Optical Review (2018) 25:473–485
https://doi.org/10.1007/s10043-018-0436-1

SPECIAL SECTION: REGULAR PAPER
Optics Awards 2017 (OA 2017)

Phase-detected Brillouin optical correlation-domain reflectometry
Yosuke Mizuno1· Neisei Hayashi2 · Hideyuki Fukuda3 · Kentaro Nakamura1

Received: 21 November 2017 / Accepted: 3 April 2018 / Published online: 2 May 2018
© The Optical Society of Japan 2018

Abstract
Optical fiber sensing techniques based on Brillouin scattering have been extensively studied for structural health monitoring
owing to their capability of distributed strain and temperature measurement. Although a higher signal-to-noise ratio (leading
to high spatial resolution and high-speed measurement) is generally obtained for two-end-access systems, they reduce thedegree of freedom in embedding the sensors into structures, and render the measurement no longer feasible when extremely
high loss or breakage occurs at a point of the sensing fiber. To overcome these drawbacks, a one-end-access sensing technique called Brillouin optical correlation-domain reflectometry (BOCDR) has been developed. BOCDR has a high spatial
resolution and cost efficiency, but its conventional configuration suffered from relatively low-speed operation. In this paper,
we review the recently developed high-speed configurations of BOCDR, including phase-detected BOCDR, with which wedemonstrate real-time distributed measurement by tracking a propagating mechanical wave. We also demonstrate breakage
detection with a wide strain dynamic range.
Keywords Brillouin scattering · Distributed strain and temperature sensing · High-speed measurement · Nonlinear optics ·Optical fiber sensors

1 Introduction
Fiber-optic Brillouin scattering [1] has been widely exploited
to perform distributed measurement of strain and temperature. The Brillouin-based distributed sensors developed thus
far can be classified into two categories. One is “analysis”,
in which two light beams are injected into both ends of afiber under test (FUT), and the other is “reflectometry”, in* Yosuke Mizuno
ymizuno@sonic.pi.titech.ac.jp
Neisei Hayashi
hayashi@cntp.t.u‑tokyo.ac.jp
Hideyuki Fukuda
fahrenheitniku@gmail.com
Kentaro Nakamura
knakamur@sonic.pi.titech.ac.jp1

Institute of Innovative Research, Tokyo Instituteof Technology, Midori‑ku, Yokohama 226‑8503, Japan

2

Research Center for Advanced Science and Technology, The
University of Tokyo, Meguro‑ku, Tokyo 153‑8904, Japan

3

Servo Laboratory, FANUC Corporation, Oshino‑mura,
Yamanashi 401‑0597, Japan

which a single light beam is injected into one end of an
FUT. The former includes Brillouin optical time-domain
analysis (BOTDA) [2–12], Brillouin optical frequencydomain analysis (BOFDA) [13–16], and Brillouin opticalcorrelation-domain analysis (BOCDA) [12, 17–24], while
the latter includes Brillouin optical time-domain reflectometry (BOTDR) [25–29], Brillouin optical frequency-domainreflectometry (BOFDR) [30], and Brillouin optical correlation-domain reflectometry (BOCDR) [31–36]. A varietyof configurations have been proposed for each technique toimprove the performance, such as spatial resolution, measurement range, signal-to-noise ratio (SNR), sampling rate,
repetition rate, strain and temperature sensitivity, strain (ortemperature) dynamic range, system simplicity, and cost efficiency. Here, let us focus mainly on Brillouin sensors with ahigh sampling (or repetition) rate.
Extremely high sampling rates have already beenachieved in BOTDA [3–11] and BOCDA [19, 20, 24].
Inspired by the idea of Bernini et al. [4], Peled et al. [5]have also developed a high-speed configuration of BOTDA
called slope-assisted BOTDA, which interrogates the FUTwith a single frequency located at the middle of the slope ofthe local Brillouin gain spectrum (BGS) and enables a single

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pump pulse to sample the strain distribution along the fulllength of the FUT. By synthesizing an adaptable probe wave,
an FUT with an arbitrary Brillouin frequency shift (BFS)
distribution can be interrogated, though the strain dynamic
range is limited by the size of the linear section of the BGS.
Peled et al. [6] have also demonstrated dynamic BOTDA
measurements by implementing the fast switching of opticalfrequency using high-performance arbitrary-waveform generators, though the measurement time becomes longer withincreasing FUT length. Furthermore, Taki et al. [7] have proposed using cyclic pulse coding based on quasi-periodic bit
sequences in BOTDA for an FUT with a length on the orderof kilometers. This configuration can substantially reduce
the number of averages required to guarantee an acceptable
SNR. Many other efforts to improve the performance ofhigh-speed BOTDA have been made [3, 8–11]. High-speed
operation has also been achieved by BOCDA. Song et al.
[19] have obtained a 1 kHz sampling rate at a single location
using a time-division pump–probe generation scheme with
optimized temporal gates and an unbalanced Mach–Zehnderdelay line. They have also demonstrated a distributed measurement over a 100 m long FUT at a 20 Hz repetition ratewith 80 cm spatial resolution using differential frequency
modulation [20]. In addition, random accessibility with a
5 kHz sampling rate has been demonstrated at the cost of
the employment of a high-speed lock-in amplifier [24]. Thus,
only analysis systems based on stimulated Brillouin scattering (SBS), not on spontaneous scattering, have been used toperform real-time Brillouin measurement with higher (submeter) spatial resolution.
In general, SBS is induced by a so-called pump–probe
configuration, which has, however, two problems from apractical viewpoint. First, the pump and probe lights need
to be injected into both ends of the FUT, which is not convenient for embedding the sensors into materials and structures; besides, the measurement turns out no longer feasible
when breakage (or extremely high loss) occurs at a point
along the FUT. Some quasi-one-end-access configurations[16, 21, 22] developed by exploiting the Fresnel reflection
at the open end of an FUT cannot perfectly overcome themeasurement difficulties associated with a breakage point.
The other problem is that a relatively expensive electro-optic
modulator (EOM), such as a single-sideband modulator, isoften needed to accurately produce the frequency difference
between the pump and probe lights [3–18, 20–24]. Moreover, most configurations require an additional EOM for optical pulse generation [2–12] or chopping for lock-in detection [17, 18, 20–24]. Although a time-division pump–probe
generation scheme [19] does not require an EOM for pump/probe generation, the driving current of the laser diode (LD)needs to be precisely controlled. Furthermore, it is notablethat some systems suffer from another problem—a limited
strain dynamic range. For instance, the largest strain that13

Optical Review (2018) 25:473–485

can be measured is, in principle, limited by the Brillouinbandwidth in slope-assisted BOTDA [4, 5] and by the nonnegligible correlation peak sidelobes in BOCDA [23]. Thus,
it is crucial to develop distributed Brillouin sensing technology that simultaneously provides a high sampling/repetition
rate, intrinsically one-end-access interrogation, high spatial
resolution, wide strain dynamic range, high SNR, and system simplicity (thereby leading to cost efficiency). Considering the first two requirements, it appears that BOTDR isnot suitable because the spontaneous-Brillouin-scatteredsignal generated by incident optical pulses is so weak thatthe signal needs to be integrated a number of times [25–29],
although there are some trials reported [37].
In this review paper, we describe recently developed highspeed configurations of BOCDR [38, 39], which potentiallysatisfy all the aforementioned advantages simultaneously.
BOCDR is known to be an intrinsically one-end-access sensing technique with high spatial resolution, system simplicity,and cost efficiency. Here, the frequency sweeping for acquiring the BGS is performed at high speed without using theinherent sweeping function of an electrical spectrum analyzer (ESA), which is used only to detect the signal power
at a fixed frequency component. The noise caused by theexternal frequency sweeping is suppressed by obtaining thedifferential signal between periods of operation with andwithout the Brillouin signal, resulting in a high SNR andenhanced 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 andcost efficiency. We also implement high-speed BFS acquisition from the BGS (“phase-detected BOCDR”) and showthat the BFS (at one position) can be derived at > 100 kHz
at the sacrifice of the limited range of measurable strain. A
variety of measurement examples (including breakage detection and the tracking of a propagating mechanical wave)
are presented, demonstrating the high performance of thishigh-speed BOCDR.

2 High‑speed configuration
2.1 Principle and proposal
As mentioned above, BOCDR [31, 32] is a distributed
sensing technique with intrinsic one-end accessibility andhigh spatial resolution. Its operating principle is based on
the correlation control of continuous lightwaves in a selfheterodyne scheme. Specifically speaking, the pump lightand the reference light (instead of the probe light usedin BOCDA) are sinusoidally frequency-modulated at f m,
and periodical “correlation peaks [40]” are generated inOptical Review (2018) 25:473–485

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the FUT. The measurement range dm is determined by theinterval between the correlation peaks, which is inversely
proportional to fm as [32]:

dm =

c
,
2 n fm(1)

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, which can be regarded as the sensing position,is scanned along the fiber to acquire a BGS or BFS distribution. The spatial resolution Δz is given by [32]:

Δz =

c ΔvB
,
2 𝜋 n fm Δf(2)

where ΔνB is the Brillouin bandwidth [typically 30 MHz insilica single-mode fibers (SMFs)] and Δf is the modulation
amplitude of the optical frequency. Note that Δf is practically limited to a half of the BFS of the FUT due to theRayleigh noise [31, 32]. The highest sampling rate reported
so far, including data acquisition to the computer, was 19 Hz[33], and was limited by the frequency sweeping speed of an
ESA. The maximal measurable strain experimentally confirmed was 0.3% (low SNR) in a temporal gating scheme[34], 0.32% (higher SNR) with noise floor compensation[35], and 0.7% (low SNR) in an intensity-modulation-based
apodization scheme [36].
Figure 1a depicts the experimental setup of the highspeed configuration of BOCDR [38]. The fiber-optic parts
are similar to those previously reported [35]. The 7 dBm
output from a distributed-feedback LD with a line widthof ~ 1 MHz was divided into two lights (pump and reference) using an optical coupler. The pump light was led toa polarization scrambler (PSCR) to suppress the polarization-dependent fluctuations of the BGS, amplified using
an erbium-doped fiber amplifier (EDFA) to 23 dBm, andthen injected into the FUT. The reflected light (Stokes)
was amplified using another EDFA to 3 dBm and mixedusing another optical coupler with the reference light, which
passed through a ~ 1 km long delay line. The heterodyned
Brillouin signal, which was converted into an electrical signal with a photo detector (PD), was mixed with the output

Fig. 1  Basic configuration ofhigh-speed BOCDR. a Experimental setup. AMP amplifier;
BPF band-pass filter; DAQ dataacquisition; 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; NFLnoise floor; OSC oscilloscope;
PD photo detector; PSCR
polarization scrambler; RF radio
frequency. b Normalized direct
output from the ESA (green),noise floor (blue), and the
intrinsic BGS obtained as theirdifference (red). c Measured
BFS dependence on a relatively
large strain of up to 2.0%. Theorange solid circles are measured points, whereas the blueline is a linear fit. Copyright(2017) The Japan Society ofApplied Physics (JSAP)

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of a microwave frequency sweeper (MFS), the frequencyof which can be repeatedly swept over a range of several
hundred megahertz. The power at a fixed frequency component (carefully selected) of the mixed signal was then output
from the ESA, which repeatedly provided the BGS in thetime domain (see Fig. 2). Next, it was input to the computer
via an inexpensive sound board (XONAR U7; ASUSTek)and monitored using a virtual oscilloscope triggered at therepetition frequency of the MFS. The BGS was observed ona real-time basis, but was largely distorted because of theconsiderable intensity noise of the MFS output, which wasdependent on the applied voltage (or output frequency). To
suppress this noise and clearly observe the BGS, the differential signal between periods of operation with and without
the Brillouin signal (i.e. between the two spectra for which
the 980 nm pump LD of the EDFA in the Brillouin-pump
path is and is not working) was calculated. It was finallyacquired as the BGS after low-pass filtering.

2.2 Fundamental characterization
Here, an MFS with a maximal repetition rate of 50 Hz wasused. First, we verified that the local BGS can be correctly
observed at this frequency. A 7.0 m long silica SMF (composed of a 0.8 m long pigtail of an optical circulator connected to a 6.2 m long SMF using an FC/APC adaptor) wasused as the FUT. The open end was cut at an angle of 8° tosuppress 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 from the circulator. The measurement
range dm was calculated to be 7.6 m according to Eq. (1).
The modulation amplitude Δf 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

Fig. 2  Principle of high-speed
BGS acquisition. By mixing with a frequency-swept
microwave, the BGS originallyobserved in the frequency
domain can be obtained in thetime domain at high speed.
Copyright (2016) Nature Publishing Group (NPG)

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Optical Review (2018) 25:473–485

resolution was 51, which can be extended to ~ 570 simply
by increasing Δf to half of the BFS, i.e. ~5.4 GHz (The Δfvalues were kept below 1.5 GHz in this paper to avoid damage to the laser, which was not designed for high-frequency
modulation use). The 64th correlation peak was used; thetemperature of the room was 18 °C.
The green curve in Fig. 1b shows the direct output fromthe ESA (the vertical axis was normalized in a linear scale sothat the maximal voltage of this spectrum became 1), whichsuffers from considerable noise, making it difficult to determine the BFS. To suppress this noise, the noise floor, i.e. thespectrum when the EDFA in the pump path was switched
off, was recorded (blue curve in Fig. 1b) and subtracted fromthe ESA output, resulting in the intrinsic BGS as shown the
red curve in Fig. 1b. The dip at ~ 11.3 GHz, which originatesfrom the sharp spectral noise due 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. Note that a higher sampling
rate can be achieved simply using a higher-speed frequency
sweeper (also refer to Sect. 3.1).

2.3 Demonstration of wide strain dynamic range
We then demonstrated a measurement with a wide strain
dynamic range. The experimental conditions were the sameas those for evaluating the noise floor described above. In the
7.0 m long SMF used as the FUT, strains of up to 2.0% wereapplied to a 0.4 m long section (4.5–4.9 m away from thecirculator) fixed on a translation stage (Fig. 3a). The correlation peak was located at the midpoint of the strained section.
The average of 5 measurements revealed the sampling rateof 10 Hz. Figure 4 shows the measured BGS dependence
on strain. Note that all the BGSs were normalized. With

Optical Review (2018) 25:473–485

Fig. 3  Structures of FUTs. a Fixed-point measurement of static strain.
b Distributed measurement with a higher SNR. c Distributed measurement with a higher spatial resolution. d Fixed-point measurement477

of dynamic strain (not sinusoidally applied; see Fig. 6). e Distributed
measurement for breakage detection. f Fixed-point measurement ofdynamic strain (sinusoidally applied). Copyright (2017) JSAP

engendered at the fixed (and pressed) positions, resulting ina weak BGS, which was distorted if normalized. Figure 1cshows 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
with the previously reported values at 1550 nm [31, 41].
Thus, the wide strain dynamic range of ~ 2.0% was demonstrated. The maximal measurable strain, which is about threetimes the largest value previously reported for BOCDR [36],is even close to the fracture strain of silica SMFs, which is
generally several percent.

2.4 Distributed and dynamic strain measurements

Fig. 4  Measured BGS dependence on a relatively large strain. Copyright (2017) JSAP

increasing strain, the BGS shifted to a higher frequency. At
2.0% strain, the translation stages began to slightly slip, at
which point the correct amount of strain could not applied;
besides, at such large strain, considerable optical loss was

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. 3b). 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
Δf 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 was200, resulting in a measurement time of ~ 4 s. The measured

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BGS distributions (normalized) are shown in Fig. 5a–c, andthe corresponding BFS distributions are shown in Fig. 5d,
where the strained sections were correctly detected witha high SNR. The non-normalized data are discussed in
Sect. 2.5.
Next, a 0.6% strain was applied to an 80 mm long section(5.58–5.66 m) of the same FUT (Fig. 3c), and Δf was set to
1.5 GHz, corresponding to Δz of ~ 48 mm. The measured
BGS distribution is shown in Fig. 5e, and the magnified
view of the BFS distribution around the strained section isshown in Fig. 5f. Although the SNR deteriorated becauseof the enhanced spatial resolution, the strained section was
successfully detected.
We also demonstrated the capability of dynamic strain
sensing. A 0.4% static strain was applied in advance to a
1.0 m long section (3.0–4.0 m) of a 7.0 m long silica SMF(Fig. 3d); 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 thestrained section, not sinusoidally, using a rotor with a 33 mm
long arm (Fig. 6). By setting fm to 13.503 GHz, the correlation peak was placed at the midpoint of the strained section (dm = ~ 7.6 m). The spatial resolution Δz was ~ 0.36 m

Optical Review (2018) 25:473–485(Δf = 0.2 GHz), and the sampling rate was 50 Hz. No averaging was performed. The measured temporal variations of the
BGS and BFS at 2.2 Hz (Fig. 7a, b) and at 4.0 Hz (Fig. 7c, d)indicate that the dynamic strains were successfully detectedwith a much higher SNR than in a previous report [33].

2.5 Breakage detection
We finally demonstrated the detection of a breakage pointcaused during a distributed measurement. A 7.0 m longsilica 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 the nominal spatial resolution Δz was~ 0.15 m (Δf = 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. 3e).
The normalized BGS distributions along the FUT measured
before and after the FUT was broken (Fig. 8a, b) clearly
indicate that, when the FUT is broken at a point, the measurement can still continue to the breakage point. However,in the section beyond the breakage point, the normalized
spectra were significantly distorted.

Fig. 5  Distributed measurement results. a–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 (magnified). Copyright (2017) JSAP13

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479

Fig. 6  Method for applying
dynamic strain (0.4–0.6%) to a
fixed section of the FUT using
an armed rotor. Copyright(2017) JSAP

Fig. 7  Measurement results of dynamic strain. Temporal variation 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. Copyright (2017) JSAP
Fig. 8  Normalized BGS distributions. Data measured a before
and b after the FUT was broken
at 5.66 m. Copyright (2017)
JSAP

Figure 9a, b show the non-normalized data corresponding to Fig. 8a, b, respectively. From these figures, the peakpower distributions were derived (Fig. 9c). Before the
FUT was broken, a slight spectral jump was observed at
0.8 m, which corresponds to the location at which the FC/
APC adaptor connects the pigtail of the circulator and thesilica 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 non-zero Brillouinsignals of the correlation peak sidelobes (generally locatedat the unstrained sections) do not overlap. After the FUT was
broken at 5.66 m, and while the spectral jump at 0.8 m andthe dip at ~ 4.5 m were maintained, the peak power at thesection beyond the breakage point decreased drastically. Thisis reasonable considering that almost no Brillouin signal

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Fig. 9  Results of breakage detection. Non-normalized BGS distributions measured a before and b after the FUT was broken at 5.66 m. c Peakpower distribution measured before the FUT was broken (red), and after the FUT was broken (blue). Copyright (2017) JSAP

returns from where the correlation peak is located, which
also explains the distorted normalized spectra (Fig. 8b).
Therefore, if a certain threshold power is appropriately set(for instance, 0.4 in Fig. 9c), we can perform real-time detection of the breakage location.

3 Ultrahigh‑speed configuration:phase‑detected BOCDR
3.1 Proposal
It is crucial to distributed measurements of dynamic strain
at higher frequencies (up to the order of kilohertz) to further enhance the operating speed of the BOCDR system.
One simple solution is to replace the MFS with a VCO,
the frequency of which can be swept across a range of several hundred megahertz at a much higher repetition rate. As
the output frequency of the VCO is not necessarily linearly
dependent on the input voltage, a so-called pre-distortion
technique should be employed. However, the enhancementof the sampling rate of the BFS is limited to several hundreds of hertz by the mere use of the VCO, because the BGS
acquisition (determined by the repetition rate of the VCO)
must be slower than the calculation time required for the
BFS derivation from the BGS.
To tackle this issue, we developed a method for simultaneously deriving the BFS and the BGS [39]. The experimental configuration is depicted in Fig. 10a, where the MFS
in Fig. 1a is replaced by a VCO that is driven with a sinusoidal output from a function generator. The basic conceptof the subsequent signal processing is shown in Fig. 11.
The ESA output, which can be regarded as the BGS, wasapproximated by a one-period sinusoidal waveform using
a band-pass filter (BPF) with the same central frequency as

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the repetition rate of the VCO. At this stage, the BFS wasin one-to-one correspondence with the phase delay of thesinusoidal waveform (at the expense of the limited range ofmeasurable strain), which was detected using an electrical
amplifier, an exclusive-OR (XOR) logic gate, and an LPF.
With this configuration, only a phase delay smaller than
180° can be properly detected. The phase delay was finallymonitored as a quasi-DC voltage using a virtual oscilloscope implemented in a personal computer, triggered by therepetition frequency for the distributed measurement. Notethat all of the additionally employed devices are relatively
inexpensive. This ultrahigh-speed configuration is named
phase-detected BOCDR.

3.2 Operation confirmation
First, the VCO operation with a BFS sampling rate of
100 kHz was confirmed (An even higher sampling rate wasachievable if the SNR deterioration was admitted). When thefrequency-control voltage was linearly applied to the VCO(blue line in Fig. 12a), the output frequency was distorted(blue curve in Fig. 12b) because of the nonlinear frequency
dependence on the applied voltage. Therefore, we applieda pre-distorted voltage (red curve in Fig. 12a) and obtained
the linear output frequency (red line in Fig. 12b).
We then verified that the phase delay and the final DC
voltage were in one-to-one correspondence with the applied
strain. In the 12.8 m long SMF used as the FUT, strains of
up to 0.3% were applied to a 1.0 m long section (6.3–7.3 m)(Fig. 3f). The correlation peak was located at the midpoint of the strained section by setting fm to 7.222 MHz(dm = ~ 14.2 m), and the spatial resolution Δz was ~ 0.38 m(Δf = 0.35 GHz). The strain dependence of the normalized
BGS approximated by a one-period sinusoidal waveform(Fig. 13) shows that the phase is delayed with increasing

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Fig. 10  Configuration ofultrahigh-speed BOCDR. a
Experimental setup. AMP
amplifier; BPF band-passfilter; DAQ data acquisition;
EDFA erbium-doped fiber
amplifier; ESA electrical
spectrum analyzer; FG function
generator; FUT fiber under test;
IF intermediate frequency; LOlocal oscillator; LPF low-passfilter; MG microwave generator;
OSC oscilloscope; PD photodetector; PSCR polarization
scrambler; RF radio frequency;
VCO voltage-controlled oscillator. b Measured phase delay
and c output DC voltage plotted
as functions of applied strain.
Copyright (2016) NPG

strain. The strain dependence of the phase delay (Fig. 10b)
was almost linear up to 0.2% with a slope of 943°/% (calculated excluding the data at > 0.2%, which deviated from thelinear trend). The DC voltage, i.e. the output from the LPF(cut-off frequency = 1.2 kHz) was also plotted as a functionof applied strain (Fig. 10c). The voltage increases linearlywith increasing strain (slope = 3.2 V/%) and reaches a maximum at ~ 0.2%, where the phase delay becomes 180°. At
this stage, it decreases for strains beyond 0.2%. Thus, theDC voltage was verified to correspond to the applied strain.

3.3 Dynamic strain measurement
A static strain of 0.1% was applied to a 1.0 m long section of
the 12.8 m long FUT (6.3–7.3 m) (Fig. 3f). Dynamic strains
at 30, 100, 300 Hz, and 1 kHz were then additionally applied
to the same section using a vibration generator. Averaging

was performed 10 times on the virtual oscilloscope. The
other experimental conditions were the same as those for theoperation confirmation described above. The measured temporal variations of the output voltage (Fig. 14a–d) indicatethat the dynamic strains of up to 1 kHz were successfully
detected, though the data at 1 kHz were somewhat distorted.
The vibration amplitude, which is dependent on the vibration frequency, can be derived from Fig. 10c; for instance,
at 30 Hz, the applied strain ranges from 0.02 to 0.18%, corresponding to a vibration amplitude of 0.8 mm. This value
moderately agreed with the directly measured value using alaser Doppler velocimeter.

3.4 Tracking of mechanical wave propagation
Finally, to demonstrate the system’s capability for ultrahighspeed acquisition of the strain distribution, we attempted

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Fig. 11  Principle of high-speed
BFS acquisition. The BGS isapproximated by a one-period
sinusoidal waveform andrectified. Its phase delay, whichcorresponds to the BFS, is thendetected using an XOR logic
gate and a LPF. The output voltage is in one-to-one correspondence with the phase delay in therange from 0 to 180°. Copyright(2016) NPG

Fig. 12  VCO operation with a
BFS sampling rate of 100 kHz.
a Linearly applied voltage (blue) and pre-distorted
applied voltage (red). b Output
frequencies when the voltage
was applied in linear (blue) andpre-distorted (red) manners.
Copyright (2016) NPG

to track the propagation of a mechanical wave [42] along
the FUT. The structure of the FUT is depicted in Fig. 14e,
where a 3.2 m long section (6.0–9.2 m) was tightly adhered
to a 0.1 m wide, 1 mm thick rubber sheet using tape. Notethat the newly employed 4.8 m long SMF (0.8–5.6 m) had aslightly lower BFS (~ 10.8 GHz) than that of the 12.0 m long
SMF (5.6–17.6 m). A mechanical wave was manually generated (Fig. 14f) and propagated along the rubber-adheredsection at a relatively slow speed. The sampling rate of the
BFS (or strain) was set to 100 kHz, and the repetition rateof the distributed measurement was set to 100 Hz. Averaging was performed 10 times. The measurement range d m
was ~ 20.4 m (fm swept from 5.048 to 5.153 MHz; 24th correlation peak) and the nominal spatial resolution Δz was~ 0.39 m (Δf = 0.5 GHz).
The measured temporal variation of the strain distribution around the 3.2 m long section attached to the rubber

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sheet (Fig. 14g) shows that the mechanical wave propagation was detected, though the SNR was low. The propagation speed was calculated to be approximately 10 m/s.
The length of the strained section and the amplitude of
the strain decreased as the mechanical wave propagated.
The amount of strain caused by the wave propagation(< 0.03%) seems to be valid, as it is close to that measured by two-end-access BOTDA [42]. The amount ofstrain along the 4.8 m long SMF with a lower BFS was
not correctly detected (negative values obtained), whichis reasonable considering the operating principle that limits the measurable strain range. Thus, the ultrahigh-speed
distributed strain sensing capability of this system wasdemonstrated.

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4 Conclusions

Fig. 13  Strain dependence of the normalized BGS approximated by aone-period sinusoidal waveform. Copyright (2016) NPG

Fig. 14  Ultrahigh-speed measurement results. Temporal variations of
the output voltage when dynamic strains were applied to a 1.0 m longsection at a 30 Hz, b 100 Hz, c 300 Hz, and d 1 kHz. e Schematic

We reviewed the recent development of high-speed configurations of BOCDR, including phase-detected BOCDR,
which can perform real-time distributed measurement withintrinsic one-end accessibility, high spatial resolution, andwide strain dynamic range. Compared to conventional twoend-access systems, BOCDR offers a high degree of freedom in embedding the sensors into materials and structures, and can continuously perform measurements evenwhen extremely high loss or breakage occurs at a point
along the sensing fiber. Another advantage to be emphasized here is the cost efficiency of the system. Besides thefact that the experimental setup does not involve EOMs,
which are used in most conventional Brillouin sensors, allof the newly employed electrical devices, which include a
VCO, filter, mixer, sound board, etc., are relatively inexpensive. Further cost reduction can be achieved by replacing the ESA with a specialized circuit, which also contributes to downsizing the whole system. In the meantime,

structure and f photograph of the FUT while tracking the propagationof a mechanical wave. g Measured temporal variation of the strain
distribution. Copyright (2016) NPG

13

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this system suffers from a trade-off between the spatial
resolution and the measurement range. Their ratio is limited to ~ 570 in a basic configuration, though some techniques for improving this value have been developed [12,34]. We thus anticipate that phase-detected BOCDR will
be a promising technique for distributed dynamic strain
and temperature sensing with high spatial resolution,
especially for a relatively short measurement range. We
have also been developing another real-time configuration of BOCDR, called slope-assisted BOCDR [43–46],
the detailed description of which is out of the scope of
this paper.
Acknowledgements The authors wish to acknowledge Tomohito
Kawa, Heeyoung Lee, Shumpei Shimada, Makoto Shizuka, Kazunari
Minakawa, Hiroki Tanaka, Sho Ikeda, Daisuke Yamane, Hiroyuki Ito,
Shiro Dosho, and Kazuya Masu (Institute of Innovative Research,
Tokyo Institute of Technology) for their experimental assistance. This
work was supported by JSPS KAKENHI Grant Numbers 25709032,
26630180, 25007652, and 17H04930, and by research Grants from the
Iwatani Naoji Foundation, the SCAT Foundation, the Konica Minolta
Science and Technology Foundation, 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.

Compliance with ethical standards
Conflict of interest On behalf of all authors, the corresponding author
states that there is no conflict of interest.

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