LETTER

Applied Physics Express 12, 062007 (2019)https://doi.org/10.7567/1882-0786/ab1e92

Infrared thermometry for breakage detection of optical fibers embedded instructures
Yosuke Mizuno1* , Sonoko Hagiwara1, Heeyoung Lee1 , Yutaka Ochi2, Takahiro Matsui3, Yukihiro Matsumoto4,
Yosuke Tanaka5 , Hitoshi Nakamura6, 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, Japan3
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: ymizuno@sonic.pi.titech.ac.jp

Received March 26, 2019; revised April 16, 2019; accepted May 1, 2019; published online May 20, 2019
We show that infrared thermometry can be used to detect a breakage point in an optical fiber embedded in a structure. Around the breakage point,incident light induces heat, which is detected using an infrared thermometer. By this method, we can obtain position information of a breakage
point, including accurate information on where it is located in the actual structure, on a real-time basis. We experimentally show the practicalusefulness of this method by detecting a breakage point in an optical fiber embedded in a composite structure comprising carbon fiber-reinforcedplastics. © 2019 The Japan Society of Applied Physics

Health monitoring of civil infrastructure such asbuildings, tunnels, bridges, dams, levees, pipelines,
wind turbines, and railroad tracks, has become one of
the most important research topics in modern society. To
date, as structural monitoring tools, optical ﬁber sensors havebeen extensively studied and applied to the measurementof strain,1–4) temperature,1–4) pressure,5,6) stress,7,8)
humidity,9,10) acoustic impedance,11,12) and many others.
Needless to say, these physical parameters can be correctly
measured under the assumption that the optical ﬁbers
embedded in structures have no breakage points. Once partof the embedded ﬁber is cut by large shear stress, etc., it
becomes difﬁcult to continue most of these measurements.
Especially when the sensor is a two-end-access type, inwhich two light beams need to be injected into both ends ofthe ﬁber under test, one breakage point completely disables
its operation. Even if the sensor is a single-end-access type,no information can be obtained from the distal part of thebreakage. Thus, breakage of an embedded ﬁber should be
avoided, but in case it is accidentally caused for someunavoidable reason, the breakage point should be detected
as soon as possible.
We can obtain position information on the breakage point ofan embedded optical ﬁber using single-end-access distributed
loss sensors, such as optical time-domain reﬂectometry,13–17)
optical frequency-domain reﬂectometry,18–22) and optical correlation-domain reﬂectometry.23–28) Optical interferometry
based on multiple Fresnel reﬂections29) and some of thedistributed strain sensors30) can also be used for this purpose.
All of these sensors can determine the position of the breakage
point with high sampling rates and high spatial resolutions; thesystem cost has also been decreasing. However, at the site of
the actual measurement, these methods suffer from a serious
issue from a practical viewpoint: even if we obtain theinformation that the embedded ﬁber has a breakage point, forexample, 13.5 m away from a certain point, it is difﬁcult toknow immediately which part of the actual structure that point
is located in.
In this paper, to resolve this practical problem, we develop
a new method, using an infrared thermometer (IRT) to detecta breakage point in an optical ﬁber embedded in a structure.
The principle is simple—at the breakage point of an opticalﬁber, optical energy is converted into thermal energy, heating
the surrounding part of the structure, which is detected using
an IRT. By this method, we can obtain the position
information of a breakage point, including accurate information on where it is in the actual structure, on a real-time basiswith high cost efﬁciency. We show the practical usability ofthis method by detecting a breakage point in an optical ﬁber
embedded in a composite structure comprising a steel plateand carbon ﬁber-reinforced plastic (CFRP) strips.
At a breakage point in an optical ﬁber, the optical energy isconverted into thermal energy and heats the surrounding
structural area. Here, we give a simple quantitative expression employing a model based on Newton’s law ofcooling.31) Suppose light is incident at t = 0, the temperature
at the breakage point T(t) is given byT (t ) = T (0 ) +⎛ hA ⎞ ⎫Q ⎧⎨1 - exp ⎜ - t ⎟ ⎬ ,⎝ C ⎠⎭hA ⎩(1 )where Q is the thermal energy, h is the heat transfer
coefﬁcient, A is the heat transfer surface area, and C is theheat capacity of the structure; T(0) corresponds to room
temperature. It is notable that, at t → ∞, the increase intemperature is proportional to Q, i.e. the power of theincident light.
The temperature at the breakage point can be roughly
measured with an IRT. In general, a blackbody is a perfectabsorber of any radiation incident on it and can emit
radiation.32,33) When heated, it generates blackbody radiation, the characteristics of which depend only on temperature.
The total radiant emittance W of an object (both blackbody
and non-blackbody) is given by the Stefan–Boltzmann law as
W = esT 4,(2 )where ε is the emissivity of the object (1 for a blackbody), σ
is the proportional constant, and T is the surface temperatureof the object. By measuring the infrared energy emitted bythe object, its temperature can be determined. The surface
temperature of a heated part of a structure can thus be062007-1© 2019 The Japan Society of Applied PhysicsAppl. Phys. Express 12, 062007 (2019)

Y. Mizuno et al.

(a)

(b)

(c)
Fig. 1. (Color online) (a) Sample geometry. (b) Experimental setup. EDFA: erbium-doped ﬁber ampliﬁer. (c) Schematic of ﬁber breakage caused by peelingof CFRP strips.

measured with an IRT. Though this method suffers from a
relatively large temperature error caused by the surface stateof the object (resulting in different ε values), it has
advantages such as cost efﬁciency, real-time operation, andvisual display of the heat.
Owing to their high strength, high elastic modulus, lightweight, and high corrosion resistance, CFRPs have been usedas materials for repairing or strengthening steel structures.34)
In this experiment, as a specimen, a CFRP-strengthened steel
plate was fabricated using what is called a vacuum-assisted
resin transfer molding (VaRTM) technique,35–38) which isknown as a reliable method for fabricating composite
materials. The VaRTM process was performed by infusing
a liquid resin into laminated ﬁbers or woven fabrics
exploiting the pressure difference between atmosphere andvacuum; the resin was then hardened by heat (see Refs. 35
and 36 for more details).
Figures 1(a) and 1(b) show the schematic structures of thespecimen. Three pieces of 1.4-m-long silica single-mode
ﬁbers (SMFs; core diameter: 8 μm, cladding diameter:

125 μm, outer diameter: 250 μm) were embedded in parallel
between a steel plate and CFRP strips [containing carbon
ﬁber cloth (UM46-40G, Torayca)]. The intervals between thethree SMFs were 9 mm, and the lengths of the embedded
ﬁber sections were all 320 mm. This structure was fabricatedby placing the SMF before the infusing liquid resin during
the VaRTM process. The depth of the structure (both the steelplate and the CFRP strips) was 39 mm, and the thickness ofthe steel plate was 12 mm. The total thickness of the CFRP
strips involving seven tapered layers (the length of each strip
differed by 10 mm) was approximately 3 mm.
To break the embedded ﬁbers by peeling of the CFRP
strips, large compressive strain was applied to the middle ofthe specimen by tightening a top screw of a three-point
bending device (Fig. 3). After breakage, we injected light into
one of the SMFs embedded on the sides of the specimen(Fig. 2). The light source was a semiconductor laser at
1551 nm, and its output power was ampliﬁed to 15–27 dBm
using an erbium-doped ﬁber ampliﬁer. The surface temperature of the CFRP strips was then measured with an IRT

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Appl. Phys. Express 12, 062007 (2019)

Y. Mizuno et al.

(a)(b)

Fig. 2. (Color online) IRT images taken (a) 30 s and (b) 180 s after light injection at 27 dBm. The dotted lines are the boundaries of the surface of thespecimen.

(a)

(b)

Fig. 3. (Color online) (a) Temporal variations of temperature change at ﬁve different incident powers. The solid curves are exponential ﬁts. (b) Temperaturechange (180 s after light injection) plotted as a function of incident power. The solid line is a linear ﬁt.

(FLIRi7, FLIR). The emissivity of the CFRP surface was set
to 0.85, and the room temperature was 28 °C.
The IRT images taken 30 s and 180 s after light at 27 dBm(=500 mW) was injected are shown in Figs. 2(a) and 2(b),respectively. A clear heated spot was detected on the side of
the specimen, and its central position agreed with thebreakage point of the embedded SMF. The shape of theheated area was not completely circular but highly elliptic,because of the thermal anisotropy of the CFRP strips. As timeproceeded, the temperature at the hottest part increased.
Subsequently, we measured the temporal dependencies of
the temperature change at the heated spot when the incident
optical power was varied [Fig. 3(a)]. Regardless of theincident power, by lapse of time, the temperature change
increased and became almost constant at approximately 150 s
after light injection. These trends were well ﬁtted byexponential curves, as suggested by Eq. (1). The temperaturechange at the same timing was larger as the incident power
was higher.
Finally, we plotted the saturated temperature change,deﬁned as the temperature change 180 s after light injection,
as a function of the incident optical power [Fig. 3(b)]. Notethat the optical power on the horizontal axis is indicatedusing a linear unit (mW). With increasing incident power, thetemperature linearly increased with a coefﬁcient of
9.2 °C W−1. This linear trend agrees with the aforementioned
theory. Thus, higher-power light injection was shown toenable detection of a breakage point with a higher contrast.
In conclusion, we demonstrated IRT-based detection of abreakage point of an optical ﬁber embedded in a structure.
Using an IRT, we simply detect the heated area of thestructure around the breakage point of the ﬁber. The uniqueadvantage of this method is that we can obtain the position
information of a breakage point, including accurate information on where it is located in the actual structure. Other
advantages are its real-time operation and cost efﬁciency. We
showed the practical usefulness of this method by detecting abreakage point of a ﬁber embedded in a CFRP-basedstructure. The obtained quantities, such as the time constant(∼150 s) and the dependence coefﬁcient (9.2 °C W−1), willvary according to the materials and conditions of the structurein which optical ﬁbers are embedded. In particular, when
optical ﬁbers are embedded deep in structures, this method
will be less effective. Injection of extremely high-power light

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Appl. Phys. Express 12, 062007 (2019)

Y. Mizuno et al.

may enhance the limited depth to some extent, but it is notcost-effective and sometimes causes a so-called ﬁber fusephenomenon.39,40) These points need to be quantitatively
analyzed further in the future. We believe that, nevertheless,our IRT-based technique remains of signiﬁcant use indetecting ﬁber breakages on site. Note that this technique
may also be useful for detecting extremely high optical loss
caused locally when a crack is induced in the ﬁber-embedded
structure.
Acknowledgments This work was supported by the MLIT Construction
Technology Research and Development Subsidy Program, by JSPS KAKENHI
Grant Numbers 17H04930 and 17J07226, and by a research grant from theFujikura Foundation.
ORCID iDs Yosuke Mizuno
https://orcid.org/0000-0002-33624720 Heeyoung Lee
https://orcid.org/0000-0003-31790386 Yosuke Tanaka
https://orcid.org/0000-0002-65395977 Kentaro Nakamura
https://orcid.org/0000-0003-2899-4484

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062007-4© 2019 The Japan Society of Applied Physics