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Japanese Journal of Applied Physics 53, 042502 (2014)
http://dx.doi.org/10.7567/JJAP.53.042502

Wide-range temperature dependences of Brillouin scattering properties
in polymer optical ﬁber
Kazunari Minakawa, Neisei Hayashi, Yuri Shinohara, Masaki Tahara,
Hideki Hosoda, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received October 31, 2013; accepted February 16, 2014; published online March 27, 2014
We investigate the temperature dependences of the Brillouin scattering properties in a perﬂuorinated graded-index (PFGI-) polymer optical ﬁber(POF) in a wide temperature range from %160 to 125 °C. The temperature dependences of the Brillouin frequency shift, linewidth, and Stokes
power are almost linear at lower temperature down to %160 °C; while they show nonlinear dependences at higher temperature. These behaviors
appear to originate from the partial glass transition of the polymer material. © 2014 The Japan Society of Applied Physics

1.

Introduction

Fiber-optic distributed temperature sensing has been demonstrated based on a number of principles including ﬁber
Bragg gratings (FBGs),1–3) Rayleigh scattering,4) Raman
scattering,5,6) Brillouin scattering,7–11) etc. Among them,
Brillouin-based techniques have recently attracted signiﬁcantattention because of their capability of distributed temperature and/or strain sensing on the basis of frequency information leading to high measurement stability.12–14) Conventionally, their sensing heads were mainly composed of glass
optical ﬁbers such as silica glass ﬁbers,7–14) tellurite glassﬁbers,15) bismuth-oxide glass ﬁbers,15) and photonic crystal
ﬁbers,16) all of which are so fragile that they cannot withstandstrains of over several percent. To enhance the limitationof the measurable strain, we have been studying the sensor
applications of Brillouin scattering in polymer optical ﬁbers(POFs),17–21) which are sufﬁciently ﬂexible to withstandstrains of up to 50%.21)
Up to now, Brillouin scattering in perﬂuorinated gradedindex (PFGI-) POFs have been successfully observed at
1.55 µm19) and its sensing characteristics have been clariﬁed,
which include the Brillouin frequency shift (BFS) dependences on strain (relatively small strains from 0 to 1%;20) andlarge strains of up to ³20%21)) and temperature (relatively
narrow range from 30 to 80 °C20)). However, the wide-range
temperature dependences of the Brillouin scattering properties in POFs have not been reported yet, which are extremely
important for their practical applications to temperaturesensing in cryogenic or high-temperature environments.
In this study, we fully investigate the temperature dependences of the Brillouin scattering properties, i.e., the BFS,
linewidth, and Stokes power, in a PFGI-POF in a wide temperature range from ¹160 to 125 °C. Although the temperature dependences of these three parameters are almost linear
at lower temperature down to ¹160 °C, they are found to
be nonlinear at higher temperature of over ³80 °C. Thesebehaviors appear to originate from the low glass-transition
temperature of the polymer that constitutes the POF. Our
results indicate that the Brillouin scattering in PFGI-POFs
can be exploited to perform cryogenic-temperature sensing
as well as higher-temperature sensing.
2.

Principle

When pump light is injected into an optical ﬁber, backscattered light called Stokes light is generated due to theinteraction with acoustic phonons, and it propagates inthe direction opposite to the pump light. This phenomenon
is known as Brillouin scattering,12) and the Stokes light
spectrum is called the Brillouin gain spectrum (BGS). The
center frequency of the BGS is known to be downshiftedfrom that of the pump light by the amount called the BFS ¯B,which is given as12)
B ¼

2nvA
;


ð1Þ

where n is the core refractive index, vA is the acoustic
velocity in the ﬁber, and ­ is the pump wavelength.
If temperature change (or strain) is applied to the ﬁber, the
BFS shifts toward higher or lower frequency depending on
the ﬁber core material, which is the basic principle of ﬁberoptic Brillouin sensing. The BFS dependence on temperature
has been investigated for many kinds of optical glass ﬁbers,
which include silica single-mode ﬁbers (SMFs; +1.18
MHz/K13)), tellurite glass ﬁbers (¹1.14 MHz/K15)), bismuth-oxide glass ﬁbers (¹0.88 MHz/K15)), and germaniumdoped PCFs (+0.82 MHz/K) (all the coefﬁcients describedhere have been recalculated using Eq. (1) under the assumption that the wavelength of the pump light is 1.55 µm and thatn is not dependent on the wavelength). The coefﬁcient of
the temperature dependence of the BFS in a PFGI-POF has
also been clariﬁed to be ¹4.09 MHz/K,20) but only in thetemperature range of 30 to 80 °C. As for silica SMFs, thedependences of the Brillouin properties on extremely low
temperature (down to 1.4 K) have been reported.22)
The Stokes power as well as the BFS is also temperaturedependent; for example, the Stokes power in a silica SMF isreported to be enhanced with increasing temperature.23) The
Stokes power or the Brillouin gain coefﬁcient is inﬂuenced
by many structural quantities,12) one of which is the effective
length Leff deﬁned as
Leff ¼

1  eL;

ð2Þ

where ¡ is the propagation loss and L is the ﬁber length.
3.

Experimental methods

We employed an 8-m-long PFGI-POF as a sample, which
consists of a core (50 µm diameter), cladding (100 µm
diameter), and overcladding (750 µm diameter) encased inpolyvinyl chloride. The core and cladding layers are composed of doped and undoped polyperﬂuorobutenylvinyl042502-1© 2014 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 53, 042502 (2014)

K. Minakawa et al.

Fig. 1. (Color online) Experimental setup for observing Brillouinscattering in the PFGI-POF sample: DAQ, data acquisition; LD, laser diode;
EDFA, erbium-doped ﬁber ampliﬁer; ESA, electrical spectrum analyzer; PD,
photo-diode; PM, powermeter; PSCR, polarization scrambler.

(a)ether, respectively. It has a numerical aperture (NA) of 0.185,
a core refractive index of ³1.35, a propagation loss of ³250
dB/km at 1.55 µm, and a core glass-transition temperatureof <108 °C. According to the speciﬁcation sheet provided
by the manufacturer, this POF is suitable for telecom use onlyin the temperature range from 0 to 70 °C because of theincreased loss.
The experimental setup shown in Fig. 1 was basicallythe same as that previously reported,19) where self-heterodyne detection was utilized for high-resolution monitoringof the BGS. The laser wavelength was 1.55 µm, and the
POF sample was placed in a thermostatic chamber. The
pump power injected into the sample was ﬁxed at 20 dBm(= 100 mW), and the inﬂuence of the polarization state was
mitigated using a polarization scrambler.
4.

Experimental results

4.1

Brillouin gain spectrum

After being set to ¹160 °C, the temperature was graduallyraised. Figure 2(a) shows the measured BGS dependence on
low temperature from ¹160 to 0 °C. With increasing temperature from ¹160 °C, the BGS shifted to lower frequency,
and the Stokes power was raised. Figure 2(b) shows themeasured BGS dependence on relatively high temperaturefrom 70 to 125 °C. As the temperature was increased, the
BGS shifted to lower frequency; the Brillouin linewidth wasbroadened and the Stokes power was reduced.
Brillouin frequency shift
From Fig. 2, we can extract the detailed temperature dependences of the BFS, linewidth, and Stokes power. Figure 3
shows the BFS dependence on wide-range temperature from
¹160 to 125 °C, which was obtained by ﬁtting each BGS
with a Lorentzian curve. In the temperature range below³85 °C, the dependence was linear with no hysteresis. The
slope was about ¹3.2 MHz/K, which is smaller than that ofour previous report.20) This discrepancy seems to be causedby the differences in material and structure of the POFs. At>85 °C, the slope became steeper, reaching ¹13.8 MHz/K
at 125 °C. Considering that the glass-transition temperatureof the core material is <108 °C, this nonlinear behavior isprobably due to the change in physical properties of thesoftened core. After being kept at 125 °C for 5 min, the temperature was decreased, during which the BFS was reduced(b)Fig. 2. (Color online) Measured dependence of the BGS on (a) low
temperature from 0 to ¹160 °C and (b) higher temperature from 70 to 125 °C.

4.2

Fig. 3. (Color online) Temperature dependence of the BFS. The solid
circles and the open squares indicate the measured points with increasing
temperature from ¹160 °C and with decreasing temperature from 125 °C,
respectively.

with no hysteresis. The BGS was buried by the noise ﬂoor at³80 °C, and the BFS was no longer measurable. This result
indicates that highly-sensitive temperature sensing based

042502-2© 2014 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 53, 042502 (2014)

K. Minakawa et al.

(a)Fig. 4. (Color online) Temperature dependence of the 1-dB Brillouin
linewidth.

(b)Fig. 5. (Color online) Temperature dependence of the Stokes power.
The solid circles and the open squares indicate the measured points withincreasing temperature from ¹160 °C and with decreasing temperature from
125 °C, respectively.

Fig. 6. (Color online) Temperature dependences of (a) the propagation
loss and (b) the effective length of the POF. The solid circles and the open
squares indicate the measured points with increasing temperature from
¹160 °C and with decreasing temperature from 125 °C, respectively.

4.4

on Brillouin scattering in PFGI-POFs is feasible in thewide temperature range including cryogenic temperature. The
nonlinear behavior of the BFS at higher temperature is
physically interesting, and could be exploited to enhance themeasurement sensitivity in future.
4.3

Brillouin linewidth
Next, the temperature dependence of the Brillouin linewidth
was plotted as shown in Fig. 4. Since the Stokes power
was not sufﬁciently high compared with the noise ﬂoor, we
utilized not 3-dB but 1-dB linewidth. While the temperature
was being raised from ¹160 °C, the linewidth was gradually
increased. The sign of this dependence was the oppositeof that of silica SMFs,23) either because of the phonon
absorption peak23) in the POF at high temperature or becauseof the multimode nature of the POF. The increase in the
linewidth became drastic at >70 °C, apparently caused by
the gradual glass transition of the core that reduces
the uniformity of the ﬁber cross-section. Note that the
linewidth at 125 °C was not fairly estimated, because the
Stokes power was extremely low (as is discussed in the next
section).

Stokes power
Finally, the temperature dependence of the Stokes poweris shown in Fig. 5. As the temperature was raised from
¹160 °C, the Stokes power was gradually increased andreached its maximum at ³60 °C. This behavior is the sameas that of silica SMFs.23) However, as the temperature
was further raised, the Stokes power started to decrease; at>120 °C, the decrease became drastic. When the temperature
was lowered after being kept at 125 °C for 5 min, the Stokes
power was no longer restored to the initial value.
In order to clarify the reason for the behavior at higher
temperature, the temperature dependence of the propagation
loss of the POF was measured, as shown in Fig. 6(a). The
loss was almost constant in the temperature range from ¹160
to 120 °C, but it abruptly started to increase at >120 °C neverto return to the initial value by decreasing the temperaturelater. The temperature dependence of the Brillouin effective
length Leff calculated using Eq. (2) is also shown in Fig. 6(b),the trend of which is quite similar to Fig. 5 at >60 °C. Thus,
we may naturally consider that the hysteresis of the Stokes
power is caused by the destroyed wave-guiding structure(core and cladding) owing to the partial glass transition of thepolymer material.

042502-3© 2014 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 53, 042502 (2014)

5.

K. Minakawa et al.

Conclusions

The temperature dependences of the Brillouin scattering
properties such as the BFS, linewidth, and Stokes power, in a
PFGI-POF were investigated in a wide temperature range
from ¹160 to 125 °C. Although the temperature dependencesof these three parameters can be regarded as almost linear
at lower temperature down to ¹160 °C, they are found to be
nonlinear at higher temperature of over ³80 °C. The Stokes
power displayed a hysteresis characteristic at ³125 °C. Thesebehaviors seem to originate from the partial glass transitionof the core material of the POF. Brillouin scattering in PFGIPOFs can be expected from our results to have a big potential
for cryogenic-temperature sensing as well as higher-temperature sensing.
Acknowledgements

The authors wish to thank Dr. Kotaro Koike (Keio Photonics
Research Institute, Keio University) for his helpful comments. This work was partially supported by a Grant-in-Aid
for Young Scientists (A) (No. 25709032) from the Japan
Society for the Promotion of Science (JSPS) and by research
grants from the 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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042502-4© 2014 The Japan Society of Applied Physics