Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 28030

Brillouin characterization of slimmed polymer
optical fibers for strain sensing withextremely wide dynamic range
YOSUKE MIZUNO,1,* NATSUKI MATSUTANI,1 NEISEI HAYASHI,2 HEEYOUNG
LEE,1 MASAKI TAHARA,1 HIDEKI HOSODA,1 AND KENTARO NAKAMURA1
1Institute of Innovative Research, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku,

Yokohama 226-8503, Japan
2Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba,

Meguro-ku, Tokyo 153-8904, Japan*ymizuno@sonic.pi.titech.ac.jp

Abstract: To date, most distributed Brillouin sensors for structural health monitoring have
employed glass optical fibers as sensing fibers, but they are inherently fragile and cannot
withstand strains of >3%. This means that the maximal detectable strain of glass-fiber-based
Brillouin sensors was ~3%, which is far from being sufficient for monitoring the possible
distortion caused by big earthquakes. To extend this strain dynamic range, polymer opticalfibers (POFs) have been used as sensing fibers. As POFs can generally withstand even ~100%
strain, at first, Brillouin scattering in POFs was expected to be useful in measuring such large
strain. However, the maximal detectable strain using Brillouin scattering in POFs was foundto be merely ~5%, because of a Brillouin-frequency-shift hopping phenomenon accompanied
by a slimming effect peculiar to polymer materials. This conventional record of the strain
dynamic range (5%) was still far from being sufficient. Here, we have thought of an idea thatthe strain dynamic range can be further extended by employing a POF with its whole length
slimmed in advance and by avoiding the Brillouin-frequency-shift hopping. The experimental
results reveal that, by applying 3.0% strain to a slimmed POF beforehand, we can achieve a>25% strain dynamic range, which is >5 times the conventional value and will greatly extend
the application fields of fiber-optic Brillouin sensing.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction
To implement smart materials and structures, fiber-optic strain and temperature sensing basedon Brillouin scattering [1] has been a major target of research owing to its distributed
measurement capability [2–12]. Most of the distributed Brillouin sensors reported so far have
employed glass optical fibers (GOFs) as fibers under test (FUTs), and their Brillouinscattering properties have been well documented. However, GOFs cannot withstand strains oflarger than ~3%, and consequently, it has been difficult to measure such large strains using
GOF-based Brillouin sensors. In order to tackle this problem, some research groups [13,14]
have attempted to use polymer optical fibers (POFs) as the FUTs of Brillouin sensors. POFs
are known to be highly flexible and can sometimes withstand large strains of dozens ofpercent without considerable additional loss [15]. POF-based large-strain sensing has been
already demonstrated using non-Brillouin methods by some research groups [16,17], but here
we focus on Brillouin-based large-strain sensing using POFs, which can potentially be
exploited to achieve distributed measurements in future.
Let us briefly trace the history of the study on Brillouin scattering in POFs. In 2010, thefirst observation of Brillouin scattering in a perfluorinated graded-index (PFGI-) POF [18],which is the only POF in which light can propagate with relatively low loss at telecom
wavelength, was reported [19]. The Brillouin frequency shift (BFS) was found to beapproximately 2.8 GHz at 1550 nm [19]. Then, the strain and temperature dependencies of#341958
Journal © 2018https://doi.org/10.1364/OE.26.028030
Received 9 Aug 2018; revised 4 Oct 2018; accepted 5 Oct 2018; published 10 Oct 2018

Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 28031

the BFS in the POFs were measured to be –122 MHz/% and –4.1 MHz/K at 1550 nm,
respectively [20]. The absolute values are 0.2 and 3.5 times larger than those of silica GOFs,
which indicates that it is difficult to employ Brillouin scattering in POFs to achieve highly
sensitive strain sensing (when strain is relatively small). The BFS dependencies on larger
strain [21] and wider-range temperature [22] have also been reported, which showed uniquenon-linear behaviors. To date, POFs have been successfully used to implement distributed
sensing systems in the time [13], frequency [14], and correlation domains [23–25].
Thus, at the early stage of research, Brillouin scattering in POFs was expected to be usefulin measuring large strains. However, as the large-strain dependence of the BFS [21] wasfurther studied, it was found that, by applying strain of larger than ~5%, a POF locally starts
to become slim with a constant outer diameter [26]. With increasing strain up to >60%, theslimmed section spreads along the whole length of the POF. The slimmed section has a BFS
of approximately 3.2 GHz at 1550 nm, and thus, this slimming effect is inevitably
accompanied by so-called BFS hopping. As the BFS is kept constant (only the Stokes powers
change at ~2.8 and ~3.2 GHz) during this irreversible effect, the BFS in POFs cannot bedirectly used to measure strain of approximately 5–60%. Against this background, we havethought of an idea that the 5% limitation of the measurable strain can be extended further by
employing a POF with its whole length slimmed in advance. The strain dependence of the
BFS in the slimmed POF has been already measured, and the BFS is reported to linearly
depend on strain with a coefficient of approximately –65.6 MHz/% at 1550 nm [26].
However, this report was only for relatively small strain of <1%. The BFS dependence on
strain of >1% in the slimmed POF has not been clarified yet.
In this work, to extend the strain dynamic range, we investigate the BFS dependence onlarge strain in the slimmed POF. The BFS is found to be non-monotonically dependent on
strain, in the same manner as in a non-slimmed POF, but without the BFS hopping
phenomenon. With increasing strain, the BFS first decreases with a coefficient of –76.5
MHz/% (0–1.2% strain) and then increases with a coefficient of + 16.6 MHz/% (up to 30%).
The BFS at 1.2–3.0% strain is almost independent of strain and cannot be directly used forstrain sensing. In contrast, by applying 3.0% strain to a slimmed POF in advance, as large as>25% strain can be measured simply by using the one-to-one correspondence of the BFS to
the strain. This strain dynamic range is at least 5 times as wide as the conventional value
using a non-slimmed POF.
2. Principles
Light propagating along an optical fiber is in part returned via Brillouin scattering [1], and thespectrum of the back-propagated Stokes light is referred to as a Brillouin gain spectrum(BGS). The central frequency of the BGS is slightly lower than the pump frequency by theamount called a BFS. The BFSs of a silica single-mode fiber (SMF) and a PFGI-POF at 1550
nm are reported to be ~10.8 GHz [1] and ~2.8 GHz [19], respectively. If relatively small
strain is applied to the SMF and the POF, their BFSs change with coefficients of 580 MHz/%[27] and –122 MHz/% [20], respectively. In the same manner, if the SMF and the POF areheated around room temperature, their BFSs change with coefficients of 1.2 MHz/K [28] and–3.2 MHz/K (or –4.1 MHz/K, depending on the fiber structure) [20,22], respectively.
An interesting phenomenon concerning Brillouin scattering in the largely strained POF isa BFS hopping phenomenon via a slimming effect [26]. When strain of larger than ~5% isapplied to the POF, it partially starts to be slimmed in a stepwise manner (the slope of astress-strain curve becomes almost zero at the elastic-to-plastic transition; refer to Appendix),
maintaining a constant outer diameter (reduced by ~16% from a non-strained state). When theapplied strain further increases, the slimmed section gradually spreads along the whole lengthof the POF. This slimming effect is reported to cause almost no additional optical loss [15].
This effect increases the density of the core of the POF, increasing the acoustic velocity,
resulting in the BFS hopping from ~2.8 GHz to ~3.2 GHz at 1550 nm (corresponding to theVol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 28032

acoustic velocities of ~1600 and 1840 m/s, respectively; the BFS value after the slimming
effect is highly repeatable). After this irreversible process, the coefficients of the BFS
dependencies on strain (reported only for relatively small strain of <1%) and temperature (ataround room temperature) have been reported to be –65.6 MHz/% and –4.04 MHz/K,
respectively.
3. Methods
In the experiment for investigating the BFS dependence on larger strain in a slimmed POF,
we employed a 0.58-m-long multimode PFGI-POF [18] as the FUT. The length of the POF
was limited owing to the size of an experimental apparatus (guide rail) for applying strain.
This POF had a three-layered structure consisting of core, cladding, and overcladding(diameters: 50, 70, and 490 μm; refractive indices: ~1.35, ~1.34, and 1.59, respectively). The
materials of the core/cladding layers and the overcladding layer were doped/undoped
amorphous perfluorinated polymers and polycarbonate, respectively. The numerical aperture
was 0.185, and the propagation loss at 1550 nm was ~0.25 dB/m.
Figure 1 depicts an experimental setup, which is based on self-heterodyne detection andbasically the same as that reported in our previous literature [19]. All the optical paths except
the FUT were silica SMFs. The output from a laser diode (LD) at 1552 nm (linewidth: ~1
MHz) was divided into two beams: pump and reference. The pump light was amplified to 24
dBm using an erbium-doped fiber amplifier (EDFA) and injected into the POF (higher-power
injection could increase the Brillouin signal, but we suppressed the power to avoid a so-called
POF fuse effect with a relatively low threshold power) [29,30]. The Stokes light was
amplified to 1 dBm using another EDFA. The reference light was also amplified to 1 dBm
and coupled with the Stokes light for heterodyne detection. A polarization scrambler wasinserted in the pump path to suppress the polarization-dependent signal fluctuations and to
improve the measurement stability. The open end of the POF was attached to amorphous
fluoropolymer to minimize the Fresnel reflection [31]. The optical beat signals wereconverted into electrical signals using a photodetector (PD) and observed using an electrical
spectrum analyzer (ESA) as the BGS. The BGS was acquired after 100 times averaging on
the ESA and fitted by a Lorentzian curve [1] to precisely deduce the BFS. The room
temperature was 16 °C.

Fig. 1. Experimental setup for investigating the strain dependence of the BFS in a slimmed
POF. EDFA: erbium-doped fiber amplifier, ESA: electrical spectrum analyzer, LD: laser
diode, PD: photodetector.

The POF was fixed on translation stages using clamps. First, in order to create a slimmed
section, strain of up to 80% was applied to the 0.36-m-long section around the middle of the
POF at the strain rate of 2 mm/s. The applied strain of 80% was kept for 5 min, and thenreleased, leading to the slimmed section with a length of 0.62 m. The BGS and BFS
dependencies on strain were measured by applying strain to the 0.53-m-long section aroundthe center of the slimmed section. The BGS and BFS showed transient response to the applied
strain, but in this measurement, the convergent values (5 min after each strain was applied)

Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 28033

were used. The Stokes light backscattered from the POF included the Brillouin signals fromthe non-strained non-slimmed, non-strained slimmed, and strained slimmed sections. The
third component is our target signal. The first component was automatically filtered out
because its BFS is ~2.8 GHz. The second component cannot be easily filtered out, but its
influence on the third is not significant, considering that its length (0.09 m) is much shorter
than that of the third (0.53 m).
4. Results and discussions
Figure 2 shows the example of the BGS measured at 0.4% strain. In spite of the 100 times
averaging, the spectral power was largely fluctuated because the height of the BGS was
extremely low (much less than 1 dB). This is partly because the POF length was much shorter
than those of previous reports [19,20] and partly because the clamped parts for applying large
strain induced non-negligible optical loss. However, the BGS shape was well fitted by a
Lorentzian curve, which resulted in the BFS of ~3.15 GHz. The standard deviation of the BFS
fluctuations at relatively small strain (< 1%) was approximately 0.02% (evaluated using 10
pieces of data measured with an interval of 1 min).

Power [dBm]

-65.2

-65.4

-65.6

Raw
Lorentzian

-65.8
3.08

3.12

3.16

Frequency [GHz]3.20

Fig. 2. BGS measured at 0.4% strain; raw data and its Lorentzian fit.

Figures 3(a) and 3(b) show the Lorentzian-fitted BGSs measured at strains of 0–2.0% and5–30%, respectively. As the raw data contained non-essential noise-floor fluctuations, thevertical axis was normalized so that the maximal powers of each BGS became 0 dB. In Fig.
3(a), when the strain was small, the BGS shifted to lower frequency with increasing strain,
but this trend ceased when the strain became larger. In Fig. 3(b), with increasing strain, the
BGS shifted to higher frequency. The height of the BGS became gradually low probably
because of the strain-induced acoustic loss (note that, as mentioned in Introduction, opticalpropagation loss is not significantly increased by large strain) [15]; when >35% strain wasapplied, the BGS was almost completely buried by the noise floor. The fluctuations of theBrillouin bandwidth are not essential and seem to have been caused by the noise-floor
fluctuations and the modal instability. Note that the re-slimming effect of the once slimmed
POF was not observed during this experiment (refer to Appendix).

Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 28034

Fig. 3. Strain dependencies of the normalized Lorentzian-fitted BGSs measured in the strain
ranges of (a) 0–2% and (b) 5–30%.

In order to quantitatively evaluate this behavior, the BFS was plotted as a function of theapplied strain from 0 to 30% (Fig. 4). With increasing strain, the BFS first decreased (at strain
smaller than ~1.2%) and then increased (up to 30%). This trend can be reasonably explainedby its stress-strain curve (refer to Appendix) in the same manner as the trend in a nonslimmed POF (where the sign of the strain coefficient turns from negative to positive at~2.5% (smaller than the strain at which the slimming effect starts)) [21]. We decided toinvestigate this trend by dividing the data into three strain ranges: 0–1.25%, 1.25–3.0%, and3.0–30.0%.

Fig. 4. BFS dependence on strain in the slimed POF. The colored regions indicate the threestrain ranges magnified in Figs. 5(a)–(c).

Figure 5(a) shows the BFS dependence on strain at 0–1.25%. In this range, the BFS was
almost linearly dependent on strain with a coefficient of –76.5 MHz/%. Its absolute value,
which is ~7.6 times as small as that in a typical silica SMF [27], is larger than the previous
report (65.6 MHz/% at <1.0%) measured at 20 °C. This could be valid considering thedifference of the POF manufacturers (Asahi Glass in [26] and Sekisui Chemical in this paper)and the strain-temperature cross effect (specifically, the absolute value of the strain sensitivity
is reported to become smaller with increasing temperature with a coefficient of 1.5
MHz/%/°C in a non-slimmed POF; see Fig. 3 of [26]). Subsequently, Fig. 5(b) shows the BFS
dependence on strain at 1.25–3.0%. The BFS in this range was almost independent of strain,
as the change in the BFS was comparable to the signal fluctuations caused by the low signalto-noise ratio (see Fig. 2). Therefore, although the measurement precision is not high, thisfeature may potentially enable strain-insensitive temperature sensing by applying, forexample, 2.0% strain to the slimmed POF beforehand. Finally, Fig. 5(c) shows the BFS

Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 28035

dependence on strain at 3.0–30.0%. In this range, with increasing strain, the BFS clearly
increased. The strain coefficient became slightly smaller especially at >20.0% strain, butlinear approximation roughly gave a coefficient of + 16.6 MHz/%. This value isapproximately 1/35, 1/7.3, and 1/4.6 times the values (absolute values) in a typical silica SMF[27], a non-slimmed POF (at small strain) [20], and the slimmed POF (at <1.25% strain),respectively. Thus, although the strain sensitivity is extremely low, this behavior of theslimmed POF appears to be useful for rough measurement of large strain (note that thestandard deviations of the BFS fluctuations in this strain range were > 0.1%). By applying
3.0% strain in advance, the one-to-one correspondence of the BFS to strain will be beneficial
for simple implementation of such large-strain sensors (which can measure at least ~26%
strain ( = ~27 / 103)). Then, the achievable strain dynamic range will be >5 times wider thanthe conventionally widest value (~5%).

Fig. 5. Magnified views of the BFS dependence on strain in the slimed POF in the strain
ranges of (a) 0–1.25%, (b) 1.25–3.0%, and (c) 3.0–30.0%. The solid lines in (a) and (c) arelinear fits.

5. Conclusion
The BFS dependence on large strain in a slimmed POF was investigated to extend a strain
dynamic range, which was conventionally limited to ~5%. We found that a BFS hopping
phenomenon is not induced in the slimmed POF, and that with increasing strain, the BFS firstdecreases with a coefficient of –76.5 MHz/% (0–1.2% strain) and then increases with acoefficient of + 16.6 MHz/% (up to 30%). The BFS at 1.2–3.0% strain was almost
independent of strain, which indicates strain sensing cannot be directly performed in thisstrain region unless the slimmed POF is pre-strained. We also showed that, by applying 3.0%
strain to the slimmed POF beforehand, up to ~25% strain can be measured simply based on
the one-to-one correspondence of the BFS to the strain. This strain dynamic range is at least 5times as wide as the conventional value using a non-slimmed POF. We anticipate that thisfinding is of crucial importance in developing wide-dynamic-range strain sensing systems
based on Brillouin scattering in POFs in the future. As a final remark, based on theexperimental results, the research motivation of POF-based Brillouin sensing can besummarized as follows: when strain is relatively small and silica fibers can be directly (bytheir own deformation) used to measure the strain, high-sensitivity temperature sensing
becomes feasible; in contrast, when strain is so large that silica fibers cannot be directly used,strain sensing is still feasible (even though the sensitivity is low).
Appendix: stress-strain curve of POF
The stress-strain curves of an 8.80-mm-long non-slimmed PFGI-POF in the strain ranges of
0–85% and 83–140% are shown in Figs. 6(a) and 6(b), respectively. These two figures wereobtained continuously. The physical parameters of the PFGI-POF sample, including the core
diameter (50 μm), were the same as those mentioned in Methods. The stress-strain curve wasmeasured using a material testing machine (AG-5KN, Shimadzu). The strain rate was 25
μm/s, which is much slower than that of our previous report [21], leading to a more accurate

Vol. 26, No. 21 | 15 Oct 2018 | OPTICS EXPRESS 28036

result. In Fig. 6(a), although the basic tendency was similar to the previous report, two dips
were observed at ~9.5% and ~83%, which corresponded to the starting and ending points ofthe slimming process, respectively. During the slimming process, the stress was almost
constant. In Fig. 6(b), with increasing strain, the stress basically increased and abruptly
dropped to zero at ~137%. This drop was due to the breakage of the POF (at one of theclamped parts); when we set the strain of the slimmed POF (of which the whole length has
just been slimmed) to 0%, the breakage strain can be calculated to be approximately 30% (=(237–183) / 183), which is close to the upper value of the strain range used in the BFS
measurement (Fig. 4). We confirmed that the distortions of the stress-strain curve from thestraight line in Fig. 6(b) (i.e., the dips at 93% and 120%, etc) were caused by the slipping of
the POF at the clamped parts, indicating that, once a POF is slimmed, it does not experience aslimming effect again. This is because the slimming effect of this POF is caused by theyielding of the overcladding layer composed of polycarbonate. The core/cladding layers may
exhibit the same effect, but along with the difference in material, their core diameters were sosmall compared to that of the overcladding that the re-slimming effect was not experimentallyobserved.

Fig. 6. Measured stress-strain curves of the POF in the strain ranges of (a) 0–85% and (b) 83–
140%.

Funding
JSPS KAKENHI (17H04930, 17J07226, 16J09080, 26220907); Japan Gas Association,
ESPEC Foundation for Global Environment Research and Technology, Association for
Disaster Prevention Research, Fujikura Foundation; Japan Association for Chemical
Innovation.
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