Vol. 25, No. 3 | 6 Feb 2017 | OPTICS EXPRESS 2239

Experimental study on depolarized GAWBS
spectrum for optomechanical sensing ofliquids outside standard fibers
NEISEI HAYASHI,1,* YOSUKE MIZUNO,2 KENTARO NAKAMURA,2 SZE YUN
SET,1 AND SHINJI YAMASHITA1
1

Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba,
Meguro-ku, Tokyo 153-8904, Japan
2
Institute of Innovative Research, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku,
Yokohama 226-8503, Japan*hayashi@cntp.t.u-tokyo.ac.jp

Abstract: We report an experimental study on the spectral dependence of depolarized guided
acoustic-wave Brillouin scattering (GAWBS) in a silica single-mode fiber (SMF) on acoustic
impedance of external materials. The GAWBS spectrum was measured when the acoustic
impedance was changed from 1.51 to 2.00 kg/s·mm2. With increasing acoustic impedance, thelinewidth increased; the dependence was almost linear with an acoustic impedance
dependence coefficient of 0.16 MHz/kg/s·mm2. Meanwhile, with increasing acoustic
impedance, the central frequency linearly decreased with an acoustic impedance dependencecoefficient of −0.07 MHz/kg/s·mm2. These characteristics are potentially applicable toacoustic impedance sensing.
© 2017 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (120.0280) Remote sensing and sensors; (280.4788) Optical sensing
and sensors.

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#280890
Journal © 2017https://doi.org/10.1364/OE.25.002239
Received 9 Dec 2016; revised 22 Jan 2017; accepted 24 Jan 2017; published 27 Jan 2017

Vol. 25, No. 3 | 6 Feb 2017 | OPTICS EXPRESS 224014. Y. Mizuno, N. Hayashi, H. Fukuda, K. Y. Song, and K. Nakamura, “Ultrahigh-speed distributed Brillouinreflectometry,” Light Sci. Appl. 5(12), e16184 (2016).
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1. Introduction
Guided acoustic-wave Brillouin scattering (GAWBS), one of the nonlinear phenomena in an
optical fiber, occurs by the interaction between incident light and acoustic waves propagatingin the cross-sectional area of the fiber [1]. This scattering is known to be forward scatteringthat accompanies multiple spectral peaks caused by acoustic resonance. Its spectrum depends
on several fiber structural parameters, such as a fiber outer diameter, a core diameter, and arefractive index profile [2]. GAWBS can be categorized into two types: one is polarized
GAWBS caused by the radial-mode (R0,m) acoustic waves [1], and the other is depolarized
GAWBS caused by the torsional-radial-mode (TR2,m) acoustic waves [1], where m denotes theorder of the acoustic resonance. The R0,m mode perturbs the refractive index of the fiber crosssection, while the TR2,m mode perturbs not only the refractive index but also the birefringence[1].
Very recently, the idea of acoustic impedance sensing based on polarized GAWBS hasalso been proposed [3]. The basic principle of this acoustic impedance sensing lies in theamplitude dependence of the R0,m-mode acoustic waves on the acoustic reflection between thecladding and overcladding/outside material. However, this sensing technique suffers from adrawback that it is difficult to apply existing distributed sensing techniques [4–14] because of
the ring-shaped configuration of the fiber under test (FUT). Distributed acoustic impedance
sensing based on GAWBS will open up new applications in future, such as underground oillayer detection without employing external vibrations [15] and cancer detection in the humanbody exploiting the stiffness difference between normal cells and diseased cells [16,17].
In order to overcome these drawbacks, here we focus on the use of depolarized GAWBS,
which has been experimentally shown to efficiently couple with backscattered stimulated
Brillouin scattering, providing a potential for distributed GAWBS sensing [18,19]. To date,some sensing characteristics of depolarized GAWBS have been investigated, including thecentral frequency dependence on strain, ambient temperature, and fiber outer diameter [20–22]. In [2], the linewidth is shown to change by external acoustic impedance. However, adetailed experimental study on the acoustic impedance dependence of the central frequency
and the linewidth of the depolarized GAWBS has not been performed yet to the best of ourknowledge.

Vol. 25, No. 3 | 6 Feb 2017 | OPTICS EXPRESS 2241

In this paper, we measure the external acoustic impedance dependence of depolarized
GAWBS in an unjacketed silica single-mode fiber (SMF) immersed into sucrose solution with
0–65% concentration (corresponding to the acoustic impedance ranging from 1.51 to 2.00
kg/s·mm2). The spectral linewidth increases nonlinearly with increasing acoustic impedance;
the dependence in the range is almost linear with a coefficient of 0.16 MHz/kg/s·mm2. In themeantime, as the acoustic impedance increases, the central frequency decreases, which
probably originates from the increase in the effective outer diameter of the fiber. The
dependence is almost linear with a coefficient of −0.07 MHz/kg/s·mm2, the sign of which isopposite to the strain and temperature coefficients. These features could be exploited toacoustic impedance sensing. As both the linewidth and the central frequency of thedepolarized GAWBS spectrum are independently changed with acoustic impedance, by using
these two parameters simultaneously, discriminative sensing of acoustic impedance and strain(or temperature) might be feasible.
2. Principle
There exist various kinds of acoustic waves in optical fibers caused by thermal fluctuations[1,2]. Among these acoustic waves, that of the torsional-radial acoustic mode (TR2,m, where mis the acoustic resonance mode), which literally induces displacements not only in the radial
direction but also in the torsional direction, perturbs the refractive index and birefringence inthe fiber [1,2]. The optical scattering by this acoustic mode is called depolarized GAWBS,
which poses multiple spectral peaks of up to several gigahertz. The central frequency(GAWBS-induced frequency shift) of the m-th acoustic mode (νGB,m) is given by [1]vGB , m =vs y m,πd(1)where d is the fiber outer diameter, v is the velocity of the shear acoustic waves vs (fordepolarized GAWBS; this should be interpreted as the velocity of longitudinal acoustic waves
vL for polarized GAWBS). The symbol ym is the value derived from the following equation[1]:(3 −ym2) J 2 (α ym )2ym2) J 2 ( ym ) − 3 y m J 3 ( y m )2= 0,ym2(2 − ) J 2 ( ym ) − ym J 3 ( ym )2(6 −J 2 (α ym ) − α ym J 3 (α ym )(2)where α is vs/vL ( = 0.624 [3]), and J2 and J3 are the second- and third-order Bessel functions.
The central frequency is known to depend on strain and temperature with dependencecoefficients (m = 5) of 1.9 MHz/% and 11 kHz/K, respectively [20,21]. Meanwhile, thespectral linewidth ∆GB can be estimated by [23]:Δ GB =vs βπ,(3)where β is the acoustic loss, which includes the propagation loss in the optical fiber and thereflection loss at the boundary between the cladding and the external material [2,23].
3. Experimental setup and method

As an FUT, we employed an unjacketed 17-m-long silica SMF with a core diameter of 10.0μm, a cladding diameter of 125.8 μm, a core refractive index of ~1.46, and a propagation lossof ~0.2 dB/km at 1.55 μm. The FUT was immersed into sucrose solution in a water bath, theconcentration of which was controlled in the range from 0 to 45% to obtain arbitrary acoustic

Vol. 25, No. 3 | 6 Feb 2017 | OPTICS EXPRESS 2242

impedance (see Table 1; cited from [24]). The temperature of the sucrose solution was kept tothe 26.5 °C (which equals room temperature).
A schematic setup for observing the depolarized GAWBS spectrum is depicted in Fig. 1,
which is basically the same as that previously reported [20,21]. All the optical paths including
the FUT were silica SMFs. The output from a distributed-feedback laser diode at 1.55 μm(linewidth: 15 kHz, power: 15 dBm (maximal output)) was injected into the FUT as pumplight. Subsequently, the optical beat signal of the GAWBS forward-scattered light and thepump light was guided to a polarizer (PL) and then to a photo detector (PD) for optical-toelectrical conversion. The signal was finally monitored as GAWBS spectrum using anelectrical spectrum analyzer (ESA) with a 1-kHz frequency resolution and a 54-kHz video
bandwidth. Averaging was performed 1000 times. The peak power of the depolarized
GAWBS spectrum was maximized using polarization controllers (PCs), which indicates thatthe light beam can be regarded as a depolarized beam [20,21].
In this measurement, we selectively observed the spectral peak corresponding to the TR2,5mode [20], which exhibited the highest signal-to-noise ratio of all the other peaks [1]. We
performed noise-floor compensation in the following procedure: (i) the raw data of theGAWBS spectrum was obtained using the ESA, (ii) the noise floor (the spectrum when theFUT was removed from the experimental setup) was separately obtained, (iii) the noise floor
was subtracted from the raw data, and (iv) the obtained spectrum was fitted by a Lorentzian
curve [25]. Figure 2(a) shows the raw GAWBS spectrum of theTR2,5 mode. A clear peak wasobserved at 108.48 MHz (corresponding to vs = 3765 m/s; theoretical value: 107.7 MHz), but
the spectral shape was slightly distorted. The measurement error of the center frequency wasless than ± 0.006 MHz, which was calculated as a standard deviation of eight measured data
(obtained every 5 min). Figure 2(b) then shows the obtained noise floor, which included asmall peak at 107.87 MHz. Finally, the subtracted spectrum is shown in Fig. 2(c). The
spectral distortion was mitigated; the Lorentzian fit of this spectrum was used for GAWBScharacterization. The linewidth ∆GB was measured to be 0.211 MHz, and the acoustic loss β
was estimated to be 178 m–1. The measurement error of the linewidth (calculated in the same
method as that of the center frequency) was less than ± 0.001 MHz.
Table 1. Acoustic impedance of sucrose solution.

aConcentration [%]Acoustic impedance [kg/s·mm2] aRef. 24

01.51

10
1.58

20
1.68

30
1.79

40
1.92

45
2.00

Fig. 1. Schematic setup for observing depolarized GAWBS. ESA, electrical spectrum analyzer;PC, polarization controller; PD, photo detector; PL, polarizer.

Vol. 25, No. 3 | 6 Feb 2017 | OPTICS EXPRESS 2243

Fig. 2. Depolarized GAWBS spectra corresponding to the TR2,5 mode. (a) Raw data, (b) noisefloor, and (c) compensated plot. The dotted line in (c) indicates its Lorentzian fit.

4. Experimental results

The normalized depolarized GAWBS spectral dependence on external acoustic impedance isshown in Fig. 3(a). The spectrum changed with the increase of acoustic impedance. The
spectral peak power did not show a clear dependence on the acoustic dependence. From thismeasurement, the dependences of the linewidth and the central frequency on external acoustic
impedance can be plotted. Figure 3(b) shows the linewidth dependence on acoustic
impedance; here, the linewidth was defined as the full width at half maximum of the 3-dB
spectral bandwidth. The linewidth increased as the acoustic impedance increased, and thedependence in the acoustic impedance range below 2.0 kg/s·mm2 was almost linear with acoefficient of 0.16 MHz/kg/s·mm2. Note that this is a relative measurement, but a quantitative
absolute measurement might be feasible by theoretically analyzing the TR2,5-mode acousticwave model with boundary conditions, referring to the method proposed in [3]. Figure 3(c)shows the central frequency dependence on acoustic impedance. With increasing acousticimpedance, the central frequency linearly decreased with a coefficient of −0.07
MHz/kg/s·mm2. The sign of this value was opposite to the strain and temperature coefficients[3,20,21]. Considering Eq. (1), this may be elucidated by the solutions of the elastic wave
equations with the boundary conditions of a finite (non-vacuum) surrounding medium; further
study is required on this point. As the linewidth and the central frequency of the depolarized
GAWBS spectrum were linearly changed with external acoustic impedance, they can bepotentially used to perform acoustic impedance sensing. Besides, combined use of thelinewidth and the central frequency may enable discriminative sensing of acoustic impedance
and another parameter, such as strain and temperature, as stated in [3]. Note that therepeatability of these results was confirmed to be high by performing the same measurements
using other FUTs.

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Fig. 3. Measured acoustic impedance dependences of (a) depolarized GAWBS spectrum, (b) its
linewidth, and (c) its central frequency.

5. Conclusions

The acoustic impedance dependence of the depolarized GAWBS spectrum in an unjacketedsilica SMF was experimentally studied. The FUT was immersed into sucrose solution with theacoustic impedance ranging from 1.51 to 2.00 kg/s·mm2. As the acoustic impedance wasincreased, the spectral linewidth linearly increased with a coefficient of 0.16 MHz/kg/s·mm2.
In contrast, as the acoustic impedance increased, the central frequency linearly decreased witha coefficient of −0.07 MHz/kg/s·mm2, the sign of which was opposite to those of thetemperature and strain coefficients; this trend may be caused by the increase in the effective
outer diameter of the SMF. These results indicate that both the linewidth and the central
frequency of the depolarized GAWBS spectrum can be exploited to perform acoustic
impedance sensing. We also showed a potential for discriminative sensing of acoustic
impedance and another physical parameter. We believe that these findings will be of great use
in developing depolarized-GAWBS-based distributed acoustic impedance sensors in the near
future for underground oil layer detection and cancer detection in the human body.
Funding

JSPS KAKENHI (JP15H4012, 25709032, 26630180, 25007652); Japan Gas Association;
ESPEC Foundation for Global Environment Research and Technology; Association for
Disaster Prevention Research.