Applied Physics Express 11, 012502 (2018)https://doi.org/10.7567/APEX.11.012502

Hydrostatic pressure dependence of Brillouin frequency shift in polymer optical ﬁbers
Yosuke Mizuno1*, Heeyoung Lee1, Neisei Hayashi2, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Tokyo 153-8904, Japan2*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received November 6, 2017; accepted November 29, 2017; published online December 14, 2017
We experimentally investigate the pressure dependence of the Brillouin frequency shift (BFS) in a polymer optical ﬁber. The BFS dependence onpressure shows a hysteresis, but after several cycles of increasing/decreasing pressure, the hysteresis is mitigated. The pressure dependence
coefﬁcient at this state is +4.3 MHz/MPa, the absolute value of which is 5.8 times as large as that of bare silica ﬁbers (the sign is opposite). The
reason for this unique behavior is discussed. This result indicates that, by using plastic optical ﬁbers instead of silica ﬁbers, distributed pressure
sensing with a higher sensitivity is potentially feasible. © 2018 The Japan Society of Applied Physics

rillouin scattering in optical ﬁbers has been thesubject of extensive research owing to its capabilityof distributed measurement of strain and temperature.1–12) To date, glass optical ﬁbers have mainly been used
for the sensor heads of Brillouin sensors, but in order toenhance the ﬂexibility (i.e., strain dynamic range), someresearch groups13–22) have started to employ polymer opticalﬁbers (POFs) as the sensor heads. One report has proven thatsometimes POFs can withstand extremely large strains ofmore than 100%.13)
POF-based sensors have another unique feature calledstrain and thermal “memory” functions,14,15) with which theinformation on the applied large strain (or overheat) can be
stored owing to their plastic deformation. For instance, if we
exploit the strain memory function, we can propose a novel
concept: “we need not always place expensive analyzers at theends of the sensing ﬁbers; after earthquakes, an oﬃcer has
only to go round with a single interrogator”. This concept will
extend the application range of ﬁber-optic sensing technology, which has been limited only to large-scale civil structures
owing to its high cost, to small-scale multifamily residences
and individual houses. Consequently, for instance, theevacuation period of people after large earthquakes might
be shortened, because people can judge rapidly and accurately
whether the houses and buildings are severely damaged.
In addition to such memory functions, previous researchstudies on Brillouin scattering in POFs16–19) have clariﬁed
its potential applicability to large-strain sensing,17) to highprecision temperature sensing with less sensitivity to strain,18)
and to low-temperature sensing.19) Distributed strain andtemperature sensing has also been demonstrated by threeresearch groups. Minardo et al.20) and Dong et al.21)
attempted to demonstrate distributed strain and temperaturesensing in a POF based on Brillouin optical frequency-domain
analysis (BOFDA)6) and Brillouin optical time-domain
analysis (BOTDA),1) respectively, but the spatial resolution
and signal-to-noise ratio (SNR) of their results were not
suﬃciently high for practical use. In contrast, our group
employed Brillouin optical correlation-domain reﬂectometry(BOCDR)11) to perform distributed strain and temperature
measurement in a POF. A 10-cm-long heated section of a 1.3m-long POF was detected with a theoretical spatial resolution of 6.0 cm,22) and further performance improvement of
distributed Brillouin sensing using a POF is being conductedfrom various aspects, one of which is the increase in thenumber of kinds of measurands. In addition to strain andBtemperature, we have experimentally proved that a local loss
can also be detected using slope-assisted BOCDR.23–25)
Here, we focus on pressure sensing based on Brillouinmeasurement. In bare silica single-mode ﬁbers (SMFs), theBrillouin frequency shift (BFS) is reported to depend linearlyon pressure with a coeﬃcient of −0.74 MHz=MPa at 1.55
µm.26–28) However, the pressure dependence of the BFS in a
POF has not been clariﬁed yet. As polymer materials are
generally softer than glass materials, the pressure-induced
distortion of a POF is anticipated to be larger than that of asilica SMF, which may result in a larger pressure dependence
coeﬃcient. Clarifying this point is extremely important as aﬁrst step to perform POF-based distributed pressure sensing.
In this work, we experimentally investigated the hydrostatic pressure dependence of the BFS in a POF. As pressure
increased, the BFS generally increased; but the case was not
so simple. The dependence coeﬃcients were initially diﬀerent between increasing and decreasing pressures. After threecycles of increasing=decreasing pressure, the BFS dependence on pressure became almost the same, which could be
used for practical pressure sensing. The pressure dependence
coeﬃcient at this state was +4.3 MHz=MPa, the absolutevalue of which was 5.8 times as large as that of bare silica
SMFs. This result indicates that by using POFs insteadof silica SMFs, distributed pressure sensing with a higher
sensitivity is potentially feasible in the future.
Light propagating in an optical ﬁber is partially returned
via spontaneous Brillouin scattering. The backscattered
Stokes light spectrum is called a Brillouin gain spectrum(BGS),29) and the central frequency of the BGS is downshifted from the incident frequency by the amount called a
BFS. At 1.55 µm, the BFS of a silica SMF is known to be∼10.8 GHz29) and that of a perﬂuorinated graded-index(PFGI-) POF (the only type of POF in which Brillouinscattering has been experimentally observed) is ∼2.8 GHz.16)
If temperature change (or strain) is applied to the ﬁber, the
BFS shifts toward a higher or lower frequency according tothe ﬁber core material, in which lies the basic principle of
Brillouin temperature (or strain) sensing. The temperaturedependence coeﬃcient of the BFS of a PFGI-POF is reported
to be −3.2 MHz=K (or −4.1 MHz=K, depending on the ﬁber
structure)16,19) at 1.55 µm, the absolute value of which is ∼3
times that of a silica SMF. Moreover, the strain dependence
coeﬃcient of the BFS of a PFGI-POF is −121.8 MHz=%,16)
the absolute value of which is about one-ﬁfth that of a silica
SMF. Thus, Brillouin scattering in a PFGI-POF has been

012502-1© 2018 The Japan Society of Applied Physics

Appl. Phys. Express 11, 012502 (2018)

Y. Mizuno et al.

Fig. 1. Experimental setup for investigating the hydrostatic pressure
dependence of the BFS in a POF. EDFA, erbium-doped ﬁber ampliﬁer; ESA,
electrical spectrum analyzer; PSCR, polarization scrambler; PD, photodiode.

shown to have a big potential for high-precision temperaturesensing with reduced strain sensitivity.
In bare silica SMFs, the BFS is reported to depend on
not only strain and temperature but also pressure.26–28)
With increasing pressure, the BFS linearly decreases with adependence coeﬃcient of −0.74 MHz=MPa at 1.55 µm. Thisdependence is reported to be similar to that of a bulk silica,
and this coeﬃcient is known to originate from the change inlongitudinal acoustic velocity rather than from the change inrefractive index.26)
We employed a 3.0-m-long PFGI-POF30) as a ﬁber under
test (FUT). The POF had a three-layered structure consistingof core, cladding, and overcladding layers (diameters: 50, 70,
and 490 µm, respectively). It had a numerical aperture of
0.185, a core refractive index of ∼1.35, and a propagation
loss of ∼0.25 dB=m at 1.55 µm. The core=cladding and overcladding layers were composed of doped=undoped amorphous perﬂuorinated polymer and polycarbonate, respectively. The water absorption ratios of these polymers are
known to be negligibly low.31)
Figure 1 schematically shows the experimental setup
for investigating the hydrostatic pressure dependence of the
BFS in the POF. This setup operates on the basis of selfheterodyne in the same manner as that used for the ﬁrst
observation of Brillouin scattering in a POF.16) All the opticalpaths except the FUT are composed of silica SMFs. A laser
diode at 1.55 µm with a linewidth of ∼1 MHz was used as alight source. Its output was divided into two light beams:
pump and reference. The pump light was ampliﬁed to∼22 dBm using an erbium-doped ﬁber ampliﬁer (EDFA) andinjected into the FUT. The backscattered 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. Using a polarization
scrambler (PSCR), the polarization state was averaged toincrease the measurement stability. The optical beat signals
were converted into electrical signals using a photodiode(PD) and observed using an electrical spectrum analyzer(ESA). When the BGS was acquired from the ESA to apersonal computer, averaging was performed 1000 times for
precise measurement. The raw data around the BGS peak was
ﬁtted by a Gaussian curve to deduce the BFS.
A hydraulic system was used to apply pressure on the POF
from 0.1 MPa (atmospheric) to 0.6 MPa. With a pump, water
was pushed through a one-way valve to a sealed hydraulictank (diameter: 250 mm; height: 450 mm), in which the FUT
was placed. The pressure was measured using a commercialpressure gauge, and a hand valve was used to release theFig. 2. Example of raw and ﬁtted BGS. Data of the 4th cycle (0.3 MPa;
increasing). The solid curve is a Gaussian ﬁt.

(a)

(b)
Fig. 3. Examples of ﬁtted BGS dependence on (a) increasing pressure(0.1, 0.3, and 0.5 MPa) and (b) decreasing pressure (0.6, 0.4, and 0.2 MPa).
Data of the 4th cycle. The insets show the magniﬁed views around the peaks.

pressure if necessary. The silica SMF connected to the POF(via butt-coupling)16) was passed through a hole on the tankcover. The water temperature was ∼10 °C.
First, the validity of the Gaussian ﬁtting of the measured
BGS was veriﬁed. An example is shown in Fig. 2, where theraw BGS was obtained when the pressure was 0.3 MPa (4thcycle; refer to the following paragraphs for the meaning of“cycle”). The raw data around the BGS peak was well ﬁtted
with the Gaussian curve, leading to the objective acquisitionof the BFS. In the same way, examples of the pressure
dependence of the BGS (4th cycle) are shown in Figs. 3(a)and 3(b). As the pressure increased (or decreased), the BGS
also increased (or decreased).

012502-2© 2018 The Japan Society of Applied Physics

Appl. Phys. Express 11, 012502 (2018)

Y. Mizuno et al.

Fig. 4. Pressure dependence of the BFS in the POF. The pressure wasincreased and then decreased 4 times. The dotted lines are linear ﬁts of eachprocess. The numbers indicated next to each trend are the pressure
dependence coeﬃcients (unit: MHz=MPa).

Subsequently, the BFS dependence on pressure was measured, as shown in Fig. 4. Interestingly, the dependence coeﬃcient was initially diﬀerent between increasing and decreasing pressures; therefore, we performed the same measurement while repeating the increasing=decreasing processes ofthe pressure 4 times. At each data plot, we waited ∼5 min for
the BGS to become stable.
When the pressure was ﬁrst applied to the POF, the BFS
was not stable and its pressure dependence was not linear. Thisinstability might be caused by the unideal interface between
the core=cladding and overcladding layers (the fabrication
technique for PFGI-POFs has not matured yet), whereas thepressure dependence is relatively small probably because theovercladding layer absorbs most of the pressure and thepressure applied to the core=cladding layer itself is low.
After the measurement at 0.6 MPa was ﬁnished, theapplied pressure was reduced to 0.1 MPa (atmospheric);
during this process, the BGS was relatively stable and the
BFS decreased almost linearly with decreasing pressure witha coeﬃcient of 6.93 MHz=MPa. The stability was improved
probably because the interface between the core=cladding
and overcladding layers became relatively uniform withpressing. The larger dependence coeﬃcient might originate
from the plastic deformation of the POF. In this case, plastic
deformation has two types, one is radial (i.e., diameterchange) and the other is longitudinal (i.e., length change).
If the diameter of the core is reduced by pressing, the densityof the core increases, leading to the increases in acoustic
velocity and in BFS (because the material becomes lesssoft).32) In contrast, if the POF length increases (or the POF
is strained), the BFS decreases (through the reduction in
Young’s modulus).18) Considering that the BFS is lowered
compared with its initial value, plastic deformation in thelongitudinal direction (elongation of the POF) appears to be
the reason for this diﬀerence between the increasing anddecreasing processes.
In the 2nd cycle of measurement, the BGS was relatively
stable even when pressure increased. This may verify our
speculation regarding the instability in the ﬁrst cycle. The
coeﬃcients were 4.52 and 6.95 MHz=MPa for the increasing

and decreasing processes, respectively. In the 3rd cycle, thecoeﬃcients were 3.82 and 6.25 MHz=MPa for the increasing
and decreasing processes, respectively. As a cycle is added,
the coeﬃcients tended to decrease regardless of the process.
This behavior is somewhat in analogy with that for thethermal treatment of intensity-based sensors using polymethyl
methacrylate POFs,33) in which ﬁve cycles of heating=coolingprocesses led to constant sensing performance.
In the 4th cycle, the BFS dependence on pressure showedalmost no diﬀerence between the increasing and decreasing
processes. This state with almost no hysteresis could be
exploited for practical pressure sensing. The pressure dependence coeﬃcient at this state was ∼4.3 MHz=MPa. The
sign was opposite to that of bare silica SMFs, the reason forwhich is that the increased density of the pressed polymer
material leads to the increased acoustic velocity and that thiseﬀect is more dominant than that in the case of silica SMFs.
What is important for sensing application is the absolute valueof the dependence coeﬃcient, which was 5.8 times as large
as that of silica SMFs,26) indicating that Brillouin sensing
with high pressure sensitivity could be achieved by using
POFs. The decrease in BFS at zero pressure corresponds toan approximately 0.1% increase in POF length,18) which was
diﬃcult to precisely measure experimentally because of the
ﬂexibility of the POF and the thermal expansion caused bytemperature change. Note that the trend in the 5th cycle wascomparable to that in the 4th cycle.
Finally, we discuss the methods for further enhancing
the pressure sensitivity of the BFS in a POF. One easy
method is to use POFs with a thinner overcladding layer or alarger core. Then, the external pressure will be more directly
applied to the core. Another method is to remove the overcladding layer of the POF. Such POFs without overcladding
layers are not commercially available, and some researchgroups tried to etch the overcladding layer selectively using
chloroform,34,35) but it generally induces considerable loss;
further study on this point is needed. Yet another method is toemploy POFs after the so-called “BFS hopping” eﬀect caused
by a large strain,32) with which the outer diameter is reduced
by 0.8 times (with a jump of the BFS to ∼3.2 GHz). Thismeans that the overcladding layer is thinned, and a higher
pressure sensitivity is anticipated. Note that such a high pressure sensitivity has been achieved using Brillouin dynamic
gratings (BDGs); 36,37) but compared with the relatively
simple BFS measurement, the BDG measurement requires
a far more complicated setup, and the FUT generally needs tobe a polarization-maintaining ﬁber.
In conclusion, the hydrostatic pressure dependence of
the BFS in a POF was experimentally investigated. The BFS
dependence on pressure showed a hysteresis, the reason forwhich appears to be the unideal interface between the core=cladding and overcladding layers, and the plastic deformationin both radial and longitudinal directions. After three cycles
of increasing=decreasing pressure, the hysteresis was mitigated. The pressure dependence coeﬃcient at this state was+4.3 MHz=MPa. Compared with the case of silica SMFs,
the sign was opposite and the absolute value was 5.8 times
as large. In addition, we discussed some methods for further
enhancing the pressure sensitivity. Further increase inmaximal applied pressure may cause a new phenomenon,32)
which should be studied in the future. Thus, we believe that012502-3© 2018 The Japan Society of Applied Physics

Appl. Phys. Express 11, 012502 (2018)

Y. Mizuno et al.

this work will open up a new way to ﬁber-optic distributedpressure sensing with higher sensitivity.
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.

16)
17)
18)
19)
20)
21)
22)

1) T. Horiguchi and M. Tateda, J. Lightwave Technol. 7, 1170 (1989).
2) T. Kurashima, T. Horiguchi, H. Izumita, S. Furukawa, and Y. Koyamada,
IEICE Trans. Commun. E76-B, 382 (1993).
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. Lopez-Gil, M. A. Soto, X. Angulo-Vinuesa, A. Dominguez-Lopez, S.
Martin-Lopez, L. Thévenaz, and M. Gonzalez-Herraez, Opt. Express 24,
17200 (2016).
6) D. Garus, K. Krebber, F. Schliep, and T. Gogolla, Opt. Lett. 21, 1402
(1996).
7) A. Minardo, R. Bernini, R. Ruiz-Lombera, J. Mirapeix, J. M. LopezHiguera, and L. Zeni, Opt. Express 24, 29994 (2016).
8) K. Hotate and T. Hasegawa, IEICE Trans. Electron. E83-C, 405 (2000).
9) Y. H. Kim, K. Lee, and K. Y. Song, Opt. Express 23, 33241 (2015).
10) E. Preter, D. Ba, Y. London, O. Shlomi, Y. Antman, and A. Zadok, Opt.
Express 24, 27253 (2016).
11) Y. Mizuno, W. Zou, Z. He, and K. Hotate, Opt. Express 16, 12148 (2008).
12) Y. Mizuno, N. Hayashi, H. Fukuda, K. Y. Song, and K. Nakamura, Light:
Sci. Appl. 5, e16184 (2016).
13) H. Ujihara, N. Hayashi, M. Tabaru, Y. Mizuno, and K. Nakamura, IEICEElectron. Express 11, 20140707 (2014).
14) K. Nakamura, I. R. Husdi, and S. Ueha, Proc. SPIE 5855, 807 (2005).
15) K. Minakawa, N. Hayashi, Y. Mizuno, and K. Nakamura, IEEE Photonics

23)
24)
25)
26)
27)
28)
29)
30)
31)
32)
33)
34)
35)
36)
37)

012502-4

Technol. Lett. 27, 1394 (2015).
Y. Mizuno and K. Nakamura, Appl. Phys. Lett. 97, 021103 (2010).
N. Hayashi, Y. Mizuno, and K. Nakamura, Opt. Express 20, 21101 (2012).
Y. Mizuno and K. Nakamura, Opt. Lett. 35, 3985 (2010).
K. Minakawa, N. Hayashi, Y. Shinohara, M. Tahara, H. Hosoda, Y.
Mizuno, and K. Nakamura, Jpn. J. Appl. Phys. 53, 042502 (2014).
A. Minardo, R. Bernini, and L. Zeni, IEEE Photonics Technol. Lett. 26, 387(2014).
Y. Dong, P. Xu, H. Zhang, Z. Lu, L. Chen, and X. Bao, Opt. Express 22,
26510 (2014).
N. Hayashi, Y. Mizuno, and K. Nakamura, J. Lightwave Technol. 32, 3999
(2014).
H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, IEEE Photonics J. 8,
6802807 (2016).
H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, Opt. Express 24, 29190(2016).
H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, J. Lightwave Technol.
35, 2306 (2017).
H. Gu, H. Dong, G. Zhang, J. He, N. Xu, and D. J. Brown, Chin. Opt. Lett.
10, 100604 (2012).
G. Zhang, H. Gu, H. Dong, L. Li, and J. He, IEEE Sens. J. 13, 2437 (2013).
S. L. Floch and P. Cambon, Opt. Commun. 219, 395 (2003).
G. P. Agrawal, Nonlinear Fiber Optics (Academic, New York, 2001).
Y. Koike and M. Asai, NPG Asia Mater. 1, 22 (2009).
S. Ando, T. Matsuura, and S. Sasaki, Chemtech 24, 20 (1994).
N. Hayashi, K. Minakawa, Y. Mizuno, and K. Nakamura, Appl. Phys. Lett.
105, 091113 (2014).
N. Zhong, Q. Liao, X. Zhu, M. Zhao, Y. Huang, and R. Chen, Sci. Rep. 5,11508 (2015).
R. Gravina, G. Testa, and R. Bernini, Sensors 9, 10423 (2009).
N. Hayashi, Y. Mizuno, and K. Nakamura, Electron. Lett. 49, 1630 (2013).
Y. H. Kim, H. Kwon, J. Kim, and K. Y. Song, Opt. Express 24, 21399(2016).
L. Teng, H. Zhang, Y. Dong, D. Zhou, T. Jiang, W. Gao, Z. Lu, L. Chen,
and X. Bao, Opt. Lett. 41, 4413 (2016).

© 2018 The Japan Society of Applied Physics