Measurement Science and Technology
PAPER
A distributed optical fibre dynamic strain sensor
based on phase-OTDR
To cite this article: A Masoudi et al 2013 Meas. Sci. Technol. 24 085204
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IOP PUBLISHING MEASUREMENT SCIENCE AND TECHNOLOGY
Meas. Sci. Technol. 24 (2013) 085204 (7pp) doi:10.1088/0957-0233/24/8/085204
A distributed optical fibre dynamic strain
sensor based on phase-OTDR
A Masoudi, M Belal and T P Newson
Optoelectronics Research Centre (ORC), University of Southampton, Southampton, SO17 1BJ, UK
E-mail: am10g09@orc.soton.ac.uk
Received 9 April 2013, in final form 12 May 2013
Published 5 July 2013
Online at stacks.iop.org/MST/24/085204
Abstract
A distributed optical fibre sensor is introduced which is capable of quantifying multiple
dynamic strain perturbations along 1 km of a sensing fibre simultaneously using a standard
telecommunication single-mode optical fibre. The technique is based on measuring the phase
between the Rayleigh scattered light from two sections of the fibre which define the gauge
length. The phase is spatially determined along the entire length of the fibre with a single
pulse. This allows multiple moving strain perturbation to be tracked and quantified along the
entire length of the fibre. The demonstrated setup has a spatial resolution of 2 m with a
frequency range of 500–5000 Hz. The minimum detectable strain perturbation of the sensor
was measured to be 80 n.
Keywords: optical fibre sensors, Rayleigh scattering, dynamic strain sensor, distributed sensor
(Some figures may appear in colour only in the online journal)
1. Introduction
Distributed temperature and strain sensors have proven to be
an efficient means for interrogating a large number of points
along a single optical fibre, a capability which has attracted
substantial interest from industries involved in oil, gas and
structural health monitoring. The majority of such distributed
optical fibre sensors have utilized either Raman scattering
[1, 2] or Brillouin scattering [3–6] and have operated at low
bandwidths.
Dynamic strain sensing has most successfully been
explored using coherent Rayleigh scattering [7] and a number
of commercial devices are now available that are capable
of detecting strain perturbations. These have applications in
areas such as perimeter monitoring by identifying disturbance
caused by intruders as well as leak detection in oil and gas
pipelines. The induced strain causes a change in the coherent
Rayleigh backscattered trace and the change is detected
and spatially located. Whilst multiple disturbances can be
detected simultaneously allowing a moving disturbance to be
tracked, such systems to date do not provide a measure of
the magnitude of strain-induced perturbation. An optical time
domain reflectometry (OTDR) system based on the phase of
the coherent Rayleigh noise (CRN), first proposed by Posey
et al [8], provided a means to quantify the strain, but in
the presented form was only able to monitor the dynamic
strain at a single section of the fibre at any one time. This
paper builds on this work, utilizing the same proposed phase
demodulation scheme but capturing the entire backscattered
trace and hence the true distributed measurement of dynamic
strain. It is this fact which makes it possible to quantify and
track multiple moving objects along the fibre. The underlying
principles of the proposed scheme are detailed in section 2,
followed by a description of the experimental layout in
section 3. Experimental results including the 3D plot of the
sensing fibre under strain are presented in section 4, while
section 5 discusses and analysis the experimental results.
Finally, section 6 highlights the outcome of the study.
2. Principles
The basic principle governing the operation of the dynamic
strain sensor is based on detecting the phase change induced
between the coherent Rayleigh scattering from two points of
the sensing fibre. Like any OTDR system, a short pulse of
light is launched into the fibre and the backscattered coherent
Rayleigh light is fed through an imbalanced Mach–Zehnder
fibre interferometer and onto a detector. The signal is therefore
the summation of light emanating from two regions of fibre
0957-0233/13/085204+07$33.00 1 © 2013 IOP Publishing Ltd Printed in the UK & the USA
Meas. Sci. Technol. 24 (2013) 085204 A Masoudi et al
Figure 1. Principle of dynamic strain sensing using ϕ-OTDR. This
figure shows the graphical representation of two groups of scatterers
in a section of the sensing fibre (a) before external perturbation and
(b) after external perturbation.
and its magnitude is dependent on their relative phases. The
change in phase is directly related to the change in strain
between the two points. Therefore, the gauge length is defined
by L
delay
/2, where L
delay
is the length of the delay fibre of
the Mach–Zehnder interferometer (MZI). This assumes that
the pulse width is shorter compared to the length of the delay
fibre.
As the pulse of light propagates along the sensing fibre,
the detector output tracks the phase between the two points
separated by the gauge length. To avoid signal fading, the
differential and cross-multiply phase demodulation system is
used in which a three port MZI and three detectors are used.
Three channels of information are stored for each pulse. Post-
processing allows the changing phase and hence dynamic
strain at any particular position to be determined. Figure 1
illustrates this concept. In this figure, two groups A and B of
scattering centres with separation of L metres are shown. To
compare the phase difference between the backscattered light
from the two sections, an MZI with a path imbalance of twice
this length 2L is used. Neglecting fibre loss, the backscattered
electric field from A and B at the photodetector is given by
E
A
= E
0
exp[i(ωt + φ
A
)]
E
B
= E
0
exp[i(ωt + φ
B
)], (1)
where φ
A
and φ
B
are the phases of the backscattered light
from A and B, respectively, ω is the angular frequency of the
backscattered light and E
0
is the magnitude of the electric
field of the backscattered light. The intensity of the combined
backscattered light is given by
I
det
= (E
A
+ E
B
)(E
A
+ E
B
)
= 2E
2
0
cos(φ
A
− φ
B
). (2)
Figure 2. Schematic of the setup used to eliminate signal fading in
interferometer by using a symmetric 3 × 3 coupler at the output of
the interferometer.
Equation (2) shows that the detected intensity depends on
the cosine of the phase difference between the two groups of
scatterers. Any external disturbances (e.g. acoustic vibration)
within the gauge length changes the phase of the backscattered
light. This phase change φ can be detected by monitoring
the backscattered signal after i nteraction. The advantage of
this technique which is referred to as phase-OTDR (ϕ-OTDR)
is its capability of not only detecting the perturbation, but also
of quantifying its magnitude and frequency.
To avoid the fading problem, an MZI with a three output
port coupler was used (figure 2). The output intensity of the
threearmsofthe3× 3 coupler can be written as [9]
I
1
= I
0
[M + N cos(ϕ)]
I
2
= I
0
M + N cos
ϕ +
2π
3
I
3
= I
0
M + N cos
ϕ −
2π
3
, (3)
where M and N are constant, ϕ is the phase difference of the
light returning through the two arms of the MZI and I
0
is the
intensity of the input signal.
The phase detection scheme known as the differentiate and
cross-multiply demodulation scheme [10] was used which is
depicted in figure 3. By using the three outputs of the 3 × 3
coupler, the phase difference between the two arms of the MZI
(i.e. ϕ in equation (3)) can be measured directly. The output
voltage V
Ph
is directly proportional to the phase:
V
Ph
=
√
3ϕ. (4)
3. Experimental arrangement
3.1. Experimental setup
The experimental setup is shown in figure 4.
10 ns optical pulses with a peak power of 10 mW at a
repetition rate of 10 μs were generated by directly modulating
the injection current of a DFB laser diode operating at 1550 nm
biased just below the threshold. 90% of the output pulses were
fed into a 28 dB gain erbium-doped fibre amplifier (EDFA1).
An optical isolator after the laser diode was used to prevent any
backward ASE (amplified spontaneous emission) generated in
the EDFA from causing instabilities or damage to the laser
diode. 10% of the optical output from the laser diode was used
to trigger the oscilloscope.
2
Meas. Sci. Technol. 24 (2013) 085204 A Masoudi et al
Figure 3. Block diagram of the phase detector used, adopted from [9].
Figure 4. Experimental arrangement. EDFA: erbium-doped fibre amplifier, PD: photodetector, FBG: fibre Bragg grating, PC: polarization
controller, C: circulator, DFB: distributed feedback.
ASE from EDFA1 was filtered out (3 dB bandwidth
of 7 nm), and the optical pulses (∼5 W peak power) were
launched into the sensing fibre.
The sensing fibre included two regions of lengths 3.5m
and 7 m, respectively, subjected to dynamic strain using a
disc PZT arrangement and a cylindrical PZT arrangement,
respectively. 320 m of unstrained–unheated fibre was used
to separate the two strained regions and further 400 m and
280 m lengths of unstrained–unheated fibre were added before
and after the 7 m and the 3.5 m strained sections of fibre,
respectively.
The backscattered light from the sensing fibre was fed via
a circulator into a 20 dB gain erbium-doped fibre amplifier,
EDFA2. ASE from the amplified backscattered signal was
removed by reflecting the amplified signal off a fibre Bragg
grating (FBG), before being fed into the imbalanced MZI via a
3
Meas. Sci. Technol. 24 (2013) 085204 A Masoudi et al
(a)
(b)
Figure 5. Rearrangement of a two-dimensional train of traces,
(a) i nto a three-dimensional diagram, (b) depicting the sequence of
traces.
3 dB coupler. The output of the 3×3 coupler was connected to
three similar detectors (90 dB gain, 125 MHz bandwidth) and
fed into an oscilloscope with a sampling rate of 300 MSa s
−1
.
The acquired signals on each detector consisted of a
train of backscattered traces. The length of each trace was
proportional to the length of the sensing fibre, and the repetition
rate was equal to the repetition rate of the pulse generator
(figure 5(a)). The sequence of traces was redrawn to provide
a 3D plot of the backscattered traces and allows the output at
a particular position to be determined as a function of time
(figure 5(b)).
The differentiate and cross-multiply demodulation
scheme was implemented digitally along the lines of the
algorithm as shown in figure 3 for one particular position along
the fibre. A fast Fourier transform was performed to identify
the frequency components of any phase perturbation occurring
within the gauge length of this point. This process was then
repeated for each point along the sensing fibre and drawn as a
3D plot showing the frequency spectrum of the perturbations
along the sensing fibre.
3.2. Experimental procedure
Both the disc and the ring PZTs were initially characterized
using an MZI to determine the voltage–strain relationship of
them at different frequencies before being included in the
experimental setup. The applied voltages to the PZTs were
initially adjusted to stretch the fibre by approximately 750 nm
or π rad corresponding to 350 n strain. Data were acquired
for a range of frequencies with the repetition rate of 10 μs
for 12 ms. To investigate the linearity of the sensor, the input
frequency of the disc PZT was set to a fixed value and different
voltages were applied to strain the fibre over the range of
100 n–1.5 μ.
4. Results
Figure 6(a) depicts the FFT of the processed signal in a 3D
diagram where the axes show the distance and the frequency
of perturbation and its magnitude. The two peaks in the figure
correspond to the two strained regions in the sensing fibre, i.e.
one at a distance of 400 m and the other at a distance of 720 m
from the front end of the fibre under test (FUT). Figure 6(b)
shows the output of the phase detector for the 3.5 m section of
the sensing fibre, which is located at a distance of 720 m from
the front end of the sensing fibre.
A 2D view of these 3D plots is depicted in figure 7.
Figure 7(a) shows the frequency components corresponding
to the applied strain at 400 m (red trace) and 720 m (blue
trace), respectively, from the front end. Figure 7(b)showsthe
phase-detector output at the same points.
Figure 8 shows the response of the sensor when the disc
PZT is subjected to a sinusoidal disturbance at 900 Hz sinwave
of varying amplitude. A correlation coefficient of 0.9979 was
determined.
The frequency response of the disc PZT for the 0.7V
PP
input voltage is plotted in figure 9. In this figure, the distributed
sensor output is shown in the blue trace, while the red trace
shows the disc PZT frequency response characterized by using
an MZI.
5. Discussion
5.1. Interpretation of 3D figures
The two peaks of figure 6(a) correspond to the strains applied
by the two PZTs to the FUT. The 2D plot shown in figure 7(a)
confirms that the peaks on the 3D diagram are accurately
indicating the frequency and the amplitude of the strains
applied to the FUT. In addition, the 3D diagram of figure 6(b)
shows that the output of the phase detector follows the
sinusoidally applied strain to the fibre. The 2D cross sections
of the phase-detector output at 400 and 720 m are shown in
figure 7(b), which illustrates the behaviour of fibre at those
points as a function of time.
5.2. Strain range and strain resolution of the sensor
Figure 8 shows a linear relationship between the amplitude of
the applied strain on the FUT and the sensor output at 900 Hz.
Similar experiments at other frequencies (i.e. 1400, 1900 and
2300 Hz) verified that this linear relationship is independent
of the frequency. The R
2
value of 0.9979 for the fitted line
confirms a high level of linearity. The minimum detectable
strain was measured to be 80 nε with a signal-to-noise ratio
of 1. The strain resolution was measured to be 20 nε.
The limiting factors for the minimum detectable strain
and the strain resolutions are the digitization level of the
oscilloscope and noises in the setup such as ASE noise,
detector noise and CRN. The CRN noise depends on the
linewidth of the DFB laser source and the pulse width. A
broad band laser source or a shorter pulse width can be used
to reduce the CRN. In addition, ASE noise can be improved
by using a narrower FBG filter.
The maximum detectable strain depends on both the
length of the FUT and the frequency of the strain. The length
of the FUT determines the repetition rate of the interrogating
pulse which itself determines the sampling rate for each point
along the FUT. In addition, since the magnitude of the strain
perturbation governs the phase change and hence the number
of fringes, perturbation with higher magnitudes behaves like a
4
