LETTERIEICE Electronics Express, Vol.11, No.17, 1–4

Measurement of large-strain
dependence of opticalpropagation loss in
perﬂuorinated polymer ﬁbers
for use in seismic diagnosis
Hiroki Ujihara, Neisei Hayashi, Marie Tabaru, Yosuke Mizunoa),
and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology,
Yokohama 226–8503, Japana) ymizuno@sonic.pi.titech.ac.jp

Abstract: Brillouin scattering in perﬂuorinated graded-index (PFGI-) polymer optical ﬁbers (POFs) has been extensively studied for structural healthmonitoring, including seismic diagnosis. Here, we measure the propagation
loss of PFGI-POFs at telecom wavelengths as a function of large appliedstrain (up to 100%) at three optical powers and as a function of strain rate at aconstant optical power. The strain dependence of the propagation loss inPFGI-POFs is found to be independent of the incident power (<27 dBm) andthe strain rate (<500% s−1), indicating that PFGI-POF-based Brillouin sensors are potentially applicable to actual seismic monitoring.
Keywords: polymer optical ﬁbers, strain sensors, propagation loss
Classiﬁcation: Fiber optics, Microwave photonics, Optical interconnection, Photonic signal processing, Photonic integration and systems
References© IEICE 2014
DOI: 10.1587/elex.11.20140707
Received July 22, 2014
Accepted July 31, 2014
Publicized August 13, 2014
Copyedited September 10, 2014[1] T. Kurashima, T. Horiguchi, H. Izumita, S. Furukawa and Y. Koyamada: IEICE
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IEICE Electronics Express, Vol.11, No.17, 1–4

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006516

1© IEICE 2014
DOI: 10.1587/elex.11.20140707
Received July 22, 2014
Accepted July 31, 2014
Publicized August 13, 2014
Copyedited September 10, 2014

Introduction

Owing to their distributed strain and temperature measurability, ﬁber-optic sensors
based on Brillouin scattering have recently attracted considerable attention as apromising candidate for monitoring the damage of civil structures caused by earthquakes. Researchers have developed various types of sensing schemes, including
time- [1, 2], frequency- [3], and correlation-domain [4, 5] techniques. Conventionally, their sensing heads were composed of glass ﬁbers (especially standard silica
glass ﬁbers), resulting in ﬁber fracture at relatively small strains of ≲3%. To extend
the maximal measurable strain, we have been exploring the possibility of using
Brillouin scattering in polymer (or plastic) optical ﬁbers (POFs) [6], which canwithstand strains of up to 100%.
Commercially available POFs are classiﬁed into two types: poly(methyl methacrylate)-based (PMMA-) POFs [6] and perﬂuorinated graded-index (PFGI-) POFs
[7]. The former predominantly transmit visible light at ∼650 nm, while the lattertransmit both visible light and telecom-wavelength light at ∼1550 nm. As someof the optical devices (high-power ampliﬁers, circulators, etc.) required for thesemeasurements are extremely difﬁcult to design for operation at visible wavelengths,
Brillouin scattering has been experimentally observed only in PFGI-POFs [8].
However, the Brillouin signal in PFGI-POFs is quite weak because of their large
core diameters, multimode nature, and relatively high losses (∼250 dB/km at1550 nm). In order to extend the measurement range or to improve the signal-tonoise ratio of the sensors, we need to transmit high-power light of sub-watt order(note that transmitting light above watt-order power raises the probability of POF
fuse initiation [9] and should be avoided for sensing purposes). To date, no reporthas been provided on the large-strain dependence of the propagation loss in PFGIPOFs with such high-power propagating light. Meanwhile, in actual great earthquakes, such as the Great East Japan Earthquake in 2011, several-Hz shaking ofcomponents was reported to have caused tremendous damage to civil structures[10]. To verify that POF-based sensors can detect strains applied at such frequency
or velocity (corresponding to ∼200% s−1, if we assume the use of a 1-m-longsensing ﬁber [11, 12]), the dependence of propagation loss on strain rate also needs
to be clariﬁed.
In this paper, we investigate the propagation loss of PFGI-POFs as a function ofa large applied strain (up to 100%) at three different optical powers. The strain-rate
dependence of the loss at a constant optical power is also reported. Irrespective of
the strain magnitude, the propagation loss of PFGI-POFs is found to remain
relatively low even when high-power light is propagating or when the strain is2

IEICE Electronics Express, Vol.11, No.17, 1–4

Fig. 1.

Schematic of experimental setup. EDFA, erbium-doped ﬁber
ampliﬁer; LD, laser diode; PFGI-POF, perﬂuorinated gradedindex plastic optical ﬁber; SMF, single-mode ﬁber.

applied at a high rate (up to 500% s−1). This result suggests that Brillouin sensors
using the PFGI-POFs may have potential as a practical monitor for earthquakes.
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Experimental setup

We employed 4-cm-long PFGI-POFs (Fontex, Asahi Glass) with a core refractive
index of 1.35, a numerical aperture of 0.185, a core diameter of 50 µm, a cladding
diameter of 70 µm, and an overcladding diameter of 500 µm. The core and cladding
layers were composed of doped and undoped polyperﬂuorobutenylvinyl ether,respectively, and the reinforcement overcladding layer consisted of polycarbonate.
The experimental setup is shown in Fig. 1. The output of a laser diode (LD) at
1552 nm was ampliﬁed with an erbium-doped ﬁber ampliﬁer (EDFA) and injected
into the PFGI-POF, which was ﬁxed on motorized stages. The transmitted power
was monitored with an optical power meter. The PFGI-POF was connected to silica
single-mode ﬁbers (SMFs) by butt-coupling [8], namely, the ends of the SMFs(ﬁtted with ‘FC’ connectors) were connected to both ends of the POF (ﬁtted with‘SC’ connectors) via ‘FC/SC’ adaptors. Strains were applied to the whole length ofthe PFGI-POF at an arbitrary rate using a vertical machining center (Vertex 550-5X,
Mitsui Seiki; a strain rate up to 500% s−1 was achievable thanks to the short ﬁberlength). The operating temperature was 27°C.
3© IEICE 2014
DOI: 10.1587/elex.11.20140707
Received July 22, 2014
Accepted July 31, 2014
Publicized August 13, 2014
Copyedited September 10, 2014

Experimental resultsFig. 2(a) shows the measured propagation loss dependences on strain at 1.5, 15,
and 27 dBm incident powers. The strain rate was ﬁxed at 0.33% s−1. Some negativevalues were measured as losses, which probably originated from the error causedby the vibration of the system. Regardless of the incident power, at a strain of∼90%, the loss increased drastically, resulting in the fracture of the POFs. A
magniﬁed view of the strain at <80%, shown in Fig. 2(b), also indicates that theincident power has no quantiﬁable inﬂuence on the observed loss.
We then investigated the strain dependence of the propagation loss at strain
rates of 0.33, 13.3, 42.6, 200, 420, and 500% s−1, as shown in Figs. 3(a) and (b).
The incident power was ﬁxed at 27 dBm. The measurement error tended to increase
proportionally with the strain rate. The propagation loss remained relatively low atstrains of <80%, and the loss characteristics were not inﬂuenced by the strain rate
(at least in this range). This result suggests that PFGI-POFs can propagate lighteven when large strains are applied at a rate of ∼200% s−1 (corresponding toseismic vibration).

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IEICE Electronics Express, Vol.11, No.17, 1–4(a)Fig. 2.

(a) Propagation loss vs applied strain at three different opticalpowers, and (b) a magniﬁed view of the strain from 0 to 80%.

(a)Fig. 3.

4(b)(b)(a) Propagation loss vs applied strain at six different strain rates,and (b) its magniﬁed view in the strain range below 80%.

Conclusion

The propagation loss of the PFGI-POFs was measured as a function of a large
applied strain (up to 100%) at three different optical powers and as a function of thestrain rate at a constant optical power. The strain dependence of the propagation
loss in PFGI-POFs was affected by neither the incident power (<27 dBm) nor by
the strain rate (<500% s−1). This fact implies that high-transmission-power Brillouin sensors using PFGI-POFs are potentially applicable in the monitoring ofstrains of several hundred percent per second, which could occur in civil structuresduring earthquakes.
Acknowledgments© IEICE 2014DOI: 10.1587/elex.11.20140707
Received July 22, 2014
Accepted July 31, 2014
Publicized August 13, 2014
Copyedited September 10, 2014

This work was partially supported by Grants-in-Aid for Young Scientists (A)(no. 25709032) and for Challenging Exploratory Research (no. 26630180) fromthe Japan Society for the Promotion of Science (JSPS) and by research grants fromthe General Sekiyu Foundation, the Iwatani Naoji Foundation, and the SCAT
Foundation. N. H. acknowledges a Grant-in-Aid for JSPS Fellows (no. 25007652).

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