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Distributed Fiber-Optic Sensor for Detection and Localization of Acoustic Vibrations

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A sensing system utilizing a standard optical fiber as a distributed sensor for the detection and localization of mechanical vibrations is presented. Vibrations can be caused by various external factors, like moving people, cars, trains, and other objects producing mechanical vibrations that are sensed by a fiber. In our laboratory we have designed a sensing system based on the Φ-OTDR (phase sensitive Optical Time Domain Reflectometry) using an extremely narrow laser and EDFAs.
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Metrol. Meas. Syst., Vol. XXII (2015), No. 1, pp. 111–118.
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Article history: received on Jul. 13, 2014; accepted on Nov. 13, 2014; available online on Mar. 15, 2015; DOI: 10.1515/mms-2015-0009.
METROLOGY AND MEASUREMENT SYSTEMS
Index 330930, ISSN 0860-8229
www.metrology.pg.gda.pl
DISTRIBUTED FIBER-OPTIC SENSOR FOR DETECTION AND LOCALIZATION
OF ACOUSTIC VIBRATIONS
Radim Sifta, Petr Munster, Petr Sysel, Tomas Horvath, Vit Novotny, Ondrej Krajsa
,
Miloslav Filka
Brno University of Technology, Faculty of Electrical Engineering and Communication, Department of Telecommunications, Technic
ka 12,
616 00, Brno, Czech Republic ( sifta@feec.vutbr.cz, +420 54114 6923)
Abstract
A sensing system utilizing a standard optical fiber as a distributed sensor for the detection and
localization of
mechanical vibrations is presented. Vibrations can be caused by various external factors,
like moving people,
cars, trains, and other objects producing mechanic
al vibrations that are sensed by a fiber. In our laboratory we
have designed a sensing system based on the Φ-OTDR (phase sensitive
Optical Time Domain Reflectometry)
using an extremely narrow laser and EDFAs.
Keywords: optical fiber sensor, acoustic vibrations, detection, localization, Φ-OTDR.
© 2015
Polish Academy of Sciences. All rights reserved
1. Introduction
Optical fibers have been used in a wide range of applications during several last decades.
The most visible and known utilization is in the area of telecommunications as the optical
fiber technologies can offer bit rates in the range of terabits per second [1]. Fibers have been
deployed not only in the area of optical networks but they are also more and more common in
the fiber sensor technologies.
Sensor applications are an attractive area of the optical fiber usage. The fiber construction,
the principle of operation (the total reflection) and the signal form (radiation) make the
transmission of data very safe and resistant to many sources of disturbances. At the same time
transmitted signals are sensitive to ambient conditions, like the temperature, strain, vibrations
or strong ambient electromagnetic field, which make them suitable for sensing purposes.
Fiber sensors can be divided into two groups - extrinsic (hybrid) fiber optic sensors and
intrinsic fiber optic sensors [2].
1.1 Distributed fiber-optic sensors
Distributed fiber sensors are a very important part of the fiber optic sensor area as a fiber
behaves like hundreds or thousands of sensors spread along it [2]. To enable this, the fiber
response to the test signal has to provide an information not only about the value of
the measured quantity but also about the created response location. The most common
solutions of these systems are based on the reflectometry principle [3].
Using the distributed fiber sensors it is possible to measure such quantities, like the fiber
loss pattern, temperature, pressure or vibrations along the distance from several meters up to
tens of kilometers, depending on a particular solution. The spatial resolution that can be
obtained spans from micrometers for short sensor lengths (equal to single meters) to tens
kilometers for long range sensors [4].
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The localization of a physical quantity occurrence is based on the transmission of short-
time and high-power pulses that travel along the fiber while a portion of the radiation (light) is
scattered due to elastic or inelastic effects that are influenced by the physical quantity
(temperature, pressure, radiation, strain, etc.), which can be measured by processing the
scattered signal [5]. A portion of this scattered signal is re-captured by the fiber and is being
spread farther. Capturing and processing the back-scattered signal is the most common
technique. The best known elastic effect is the Rayleigh scattering; the Brillouin and the
Raman scatterings belong to the inelastic ones [3].
An optical time domain reflectometer (OTDR) was demonstrated over three decades ago
[6], [7]. Now it is widely used for localizing fiber breaks and other types of anomalies
occurring in the optical fiber. The system detects the presence and location of perturbations,
which were affected by the intensity of the radiation (light) returned from the fiber, but do not
respond to phase changes of the radiation (light) [8].
The distributed sensor system presented in this paper utilizes a Φ-OTDR designed to
enhance coherent effects rather than avoid them [3]. The core of this system is a floating
interferometer – a phase sensitive device measuring the interference of the radiation (light)
back-scattered from different parts of the optical fiber, which arrives at the same time to the
photo-detector [8]. That is the reason why the Φ-OTDR can detect much smaller perturbations
than a conventional OTDR.
Many distributed sensors based on the phase-sensitivity have been proposed. Nowadays it
is the most advanced principle which can be used for monitoring of more than one vibration
source along the optical route [10]. For example, in [3] a system based on the phase-
sensitivity for 19 km long perimeters was described. The system used a real 8.5 km fiber
cable and a 10.5 km fiber on coils. The real fiber cable was buried in the ground at the 20-46
cm depth. In [4] a distributed vibration sensor based on the coherent detection of Φ-OTDR
was proposed. This sensor achieved the spatial resolution of 5 m and the maximum frequency
response of 1 kHz for a 2 km long optical route. In [8] a system for 128 km with the spatial
resolution of 15 m, using the Raman amplifying, was proposed.
2. System description and experimental measurement
We have designed the Φ-OTDR according to the scheme shown in Fig. 1. The key element
of this scheme is a highly coherent and frequency stable DFB (Distributed Feedback) laser
source Koheras Adjustik with an ultra-narrow spectral line width of about 100 Hz.
AOM
Laser
Adjustic
2000 m 2000 m
10 m
G
2
G
1
DFB
Laser
AOM
driver
f
BW
Fig. 1. A scheme of the tested topology.
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The second most important element is an acoustic-optic modulator (AOM) with a high
extension ratio > 50 dB. This modulator must be driven by a special driver, which needs a
precise source of the DC voltage and a generator of the RF (Radio Frequency) signal. We
have used two EDFA (Erbium Doped Fiber Amplifier) modules. The first one is used as a
booster in a direct branch for amplifying the pulsed signal and the second one is used as a pre-
amplifier for amplifying the back-scattered signal. We have also used three couplers. The first
coupler, which is situated behind the second auxiliary DFB laser, is used for separating the
optical signal from this laser. The couplers in front of the amplifiers are used for merging the
useful signal with the signal from the second auxiliary laser. The variable attenuator is used
for optional power level setting of the second laser.
The CW (Continual Wave) signal from the Koheras Adjustik laser (with the central
wavelength of 1540 nm) goes through an optical isolator into the acoustic-optic modulator,
where pulses are created using a special driver and a signal generator. It is very important to
use an optical isolator behind the laser to prevent the laser from damage by back-scattered
optical signals. The pulse width was set to 500 ns with the frequency of 17 kHz. This
frequency is given by the equation (1) and is adequate to the fiber length of 4 km. The pulsed
optical signal is then amplified by an EDFA and goes through an optical band-pass filter and a
circulator into the FUT (Fiber Under Test). We have used a common telecommunication
optical fiber G.652.D.
],[
1Hzf
τ
(1)
where [3]:
].[
2s
v
s
=τ (2)
The total fiber length is s and v is the velocity of light in an optical fiber, which is given by
the equation (3):
],/[102 8
sm
n
c
v== (3)
where c is the velocity of light in the vacuum and n is the refractive index.
The spatial resolution for the pulse width of Tp = 500 ns is given by the equation (4):
m
Tv
zp50
2
105102
2
78
=
=
=
. (4)
One sample is adequate to the fiber length:
,67,6
1030
102
6
8
m
F
v
l
s
=
== (5)
where l is the adequate fiber length and Fs is the sampling frequency in Hz.
Since it is not possible to directly amplify a pulsed optical signal with a pulse repetition of
17 kHz we have proposed and designed a system with a second DFB-CW laser to suppress
this problem. The principle is that the CW laser (a standard laser with the central wavelength
of 1550 nm) with a different wavelength is merged with the pulsed signal by a coupler. The
EDFA is then optically suspended on the signal from the CW laser and power fluctuations are
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R. Sifta, P. Munster, P. Sysel, T. Horvath, V. Novotny, O. Krajsa: DISTRIBUTED FIBER SENSOR FOR DETECTION …
minimal. The power of the second laser must be a little bit higher than the power of the pulsed
signal.
Acoustic vibrations were created by a speaker and a generator in the middle of the 4 km
optical fiber cable. The back-scattered signal is distributed by the Rayleigh scattering from
each point of the fiber. This signal passes through an optical circulator to a coupler where it is
merged with the optical signal from the second CW laser. The signal is then amplified,
filtered by a band-pass filter and converted to the electrical signal by a balanced photo-
detector. Both filters are used for filtering-out the optical signal from the second auxiliary
laser.
3. Measurement results
It is very difficult to set power levels of both lasers and gains of amplifiers. If the gain of
the first amplifier is too high, there is a risk of the Brillouin scattering generation. That is the
reason why the power levels in each part of the fiber have to be balanced and optical filters
should be used for adverse signal filtering.
After the opto-electrical conversion the signal is displayed on an oscilloscope and recorded
by a two-channel digital acquisition card (DAQ) with the sampling rate of 30 MS/s. We tested
our proposed scheme for different pulse widths, frequencies and fiber lengths. The best results
were achieved for the pulse width of 500 ns and the total fiber length of 4 km. With this set up
it is possible to see the vibrations with a naked eye on the oscilloscope. After DAQ recording
and post processing we achieved the location of vibrations with the spatial resolution of 50 m.
The most important part of this system is signal processing.
3.1 Average fiber response
At first the individual fiber responses hm[n] to the measured pulse were recorded. The
responses do not have a monotonically decreasing character, but due to reflections and
scatterings on the fiber non-homogeneities the maxima or minima appear at different
distances from the near end of the fiber. If we calculate the average of all responses, we can
get a typical fiber response to the measured pulse.
].[
1
][
1
0
nh
M
nh
M
m
m
=
= (6)
The average response for the total number of M = 40,000 responses is shown in Fig. 2.
Fig. 2. The average long-time fiber responses to the measured pulses.
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3.2 Detection and localization of vibrations
If the individual responses are synchronized with the measured pulse we can get a
waveform which is shown in Fig. 3. The positions of maxima approximately correspond to
the positions of the peaks in the fiber response average (see Fig. 2). Approximately at the half
of the fiber length, i.e. at a distance of 2,000 m from the near end of the fiber there is an
evident maximum but with a periodically varying value. This fast changing maximum
corresponds to the vibration on the fiber.
We can remove maxima with the constant value and position. For removing the offset the
1st order high-pass filter is used. The transfer function H(z) is given by the equation (7):
,
99
.
0
995.0995.01
2
1
)(
=
+
=
z
z
a
z
za
zH (7)
where a is the coefficient of the transfer function and z is the operator of the transfer function.
Fig. 3. The waveform of individual responses.
The filtration is carried out along the vertical axis, so there is actually a subsampling of the
fiber response. The input sample is always got from the same position of the next fiber
response. In the original sampling frequency of 30 MHz and the length of one response equal
to 1,305 samples, the sampling rate drops to 22.989 kHz. The cutoff frequency of a high-pass
filter with the transfer function (6) is then about 30 Hz. The filtered responses ][
ˆ
nhmare
shown in Fig. 4.
After offset filtering the vibrations at the distance of 2,000 m are clearly visible.
Nevertheless, the waveforms are still interfered by the wideband noise. To remove this
component a low-pass filter was used. The cutoff frequency was set to 200 Hz. The filter was
realized as a floating average of 50 responses:
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R. Sifta, P. Munster, P. Sysel, T. Horvath, V. Novotny, O. Krajsa: DISTRIBUTED FIBER SENSOR FOR DETECTION …
Fig. 4. The waveform of individual responses without the offset.
Fig. 5. The waveform of individual responses without the offset and after low-pass filtering.
=
=
49
0
].[
ˆ
50
1
][
k
kmm nhng (8)
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The floating average was applied along the vertical axis like in the case of a high-pass
filter. The final responses are shown in Fig. 5.
The final step before the automatic detection was thresholding:
.)],[max(][
ˆθθ = ngng mm (9)
The threshold value θ was suitably selected, so that for the measurement on the fiber
without vibrations all values gm[n] were thresholded. The results after thresholding are shown
in Fig. 6. The only non-zero samples are at the distance of 2,000 m from the near end of the
fiber, where the vibrations really occurred. It is possible to convert the detection and
localization into searching the non-zero values.
Fig. 6. The waveform of individual responses after filtering and thresholding.
4. Conclusion
Nowadays optical fibers are increasingly used in the area of sensors. Using the distributed
fiber sensors it is possible to measure such quantities, like the fiber loss pattern, temperature,
pressure or vibrations along the distance from several meters up to tens of kilometers,
depending on a particular solution.
In this paper we have proposed a distributed fiber sensor system based on the Φ-OTDR.
This system was tested on the fiber length of up to 4 km with the spatial resolution of 50 m.
The vibrations were created by a speaker for different frequencies from 100 Hz up to 1 kHz
with a successful sensitivity. A practical usage for the buried optical cable is the aim of the
further research. The system should be used for the detection, localization and classification
of acoustic vibrations caused, for example, by moving people, cars, trains and other objects
producing mechanical vibrations that are sensed by a fiber. The system could be used also for
monitoring pipeline damages, seismically active areas, etc.
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The recorded data were post-processed in the Matlab environment. It is necessary to filter
all undesirable components of the signal and noise to get a clearly visible location of occurred
vibrations. For this, high-pass and low-pass filters, as well as thresholding, were applied.
Many systems were proposed for monitoring acoustic vibrations, based on Rayleigh’s
scattering. A general problem of these systems is the amplification of a pulsed signal with
respect to the OSNR (Optical Signal to Noise Ratio). Our proposed system uses EDFAs
with the second auxiliary laser and optical filters to solve this problem.
The system described in this paper has a significant potential and will be gradually
improved in the future.
Acknowledgements
This paper was supported by the projects of the Ministry of Industry and Trade of the
Czech Republic: MPO FR-TI4/696 and SIX CZ.1.05/2.1.00/03.0072.
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... Vibration signal detection is critical for fault diagnosis of electromechanical devices, such as railway roller bearings [1] and internal combustion engines [2]. Owing to the ability of distributed sensing and transmission [3], anti-electromagnetic interference [4], and convenient installation, the optical fiber vibration sensor has grown significantly and has great potential in the field of fault diagnosis [5]. In practical application, because the position information and time-frequency characteristics of vibration signal need to be obtained simultaneously, the optical fiber vibration sensor based on coherent optical time domain reflectometry (COTDR) becomes an important choice, which is suitable for long-distance vibration positioning [6] and vibration phase recovery [7]. ...
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... However, the phase information of the return light is not available in the standard OTDR. In general, FOS technology for distributed parameter detection spans from interferometry-based systems to Rayleigh and/or Brillouin scattering-based phase-OTDR and polarization-OTDR systems, using either coherent or direct detection methods [9][10][11]. ...
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Since the technology has moved strongly into a number of different areas a textbook of this sort could be used by a wide variety of academic departments including physics, electrical engineering, mechanical engineering, civil engineering, aerospace engineering and bioengineering. To make the second edition as widely appealing as possible a series of significant upgrades were made. 1. The book is structured to support a variety of academic programs and it can also be used as a general reference by practicing engineers and scientists. 2. The introductory chapter has been revised to outline the new content of the second edition and provide a overview of the current status of fiber optic sensor technology. 3. A new, extensive chapter has been added covering fiber optic grating sensor technology and its application to aerospace, civil structures, oil and gas and power generating applications. 4. A second new chapter has been added on the emerging field of biomedical fiber optic sensors. This is one of the most rapidly growing fields of use for fiber optic sensors and with rising health costs and medical advances promises to be an important area for many years to come.
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