characteristics based on the
coefficients of
strain and temperature as well as the measured Brillouin frequencies for the
fibers.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph showing the Brillouin frequency spectrum of an optical
fiber as
well as the peak power thereof.
,
Figure 2 is a graph showing Brillouin loss as a function of the Brillouin
frequency shift
for PC and LEA fibers.
Figure 3 is a graph showing the effect of temperature on Brillouin
measurements.
Figure 4 is a graph showing the effect of strain on Brillouin measurements.
Figure 5 shows a first arrangement of optical fibers in accordance with the
present
invention.
Figures 6a and 6b show alternative arrangements of optical fibers in
accordance with
the present invention.
Figures 7A and 7B are graphs showing strain coefficients for optical fibers
having
different refractive indices in an arrangement of the present invention.
Figure 8 shows a pair of optical fibers in accordance with the present
invention,
spliced together at one end and installed on a section of a steel pipeline.
Figures 9A and 9B are graphs showing vB for the two optical fibers used in the
example of Figure 8.
DETAILED DESCRIPTION OF THE INVENTION
The present invention utilizes a pair of fibers connected or installed
together, with
one of the fibers having a refractive index that differs from that of the
other fiber. For
example, a first fiber uses pure silica as the cladding and pure silica doped
with Ge as the
core, and the second fiber uses pure silica doped with F as the cladding and
pure silica as
the core. Another example has a first fiber using pure silica as the cladding
and pure silica
doped with Ge as the core, and a second fiber using pure silica as the
cladding and pure
silica doped with a different dose of Ge as the core, such as SMF-28 and LEAF.
Preferred
fibers for this invention are single mode fibers (SMF), because they are
cheaper and more
conventional. The fibers are connected together at at least one end thereof
and laser light
will be pumped into at least one of the fibers, with suitable means being
provided for
measuring the Brillouin frequencies of the respective fibers.
Figure 5 shows a first example 10 of first 12 and second 14 single mode fibers
connected to a splitter 16 at each end, with the splitters being used as
input/output or
inputs of probe and pump lasers 18, 20.
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Figures 6a and 6b show alternative arrangements 22, 24 of first 26 and second
28
single mode fibers spliced together at one end 30, with the other ends 32
being used for
input/output of a single laser 34 or inputs of probe and pump lasers 36, 38.
In each of these examples the first and second single mode fibers have
different
refractive indices.
Each of the two fibers will provide one peak of the Brillouin spectrum but the
two
Brillouin spectra will have different Brillouin frequencies. The two peaks
coming from the
two fibers will have different strain coefficients CE and temperature
coefficients Gr. These
two peaks are associated with a single set of local strain and temperature
information.
The following set of equations can be used to solve for both the strain and
temperature as detected in the pair of fibers:
pk
[Av C 1 cipk 1 Ac
13 = E
(3)
A vBpk 2 cpk 2 cfk 2 AT
rid k () = v pBk12
()
where A v 12
(c, T) - VPBko 1 (2) (0 , To), Ac = c - co , AT = T -To , eo and To are the
strain and temperature corresponding to a reference Brillouin frequency
vPBkol(2)(c0,T0). If
the strain coefficients CePk I and CePk2 and temperature coefficients Crl and
Cr2 for peaks
1 and 2, respectively, satisfy
cpk 1 cpk 1
(4)
cpk 2 pk 2
ci
the change in temperature AT can be given by
Avpk2 .cpkl A vpBk1 .cpk2
AT= B
(5)
cpk 1 c -)k 2 cpk 2 c '
ipk 1
l.
and the change in fiber strain can also be obtained by
A vpk 1 epk2 A vpk 2 ,---q)k 1
Ac = B B I
(6)
cpk 1 c.f k 2 cpk 2 c Ipk 1 =
CA 02629446 2014-08-21
A practical example of the present invention would involve monitoring a steel
pipeline to ascertain strain and temperature characteristics thereof in order
to predict
whether the pipeline would be susceptible to buckling. Two different kinds of
single mode
fiber are utilized, one being SMF-28, and the other being a single mode fiber
with a different
doping dose of Ge. There are different central Brillouin frequencies at room
temperature,
namely 12796 MHz for SMF-28 and 12479 MHz for the other fiber, as well as
different strain
and temperature coefficients (see Figures 7A and 7B). The two fibers 40, 42
are spliced
together at one end 44 and then installed on a steel pipeline 46 (Figure 8).
When laser beams are directed into the fibers there will be two Brillouin
spectra
corresponding to the two fibers appearing at the same real location, but in
the time domain
they will appear at different times because the fibers were spliced together
at one end.
Figures 9A and 9B show that VB for the SMF-28 fiber is 12980 MHz at 354.5 ns,
whereas vB
for the other fiber is 12935 at 412 ns. This data, when utilized in the
previous equations
will determine the strain and temperature characteristics of the pipeline at a
single point in
time, to help determine whether the operating conditions of the pipeline are
well within
standard acceptable conditions.
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