LetterVol. 43, No. 11 / 1 June 2018 / Optics Letters2539

Design and characterization of a curvature sensor
using fused polymer optical fibers
ARNALDO LEAL-JUNIOR,1,* ANSELMO FRIZERA,1 HEEYOUNG LEE,2 YOSUKE MIZUNO,2KENTARO NAKAMURA,2 CÁTIA LEITÃO,3 MARIA FÁTIMA DOMINGUES,3 NÉLIA ALBERTO,3 PAULO ANTUNES,3,4PAULO ANDRÉ,5 CARLOS MARQUES,3 AND MARIA JOSÉ PONTES1
1Telecommunications Laboratory (LABTEL), Electrical Engineering Department, Federal University of Espı́rito Santo, Fernando Ferrari avenue,29075-910, Vitória-ES, Brazil2Institute of Innovative Research, Tokyo Institute of Technology, Japan
3
Instituto de Telecomunicações, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
4Physics Department & I3N, Universidade de Aveiro, 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*Corresponding author: arnaldo.leal@aluno.ufes.br
Received 11 April 2018; accepted 23 April 2018; posted 26 April 2018 (Doc. ID 327976); published 21 May 2018

This Letter demonstrates the application of polymer opticalfibers (POFs) damaged by the fiber fuse effect to curvaturesensing and dynamic angular monitoring. The curvaturesensing performance using the fused-POF is compared toPOF without the fuse effect. Both POFs are submitted
to angles of up to 90 deg in flexion/extension cycles withangular velocities ranging from 0.48 rad/s to 5.61 rad/s.
The fused POF is found to show higher performance withrespect to sensitivity, correlation coefficient with linear regression, and hysteresis. For instance, at the angular velocityof 0.48 rad/s, the fused POF shows >3 times higher sensitivity and significantly lower hysteresis than those of thenon-fused POF. In addition, the fused POFs have lower
cross-sensitivity and hysteresis variations on the tests withdifferent angular velocities. These results indicate that thefused POFs are potential candidates to develop curvature
sensors with various advantages over non-fused POFs,
for applications such as gait analysis and wearable
robotics. © 2018 Optical Society of America
OCIS codes: (160.5470) Polymers; (130.5460) Polymer waveguides;(280.4788) Optical sensing and sensors; (060.2370) Fiber optics
sensors.
https://doi.org/10.1364/OL.43.002539

Optical fiber sensors have been used to measure various physical parameters, such as temperature [1], strain [2], pressure [3],refractive index [4], and curvature [5], among others. In addition to silica optical fibers, polymer optical fibers (POFs) havealso been used for sensing purposes. Although POFs generally
suffer from higher optical propagation losses, they have someadvantages over silica fibers, which include a non-brittle nature,higher strain limits, flexibility in bending, and fracture toughness [6]. By exploiting these advantages, numerous kinds of0146-9592/18/112539-04 Journal © 2018 Optical Society of America

POF sensors have been developed for the past several decades.
They are, for instance, based on short and long period gratings[7,8], interferometry [9], Brillouin scattering [10], and intensity variations [11].
A catastrophic fiber fuse effect is a self-destruction process ofan optical fiber [12], where high-power light causes an opticaldischarge under certain conditions, such as tight bending or
damaged connection. POFs are generally composed of variousmaterials, such as polymethyl methacrylate (PMMA), polycarbonate, Zeonex, thermoplastic olefin polymer of amorphousstructure, and cyclic transparent optical polymer (CYTOP),
where these materials are further discussed in [13]. From allthese types of POFs, the fiber fuse effect has been observed onlyin the CYTOP-based POF [14,15]. The POF fuse was initiatedwith the amplified signal from a laser diode at 1546 nm to200 mW using an erbium-doped fiber amplifier. One extremityof the POF was connected to a silica single-mode fiber, whereas
the other end was covered with 0.5-μm alumina powder for thefuse ignition [14].
In silica glass fibers, periodical voids are created inside thecore after the fuse effect [12]. Although many sensing applications of these voids in fused silica fibers have been reported[16,17], fused POFs are yet to be further explored for sensing
applications. Unlike the case of the silica fiber fuse, light canstill propagate through the fused POFs, but with a high attenuation of ∼1.4 dB∕cm [15]. In addition, the mechanical characterization of the fused POFs [18] indicates that they havesome advantages over the same type of non-fused POFs for elastic deformation sensing applications (strain, curvature, etc.),
especially when the deformation is dynamically applied.
In this work, we report, for the first time, to the best of ourknowledge, the development of a curvature sensor using a fused
POF. This sensor is based on the light power attenuation when
the POF is under curvature. In this sensor, one of the mechanisms for light power attenuation is the variations of the refractive index induced by the increased stress, via the stress-optic

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Vol. 43, No. 11 / 1 June 2018 / Optics Letterseffect (changes in the refractive index with increasing stress) [11].
Therefore, the POF material properties play a major role in thesensor performance. In order to compare the performances (suchas sensitivity, hysteresis, and correlation coefficient with linearregression) of the fused-POF-based sensor with the non-fusedones, curvature tests with different angular ranges and velocities
were conducted similarly in both POFs.
Figure 1 schematically shows the experimental setup forevaluating the curvature sensing performance of fused andnon-fused POF samples. The sample lengths were both set to5 cm, as the propagation loss of the fused POF is relatively high.
The light source was a low-cost laser with a central wavelength at
650 nm with 5 mW power, where it is also possible to employ
light emitting diodes (LEDs) such as the one presented in [19].
The power of the POF-transmitted light was converted into anelectrical signal using a photodiode (IF-D91, Industrial FiberOptics, USA) with a trans-impedance amplifier. The acquisition
was made at a sampling rate of 200 Hz using a data-acquisitionboard (USB-6008, National Instruments, USA). The angle andthe angular velocity were controlled using a direct current (DC)motor, and the reference angle was measured using a potentiometer-based goniometer with angle and angular velocity uncertainties of ∼0.1 deg and ∼0.01 rad∕s, respectively.
We performed annealing of the fused and non-fused POF
samples before their installation in the setup. Annealing ofPOFs (thermal treatment process) is an effective method tosuppress the measurement hysteresis [20] and increase theirsensitivity to mechanical parameters [21]. In the annealing process, POFs are generally kept at a temperature that is close to,but lower than, their glass transition temperature for several
hours [20]. In this experiment, both POF samples were placed
inside a thermostatic chamber (Ethik Technology, Brazil) at
95°C for 48 h (note that the glass transition temperature ofCYTOP-based POFs is higher than 100°C). In addition, inorder to increase the measurement sensitivity and linearity,
which is defined as the determination coefficient with linear
regression (R 2 ), lateral sections were machined in both thePOF samples through abrasive removal of material to create
sensitive zones with a length of ∼14 mm and a depth of∼0.1 mm [5].
After the sample preparation, curvature tests were conducted in the angular range from 0 deg to 90 deg. To evaluate
the sensitivity, R 2 , and hysteresis, the angular velocity was kept

Fig. 1. Experimental setup for evaluating the curvature sensingcharacteristics of fused and non-fused POF samples. The inset is a
microscopy photograph of the fused POF.

Letterconstant. Subsequently, additional tests were performed toevaluate the sensitivity and hysteresis. The angular interval
was the same, but different angular velocities, ranging from
0.48 rad/s up to 5.61 rad/s, were applied.
First, for fused and non-fused POF samples, the transmitted
powers were plotted as a function of angles from 0 deg to
90 deg at a constant angular velocity of 0.48 rad/s (Fig. 2).
The vertical axis was normalized so that the optical power at
0 deg became 1. With the increasing angle, both POF samples
showed optical power attenuation, but the angle sensitivity of
the fused POF was 0.0067 deg−1 , which was >3 times larger
than that of the non-fused POF (0.0019 deg−1 ). In addition,
the R 2 between the measured data and its linear regression for
the fused POF was 0.999, which was much closer to 1 (perfectly linear) than that of the non-fused POF (0.995). The
higher sensitivity of the fused POF may be attributed to its
higher optical propagation losses. Also, the higher R 2 of thefused POF may be originated from its highly linear relationshipbetween stress and strain, when compared with the non-fused
POF [18].
Subsequently, in order to evaluate the hysteresis, both POF
samples were submitted to flexion and extension cycles, at anangular velocity of 0.48 rad/s, in order to obtain the sensor response with the lowest influence of the angular velocity. The
measurements were repeated for three cycles, to show the sensor
repeatability, and the averaged values were used. The results for
non-fused and fused POFs are shown in Figs. 3(a) and 3(b), respectively. The hysteresis (defined as the highest normalized distance between the flexion and extension curves [22]) of the fused
POF was 0.01% (within the detectable minimal limit), which
was much lower than that of the non-fused (0.21%). Note that
this value is two orders of magnitude lower than that reported for
PMMA-based POF at this angular velocity (∼1.2%) [5]. The
reason for this difference may be related to the higher R 2 ofthe stress–strain curve of the fused POF [18]. Another reason
may be the fact that the fused POF shows lower Young’s modulus dependence with the frequency variations than the non-fused
POF [18].

Fig. 2. Angle dependencies of the normalized transmitted power for
fused (red) and non-fused (blue) POFs at a constant angular velocity of
0.48 rad/s. The points are the measured data, and the lines are thelinear fits.

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Fig. 4. Normalized angle sensitivities plotted as functions of angularvelocity for non-fused (blue) and fused (red) POFs.

Fig. 3. Normalized transmitted power as a function of the angle forflexion/extension (blue/red) cycles for the: (a) non-fused and (b) fused
POFs. The angular velocity was 0.48 rad/s. The points are the measured data, and the lines are the linear fits.

Next, the influence of flexion/extension cycle frequency on
the angle sensitivity of the POF samples were analyzed bychanging the angular velocities (0.48 rad/s, 0.55 rad/s,
1.05 rad/s, 4.55 rad/s, and 5.61 rad/s made in the range between 0 deg and 90 deg, whereas 2.55 rad/s and 3.35 rad/s
performed in the range of 0 deg to 50 deg due to experimentallimitations). Since the power-attenuation mechanism of thePOFs includes the stress-optic effect [11], the variations in
the Young’s modulus [18] can lead to stress variations, which
result in different attenuation characteristics. Hence, if thematerial properties of the POF change with the frequency
or, in this case, angular velocity, there may be a cross-sensitivityof the angle sensitivity with respect to the angular velocity.
Figure 4 shows the angle sensitivity dependencies on theangular velocity for fused and non-fused POFs. The verticalaxis was normalized so that the sensitivity (absolute value) became 1 at 0.48 rad/s, and the error bars represent the propagation of the experimental uncertainty due to the materials
employed in the setup. Such uncertainty is about 2% of themeasured values. The sensitivity of the non-fused POF clearly

increased with the increase of the angular velocity, whereas thesensitivity of the fused POF remained relatively unchanged anddid not show a significant variation. The reason for this behavior is the low variation of the fused POF Young’s modulus, with
the increasing strain-applying cycle frequency, when comparedwith the same POF without the fuse effect [18]. Such lower
variations in the Young’s modulus lead to lower variations of
the refractive index with increasing angular velocity, resulting
in the lower cross-sensitivity. Therefore, from the viewpoint ofa stable operation without the influence of the cross-sensitivity,
the fused POF is preferable to the non-fused POF (especiallyfor the applications at angular velocities higher than ∼1 rad∕s).
This feature is particularly advantageous in gait analysis applications, where the human knee develops a high variation of its
angular velocity (∼3.5 rad∕s [5]), which will lead to errors in
the angle measurement if the sensor presents a high angularvelocity cross-sensitivity. Similarly, lower limb exoskeletons
work in a range of angles that requires a sensor with lower
cross-sensitivity. Therefore, the fused POF application as curvature sensor for movement analysis and in wearable robotics
provides practical advantages when compared with the nonfused POFs.
Finally, the hysteresis dependencies on the angular velocity
were also investigated for the fused and non-fused POFs
(Fig. 5). Once again, the error bars represent the propagationof the experimental uncertainty (below 2% of the measuredvalues). With an increasing angular velocity, the hysteresis of
the non-fused POF clearly increased, whereas that of the fused
POF was negligibly low in the whole range of tested angularvelocities. Even the highest hysteresis obtained for the fused
POF was as low as 0.04% (when the angular velocity was0.55 rad/s). The lower variations in the hysteresis of the fused
POF can also be attributed to the lower differences in theYoung’s modulus. These experimental results indicate that
fused POFs have many advantages over standard non-fused
POFs, and as so, are a promising candidate for intensityvariation-based curvature sensing.
In summary, we characterized the fused and non-fused POFs
as intensity-variation-based curvature sensors. The flexion/extension cycle tests revealed that the fused POF exhibited higher

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Vol. 43, No. 11 / 1 June 2018 / Optics Letters

Letter
REFERENCES

Fig. 5. Hysteresis dependencies on angular velocity for non-fused(blue) and fused (red) POFs.

sensing performance in all the measured parameters, including
higher angle sensitivity, higher R 2, and lower hysteresis thanthose of the non-fused POF. In addition, measurements at different angular velocities showed that the fused POF has lower crosssensitivity and lower hysteresis than those of the non-fused POF.
It is also worth mentioning that multiple curvature sensors canbe used if additional photodetectors are employed or by using
multiplexing techniques. Thus, the presented results demonstrate that fused POFs will be of significant use in developing
low-cost curvature sensors with high sensitivity and stability.
Funding. Coordenação de Aperfeiçoamento de Pessoal deNível Superior (CAPES) (88887.095626/2015-01); Fundação
Estadual de Amparo à Pesquisa do Estado do Espírito Santo(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, UID/EEA/50008/2013); Fujikura Foundation Japan Association
for Chemical Innovation (JACI); JSPS KAKENHI (17H04930,17J07226); FCT, IT-LA (PREDICT scientific action).

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