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Applied Physics Express 6 (2013) 052501
http://dx.doi.org/10.7567/APEX.6.052501

Potential Applicability of Brillouin Scattering in Partially Chlorinated Polymer Optical Fibers
to High-Precision Temperature Sensing
Kazunari Minakawa, Neisei Hayashi, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received February 22, 2013; accepted April 2, 2013; published online April 18, 2013
Using the ultrasonic pulse–echo technique, we estimate the Brillouin frequency shift (BFS) and its temperature dependence in partially chlorinatedgraded-index polymer optical fibers (PCGI-POFs), which have high thermal stability compared with other POFs. At 1550 nm, the estimated BFS is4:43 GHz with its temperature coefficient of approximately 6:9 MHz/K, which is 5:8 times as large as that in silica fibers. Since the fracturestrain is 3:0% (mostly in the elastic region), the BFS dependence on strain in PCGI-POFs cannot be estimated by this technique. These results
indicate that Brillouin scattering in PCGI-POFs has a big potential for distributed high-precision temperature sensing.
# 2013 The Japan Society of Applied Physics

rillouin scattering in optical ﬁbers1) has been
exploited to develop a remarkably wide variety ofdevices and systems, such as lasers,2) signal
processors,3) phase conjugators,4) slow light generators,5)
optical storages,6) core aligners,7) optical compressors,8)delay lines,9) coolers,10) and gyroscopes.11) Distributed strain
and temperature sensing is also one of its most important
applications,12–17) and extensive research has been performed for the past several decades. In conventional ﬁberoptic Brillouin sensors, their sensing heads are composedof glass optical ﬁbers (GOFs) such as silica single-mode
ﬁbers (SMFs), which cannot withstand strains of larger
than several percent. To extend the applicable strain range,
we have been studying Brillouin sensors based on polymer
optical ﬁbers (POFs).18–21) POFs are generally so ﬂexible
compared with GOFs that they can withstand strains ofover 50%. Besides, they have many attractive features suchas low cost, ease of installation, and high safety.
Commercially available POFs are classiﬁed into three:poly(methyl methacrylate)-based (PMMA-) POFs, perﬂuorinated graded-index (PFGI-) POFs, and partially chlorinated(PC) GI-POFs. Brillouin scattering in PFGI-POFs withrelatively low loss at 1550 nm has already been observed
and investigated.18,19) Brillouin scattering in PMMA-POFs
cannot be directly observed due to their large core diameterand extremely high loss at 1550 nm, but their Brillouinproperties have been estimated using the ultrasonic pulse–
echo technique.20,21) PCGI-POFs have recently attracted considerable attention because of their low propagation loss at
the 650–700/750–800 nm region as well as their high thermal
resistance of up to 100  C (cf. 70  C in other POFs),22,23)
which enables the implementation of Brillouin sensors with awide temperature range. However, since their propagation
loss is high at 1550 nm, where various devices are available
for Brillouin detection with high frequency resolution,
Brillouin scattering in PCGI-POFs has not been observed yet.
In this paper, we measure the acoustic velocity in a PCGIPOF using the ultrasonic pulse–echo technique, based on
which the Brillouin frequency shift (BFS) and its temperature dependence are estimated. The BFS is calculated tobe 4:43 GHz at 1550 nm. Its temperature coeﬃcient isapproximately 6:9 MHz/K, the absolute value of which is
1:7 times as large as that of a PFGI-POF. The fracturestrain of the PCGI-POF is 3:0% (basically in the elastic
region), and so the BFS dependence on strain cannot be
estimated by this technique. These measurement results

B

Fig. 1.

Experimental setup for acoustic velocity measurement in PCGI-

POF.

predict that the Brillouin scattering in PCGI-POFs is highly
suitable for the implementation of high-precision temperature-sensing systems.
When pump light is injected into an optical ﬁber,backscattered light called Stokes light is generated due tothe interaction with acoustic phonons, and it propagates inthe direction opposite to the pump light. This phenomenon iscalled Brillouin scattering.1) The central frequency of the
Stokes light spectrum, called the Brillouin gain spectrum(BGS), shifts to a lower frequency than the pump frequency.
The magnitude of this frequency downshift is called BFS,
which is expressed by1)
BFS ¼

2nvA
;
p

ð1Þ

where n is the core refractive index, vA is the acoustic
velocity in the ﬁber, and  p is the pump wavelength. Thus,by measuring vA in a target ﬁber with known n, the BFS at
an arbitrary pump wavelength can be estimated.
We employed a 1.00-mm-long PCGI-POF sample with anouter diameter of 750 m, a core diameter of 120 m, and acore refractive index of 1:52. Both ends of the sample were
carefully polished with 3 m alumina powders. Figure 1
depicts the experimental setup for measuring the acoustic
velocity in the PCGI-POF sample, which is basically thesame as that in Ref. 20. A pulsed converging ultrasonicwave with a center frequency of 20 MHz generated with afocus-type transducer (Panametrics M316) connected to apulser/receiver (Panametrics 5900PR) was launched into thesample ﬁxed on an aluminum plate in degassed water.

052501-1# 2013 The Japan Society of Applied Physics

Person-to-person distribution (up to 10 persons) by the author only. Not permitted for publication for institutional repositories or on personal Web sites.

Appl. Phys. Express 6 (2013) 052501

K. Minakawa et al.

(a)

Estimated BFS in PCGI-POF at 1550 nm as a function ofincreasing (hollow circles) and decreasing (solid squares) temperature.
Fig. 3.

(b)
Fig. 2.

Measured echo waveforms (a) with and (b) without PCGI-POF at

22  C.

The temperature of the degassed water was controlled witha heater. The eﬀective diameter of the focused wave was
720 m. The reﬂected waves from the top and bottom of thesample were detected with the same transducer and observedwith an oscilloscope. Using the time delay between theobserved reﬂected waves and the length of the sample, theacoustic velocity can be calculated.
Figures 2(a) and 2(b) show the measured echo waveforms
at room temperature (22  C) with and without the PCGI-POF
sample on the stage, respectively. In Fig. 2(a), the clear
peak, where the relative time was deﬁned as 0 s, represents
the reﬂected wave from the top of the sample; the relatively
small peak at 0:9 s indicates the reﬂected wave at its
bottom. The large ﬂuctuations at 1:3 s were caused by thereﬂection from the stage surface, because similar ﬂuctuations were observed without the sample as shown in
Fig. 2(b). Thus, we obtained a time delay of 886 ns, withwhich, using the sample length (¼ 1:00 mm), the acoustic
velocity in the PCGI-POF sample was calculated to be
2.26 km/s. Then, using Eq. (1), the BFS in the PCGI-POF,
which is inversely proportional to the pump wavelength, wasalso calculated to be 4:43 GHz at 1550 nm (10:57 GHz
at 650 nm). This value is 0.82 times the BFS in PMMAPOFs,20) which indicates that the acoustic velocity or BFS
can be moderately controlled by adjusting the doping concentration of chlorine to PMMA-POFs. In the meantime,since the BFS in the PCGI-POF is 1.6 times as high as that in
PFGI-POFs,18) we can expect that the signal-to-noise (SNR)
deterioration caused by Rayleigh noise24) will be mitigatedby employing PCGI-POFs.
Next, by changing the temperature of the sample in therange from 20 to 80  C, the BFS dependence on temperature
in a PCGI-POF at 1550 nm was estimated, as shown in
Fig. 3. The refractive-index dependence on temperature
was not taken into account, under the assumptions that its
coeﬃcient is as negligibly small as that in bulk PMMA.25)
The hollow-circle and solid-square points were measured
with increasing and decreasing temperature, respectively. In
this temperature range, the BFS showed a linear change with

(a)

(b)
(a) Measured stress–strain curves of PCGI-POF and PMMAPOF,7) and (b) their magniﬁed view in the range below 4% strain.
Fig. 4.

temperature with no hysteresis. The temperature coeﬃcient
was approximately 6:9 MHz/K, the absolute value of
which is 1:7 times as large as that of PFGI-POFs19) (andeven 5:8 times as large as that of silica SMFs26)). Thus,
Brillouin scattering in PCGI-POFs seems to be potentiallyapplicable to high-precision temperature sensing.
Finally, to estimate the BFS dependence on strain, weinvestigated a stress–strain curve of a 100-mm-long PCGIPOF using a tension tester (Shimadzu AG-500N), as shownin Fig. 4(a), where a stress–strain curve of a 100-mm-long
PMMA-POF21) is also shown for comparison. The tensile052501-2# 2013 The Japan Society of Applied Physics

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Appl. Phys. Express 6 (2013) 052501

K. Minakawa et al.

speed was set to 100 mm/min (Note that no signiﬁcantchange was observed in the stress–strain curve of the PCGIPOF when the tensile speed was reduced to 12 mm/min.).
The fracture strain of the PCGI-POF was 3:0%, which ismuch smaller than that of the PMMA-POF (55%). The
magniﬁed view of the stress–strain curve of the PCGIPOF is shown in Fig. 4(b). Its fracture strain seems tobe even smaller than the upper yield point (4:5% in
the PMMA-POF), indicating that PCGI-POFs lie mainlyin the elastic region, that their BFS dependence on strain
cannot be estimated by the ultrasonic pulse–echo technique, and that they are not suitable for large-strain sensing
with a so-called memory eﬀect exploiting plastic deformation.27)
In conclusion, the acoustic velocity in a PCGI-POF was
measured using the ultrasonic pulse–echo technique, and the
BFS was estimated together with its temperature dependence. The acoustic velocity at room temperature was 2.26
km/s, corresponding to the BFS of 4:43 GHz at 1550 nm
and 10:57 GHz at 650 nm. The coeﬃcient of its temperature dependence was as large (absolute value) as 6:9
MHz/K at 1550 nm, which is 1:7 times as high as that ofa PFGI-POF. By measuring the stress–strain curve of the
PCGI-POF, its fracture strain was found to be 3:0%, which
is much smaller than those of other POFs. Thus, although
PCGI-POFs do not appear to be suitable for large-strain
sensing, we believe that they will be of great use indeveloping distributed Brillouin temperature sensors, exploiting their high thermal stability and high-precision
temperature-sensing capability.

research grants from the JFE 21st Century Foundation, the General Sekiyu
Foundation, and the Iwatani Naoji Foundation.

Acknowledgments We are indebted to Sekisui Chemical Co., Ltd., forproviding us with PCGI-POF samples. We are also grateful to Professor H.
Hosoda at Tokyo Institute of Technology for lending us the tension tester. This
work was partially supported by a Grant-in-Aid for Research Activity Start-up(24860029) from the Japan Society for the Promotion of Science (JSPS), and by

24)
25)

1) G. P. Agrawal: Nonlinear Fiber Optics (Academic Press, San Diego, CA,

2001).
2) G. Wang, L. Zhan, J. Liu, T. Zhang, J. Li, L. Zhang, J. Peng, and L. Yi:

Opt. Lett. 38 (2013) 19.
3) S. Norcia, S. Tonda-Goldstein, D. Dolﬁ, J.-P. Huignard, and R. Frey: Opt.

Lett. 28 (2003) 1888.
4) E. A. Kuzin, M. P. Petrov, and B. E. Davydenko: Opt. Quantum Electron.

26)
27)

17 (1985) 393.
K. Y. Song, M. Herráez, and L. Thévenaz: Opt. Express 13 (2005) 82.
Z. Zhu, D. J. Gauthier, and R. W. Boyd: Science 318 (2007) 1748.
Y. Mizuno and K. Nakamura: J. Lightwave Technol. 29 (2011) 2616.
Y. Okawachi and A. L. Gaeta: Nat. Photonics 6 (2012) 274.
W. Zou, Z. He, and K. Hotate: Appl. Phys. Express 3 (2010) 012501.
G. Bahl, M. Tomes, F. Marquardt, and T. Carmon: Nat. Phys. 8 (2012) 203.
F. Zarinetchi, S. P. Smith, and S. Ezekiel: Opt. Lett. 16 (1991) 229.
P. Dragic, T. Hawkins, P. Foy, S. Morris, and J. Ballato: Nat. Photonics 6(2012) 627.
T. Horiguchi and M. Tateda: J. Lightwave Technol. 7 (1989) 1170.
T. Kurashima, T. Horiguchi, H. Izumita, S. Furukawa, and Y. Koyamada:
IEICE Trans. Commun. E76-B (1993) 382.
D. Garus, K. Krebber, F. Schliep, and T. Gogolla: Opt. Lett. 21 (1996)
1402.
K. Hotate and T. Hasegawa: IEICE Trans. Electron. E83-C (2000) 405.
Y. Mizuno, W. Zou, Z. He, and K. Hotate: Opt. Express 16 (2008) 12148.
Y. Mizuno and K. Nakamura: Appl. Phys. Lett. 97 (2010) 021103.
Y. Mizuno and K. Nakamura: Opt. Lett. 35 (2010) 3985.
N. Hayashi, Y. Mizuno, D. Koyama, and K. Nakamura: Appl. Phys.
Express 4 (2011) 102501.
N. Hayashi, Y. Mizuno, D. Koyama, and K. Nakamura: Appl. Phys.
Express 5 (2012) 032502.
H. Yoshida and Y. Koike: Proc. POF2012, p. 266.
K. Koike, F. Mikes, Y. Okamoto, and Y. Koike: J. Polym. Sci., Part A 47
(2009) 3352.
Y. Mizuno, N. Hayashi, and K. Nakamura: Electron. Lett. 49 (2013) 56.
J. Brandrup, E. H. Immergut, and E. A. Grulke: Polymer Handbook(Wiley, New York, 1999).
T. Kurashima, T. Horiguchi, and M. Tateda: Appl. Opt. 29 (1990) 2219.
K. Nakamura, I. R. Husdi, and S. Ueha: Proc. SPIE 5855 (2005) 807.

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