Vol. 26, No. 10 | 14 May 2018 | OPTICS EXPRESS 12939

Strain, temperature, moisture, and transverse
force sensing using fused polymer opticalfibers
ARNALDO LEAL-JUNIOR,1,* ANSELMO FRIZERA,1 HEEYOUNG LEE,2 YOSUKEMIZUNO,2 KENTARO NAKAMURA,2 TIAGO PAIXÃO,3 CÁTIA LEITÃO,3,4 M.
FÁTIMA DOMINGUES,4 NÉLIA ALBERTO,4 PAULO ANTUNES,3,4 PAULO
ANDRÉ,5 CARLOS MARQUES,4 AND MARIA JOSÉ PONTES1
1

Telecommunications Laboratory (LABTEL), Graduate Program of Electrical Engineering of Federal
University of Espírito Santo, Brazil
2
Institute of Innovative Research, Tokyo Institute of Technology, Japan3
Physics Department & I3N, University of Aveiro, Portugal
4
Instituto de Telecomunicações, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
5
Instituto de Telecomunicações and Department of Electrical and Computer Engineering, Instituto
Superior Técnico, University of Lisbon, Portugal*
arnaldo.leal@aluno.ufes.br

Abstract: This paper presents the characterization of polymer optical fibers (POFs) submitted
to the catastrophic fuse effect towards intensity-variation-based sensing of strain, transverse
force, temperature, and moisture. In the experiments, POFs with and without the fuse effectare tested and the results are compared with respect to the sensitivity, linearity, and root meansquared error (RMSE). The fused POFs have higher linearity and lower RMSE than nonfused POFs in strain and transverse force sensing. Also, the sensitivity of the fused POFs ishigher in transverse force and temperature sensing, which can be related to the higher
sensitivity to the curvature that the transverse force creates on the POF and to the moresignificant variations of the refractive index with temperature increase. Additionally, thefused POFs present lower moisture absorption than the non-fused POFs. The presented results
indicate a great potential of the fused POFs intensity-variation-based sensing applications ofvarious physical parameters.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
OCIS codes: (160.5470) Polymers; (130.5460) Polymer waveguides; (280.4788) Optical sensing and sensors;(060.2370) Fiber optics sensors.

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#326203
Journal © 2018https://doi.org/10.1364/OE.26.012939
Received 15 Mar 2018; revised 20 Apr 2018; accepted 25 Apr 2018; published 4 May 2018

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1. Introduction
The catastrophic fuse effect is a self-destruction process of an optical fiber, triggered whenextremely high-power optical signals are injected into a fiber submitted to non-ideal
conditions, such as tight bendings, and damaged connectors [1,2]. Once an optical discharge
is initiated, it is trapped in the fiber core and travels back toward the light source, consuming
the light energy and permanently damaging the fiber. This effect was first observed in silica
single-mode fibers (SMFs) in 1987 [1]. Thereafter, the fuse effect has been continuously
studied in different glass fibers, including micro structured [3], erbium-doped [4], fluoride[5], chalcogenide [5], and photonic crystal [6]. Recently, the fuse effect has been observed

Vol. 26, No. 10 | 14 May 2018 | OPTICS EXPRESS 12941

and characterized in polymer optical fibers (POFs) [7–9]. At 1550 nm, the propagation
velocity of the POF fuse was estimated to be approximately 20 mm/s (one to two orders
slower when compared with silica fiber fuse), and the threshold power density was 6.6
kW/cm2 (>150 times lower than that of the silica fiber fuse) [7]. The temperature of the
optical discharge was estimated to be ~3600 K, which was much lower than for the silica
fiber fuse [8]. Thus, the physical properties of the POF fuse are unique.
The fuse effect in silica SMFs produces voids along the fibers, which can be used as a
cost-effective alternative for Fabry-Perot interferometry (FPI) [10]. For this reason, fusedsilica SMFs have been employed in sensing applications of strain [10], liquid level [11],relative humidity [12], refractive index [13], and temperature [14]. Another application of the
fused silica SMFs is as an edge filter for low-cost interrogation of fiber Bragg gratings [15].
Although the referred voids are also observed in fused POFs, they formation are not stable[8], which may inhibit their applications as FPI micro-cavities. However, the POFs in which
the fuse effect occurred (fused POFs) have a unique advantage, since the optical propagation
loss is lower than the values for the fused silica SMFs. This optical propagation loss value forthe fused POFs is approximately 1.4 dB/cm, which is adequate to enable the signal
propagation for a few centimeters [7,8]. In addition, POFs present advantages over silica
SMFs regarding their material features, such as higher strain limit, fracture toughness, andresistance to impact loads [16]. Therefore, fused POFs could be applicable to physical sensing
with new approaches, different from the ones used with the fused silica SMFs.
One possible interrogation technique for fused-POF-based sensors is to exploit the lightintensity variations with respect to physical parameters, such as refractive index [17],acceleration [18], angle [19], temperature [20], torque [21], and strain [22]. Intensityvariation-based fiber-optic sensors generally present the advantages of cost efficiency andsimplicity of signal processing [23]. Recently, a dynamic mechanical analysis on the fusedand non-fused POFs has been performed [24], and the fused POFs have been shown to bepotentially higher in performance for strain sensing applications.
Considering this background, here we characterize the fused POFs for strain andtransverse force sensing. The response of the fused POFs to these parameters are comparedwith those of the same POFs without the fuse effect. Subsequently, both fused and non-fused
POFs are also characterized with respect to temperature and moisture absorption, in order toevaluate the cross-sensitivity of the strain and transverse force sensors to these environmental
parameters.
2. Experimental setup
The POF employed in the experiment had a perfluorinated graded-index core with 50 µm
diameter made of doped polyperfluorobutenylvinyl ether, a cladding of the same material
without doping with 25 µm thickness, and an overcladding (composed of polycarbonate) that
resulted in an outer diameter of 750 µm. The fuse was initiated using the experimental setup
and methods described by Y. Mizuno et al [7], where there is an amplified optical power of
200 mW injected into the POF that presents one end connected to a silica single mode fiberusing an FC/SC adaptor and the other end polished with a 0.5 µm alumina powder for thefuse effect ignition. Since fused POFs have much higher optical propagation loss than nonfused POFs (0.25 dB/m at 1550 nm for fused POFs, where the one for non-fused POF isabout 250 dB/km), the length of the fused and non-fused POF samples was set to 3.4 cm, fora reliable performance comparison.
In order to reduce the internal stress generated during the fiber drawing process and toenhance its mechanical sensitivity [25], the annealing was performed to both the fused andnon-fused POF samples. Using a thermostatic chamber (1/400 ND, Ethik Technology), thetemperature around the samples were maintained at 95°C (close to their glass transition
temperatures) for 48 hours [26], where the annealing time chosen is large enough to guarantee
the molecular alignment relaxation required for the Young’s modulus reduction [27]. After

Vol. 26, No. 10 | 14 May 2018 | OPTICS EXPRESS 12942

preparation of the samples, they were installed in the setup presented in Fig. 1(a) for strain
tests. One end of the POF was fixed, with acrylate glue, on a fixed stage and the other on atranslation stage with controllable displacements. The applied displacements resulted in astrain ranging from 0.5 to 1.5%. For transverse force tests, another setup depicted in Fig. 1(b)was used. Calibrated weights corresponding to transverse forces of 5, 10, 15, and 20 N wereapplied to each POF sample installed in the setup. It is worth to mention that higher forces
can be applied in the fiber, if its strain limit (~8%) is not surpassed. However, these forceswere not applied due to experimental limitations of the employed setup.

Fig. 1. Schematics of the experimental setups for (a) strain and (b) transverse force
characterizations of fused and non-fused POFs.

The optical signal source was a low-cost laser peaking at 650 nm with an optical power of
5 mW. The signal acquisition was performed with a photodetector (IF-D91, Industrial Fiber
Optics) with a trans-impedance amplifier, which was connected to an acquisition board(USB-6008, National Instruments) for signal acquisition at 200 Hz. Regarding thetemperature characterization, the POF samples were placed over a thermoelectric Peltier plate(TEC1-12706, Hebei IT) with a temperature controller (TED 200 C, Thorlabs). Using asimilar setup as depicted in Fig. 1, the optical power transmitted through the POF was
measured continually, at 25, 30, 40, and 50°C, for a period of 3 hours. The temperature upper
limit (50°C) is the highest temperature of the Peltier plate employed. Nevertheless, the sensor
is able to operate in higher temperatures, where the temperature limitation is the fiber glass

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transition temperature. The moisture absorption was also evaluated by immersing each POF
sample inside a container filled with distilled water. Both samples were straightly placed to
avoid the optical loss caused by bending.
3. Results and discussion
The strain characterization results are shown in Fig. 2. The transmitted optical power, throughthe fused and non-fused POF samples, were plotted as function of the applied strain. The
transmitted optical power (in mW) was normalized using its zero-strain value as a reference.
The error bars were defined as the standard deviations of the three measurements under thesame condition. With increasing strain, both samples exhibited almost linear negative
dependencies, with correlation factor closers to the unit. The strain sensitivity of the nonfused POF was approximately 1.4 times higher than that of the fused POF, while the fused
POF had higher linearity. The root mean squared errors (RMSEs) between the reference strain
applied and the one measured by the POF sensors for the fused and non-fused POFs withrespect to the strain were 0.002% and 0.111%, respectively. In this case, the reference strain
is the translation stage axial movement divided by the initial fiber length.
One possible reason for the high linearity of the fused POF is the linear relation between
stress and strain in this strain range. When the POF is under strain, it is induced stress, leading
to the variations of its refractive index due to the stress-optic effect, which generally induceschanges on the transmitted power due to the variation of the critical angle and number ofmodes [28]. Thus, the linear deviations on the fiber stress may lead to linear variations of the
POF power, which can also explain the lower errors of the fused POF. In contrast, the lower
sensitivity of the fused POF can be explained assuming that both POFs are in the elastic
regions, by the Hooke’s law presented in Eq. (1).

σ = Eε ,(1)

where σ is the stress, E is the Young’s modulus, and ε is the strain. This equation indicates
that, if submitted to the same strain, the material with a lower Young’s modulus has lower
stress than that with a higher Young’s modulus. As it has been reported in [24], the fused
POF has a smaller value of Young’s modulus, therefore the stress on the POF becomes lower,
leading to lower power attenuation than the non-fused POF. For this reason, the fused POF
presented a lower sensitivity (0.0042%−1) than the one of non-fused POFs (0.0060%−1), butwith higher linearity (0.999 for the fused POF and 0.985 for the non-fused ones) and,
consequently, lower errors.

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Fig. 2. Normalized transmitted optical power plotted as function of strain applied to fused (red)and non-fused (blue) POFs. The points are experimental data, and the lines are linear
regressions.

Subsequently, the transmitted power dependence on the transverse force is shown in Fig.
3. The transmitted optical power was normalized using as a reference the situation without
loading. The force sensitivity of the fused POF was approximately 1.3 times higher than thatof the non-fused POF. The sensitivity of fused POFs is 0.024 N−1, whereas the non-fused
POFs presented a sensitivity of about 0.019 N−1. The force sensitivity is related to thecurvature that the POFs are subjected when the force is applied, and the higher force
sensitivity of the fused POF is probably originated from its higher sensitivity to curvature.
Additionally, the fused POF also presented higher linearity than that of the non-fused POF,
which seems to be related to the linear stress-strain relation of the fused POF. The RMSE forthe non-fused POFs with respect to the transverse force was 0.64 N, while that for the fused
fibers was 0.27 N. Therefore, the fused POF has been shown to have many advantages overthe non-fused POF for transverse force sensing applications, including higher sensitivity,
higher linearity, and lower measurement error. Furthermore, the non-fused POF presented anonlinear behavior under transverse force with a linearity of 0.923. This low linearity also
leads to a normalized transmitted power higher than 1 in the linear regression curve when theforce is zero. Thus, it is only related to the sensor nonlinearity, since the measured value atzero force is 1 for both sensors.

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Fig. 3. Normalized transmitted optical power plotted as a function of transverse force applied
to fused (red) and non-fused (blue) POFs. The points are experimental data, and the lines arelinear regressions.

The transmitted optical power with respect to temperature was also measured as is shownin Fig. 4. The transmitted optical power was normalized using as a reference its value at
25°C. With increasing temperature, the POF optical propagation loss increases, irrespective of
the samples, but the temperature sensitivity of the fused POF was >20 times higher than the
non-fused POF. The intensity variation obtained with the temperature increase is related tothe thermo-optic effect, which is the POF refractive index variation with respect to thetemperature, as characterized in [29]. Although the fused POFs did not exhibit a sharp
decrease in the Young’s modulus at temperature close to the glass-transition temperature,they showed larger variations of the Young’s modulus at temperature of <60°C than those of
the non-fused POFs [24]. Therefore, there might be larger power variations in the fused POFs
due to thermally induced strain. In addition, the temperature increase generally causes
variations of the refractive index, resulting in deviations of the transmitted power [30]; suchvariations may be larger in the fused POFs. Thus, the fused POFs presented a sensitivity morethan 10 times higher when compared with the non-fused POFs, where the sensitivities are
0.016 °C−1 and 0.0014 °C−1 for the fused and non-fused POFs, respectively. The lower
linearity of the fused POF (0.976 for the fused and 0.985 for non-fused ones) may be
attributed to the nonlinearity of the refractive index variations with temperature, which also
leads to larger measurement errors [30].

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Fig. 4. Fused (red) and non-fused (blue) POF response to temperature (points are experimental
data, and the lines are linear regressions).

Finally, the moisture absorption of the POF samples was evaluated. Figure 5 shows thenormalized transmitted power of the fused and non-fused POFs plotted as functions of time.
Both POF samples showed significant power reduction within 20 minutes subsequently totheir immersion in distilled water at 25°C. After that period, the transmitted power became
relatively stable. Regardless of time, the variations of the normalized transmitted power of thenon-fused POF (maximum: 10.8%) were larger than those of the fused POF (maximum:
7.8%), which indicates that fused POF absorbs less water. Such lower variations can be
ascribed to the lower sensitivity to humidity [24]. Additionally, the fused POFs presented ahigher stability than the non-fused ones after 30 minutes, where the standard deviation in theinterval from 30 minutes to 180 minutes is 2.03% for the non-fused and 0.12% for the nonfused POFs.

Fig. 5. Fused (red) and non-fused (blue) POF response to time after the POFs were immersed
into distilled water.

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4. Conclusions
We characterized the fused POF as a candidate of an intensity-variation-based sensing
element for strain and transverse force, as well as temperature and moisture absorption. The
fused POF showed 1.4 times smaller dependence on strain than that of the non-fused POF,
but their sensitivities to transverse force and temperature were 1.3 times and >20 times higher
than those of non-fused POFs, respectively. The linearity and measurement error were also
evaluated. In addition, the moisture absorption of the fused POF was lower than that of thenon-fused POF. These results prove the advantages of the fused POF as an element ofintensity-variation-based sensors, especially for highly sensitive transverse force andtemperature sensing. We foresee the use of these properties on the development ofinterferometry-based sensors and the use of the electric conductivity of the fused POF [8]may lead to the development of novel sensing systems. In addition, future works also include
the inscription of short or long period gratings in the fused POF, where the core carbonization
due to the fuse can be a kind of hollow core fiber.
Funding
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)(88887.095626/2015-01); Fundação Estadual de Amparo à Pesquisa do Estado do Espírito
Santo (FAPES) (72982608); Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq) (304192/2016-3 and 310310/2015-6); Fundação para a Ciência e a
Tecnologia (FCT) (PD/BD/128265/2016 (DAEPHYS), SFRH/BPD/101372/2014, andSFRH/BPD/109458/2015); Fundação para Ciência e a Tecnologia / Ministério da Educação eCiência (UID/EEA/50008/2013); European Regional Development Fund (PT2020
Partnership Agreement); JSPS KAKENHI (17H04930 and 17J07226); Fujikura Foundation;
Japan Association for Chemical Innovation; FCT, IT-LA (PREDICT scientific action).