Sensors and Actuators A 268 (2017) 68–71

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Sensors and Actuators A: Physicaljournal homepage: www.elsevier.com/locate/sna

Measurement sensitivity dependencies on incident power and spatial
resolution in slope-assisted Brillouin optical correlation-domain
reﬂectometry
Heeyoung Lee ∗ , Yosuke Mizuno, Kentaro Nakamura
Institute of Innovative Research, Tokyo Institute of Technology, 4259, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japana r t i c l e

i n f oArticle history:
Received 13 July 2017
Received in revised form 12 October 2017
Accepted 31 October 2017
Available online 3 November 2017
Keywords:
Brillouin scattering
Optical ﬁber sensors
Distributed measurement
Strain and temperature sensing
Sensitivity
Modulationa b s t r a c t
To enable high-speed distributed strain and temperature measurements, we have recently developed a
new conﬁguration of Brillouin optical correlation-domain reﬂectometry (BOCDR), called slope-assisted(SA-) BOCDR, which exploits the slope power of the Brillouin gain spectrum. Although its fundamental
operations have been already clariﬁed, a detailed study on the measurement sensitivity has not been
performed yet. Here, we investigate the inﬂuences of the incident power and the spatial resolution
on the measurement sensitivity of SA-BOCDR. The sensitivity is found to be improved with increasing incident power and/or lowering spatial resolution, which is veriﬁed through distributed temperature
measurements.
© 2017 Elsevier B.V. All rights reserved.

1. Introduction
As a promising method for detecting damages of civil infrastructures, distributed ﬁber-optic sensing techniques based on Brillouinscattering [1] have been extensively studied for the past several
decades, and numerous conﬁgurations have been developed formore sophisticated damage inspections [2–7]. Each conﬁguration
has its own advantages and disadvantages and is being extensively
studied [8–20]. In particular, Brillouin optical correlation-domain
reﬂectometry (BOCDR) [6,8] is the only technique that can simultaneously achieve intrinsically single-end accessibility and highspatial resolution. Hence, ever since it was ﬁrst proposed in 2008,
various schemes of BOCDR have been implemented to improve its
performance, such as measurement range [13], spatial resolution[21], and sampling rate [22]. One of the newly developed conﬁgurations is slope-assisted (SA-) BOCDR [23], which uses the slope
power of the Brillouin gain spectrum (BGS) to deduce the Brillouinfrequency shift (BFS), leading to a high sampling rate, loss-point
detectability, and beyond-nominal-resolution effect [24].
Although the basic operations of SA-BOCDR have been well characterized using both silica ﬁbers [23,24] and polymer optical ﬁbers∗ Corresponding author.
E-mail address: hylee@sonic.pi.titech.ac.jp (H. Lee).
https://doi.org/10.1016/j.sna.2017.10.063
0924-4247/© 2017 Elsevier B.V. All rights reserved.

[25], no reports have been provided regarding the inﬂuences of theexperimental conditions on the measurement sensitivity. Unlikein the case of standard BOCDR, the measurement sensitivity of SABOCDR is susceptible to the variations of incident power and spatial
resolution, which determine the Brillouin signal power.
In this work, ﬁrst, by analyzing the BGS shape, we investigate themeasurement sensitivity dependencies on the incident power andthe spatial resolution in SA-BOCDR. We show that the sensitivity isenhanced with higher incident power and/or lower spatial resolution. Then, this result is veriﬁed through distributed temperature
measurements.

2. Principles
Distributed Brillouin sensors generally operate based on the
BFS dependencies on strain and temperature [2–7]. As alreadymentioned, BOCDR is the only technique with intrinsic singleend accessibility and relatively high spatial resolution [6,8]. Thissystem resolves the sensing positions using what is called a correlation peak, which is generated in a ﬁber under test (FUT) by
sinusoidally modulating the laser output frequency. The position of
the correlation peak can be continuously scanned along the FUT by
sweeping the modulation frequency fm ; in this manner, distributed
BFS measurements can be conducted. Sinusoidal frequency modulation leads to multiple correlation peaks generated along theH. Lee et al. / Sensors and Actuators A 268 (2017) 68–71

69

Fig. 1. Experimental setup for SA-BOCDR.

FUT periodically, and the measurement range dm (interval of thecorrelation peaks) is given by the modulation frequency fm as [8]
dm =

c
,
2 n fm(1)

where c is the velocity of light in a vacuum and n is the core refractive index. When fm is lower than the Brillouin bandwidth nB(approximately 30 MHz in silica single-mode ﬁbers (SMFs); dependent on incident optical power [26]), the spatial resolution z isgiven by [8]
z =

c B
,
2  n fm f(2)

where f is the modulation amplitude of the optical frequency.
In standard BOCDR, the strain (or temperature) information at
one sensing position is determined from a peak frequency (i.e., BFS)
after acquiring the whole BGS [6,8]. In contrast, in SA-BOCDR, thestrain (or temperature) information is obtained using not the BFS
itself but the spectral power change at a ﬁxed frequency vB0 (basedon its one-to-one correspondence with the BFS) [23]. As a result,the BFS distribution along the FUT is obtained as a power-change
distribution in SA-BOCDR. The measurement range and the nominal spatial resolution are given by the same equations as those forstandard BOCDR (Eqs. (1) and (2)).
3. Experimental setup
In the experiment, we employed a 3.0-m-long silica SMF as an
FUT. The experimental setup depicted in Fig. 1 is basically the sameas that previously used [23,24]. The output from a 1.55 ␮m laser(3 dB bandwidth: ∼1 MHz, power: 4 dBm) was divided into twolight beams, pump and reference. The incident power to the FUT
was changed by adjusting the output power of an erbium-dopedﬁber ampliﬁer (EDFA) in the pump path. The backscattered Stokes
light was ampliﬁed to ∼1 dBm using another EDFA and heterodyned with the reference light, which was ampliﬁed to ∼3 dBm
after passing through a 1-km-long delay line (used to control theorder of the correlation peak generated in the FUT). The heterodyned signal was converted to an electrical signal using a photo
diode (PD). The polarization state was scrambled to suppress thepolarization-dependent signal ﬂuctuations. The video bandwidth
and the resolution bandwidth of the electrical spectrum analyzer(ESA) were set to 10 kHz and 10 MHz, respectively.
The modulation frequency fm and amplitude f were set to10.858 MHz (swept from 10.825 to 10.891 MHz for distributed
measurement) and 0.06–3.08 GHz, respectively, corresponding toa measurement range of ∼9.5 m and a theoretical spatial resolution of 1.65–0.03 m according to Eqs. (1) and (2). For eachmeasurement, vB0 was adjusted to maximize the linear range (i.e.,
strain/temperature dynamic range) by differentiating the spectral

Fig. 2. Examples of the BGS when the spatial resolution z was 1.65 (purple), 0.43
(aqua), 0.18 (green), 0.09 (yellow), and 0.03 m (red). (a) Measured at 15-dBm incident power. (b) Measured at 24-dBm incident power. (For interpretation of thereferences to colour in this ﬁgure legend, the reader is referred to the web versionof this article.)slope; refer to [23] for the details. The repetition rate was set to
100 Hz, and 128 times averaging was performed on the oscilloscope to obtain a sufﬁciently high signal-to-noise ratio. The room
temperature was 21 ◦ C.
4. Experimental results
First, to clarify the measurement sensitivity (deﬁned as a spectral slope at vB0 ) at each incident power and spatial resolution,
we obtained the BGS at incident powers from 9 to 27 dBm (step:3 dB), while the spatial resolution was simultaneously varied in
the range from 0.03 to 1.65 m. Examples of the BGS measured at
15 and 24 dBm are shown in Figs. 2 (a) and 1 (b), respectively. In
both cases, the BGS gradually became weaker and broader as thespatial resolution grew higher (the value itself became smaller),
which leads to the reduction in its spectral slope. Here, the reduction in the peak power is natural if we simply consider that theBrillouin signal returns only from the ﬁber section roughly equal to
the spatial resolution. The broadening of the bandwidth caused by
the frequency modulation has also been reported [8]. Note that, inFig. 1(b), when the spatial resolution was 1.65 m, an irregular peak
was observed at ∼11.1 GHz. This peak, which originated from the

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H. Lee et al. / Sensors and Actuators A 268 (2017) 68–71

Fig. 4. Normalized power-change distributions along the FUT measured at four
experimental conditions. The solid curves are measured data, and the dotted curves
are calculated trends. The inset indicates the structure of the FUT.

Fig. 3. Measurement sensitivities plotted as functions of the spatial resolution atincident powers from 9 to 27 dBm (step: 3 dB).

2nd-order Brillouin peak [26], immediately disappeared by increasing modulation amplitude and did not inﬂuence the results of thismeasurement.
We then precisely analyzed the spectral slopes of each BGS andinvestigated the measurement sensitivity dependence on the spatial resolution. Here, the values of the optimal ﬁxed frequency vB0(at which the slope power is measured) and the bandwidth of thelinear region wL (which is evaluated by differentiating the spectral slope; refer to [23] for its detailed deﬁnition) were calculated
for each BGS. For instance, when the incident power was 24 dBm
and the spatial resolution was 1.65 m, vB0 was set to 10.83 GHz andwL was approximately 50 MHz (10.78–10.83 GHz). Fig. 3 shows themeasurement sensitivities plotted as functions of the spatial resolution at seven different incident powers. Regardless of the incident
power, the measurement sensitivity decreased and ﬁnally converged to 0 dB/GHz as the spatial resolution grew higher (the valueof the resolution itself grew smaller). At 9-dBm incident power, it
was difﬁcult to calculate the BGS slopes when the spatial resolution was lower than ∼0.3 m, because the BGS was buried by thenoise. Note that, as Brillouin-scattered power does not generally
increase in proportion to increasing incident power [1], we did notnormalize the plots in Fig. 3. Note also that, though the temperature
dynamic range seems to be dependent on incident power and spatial resolution as well, it cannot be simply evaluated by the rangeof the linear region because of the structural noise ﬂoor unique tocorrelation-domain techniques [27].
Subsequently, we performed distributed temperature measurements to verify that the results shown in Fig. 3 (indirectly acquired
based on the analysis of the BGS slope) can be practically employed.
The structure of the FUT is depicted in the inset of Fig. 4. While maintaining the temperature of a 0.30-m-long section (1.7 m away fromthe proximal FUT end) at 60 ◦ C using a heater, the power-change
distributions along the FUT were obtained under the following four
experimental conditions: (incident power Pin , spatial resolutionz) = (15 dBm, 0.09 m), (15 dBm, 0.18 m), (24 dBm, 0.09 m), and(24 dBm, 0.18 m). Fig. 4 shows the power-change distributions; thevertical axis was normalized so that the maximal value became 1
when (Pin , z) = (24 dBm, 0.18 m). Note that the proportions among
the four data on the vertical axis were maintained. The maximal
value of each data were in good agreement with the trends (shownas dotted curves in Fig. 3) calculated using Fig. 3. Note that, in thecalculation, the shape of the correlation peak was assumed to bea rectangle (refer to [24]), which is not accurate. The discrepancy
between the measured data and the calculated trends seems to bepartially caused by this assumption. For each power-change distribution, we measured the standard deviation of the noise ﬂoor
(signal ﬂuctuations of the non-heated sections). Under the four
experimental conditions: (Pin , z) = (15 dBm, 0.09 m), (15 dBm,
0.18 m), (24 dBm, 0.09 m), and (24 dBm, 0.18 m), the standard deviations (unit: ◦ C) were calculated to be approximately 4.8, 3.3, 1.2,
and 0.9 ◦ C, respectively. The sensing error decreased with increasing measurement sensitivity (increasing incident power and/or
lowering spatial resolution). Note that these values are also affectedby other experimental parameters, such as the measurement speed,
the number of averaging, etc. Thus, higher-power light was conﬁrmed to be required to enhance the measurement sensitivity as
well as the signal-to-noise ratio. However, injection of extremely
high-power light sometimes induces a so-called optical ﬁber fusephenomenon, which destroys the sensing ﬁber [28–30]. Consequently, the optimal incident power will be approximately 25 dBm,
which should be increased or decreased considering the actual situations.
5. Conclusion
Based on the analysis of the BGS shape, we investigated the measurement sensitivity dependencies on the incident power and thespatial resolution in SA-BOCDR. The sensitivity decreased and thenconverged to 0 dB/GHz as the incident power decreased and/or the
spatial resolution grew higher. Then, we veriﬁed this result throughdistributed temperature measurements. Thus, we believe that thiswork will provide a useful guideline in setting the experimental
conditions of SA-BOCDR and then in achieving single-end-accesshigh-speed distributed sensing with high spatial resolution in the
future.
Acknowledgements
This work was supported by JSPS KAKENHI Grant Numbers
17H04930 and 17J07226, and by research grants from the Japan
Gas Association, the ESPEC Foundation for Global Environment
Research and Technology, the Association for Disaster Prevention
Research, the Fujikura Foundation, and the Japan Association for
Chemical Innovation.
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Biographies

Heeyoung Lee received the B.E. degree in mechanicalengineering from Kyungpook National University, Korea,
in 2014, and the M.E. degree in information processing
from Tokyo Institute of Technology, Japan, in 2017. She
has been continuing to study ﬁber-optic sensors for her
Dr.Eng. degree at Tokyo Institute of Technology. She is currently a Research Fellow (DC1) of the Japanese Society for
the Promotion of Science (JSPS).

Yosuke Mizuno received the B.E., M.E., and Dr.Eng.
degrees in electronic engineering from the University ofTokyo, Japan, in 2005, 2007, and 2010, respectively. Since
2012, he has been an Assistant Professor at the Precision
and Intelligence Laboratory (presently, Institute of Innovative Research), Tokyo Institute of Technology, where heis active in ﬁber-optic sensing, polymer optics, and ultrasonics.

Kentaro Nakamura received the B.E., M.E., and Dr.Eng.
degrees from Tokyo Institute of Technology, Japan, in
1987, 1989, and 1992, respectively. Since 2010, he has
been a Professor at the Precision and Intelligence Laboratory (presently, Institute of Innovative Research), Tokyo
Institute of Technology. His research ﬁeld is the applications of ultrasonic waves, measurement of vibration andsound using optical methods, and ﬁber-optic sensing.