Low-cost Brillouin opticalcorrelation-domain reflectometry involving
merely one fibre amplifier

1

laser

Introduction: Brillouin scattering in optical ﬁbres has been extensively
studied and exploited to develop distributed strain and temperature
sensors [1]. A number of conﬁgurations including time-, frequency-,
and correlation-domain reﬂectometry (or analysis) have been reported
thus far [2–7]. Here we focus on Brillouin optical correlation-domain
reﬂectometry (BOCDR), which is the only technique that can simultaneously achieve operation by single-end light injection, random
accessibility to sensing positions, high spatial resolution, and cost efﬁciency [6, 8]. Unlike two-end-access analysis based on stimulated
Brillouin scattering, BOCDR is based on relatively weak spontaneous
Brillouin scattering, resulting in a lower signal-to-noise ratio (SNR).
To improve the SNR, conventional setups of BOCDR involved
multiple – typically three – erbium-doped ﬁbre ampliﬁers (EDFAs) in
the reference path, pump path, and Stokes path (deﬁned in the next
section); in addition, in most cases, the EDFA inserted in the pump
path was designed for high-power use (>20 dBm) [6, 8–12]. However,
the employment of multiple EDFAs including a high-power type has
limited the size and the cost efﬁciency of the whole system. Thus,
it is crucial for practical applications to reduce the number of EDFAs
used in the BOCDR setup.
In this work, ﬁrst, we experimentally clarify that, of all the three
EDFAs conventionally used in the BOCDR, the EDFA in the Stokes
path plays the most important part in increasing the SNR of thesystem. Then, we implement the BOCDR setup excluding the othertwo EDFAs and prove that distributed strain sensing can be performed
even by use of a single EDFA if the output power of the light source is
sufﬁciently high. The measurement error is also evaluated.
Principle: Brillouin sensing generally exploits the dependence of theBrillouin frequency shift (BFS) on strain and temperature. BOCDR
is one of the distributed Brillouin sensors and operates based on thesynthesis of optical coherence functions. By sinusoidally modulating
the output frequency of the light source, what is called a correlation
peak (i.e. sensing position) is generated in a ﬁbre under test (FUT)[6, 8]. As the location of the correlation peak can be arbitrarily set in
the FUT by adjusting the modulation frequency fm, distributed sensing
is feasible by sweeping fm. The measurement range is simply determinedby fm as the interval of neighbouring correlation peaks. In contrast,
the nominal spatial resolution is determined not only by fm but also
by the modulation amplitude Δf [6, 8].
A typical setup of standard BOCDR is depicted in Fig. 1. The laser
output is divided into two; one is pump light injected into an FUT,and the other is reference light. The Brillouin-scattered Stokes light isheterodyned with the reference light, and their optical beat signal is converted into an electrical signal using a photodiode and observed using
an electrical spectrum analyser (ESA). A delay line is inserted in thereference path to control the order of the correlation peak; it is difﬁcult
to use the 0th correlation peak to perform distributed measurement[6, 8]. What is notable here is that, in this conventional setup, three
EDFAs (referred to as EDFA-ref, EDFA-pump, and EDFA-Stk) are
inserted in the reference, pump, and Stokes paths, respectively, toenhance the SNR of the measurement. However, the number of
EDFAs should be minimised from the viewpoint of the system sizeand cost efﬁciency. To date, no reports have been provided regarding
the performance of distributed measurement when some (or all) of
these EDFAs are removed.

referenceACfibre under test
stokesdelay

bias teeDC

2
3

EDFA-pump

Y. Mizuno✉, G. Han, K. Noda, H. Lee and K. Nakamura
Brillouin optical correlation-domain reﬂectometry (BOCDR) is oneof the ﬁbre-optic distributed strain and temperature sensing systems.
Conventionally, BOCDR has been implemented using three
erbium-doped ﬁbre ampliﬁers (EDFAs) to enhance the signal-tonoise ratio (SNR) of the measurement. However, the use of multiple
EDFAs has restricted the size and cost efﬁciency of the system.
In this work, the authors show that, among the three EDFAs previously
employed, the one in the Stokes path plays the most important rolein enhancing the SNR. Subsequently, they implement BOCDR using
only one EDFA and demonstrate distributed strain sensing. The
measurement error is also investigated as a function of the output
power of a light source.

correlation peakscannedpump

EDFA-refEDFA-Stk

photodiodeelectrical
spectrum
analyser

Fig. 1 Schematic setup of conventional BOCDR involving three EDFAs.
AC, alternating current; DC, direct current

Setup: The ﬁrst experiment for showing that the EDFA-Stk is the mostimportant of the three EDFAs was performed using the almost the same
setup as Fig. 1, except for the following two points. One is that some(or all) of the EDFAs were removed from the setup. The other differenceis that the relative polarisation state was scrambled by inserting apolarisation scrambler (PSCR) in the pump path right after an optical
coupler to suppress the polarisation-dependent signal ﬂuctuations
and to evaluate the observed Brillouin gain spectra (BGSs) fairly.
A 1.0-m-long silica single-mode ﬁbre (SMF) was used as the FUT,and the Brillouin signal returned from the whole length of the FUT
was observed (i.e. the driving current of the laser was not modulated).
The output power of the laser at 1552 nm was 8 dBm. The video bandwidth and the resolution bandwidth of the ESA were 100 kHz and
10 MHz, respectively. Spectral averaging was performed 15 times.
The room temperature was 27°C.
The second experiment for demonstrating distributed strain sensing
using the EDFA-Stk only was also performed using an almost identicalsetup as that of the ﬁrst experiment. One difference is that a polarisation
controller was used instead of the PSCR to reduce the system cost. The
output power of the EDFA-Stk was 0 dBm. The structure of the FUT isshown in Fig. 2. Strain was applied to a 0.5-m-long section of a
3.9-m-long silica SMF. A bending was applied near the open end of
the FUT to suppress the Fresnel reﬂection. The modulation amplitude
Δf was ∼3.0 GHz, and the modulation frequency fm was swept from1.051 to 1.059 MHz. These modulation conditions corresponded tothe measurement range of ∼97 m and the spatial resolution of ∼0.4 m.
This resolution is higher than a typical resolution of standard
Brillouin optical time-domain reﬂectometry (∼1 m) [3, 13]. The timerequired to obtain one distributed BFS trace was ∼10 s. The other conditions were the same as those of the ﬁrst experiment.
correlation peak12
3

angledconnection

0.8 m

1.7 m

strained
0.5 m

1.7 m

Fig. 2 Structure of FUT for distributed measurement

Experimental results: When no EDFAs were employed in the setup, theBrillouin signal returned from the whole length of the 1.0-m-long FUT
was not observed. In the same manner, when only the EDFA-ref wasused, the Brillouin signal was not observed, either. In contrast, wheneither the EDFA-pump or the EDFA-Stk was employed, the BGSs
were clearly observed as shown in Figs. 3a and b, respectively. In
both cases, as the output power of the EDFA increased, the peak
power of the BGS increased. However, the output power of the
EDFA-Stk required to obtain the same BGS peak power was >20 dB
lower than that of the EDFA-ref. This result indicates that the
EDFA-Stk plays the most important role in improving the SNR of theBrillouin measurement and that, if we desire to construct a setup
involving merely one EDFA, the most efﬁcient way is to use the
EDFA-Stk.
Subsequently, distributed strain sensing was performed using the
setup involving only the EDFA-Stk while the output power of thelaser was varied. Fig. 4a shows the BFS distributions measured when
the laser power was 6, 12, and 18 dBm. A 0.25% strain was applied.
At 12 and 18 dBm, the BFS along the strained section was upshifted

ELECTRONICS LETTERS 27th June 2019 Vol. 55 No. 13 pp. 754–756

for ∼110 MHz (corresponding well to the 0.25% strain), but themeasurement failed at 6 dBm. To evaluate this more quantitatively,
the measurement error (deﬁned as the standard deviation of each BFS
distribution from its theoretical trend) was calculated and plotted as afunction of the laser power (Fig. 4b). With increasing laser power, theerror initially decreased and then became almost constant. Suppose
the measurement error threshold is, for instance, ∼50 MHz (corresponding to ∼0.1%; this requirement differs depending on applications), thisresults suggests that the laser power of ∼10 dBm or higher is desirable inthis experiment. Note that this measurement error can be improved byincreasing the number of averaging at the cost of the measurement time.
0.20

0.20

EDFA-Stk only

26 dBmEDFZ-pump only

0.10

22 dBm

0.05

18 dBmrelative power, dB

relative power, dB

0.15

0.10

–5 dBm

0.05–10 dBm

0

–0.05
10.75

–0.05
10.75

10.83

10.99

10.91

0 dBm

0.15

0

10.83

10.91

frequency, GHz

frequency, GHzab

Acknowledgments: This work was supported by JSPS KAKENHI grant
numbers 17H04930 and 17J07226.

10.99

Fig. 3 Dependence of the measured BGSs on the output power of the EDFAs
a When only the EDFA-pump was usedb When only the EDFA-Stk was used200
frequency, GHz

frequency, GHz

10.7

150
100
50

10.5

0
0

1

2
3
position, m

4

5

10

15
20
position, m

25

b

a

Fig. 4 Results of distributed strain measurement (1)a BFS distributions measured at three different output powers of the laserb Measurement error plotted as a function of the laser output power

Finally, at the laser power of 12 dBm, we measured the BFS distributions when applied strain increased (Fig. 5a). The upshift of the
BFS at the strained section became larger with increasing strain.
Fig. 5b shows the BFS (averaged along a 0.3-m-long section aroundthe midpoint of the strained section) plotted as a function of strain.
The dependence was almost linear with a coefﬁcient of ∼436 MHz/%,which moderately agreed with the previously reported coefﬁcient [14].
Thus, distributed strain sensing was demonstrated using a low-cost
setup of BOCDR involving a single EDFA in the Stokes path.
11.15

11.14
0.6 %

11.04

frequency, GHz

frequency, GHz

EDFA-Stkonlystrained
step: 0.1 %

10.94

0%

10.84

11.05

10.95
slope: 436.3 MH/%

10.85
0

1

2
3
position, m

4

Y. Mizuno, G. Han, K. Noda, H. Lee and K. Nakamura (Instituteof Innovative Research, Tokyo Institute of Technology, Yokohama
226-8503, Japan)

References

250
18 dBm

10.9© The Institution of Engineering and Technology 2019
Submitted: 14 February 2019 E-ﬁrst: 21 May 2019
doi: 10.1049/el.2019.0572
One or more of the Figures in this Letter are available in colour online.

✉ E-mail: ymizuno@sonic.pi.titech.ac.jp

11.1
12 dBm
6 dBm

Conclusion: We experimentally proved that, of all the three EDFAs
used in the conventional BOCDR setup, the EDFA in the Stokes pathplays the most important part in increasing the SNR. We thenimplemented the setup involving a single EDFA in the Stokes pathand demonstrated distributed strain measurement with a spatial
resolution of 0.4 m. The measurement error was also evaluated withvarying the output power of the laser, and the laser power of>10 dBm was shown to be required in this experiment if the error of<0.1% is to be obtained. Note that this power can be obtained without
difﬁculty using inexpensive commercially available lasers. As the
reduction in the number of the EDFAs involved in the setup is beneﬁcialfrom the viewpoints of cost and size, we believe that this study will be auseful contribution to the development of cost-effective/compact
BOCDR conﬁgurations in the near future.

0

a

0.2

0.4
strain, %0.6

bFig. 5 Results of distributed strain measurement (2)a BFS distributions measured when different strains were appliedb BFS versus applied strain. The grey plots are raw data along the strained
section, the blue circles are their averages, and the red line is a linear ﬁt

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