Applied Physics Express 11, 062502 (2018)https://doi.org/10.7567/APEX.11.062502

Experimental observation of spontaneous depolarized guided acoustic-wave
Brillouin scattering in side cores of a multicore ﬁber
Neisei Hayashi1*, Yosuke Mizuno2, Kentaro Nakamura2, Sze Yun Set1, and Shinji Yamashita1
1Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Tokyo 153-8904, Japan
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan2*E-mail: hayashi@cntp.t.u-tokyo.ac.jp
Received March 28, 2018; accepted May 2, 2018; published online May 21, 2018
Spontaneous depolarized guided acoustic-wave Brillouin scattering (GAWBS) was experimentally observed in one of the side cores of an
uncoated multicore ﬁber (MCF). The frequency bandwidth in the side core was up to >400 MHz, which is 0.5 times that in the central core. The
GAWBS spectrum of the side core of the MCF included intrinsic peaks, which had diﬀerent acoustic resonance frequencies from those of thecentral core. In addition, the spontaneous depolarized GAWBS in the central/side core was unaﬀected by that in the other core. These results willlead to the development of polarization/phase modulators using an MCF. © 2018 The Japan Society of Applied Physics

onlinear optical phenomena in optical ﬁbers havebeen used extensively for sensing and nonsensing
applications, such as distributed temperature andstrain sensors and radio-frequency (RF) generators. This isbecause optical ﬁbers have high electromagnetic tolerance anda compact and simple structure.1–12) Fiber-optic guided
acoustic-wave Brillouin scattering (GAWBS), one of such
nonlinear optical phenomena,8–20) is caused by the interactionof incident light and acoustic waves in the core of the opticalﬁber. It occurs as either depolarized or polarized GAWBS.8)
Owing to the high signal-to-noise ratio (SNR) of depolarized
GAWBS compared with that of polarized GAWBS,8) theformer is often used in applications such as temperature,13)
strain,14) and optomechanical sensing.15,16) The depolarized
GAWBS can be categorized as either spontaneous or stimulated. Stimulated depolarized GAWBS has been observed indual-mode single-core ﬁbers17) and small-core (2-µm-diameter) photonic crystal ﬁbers (PCFs).18) Recently, stimulated
depolarized GAWBS in multicore ﬁbers (MCFs) has been
observed by the pulse excitation method.10) The observed
spectrum in the central MCF core was almost the same as thatof a silica single-mode ﬁber (SMF). The spectrum in the sidecore was observed in the frequency range up to ∼400 MHz,
which did not agree with theoretical calculations, assuming
the use of a CW light source, because the stimulated backward
Brillouin scattering was generated by the pulse excitation
method.10) An interaction occurs between stimulated depolarized GAWBSs in both the central and side cores, which act
as the cross-phase modulation (XPM).10) On the other hand,spontaneous depolarized GAWBS has been observed invarious optical ﬁbers, such as silica SMFs,8) polarizationmaintaining ﬁbers,19) and few-mode ﬁbers.20) In the MCF, thespontaneous depolarized GAWBS has been observed in thecentral core, but not in the side cores of the MCF, because theSNR is signiﬁcantly reduced by acoustic wave dumping in thepolymer coat.8–10,20)
In this study, we successfully observe spontaneous
depolarized GAWBS in one of the side cores of an MCF
with its polymer coat removed. The spectrum in the side core
is observed in the low frequency range up to ∼400 MHz,
which is diﬀerent from that of stimulated depolarized
GAWBS in MCFs because stimulated backward Brillouinscattering does not appear as noise with the use of a continuous-wave (CW) light source. The spontaneous depolarized GAWBS in the side core contains peaks with the sameNcentral frequencies as that in the central core (which we call“intrinsic peaks”) and peaks with central frequencies diﬀerent
from those in the center core (which we call “shear peaks”).
We then measure the interaction of the spontaneous depolarized GAWBS between the center and side cores, with theresult of no interaction detected. These ﬁndings will be usefulin the development of various optical devices, includingstrain, temperature, and acoustic impedance sensors, phase=polarization modulators, and RF oscillators.
Several acoustic waves that have their origin in thermalﬂuctuations can exist in optical ﬁbers.8,9) Among theseacoustic waves, the torsional-radial acoustic mode (TR2,m,where m is the acoustic resonance mode), which induces
displacements not only in the radial direction but also in thetorsional direction, perturbs the refractive index and birefringence in the ﬁber.8,9) The optical scattering caused bythis acoustic mode is termed depolarized GAWBS. In thiscase, multiple spectral peaks of up to several GHz are observed. The central frequency (GAWBS-induced frequency
shift) of the m-th acoustic mode (νGB,m) is given by 8)
GB,m ¼

vym;
d

ð1Þ

where d is the ﬁber outer diameter and vs is the velocity of theshear acoustic waves for depolarized GAWBS. The symbol
ym is the value derived from the following equation:8)



y2
y2
3  m J2 ðym Þ
6  m J2 ðym Þ  3ym J3 ðym Þ22y22  m J2 ðym Þ  ym J3 ðym ÞJ2 ðym Þ  ym J3 ðym Þ2¼ 0;

ð2Þ

where α is vs=vL (= 0.62415)) and J2 and J3 are the secondand third-order Bessel functions, respectively. The central
frequency is known to depend on strain and temperature
with dependence coeﬃcients (m = 5) of 1.9 MHz=%14) and11 kHz=K,13) respectively.
The ﬁber under test (FUT) was a 20-m-long standard MCF
uncoated with a heater, which has a central core, and six side
cores that are equally spaced on a hexagonal grid as shown inFig. 1. The outer diameter was 142.4 µm and the centers of theside cores are 40.4 µm away from the ﬁber center. The modeﬁeld diameter in all the cores is 7.4 µm at 1550 nm. The opticalpower coupling between any pair of cores, in the ﬁber itself,062502-1© 2018 The Japan Society of Applied Physics

Appl. Phys. Express 11, 062502 (2018)

N. Hayashi et al.

Fig. 1. Cross section of the multicore ﬁber under test.

and in the fan-out units (maximum input power: 20 dBm CW)
at both ﬁber ends was veriﬁed as lower than −40 dB.
A schematic of the setup for observing the spontaneous
depolarized GAWBS spectrum in the MCF is depicted inFig. 2; this is basically the same as that previously reported,10)
except that a CW source was used. The output of a distributedfeedback laser diode (wavelength: 1550 nm; linewidth: 15kHz; maximum output power: 15 dBm) was divided using a70=30 optical coupler. One output component was used as theprobe source and the other as the pump. The probe signal wasdirectly injected into the core of the FUT, where the measured
core was arbitrarily selected. The pump light was injected into
the core through an erbium doped optical ﬁber ampliﬁer(EDFA) and an optical switch, and the measured core was also
arbitrarily selected. Subsequently, the optical beat signal of
the GAWBS forward-scattered light and pump light wasguided to a polarizer (PL), and then to a photodetector (PD)for optical-to-electrical conversion. All the optical paths
excluding the FUT consisted of silica SMFs. The signal was
ﬁnally monitored as a spontaneous depolarized GAWBS
spectrum using an electrical spectrum analyzer (ESA) with100 kHz frequency resolution and 100 kHz video bandwidth.
Averaging was performed 1000 times. The peak power of thespontaneous depolarized GAWBS spectrum was maximized
using polarization controllers (PCs), indicating that the lightbeam can be regarded as a depolarized beam.3,4)
In this investigation, we performed noise-ﬂoor compensation using the following procedure: (i) the raw data of the
spontaneous depolarized GAWBS spectrum was obtainedusing the ESA, (ii) the noise ﬂoor (the spectrum when theFUT was removed from the experimental setup) wasseparately obtained, and (iii) the noise ﬂoor was subtracted
from the raw data. The vertical axis of the spectrum calculated in this manner is deﬁned as an SNR in this paper.
The measurement temperature was 26 °C. The spontaneous
depolarized GAWBS spectrum in the uncoated MCF isshown in Fig. 3. The green curve represents the measured
GAWBS spectrum in the central core. A blue curve shows
the spectrum of the GAWBS in a side core. The frequency
range of the spectrum extended to ∼400 MHz, which was 0.5
times larger than the frequency range for the same phenomenon in a silica SMF because the spectrum in the higher
frequency range has a low SNR compared with that in thelower frequency range.1) The maximum SNR in the centercore was 8 dB at 105 MHz. The spectrum of the side coreincluded some intrinsic and sheared GAWBS peaks (notethat no signiﬁcant diﬀerence was observed among diﬀerent
side cores). An accurate calculation of the theoretical center
frequency of each peak might be possible using thecombination of TRm,n (m ¼ 2; 3; 4; . . . , n ¼ 1; 2; 3; . . .).11)
The maximum SNR in the side core was 2 dB at 30 MHz,Fig. 2. Schematic setup for observing spontaneous depolarized GAWBS.
EDFA, erbium-doped ﬁber ampliﬁer; ESA, electrical spectrum analyzer;
FUT, ﬁber under test; MCF, multicore ﬁber; PC, polarization controller; PD,photodetector; PL, polarizer; PSCR, polarization scrambler.

which was smaller than that in the central core owing to theunsymmetrical structure around the side core.11) If we use
these intrinsic peaks for acoustic impedance sensing, we canexpect a more accurate acoustic frequency response. The
spontaneous depolarized GAWBS spectrum of the centralcore is the same as that of this nonlinear event in silica SMFs,
which can be calculated using Eq. (1), assuming that thelongitudinal acoustic velocity in the MCF is 5998 m=s andthe ratio of the transverse acoustic velocity to the longitudinal
acoustic velocity in the MCF is 0.624.
Subsequently, we measured the interaction between
spontaneous depolarized GAWBSs in the side and centercores as shown in Fig. 4. The depolarized GAWBS spectra in
the side core, central core, and side core in the vicinity of theﬁrst were measured as shown in Figs. 4(a)–4(c), respectively,
when the optical switch was OFF; such spectra were almost
the same as those in Fig. 3. The pump power was 20 dBm,
which was the maximum input power of the splitter of theMCF for avoiding the cross talk between cores to other cores.
First, we injected the pump signal into the central core andthe probe light into the side core, but there was no observed
interaction [Fig. 4(d)]. When the pump light was injected
into the side core and the probe light into the center core, westill did not observe any interactions [Fig. 4(e)]. Finally, thepump signal was injected into the side core and the probe
light was injected into a separate side core in the vicinityof the ﬁrst. This did not yield any observable interactions[Fig. 4(f)]. These results are consistent with the basic
principle of active phase modulation using GAWBS.
Finally, we discuss how we can potentially implement(i) active phase modulators, (ii) RF oscillators, and (iii)
multiparameter sensors, using the spontaneous depolarized
GAWBS in an MCF. As for (ii) and (iii), the explanations
are relatively simple. The GAWBS signal can be simply
converted into an electrical signal (RF signal) using aphotodetector, which can be regarded as an RF oscillator.
In the meantime, the GAWBS signal is dependent on theambient acoustic impedance, for instance, and thus, discriminative sensing of multiple physical parameters will be
feasible if we combine the dependence of standard Brillouin062502-2© 2018 The Japan Society of Applied Physics

Appl. Phys. Express 11, 062502 (2018)

N. Hayashi et al.

Fig. 3. Measured spontaneous depolarized GAWBS: spectra in side (blue) and center (green) cores.

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 4. Depolarized GAWBS spectra in the (a) side core, (b) central core, and (c) side core in the vicinity of the ﬁrst. GAWBS spectra with the interaction
between acoustic waves in the (d) side core and central core (pump light into the center core and probe light into the side core), (e) side core and central core(pump light into the side core and probe light into the central core), and (f) side core and another side core (in the vicinity).

062502-3© 2018 The Japan Society of Applied Physics

Appl. Phys. Express 11, 062502 (2018)

N. Hayashi et al.

scattering on strain=temperature in another core of a single
MCF. As for (i), the basic concept has been proposed by
Zadok’s group.10) They have reported XPM in an MCF;
when a strong optical pump is injected into the central coreof an MCF, stimulated GAWBS is induced, generating
the strong radial acoustic wave. This strong radial acoustic
wave aﬀects the birefringence of the side cores, and then thepolarization state of the optical signal in the side coresis changed. In this paper, we have clariﬁed that the XPM
is not caused by using the spontaneous GAWBS. Thus,
we speculate that the XPM can be controlled by switching
stimulated=spontaneous GAWBS. In addition, if we deﬁne
the generation of stimulated GAWBS as “1” and that of
spontaneous GAWBS as “0”, the modulator will potentiallyact as a communication device.
In conclusion, we successfully observed the spontaneous
depolarized GAWBS in one of the side cores of an uncoatedMCF. The spontaneous depolarized GAWBS spectrum in theside cores was observed to be in the low frequency range
up to ∼400 MHz, which was diﬀerent from the stimulated
depolarized GAWBS in MCFs. We did not observe any
interaction between spontaneous depolarized GAWBSs in
the center and side cores. The physical aspects of theseresults are interesting and will be of great signiﬁcance inthe development of various optical devices such as multiparameter sensors, active phase=polarization modulators, andRF oscillators.
Acknowledgments We would like to thank JSPS KAKENHI Grant
Numbers JP15H4012, 17H04930, 25009080, and 17K14692, Japan Gas
Association, Fujikura Foundation, and Japan Association for Chemical Innovation
for supporting this study. We are profoundly grateful to K. Omichi and Dr. N.

Ishikura, Opt. & Electron. Lab., Fujikura Ltd., Sakura, Japan, for providing multicore ﬁbers with fan-out units.

1) G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, San Diego, CA,
2001) Chap. 9.
2) T. Horiguchi and M. Tateda, J. Lightwave Technol. 7, 1170 (1989).
3) D. Garus, K. Krebber, F. Schliep, and T. Gogolla, Opt. Lett. 21, 1402
(1996).
4) K. Hotate and T. Hasegawa, IEICE Trans. Electron. E83-C, 405 (2000).
5) Y. Mizuno, N. Hayashi, H. Fukuda, K. Y. Song, and K. Nakamura, Light:
Sci. Appl. 5, e16184 (2016).
6) H. Lee, N. Hayashi, Y. Mizuno, and K. Nakamura, Opt. Express 24, 29190(2016).
7) Z. Zhao, M. A. Soto, M. Tang, and L. Thévenaz, Opt. Express 24, 25211(2016).
8) R. M. Shelby, M. D. Levenson, and P. W. Bayer, Phys. Rev. B 31, 5244
(1985).
9) A. J. Poustie, J. Opt. Soc. Am. B 10, 691 (1993).
10) H. H. Diamandi, Y. London, and A. Zadok, Optica 4, 289 (2017).
11) Y. London, H. H. Diamandi, and A. Zadok, APL Photonics 2, 041303(2017).
12) A. Tehranchi, V. L. Iezzi, S. Loranger, and R. Kashyap, IEEE J. Quantum
Electron. 52, 1 (2016).
13) Y. Tanaka and K. Ogusu, IEEE Photonics Technol. Lett. 11, 865 (1999).
14) Y. Tanaka, H. Yoshida, and T. Kurokawa, Meas. Sci. Technol. 15, 1458
(2004).
15) Y. Antman, A. Clain, Y. London, and A. Zadok, Optica 3, 510 (2016).
16) N. Hayashi, Y. Mizuno, K. Nakamura, S. Y. Set, and S. Yamashita, Opt.
Express 25, 2239 (2017).
17) P. St. J. Russell, R. Culverhouse, and F. Farahi, Electron. Lett. 26, 1195
(1990).
18) M. S. Kang, A. Brenn, and P. St. J. Russell, Phys. Rev. Lett. 105, 153901(2010).
19) N. Nishizawa, S. Kume, M. Mori, T. Goto, and A. Miyauchi, Opt. Rev. 3,29 (1996).
20) T. Matsui, K. Nakajima, and F. Yamamoto, Appl. Opt. 54, 6093 (2015).

062502-4© 2018 The Japan Society of Applied Physics