BRIEF NOTEJapanese Journal of Applied Physics 57, 038002 (2018)https://doi.org/10.7567/JJAP.57.038002

Strain dependence of perﬂuorinated polymer optical ﬁber Bragg grating measured
at different wavelengths
Ryo Ishikawa1, Heeyoung Lee1, Amédée Lacraz2, Antreas Theodosiou2,
Kyriacos Kalli2, Yosuke Mizuno1*, and Kentaro Nakamura1
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Nanophotonics Research Laboratory, Cyprus University of Technology, Limassol 3036, Cyprus

1
2*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received November 14, 2017; accepted January 9, 2018; published online February 14, 2018
We measure the strain dependence of multiple Bragg wavelengths (corresponding to different diffraction orders) of a ﬁber Bragg grating (FBG)
inscribed in a perﬂuorinated graded-index polymer optical ﬁber (PFGI-POF) in the wavelength range up to 1550 nm. On the basis of this result, weshow that the fractional sensitivity, which has been conventionally used as a wavelength-independent index for fair comparison of the FBG
performance measured at different wavelengths, is dependent on wavelength in this range. The reason for this behavior seems to originate fromthe non-negligible wavelength dependence of refractive index and its strain-dependence coefﬁcient. Using the wavelength dependence of therefractive index already reported for bulk, we deduce the wavelength dependence of the strain coefﬁcient of the refractive index. This information
will be a useful archive in implementing PFGI-POF-based strain sensors based on not only FBGs but also Brillouin scattering in the future.
© 2018 The Japan Society of Applied Physics

For the past several decades, ﬁber-optic sensing technology
has been extensively studied to measure various physicalparameters.1–26) In general, optical ﬁber sensors are categorized into two conﬁgurations. One is distributed sensing,1–14)
which includes strain and temperature sensing based on
Brillouin3–12) or Raman scattering.13,14) Its distributed measurement capability is an attractive feature, but its sensitivity issometimes insuﬃcient. The other conﬁguration is singlepoint or multiplexed sensing,15–26) which includes sensing
based on ﬁber Bragg gratings (FBGs).17–26) Although it
suﬀers from dead zones where measurements cannot be
performed, its sensitivity is generally higher than that of
distributed sensing, rendering FBG sensing more suitable forsome applications, such as high-frequency dynamic measurements. Here, we focus on FBG-based sensing.
FBGs have been used to measure a variety of parameters,such as strain,19,20) temperature,19) humidity,21) pressure,22)
refractive index,23) and many others.24–26) Among them,strain sensing is one of the most common purposes of FBG
sensors, and numerous reports have been provided especiallyon the FBGs inscribed in standard silica single-mode ﬁbers(SMFs).19) For instance, a strain sensitivity of 6.3 nm=% has
been reported at ∼800 nm.20) However, silica SMFs are
relatively fragile and cannot withstand strains larger than∼3%. One solution to this problem is to employ polymer
optical ﬁbers (POFs),27) which have extremely high ﬂexibility
and can sometimes withstand strains of 50% or larger.28)
Therefore, FBGs inscribed in POFs (POF-FBGs) haveattracted considerable attention to enhance the strain dynamic
range of the sensors.29)
The most widely used POFs are poly(methyl methacrylate)(PMMA-) POFs,27) and PMMA-POF-FBGs have been reported to have a strain sensitivity of 7.1 nm=% at ∼800 nm30)
and a strain dynamic range of 13% (potential).31) However, theoptical propagation loss of PMMA-POFs is lowest at visible
wavelengths (∼600 nm) and is extremely high (≫100 dB=m)
at 1550 nm. Consequently, many high-performance yet relatively inexpensive devices designed to be optimal at telecom
wavelength, including ampliﬁed spontaneous emission sources, cannot be used to interrogate the Bragg wavelengths. To
tackle this issue, the use of FBGs inscribed in perﬂuorinatedgraded-index (PFGI-) POFs32) has recently become one of the
hot topics in the POF-FBG sensing community.
PFGI-POFs are designed for short distance communication
systems,32) with their propagation loss relatively low (0.25dB=m) even at 1550 nm. In addition, the refractive index of
their core is close to that of water; this property is potentially
useful in some biosensing applications.33) However, theinscription of FBGs in PFGI-POFs was extremely diﬃcult,because they are commercially available only as multimodeﬁbers and are sometimes not photosensitive in the ultraviolet
wavelength region.34) Only recently, some inscription techniques for PFGI-POF-FBGs have been developed,34,35) including a femtosecond-laser-based method.34) To date, theirstrain, temperature, and pressure sensing performances havebeen investigated and found to be quite diﬀerent from those ofsilica-SMF-FBGs and PMMA-POF-FBGs.36–38)
In addition to the fundamental characterization at 1550
nm,36) the strain sensing properties of PFGI-POF-FBGs havebeen investigated in a wide range of wavelengths from 517 to
883 nm.36) The Bragg wavelengths of six peaks (corresponding to the diﬀraction orders39) from 7th to 12th) were shown tohave diﬀerent strain sensitivities, which were almost linearly
dependent on the wavelength. This result is natural if weneglect the dependence of the refractive index (and its straindependence coeﬃcient) on wavelength.39) Therefore, in orderto fairly compare the strain sensitivities measured at diﬀerent
wavelengths, researchers have deﬁned a “fractional sensitivity” as the value of the strain sensitivity divided by the wavelength.38,40–44) For instance, the strain sensitivity of 8.12
nm=% at 883 nm and that of 4.82 nm=% at 517 nm (corresponding to the fractional sensitivities of 9.20 × 10−3=% and9.32 × 10−3=%, respectively) were regarded as almost thesame.36) However, it has not been guaranteed that the fractional sensitivity is constantly used as an indicator for a faircomparison of the strain sensitivities measured at diﬀerent
wavelengths of up to 1550 nm.
In this work, using a PFGI-POF-FBG, we measure thestrain dependence of four Bragg wavelengths (corresponding
to the orders from 4th to 7th) up to 1550 nm, and show that thefractional sensitivity is not constant but dependent on wavelength in this range. The wavelength dependence of the frac-

038002-1© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 038002 (2018)

R. Ishikawa et al.

tional sensitivity may originate from the non-negligible dependence of refractive index and its strain-dependence coeﬃcient on wavelength. Using the refractive index dependence on
wavelength already reported for bulk perﬂuorinated polymers,
we then deduce the wavelength dependence of the strain
coeﬃcient of the refractive index. This result will be of crucial
importance in fairly evaluating the PFGI-POF-FBG performance at diﬀerent wavelengths and in developing PFGI-POFbased sensors working at diﬀerent wavelengths in the future.
We employed a PFGI-POF (Chromis Fiberoptics GigaPOF62SR)32) with a length of 1.4 m. This POF has 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 are composed of
doped and undoped amorphous ﬂuoropolymer (CYTOP®),
respectively, while the overcladding is composed of polycarbonate. The optical propagation loss is ∼0.25 dB=m at 1550
nm. An FBG was inscribed in the middle of this PFGI-POF
directly, without removing the overcladding, with a femtosecond laser system (High Q Laser High Q femtoREGEN) at
517 nm.34,45) The pulse duration was 220 fs, the repetition
rate was 1 kHz, and the pulse energy was close to 100 nJ.
The PFGI-POF was placed on a two-axis translation system(Aerotech) with high resolution and high accuracy. Using along-working-distance objective (×50) mounted on the thirdaxis, the laser beam was focused into the PFGI-POF. By synchronizing the laser pulse repetition rate and the stage motion
accurately, plane-by-plane gratings were inscribed with adesired length and an index-modulation value.34,45,46) In thisexperiment, the FBG-inscribed length was 2 mm. The experimental setup is shown in Fig. S1 in the online supplementary
data at http://stacks.iop.org/JJAP/57/038002/mmedia.
First, we measured the wide-wavelength-range optical
spectrum of the FBG-reﬂected light. Two partial spectra(885–1090 and 1100–1680 nm) measured using diﬀerent
circulators are shown in Fig. 1. The whole spectra were
somewhat distorted because of the non-ﬂat output spectrum of
the light source (note that the relatively broad peak at ∼1065
nm was due to the seed of the supercontinuum generation) andof the transmission band of the circulators. The FBG-reﬂected
spectra were, however, still clearly observed at 895, 1043,
1248, and 1560 nm. These peaks correspond to the diﬀraction
orders of 7th, 6th, 5th, and 4th, respectively. The peaks of theother orders (for instance, the peak of the 8th order is expected
to appear at 781 nm) were not observed in this measurement
because of the limited bandwidths of the circulators used in
the experiment (although high-order peaks are potentiallymeasureable in the visible wavelength spectrum).36)
The magniﬁed view of the FBG-reﬂected spectrum around1043 nm is shown in Fig. S2(a) in the online supplementary
data at http://stacks.iop.org/JJAP/57/038002/mmedia. Multiple peaks and dips, caused by the multimode nature of the
POF,47) were observed in the spectrum. We selected the clear
and highest peak (which we call the main peak hereafter) at
1043.03 nm and deﬁned its central wavelength as the Bragg
wavelength of this diﬀraction order (6th). We then measured
the strain dependence of the main peak as shown in Fig. S2(b)in the online supplementary data at http://stacks.iop.org/JJAP/
57/038002/mmedia, where the other peaks were not shown.
As the applied strain was increased, the main peak shifted tolonger wavelengths. The change in the spectral shape (peakFig. 1. (Color online) Wide-wavelength-range optical spectrum of the
FBG-reﬂected light. Two partial spectra (885–1090 and 1100–1680 nm) were
measured using diﬀerent circulators. Four FBG-reﬂected spectral peaks that
correspond to the diﬀraction orders from 4th to 7th were clearly observed.

(a)

(b)

Fig. 2. (Color online) (a) Strain sensitivity of the PFGI-POF-FBG plotted
as a function of wavelength. The solid line is a linear ﬁt. The error bars
were calculated using the maximal and minimal slopes of Fig. S2(c) in the
online supplementary data at http://stacks.iop.org/JJAP/57/038002/mmedia.
(b) Wavelength dependence of the strain coeﬃcient of the refractive index.

power and linewidth) was probably caused by the multimodenature of POF.47) Figure S2(c) in the online supplementary
data at http://stacks.iop.org/JJAP/57/038002/mmedia shows
the Bragg wavelength dependence on strain. The Bragg
wavelength increased linearly with increasing strain with adependence coeﬃcient of 8.56 nm=%. Subsequently, the samemeasurement was performed for the FBG-reﬂected peaks atthree other wavelength regions (895, 1248, and 1560 nm). The
main peaks were located at 895.32, 1248.78, and 1560.65 nm,
and the obtained strain-dependence coeﬃcients of the Bragg
wavelengths were 6.52, 10.95, and 14.32 nm=%, respectively.
Figure 2(a) shows the strain sensitivity plotted as a function of wavelength. With increasing wavelength, the strain
sensitivity increased almost linearly with a dependence coeﬃcient of 0.0116=%. We subsequently calculated the fractional sensitivity by dividing the strain sensitivity with thewavelength, and its dependence on wavelength is shown in
Fig. S3 in the online supplementary data at http://stacks.iop.
org/JJAP/57/038002/mmedia. Conventionally, the fractional
sensitivity has been treated as a constant that does not depend
on wavelength, but in this experiment, it exhibited clear
dependence on wavelength (it slowly saturated towards
longer wavelengths). According to the theory, this result
seems to have been caused by two factors: (i) the wavelength
dependence of the refractive index, and (ii) the wavelength
dependence of the strain coeﬃcient of the refractive index.
The former (i) has already been studied in perﬂuorinatedpolymer bulk, and the dependence coeﬃcient is reported to

038002-2© 2018 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 57, 038002 (2018)

R. Ishikawa et al.

be 4.25 × 10−6=nm.48) Therefore, by using this value and our
result, the inﬂuence of the latter (ii) can be calculated. As
shown in Fig. 2(b), the strain coeﬃcient of the refractive
index in the PFGI-POF was found, in this wavelength range,
to depend on wavelength, saturating toward longer wavelength, and to constantly take a negative value (this isvalid considering that, when strain is applied, the density is
reduced, leading to a lower refractive index). As n becomes
smaller with increasing wavelength (in bulk), it is natural thatthe absolute value of ∂n=∂ε should also become smaller withincreasing wavelength. In addition, if we assume that ∂n=∂εdoes not saturate at longer wavelengths, it needs to change
from negative to positive at a certain wavelength, which
contradicts the fact that ∂n=∂ε constantly takes a negativevalue; thus the saturation behaviour is also natural.
This newly obtained data will be a useful archive for the
future development of PFGI-POF-based strain sensors basedon not only FBGs but also Brillouin scattering (and combinations of the two), because the Brillouin frequency shift is
proportional to the refractive index in the same way as the
Bragg wavelength.49) Note that the strain dependence of theBrillouin frequency shift in PFGI-POFs has been reported
only at the 1550 nm region,50,51) and that its wavelength
dependence is unknown as yet (the propagation loss of PFGIPOFs becomes approximately 10 times lower at 1000 nm
than at 1550 nm;32) this property will be beneﬁcial for thedevelopment of PFGI-POF-based long-measurement-range
Brillouin sensors working at nontelecom wavelengths).
In conclusion, the strain dependence of the four Bragg
wavelengths of an FBG inscribed in a PFGI-POF was investigated in the wavelength range up to 1550 nm, and the fractional sensitivity, which was conventionally regarded as awavelength-independent gauge for fair performance comparison, was shown to clearly depend on wavelength in thisrange. This result probably originated from the non-negligible dependence of refractive index and its strain-dependence coeﬃcient on wavelength. On the basis of the wavelength dependence of the refractive index already reported for
perﬂuorinated polymer bulk, the wavelength dependence ofthe strain coeﬃcient of the refractive index was calculated.
We believe that this information will be of great use not only
in fairly evaluating the PFGI-POF-FBG performance at
diﬀerent wavelengths but also in implementing PFGI-POFbased strain sensors exploiting the mechanism of Brillouinscattering, as well as FBGs, in the future.
Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 17H04930 and 17J07226, and by research grants from the Japan Gas
Association, the ESPEC Foundation for Global Environment Research andTechnology, the Association for Disaster Prevention Research, the Fujikura
Foundation, and the Japan Association for Chemical Innovation. Partial funding
was received from the People Programme (Marie Curie Actions) of the European
Union’s Seventh Framework Programme FP7=2007-2013=under REA grant
agreement No. 608382 (“TRIPOD”) and the Cyprus University of Technology.

1) R. Rathod, R. D. Pechstedt, D. A. Jackson, and D. J. Webb, Opt. Lett. 19,
593 (1994).
2) Y. Koyamada, M. Imahama, K. Kubota, and K. Hogari, J. Lightwave
Technol. 27, 1142 (2009).
3) A. Motil, A. Bergman, and M. Tur, Opt. Laser Technol. 78, 81 (2016).
4) A. Denisov, M. A. Soto, and L. Thévenaz, Light: Sci. Appl. 5, e16074(2016).
5) A. Minardo, R. Bernini, R. R. Lombera, J. Mirapeix, J. M. L. Higuera, and
L. Zeni, Opt. Express 24, 29994 (2016).

6) K. Hotate and T. Hasegawa, IEICE Trans. Electron. E83-C, 405 (2000).
7) Y. H. Kim, K. Lee, and K. Y. Song, Opt. Express 23, 33241 (2015).
8) E. Preter, D. Ba, Y. London, O. Shlomi, Y. Antman, and A. Zadok, Opt.
Express 24, 27253 (2016).
9) Y. Mizuno, W. Zou, Z. He, and K. Hotate, Opt. Express 16, 12148 (2008).
10) Y. Mizuno, N. Hayashi, H. Fukuda, K. Y. Song, and K. Nakamura, Light:
Sci. Appl. 5, e16184 (2016).
11) H. Takahashi, F. Ito, C. Kito, and K. Toge, Opt. Express 21, 6739 (2013).
12) L. Teng, H. Zhang, Y. Dong, D. Zhou, T. Jiang, W. Gao, Z. Lu, L. Chen,
and X. Bao, Opt. Lett. 41, 4413 (2016).
13) M. A. Farahani and T. Gogolla, J. Lightwave Technol. 17, 1379 (1999).
14) J. P. Dakin, D. J. Pratt, G. W. Bibby, and J. N. Ross, Electron. Lett. 21, 569(1985).
15) K. S. Kim, Y. Mizuno, M. Nakano, S. Onoda, and K. Nakamura, IEEE
Photonics Technol. Lett. 23, 1472 (2011).
16) Y. Mizuno, H. Lee, S. Shimada, Y. Matsumoto, Y. Tanaka, H. Nakamura,and K. Nakamura, IEICE Electron. Express 14, 20170962 (2017).
17) R. Kashyap, Fiber Bragg Gratings (Academic, San Diego, CA, 1999).
18) A. Othonos and K. Kalli, Fiber Bragg Gratings: Fundamentals andApplications in Telecommunications and Sensing (Artech House, Boston,MA, 1996).
19) F. Farahi, D. J. Webb, J. D. C. Jones, and D. A. Jackson, J. Lightwave
Technol. 8, 138 (1990).
20) Y.-J. Rao, Meas. Sci. Technol. 8, 355 (1997).
21) P. Kronenberg, P. K. Rastogi, P. Giaccari, and H. G. Limberger, Opt. Lett.
27, 1385 (2002).
22) M. G. Xu, L. Reekie, Y. T. Chow, and J. P. Dakin, Electron. Lett. 29, 398(1993).
23) A. Iadicicco, A. Cusano, A. Cutolo, R. Bernini, and M. Giordano, IEEE
Photonics Technol. Lett. 16, 1149 (2004).
24) D. C. Betz, G. Thursby, B. Culshaw, and W. J. Staszewski, Smart Mater.
Struct. 12, 122 (2003).
25) X. Sang, C. Yu, T. Mayteevarunyoo, K. Wang, Q. Zhang, and P. L. Chu,
Sens. Actuators B 120, 754 (2007).
26) K. Krebber, H. Henschel, and U. Weinand, Meas. Sci. Technol. 17, 1095
(2006).
27) M. G. Kuzyk, Polymer Fiber Optics: Materials, Physics, and Applications(CRC Press, Boca Raton, FL, 2006).
28) H. Ujihara, N. Hayashi, M. Tabaru, Y. Mizuno, and K. Nakamura, IEICEElectron. Express 11, 20140707 (2014).
29) D. J. Webb, Meas. Sci. Technol. 26, 092004 (2015).
30) I. P. Johnson, K. Kalli, and D. J. Webb, Electron. Lett. 46, 1217 (2010).
31) Z. Xiong, G. D. Peng, B. Wu, and P. L. Chu, IEEE Photonics Technol. Lett.
11, 352 (1999).
32) Y. Koike and M. Asai, NPG Asia Mater. 1, 22 (2009).
33) G. Zhou, C. F. J. Pun, H. Y. Tam, A. C. L. Wong, C. Lu, and P. K. A. Wai,IEEE Photonics Technol. Lett. 22, 106 (2010).
34) A. Lacraz, M. Polis, A. Theodosiou, C. Koutsides, and K. Kalli, IEEE
Photonics Technol. Lett. 27, 693 (2015).
35) M. Koerdt, S. Kibben, J. Hesselbach, C. Brauner, A. S. Herrmann, F.
Vollertsen, and L. Kroll, Procedia Technol. 15, 138 (2014).
36) P. Stajanca, A. Lacraz, K. Kalli, M. Schukar, and K. Krebber, Proc. SPIE
9899, 989911 (2016).
37) A. Theodosiou, M. Komodromos, and K. Kalli, J. Lightwave Technol. 35,
3956 (2017).
38) R. Ishikawa, H. Lee, A. Lacraz, A. Theodosiou, K. Kalli, Y. Mizuno, and K.
Nakamura, IEEE Photonics Technol. Lett. 29, 2167 (2017).
39) M. Royon, D. Piétroy, E. Marin, and A. Sailot, Tribology Int. 115, 417(2017).
40) S. E. Kanellopoulos, V. A. Handerek, and A. J. Rogers, Opt. Lett. 20, 333(1995).
41) G. W. Yoﬀe, P. A. Krug, F. Ouellette, and D. A. Thorncraft, Appl. Opt. 34,
6859 (1995).
42) K. O. Hill and G. Meltz, J. Lightwave Technol. 15, 1263 (1997).
43) B. Sutapun, M. T. Azar, and A. Kazemi, Sens. Actuators B 60, 27 (1999).
44) C. Fernández-Valdivielso, I. R. Matías, and F. J. Arregui, Sens. Actuators A
101, 107 (2002).
45) A. Theodosiou, A. Lacraz, M. Polis, K. Kalli, M. Tsangari, A. Stassis, andM. Komodromos, IEEE Photonics Technol. Lett. 28, 1509 (2016).
46) A. Lacraz, A. Theodosiou, and K. Kalli, Electron. Lett. 52, 1626 (2016).
47) C. Lu and Y. Cui, J. Lightwave Technol. 24, 598 (2006).
48) T. Ishigure, Y. Koike, and J. W. Fleming, J. Lightwave Technol. 18, 178(2000).
49) G. P. Agrawal, Nonlinear Fiber Optics (CRC Press, Boca Raton, FL, 2006).
50) Y. Mizuno and K. Nakamura, Appl. Phys. Lett. 97, 021103 (2010).
51) N. Hayashi, Y. Mizuno, and K. Nakamura, J. Lightwave Technol. 32, 3999
(2014).

038002-3© 2018 The Japan Society of Applied Physics

Supplementary Information

Strain dependence of perfluorinated polymer optical fiber
Bragg grating measured at different wavelengths
Ryo Ishikawa1, Heeyoung Lee1, Amédée Lacraz2, Antreas Theodosiou2,
Kyriacos Kalli2, Yosuke Mizuno1*, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan

2

Nanophotonics Research Laboratory, Cyprus University of Technology, Limassol 3036, Cyprus*

E-mail: ymizuno@sonic.pi.titech.ac.jp

TheoryIn an FBG, its periodical refractive index perturbation reflects a narrow band, the central
wavelength of which is called the Bragg wavelength 𝜆𝐵 and is given by17,39)2𝑛𝛬𝜆𝐵 = 𝑚 ,(1)

where n is the effective refractive index of the fiber core, 𝛬 is the grating pitch, and m (= 1, 2,3, …) is the diffraction order. When a relatively small strain 𝜀 (<< 1) applied to an FBG is
increased by 𝛥𝜀, the Bragg wavelength shift 𝛥𝜆𝐵 is given by17,39)
1 𝜕𝑛

1 𝜕𝛬

1 𝜕𝑛

𝛥𝜆𝐵 = 𝜆𝐵 (𝑛 𝜕𝜀 + 𝛬 𝜕𝜀 ) 𝛥𝜀 = 𝜆𝐵 (𝑛 𝜕𝜀 + 1) 𝛥𝜀 .

(2)

As the strain sensitivity defined as 𝛥𝜆𝐵 /Δ𝜀 is thus in proportion to 𝜆𝐵 , strain sensitivities
measured at different wavelengths cannot be directly compared. To overcome thisinconvenience, conventionally, we have made use of the fractional strain sensitivity (=𝛥𝜆𝐵 /Δ𝜀 ) 38,40–44) that unambiguously compares the sensitivities measured at different
wavelengths. The fractional sensitivity is given by
1 𝛥𝜆𝐵
𝜆𝐵 Δ𝜀

1 𝜕𝑛

= 𝑛 𝜕𝜀 + 1 ,(3)which indicates that the fractional sensitivity can be regarded as a wavelength-independent
constant only when we assume that n and 𝜕𝑛/𝜕𝜀 are not dependent on wavelength. Therefore,
it is not necessarily reasonable to compare the strain sensitivities at different wavelengths
using the fractional sensitivity.

1Experimental setup
Figure S1 shows the experimental setup for measuring the Bragg wavelengths of the
PFGI-POF-FBG. All the optical paths except the PFGI-POF were silica SMFs. One end of
the PFGI-POF was connected to a silica SMF using the so-called butt-coupling technique,50)
and the other end was kept open. The output from a supercontinuum source (SC-5, Yangtze
Soton Laser; wavelength range: 460–2000 nm; output power: 200 mW) was injected into the
PFGI-POF, and the reflected light was guided to an optical spectrum analyzer (AQ6370,
Yokogawa Electric) via an optical circulator. According to the wavelength range where theBragg wavelengths were located, two optical circulators with different operating wavelength
ranges (central wavelengths: approximately 980 and 1550 nm) were employed. Strains from0 to 0.4% were applied to the whole length of the PFGI-POF fixed on translation stages.
Considering the possible modal dispersion, a mode scrambler should be employed in thesystem,S1) but its considerable loss prevents us from measuring the FBG reflected spectra atshorter wavelengths (the modal dispersion was experimentally confirmed to be negligibly
smaller than the chromatic dispersion, at least at 1560 nm).

Reference
S1.

M. J. Schmid and M. S. Müller, Opt. Express 23, 8087 (2015).

2

Supplementary Figures

Supplementary Figure S1 | Experimental setup for measuring the Bragg wavelengths of the
PFGI-POF-FBG.

(a)

(b)

(c)

Supplementary Figure S2 | (a) FBG-reflected spectrum magnified around 1043 nm. The
highest peak was defined as the main peak. (b) Strain dependence of the main peak. (c) Bragg
wavelength dependence on strain. The solid line is a linear fit. The error bars were calculatedusing the standard deviations of 20 measurements.

Supplementary Figure S3 | Dependence of the fractional strain sensitivity on wavelength.
3