Applied Physics Express 7, 112501 (2014)
http://dx.doi.org/10.7567/APEX.7.112501

Suppression of ghost correlation peak in Brillouin optical correlation-domain reﬂectometry
Neisei Hayashi, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received September 2, 2014; accepted October 14, 2014; published online October 31, 2014
In Brillouin optical correlation-domain reﬂectometry (BOCDR), an undesired “ghost” correlation peak is automatically generated at the open end of
a sensing ﬁber in addition to the desired correlation peak used for distributed measurement. The Brillouin spectrum of the light scattered from thisghost peak, which has a ﬁxed position, overlaps that from the desired peak, inducing noise and lowering the signal-to-noise ratio. The importanceof ghost-peak suppression is experimentally shown by demonstrating that a distributed strain measurement can only be successful when abending loss of 6 dB or larger is applied near the open end. © 2014 The Japan Society of Applied Physics

rillouin scattering in optical ﬁbers1) has been
extensively studied for over 40 years and has been
applied in the development of several related devices
and systems1–4) including strain and temperature sensors.5–9)
To date, the variety of Brillouin-based distributed sensing
techniques that have been demonstrated can be classiﬁed into
two categories: reﬂectometry and analysis. In reﬂectometry,
which is based on spontaneous Brillouin scattering, a lightbeam is injected into only one end of the ﬁber under test
(FUT), whereas in analysis, which is based on stimulated
Brillouin scattering, two light beams are injected into
both ends of the FUT. Analysis systems include Brillouin
optical time-, frequency-, and correlation-domain analysis(BOTDA,5,10–13) BOFDA,6,14,15) and BOCDA,7,16,17) respectively) in which a relatively large signal and high signal-tonoise (SN) ratio can be achieved. However, two-end access issometimes inconvenient because the system does not work
completely when the FUT has a breakage point. Preparation of the probe light often requires relatively expensive
devices such as single-sideband modulators, which can beproblematic for practical applications. In contrast, eventhough the signal is weaker, reﬂectometry systems such as
Brillouin optical time- and correlation-domain reﬂectometry(BOTDR8,18,19) and BOCDR,9,20–26) respectively) are freefrom these drawbacks. Though BOTDR has a measurement
range of several tens of kilometers or longer, its spatial
resolution is inherently limited to ³1 m27,28) even thoughseveral methods have recently been developed to enhance theresolution.10–13,18,19) Thus, we focus on BOCDR in this study.
The BOCDR9,20–26) operation is based on the so-called“correlation peak”29) synthesized by the correlation control of
two continuous waves (CWs): Brillouin Stokes light andreference light. Furthermore, BOCDR has substantial oneend accessibility, high spatial resolution (13 mm with silica
ﬁbers20) and 6 mm with tellurite ﬁbers22)), and high sampling
rate (i.e., fast measurement speed). One drawback is thetrade-off relation between the spatial resolution and themeasurement range, which can be mitigated by temporalgating24) and dual modulation25) techniques. Another practical merit of BOCDR is the system simplicity, leading tocost efﬁciency. To further simplify the BOCDR system, we
have previously proposed an even simpler implementationof BOCDR, named S-BOCDR,30) where the light Fresnelreﬂected at the open end of the FUT is exploited as thereference light without using the conventional reference path.
In S-BOCDR, the zeroth correlation peak is ﬁxed at the FUTend, and the ﬁrst correlation peak is scanned along the FUT;
to avoid spectral overlap and achieve a correct measurement,Bthe inﬂuence of the zeroth peak needs to be suppressed eitherby (1) artiﬁcially applying loss near the open end or by (2)cutting the open end at an angle [or by using an angled
physical contact (APC)-type connector]. It is notable that asimilar situation occurs in the standard BOCDR conﬁguration. To the best of our knowledge, all BOCDR-based
distributed measurements along the whole length of the
FUT with polarization scrambling (or diversity) have been
performed using a specially devised open end [i.e., case (1) or(2)]23) without any speciﬁc reason. Note that if the polarization state is optimized for the desired correlation peak, themeasurement can sometimes be correctly performed using theopen end with a physical contact (PC)-type connector.9)
Further clariﬁcation of this point is extremely important for
future optimal system design.
In this work, we investigate the inﬂuence of this “ghost”
correlation peak on BOCDR performance. By measuring theBrillouin spectrum dependence on the bending loss applied
near the open end of the FUT, we show that loss greater than
5 dB should be applied to suppress the inﬂuence of the ghost
peak in this experiment. We also demonstrate a distributed
strain measurement with an applied bending loss of 6 dB andpresent the importance of ghost-peak suppression.
Light propagating in an optical ﬁber is partially returned via
spontaneous Brillouin scattering. The backscattered Stokes
light spectrum is called the Brillouin gain spectrum (BGS),1)
and the central frequency of the BGS is shifted downward
relative to the incident frequency by an amount termed
the Brillouin frequency shift (BFS). The BFS in a standard
silica single-mode ﬁber (SMF) is known to be ³10.8 GHz at
1.55 µm.1) If either strain or temperature change is applied
to a silica SMF, the BFS shifts toward a higher frequency
with dependence coefﬁcients of ³493 MHz/%31) and ³1.0
MHz/K,32) respectively, at 1.55 µm. Thus, by measuring the
BFS, the magnitude of the applied strain or the temperaturechange can be derived.
To perform a distributed measurement, strain and temperature need to be spatially resolved. The BOCDR is one of thepromising Brillouin-based distributed sensing techniques,
offering many advantages including one-end accessibility,
high spatial resolution, high sampling rate, and cost efﬁciency. A conceptual schematic of the standard BOCDR setup
is depicted in Fig. 1. Its operating principle is based on thecorrelation control of CWs;9,21,29) namely, the pump light andthe reference light in a standard self-heterodyne scheme foranalyzing Brillouin signals9,33,34) are sinusoidally frequencymodulated at fm, producing periodic correlation peaks along
the FUT. Because only one of these correlation peaks (i.e.,

112501-1© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 112501 (2014)

N. Hayashi et al.

Fig. 1. Conceptual schematic of a standard BOCDR setup forinvestigating the inﬂuence of a ghost correlation peak.

sensing positions) should remain in the FUT, the measurement range dm is determined by their interval, which isinversely proportional to f m as
dm ¼

c
;
2n f mFig. 2. Experimental setup of a BOCDR system containing an opticalattenuator (ATT), EDFAs, an ESA, a PD, and a polarization scrambler(PSCR).

ð1Þ

where c is the velocity of light in vacuum and n is therefractive index of the ﬁber core. By sweeping f m , thecorrelation peak can be scanned along the ﬁber to acquire
the BGS or BFS distribution. When f m is lower than the
Brillouin bandwidth ¦¯B (³30 MHz in a silica SMF1)), thespatial resolution ¦z is given by21)
z ¼

cB
;
2nf m  f

ð2Þ

where ¦ f is the modulation amplitude of the optical frequency. This is the basic theory regarding BOCDR operation,
but we note here that an undesired “ghost” correlation peakexists at the open end of the FUT, as shown in Fig. 1. This
peak occurs because the zeroth correlation peak is generally
generated at the zero-optical-path-length-difference position,
which is the open end of the FUT where the Fresnel-reﬂected
light acts as a reference light (refer to the S-BOCDR
scheme30) for more details on the zeroth-peak generation).
This ghost peak produces a noise source and deteriorates the
SN ratio of a distributed measurement because the Brillouinspectrum from the ghost peak constantly overlaps that fromthe desired peak.
The experimental BOCDR setup for investigating theinﬂuence of the ghost correlation peak is shown in Fig. 2. All
optical paths were composed of silica SMFs. A distributedfeedback laser diode at 1.55 µm with 1 MHz linewidth wasused as a light source, and the optical frequency of its output
was sinusoidally modulated by direct modulation of thedriving current. The frequency-modulated output was divided
into two beams with a 3 dB coupler. One beam was guided
through a 1 km delay line to adjust the correlation peak order,
ampliﬁed to ³22 dBm with an erbium-doped ﬁber ampliﬁer(EDFA), and used as the reference light of heterodyne
detection. The other beam was polarization-scrambled,23)
ampliﬁed with another EDFA, and injected into the FUT as
the pump light with an incident power of 28 dBm. A 99:1
coupler was used to mix the Stokes light and the reference
light to obtain their optical beat signal, which was attenuatedby 12.8 dB, converted to an electrical signal with a photodetector (PD), and ﬁnally monitored with an electrical
spectrum analyzer (ESA) with 300 kHz frequency resolution.
Here, the attenuation was required to avoid gain saturation of
the PD output.

Fig. 3. Structure of the FUT.

For the FUT, we used a silica SMF with a length of 14 m,
a numerical aperture of 0.13, a core diameter of 9 µm, a
cladding diameter of 125 µm, a core refractive index of³1.46, and a propagation loss of ³0.5 dB/km at 1.55 µm.
The modulation frequency f m was swept from 2.8557 to
2.9394 MHz, corresponding to the measurement range dm of
35 m according to Eq. (1). The modulation amplitude ¦ f was
set to 0.7 GHz, resulting in the theoretical spatial resolution¦z of 48 cm from Eq. (2). The fourteenth correlation peak
was used. The overall sampling rate of a single-location
measurement was ³3.3 Hz; as the number of the sensing
points was 280, the total measurement time was ³84 s.
First, we performed a distributed strain measurement
without suppressing the ghost correlation peak. The FUT
structure is shown in Fig. 3. A strain of 0.20% was applied toa 1.5 m long section of the SMF. The end of the FUT was
kept open, meaning a ghost peak exists at the open end.
Figure 4 shows the measured BGS distribution along the
FUT, where the strained section was not detected.
Next, using the same FUT, we applied a variable bending
loss near the open end, as shown in Fig. 3. Figure 5(a) shows
the BGS dependence on the bending loss measured when
the desired correlation peak was ﬁxed at the midpoint of
the strained section. Owing to the scrambled polarization
state, the spectral power ﬂuctuations were less than «0.05
dB. Without the bending loss, the BGS peak was observed at³10.87 GHz, which corresponds to the BFS of an unstrained
silica SMF near the open end, where the ghost peak was ﬁxed
(the shape of the peak was slightly distorted because of thenoise ﬂoor of the ESA23)). However, with increasing bending
loss, the BGS peak at ³10.87 GHz gradually disappeared. At³6 dB, a BGS peak at ³10.98 GHz was clearly observed,112501-2© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 112501 (2014)

N. Hayashi et al.

(a)Fig. 4. The BGS distribution measured with an open FUT end. A 0.20%
strain was partially applied to the FUT.

(b)
(a)

Fig. 6. (a) BGS distribution measured when a 6 dB bending loss wasapplied near the FUT end. A 0.20% strain was partially applied to the FUT.
(b) BFS distributions measured when the applied strains were 0.15, 0.20, and
0.27%.

(b)
Fig. 5. (a) BGS dependence on the bending loss measured when thedesired correlation peak exists at the midpoint of the strained section(0.20%). (b) BGS peak powers at ³10.98 GHz (desired) and ³10.87 GHz(ghost) measured as functions of bending loss.

which corresponded to the BFS of the strained silica
SMF section, where the desired correlation peak existed.
Figure 5(b) shows the BGS peak powers at ³10.98 GHz(desired) and ³10.87 GHz (ghost) plotted as functions of thebending loss. As the loss increased, both powers decreased,
and the power from the desired peak became higher thanthat from the ghost peak when the bending loss exceeded
approximately 5 dB. This suggests that a loss greater than
5 dB needs to be induced near the open end of the FUT tocorrectly perform a distributed strain measurement in thisconﬁguration.

Finally, a distributed measurement was performed when
6 dB bending loss was applied near the FUT end. Other
experimental conditions were the same as those describedabove (i.e., those used to obtain Fig. 4). Figure 6(a) shows the
BGS distribution measured when the applied strain was
0.20%; the strained section was correctly detected. Note that
the SN ratio of this measurement is lower than that of somepreviously reported results.9,20,23) This low SN ratio originatesfrom the fact that the optical heterodyne signal was artiﬁciallyattenuated by 12.8 dB to avoid gain saturation of the PD when
the high-power light Fresnel-reﬂected at the FUT end (without
the bending loss) was returned. When a greater than 6 dB
bending loss is applied, this attenuator can be removed,
leading to a drastic enhancement of the SN ratio. Figure 6(b)shows the BFS distributions measured when the applied
strains were 0.15, 0.20, and 0.27%. As shown in the inset, theBFS was linearly dependent on the applied strain with aproportionality coefﬁcient of ³491 MHz/%, which is in good
agreement with the previous report.31) Thus, we clariﬁed thatthe ghost peak must be sufﬁciently suppressed to correctly
perform distributed measurements using BOCDR.
In conclusion, the inﬂuence of the ghost correlation peakon the BOCDR performance was experimentally investigated. First, a correct distributed strain measurement was
shown to be infeasible if the FUT end was simply kept open.
The BGS dependence on the bending loss applied near the
open end to suppress the ghost peak was also measured, and aloss greater than 5 dB was required. A successful distributed

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N. Hayashi et al.

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112501-4© 2014 The Japan Society of Applied Physics

strain measurement was then demonstrated when the ghost
peak was sufﬁciently suppressed. From these results, ghostpeak suppression was shown to be crucial for correct
BOCDR operation. Theoretically, the value of 5 dB is almost
independent of the FUT length or the spatial resolution(or measurement range). The former case occurs because thepropagation loss of the SMF is as small as ³0.5 dB/km (notethat, in BOCDR, an FUT with a length of several tens ofkilometers is seldom used because it results in quite poor
spatial resolution (greater than several meters) owing to theresolution/range trade-off relation). The latter case occurs
because the resolution affects only the width of the correlation peak. If the resolution is extremely low, the positionof the bending loss needs to be farther from the FUT end. In
particular, the following criterion can be used for determining
the position at which bending loss should be applied, i.e.,
the loss should be applied at a position that is approximately
half of the spatial resolution far from the FUT open end (asin this experiment). Though bending loss was used in thiswork, cutting the FUT end at an angle or using an APC-typeconnector at the FUT end could be an alternative method.
Our results also suggest that the FUT should not containreﬂective spots such as badly coupled or spliced points andinherent ﬁber defects because another ghost peak may begenerated. We believe that this study will provide a useful
guideline for optimal BOCDR system design in the future.
Acknowledgments This work was partially supported by Grants-in-Aid
for Young Scientists (A) (No. 25709032) and for Challenging Exploratory
Research (No. 26630180) from the Japan Society for the Promotion of Science(JSPS) and by research grants from the General Sekiyu Foundation, the Iwatani
Naoji Foundation, and the SCAT Foundation. N.H. acknowledges a Grant-in-Aid
for JSPS Fellows (No. 25007652).

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