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Japanese Journal of Applied Physics 56, 072501 (2017)https://doi.org/10.7567/JJAP.56.072501

Single-end-access distributed strain sensing with wide dynamic range
using higher-speed Brillouin optical correlation-domain reﬂectometry
Yosuke Mizuno1*, Neisei Hayashi2, Hideyuki Fukuda3, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Tokyo 153-8904, Japan
3
Servo Laboratory, FANUC Corporation, Oshino, Yamanashi 401-0597, Japan2*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received February 24, 2017; revised April 10, 2017; accepted May 8, 2017; published online June 13, 2017
Single-end-access real-time ﬁber-optic distributed strain sensing has recently been demonstrated using an ultrahigh-speed conﬁguration of
Brillouin optical correlation-domain reﬂectometry (BOCDR). Its extremely high sampling rate was, however, achieved at the cost of a limited strain
dynamic range (<0.2%). Here, by employing a noise-ﬂoor compensation technique, we develop a new cost-effective higher-speed conﬁguration of
BOCDR with a wide strain dynamic range (up to 2.0%; evaluated by static strain measurement). This value is larger than that of any other BOCDR
conﬁguration. Using this conﬁguration, we demonstrate some fundamental distributed strain measurements and breakage detection.
© 2017 The Japan Society of Applied Physics

1.

Introduction

Brillouin scattering1) has been extensively exploited todevelop ﬁber-optic sensors due to the feasibility of distributed strain and temperature measurement. Various kinds of
distributed Brillouin sensing techniques have been implemented thus far, including Brillouin optical time-domain
analysis (BOTDA),2–12) Brillouin optical frequency-domain
analysis (BOFDA),13–16) Brillouin optical correlation-domain
analysis (BOCDA),12,17–24) Brillouin optical time-domain
reﬂectometry (BOTDR),25–29) and Brillouin optical correlation-domain reﬂectometry (BOCDR).30–36) Among thesetechniques, “analysis” systems require injection of two lightbeams into both ends of a ﬁber under test (FUT) to induce
stimulated Brillouin scattering leading to a high signalto-noise ratio (SNR). In contrast, “reﬂectometry” systems,
which operate based on spontaneous Brillouin scattering,
require injection of a light beam into only one end of an FUT.
In general, two-end-access analysis systems pose two major
problems. First, they are lacking in convenience for practically embedding the sensors into materials and structures;
furthermore, the measurement can no longer be performed
when extremely high loss or breakage occurs at a point along
the FUT. Although some quasi-one-end-access conﬁgurations16,21,22) have been developed by using the Fresnel
reﬂection at the open end of an FUT, the measurement difﬁculties associated with a breakage point cannot be perfectly
overcome. The second problem is that a relatively expensive
electro-optic modulator (EOM), such as a single-sideband
modulator, is often needed to accurately produce the frequency diﬀerence between the two light beams (pump andprobe).3–18,20–24) Moreover, most conﬁgurations require an
additional EOM for optical pulse generation2–12) (also true for
BOTDR)25–29) or chopping for lock-in detection.17,18,20–24)
Although a time-division pump–probe generation scheme19)
does not require an EOM for pump=probe generation, thedriving current of the laser diode (LD) needs to be precisely
controlled. Thus, considering the practical convenience andcost eﬃciency, here we focus on BOCDR.
BOCDR is known to be an inherently single-end-accesssensing technique with high spatial resolution, system simplicity, and cost eﬃciency, but conventionally, its sampling

rate was limited to 19 Hz due to the relatively slow frequency
sweeping.32) In 2016, we developed an ultrahigh-speed conﬁguration of BOCDR,38) in which the Brillouin gain spectrum (BGS) in the time domain was approximated by a oneperiod sinusoidal waveform. The Brillouin frequency shift(BFS) was converted into its phase delay, which was subsequently converted into a voltage so that the BFS information can be directly obtained. In this conﬁguration, a strain
sampling rate of up to 100 kHz at an arbitrary position was
experimentally veriﬁed. However, the strain dynamic range
was limited to 0–0.2% because of the nature of phasedetection. Another high-speed conﬁguration of BOCDR
exploits the spectral power of the BGS,39,40) but its strain
dynamic range was narrower than 0–0.1% because of thelimited linear range of the BGS slope. Note that even thewidest strain dynamic range of lower-speed BOCDR conﬁgurations was 0–0.7%,35,36) limited by the non-negligible
correlation-peak sidelobes (the inﬂuence of which can be
mitigated by apodization).35)
In this work, we develop a new cost-eﬀective higher-speed
conﬁguration of BOCDR with a wide strain dynamic range ofup to 2.0%. Here, the frequency sweeping for acquiring the
BGS is performed at higher speed without using the inherent
sweeping function of an electrical spectrum analyser (ESA),
which is used only to detect the signal power at a ﬁxed
frequency component. The noise caused by the external frequency sweeping is suppressed by obtaining the diﬀerential
signal between periods of operation with and without theBrillouin signal, resulting in a high SNR and enhanced strain
dynamic range. The additionally required main devices
only include a relatively low-performance frequency sweeper[or a standard voltage-controlled oscillator (VCO)] and an
inexpensive sound board connected to a computer, thereby
maintaining the system simplicity and cost eﬃciency. Notethat this work is an evolutionary version of our previous
work published in a conference proceeding,41) where similar
experimental results are presented but not in detail becauseof the strictly limited space. Here, mainly focusing on thestrain dynamic range, more detailed background, experimental conditions, and discussions are described; furthermore,some experimental results on breakage detection using thisconﬁguration are presented for the ﬁrst time.

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Jpn. J. Appl. Phys. 56, 072501 (2017)2.

Y. Mizuno et al.

Principles and methods

BOCDR operates based on the controlled correlation of continuous light beams. As described in detail in literatures,30,31)
by sinusoidally modulating the frequencies of pump lightand reference light at modulation frequency ( fm), so-called
correlation peaks37) (which can be regarded as sensing points)
are periodically generated in the FUT. The measurement
range dm corresponds to the interval between the correlation
peaks, which is in inverse proportion to fm, as follows:31)
dm ¼

c
;
2nfm

ð1Þ

where c is the velocity of light in a vacuum and n is therefractive index of the ﬁber core. By sweeping fm, the correlation peak can be scanned along the FUT to derive a BGS or
BFS distribution. The spatial resolution Δz is expressed as31)
z ¼

cvB
;
2nfm fð2Þ

where ΔvB is the Brillouin bandwidth [∼30 MHz in silica
single-mode ﬁbers (SMFs)] and Δ f is the modulation
amplitude of the optical frequency. The upper limit of Δ f
is half the BFS of the FUT due to the Rayleigh noise.30,31) By
the sweeping speed of an ESA, the sampling rate including
data acquisition to the computer was limited to 19 Hz.32) In
the meantime, the maximal measurable strain experimentally
conﬁrmed was approximately 0.1% in a slope-assisted conﬁguration,39) 0.2% in an ultrahigh-speed conﬁguration,38)
0.3% in a temporal gating scheme,33) and 0.7% in an intensity-modulation-based apodization scheme35) or in a lock-in
detection scheme.36)
The experimental setup of the newly conﬁgured BOCDR
is schematically shown in Fig. 1. The ﬁber-optic parts were
almost the same as those in a conventional setup.34) After
optical-to-electrical conversion of the heterodyned Brillouinsignal using a photo detector (PD), the BGS was acquired athigh speed by converting the signal from the frequency
domain to the time domain (note that this step is essentiallythe same as that in Ref. 38). The Brillouin signal was mixed
with the output of a microwave frequency sweeper (MFS); its
maximal repetition rate was 50 Hz in this experiment, which
can be enhanced further by employing a higher-performance
MFS or a VCO. The frequency of the MFS was repeatedly
swept for several hundred megahertz to scan the BGS in thefrequency domain. The power at a ﬁxed frequency component (carefully selected so that the whole BGS might becovered even when strain or temperature change was applied)of the mixed signal was subsequently output from the
ESA using a zero-span mode, and the BGS was repeatedly
reproduced in the time domain. Needless to say, the BGS
acquisition must be slower than the calculation time required
for the BFS derivation from the BGS (in our previous
setup,38) the BGS was then converted into a one-period
sinusoidal waveform to achieve an even higher sampling rate
at the cost of the limited strain dynamic range, but here we
did not perform this step to maintain a wider dynamic range).
Next, the BGS signal in the time domain is input to the computer via an inexpensive sound board (ASUSTek XONAR
U7) and monitored using a virtual oscilloscope triggered at
the repetition frequency of the MFS. The BGS was observed

Fig. 1. (Color online) Experimental setup of high-speed BOCDR. AMP,
ampliﬁer; BPF, band-pass ﬁlter; DAQ, data acquisition; EDFA, erbiumdoped ﬁber ampliﬁer; ESA, electrical spectrum analyzer; FG, function
generator; FUT, ﬁber under test; IF, intermediate frequency; LO, local
oscillator; LPF, low-pass ﬁlter; MFS, microwave frequency sweeper; MG,microwave generator; NFL, noise ﬂoor; OSC, oscilloscope; PD, photodetector; PSCR, polarization scrambler; RF, radio frequency.

on a real-time basis, but was largely distorted because of theconsiderable intensity noise of the MFS output, which wasdependent on the output voltage (or frequency). To suppress
this noise and clearly observe the BGS, the diﬀerential signal
between periods of operation with and without the Brillouinsignal (i.e., between the two spectra for which the 980 nm
pump LD of the EDFA in the Brillouin-pump path is andis not working) was calculated; after low-pass ﬁltering, it
was ﬁnally acquired as the BGS used for the measurements(note that this noise-ﬂoor compensation technique was not
required in the ultrahigh-speed conﬁguration,38) in which thedistorted BGS was directly approximated by a sinusoidal
waveform; this was feasible because the frequency span was
signiﬁcantly limited).
3.

Experimental results

3.1

Fundamental characterization
First, we veriﬁed that the local BGS can be correctly observed at 50 Hz (maximal repetition rate of the MFS). A
7.0 m long silica SMF (composed of a 0.8 m long pigtail ofan optical circulator connected to a 6.2 m long SMF using an
FC=APC adaptor) was used as the FUT. The open end was
cut at an angle of 8° in order to suppress the Fresnel reﬂection. The modulation frequency fm was set to 13.536 MHz,
with a correlation peak located at the point 4.7 m away fromthe circulator. The measurement range dm was calculated tobe 7.6 m according to Eq. (1). The modulation amplitude Δ f
was set to 0.5 GHz, resulting in a theoretical spatial resolution
Δz of ∼0.15 m from Eq. (2). The ratio of the measurement
range to the spatial resolution was 51, which can be extended
to ∼570 simply by increasing Δ f to half of the BFS, i.e.,∼5.4 GHz (the Δ f values were kept below 1.5 GHz in thispaper to avoid damage to the LD, which was not designed forsuch modulation use). The 64th-order correlation peak wasused, and the temperature of the room was 18 °C.
The green curve in Fig. 2(a) shows the direct output fromthe ESA, which suﬀers from so much noise that the BFS
cannot even be determined. Note that the vertical axis was
normalized in a linear scale so that the maximal voltage of072501-2© 2017 The Japan Society of Applied Physics

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Y. Mizuno et al.

(a)

(a)

(b)

(c)
(b)

(d)

(c)
Fig. 2. (Color online) (a) Normalized direct output from the ESA (green),noise ﬂoor (blue), and the intrinsic BGS obtained as their diﬀerence (red).
(b, c) Measured BGS and BFS dependences on a relatively large strain of up
to 2.0%, respectively. In (c), the orange solid circles are measured points,
whereas the blue line is a linear ﬁt.

this spectrum became 1. To suppress this noise, the noise
ﬂoor, i.e., the spectrum when the EDFA in the pump path
was switched oﬀ, was recorded [blue curve in Fig. 2(a)] andsubtracted from the ESA output, resulting in the intrinsic
BGS as shown as the red curve in Fig. 2(a). The dip at∼11.3 GHz, which originates from the sharp spectral noisedue to the frequency range switching of the MFS, can be
ﬂattened with an appropriate low-pass ﬁlter (LPF). Thus, it
was conﬁrmed that the local BGS was obtained at 50 Hz(we conﬁrmed that a >200-Hz sampling rate can be achieved
simply by replacing the MFS with a VCO; refer to Sect. 2 forthe limiting factor).
Next, a wide strain dynamic range was demonstrated.
The experimental conditions were the same as those for(e)Fig. 3. (Color online) Structures of FUTs. (a) Fixed-point measurementof static strain. (b) Distributed measurement with a higher signal-to-noise
ratio. (c) Distributed measurement with a higher spatial resolution. (d) Fixedpoint measurement of dynamic strain (not sinusoidally applied).
(e) Distributed measurement for breakage detection.

evaluating the noise ﬂoor described above. In the 7.0 m long
SMF used as the FUT, strains of up to 2.0% were applied to a
0.4 m long section (4.5–4.9 m away from the circulator) ﬁxed
on a translation stage [Fig. 3(a)]. The correlation peak was
located at the midpoint of the strained section. The averageof 5 measurements revealed the sampling rate of 10 Hz.
Figure 2(b) shows the measured BGS dependence on strain.
Note that all the BGSs were normalized. With increasingstrain, the BGS shifted to a higher frequency. At 2.0% strain,
the translation stages began to slightly slip, at which point the072501-3© 2017 The Japan Society of Applied Physics

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Y. Mizuno et al.

correct amount of strain was not applied; besides, at suchlarge strain, considerable optical loss was engendered at
the ﬁxed (and pressed) positions, resulting in a weak BGS,
which was distorted if normalized. Figure 2(c) shows the
BFS dependence on strain, which was almost linear. The
slope calculated using the data (excluding that of the 2.0%
strain) was 509.5 MHz=%, which moderately agrees withthe previously reported values at 1550 nm.30) In this manner,
the wide strain dynamic range of ∼2.0% was demonstrated.
The maximal measurable strain, which is about three times
the largest value previously reported for BOCDR,35,36) iseven close to the fracture strain of silica SMFs, which is
generally several percent.
3.2 Distributed and dynamic strain measurements
Distributed measurements were subsequently demonstrated.
First, a 7.0 m long silica SMF was used as the FUT, in which
strains of 0.3 and 0.6% were applied to a 0.4 m long section(4.5–4.9 m) [Fig. 3(b)]. The modulation frequency fm was
swept from 13.406 to 13.601 MHz. The measurement range
dm was calculated to be ∼7.6 m. The modulation amplitude
Δ f was set to 0.5 GHz, corresponding to the nominal spatial
resolution Δz of ∼0.15 m. The sampling rate for obtaining
one BGS was 50 Hz and the number of sensing points was set
to 200, leading to a measurement time of ∼4 s. The measured
BGS distributions (normalized) are shown in Figs. 4(a)–4(c),and the corresponding BFS distributions are shown in
Fig. 4(d), where the strained sections were correctly detectedwith a high SNR. The non-normalized data are discussed in
the following Section (i.e., breakage detection).
Next, a 0.6% strain was applied to an 80 mm long section(5.58–5.66 m) of the same FUT [Fig. 3(c)], and Δ f was set to
1.5 GHz, corresponding to Δz of ∼48 mm. The measured
BGS and BFS distributions are shown in Figs. 4(e) and 4(f),respectively, and the magniﬁed view of the latter around thestrained section is shown in Fig. 4(g). Although the SNR
deteriorated because of the enhanced spatial resolution, thestrained section was nevertheless successfully detected. Notethat the plot pattern slightly varies measurement by measurement and that the non-sharp BFS change is sometimes
obtained probably because of the inﬂuence of the strain
distribution inside the glue.
We then demonstrated the system’s capacity for dynamic
strain sensing. A 0.4% static strain was applied in advanceto a 1.0 m long section (3.0–4.0 m) of a 7.0 m long silica
SMF [Fig. 3(d)]; the static strain was applied because it
was diﬃcult to stably apply dynamic strains to an unstrained
section. Dynamic strains at 2.2 and 4.0 Hz were then applied
to the strained section, not sinusoidally, using a rotor with a
33 mm long arm (Fig. 5). By setting fm to 13.503 GHz, thecorrelation peak was placed at the midpoint of the strained
section (dm ≃ 7.6 m). The spatial resolution Δz was ∼0.36 m(Δ f = 0.2 GHz), and the sampling rate was 50 Hz. No
averaging was performed. The measured temporal variationsof the BGS and BFS at 2.2 Hz [Figs. 6(a) and 6(b)] and at
4.0 Hz [Figs. 6(c) and 6(d)] indicate that the dynamic strains
were successfully detected with a much higher SNR than thatof a previous report.32)
We here discuss the measurement error of this system bycomparing it with that of the ultrahigh-speed conﬁguration of
BOCDR.38) As already described in the Introduction, in theultrahigh-speed conﬁguration, the BFS is converted into the

(a)

(b)

(c)

(d)

(e)

(f)

(g)
Fig. 4. (Color online) Distributed measurement results. (a, b, c) BGS
distributions for locally applied strains of 0.0, 0.3, and 0.6%, respectively. (d)Corresponding BFS distributions. (e) BGS distribution for a locally applied
0.6% strain. (f) Corresponding BFS distribution and (g) its magniﬁed view.

Fig. 5. (Color online) Method for applying dynamic strain (0.4–0.6%) toa ﬁxed section of the FUT using an armed rotor.

phase of a sinusoidal waveform. Although we attempt toseek a frequency range which linearly depends on the BFS,
the linearity is not perfect, which inevitably results in some072501-4© 2017 The Japan Society of Applied Physics

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Y. Mizuno et al.

(a)

(b)

(a)

(b)

(c)

(d)

(c)

(d)

Fig. 6. (Color online) Measurement results of dynamic strain. Temporalvariation of (a) BGS and (b) BFS at 2.2 Hz, as well as (c) BGS and (d) BFS
at 4.0 Hz. The circles are measured points, whereas the solid curves indicate
the theoretical trends.

measurement error. On the other hand, in the conﬁguration
presented here, the BFS value is directly used for strain
evaluation, and consequently, the nonlinearity-induced measurement error can be inherently avoided. The actual sensing
error of the system depends on many parameters including
the number of averaging. When we need to achieve highspeed measurement, the measurement error becomes relatively serious [see Figs. 6(b) and 6(d)]; however, this error
can be suppressed by averaging the data suﬃcient times.
3.3 Breakage detection
We also demonstrated the detection of a breakage pointcaused during a distributed measurement. This kind of
demonstration has not been reported anywhere at least incorrelation-domain schemes, and thus it is important toexperimentally prove the merit of single-end accessibility(the light reﬂected at the breakage point may result in
deterioration of the measurement). A 7.0 m long silica SMF
was used as the FUT, in which a 0.6% strain was applied to a
0.4 m long section (4.3–4.7 m). The measurement range dm
was ∼7.6 m ( fm swept from 13.406 to 13.601 MHz) and thenominal spatial resolution Δz was ∼0.15 m (Δ f = 0.5 GHz).
Then, during a distributed measurement, the FUT was broken
by being crushed with a hammer at a location 5.66 m
away from the circulator [Fig. 3(e)]. The normalized BGS
distributions along the FUT measured before and after the
FUT was broken [Figs. 7(a) and 7(b)] clearly indicate that,when the FUT is broken at a point, the measurement canstill continue to the breakage point. However, in the section
beyond the breakage point, the normalized spectra weresigniﬁcantly distorted.
Figures 7(c) and 7(d) show the non-normalized data
corresponding to Figs. 7(a) and 7(b), respectively. Fromthese ﬁgures, the peak-power distributions were derived[Fig. 7(e)]. Before the FUT was broken, a slight spectral jump
was observed at 0.8 m, which corresponds to the location at(e)Fig. 7. (Color online) Results of breakage detection. (a, b) Normalizedand (c, d) non-normalized BGS distributions measured (a, c) before and(b, d) after the FUT was broken at 5.66 m. (e) Peak-power distribution measured before the FUT was broken (red), and after the FUT was broken (blue).

which the FC=APC adaptor connects the pigtail of thecirculator and the silica SMF. A spectral dip was also
observed at ∼4.5 m, corresponding to the applied strain. The
peak power of the BGS at the strained section becomes lower
than that at the unstrained section, because the weak but nonzero Brillouin signals of the correlation peak sidelobes(generally located at the unstrained sections) do not overlap.
After the FUT was broken at 5.66 m, and while the spectral
jump at 0.8 m and the dip at ∼4.5 m were maintained, the peak
power at the section beyond the breakage point decreased
drastically. This is reasonable considering that almost no
Brillouin signal returns from where the correlation peak islocated, which also explains the distorted normalized spectra[Fig. 7(b)] (this result indicates at the same time that thelight reﬂected at the breakage point does not deteriorate
the performance of distributed measurement when we use ahammer, which is closer to a practical situation than a sharp
cutter). Therefore, if a certain threshold power is appropriately
set [for instance, 0.4 in Fig. 7(e)], we can detect the locationof a breakage point on a real-time basis.
4.

Conclusions

By employing a noise-ﬂoor compensation technique, a newhigher-speed conﬁguration of BOCDR with a wide strain

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Y. Mizuno et al.

dynamic range and cost eﬃciency was developed and its
basic operation was demonstrated. A strain dynamic range of
0–2.0% was experimentally achieved; this value was muchwider than that of any other previous conﬁguration. As forthe cost eﬃciency, besides the fact that the experimental
setup does not involve EOMs, which are used in mostconventional Brillouin sensors, all of the newly employedelectrical devices, which include an MFS, ﬁlter, mixer, soundboard, etc., are relatively inexpensive. Further cost reduction
can be achieved by replacing the ESA with a specialized
circuit, which also contributes to downsizing the wholesystem. Brillouin-based breakage detection was also demonstrated for the ﬁrst time to the best of our knowledge. The
sampling rate of this conﬁguration described in this paper
was 50 Hz, but we veriﬁed that a >200-Hz sampling rate canbe achieved simply by replacing the MFS with a VCO, whichis over 10 times the highest value of standard conﬁgurations.
We believe that this cost-eﬀective higher-speed conﬁgurationof BOCDR will be of great use for practical distributed strain
and temperature sensing with high spatial resolution and wide
dynamic range in future.
Acknowledgments

This work was supported by JSPS KAKENHI Grant Numbers
25709032, 26630180, 25007652, 15J11445, and 17H04930,
and by research grants from the Japan Gas Association, the
ESPEC Foundation for Global Environment Research andTechnology, and the Association for Disaster Prevention
Research. The authors are grateful to Heeyoung Lee, Shumpei
Shimada, Makoto Shizuka, Kazunari Minakawa, and Hiroki
Tanaka (Institute of Innovative Research, Tokyo Institute ofTechnology) for their experimental assistance.

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