4458

IEEE SENSORS JOURNAL, VOL. 19, NO. 12, JUNE 15, 2019

Potential of Discriminative Sensing of Strain andTemperature Using Perfluorinated Polymer FBG
Yosuke Mizuno , Senior Member, IEEE, Ryo Ishikawa, Heeyoung Lee ,Antreas Theodosiou , Student Member, IEEE, Kyriacos Kalli , Member, IEEE,
and Kentaro Nakamura , Member, IEEE

Abstract— Though the strain characteristics of fiber Bragg
gratings (FBGs) inscribed in perfluorinated graded-index (PFGI)
polymer optical fibers (POFs) have been reported, their temperature characteristics have not yet been detailed. In this paper,
we experimentally investigate the temperature dependence of the
Bragg wavelength of a PFGI-POF-FBG. With the increasing
temperature, each peak of the FBG-reflected spectrum shiftedto longer wavelength with different coefficients. The temperature
coefficient of one of the clearest peaks was 0.09 nm/◦ C, which waseight times larger than those of FBGs in silica single-mode fibers
and almost the same as those of FBGs in polymethyl methacrylate
POFs. A temperature-independent but strain-dependent peak
was also observed, which indicates the potential of discriminative
sensing of strain and temperature.
Index Terms— Polymer optical fibers, fiber Bragg gratings,
temperature sensing, strain sensing.

I. I NTRODUCTIONSENSING of a variety of physical, chemical, and biologicalparameters has been one of the major applications of
optical fibers [1], [2]. Configurations of fiber-optic sensors
are generally categorized into two: distributed sensors andsingle- (or multiple-) point sensors. The former include strain
and/or temperature sensors based on some nonlinear phenomena [3]–[5], which operate in a distributed manner at the costof relatively low sensitivity (for instance, it is difficult for such
distributed sensors to detect small strain of << 10 με). The
latter include fiber-grating-based sensors [6]–[13]. Although
the number of sensing points is limited, their sensitivity is
generally much higher than that of distributed sensors. Here,let us focus on fiber Bragg grating (FBG) sensors.
FBGs have been reported to have measurement capabilityof strain [8], [13], temperature [8], humidity [9], refractive
index [10], pressure [11], [12], and many others. Of all these

Manuscript received January 10, 2019; revised February 7, 2019; accepted
February 16, 2019. Date of publication February 21, 2019; date of current
version May 16, 2019. This work was supported in part by the Japan
Society for the Promotion of Science KAKENHI under Grant 17H04930 and
Grant 17J07226 and in part by the Fujikura Foundation. The associate editor
coordinating the review of this paper and approving it for publication was
Prof. Agostino Iadicicco. (Corresponding author: Yosuke Mizuno.)
Y. Mizuno, R. Ishikawa, H. Lee, and K. Nakamura are with the Instituteof Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503,
Japan (e-mail: ymizuno@sonic.pi.titech.ac.jp; rishikawa@sonic.pi.titech.ac.jp;
hylee@sonic.pi.titech.ac.jp; knakamur@sonic.pi.titech.ac.jp).
A. Theodosiou and K. Kalli are with the Photonics and Optical Sensors
Research Laboratory, Cyprus University of Technology, Limassol 3036,
Cyprus (e-mail: theodosiou.antreas@gmail.com; kyriacos.kalli@cut.ac.cy).
Digital Object Identifier 10.1109/JSEN.2019.2900464

parameters, strain and temperature are two of the most common measurands of FBG sensors. To date, an FBG inscribed ina standard silica single-mode fiber (SMF) has been reported
to have a strain sensitivity of 6.3 nm/% at ∼800 nm [14](corresponding to 12.2 nm/% at ∼1550 nm) and a temperaturesensitivity of ∼0.011 nm/◦ C at ∼1550 nm [6], [7]. Silica
SMFs, however, suffer from their fragility and easily break
if a ∼3% strain is applied. One method for tackling thisproblem is to use polymer optical fibers (POFs) [15], which
are so flexible that they do not break even if a large strainof tens of percent is applied [16]. Therefore, POF-FBGs havegathered a lot of attention to extend the measurable maximal
strain [17].
POFs have several types, of which the most widely used arepoly(methyl methacrylate) (PMMA)-POFs. FBGs inscribed in
PMMA-POFs have been reported to have a strain sensitivityof 7.1 nm/% at ∼800 nm [18] (corresponding to 13.8 nm/%at ∼1550 nm) and a temperature sensitivity of 0.088 nm/◦ C
at ∼1560 nm [19]; their potential strain dynamic range is
13% [20]. However, PMMA-POF-FBGs suffer from extremely
high propagation loss (>>100 dB/m) at telecom wavelength.
Therefore, it is not easy to employ high-performance but relatively inexpensive devices designed for telecom use, such as
amplified spontaneous emission (ASE) sources, to observe the
FBG-reflected spectra. To solve this issue, recently, FBGs havebeen inscribed not only in PMMA-POFs but also in perfluorinated graded-index (PFGI-) POFs, which have been developed
for short distance communication systems (relatively low
propagation loss (0.25 dB/m) even at 1550 nm) [21], [22].
As PFGI-POFs are commercially available only as
multimode fibers and not photosensitive at ultraviolet wavelength [22], it was difficult to inscribe FBGs in PFGI-POFs.
However, by using femtosecond laser irradiation, FBG inscription in PFGI-POFs has now turned out feasible [22]–[30].
PFGI-POF-FBGs have lower optical loss, and their Bragg
wavelengths at ∼1550 nm can be investigated using ASE
sources. In addition, by exploiting their core refractive index
close to that of water, they are sometimes beneficial forbio-sensing applications [23]. To date, their sensing characteristics of strain, pressure, bending, etc., have been well documented [12], [13], [22], [27], [31], [32], but no detailed reports
have been given to their temperature dependence. More specifically, the temperature dependence of the Bragg wavelengthof a PFGI-POF-FBG has been roughly reported [22]–[24],1558-1748 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

MIZUNO et al.: POTENTIAL OF DISCRIMINATIVE SENSING OF STRAIN AND TEMPERATURE

but more precise measurement of each Bragg wavelength ofmultiple spectral peaks, corresponding to different propagation
modes, has not yet been clarified.
Motivated by this situation, in this work, we precisely investigate the temperature dependence of the Bragg wavelength of
a PFGI-POF-FBG at 1550 nm. We show that, with increasing
temperature, all the spectral peaks shift to longer wavelengthwith different coefficients. One of the clearest peaks exhibitsa temperature coefficient of 0.09 nm/◦ C, which is 8 times thevalue of an FBGs in a silica SMF and almost identical tothat of a PMMA-POF-FBG. One of the other peaks is foundto be temperature-independent but strain-dependent, whichindicates that discriminative sensing of strain and temperature
is potentially feasible by using multiple spectral peaks of asingle PFGI-POF-FBG.

4459

Fig. 1. Experimental setup for measuring the temperature dependence of the
Bragg wavelength of the PFGI-POF-FBG.

II. P RINCIPLE
FBGs can be inscribed not only in SMFs but also in MMFs.
Suppose a GI-MMF with a refractive index profile expressed
as (0 < r < R): r g,(1)n (r ) = n 1 1 − 2R
and as (R ≤ r ):√n (r ) = n 1 1 − 2,(2)where n 1 is the maximal value of the core refractive index,
R is the core radius,  is the relative index difference between
core and cladding, and g is the parabolic profile parameter.
The approximate number of modes in the GI-MMF M is thengiven by [33]:


2
g
2πa
n1 
,
(3)
λ
g+2
where λ is the FBG reflection wavelength. By substituting the parameters of a standard PFGI-POF (n 1 = 1.347,
a = 25 um,  = ∼0.01, g = 2, λ = ∼1560 nm),the number of modes is roughly estimated to be 90 at thiswavelength. Without special control for exciting only thefundamental mode [34], light injected into this PFGI-POF
excites some of these modes (especially, lower-order modes)
and generates multiple peaks at slightly different wavelengths
in the FBG-reflected spectrum.
When an PFGI-POF-FBG is strained or heated, each
spectral peak shifts with its own dependence coefficient.
At 1550 nm, a typical strain-dependence coefficient is∼14 nm/% [22], [28], which is ∼1.1 times the value of an
FBG in a silica SMF [6]. A temperature dependence coefficient is reported to be ∼0.015 nm/◦C [22] or 0.028 nm/◦ C[23], [24]. However, here we should note that these values
were obtained using some demodulation techniques [28], [35];namely, these values were kinds of averaged values of all
the peaks in the spectrum. In the case of strain, each peakhas been shown to exhibit its own dependence coefficient,ranging from 12.6 nm/% to 14.3 nm/% [35]. In contrast, such a
detailed measurement has not been performed for temperaturedependence.
M=

Fig. 2. (a) Measured spectrum of the FBG-reflected light. (b) Magnified
view of the red-circled part in (a), around the FBG-induced peaks.

III. FBG I NSCRIPTION AND M EASUREMENT S ETUPAn FBG was inscribed in a 1.2-m-long PFGI-POF using a
femotosecond laser irradiation method [29], [34]. The lengthof the FBG was 2 mm. The PFGI-POF (GigaPOF−50SR,
Chromis Fiberoptics) was composed of three layers: core(diameter: 50 μm; refractive index: ∼1.35), cladding(diameter: 70 μm, refractive index: ∼1.34), and overcladding(diameter: 490 μm). The core and cladding layer were
doped and undoped amorphous fluoropolymer (CYTOP®),
respectively, and the overcladding layer was polycarbonate.
The optical propagation loss was ∼0.25 dB/m at 1550 nm,
and the numerical aperture was ∼0.19. The FBG was
inscribed directly, without removal of the overcladding layer,using a femtosecond laser system (High Q femtoREGEN,
High Q Laser) at 517 nm. The pulse duration was 220 fs,
the repetition rate was 1 kHz, and the pulse energy was∼100 nJ [22], [29]. The POF was mounted on an air bearing
translation system (Aerotech), which can achieve two-axis
motion with high resolution and high accuracy. A longworking-distance objective (x50) was mounted on a thirdaxis, and the laser beam was irradiated focusedly into thefiber. Accurate synchronization of the laser pulse repetition
rate and the stage motion enabled plane-by-plane grating
inscription with a desired length and an index-modulation
value [22], [26], [29], [34].
Figure 1(a) shows an experimental setup for measuring
the temperature dependence of the FBG-reflected spectrum.
All the optical paths except the POF were silica SMFs. The
output from an ASE source was injected into the POF, and the4460IEEE SENSORS JOURNAL, VOL. 19, NO. 12, JUNE 15, 2019

Fig. 3. Temperature dependence of the POF-FBG-reflected spectrum. (a) Spectral change when temperature increased from 27.5◦ C to 60.1◦ C. (b) Spectralchange when temperature increased from 27.5◦ C to 33.2◦ C. (c) Central wavelengths of the three spectral peaks plotted as functions of temperature. The error
bars were calculated using the standard deviations of 20 measurements. The dashed lines are linear fits.

FBG-reflected light was guided via an optical circulator to an
optical spectrum analyzer (AQ6370, Yokogawa Electric Corp).
We used a reflectometric (not transmissive) configuration to
reduce the influence of modal interference [36], [37]. The
POF was placed in a thermostatic chamber. As the temperature accuracy of the chamber was not sufficiently high, athermocouple was placed near the POF-FBG to calibrate thetemperature. One end of the POF was connected to a silica
SMF outside the chamber using a butt-coupling technique [38],and the other end was cut with an 8◦ angle to suppress theFresnel reflection. Note that the following experimental results
were highly repeatable as long as the alignment of the fibers
in the setup was maintained.
IV. E XPERIMENTAL R ESULTSFirst, we measured the optical spectrum of the FBGreflected light, as shown in Fig. 2(a). Though the spectrum of
the light Fresnel-reflected mainly at the SMF-to-POF boundary was overlapped, it was clearly observed at ∼1560 nm.
Figure 2(b) shows the magnified view of the FBG-reflected
spectrum around its peaks. Multiple peaks and dips, causedby the multimode nature of the POF [39], were observed in
the spectrum.
Subsequently, we measured the FBG-reflected spectra whilechanging the temperature from 27.5◦C to 60.1◦C, as shownin Fig. 3(a). At 27.5◦C, three peaks were observed at 1557.7,
1558.7, and 1560.4 nm in this range. With increasing temperature, all these peaks moved to longer wavelength but withdifferent dependence coefficients. As some of the peaks wereburied by another peak, it was not easy to trace particularpeaks in this wide temperature range. The spectral dependence
on temperature in the narrow range from 27.5◦C to 33.2◦Cis shown in Fig. 3(b), The temperature dependence of thepeak at 1558.7 nm was almost linear, but at ∼33◦C, it
became non-observable because of the overlap with the peakat 1560.4 nm.
Figure 3(c) shows the temperature dependence of the central
wavelengths of the three peaks. The temperature-dependencecoefficients of the peaks at 1557.7 and 1558.7 were
0.062 nm/◦ C and 0.090 nm/◦ C (coefficients of determination
R 2 : 0.995 and 0.993), respectively. The latter is ∼8 times
as large as that of a silica SMF-FBG (0.011 nm/◦C) [6]

Fig. 4. (a) Strain dependence of the POF-FBG-reflected spectrum. (b) Central
wavelengths of the two peaks plotted as functions of temperature. The dashedlines are linear fits.

and almost the same as that of a PMMA-POF-FBG(0.088 nm/◦ C) [19]. In contrast, the peak at 1560.4 nm
was almost constant while the temperature increased from27.5◦C to ∼50◦ C. When the temperature increased to ∼60◦ C,
the peak exhibited a clear upshift, and the linear fit in the wide
temperature gave a dependence coefficient of 0.0076 nm/◦ C(R 2 = 0.777), which is much smaller than those of the otherpeaks.
Finally, as shown in Fig. 4(a), we measured the strain dependence of the FBG-reflected spectrum including the two peaks,
one of which is dependent on temperature (at 1558.7 nm)
and the other is not largely dependent on temperature (at1560.4 nm). Strains from 0% to 0.5% were applied to a
10-cm-long section including the FBG. With increasing strain,
both of the peaks shifted to longer wavelength. The strain
dependence of the central wavelengths of the two peaks isshown in Fig. 4(b). Another peak sometimes appeared near thetwo peaks, but we traced the initial two peaks by continuous
observation. The dependence was almost linear for both peaks,
and the coefficients were almost the same (11.2 nm/% at1558.7 nm and 11.3 nm/% at 1560.4 nm; these values are
smaller than previously reported value [22], [28], [35], but
it is not unnatural considering that the previous report estimated the Bragg wavelength using some unique demodulation
methods [28], [35]; their R 2 values were both ∼0.998).
These results indicate that highly accurate discrimination ofstrain and temperature is potentially feasible by simultaneously
employing the multiple peaks (i.e., the temperature-dependentand strain-dependent peak and the temperature-independent

MIZUNO et al.: POTENTIAL OF DISCRIMINATIVE SENSING OF STRAIN AND TEMPERATURE

but strain-dependent peak) of the POF-FBG-reflected spectrum. Note that the spectral peak at 1557.7 nm had a strain
sensitivity of 12.2 nm/%, which is slightly higher than thoseof the other two peaks, but it suffered from a relatively low R 2
value of 0.971. This indicates that there is a trade-off relationship between the temperature range and the measurement precision. One advantage of this method is its simple calculation
procedure (i.e., strain information can be directly obtained),in which we do not need to use matrix-based discrimination ofstrain and temperature [40]–[43]. The achievable temperature
range is limited by the overlap of two spectral peaks, but it
can be extended to up to ∼50◦ C, if we permit a lowered
measurement precision.
V. C ONCLUSION
The temperature dependence of the POF-FBG-reflected
spectrum was experimentally measured. As the temperature
increased, all the spectral peaks shifted to longer wavelengthwith different coefficients. The temperature coefficient of oneof the clearest peaks was 0.09 nm/◦ C (within a limited
temperature range below ∼33◦C), which was 8 times the valueof a silica SMF-FBG and almost identical to that of a PMMAPOF-FBG. In contrast, one of the peaks showed almost no
shift while the temperature increased to ∼50◦ C. Both thetemperature-dependent and independent peaks showed identical dependence on strain, which indicates that highly accurate
discriminative sensing of strain and temperature may be possible by using both the peaks simultaneously. We believe that
our finding will greatly stimulate the FBG-sensing community.
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Yosuke Mizuno (M’14–SM’17) was born in Hyogo,
Japan, in 1982. He received the B.E., M.E., andDr. Eng. degrees in electronic engineering from the
University of Tokyo, Japan, in 2005, 2007, and2010, respectively. From 2007 to 2010, he wasengaged in the Brillouin optical correlation-domainreflectometry at the University of Tokyo, for his
Dr. Eng. degree.
From 2010 to 2012, he was with the Tokyo
Institute of Technology, Japan, where he worked
on polymer optics. In 2011, he stayed at the BAM
Federal Institute for Materials Research and Testing, Germany, as a Visiting
Research Associate. Since 2012, he has been an Assistant Professor with thePrecision and Intelligence Laboratory (presently the Institute of Innovative
Research), Tokyo Institute of Technology, where he has been active infiber-optic sensing, polymer optics, and ultrasonics.
Dr. Mizuno is a member of the IEEE Photonics Society, the Japan Society ofApplied Physics, the Optical Society of Japan, and the Institute of Electronics,
Information, and Communication Engineers, Japan. He was a recipient of
the Konica Minolta Imaging Science Award 2014, the ESPEC Prize for
the Encouragement of Environmental Studies 2016, the Funai Information
Technology Award 2017, and the Optics Design Award 2018. From 2007 to
2010, he was a Research Fellow (DC1) of the Japan Society for the Promotionof Science, where he was also a Research Fellow (PD) from 2010 to 2012.

Ryo Ishikawa was born in Kanagawa, Japan,
in 1994. He received the B.E. degree in electrical andelectronic engineering from the National Institute ofTechnology, Tokyo College, Japan, in 2017. He is
pursuing the M.E. degree in electrical and electronicengineering (optical fiber sensing) with the Tokyo
Institute of Technology, Japan. His research interests include fiber Bragg gratings and polymer fiber
sensing.
Mr. Ishikawa is a Student Member of the Japan
Society of Applied Physics and the Institute ofElectronics, Information and Communication Engineers (IEICE), Japan.
He received the Outstanding Student Award 2018 from the Optical Fiber
Technologies Conference and IEICE.

IEEE SENSORS JOURNAL, VOL. 19, NO. 12, JUNE 15, 2019

Heeyoung Lee was born in Seoul, South Korea,
in 1990. She received the B.E. degree in mechanicalengineering from Kyungpook National University,
South Korea, in 2014, and the M.E. degree in
information processing from the Tokyo Institute ofTechnology, Japan, in 2017, where she is pursuing
the Dr. Eng. degree in electrical and electronic engineering (fiber-optic sensors). Her research interests
include fiber-optic sensing and polymer optics.
Ms. Lee is a Student Member of the Japan Societyof Applied Physics and the Optical Society of Japan.
She was a recipient of the Optics Design Award in 2018. She received
the Best Student Paper Award from the 26th International Conference on
Plastic Optical Fibers in 2017, the Student Award from the 22nd International
Microoptics Conference in 2017, the Outstanding Student Award 2017 fromthe Optical Fiber Technologies Conference and the Institute of Electronics,
Information and Communication Engineers, Japan, and the Student Paper
Award from the 7th Asia–Pacific Optical Sensors Conference in 2018. Since
2017, she has been a Research Fellow (DC1) of the Japan Society for the
Promotion of Science.

Antreas Theodosiou received the B.Sc. degree inelectrical engineering and the M.Sc. degree in electrical engineering, with specialization in communication systems from Frederick University, Cyprus,in 2011 and 2013, respectively, and the Ph.D. degree,with specialization in photonics and optical sensing,from the Cyprus University of Technology in 2018.
His current research interests include the development of optical fiber sensors using femtosecond
lasers.

Kyriacos Kalli (M’00) received the B.Sc. (Hons.)
degree in theoretical physics and the Ph.D. degreein physics from the University of Kent, U.K.,
in 1988 and 1992, respectively, where he studied
linear and non-linear phenomena in optical fibers.
In 2008, he joined the Cyprus University of Technology, where he is an Associate Professor with
the Department of Electrical Engineering, Computer
Engineering and Informatics and the Director of the
Photonics and Optical Sensors Research Laboratory.
Dr. Kalli has more than 230 journal and conference
publications. He is a co-author of Fiber Bragg Gratings: Fundamentals andApplications in Telecommunications and Sensing and many book contributions. His research interests are in Bragg grating and optical fiber sensors,
femtosecond laser micro-fabrication of photonic devices, microfluidics, andplasmonics. He is a Chartered Physicist and a Senior Member of OSA.
He is currently on the Board of the Directors of the Cyprus Organizationfor Standardization.

Kentaro Nakamura (M’00) was born in Tokyo,
Japan, in 1963. He received the B.E., M.E., andDr. Eng. degrees from the Tokyo Institute of Technology, Japan, in 1987, 1989, and 1992, respectively.
Since 2010, he has been a Professor with thePrecision and Intelligence Laboratory (presently theInstitute of Innovative Research), Tokyo Instituteof Technology. His research field is the applications of ultrasonic waves, measurement of vibrationand sound using optical methods, and fiber-optic
sensing.
Dr. Nakamura is a member of the Acoustical Society of Japan (ASJ),
the Japan Society of Applied Physics (JSAP), the Institute of Electronics,
Information and Communication Engineers (IEICE), and the Institute ofElectrical Engineers of Japan. He was a recipient of the Optics Design Award2018. He received the Awaya Kiyoshi Award for encouragement of researchfrom ASJ in 1996, the Best Paper Award from IEICE in 1998, the Best Paper
Award from the Symposium on Ultrasonic Electronics in 2007 and 2011, andthe Japanese Journal of Applied Physics Editorial Contribution Award from
JSAP in 2007.