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OPEN

Brillouin scattering in multicore optical fibers for sensing
applications

received: 05 December 2014
accepted: 27 April 2015
Published: 12 June 2015

Yosuke Mizuno1, Neisei Hayashi1, Hiroki Tanaka1, Yuji Wada2 & Kentaro Nakamura1
We measure the Brillouin gain spectra in two cores (the central core and one of the outer cores) of
a ~3-m-long, silica-based, 7-core multi-core fiber (MCF) with incident light of 1.55 μm wavelength,
and investigate the Brillouin frequency shift (BFS) and its dependence on strain and temperature.
The BFSs of both the cores are ~10.92 GHz, and the strain- and temperature-dependence coefficientsof the BFS in the central core are 484.8 MHz/% and 1.08 MHz/°C, respectively, whereas those in theouter core are 516.9 MHz/% and 1.03 MHz/°C. All of these values are not largely different from thosein a silica single-mode fiber, which is expected because the cores are basically composed of thesame material (silica). We then analyze the difference in structural deformation between the twocores when strain is applied to the fiber, and show that it does not explain the difference in the BFS
dependence of strain in this case. The future prospect on distributed strain and temperature sensing
based on Brillouin scattering in MCFs is finally presented.

There is a growing demand for optical fibers with high transmission capacities that can slake ourseemingly insatiable appetite for data. Although the spectral efficiency of these fibers has been extensively enhanced by using a variety of techniques1, the information-transmission capacity of a standard
silica-based single-mode fiber (SMF) is approaching its limit because of the nonlinear effects of optical fibers2–6. A recent solution to this ever-increasing demand is space-division multiplexing based on
multi-core fibers (MCFs)7–15. By exploiting such multiple cores, researchers have been drastically enhancing the transmission capacity delivered over a single fiber.
In the meantime, Brillouin scattering in silica SMFs has attracted considerable interest16–19 and has been
applied to a number of devices and systems, including distributed strain and temperature sensors20–24. To
improve the performance of these devices, Brillouin scattering properties in various special fibers havebeen investigated, and some of them have already been practically applied. Such special fibers include tellurite glass fibers25,26, chalcogenide glass fibers27,28, bismuth-oxide glass fibers26,29, photonic crystal fibers(PCFs)30, rare-earth-doped glass fibers (REDFs)31,32, polymethyl methacrylate (PMMA)-based polymer
optical fibers (POFs)33, and perfluorinated graded-index (PFGI) POFs34,35. Each special fiber has its own
unique advantage; for instance, the Brillouin-scattered Stokes powers in tellurite and chalcogenide fibers
are much higher than those in silica SMFs, owing to their large Brillouin gain coefficients25–28; the Stokes
powers in REDFs can be adjusted by controlling the pumping light power (at 980 nm in erbium-doped
fibers (EDFs))32,36; and Brillouin scattering in POFs is potentially applicable to high-sensitivity temperature measurement35 as well as large-strain measurement37. Similarly, if Brillouin-scattered signals frommultiple cores of an MCF can be simultaneously exploited, a discriminative measurement of strain andtemperature38–41 will be feasible by using a single fiber, as discussed below. To the best of our knowledge,
the Brillouin properties in MCFs have not been experimentally investigated yet. Therefore, clarifying
these properties is the first step to explore their potentials in practical applications.
1

Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama
226-8503, Japan. 2Faculty of Science and Technology, Seikei University, 3-3-1 Kichijoji Kitamachi, Musashino-shi,
Tokyo 180-8633, Japan. Correspondence and requests for materials should be addressed to Y.M. (email: ymizuno@
sonic.pi.titech.ac.jp)

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Temperature
coefficient(MHz/K)

~1.46

+580

+1.18

~7.95

~2.03

–230

–1.14

~7.95

~2.81

–

–

d

~8.83

~2.22

–

− 0.88Ge-doped PCFe

~10.29

~1.46

+409

+0.82

Er-dopedf~11.42

~1.46

+479

+0.87

g

Nd-doped~10.82

~1.46

+466

+0.73

Tm-dopedg~10.90

~1.46

+433

+0.90

PFGI-POFh

~2.83

~1.35

–122

–4.09

Fiber

BFS
(GHz)

Silica SMFa

~10.85Tellurite

ChalcogenidecbBismuth-oxide

Table 1. BFS at Room Temperature and its Strain and Temperature Coefficients in Silica SMF, Tellurite
Fibers, Chalcogenide Fibers, Bismuth-Oxide Fibers, Germanium-Doped PCFs, Erbium-Doped Fibers,
Neodymium-Doped Fibers, Thulium-Doped Fibers, and PFGI-POFs at 1.55 μm. The Refractive Index n ofEach Fiber is Also Presented. aRefs 17,18. bRefs 25,26. cRef 27. dRefs 26,29. eRef 30. fRef 31. gRef 32. hRef 35.

In this paper, the Brillouin gain spectra (BGS) in two cores (the central core and one of the outercores) of a ~3-m-long, silica-based, 7-core MCF are measured at 1.55 μ m, and the Brillouin frequency
shift (BFS) and its dependence on strain and temperature are investigated. The BFSs of both the coresare ~10.92 GHz, which are higher than that of a standard silica SMF by over 50 MHz. The strain- andtemperature-dependence coefficients of the BFS in the central core are 484.8 MHz/% and 1.08 MHz/°C,
respectively, whereas those in the outer core are 516.9 MHz/% and 1.03 MHz/°C. All of these coefficientsare nearly identical to the values in a silica SMF, as is expected considering that the cores are basicallycomposed of the same material, i.e., silica. Subsequently, using finite element analysis (FEA), the difference in applied stress between the two cores when strain is applied to the fiber is calculated to be insufficiently large to cause the difference in the BFS dependence of strain. The future vision for MCF-based
Brillouin sensors is finally discussed.

Results

Principle. Owing to an interaction with acoustic phonons, a light beam propagating through an opti-

cal fiber is backscattered, generating so-called Stokes light. This nonlinear process is known as Brillouinscattering16, where the central frequency of the Stokes light spectrum (called the BGS) becomes lower
than that of the incident light. The amount of this frequency shift is referred to as the BFS, which is, in
optical fibers, given as16

BFS =2 n vA,
λ(1)

where n is the refractive index, vA is the acoustic velocity in the fiber core, and λ is the wavelength of
the incident light. As n and vA are dependent on strain and temperature, the BFS also exhibits strain
and temperature dependence, which serves as the operating principle of strain/temperature sensors.
To date, the BFS dependence on strain and temperature has been investigated in a variety of special
fibers. Table 1 summarizes the BFS and its dependence on strain and temperature, along with therefractive index, in silica SMFs17,18, tellurite fibers25,26, chalcogenide fibers27, bismuth-oxide fibers26,29,
germanium-doped PCFs (main peak only)30, REDFs (erbium-31, neodymium-32, and thulium-doped)32,
and PFGI-POFs35. Supposing that the wavelength of the incident light is 1.55 μ m and that n is notdependent on the wavelength, all the values in Table 1 have been calculated using Eq. (1). As canbe seen, the BFS and its strain and temperature dependence drastically vary depending on the fiber
materials and structures, which is extremely important in considering the applications of Brillouinscattering in MCFs.

Experimental setup.

The experimental setup for investigating the BFS dependence on strain andtemperature in the MCF is depicted in Fig. 1, where self-heterodyne detection was used to observe
the BGS with a high frequency resolution34. The output of a laser diode at 1.55 μ m was divided into
two, one of which was amplified with an erbium-doped fiber amplifier (EDFA) to 14 dBm and used as
reference light. The other was also amplified with another EDFA to 30 dBm and injected into the MCF
as pump light. The backscattered Brillouin Stokes light was then coupled with the reference light. The
optical beat signal was then converted into an electrical signal with a photo detector (PD), and observedwith an electrical spectrum analyzer (ESA) as BGS. Each BGS measurement was performed 20 times,
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Figure 1. Schematic of the experimental setup for Brillouin measurement.

Figure 2. (a) Cross-sectional micrograph of the 7-core MCF. Structures of the fiber under test for detecting
Brillouin scattering in (b) the central core and (c) one of the outer cores of the MCF.

with the average being calculated thereafter. The relative polarization state between the Stokes light andthe reference light was adjusted with polarization controllers (PCs). The room temperature was ~28 °C.
We employed a ~3-m-long, silica-based, 7-core MCF as a fiber under test11, the cross-sectional micrograph of which is shown in Fig. 2(a). The cladding diameter was 197.0 μ m, and the core pitch was, on
average, 56.0 μ m. At 1.55 μ m, the mode-field diameter (MFD) and propagation loss of each core were11.2 ±  0.1 μ m and 0.198 ±  0.011 dB/km, respectively. When the Stokes light returned from the central
core (#1 in Fig. 2(a)) of the MCF was detected, as shown in Fig. 2(b), one end of the MCF was spliced
to a ~1-m-long silica SMF (termed as SMF-1), which was connected to a circulator using an arc-fusion
splicer (central cores were automatically aligned), and the other end of the MCF was cut on an angle
and immersed into matching oil (n =  1.46) to suppress the Fresnel reflection. In contrast, when the
Stokes light returned from one of the outer cores (#2 in Fig. 2(a)) of the MCF was detected, as shown inScientific Reports | 5:11388 | DOI: 10.1038/srep11388

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Figure 3. (a) BGS dependence on strain (0, 0.067, 0.135, 0.202, and 0.270%) and (b) BFS dependence on
strain in the central core of the MCF.

Fig. 2(c), the outer core at one end of the MCF was spliced to one end of a ~1-m-long silica SMF (termed
as SMF-2; manufactured by a company different from that of the SMF-1) using an arc-fusion splicer(accurate core alignment was required). The other end of the SMF-2 was spliced to a ~1-m-long SMF-1.
The Fresnel reflection at the other end of the MCF was suppressed in the same way. Strain was applied
to the whole length of the MCF, two parts near the ends of which were fixed on translation stages by
epoxy glue to avoid slipping. The temperature of the entire MCF was manipulated via an external heater.

Central-core characterization. First, the experimental results on the central core of the MCF arepresented. The blue curve in Fig. 3(a) shows the BGS measured at room temperature. Two clear peaks(corresponding to BFSs) were observed at 10.87 and 10.92 GHz, which originate from the SMF-1 and the
MCF, respectively. The BGS change when strain was applied only to the MCF is also shown in Fig. 3(a).
With increasing strain, the BGS of the SMF-1 hardly changed, while the BGS of the MCF shifted tohigher frequency. As the small peaks observed at 10.95 and 11.03 GHz did not exhibit strain dependence,
they are the second- and third-order Brillouin peaks of the SMF-1, respectively16,19. The instability of thepeak power of the MCF was caused by the spectral overlap with the higher-order peaks of the SMF-1
and by the polarization-dependent fluctuations. Using this result, we plotted the BFS of the central coreof the MCF as a function of strain (Fig. 3(b)). The dependence was almost linear with a proportionality constant of 484.8 MHz/% (with an error of ±~5 MHz/%; calculated from repetitive measurements),
which is in agreement with the value (~493 MHz/%) in a standard silica SMF at 1.55 μ m17.
We then measured the BGS dependence on temperature in the central core of the MCF (Fig. 4(a)).
With increasing temperature, only the BGS of the MCF shifted to higher frequency. Figure 4(b) shows the
BFS dependence on temperature, which was also linear with a proportionality constant of 1.08 MHz/°C(with a measurement error of ±~0.03 MHz/°C). This value was almost the same as that (1.00 MHz/°C)
in a standard silica SMF at 1.55 μ m18.
Outer-core characterization. Next, we present the measured results on the outer core of the MCF.
The BGS measured at room temperature is shown in Fig. 5, where the Brillouin peak of the MCF was
not clearly detected, because it overlapped with not only the peaks of the SMF-1 but also the peaks of
the SMF-2 (including their higher-order peaks). To resolve this problem, we employed a differential
measurement technique, with which the BGS of the SMF-1 and SMF-2 can be removed from the measured spectrum. The procedure was as follows: (1) obtain the BGS when local bending was applied to
the SMF-2 at the region around the MCF/SMF-2 interface to induce considerable loss of over 40 dB;(2) obtain the BGS after the bending was released; and (3) subtract the BGS obtained in (1) from thatobtained in (2) in log units. With this procedure, slight spectral distortion might be induced but the BFS
values can be more accurately measured than without subtraction. Note that the region to which bending
was applied was so short (<1 cm) that its influence on the measured results was negligible.
The BGS measured at room temperature using the differential measurement technique is shown as
the blue curve in Fig. 6(a). The BGS of the MCF was detected separately from those of the SMF-1 andSMF-2. The BFS of the outer core of the MCF was ~10.92 GHz, which is almost the same as that of thecentral core. These values are higher than any other reported value in a standard silica SMF by >50 MHz,
indicating that the cores of this MCF have slightly higher degrees of hardness, and thus slightly higher
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Figure 4. (a) BGS dependence on temperature (28, 40, 50, 60, 70, 80, 90 °C) and (b) BFS dependence on
temperature in the central core of the MCF.

Figure 5. Measured BGS in the outer core of the MCF, overlapped with those in the SMF-1 and SMF-2.

acoustic velocities. This could also be explained as follows: the MFD of the MCF is 11.2 μ m, whereas thatof a standard silica SMF (G.652 series) is 10.5 μ m at 1.55 μ m. This indicates that the cores of the MCF
are less doped with GeO2 than that of a standard silica SMF. Considering that the BFS decreases withincreasing GeO2 doping concentration42, it is natural that higher BFS should be obtained in the MCF.
Subsequently, we measured the BGS dependence on strain in the outer core of the MCF, as also
shown in Fig. 6(a). With increasing strain, the BGS of the MCF shifted to higher frequency. A dip independent of strain was generated at ~10.91 GHz by the differential measurement technique; along withthe polarization-dependent fluctuations, this caused the instability of the peak power. The BFS dependence on strain in the outer core of the MCF is shown in Fig. 6(b). The proportionality constant was
516.9 MHz/%, which is ~30 MHz/% larger than that of the central core (484.8 MHz/%). This difference
is not due to the measurement error (± ~5 MHz/%). An extensive analysis on its origin is provided in
the following section.
The BGS dependence on temperature in the outer core of the MCF is also shown in Fig. 7(a). With
increasing temperature, the BGS of the MCF shifted to higher frequency. From the temperature dependence of the BFS (Fig. 7(b)), the proportionality constant was calculated to be 1.03 MHz/°C, which was
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Figure 6. (a) BGS dependence on strain (0, 0.040, 0.069, 0.098, 0.126, 0.156, 0.184%) and (b) BFS
dependence on strain in the outer core of the MCF.

Figure 7. (a) BGS dependence on temperature (28, 40, 50, 60, 70, 80, 90 °C) and (b) BFS dependence on
temperature in the outer core of the MCF.

almost the same as those in the central core (1.08 MHz/°C) and a standard silica SMF (1.00 MHz/°C) at
1.55 μ m18.

Discussion

We discuss whether the observed difference in the BFS dependence between the central core and theouter core is caused by their structural or material difference. To investigate the difference in structuraldeformation when strain is applied to the MCF, the stress distribution in the cross-sectional direction
was calculated using a commercial FEA software, ANSYS 11.0 (ANSYS Inc.). A real-size cross section of
the MCF was modeled. For core material, the Young’s modulus E of 71 GPa, Poisson’s ratio ν of 0.166,
and mass density ρ of 2220 kg/m3 were used, whereas for cladding material, E of 73 GPa, ν of 0.167, andρ of 2220 kg/m3 were used (all these values were extracted from Figs 2 and 3 of Ref. 43). A longitudinalstrain of 0.2% was uniformly applied.
Before presenting the simulation result, we here provide concrete evidence for the fact that the
non-uniform strain distribution on the cross-section of the MCF is not the main reason. Suppose that the>5% discrepancy (observed in experiment) originates from the non-uniform strain distribution on the
cross-section, and that, when 0.2% strain is applied to a 3-m-long MCF (actual measurement condition;
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Figure 8. Simulated stress distribution in the cross-sectional direction.

see Figs 3(b) and 6(b)), the length of the central core has become 3.006 m. Then, the length of the outer
core needs to be >3.0063 m, which means that one of the two strain-applied points should have a longitudinal displacement of >150 μ m (= 300 μ m/2) between the central and outer cores. This situation ishighly unlikely if we consider the core pitch of ~56 μ m. Therefore, the non-uniform strain distribution
on the cross-section is not the main reason for the discrepancy of the strain coefficients, and it is valid
to assume that a longitudinal strain is uniformly distributed on the cross-section of the MCF.
The calculated stress distribution in the cross-sectional direction is shown in Fig. 8. The asymmetric nature seems to be caused by the meshing algorithm of the software. The stress was relatively highon the circumferences of the cores, and the stress applied to the outer cores (especially on their outersides; indicated in red) turned out to be higher than that applied to the central core. However, even thehighest stress obtained in the cross-sectional direction was approximately 6.2 kPa, which is negligibly
low compared to the calculated longitudinal stress of ~14 MPa. Thus, we speculate that the difference
in structural deformation between the central and outer cores cannot explain the difference in the BFS
dependence on strain, which probably originates from the practical difference in core material (detailed
information is not available).
In summary, we measured the BFS and its dependence on strain and temperature in the central coreand one of the outer cores of a 7-core MCF. Although the strain dependence between the two cores
was not perfectly identical, the strain- and temperature-dependence coefficients differed only slightly,
which is to be expected because the MCF used in this measurement was fabricated with the intention
that all the cores should be identical in material and structure. However, the difference in the Brillouinproperties among the cores is expected to be enhanced by fabricating an MCF using different materials(see Table 1) and structures (note that several schemes have been developed to suppress the inter-corecrosstalk by inducing phase mismatch among the cores)12–15. If this difference is sufficiently large, by
exploiting Brillouin scattering in multiple cores of the MCF, a discriminative measurement of strain andtemperature distributions will be feasible only by use of a single fiber38–41. Another potential application of Brillouin scattering in MCFs is bending/torsion sensing, because each core experiences different
strains when bending or torsion is applied to the fiber. We anticipate that this paper will be an importantarchive exploring a new field of research: MCF Brillouin sensing.

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Acknowledgments

The authors are indebted to Furukawa Electric Co., Ltd. for providing the MCF samples. They also thankHideyuki Fukuda (Tokyo Institute of Technology, Japan) for his technical support in the simulation.
This work was partially supported by Grants-in-Aid for Young 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, andthe SCAT Foundation. N.H. acknowledges a Grant-in-Aid for JSPS Fellows (No. 25007652).

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Author Contributions

Y.M. and N.H. designed and performed the experiments. Y.M. and K.N. performed the experimental
data analysis. H.T. and Y.W. performed the simulation. Y.M. wrote the manuscript with input from allco-authors.

Additional Information

Competing financial interests: The authors declare no competing financial interests.
How to cite this article: Mizuno, Y. et al. Brillouin scattering in multi-core optical fibers for sensing
applications. Sci. Rep. 5, 11388; doi: 10.1038/srep11388 (2015).
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Scientific Reports | 5:11388 | DOI: 10.1038/srep11388

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