Applied Physics Express 11, 072501 (2018)https://doi.org/10.7567/APEX.11.072501

Distributed strain measurement and possible breakage detectionof optical-ﬁber-embedded composite structure using slope-assisted
Brillouin optical correlation-domain reﬂectometry
Heeyoung Lee1*, Yutaka Ochi2, Takahiro Matsui3, Yukihiro Matsumoto4, Yosuke Tanaka5,
Hitoshi Nakamura6, Yosuke Mizuno1, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Advanced Composites Center, Toray Industries, Inc., Nagoya 455-0024, Japan
3
First Advanced Composites Technical Department, Toray Industries, Inc., Nagoya 455-0024, Japan
4
Department of Architecture and Civil Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan
5
Institute of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
6
Department of Civil and Environmental Engineering, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan2*E-mail: hylee@sonic.pi.titech.ac.jp
Received April 18, 2018; accepted May 23, 2018; published online June 7, 2018
Slope-assisted Brillouin optical correlation-domain reﬂectometry (SA-BOCDR) is a recently developed structural health monitoring technique formeasurements of strain, temperature, and loss distributions along optical ﬁbers. Although the basic operational principle of this method has beenclariﬁed, no measurements using optical ﬁbers embedded in actual structures have been reported. As a ﬁrst step towards such practical applications, in this study, we present an example of an SA-BOCDR-based diagnosis using a composite structure with carbon ﬁber-reinforced plastics.
The system’s output agrees well with the actual strain distributions. We were also able to detect the breakage of the embedded ﬁber, thus
demonstrating the promise of SA-BOCDR for practical applications. © 2018 The Japan Society of Applied Physics

ecently, the demand for structural health monitoringof deteriorated or damaged civil infrastructures has
been rapidly growing. Many types of sensing techniques have been suggested to improve human safety andmaintainability of infrastructures. As one of the methods toachieve this goal, Brillouin-scattering-based optical ﬁber
sensing techniques1–15) have attracted a signiﬁcant attention
owing to their ability to measure strain and temperature
distributions. Among various Brillouin sensors,1–6) Brillouin
optical correlation-domain reﬂectometry (BOCDR)5) is the
only technique that can simultaneously achieve single-end
access, random access, high spatial resolution, and costeﬃciency. However, one of the major drawbacks of BOCDR
has been its relatively large measurement time, owing to thefrequency sweeping function required for the acquisition of
the Brillouin gain spectrum (BGS). In order to overcome thisdrawback, we have recently developed two types of frequency-sweeping-free BOCDR conﬁgurations: phase-detected(PD) BOCDR16) and slope-assisted (SA) BOCDR.17) PDBOCDR can oﬀer sampling rates over 100 kHz; however,
the measurable maximum strain is limited to ∼0.2%. In thisstudy, we focus on SA-BOCDR, which can be used to measure larger strains [although its strain dynamic range is also
restricted by the BGS shape and Brillouin-frequency-shift(BFS) dependence on the strain].
In order to measure the strain, temperature, and local loss,
SA-BOCDR utilizes not the BFS but the power on the slopeof the BGS. In addition to its high-speed operation, thisconﬁguration oﬀers the so-called beyond-nominal-resolution
eﬀect.18) By employing this eﬀect, a strained or heated section even several tens of times shorter than the nominal
spatial resolution can be detected. We have already demonstrated the detection of a 2-mm-long strained section;19) thisis the smallest length that has ever been experimentally
detected using distributed Brillouin sensors. The use of apolymer optical ﬁber (POF) with an extremely high ﬂexibility
and high temperature sensitivity as a sensing ﬁber of SABOCDR has also been shown to be eﬀective.20) Therefore,R

the performance of SA-BOCDR has signiﬁcantly improved;
however, its relevance to optical ﬁbers embedded in actual
structures has not yet been experimentally demonstrated,which is of crucial importance to its applicability.
In this study, as a ﬁrst step towards such practical applications, we present an example of an SA-BOCDR-based
diagnosis using an optical ﬁber embedded in a composite
structure. As a specimen to be measured, we prepareda carbon-ﬁber-reinforced-plastic (CFRP)21–23)-strengthened
steel plate, in which a standard silica single-mode ﬁber
(SMF; not a POF) was embedded. Upon application of loadto the structure on its top or bottom, we detect the induced
compressive or tensile strain distributions, respectively. We
show that the breakage of the embedded ﬁber can also be
detected as an abrupt local power change in the SA-BOCDR
output.
Owing to their high strength, high elastic modulus, small
weight, and high corrosion resistance, CFRPs have been
widely used as materials to repair or strengthen steel structures.24) In this experiment, as a specimen, a CFRP-strengthened steel plate was prepared using the vacuum-assisted resintransfer molding (VaRTM) technique,25–27) which is known
as an advanced, highly reliable, and reproducible method forthe fabrication of composite materials. The VaRTM process is schematically shown in Fig. 1.26) The process was
performed by infusing liquid resin into laminated ﬁbers or
woven fabrics using a pressure diﬀerential between theatmosphere and vacuum; thereafter, the resin was hardenedby heat (see Ref. 25 for more details). This technique does
not require a prepreg or autoclave, yielding a highly eﬃcientfabrication of composite materials.
Figure 2 shows the structure of the specimen. A 1.4-mlong silica SMF with a BFS of 10.85 GHz at room temperature (23 °C) was, as a sensing ﬁber, embedded between asteel plate and CFRP strips [containing carbon ﬁber cloth(Torayca UM46-40G)]. This structure was fabricated byplacing the SMF before the infusion of liquid resin during the
VaRTM process. The depth of the structure (both steel plate

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Fig. 1. Schematic of the VaRTM process (adapted from Ref. 26).

Fig. 4. Three-point bending device.
Fig. 2. Structure of the specimen.

Fig. 3. Experimental setup for SA-BOCDR. EDFA: erbium-doped ﬁber
ampliﬁer, ESA: electrical spectrum analyzer, OSC: oscilloscope, PD: photodetector, PSCR: polarization scrambler.

and CFRP strips) was 39 mm, while the thickness of the steel
plate was 12 mm. The total thickness of the CFRP strips
composed of seven tapered layers (the length of each strip
diﬀered by 10 mm) was ∼3 mm.
Figure 3 shows the experimental setup of SA-BOCDR for
the measurement of the strain distributions along the sensing
ﬁber, which was basically equal to that used in our previous
study.17) The 1,550-nm light from a semiconductor laser was
divided into two light beams: incident and reference. After
passing through a polarization scrambler (PSCR), the incident
light was ampliﬁed to ∼28 dBm using an erbium-doped ﬁber
ampliﬁer (EDFA) and injected into the sensing ﬁber embedded in the specimen. In the meantime, the reference light
was guided to a 1-km-long delay ﬁber, ampliﬁed to ∼3 dBm,
and then heterodyned with the Stokes light (Brillouinscattered from the sensing ﬁber; ampliﬁed to ∼1 dBm). The
heterodyned optical signal was converted into an electricalsignal using a photo detector (PD), and ampliﬁed by 23 dB
using an electrical ampliﬁer to improve the signal–to–noise
ratio (SNR). Thereafter, the spectral power change at a ﬁxed
frequency vB0 (= 10.83 GHz)17) of the BGS was acquired
using the narrow band-pass ﬁltering function of an electrical
spectrum analyzer (ESA) and observed using an oscilloscope(OSC). The video and resolution bandwidths of the ESA were
3 kHz and 10 MHz, respectively. Averaging was performed
1,024 times to enhance the SNR.
In order to resolve the sensing locations, the laser output
frequency was sinusoidally modulated, and a so-called

correlation peak was generated in the sensing ﬁber.3,5) By
sweeping the modulation frequency fm (= 9.132–9.156 MHz),the correlation peak was scanned along the ﬁber, enabling a
distributed measurement. The measurement range dm was
11.3 m, which was determined by fm according to Eq. (16) inRef. 28. By setting the modulation amplitude Δf to 1.52 GHz,
the spatial resolution of the system was 7.1 cm [calculated
from Eq. (15) in Ref. 28].
We performed two diﬀerent types of experiments: one
to evaluate the magnitudes of the tensile and compressive
strains applied to the sensing ﬁber, and the other one to detect
the breakage of the ﬁber. First, using a three-point bending
device (Fig. 4), tensile and compressive strains were applied
to the middle of the specimen by tightening the top screw witha pitch of 2 mm (the specimen was upturned when tensile
strains were applied). We measured the distributed tensile andcompressive strains from 0 to 1.5 and from 0 to 2.5 turns (ofthe top screw), respectively. Measurements were performed
every 0.5 turns. According to our calculation based on a ﬁniteelement analysis, the strain increases by ∼400 µε with each
additional turn, leading to the applied strains of approximately
0, 200, 400, and 600 µε (for tensile strains) and 0, 200, 400,
600, 800, and 1,000 µε (for compressive strains).
Similar experiments were performed by applying larger
compressive strains to the specimen until the sensing ﬁber
broke. Using SA-BOCDR, the power-change distributions
were measured while the top screw was tightened from 0 to
6.0 turns, corresponding to strains of 0 to ∼2,400 µε. Throughout the measurements, the room temperature was 23 °C.
First, we present the experimental results of the evaluationof the magnitudes of the applied tensile and compressive
strains. Figure 5(a) shows the measured power-change
distributions along the whole sensing ﬁber when the tensile
strains of 0, 200, 400, and 600 µε and compressive strains
of 0, 200, 400, 600, 800, and 1,000 µε were applied to thespecimen. The vertical axis was converted into strain using
the known strain-dependence coeﬃcient of a power changeof −1.59 dB=%.17) It is worth noting that the positive andnegative signs indicate tensile and compressive strains,respectively. The horizontal axis represents the relative
distance from the midpoint of the embedded section of thesensing ﬁber (positive values correspond to distal locations).

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Fig. 6. Power-change distributions measured along the sensing ﬁber when
the compressive strains were applied until the ﬁber broke.

(a)

(a)

(b)
(b)

Fig. 5. (a) Strain distributions measured along the sensing ﬁber whentensile strains of 0, 200, 400, and 600 µε and compressive strains of 0, 200,
400, 600, 800, and 1,000 µε were applied. The dotted lines indicate thetheoretical trends. (b) Maximum tensile and compressive strains [derived
from (a)] as a function of the revolution number of the top screw.

The power changes were observed along an ∼390-mm-longsection around the embedded section. It is worth noting thatthe actually embedded length of the sensing ﬁber was ∼320
mm; the diﬀerence of ∼70 mm corresponds to the nominal
spatial resolution.18) With the increase of the applied tensileand compressive strains, the measured strains also increased
moderately, in agreement with the theoretical values (shownby dotted lines; which are not theoretical SA-BOCDR
outputs but theoretical strain distributions). In Fig. 5(b), themaximum strain obtained from each measured distribution in
Fig. 5(a) is plotted as a function of the number of revolutionsof the top screw. Irrespective of tensile or compressive
strains, the maximum strain increased almost linearly with
the number of revolutions of the screw. The calculated slopes
for the tensile and compressive strains were ∼364 and ∼385
µε=turn, respectively, in a moderate agreement with thetheoretical values (∼400 µε=turn). The discrepancy seems to
be caused by the insuﬃcient adhesion strength of the sensing
ﬁber.
Subsequently, we detected the breakage of the sensing ﬁber
by applying larger compressive strains to the specimen. The
top screw was tightened to 0, 1.5, 3.0, 4.5, 5.3, 5.5, and 6.0
turns. The measured distributions of the power change (output
as voltage) are shown in Fig. 6. The vertical axis was deﬁned
to be positive when the strain was compressive. When thenumber of revolutions was equal to or smaller than 4.5, thepower changes were observed along the correct section (i.e.,embedded ﬁber section). In this range of strains, the power(c)Fig. 7. Photographs of the CFRP strips peeled oﬀ from the steel; (a) top
view, (b, c) side views.

change was almost proportional to the number of revolutions.
However, when the number of revolutions was larger than
5.3 (∼2,120 µε), the power change exhibited a signiﬁcant
increase along the distal side from the midpoint of thespecimen. This behavior implies that a considerable opticalloss was induced,17) possibly by the ﬁber breakage. Figure 7
shows photographs of the specimen after the revolution of
6.0 turns. A part of the CFRP strips were clearly peeled oﬀ
from the steel. These results indicate that SA-BOCDR has
the capability to indirectly predict the peeling of composite
structures through ﬁber breakage detection.
In conclusion, we experimentally investigated the strain
distributions along the sensing ﬁber embedded in the CFRPstrengthened steel plate. The measured tensile and compressive strains were linearly dependent on the number of revolutions of the top screw. The dependence coeﬃcients were∼364 µε=turn for tensile strain and ∼385 µε=turn for compressive strain, which were in a moderate agreement withthe theoretical values. We also detected the breakage of theembedded sensing ﬁber when large compressive strains wereapplied. The optical loss appeared when the screw wastightened to 5.3 turns (∼2,120 µε); the embedded sensing

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ﬁber was completely broken at 6.0 turns (∼2,400 µε). We
believe that, by presenting a promising example of composite
structural diagnosis, this study provides an important basis
for the future development of SA-BOCDR-based healthmonitoring systems for practical applications.
Acknowledgments This study was supported by JSPS KAKENHI Grant
Numbers 17H04930 and 17J07226, and by research grants from the Japan Gas
Association, ESPEC Foundation for Global Environment Research andTechnology, Association for Disaster Prevention Research, Fujikura Foundation,
and Japan Association for Chemical Innovation.

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