BRIEF NOTEJapanese Journal of Applied Physics 58, 068004 (2019)https://doi.org/10.7567/1347-4065/ab0784

First demonstration of Brillouin optical correlation-domain reflectometry based on
external modulation scheme
Kohei Noda* , Heeyoung Lee , Yosuke Mizuno , and Kentaro Nakamura
Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan*

E-mail: knoda@sonic.pi.titech.ac.jp

Received December 19, 2018; revised February 8, 2019; accepted February 15, 2019; published online May 28, 2019
We demonstrate basic operation of Brillouin optical correlation-domain reflectometry (BOCDR) based on an external modulation scheme, which
can mitigate three demerits of a conventional direct modulation scheme. A 0.8 m long strain in a 2.8 m long silica fiber is successfully detected in a
distributed manner. This scheme can be employed not only in BOCDR but also in Brillouin optical correlation-domain analysis.
© 2019 The Japan Society of Applied Physics
Supplementary material for this article is available online

Fiber-optic distributed strain and temperature sensing techniques
based on Brillouin scattering have been extensively studied, anda number of conﬁgurations have been developed, including
time-, frequency-, and correlation-domain techniques.1–14) Here,
we focus on Brillouin optical correlation-domain reﬂectometry(BOCDR), which has general merits such as operation bysingle-end light injection, high spatial resolution, random
accessibility, and cost efﬁciency.15,16) Since the ﬁrst proposalof BOCDR, its performance has been drastically improved; forinstance, ultrahigh-speed distributed sensing was demonstrated
in 2016.17)
All of the previously reported conﬁgurations of BOCDR
have been based on a direct modulation scheme, in which thedriving current of a laser is directly modulated to generatefrequency-modulated light.15–17) However, despite its cost
efﬁciency, the direct modulation scheme poses three problems: (1) a suitable laser for high-speed large-amplitude
modulation needs to be selected, (2) optical power modulation is inevitably accompanied, possibly resulting in deteriorated performance, and (3) the modulation amplitude isstrongly dependent on the modulation frequency, possibly
leading to a signiﬁcant change in spatial resolution according
to measurement position.
These drawbacks are also found in optical correlationdomain reﬂectometry (OCDR),18–23) which is a Fresnelreﬂection-based technique for detecting low-quality connections, splices, and damaged parts along an optical ﬁber. We
have developed a new OCDR conﬁguration based on an
external modulation scheme, in which, instead of directly
modulating the driving current of a laser, the opticalfrequency is modulated using an electro-optic modulator(EOM).24) After demonstrating basic operation of externalmodulation-based OCDR, we have proved that at least twodrawbacks (1) and (3) can be mitigated.24)
However, in the external modulation scheme, the power ofthe frequency-modulated light becomes signiﬁcantly low
because of the limited modulation efﬁciency of an EOM
and the insertion loss of an optical narrowband-pass ﬁlter.
Therefore, although the external-modulation-based operationof OCDR with relatively strong reﬂected signal has been
already veriﬁed, that of BOCDR with relatively weak signal
has not been demonstrated yet.
In this study, by appropriately amplifying the frequencymodulated light, we demonstrate basic operation of BOCDR
based on the external modulation scheme for the ﬁrst time tothe best of our knowledge. We show that, when strain of up
to 0.71% is applied to a 0.8 m long section of a 2.8 m longsensing ﬁber, the obtained distributions of the Brillouin gain
spectrum (BGS) and the Brillouin frequency shift (BFS) arein good agreement with the simulation results.
BOCDR is a single-end-access distributed strain andtemperature sensing technique based on synthesis of an
optical coherence function (SOCF).25) By sinusoidally modulating the optical frequency of a laser output, a so-called
correlation peak is generated along a sensing ﬁber. By
controlling the modulation frequency, the position of thecorrelation peak can be scanned along the ﬁber, enabling
distributed measurement. The spatial resolution and measurement range are determined by the modulation frequency andamplitude; refer to the previous literatures15,16) for moredetails. Conventionally, the frequency-modulated light wasgenerated by directly modulating the driving current of alaser. This direct modulation scheme is cost-efﬁcient but
suffers from the aforementioned demerits. Here, we employ
the external modulation scheme, in which the opticalfrequency of the laser output is constant, while it issinusoidally modulated using an external EOM.
The experimental setup of external-modulation-based
BOCDR, depicted in Fig. 1(a), is basically identical to that
previously reported15,16) except for the preparation of thefrequency-modulated light (before its division into pump andreference lights). To modulate the frequency of the laser
output (NX8563LB, NEC), ﬁrst, a frequency-swept sawtooth electrical signal was generated using a function generator (FG) and applied to a voltage-controlled oscillator(VCO; HMC733LC4B, Analog Devices). Then the frequency-modulated microwave output from the VCO wasapplied to a double-sideband modulator (DSBM), and thegenerated upper sideband was selected using an opticalnarrowband-pass ﬁlter (BVF-300CL, Alnair). The generated
frequency-modulated light, which was signiﬁcantly weak(<−20 dBm), was boosted to 8 dBm using an erbium-dopedﬁber ampliﬁer (EDFA) and an optical ﬁlter for suppressing
ampliﬁed spontaneous emission. The polarization state wasscrambled. The power of the incident light to the sensing
ﬁber was 27 dBm. Spectral averaging was performed fourtimes. The modulation frequency was swept from 4.15 MHz
to 4.21 MHz, and the modulation amplitude was approximately 1.4 GHz, corresponding to the spatial resolution ofapproximately 0.24 m and the measurement range of 24 m068004-1© 2019 The Japan Society of Applied Physics

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K. Noda et al.

(a)(b)Fig. 1. (Color online) Schematic setup. (a) Setup of BOCDR based on
external modulation scheme. DSBM: double-sideband modulator, EDFA:
erbium-doped ﬁber ampliﬁer, ESA: electrical spectrum analyzer, FG:
function generator, FUT: ﬁber under test. PD: photo diode, PSCR:
polarization scrambler, VCO: voltage-controlled oscillator. (b) Structure ofsensing ﬁber.

(calculated using equations in Refs. 15 and 16; note that thedependence of the spatial resolution on the measurement
position is much smaller than that of the direct modulationscheme).24)
The structure of the sensing ﬁber is depicted in Fig. 1(b). A
1.8 m long unjacketed silica single-mode ﬁber (SMF;
BFS = 10.84 GHz, pre-measured strain dependence coefﬁcient of BFS = 448 MHz/%) was connected to a 1.0 m longjacketed silica SMF of the second port of an optical circulator(BFS = 10.87 GHz) via a ferrule connector/angled physicalcontact (FC/APC) connector and employed as a sensing ﬁber.
A considerable bending loss was artiﬁcially applied near theend of the ﬁber to suppress the Fresnel reﬂection. Part of thesensing ﬁber was ﬁxed on a translational stage using a clump,and strains of up to 0.71% were applied to a 0.8 m longsection of the ﬁber. We applied strain to a ﬁber section much
longer than the spatial resolution to demonstrate distributed
sensing with a high signal-to-noise ratio. The room temperature was 22 °C.
The measured BGS and BFS distributions when 0.46%
strain was applied are shown in Fig. 2. The vertical axis wasnormalized so that the maximal and minimal powers of each
BGS became 1 and 0, respectively. The relative position wasdeﬁned as the distance from the circulator. In the strained
section, the BFS changed for approximately 200 MHz. The
slight change of the BFS at the position = 1.0 m corresponds
to the aforementioned slight difference of the BFSs of the twodifferent silica SMFs.
Subsequently, we measured the BFS distributions whenstrain was varied from 0 to 0.71% [Fig. 3(a)]. As strain
increased, the BFS in the strained section also increased. TheBFS (calculated as an average of the BFS values in the 30cm-long section at the middle of the strained section) wasplotted as a function of applied strain, as shown in Fig. 3(b).
The dependence was almost linear, and the coefﬁcient was419 MHz/%, which moderately agrees with the theoreticalvalue pre-measured using the whole length. Thus, thebasic operation, i.e., distributed strain measurement, wasexperimentally demonstrated by external-modulation-based
BOCDR. Note that comparative study on the direct modulation scheme and the external modulation scheme is also
provided in the online supplementary data available online atstacks.iop.org/JJAP/58/068004/mmedia.
In conclusion, the basic operation of BOCDR based on theexternal modulation scheme was demonstrated for the ﬁrst
time. A 0.8 m long strain in a 2.8 m long sensing ﬁber wassuccessfully detected. This scheme can mitigate the generalFig. 2. (Color online) Normalized BGS distribution measured at 0.46% strain. The BFS distribution is also indicated in red.

068004-2© 2019 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 58, 068004 (2019)

K. Noda et al.

mitigated, though further study is required on this point. We
note that this external modulation scheme is applicable not
only to BOCDR but also to Brillouin optical correlationdomain analysis (BOCDA).11–14) Thus, we anticipate that theﬁrst demonstration of the external-modulation-based distributed Brillouin measurement is an important technologicalstep in optical ﬁber sensing community.
Acknowledgments This work was supported by JSPS KAKENHI Grant
No. 17H04930 and 17J07226 and by a research grant from the Fujikura
Foundation.
ORCID iDs Kohei Noda
https://orcid.org/0000-0003-49769657 Heeyoung Lee
https://orcid.org/0000-0003-31790386 Yosuke Mizuno
https://orcid.org/0000-0002-33624720 Kentaro Nakamura
https://orcid.org/0000-0003-2899-4484

Fig. 3. (Color online) Experimental results. (a) BFS distributions whenstrain was changed from 0 to 0.71%. (b) BFS (in the strained section) plotted
as a function of applied strain.

demerits of the conventional direct modulation scheme.
Speciﬁcally speaking, a laser which is not designed forhigh-frequency large-amplitude modulation can be used. Inaddition, this scheme is free from the nonlinear mutual
dependence between the modulation frequency and amplitude, and unexpected change in the spatial resolution
according to the measurement position can be avoided. The
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