APPLIED PHYSICS LETTERS 105, 091113 (2014)

Brillouin frequency shift hopping in polymer optical fiber
Neisei Hayashi,a) Kazunari Minakawa, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku,
Yokohama 226-8503, Japan(Received 22 July 2014; accepted 26 August 2014; published online 5 September 2014)
We investigated the Brillouin gain spectrum dependence on large strain of up to 60% in a polymer
optical fiber (POF) at 1.55 lm, and found that the Brillouin frequency shift (BFS) abruptly changes
from 2.7 GHz to 3.2 GHz. We named this phenomenon “BFS hopping,” and found it tooriginate from the varied acoustic velocity induced by the stepwise change in the core diameter ofthe POF. This is because of the yielding of the overcladding layer composed of polycarbonate.
After the occurrence of BFS hopping phenomenon, the BFS dependence coefficients on strain andtemperature in the POF were measured to be 65.6 MHz/% and 4.04 MHz/K respectively. Thesevalues indicate that, compared to an unstrained POF, further higher-precision temperature sensing
C 2014 AIP Publishing LLC.
with lower strain sensitivity is feasible. V[http://dx.doi.org/10.1063/1.4895041]
For the past several decades, Brillouin scattering in optical fibers have been intensively studied.1,2 Its applications
include lasers,2 optical memories,3 phase conjugators,4 slowlight generators,5 and distributed strain/temperature sensors.6–10 To date, glass optical fibers are mainly used as thesensing heads of Brillouin sensors,6–13 however, they canwithstand only small strain and cannot be applied for largestrain sensing. To resolve this limitation, we propose aunique approach to employ polymer optical fibers(POFs)14,15 as the Brillouin sensing heads.
POFs have various advantages over glass fibers, such as
high flexibility (endurable for several tens of percent),
low-cost connection, ease of handling, high safety, and a socalled memory effect.16 Two types of commercially available POFs are poly(methyl methacrylate) (PMMA)-based
POFs14 and perfluorinated graded-index (PFGI)-POFs.15 Theformer basically transmit only visible light at 650 nm,
while the latter can transmit light at telecommunication
wavelength of up to 1.55 lm in addition to visible light. Sofar, we have succeeded in observing Brillouin scattering inPFGI-POFs at 1.55 lm, and studied its fundamental properties including Brillouin frequency shift (BFS), Brillouin linewidth, and Brillouin gain coefficient.17 We have also
investigated the BFS dependence on strain and temperature
in PFGI-POFs, and showed their applicability to highprecision temperature sensing with reduced strain sensitivity.18,19 Besides, we have recently measured the Brillouingain spectrum (BGS) dependence on large strain of up to20%,20 and found that at 10% strain, the Brillouin Stokes
power starts to decrease. We attributed the reason for thisbehavior to an increased propagation loss leading to a shortened effective length.20 However, the drastic decrease(60%) in the Stokes power at 20% strain is not fully
explained by only 30% reduction of the effective length.
In this work, we investigate the BGS dependence oneven larger strain of up to 60% in a PFGI-POF at 1.55 lm,
and find that the BFS abruptly changes from 2.7 GHz toa)

Author to whom correspondence should be addressed. Electronic mail:hayashi@sonic.pi.titech.ac.jp0003-6951/2014/105(9)/091113/3/$30.00

3.2 GHz. This provides a reasonable explanation for thephenomenon described above. We name this “BFS hopping,”
the origin of which is shown to be the varied acoustic velocity induced by the stepwise change in the core diameter.
Further, from our measurements, we found the strain- andtemperature-dependence coefficients of the BFS in theslimmed-down POF (after the BFS hopping) to be
65.6 MHz/% and 4.04 MHz/K, respectively, indicatingthat with lower strain sensitivity, higher-precision temperature sensing can be achieved.
A beam of light when incident on an optical fiber interacts with the acoustic phonons generating backscattered
Stokes light, which propagates in the direction opposite to
the incident light. This phenomenon is called spontaneous
Brillouin scattering.1,2 The Stokes light spectrum, referred toas BGS, generally has a Lorentzian shape and the central frequency of the BGS shifts down relative to the incident frequency. The amount of this frequency shift is called BFS,
which is 10.8 GHz and 2.8 GHz in a silica single-mode
fiber (SMF)2 and PFGI-POF,17 respectively, at 1.55 lm
wavelength. When a temperature change (or strain) isapplied to the fiber, the BFS shifts towards a higher or lower
frequency depending on the fiber core material; this is thefundamental operating principle of fiber-optic Brillouin temperature (or strain) sensors. The strain- and temperaturedependence coefficients of the BFS at 1.55 lm are reported
to be, respectively, 493 MHz/% (Ref. 21) and 1.00 MHz/K(Ref. 22) for silica SMFs, and 121 MHz/% (Ref. 18) and3.2 MHz/K (Ref. 23) for PFGI-POFs.
We employed a 0.6 -m-long PFGI-POF with 50 lm corediameter. The core and cladding layers are composed ofdoped and undoped polyperfluorobutenylvinyl ether (trademark: CYTOP). The refractive index at the center of thecore, the numerical aperture, and the propagation loss at
1.55 lm are 1.356, 0.185, and 250 dB/km, respectively.
The polycarbonate reinforcement overcladding layer (diameter of 490 lm) reduces micro-bending losses and increases
the load-bearing capability. Instead of the standard experimental setup based on self-heterodyne detection,17 we used a
newly developed Fresnel-assisted setup,24 which can detect

105, 091113-1

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091113-2Hayashi et al.

Appl. Phys. Lett. 105, 091113 (2014)

FIG. 1. Measured dependence of BGS(a) and BFS (b) of POF with large
strain. In (b), the BFS of the initial
peak was not accurately measured withstrain >20% (colored in gray).

the BGS in POFs with a higher signal-to-noise ratio. Largestrain was applied to the POF with two computer-controlled
motorized stages at a room temperature of 20  C.
First, we measured the BGS dependence on large strainof up to 60% in the POF, as shown in Fig. 1(a). The pump
power was 26 dBm, and the strain rate was 200 lm/s. TheBrillouin peak observed at 2.8 GHz in the absence of strain
shifted to lower frequency at <2.3% strain, and then shiftedto a higher frequency; its peak power gradually reduced withincreasing strain (>10%). This behavior has been reported
earlier in Ref. 20. However, we first report the appearance ofan additional peak at 3.2 GHz when strain was >7.3%(Fig. 1(a)). At 31% strain, the power of the two peaks isalmost the same, and at 60% strain, the initial peak almost
disappeared (note that the peak at 2.85 GHz originatedfrom the 3-cm-long unstrained POF section near the connector). The BFS of the two peaks were then plotted as functions of strain, as shown in Fig. 1(b). The dependence of theinitial peak showed the same non-monotonic behavior as
previously reported20 (>20%, the BFS cannot be accuratelymeasured). The BFS of the newly appeared peak was almost
constant, independent of the applied strain in this range.
Next, side views of the POF in the presence of 7.3%
strain are shown in Fig. 2(a). Several sections were slimmed
down in a stepwise manner, and with increasing strain, theslimmed sections grew longer (i.e., spread along the POF),
while their outer diameter was maintained. This explains theindependence of BFS from large strain (>7.3%) describedabove. The core diameter of the slimmed-down POF wasestimated to be 0.84 times of that of the unstrained POF
from a cross-sectional view shown in Fig. 2(b). This phenomenon is probably caused by the yielding of the overcladding layer made up of polycarbonate, and not because of thecore or cladding layers. The upper yield point of 8.0% (seeFig. 6 in Ref. 25) agrees with the strain at which the POF
was slimmed down (according to the specification sheet, theupper yield point of CYTOP composing the core and cladding layers is approximately 20%). This abrupt change in thecore diameter seems to have induced the change in the
acoustic velocity, therefore resulting in the BFS hopping.
Further, note that the unstable stress-strain curve of thePFGI-POF in the range from 10% to 60% (see Fig. 1 inRef. 20) can also be explained by this phenomenon.
Finally, after the whole length of the POF was slimmed
down at 60% strain, its BGS/BFS dependence on strain andtemperature was investigated after the strain was released,
and the results are shown in Figs. 3(a)–3(d). Their BFS dependence coefficients were found to be 65.6 MHz/% and4.04 MHz/K (Figs. 3(b) and 3(d)), which are 0.5 times18
and 1.3 times23 the value of an unstrained POF. This result
indicates that further higher-precision temperature sensing

FIG. 2. (a) Side views of the slim-down process of the POF (taken every
4 s), and (b) cross-sectional view of the slimmed-down POF.

FIG. 3. Measured strain dependence of BGS (a) and BFS (b), and temperature dependence of BGS (c) and BFS (d) of the slimmed-down POF.

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091113-3

Hayashi et al.

with lower strain sensitivity is feasible by exploiting theBrillouin signals in the slimmed-down POFs.
In conclusion, the BGS dependence on large strain of up
to 60% in a PFGI-POF was investigated at 1.55 lm, and an
abrupt change in BFS from 2.7 GHz to 3.2 GHz wasobserved. We showed that this “BFS hopping” is probably
caused by the varied acoustic velocity induced by the stepwise change in the core diameter (because of the yield of
polycarbonate-based overcladding layer). The strain- andtemperature-dependence coefficients of the BFS in theslimmed-down POF were 65.6 MHz/% and 4.04 MHz/K,
respectively, which indicates the possibility of further
higher-precision temperature sensing with lower strain sensitivity. We believe that the physical aspects of these results
are interesting and will be of great significance in developing
POF-based large-strain sensing systems in the near future.
This work was partially supported by Grants-in-Aid forYoung Scientists (A) (No. 25709032) and for Challenging
Exploratory Research (No. 26630180) 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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