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Vol. 43, No. 8 / 15 April 2018 / Optics Letters

LetterDynamic mechanical analysis on fused polymer
optical fibers: towards sensor applications
ARNALDO LEAL-JUNIOR,1,* ANSELMO FRIZERA,1 MARIA JOSÉ PONTES,1 PAULO ANTUNES,2NÉLIA ALBERTO,3 MARIA FÁTIMA DOMINGUES,2 HEEYOUNG LEE,4 RYO ISHIKAWA,4 YOSUKE MIZUNO,4
KENTARO NAKAMURA,4 PAULO ANDRÉ,5 AND CARLOS MARQUES2
1

Telecommunications Laboratory (LABTEL), Graduate Program of Electrical Engineering of Federal University of Espı́rito Santo, Vitória,Espı́rito Santo, Brazil
2
Instituto de Telecomunicações and Physics Department & I3N, University of Aveiro, Aveiro, Portugal
3
Instituto de Telecomunicações and Centre for Mechanical Technology and Automation, Department of Mechanical Engineering,
Aveiro University, Aveiro, Portugal
4
Institute of Innovative Research, Tokyo Institute of Technology, Tokyo, Japan
5
Instituto de Telecomunicações and Department of Electrical and Computer Engineering, Instituto Superior Técnico,
University of Lisbon, Lisbon, Portugal*Corresponding author: arnaldo.leal@aluno.ufes.br
Received 19 December 2017; revised 2 March 2018; accepted 13 March 2018; posted 14 March 2018 (Doc. ID 318089); published 10 April 2018

This Letter presents, for the first time, to the best of ourknowledge, the dynamic mechanical analysis of a polymer
optical fiber (POF) that was previously damaged by thecatastrophic fuse effect. The variation of the fiber Young’s
modulus was evaluated with respect to the increase of
temperature, humidity, and frequency of strain cycles. The
obtained data for the fused POF are compared with theones for the same POF without the fuse effect. The results
show the feasibility of the fused POF for sensor applications, such as strain and acceleration measurement, since
it presents temperature sensitivity almost two times lower
in temperatures between 26°C and 90°C and Young’s
modulus 2.3 times lower than those obtained with the barefiber. The Young’s modulus variation with the humidity is
1.5 MPa/%RH in a humidity range of 66–96%. In addition, the fused POF presented a variation of its dynamic
modulus with the frequency increase four times lower than
non-fused POFs on the range of 0.01–100.00 Hz. These
results pave the way for future applications of fused POFs
as sensing elements. © 2018 Optical Society of America
OCIS codes: (160.5470) Polymers; (160.4670) Optical materials;(130.5460) Polymer waveguides; (220.0220) Optical design andfabrication; (280.4788) Optical sensing and sensors.
https://doi.org/10.1364/OL.43.001754

The catastrophic fuse effect is the continuous self-destruction
process of an optical fiber, triggered when high-power light isused in optical fibers under a tight bend or in damaged connections [1–4]. When the high-power optical signals propagating through the fiber find such defected points, this generally
results in a local heating and the creation of an optical discharge. Such a discharge is captured in the fiber core and0146-9592/18/081754-04 Journal © 2018 Optical Society of America

back-propagates towards the light source, permanently damaging the optical fiber. The first catastrophic fuse effect wasreported in silica single-mode fibers (SMFs) in 1987 [4]. Since
then, this effect has been reported in various special glass fibers,
such as microstructured [5], fluoride [6], chalcogenide [6],erbium-doped [7], and photonic crystal [8] fibers. The fuse
effect was first observed in polymer optical fibers (POFs) in2014 [8], and its propagation mechanism was discussed in detail [8–10]. The reported threshold power density was 180
times lower than that of silica fibers, and the propagation velocity was about 0.02 m/s for the fuse effect on POFs, whereas thisvalue can be about 0.5 m/s for silica fibers [11]. Furthermore,in POFs, the passing of the optical discharge leads to a dark
oscillatory path with an almost constant period [10].
Since the optical propagation loss in fused silica fibers isextremely high, the fuse effect is a critical factor that limits
the maximum optical power allowed in an optical fiber systemfor communication purposes [10]. In addition, this phenomenon restricts the application to distributed sensing, based onfiber nonlinear effects, such as Brillouin or Raman scattering[12,13]. However, the fuse effect in silica fibers also createsvoids in the fiber, which are micro-cavities that can be employed as a cost-effective alternative for Fabry–Perot interferometer (FPI) applications, since these micro-cavities areusually manufactured with high cost equipment, such as femtosecond lasers [11]. For this reason, silica fibers with the catastrophic fuse effect have been used to produce sensors formeasuring several parameters including strain [11], refractive
index [14], relative humidity (RH) [15], temperature [16], andhydrostatic pressure [17]. Additionally, the produced microcavities can be employed as edge filters for low-cost interrogation of fiber Bragg grating sensors [18].
Although the reported applications were developed in silica
fiber, POFs present advantages over silica fibers due to theirLettermaterial features, which include higher fracture toughness,strain limits, and flexibility in bending [19]. Furthermore,
the non-brittle nature and biocompatibility of POFs makethem suitable for in vivo applications [20,21].
In order to discuss the fuse effect on POF material features
for the first time, to the best of our knowledge, this Letter
presents the dynamic mechanical analysis (DMA) on a POF
before and after the catastrophic fuse effect. A stress-strain test
is performed on the non-fused and fused POFs to measure their
Young’s modulus. Then temperature tests are made to evaluate
the fiber viscoelastic properties, including the storage modulus
means of increasing the temperature during sequential strain
cycles. Furthermore, the fiber material behavior with frequency
and temperature is also measured. Since the material characterization of the fused POFs is an important analysis that should be
performed prior to their use as physical sensors, the resultspresented in this Letter can pave the way for future sensor
applications of fused POFs, exploiting the polymer viscoelastic
nature [22].
The fiber employed in the experiment is a perfluorinatedgraded index POF [23] with a core diameter of 50 μm, a cladding with 50 μm thickness, and an overcladding that results in afiber total diameter of 750 μm (ID050 Seikisui Chemical,
Japan). The materials of core and cladding are doped and undoped polyperfluorobutenylvinyl, either with a refractive index
of 1.356 or 1.342, respectively. The setup employed for thefiber fuse initiation is presented in [8]. A distributed-feedback
laser diode (output power of 5 mW; wavelength of 1546 nm) is
amplified using an erbium-doped fiber amplifier (LXM-S-21,
Luxpert Technologies) to 200 mW and injected into the POF.
One end of the POF is connected to a silica SMF using an
FC/SC adaptor. The other end is polished with a 0.5 μm
alumina powder to obtain a high surface roughness for the fuse
effect ignition. Since POF cleaving is important for the fiberconnectorization [24], all samples were cleaved with a razor
blade perpendicular to the fibers with controlled temperatureof 70°C.
Polymers are viscoelastic materials that present a combination of loss and storage behavior on their responses [25], wheresuch a behavior leads to a non-constant response with stress andstrain cycles [26] that are also observed in a long-term cyclic
response [27]. Moreover, their strain performance can beenhanced through annealing and etching treatments [28]. The
annealing can be made in the fiber or in the preform [29] and iscomposed of keeping the fiber in a temperature below the fiber
material glass transition temperature (T g ) for some hours[30,31]. In order to increase the strain performance of thePOFs, the annealing is made on both non-fused and fused
POFs, which were kept in a temperature of 90°C for 24 h.
The annealing temperature was chosen considering that theemployed POFs present a structure similar to CYTOP fibers,
which presents a T g of about 120°C. To characterize theresponse of the fibers under different conditions, DMA is performed, which is composed of the application of an oscillatory
load on the polymer with specific temperature, frequency, andhumidity range [32]. Thus, the non-fused and fused fibers are
positioned on a dynamic mechanic analyzer (DMA 8000,
Perkin Elmer, USA) for axial stress-strain cycles (see Fig. 1).
The climatic chamber is composed of the DMA 8000 heaterand cooling units for temperature control, and the TT humidity generator and controller (Triton Technology, USA), which

Vol. 43, No. 8 / 15 April 2018 / Optics Letters1755

Fig. 1. Experimental setup employed for the DMA on the POFs.

enable the variation of the temperature (ΔT ) and RH variation(ΔRH), while the movable part provides the frequency (Δf )and strain (Δε) variations.
The analyzed material properties with the DMA are thestorage moduli (E 0 ) and loss moduli (E 0 0 ). The relation
between these two moduli with the dynamic Young’s modulus
of a viscoelastic material (E  ) is presented asE   E 0  iE 0 0 :(1)

The storage modulus is related to the elastic part of the
response, and the loss modulus presents the dissipative or
viscous part of the response.
The non-fused and fused fibers are also subjected to stressstrain tests to measure the material Young’s moduli before andafter the fuse effect. Then thermal tests in the range of 26–90°C
are carried out to verify the material behavior with the temperature increase. In addition, frequency tests from 0.01 to
100.00 Hz are performed on the POFs under strain cycles toevaluate their response over the frequency increase. Finally, a
ΔRH of about 30% (in the range from 66% to 96% due
to operational limitations of the employed RH controller, since
the RH variation range that provides the highest stability is
about 30% for humidity sweep) is applied on the fibers to assess
the material sensitivity with respect to this parameter. The storage modulus variation with the RH was also evaluated under
three different temperatures (25°C, 55°C, and 75°C).
The stress-strain cycles made on the polymer enable theevaluation of the Young’s modulus. The obtained Young’s
modulus of the non-fused fiber was 3.5 GPa, while the fiber
after the fuse effect presents a Young’s modulus of 1.5 GPa.
These values were obtained by the slope of the stress-strain
curve on the strain range of 0.05–0.25%, as recommended by
ISO527-1:1996 for polymers. This reduction may be related toan additional relaxation of the polymer chains after the passageof the high-energy optical discharge that presents a temperatureof about 3327°C [10]. The lower Young’s modulus of the fused
POF can provide higher sensitivity for strain sensing applications, when compared with the non-fused POF [33].
Figure 2 presents the response of the polymer material withthe temperature increase. The temperature sensitivity obtained
for the fused POFs is 6.25 MPa/°C, which is almost two times

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Vol. 43, No. 8 / 15 April 2018 / Optics Letters

LetterFig. 2. Temperature response of fused and non-fused POFs.

Fig. 3. Frequency response of fused and non-fused POFs.

lower than the one of the non-fused POF (11.30 MPa/°C).
These results were obtained with an RH of 88% (room RH
due to operational limitations of the RH controller) in a frequency of 1 Hz and show that the fused POF presents a lower
variation of the storage modulus with the increase of temperature, which provides advantages over the non-fused POF for
intensity-variation-based sensor applications. This sensor interrogation technique relies on the optical power variation withrespect to some parameters, such as strain [34], angle [22],and refractive index [35]. Therefore, a higher variation of
the Young’s modulus, caused by the change in the storage
modulus, leads to a higher cross-sensitivity of the sensor withthe temperature, which can be a source of errors on the sensor’s
measurement. Since the fused POF can propagate light for a
few centimeters with an optical propagation loss of about
1.4 dB/cm [8], an intensity-variation-based sensor using the
fused POF provides lower temperature cross-sensitivity thanthe one with the non-fused fiber. Such a high optical propagation loss is due to the fuse effect, since the loss of the non-fused
POF is about 250 dB/km [10].
Regarding the frequency response of each fiber, Fig. 3
presents the results obtained for a load with a frequency range
between 0.01 and 100.00 Hz with a step of three points per
decade applied on POFs under constant temperature (26.0°C)
and RH (88%), which corresponds to the room temperature
and RH, respectively. The humidity controller was set to maintain the room RH due to operational limitations of its humidity
range. The response of the non-fused fiber presents a considerable variation with the increase of frequency, which starts
around 3.2 GPa when the frequency is 0.01 Hz and increases
to 3.5 GPa when the frequency is 1 Hz. Then it presents
an almost linear increase of 43.00 MPa/Hz for a frequency
range from 1.00 to 100.00 Hz. The fused POF response
presents a positive frequency dependence with a coefficientof 9.00 MPa/Hz (calculated in the range of 10 to 100 Hz),which is more than four times lower than that of the non-fused
POF. Such advantageous findings also enable the use of fused
POFs in sensors for low frequency applications.
This stable response of the fused POF with the increase of
frequency is an advantage over the non-fused ones for applications, such as accelerometers operating at frequencies below

100.00 Hz [36]. In addition, it is useful for curvature sensor
applications in different angular velocities [22], where such alower variation on the mechanical properties of the fiber understrain will provide lower errors for the tests with a wider rangeof angular velocities.
The last set of tests made is the RH variation to verify thewater absorption of the samples. Since the employed POF
presents a material composition similar to cyclic transparent
amorphous fluoropolymer (CYTOP) fibers [37], it is expectedthat such fibers do not change their mechanical properties with
the increase of this parameter [37]. Such a humidity insensitivity can reduce the cross-sensitivity of POF sensors, which isoften observed in polymethyl methacrylate (PMMA) POFs
[31]. Figure 4 presents the storage modulus variation for the
non-fused and fused POFs in an RH variation of about
30%. Such a variation was obtained due to operational limitations that lead to lower humidity variation in the climaticchamber. After setting the humidity, it remains constant for
about 30 min before the test. Then the highest humidity is
set on the equipment. In addition, the tests were made with

Fig. 4. Storage modulus as a function of RH of about 30% (from
66% to 96%) for the fused and non-fused POFs. The inset shows
the storage modulus as a function of the RH at three different
temperatures.

Letter
constant temperature of 26.0°C and frequency of 1 Hz, whichis the standard frequency of the equipment for this type of test.
In order to evaluate the POF response under low humidity conditions and for different temperatures, the inset in Fig. 4
presents the non-fused and fused POFs responses with threedifferent temperatures, namely 25°C, 55°C, and 75°C, for three
RH conditions (25%, 65%, and 90%).
Regarding water/moisture absorption, both non-fused andfused POFs presented low variations of their mechanical properties with the increase of humidity, as depicted in Fig. 4. The
maximum variation for fused fibers was below 4.5 MPa, whichmeans that the ΔRH may not lead to deviations on the measurement of fused POF sensor applications. Note that the variationof the storage modulus with the RH for the fused POF was
similar to that for the non-fused POFs that enable sensing
applications of both fused and non-fused POFs with lower
humidity cross-sensitivity than the one of the PMMA POFs, forexample. Furthermore, the obtained results under different temperatures and with lower RH also show the low absorption of the
fused POFs, where the curves presented almost constant slope,and their offsets are due to the temperature variations. The advantages related to the material features of the fused POFs pave
the way for sensor applications where the sensing element is the
fused POF itself. This can be achieved since the POFs canpropagate light for a few centimeters after the fuse effect.
In conclusion, this Letter presents the DMA of a fused POF.
The material responses with respect to temperature, load frequency, and humidity were evaluated in detail. In addition,
stress-strain tests were performed to obtain the Young’s modulus of the fused POF, which is 2.3 times lower than the one of
the bare POF. The results were compared with those obtained
for a non-fused POF. The storage modulus of the fused
POF presented a lower sensitivity two times lower than temperature variation, which may lead to lower temperature crosssensitivity. Additionally, the fused POF presented a stable
dynamic modulus with the frequency range of 0.01–
100.00 Hz, with four times lower modulus variation than thatof the non-fused POF. Furthermore, low variation with theincrease of humidity was obtained for the fused POFs, wheresuch a variation is lower than 1.5 MPa/%RH.
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, 310310/2015-6); Fundação para a Ciência e a
Tecnologia (FCT) (SFRH/BPD/101372/2014, SFRH/BPD/
109458/2015, PREDICT project (FCT, IT-LA), UID/
CTM/50025/2013, UID/EEA/50008/2013, UID/EMS/
00481/2013); European Regional Development Fund (ERDF)(PT2020); Japan Society for the Promotion of Science (JSPS)(17H04930, 17J07226); Japan Association for Chemical
Innovation (JACI); Fujikura Foundation.
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