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Light: Science & Applications (2016) 5, e16184; doi:10.1038/lsa.2016.184
Ofﬁcial journal of the CIOMP 2047-7538/16
www.nature.com/lsa

ORIGINAL ARTICLE

Ultrahigh-speed distributed Brillouin reﬂectometry
Yosuke Mizuno1, Neisei Hayashi2, Hideyuki Fukuda3, Kwang Yong Song4 and Kentaro Nakamura1
Optical ﬁbre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on
account of their capacity for distributed strain and temperature measurements. However, real-time distributed strain measurement has been achieved only for two-end-access systems; such systems reduce the degree of freedom in embedding the sensors
into structures, and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at apoint along the sensing ﬁbre. Here, we demonstrate real-time distributed measurement with an intrinsically one-end-accessreﬂectometry conﬁguration by using a correlation-domain technique. In this method, the Brillouin gain spectrum is obtained athigh speed using a voltage-controlled oscillator, and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform; the phase delay is subsequently converted into a voltage, which can be directly measured. When asingle-point measurement is performed at an arbitrary position, a strain sampling rate of up to 100 kHz is experimentally veriﬁed
by detecting locally applied dynamic strain at 1 kHz. When distributed measurements are performed at 100 points with 10 times
averaging, a repetition rate of 100 Hz is veriﬁed by tracking a mechanical wave propagating along the ﬁbre. Some drawbacks ofthis ultrahigh-speed conﬁguration, including the reduced measurement accuracy, lowered spatial resolution and limited strain
dynamic range, are also discussed.
Light: Science & Applications (2016) 5, e16184; doi:10.1038/lsa.2016.184; published online 16 December 2016
Keywords: Brillouin scattering; distributed strain and temperature sensing; high-speed measurement; nonlinear optics; optical ﬁbre
sensors

INTRODUCTION
Brillouin scattering1 is regarded as one of the most promising principles
of ﬁbre-optic sensing in terms of the feasibility it affords for measurements of strain and temperature distribution. The Brillouin-based
distributed sensors developed so far can be classiﬁed into two types:‘analysis’ in which two light beams are injected into both ends of a ﬁbre
under test (FUT) and ‘reﬂectometry’ in which a light beam is injected
into only one end of an FUT. The former category includes Brillouin
optical time-domain analysis (BOTDA)2–12, Brillouin optical frequencydomain analysis (BOFDA)13–16 and Brillouin optical correlationdomain analysis (BOCDA)12,17–25, whereas the latter category includes
Brillouin optical time-domain reﬂectometry (BOTDR)26–30 and
Brillouin optical correlation-domain reﬂectometry (BOCDR)31–37.
For each technique, a number of different conﬁgurations have been
proposed in order to improve the sensing performance in terms ofspatial resolution, measurement range, signal-to-noise ratio (SNR),
sampling rate, repetition rate, strain and temperature sensitivity, strain
dynamic range, system simplicity and cost efﬁciency. Here, we focus
on Brillouin sensors with a high sampling (or repetition) rate for
distributed dynamic strain measurement.
To date, extremely high sampling rates have been achieved in
BOTDA3–11 and BOCDA19,20,25. In particular, Voskoboinik et al.3 haveproposed the idea of using multiple pumps and probes in BOTDA to1

avoid time-consuming frequency sweeping, thus leading to a potential
increase in measurement speed at the expense of frequency granularity.
Furthermore, inspired by the idea of Bernini et al.4, Peled et al.5 havedeveloped slope-assisted BOTDA, which interrogates the FUT with asingle frequency located at the middle of the slope of the local
Brillouin gain spectrum (BGS), thereby enabling a single pump pulse
to sample the strain distribution along the full length of the FUT. By
synthesising 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 as the length of the
FUT increases. In addition, Taki et al.7 have proposed the use of cyclic
pulse coding based on quasi-periodic bit sequences in BOTDA for an
FUT with a length on the order of kilometres. This conﬁguration
allows for real-time decoding, resulting in a substantial reduction in
the number of averages required to guarantee an acceptable SNR.
Performance improvements for these distributed dynamic sensors
based on BOTDA are still being aggressively studied8–11.
As for BOCDA, Song et al.19 have achieved a 1 kHz sampling rate ata single location using a time-division pump-probe generation scheme

Institute of Innovative Research, Tokyo Institute of Technology, Midori-ku, Yokohama 226-8503, Japan; 2Research Center for Advanced Science and Technology, The Universityof Tokyo, Meguro-ku, Tokyo 153-8904, Japan; 3Servo Laboratory, FANUC Corporation, Oshino-mura, Yamanashi 401-0597, Japan and 4Department of Physics, Chung-Ang
University, Dongjak-gu, Seoul 06974, Korea
Correspondence: Y Mizuno, Email: ymizuno@sonic.pi.titech.ac.jp
Received 3 March 2016; revised 27 June 2016; accepted 29 June 2016; accepted article preview online 30 June 2016

Real-time distributed Brillouin reﬂectometry
Y Mizuno et al2with optimised temporal gates and an unbalanced Mach–Zehnderdelay line. By using differential frequency modulation, they haveadditionally demonstrated a distributed measurement over a 100-mlong FUT at a 20-Hz repetition rate with a spatial resolution of 80 cm20.
Random accessibility with a 5-kHz sampling rate has also been
demonstrated by the employment of a high-speed lock-in ampliﬁer25.
In this manner, only analysis systems based not on spontaneous but on
stimulated Brillouin scattering (SBS) have been exploited to acquire
Brillouin signals on a real-time basis with sub-metre spatial resolution.
In general, SBS is induced by a so-called pump-probe conﬁguration,
which poses one major problem. This conﬁguration requires thepump and probe lights to be injected into both ends of the FUT,
which is not convenient for practically embedding the sensors intomaterials and structures; furthermore, the measurement can no longer
be performed when extremely high loss or breakage occurs at a point
along the FUT. Some quasi-one-end-access conﬁgurations16,21,22 havebeen developed by exploiting the Fresnel reﬂection at the open end of
an FUT (cut perpendicularly to the ﬁbre axis for high reﬂection, with amirror or a small ﬁbre loop sometimes being equipped), but theycannot perfectly overcome the measurement difﬁculties associatedwith a breakage point.
Against this background, it is of paramount importance to develop
distributed Brillouin sensing technology that simultaneously provides
a high sampling/repetition rate, intrinsically one-end-access interrogation, and high spatial resolution. BOTDR does not seem to be suitablebecause the spontaneous-Brillouin-scattered signal generated by incident optical pulses is so weak that the signal needs to be integrated
numerous times26–30.
In this work, we describe a new conﬁguration for BOCDR thatsatisﬁes these three requirements simultaneously. BOCDR is known tobe an intrinsically one-end-access sensing technique with high-spatial
resolution, but its highest sampling rate reported so far was lower than
20 Hz. Here, using a voltage-controlled oscillator (VCO), the frequency sweeping for acquiring the BGS is performed at high 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 BGS is then approximated by a sinusoidal
waveform, and the BFS is converted into its phase delay, which isfurther converted into a voltage for direct measurement. When asingle-point measurement is performed at an arbitrary position, amaximal strain sampling rate of 100 kHz is experimentally veriﬁed bydetecting dynamic strain at 1 kHz. When distributed measurements
are performed at 100 points with 10 times averaging, a repetition rateof 100 Hz (corresponding to the maximal sampling rate (100 kHz)
divided by the number of points and the number of averages) is thenproved to be feasible by tracking a propagating mechanical wave. Thisultrahigh-speed conﬁguration functions at the sacriﬁce of the deteriorated measurement accuracy, lowered spatial resolution, and limited
range of measurable strain, which are fully discussed.
MATERIALS AND METHODSBOCDR31,32 is known as a distributed sensing technique with intrinsic
one-end accessibility and high spatial resolution. Its operating
principle is based on the correlation control of continuous lightwaves
in a self-heterodyne scheme. In other words, the pump light and thereference light (instead of the probe light used in BOCDA) aresinusoidally frequency-modulated at fm, producing periodical ‘correlation peaks’38 in the FUT. The measurement range dm, determined by
the interval between the correlation peaks, is inversely proportional tofm, as follows32:
dm ¼

ð1Þ

where c is the velocity of light in a vacuum and n is the refractive index
of the ﬁbre core. By sweeping fm, the correlation peak, i.e. the sensing
position, can be scanned along the ﬁbre to acquire a BGS or BFS
distribution. The spatial resolution Δz is given by32:
Dz ¼

1

Polarization
scrambler

ð2Þ

FUT

2
3

Delay

cDvB
2pnf m Df

Pump

EDFA
Laser

c
2nf m

StokesEDFA
ReferencePhotodetector

Function
generator
Sync

Sync

Function
generator

LO

RF

VCO
IF

Microwave
generator

Dataacquisition

LO
Oscilloscope

IF
LPF

Computer

BPF

Phase detector

RF

BPF
Electrical spectrum analyzer

Figure 1 Conﬁguration of ultrahigh-speed BOCDR. Experimental setup. EDFA, erbium-doped ﬁbre ampliﬁer; IF, intermediate frequency; LO, local oscillator;
RF, radio frequency.
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Real-time distributed Brillouin reﬂectometry
Y Mizuno et al3

where ΔνB is the Brillouin bandwidth (~30 MHz in silica single-mode
ﬁbres (SMFs)) and Δf is the modulation amplitude of the opticalfrequency. Note that Δf is practically limited to one half of the BFS of
the FUT because of the noise caused by the Rayleigh scattering31,32.
To date, the highest sampling rate including data acquisition to thecomputer was 19 Hz33, and was limited by the sweeping speed of
an ESA.
The experimental setup of the newly conﬁgured BOCDR isschematically shown in Figure 1. The ﬁbre-optic parts were similar
to those in a conventional setup35; a polarisation scrambler was
employed in the pump path (see Supplementary Information for
details). The heterodyned Brillouin signal was converted into anelectrical signal using a photo detector (PD). The key point of highspeed BGS acquisition is the conversion of the BGS from thefrequency domain to the time domain (Figure 2a). The PD output
was mixed with the output of a VCO, the frequency of which was
repeatedly swept over a range of several hundred megahertz using afunction generator, resulting in the BGS scanned in the frequency

domain. The power at a ﬁxed frequency component (carefully chosen)of the mixed signal was then output from the ESA using a so-calledzero-span mode, which repeatedly provided the BGS in the time
domain. The BGS acquisition (determined by the repetition rate of theVCO) must be slower than the calculation time required for the BFS
derivation from the BGS, and therefore we developed a method for
simultaneously deriving the BFS and the BGS. The basic concept of thesubsequent signal processing is shown in Figure 2b. The ESA output,
which can be regarded as the BGS in the time domain, wasapproximated by a one-period sinusoidal waveform using a bandpass ﬁlter (BPF) with the same central frequency as the repetition rateof the VCO. At this stage, the BFS was in one-to-one correspondencewith the phase delay of the sinusoidal waveform (at the expense of thelimited range of measurable strain; only a phase delay smaller than
180° can be properly detected). The phase delay was then converted
into a quasi-DC voltage using an electrical ampliﬁer (AMP), an
exclusive-OR (XOR) logic gate, and a low-pass ﬁlter (LPF), and was
ﬁnally input to the computer via a sound board and monitored using a

Power

BGS

Mixing

Power

a

BFS
Frequency

BGS

Power

Frequency

Frequency

Time

Time

b
BPF

BGS

Sine

Phase fixed

AMP

SquarePhase detection
XOR & LPF

SquareFixed

Voltage

Brillouin
XOR out

0°

180°
360°
Phase delay

LPF out

Figure 2 Principles of high-speed acquisition of BGS and BFS. (a) BGS acquisition. By mixing with a frequency-swept microwave, the BGS originallyobserved in the frequency domain can be obtained in the time domain at high speed. (b) BFS acquisition. The BGS is approximated by a one-period
sinusoidal waveform and rectiﬁed. Its phase delay, which corresponds to the BFS, is then detected using an XOR logic gate and an LPF. The output voltage
is in one-to-one correspondence with the phase delay in the range from 0 to 180°.
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Real-time distributed Brillouin reﬂectometry
Y Mizuno et al
4

RESULTS AND DISCUSSION
Operation conﬁrmation
First, the VCO operation with a BFS sampling rate of 100 kHz was
conﬁrmed (an even higher sampling rate was achievable if the SNR
deterioration was admitted). When the frequency-control voltage waslinearly applied to the VCO (blue line in Figure 3a), the output
frequency was distorted (blue curve in Figure 3b) because of thenonlinear frequency dependence on the applied voltage. Therefore, weapplied a pre-distorted voltage (red curve in Figure 3a) and obtained
the linear output frequency (red line in Figure 3b). The output
frequency was swept from 2.17 to 2.53 GHz, which was set to be
sufﬁciently lower than the BGS (~10.8 GHz) of a silica SMF. If thisfrequency were close to the BGS, the differential component of theirmixed signal would be located near DC and overlapped by its own
folded spectrum, resulting in a signiﬁcantly deteriorated SNR.
Next, we veriﬁed that the phase delay and the ﬁnal DC voltage werein one-to-one correspondence with the applied strain. A 12.8-m-longsilica SMF (composed of a 0.8-m-long pigtail of an optical circulator
connected to a 12-m-long SMF using an FC/APC adaptor) was used as
the FUT. Strains of up to 0.30% were applied to a 0.4-m-long section(6.6–7.0 m away from the circulator) (Figure 4). The open end was cut
at an angle of 8° in order to suppress the Fresnel reﬂection. The
modulation frequency fm was set to 7.222 MHz, with a correlation
peak located at the midpoint of the strained section. The measurement
range dm was calculated to be 14.2 m according to Equation (1). The
modulation amplitude Δf was set to 0.35 GHz, resulting in atheoretical spatial resolution Δz of ~ 0.38 m from Equation (2). The
ratio of the measurement range to the spatial resolution was ~ 37,
which can be extended to ~ 570 simply by increasing Δf to half of the
BFS, i.e. ~ 5.4 GHz (the Δf values were kept relatively low to avoid
damage to the laser, which was not designed for large-amplitude
modulation use). The 64th correlation peak was used, and the room
temperature was 18 °C.
The raw and sinusoidally approximated BGSs with and without
0.07% strain are shown in Figure 5. The vertical axis was normalised
so that the maximal and minimal powers of each spectrum became 1and − 1, respectively. Averaging was performed 10 times. The peaks of
the raw BGSs, i.e. BFSs, were not completely the same as those of the

a 4

0.4 m

5.8 m

Correlation peak
FC/APC

Strained

0.8 m{

0–0.30%0.10% (static) + dynamic
12.0 mFigure 4 Structures of FUT. Strains of up to 0.30% were applied toa 0.4-m-long section of a 12.8-m-long silica SMF, which was composed of a
0.8-m-long pigtail of an optical circulator connected to a 12-m-long SMF
using an FC/APC adaptor.

2

1

0

–1
w / averaging (10 times)
–2

0

2

4
6
Time (μs)

810Figure 5 Normalised raw and sinusoidally approximated BGSs. The red andblue solid curves indicate the raw BGSs with and without 0.07% strain,
respectively; whereas the orange and aqua dashed curves are theapproximated BGSs with and without 0.07% strain, respectively. Averaging
was performed 10 times.

2.5
Frequency (GHz)

Voltage (V)

6.6 m

b 2.6

3

2

1

0

approximated BGSs. This is natural considering that the approximation was performed using the whole shape of the BGS, and the strain
can still be correctly detected so long as the actual BFS and the peak of

Normalised voltage

virtual oscilloscope triggered by the repetition frequency for the
distributed measurement.

2.4
2.3
2.2

0

10
Time (μs)

20

2.1

0

10
Time (μs)

20

Figure 3 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) and pre-distorted (red) manners.
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Real-time distributed Brillouin reﬂectometry
Y Mizuno et al
5the approximated BGS are in one-to-one correspondence (to be
veriﬁed in the following paragraph). The raw and sinusoidally
approximated BGSs with no averaging and 100 times averaging arealso shown in Supplementary Fig. S1a and S1b, respectively. As theaverage number of times increased, the SNR was improved, whichindicates that a 100 kHz sampling rate can be obtained if a low SNR isadmitted.
The strain dependence of the normalised BGS approximated by aone-period sinusoidal waveform is then shown in Figure 6. Averaging
was performed 10 times. The phase was delayed with increasing strain,
but the dependence was not completely linear. The phase delay
dependence on strain of up to 0.20% (Figure 7a) was almost linearwith a slope of 952 degrees/% (calculated excluding the data at
40.20%, which deviated from the linear trend). Here, to evaluate themeasurement accuracy, we displayed the error bars calculated at eachstrain as the standard deviations of 100 data plots. The possible error
was not constant at different strains and was largest (+/ − 15.4°) at~ 0.15% strain, because of the spectral noise structure unique to the2Strained

Voltage (a.u.)

0%

0.30%

1

0

–1
Step: 0.05%
–2

0

2

4

6
Time (μs)

810Figure 6 Strain dependence of the normalised BGS approximated by a oneperiod sinusoidal waveform. The spatial resolution Δz was 0.38 m, and thelength of the strained section Lε was 0.4 m. Averaging was performed
10 times.

a 300

Demonstrations
The ﬁrst demonstration was dynamic strain measurement. A 0.10%
static strain was applied in advance to a 0.4-m-long section(6.6–7.0 m) of the 12.8-m-long silica SMF (Figure 4); the static strain
was applied because it was difﬁcult to stably apply dynamic strains to ab 2.0
Δz = 0.38 m
L ε= 0.4 m

250
200

Voltage (V)Phase delay (degrees)

ESA. In the worst case, the measurement error was approximately+/ − 10%. We admit that this value is not sufﬁciently low for accurate
strain measurements, but this system is still useful in some relatively
rough dynamic strain measurements.
The ﬁnal DC voltage, i.e. the output from the LPF (cut-off
frequency = 1.2 kHz) was also plotted as a function of applied strain(Figure 7b). The voltage increased linearly with increasing strain(slope = 3.2 V/%) and reached a maximum at ~ 0.2%, where the phasedelay became 180°. At this stage, it decreased for strains beyond~ 0.2%. The DC voltage was thus conﬁrmed to correspond to theapplied strain, but with a limited strain dynamic range.
Finally, we investigate the error of our phase-based detection, whichis possibly induced by the modiﬁcation of the BGS due to the irregularstrain distribution in the vicinity of the correlation peak. In general,
the BGS in the correlation-domain techniques consists of two spectral
contributions: one is the sharp signal peak corresponding to the strain
and the other is the sum of the noise substructure originating from all
the other positions23. Therefore, the strain distribution at the positions
near the correlation peak may modify the overall shape of the BGS,
although the peak frequency is maintained. Figure 8 shows one ofsome extreme cases where large strains are applied near the correlation
peak (refer to Supplementary Information for details), and one can see
signiﬁcant modiﬁcation of the BGS. In order to quantify the possible
error of the phase-based detection caused by such a ‘neighbourhood
effect’, we performed some simulations using the method detailed inRef. 24 with various strain distributions near the correlation peak. As
detailed in Supplementary Information, we can conclude that if weallow an absolute error of +/ − 200 με (or +/ − 10 MHz), the spatial
resolution is effectively lowered by ~ 3 times. It should be also notedthat the drop in performance is dependent on the measurement
parameters such as the frequency sweep range, FUT length, andmaximal applied strain. This ﬁnding will be a universal guideline,
which is useful not only for this method but also for the otherspectral-shape-based methods such as slope-assisted BOCDR37.

150
100

1.5

1.0

50
0

0

0.1
0.2
Strain (%)

0.3

0.5

0

0.1
0.2
Strain (%)

0.3

Figure 7 Phase delay and output DC voltage measured as functions of applied strain. (a) Strain dependence of the phase delay. The spatial resolution Δz
was 0.38 m, and the length of the strained section Lε was 0.4 m. Averaging was performed 10 times. The error bars were calculated at each strain as thestandard deviations of 100 data plots. The slope in the range from 0 to 0.20% was 952 degrees/%. (b) Strain dependence of the output DC voltage. The
slope in the range from 0 to 0.20% was 3.2 V/%.
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Real-time distributed Brillouin reﬂectometry
Y Mizuno et al
6

0-0-0

Power (a.u.)

1-1-1(Sine fit)
1-1-1

0.8

0.6
64.18 MHz
0.4
–400

–200
0
200
Relative frequency Δf (MHz)

400

500

b 100

250

50

Voltage (mV)aVoltage (mV)

Figure 8 Example of the simulated BGSs. The black dashed curve is thesimulated BGS for the FUT conﬁguration ‘0-0-0’ (refer to Supplementary
Fig. S2 for the deﬁnition), and the red solid curve is that for ‘1-1-1’. The
green dotted curve, which is the sinusoidal ﬁt of the solid curve, becomesmaximal at Δf = 64.18 MHz.

0
–250
–500

0

20 40 60
Time (ms)

80

e

0
–50
–100

0

10
20
Time (ms)30

Mechanical wave
Tape
FC/APC

0.8 m

Rubber sheet

c

40
20
0
–20
–40

0

2

4
6
Time (ms)d8Voltage (mV)

1.0

times. The other experimental conditions were the same as those for theoperation conﬁrmation described above. The measured temporalvariations (deviation from the initial value) of the output voltage(Figure 9a–9d) indicate that the dynamic strains of up to 1 kHz weresuccessfully detected, though the data at 1 kHz were somewhat
distorted. The vibration amplitude, which is dependent on the vibration
frequency, can be derived from Figure 7b; for instance, at 30 Hz, theapplied strain ranges from 0.02% to 0.18%, corresponding to avibration amplitude of 0.32 mm. This value was moderately consistent
with the value directly measured by the laser Doppler velocimetry.
Subsequently, to demonstrate the system’s capability for ultrahighspeed acquisition of the strain distribution, we attempted to track the
propagation of a mechanical wave39 along the FUT. The structureof the FUT is depicted in Figure 9e, 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. Note that the newly employed 4.8-m-long SMF(0.8–5.6 m) had a slightly lower BFS (~10.8 GHz) than that of the
12-m-long SMF (5.6–17.6 m). A mechanical wave was manually
generated (Figure 9f) and propagated along the rubber-adhered section
at a relatively slow speed. The sampling rate of the BFS (or strain) was
set to 100 kHz, and the repetition rate of the distributed measurement
was set to 100 Hz. Averaging was performed 10 times. The measurement range dm 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 neighbourhood effect was not considered).

Voltage (mV)non-strained section. Dynamic strains at 30 Hz, 100 Hz, 300 Hz and
1 kHz were then applied to the same section using a vibration
generator, which makes it unnecessary to consider the neighbourhood
effect discussed in the previous section. Averaging was performed 10

4
0
–4
–8

8

0

1
2
Time (ms)3g

FC/APC

4.8 m

0.4 m

3.2 m

8.4 m

f
Mechanical wave
propagation

0.03
Strain
(%)

0.8
0.6
0
10
8
Position
(m)

0

0.4
0.2 Time(s)

6
4

Figure 9 Ultrahigh-speed measurement results. Temporal variations (deviation from the initial value) of the output DC voltage when dynamic strains wereapplied to a 0.4-m-long section at (a) 30 Hz, (b) 100 Hz, (c) 300 Hz and (d) 1 kHz. (e) Schematic structure and (f) photograph of the FUT while tracking
the propagation of a mechanical wave. (g) Measured temporal variation of the strain distribution.
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Real-time distributed Brillouin reﬂectometry
Y Mizuno et al
7

The measured temporal variation of the strain distribution aroundthe 3.2-m-long section attached to the rubber sheet (Figure 9g) showsthat the mechanical wave propagation was detected, though the SNR
was low. The propagation speed was calculated to be ~ 10 m s − 1. The
length of the strained section and the amplitude of the strain decreased
as the mechanical wave propagated, which is consistent with the actual
observation (Figure 9f). The amount of strain caused by the wave
propagation (o0.03%) seems to be valid, as it is close to thatmeasured by two-end-access BOTDA39. The amount of strain along
the 4.8-m-long SMF with a lower BFS was not correctly detected(negative values obtained), which is reasonable considering
the operating principle that limits the measurable strain range.
The negative-strain (i.e. compressed) regions, which should appear
at the foot of the peak39 were not clearly detected probably because of
the combination of the following three reasons: (i) the dependence
coefﬁcient of the phase delay on negative strain (compression) was
smaller, as the experimental setup was optimised for strain ranging
from 0 to ~ 0.2%; (ii) the low-pass ﬁltering at the end of the signal
processing reduced the drop of the voltage; (iii) the properties of thematerials (rubber, tape and so on) used to fabricate the FUT weredifferent from those in Ref. 39. In this way, although the SNR was
not sufﬁciently high, the ultrahigh-speed distributed strain sensing
capability of this system was demonstrated.
CONCLUSIONSIn this work, real-time distributed Brillouin reﬂectometry withintrinsic one-end accessibility and high spatial resolution was developed for the ﬁrst time by use of a correlation-domain technique. In
this conﬁguration, the BGS was converted from frequency domain totime domain by mixing it with the output of a VCO. Then, the BGS in
the time domain was approximated by a one-period sinusoidal
waveform, and the BFS was converted into its phase delay, which
was subsequently converted into a voltage so that the BFS information
can be directly obtained. A strain sampling rate of up to 100 kHz at anarbitrary position was experimentally veriﬁed by detecting locallyapplied dynamic strain at 1 kHz. A repetition rate of distributed
measurement of 100 Hz (10 times averaging, 100 sensing points) was
also veriﬁed by tracking a propagating mechanical wave. We must
admit that, compared with those of other high-speed techniques,
including slope-assisted BOTDA5,6 and BOCDA19,20,25, the current
performance of our method is low because of its major drawbacks,
such as the relatively low measurement accuracy (in the worst case,approximately +/ − 10% error with 10 times averaging), spatial
resolution degraded by a factor of ~ 3 (compared with standard
BOCDR30,31 or BOCDA17), and limited strain dynamic range (0 to~ 0.2% in this experiment); these points could be a margin forimprovement in the future. However, we anticipate that intrinsic oneend accessibility, the advantage of this system that other techniques
cannot provide, will more than compensate for these shortcomings
and that this will be a promising technique for distributed dynamic
strain and temperature sensing with practical convenience, especiallyfor a relatively short measurement range.
CONFLICT OF INTEREST
The author declare no conﬂict of interest.

AUTHOR CONTRIBUTIONSYM and KN designed the study. YM and NH performed the experiments. YM
and HF analysed the data. YM and KYS performed the simulation. YM wrote
the manuscript with input from all co-authors.

ACKNOWLEDGEMENTS
We wish to acknowledge Tomohito Kawa, Heeyoung Lee, Shumpei Shimada,
Makoto Shizuka, Kazunari Minakawa, Hiroki Tanaka, Wei Qiu, Sho Ikeda,
Daisuke Yamane, Hiroyuki Ito, Shiro Dosho and Kazuya Masu (Institute of
Innovative Research, Tokyo Institute of Technology) for their experimental
assistance and Richard Nedelcov (Department of Language Arts, Tokyo
University of the Arts) for his English editing. This work was supported by JSPS
KAKENHI Grant Numbers 25709032, 26630180 and 25007652, and byresearch grants from the Iwatani Naoji Foundation, the SCAT Foundation and
the Konica Minolta Science and Technology Foundation.

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r The Author(s) 2016

Supplementary Information for this article can be found on the Light: Science & Applications’ website (http://www.nature.com/lsa).

Light: Science & Applications

doi:10.1038/lsa.2016.184

Supplementary Information

Ultrahigh-speed distributed Brillouin reflectometry
Yosuke Mizuno,1* Neisei Hayashi,2 Hideyuki Fukuda,3Kwang Yong Song,4 and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan

2

Research Center for Advanced Science and Technology, The University of Tokyo,
4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan
3

Servo Laboratory, FANUC Corporation,

3580 Shibokusa Aza-Komanba, Oshino-mura, Yamanashi 401-0597, Japan
4

Department of Physics, Chung-Ang University,

84 Heukseok-ro, Dongjak-gu, Seoul 06974, Korea*Corresponding author. E-mail: ymizuno@sonic.pi.titech.ac.jp

Experimental setup
The 7 dBm (5 mW) output from the distributed-feedback laser diode (NX8562LB; NEC;
~1 MHz linewidth) was divided into two (pump and reference lights) using a 3 dB coupler.
The pump light was led to a polarization scrambler (PCD-104; General Photonics) in order tosuppress the polarization-dependent fluctuations of the BGS, amplified using an erbiumdoped fibre amplifier (EDFA; HPA-200C-30-SM; Alnair) to 23 dBm (200 mW), and theninjected into the FUT. The reflected Stokes light was amplified using another EDFA (LXMS-21; Luxpert Technologies) to 3 dBm (2 mW) and mixed using a 3 dB coupler with thereference light that passed through a ~1 km long delay line. This optical beat signal wasconverted into an electrical signal using a photo detector (DSCR402; Discovery). For theelectrical signal processing in Figure 1 in the main text, an amplifier (CA812-202; Ciao
Wireless; 23 dB gain), a function generator (WF1946; NF Electric Instruments), a VCO(MAX2750EUA; Maxim), a dual channel programmable filter (3627; NF Electric
Instruments), an ESA (R3273; Advantest), and an XOR logic gate in an integrated circuit(model 4046) were used. The ESA output was input to the computer via a sound board(XONAR U7; ASUSTek; 192 kHz/24 bit). An oscilloscope was virtually implemented in thecomputerusing
thefreelyavailable
programming
language
Processing(https://processing.org/). For dynamic strain measurements of up to 1 kHz, a vibration
generator (512-D; EMIC) and a laser Doppler velocimeter (CLV1000; Polytec) were used.

1

Simulation
The BGS in the correlation-domain techniques generally consists of two spectral
components; one is the sharp signal peak corresponding to the strain, and the other is the sumof the noise substructure originating from all the other positions. Therefore, the strain
distribution at the positions near the correlation peak may modify the overall shape of the
BGS even when the peak frequency is kept unchanged. Thus, our phase-based method issusceptible to errors caused by the modification of the BGS because of the irregular strain
distribution in the vicinity of the correlation peak. Here, we perform simulations toquantitatively evaluate this “neighbourhood effect” using the method detailed in Ref. S1.
The simulated configuration of the strain distribution in the vicinity of the correlation
peak is depicted in Supplementary Fig. S2, where R is the theoretical spatial resolution
calculated using Equation (2) and the arrow indicates the position of the correlation peak, i.e.,
the point of detection. For simplicity, we assumed that the length of the FUT is equal to thetheoretical sensing range calculated using Equation (1), and that the BFS of the remaining
fibre is constant (∆ν = 0). The BFS at the point of detection was set to be unchanged (∆ν = 0),
while those of the nearby positions were changed (∆ν = +100 MHz, corresponding to the
maximal strain (0.2%) that can be detected with the simple configuration of this system).
Figure 8 in the main text and Supplementary Figs. S3a–S3i show the resultant BGSs with afrequency sweep range (∆f) of 800 MHz (–400 to +400 MHz). The vertical axis was
normalized so that the peak power with the uniformly non-strained FUT became 1.
Considerable variations were observed in the shape of the BGS, while the gain peakfrequency was not changed (∆f = 0). We then performed sinusoidal approximations to eachsimulated BGS, using the data in the frequency range of 360 MHz (–130 to +230 MHz; sameas that in the experiments). The approximated curves and their peak frequencies are alsopresented in Figure 8 in the main text and Supplementary Figs. S3a–S3i. From these results,
we can conclude that if an absolute error of +/–200 µε (or +/–10 MHz) is allowed, the spatial
resolution is effectively lowered by ~3 times. We should note that the performance
deterioration depends on the measurement parameters, such as the frequency sweep range,
FUT length, and applied strain distribution. This conclusion will be a universal guideline
which is also useful for other spectral-shape-based correlation-domain techniquesS2.

References
S1.

Song KY, He ZY, Hotate K. Effects of intensity modulation of light source on Brillouin
optical correlation domain analysis. J Lightwave Technol 2007; 25: 1238–1246.

S2.

Lee HY, Hayashi N, Mizuno Y, Nakamura K. Slope-assisted Brillouin opticalcorrelation-domain reflectometry: proof of concept. IEEE Photon J 2016; 8, 6802807.

2

Supplementary Figures

Supplementary Figure S1 | Normalized raw and sinusoidally approximated Brillouingain spectra (BGSs). (a) Data obtained when averaging was not performed, and (b) dataobtained when averaging was performed 100 times. The red and blue solid curves indicate
the raw BGSs with and without 0.07% strain, respectively; while the orange and aqua dashedcurves are the approximated BGSs with and without 0.07% strain, respectively.

Supplementary Figure S2 | Simulated configuration of the strain distribution in thevicinity of the correlation peak.

3

a

b

C

39.03 MHzd5.15 MHzef–6.21 MHzg

–4.06 MHz82.71 MHzh29.25 MHzi

25.29 MHz

3.64 MHz

2.89 MHz

Supplementary Figure S3 | Brillouin gain spectra (BGSs) simulated for each strain
distribution and their sinusoidal approximations. The red solid curves are the simulated
raw BGSs, and the green dotted curves are their sinusoidal fits, the peak frequencies of which
are indicated as the blue values. Strain distributions: (a) 2-2-2, (b) 3-3-3, (c) 4-4-4, (d) 5-5-5,(e) 2-1-2, (f) 3-2-3, (g) 4-2-4, (h) 4-3-4, and (i) 5-3-5.

4