Observation of Backward
Guided-Acoustic-Wave Brillouin
Scattering in Optical Fibers Using
Pump–Probe Technique
Volume 8, Number 3, June 2016
Neisei Hayashi
Heeyoung Lee
Yosuke Mizuno, Member, IEEE
Kentaro Nakamura, Member, IEEE

DOI: 10.1109/JPHOT.2016.2550321
1943-0655 Ó 2016 IEEEIEEE Photonics Journal

Observation of Backward GAWBS

Observation of Backward
Guided-Acoustic-Wave Brillouin
Scattering in Optical Fibers Using
Pump–Probe Technique
Neisei Hayashi,1 Heeyoung Lee,2 Yosuke Mizuno,2 Member, IEEE, andKentaro Nakamura,2 Member, IEEE
1

Research Center for Advanced Science and Technology, University of Tokyo,
Tokyo 153-8904, Japan
2
Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute ofTechnology, Yokohama 226-8503, Japan
DOI: 10.1109/JPHOT.2016.2550321
1943-0655 Ó 2016 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received March 21, 2016; revised March 28, 2016; accepted March 29, 2016. Date ofcurrent version April 26, 2016. This work was supported in part by the Japan Society for the Promotion of Science under KAKENHI Grant 25709032, KAKENHI Grant 26630180, and KAKENHI Grant
25007652 and in part by research grants from the Iwatani Naoji Foundation, the SCAT Foundation,and the Konica Minolta Science and Technology Foundation. Corresponding author: N. Hayashi(e-mail: hayashi@cntp.t.u-tokyo.ac.jp).

Abstract: We experimentally demonstrate the observation of backward (i.e., opposite ofthe direction of the pump) guided-acoustic-wave Brillouin scattering (GAWBS) using thepump–probe technique and investigate the spectral dependence on the pump–probe
light power. We show that the signal-to-noise ratio (SNR) of the backward GAWBS measurement can be drastically improved by using this technique. We also observe a nonmonotonic dependence of the SNR on the probe power, which indicates that optimalprobe power exists for implementing GAWBS-based distributed sensors.
Index Terms: Guided-acoustic-wave Brillouin scattering (GAWBS), optical fiber sensors,nonlinear optics, pump-probe technique.

1. Introduction
Brillouin scattering in optical fibers, which is caused by the interaction between photons andacoustic phonons, can be categorized into two varieties: 1) standard Brillouin scattering, inwhich incident light interacts with longitudinal acoustic waves; and 2) guided-acoustic-wave
Brillouin scattering (GAWBS) [1] (also referred to as forward Brillouin scattering [2], cladding
Brillouin scattering [3], and transverse stimulated Brillouin scattering [4]), in which incident lightinteracts with transverse acoustic waves. In standard Brillouin scattering, the scattered light propagates backward (i.e., opposite to the direction of the pump) accompanying the Brillouin frequency shift (BFS), which is on the order of gigahertz (although several higher order peaks
sometimes appear). On the other hand, during GAWBS, the scattered light propagates forwardwith multiple frequency shifts [which we refer to as guided-acoustic-wave Brillouin frequency
shifts (GAW-BFSs)]; these are lower than 1 GHz and correspond to the resonance modes.
To date, standard longitudinal Brillouin scattering in optical fibers has been extensively
exploited in order to develop various optical devices and systems, including not only lasers [5],Vol. 8, No. 3, June 2016

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Observation of Backward GAWBS

slow light generators [6], optical comb generators [7], and all-optical logic gates [8], but distributed strain and temperature sensors as well [9]–[18]. However, no report has been provided on
GAWBS-based distributed measurements, despite the fact that the GAW-BFS exhibits strain
and temperature dependencies in the same manner that the standard BFS does [19], [20]. By
using GAWBS, several physical parameters—such as the fiber outer diameter [21] and theacoustic impedances of the surrounding materials—can be additionally measured in a distributed manner. Thus, in order to fill this lacuna in the literature, in this study, we aim to achieve
GAWBS-based distributed sensing.
Some distributed sensing techniques have been developed for systems based on standard
Brillouin scattering, including time-domain [9]–[14] and correlation-domain techniques [15]–[18].
In the effort to directly apply these techniques to GAWBS-based systems, problems arise as aresult of the mismatch between the scattering direction required by these techniques and thatprovided by GAWBS. More specifically, all the previously developed techniques exploit the optical path difference of the scattered light beams, and the scattered light needs to propagate
backward (otherwise, the optical path difference will not occur). However, GAWBS typically generates forward-scattered light, which cannot be directly used to develop such distributed sensing
systems. Therefore, successful observation of backward GAWBS light (which we define as the
GAWBS light propagating in the direction opposite to the incident light) is extremely crucial to
the task of integrating these various distributed sensing techniques into GAWBS-based
systems.
One method of observing backward GAWBS light is to exploit stimulated Brillouin scattering(SBS), which is a phenomenon whereby the longitudinal Brillouin scattering become significantly
enhanced via electrostriction when the power of the incident pump light is higher than a certain
threshold. Tanaka et al. [22] and Dossou et al. [23] have reported the backward observation of
GAWBS light, which consists of two components: the SBS-backscattered GAWBS light inducedby the high-power pump light, and the GAWBS light induced by the SBS-backscattered highpower pump light. As the optical paths of the two components are the same, the two components need not be separated in order to develop GAWBS-based distributed sensors.
In practical applications of standard Brillouin sensors, the so-called pump-probe technique
is often employed in order to improve the signal-to-noise ratio (SNR) of the measurement[24]–[26]. According to this technique, probe light and pump light are injected into opposite
ends of the fiber under test (FUT). Using this technique, SBS can be induced at a much
lower threshold power, leading to a higher-power Brillouin-scattered signal under the same
incident pump power. Although the pump-probe technique also appears to be effective at improving the SNR of the backward GAWBS measurement, there have been no detailed reports of this phenomenon.
In this paper, we employ the pump-probe technique to observe backward GAWBS light, andinvestigate its spectral dependencies on the pump and probe light powers. We experimentallydemonstrate that the pump-probe technique is effective at improving the SNR. More specifically,
we show that the SNR dependence on the probe power is non-monotonic, which indicates theexistence of an optimal probe power to be used in implementing actual GAWBS-based distributed sensors.

2. Principle of GAWBSGAWBS, which is typically a forward scattering phenomenon, results from the interaction between incident light and the transverse modes of acoustic waves excited by small thermally induced vibrations of glass molecules [1]. The refractive index of the fiber is perturbed by the
acoustic modes, leading to phase or polarization modulation in the scattering process. The
GAW-BFSs are given by the resonance frequencies of the acoustic modes, which are mainly
determined by the acoustic velocity (including longitudinal and transverse acoustic waves) andthe fiber outer diameter. The transverse acoustic modes that cause GAWBS can be classified
into two varieties [1]: radial R0;m modes and mixed torsional-radial TR2;m modes, where m is theVol. 8, No. 3, June 2016

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order of the resonance. The R0;m modes have symmetric axes, whereas the TR2;m modes havesymmetric planes along the optical fiber. The R0;m modes perturb the refractive index radiallyand maintain the polarization of the scattered light (referred to as polarized GAWBS). In contrast, the TR2;m modes perturb the birefringence and modulate the polarization of the scattered
light (referred to as depolarized GAWBS). The GAW-BFS of the m-th acoustic mode vGB;m isgiven by [1]vGB;m ¼

vym
d

(1)

where d is the fiber outer diameter, v is the velocity of the longitudinal acoustic wave vL (for polarized GAWBS) or transverse acoustic wave vs (for depolarized GAWBS), and ym is the value
derived from the following equation [1]:




y2
y2
6  2m J2 ðym Þ  3ym J3 ðym Þ
3  2m J2 ðym Þ;


¼0
(2)
y2
2  2m J2 ðym Þ þ ym J3 ðym Þ
J2 ðym Þ  ym J3 ðym Þ;
where  ¼ vs =vL and J2 and J3 are the second- and third-order Bessel functions. The spectral
powers of the TR2;m modes are reported to be higher than those of the R0;m modes [1].
The GAW-BFSs of the TR2;m modes are known to show strain and temperature dependencies; for instance, when m is 5, the GAW-BFS linearly depends on the strain and temperature
with coefficients of ∼1.9 MHz/% (at 1319 nm) [19] and ∼11 kHz/K (at 1550 nm) [20], respectively. The linewidths of the GAWBS spectral peaks are also reported to strongly depend on the
acoustic properties (acoustic impedance, etc.) of the material attached to the optical fiber [19],[27]. For instance, the linewidth of the GAWBS peak (TR2;5 mode) observed using a bare fiber
is ∼0.3 MHz, which increases to ∼4 MHz when the fiber is attached to a polymer. This behavior
may be applicable to the acoustic impedance measurement of the attached materials.
The observation of the backward GAWBS signal has been performed by injecting high-powerpump light [3], [22], [23]. If the pump power becomes higher than the SBS threshold, we can observe part of the GAWBS signal in the backward direction. The backward GAWBS signal involves two components. One is the GAWBS light that first propagates forward as a result of thehigh-power pump light and is then backscattered by the SBS process; the other is the GAWBS
light that is induced by the SBS-backscattered high-power pump light. One useful technique toreduce the SBS threshold is the pump-probe technique [24]–[26], in which probe light is additionally injected into the FUT from the other end. By setting the probe frequency to v0  vB (v0 :pump frequency, vB : BFS of the FUT), the energy transfer from pump to probe can be drasticallyaccelerated.

3. Experimental Setup
The experimental setup for observing backward GAWBS light using the pump-probe technique
is depicted in Fig. 1. A 10 km long silica single-mode fiber (SMF) with a BFS of 10.86 GHz wasused as the FUT. The laser output at 1550 nm, which had a power of 6.0 dBm and a linewidthof ∼1 MHz, was divided into two components using a 3 dB coupler. One light beam was usedas pump light, which was polarization-controlled, amplified using an erbium-doped fiber amplifier(EDFA), and injected into the FUT. The pump light, which was transmitted through the FUT,
was guided to the air via an optical circulator in order to prevent it from being reflected and injected again into the FUT. The other light beam was used as probe light, which was frequencydownshifted by the BFS of the FUT (= 10.86 GHz) using a single-sideband modulator (SSBM;
the carrier suppression ratio was > 20 dB), amplified using another EDFA, and injected into the
FUT from the other end. The GAWBS light propagating backward was polarized and guided to a
photo detector (PD) along with the transmitted probe light. Using the PD, the beat signal between the GAWBS light and the transmitted probe light was converted into an electrical signal,
which was observed using an electrical spectrum analyzer (ESA). The polarization state wasVol. 8, No. 3, June 2016

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Fig. 1. Experimental setup for observing the backward GAWBS light using the pump–probe technique (EDFA: erbium-doped fiber amplifier; ESA: electrical spectrum analyzer; FUT: fiber under
test; PC: polarization controller; PD: photodetector; SSBM: single-sideband modulator).

Fig. 2. Wide-range view of the observed GAWBS spectrum.

adjusted using a polarization controller (PC) so that the observed GAWBS signal was maximal.
The ambient temperature was 26 °C.

4. Experimental Results
Fig. 2 shows the backward GAWBS spectrum observed for a pump power of 12.0 dBm and aprobe power of 0 dBm. Approximately 10 clear peaks appeared in the frequency range from
200 MHz to 800 MHz. The GAW-BFS values of each peak agreed well with the theoreticalvalues of each TR2;m mode [1].
Subsequently, we measured the pump power dependence of the GAWBS spectrum near the
peak at ∼320 MHz (corresponding to the TR2;18 mode, which exhibits one of the clearest peaks).
The probe power was 0 dBm. As the pump power increased, the spectral peak power increased, but the noise floor also increased [see Fig. 3(a)]. Therefore, here we define the SNR
as the ratio between the spectral peak power and the spectral power at 340 MHz. Fig. 3(b)shows the pump power dependence of the GAWBS spectrum, which was normalized so thatthe spectral power at 340 MHz was 0 dB. The spectrum at the pump power of 3.6 dBm (regarded as the noise floor of the ESA) was subtracted from all the spectra. The SNR increased

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Fig. 3. (a) Pump power dependence of the GAWBS spectrum around 320 MHz. (b) Pump power
dependence of the GAWBS spectrum normalized in order for the spectral power at 340 MHz to
be 0 dB. (c) Pump power dependence of the SNR of the backward GAWBS measurement withand without the probe light.

Fig. 4. (a) Probe power dependence of the GAWBS spectrum around 320 MHz. (b) Probe power
dependence of the GAWBS spectrum normalized in order for the spectral power at 340 MHz to be
0 dB. (c) Probe power dependence of the SNR of the backward GAWBS measurement for a pump
power of 12 dBm.

as the pump power increased. The SNR was then plotted as a function of the pump power, as
shown in Fig. 3(c), which also shows the results obtained when the probe light was not injected. The backward GAWBS became observable at a pump power of 3.6 dBm, which was∼10 dB lower than the value observed without using the probe light. The SNR increased linearly as the pump power increased with a slope of 0.18 dB/dBm, which was almost twice aslarge as the value obtained without using the probe light. This result clearly showcases the effectiveness of the pump-probe technique in improving the SNR of the backward GAWBS
measurement.
Finally, we measured the probe power dependence of the GAWBS spectrum at ∼320 MHz.
The pump power was 12.0 dBm. As shown in Fig. 4(a), as the probe power increased, the spectral peak power as well as the noise floor increased. The probe power dependence of the
GAWBS spectrum normalized using the spectral power at 340 MHz is shown in Fig. 4(b). The
spectrum at the probe power of −21.0 dBm was subtracted from all the spectra in order to suppress the influence of the noise of the ESA. Interestingly, the SNR obtained at a probe power of−9.0 dBm was higher than that obtained at 0 dBm. The SNR dependence on the probe power isshown in Fig. 4(c). The backward GAWBS signal became observable at a probe power of approximately −21 dBm. As the probe power increased up to −9.0 dBm, the SNR monotonically increased. However, as the probe power further increased above −9.0 dBm, the SNR decreased.
This behavior can be explained by the fact that the increase in the noise floor (the influence ofthe noise caused by the ESA was negligible in this measurement) was more significant than theincrease in the GAWBS signal. Note that this noise floor is actually the foot of the beat signal
between the probe light and the SBS light; the power of the SBS light is much higher than thatVol. 8, No. 3, June 2016

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of the GAWBS light. As it is infeasible in principle to suppress this noise floor while maintaining
the peak power of the backward GAWBS light, such SNR deterioration is inevitable in thisscheme. This result indicates that an optimal probe power exists for developing GAWBS-based
sensors using the pump-probe technique.

5. Conclusion
The backward GAWBS observation was successfully performed for the first time using thepump-probe technique, and the spectral dependencies on the pump and probe powers were investigated. The SNR was experimentally shown to drastically increase as a result of using thistechnique. More specifically, a non-monotonic dependence of the SNR on the probe power (at afixed pump power) suggested the existence of an optimal probe power to be used in developing
GAWBS-based sensors. These results constitute the first step in the development of backward
GAWBS-based distributed sensing, and will be useful in measuring quantities such as strain,temperature, fiber outer diameter, and acoustic impedance in the future.

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