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OPEN

Received: 23 February 2018
Accepted: 8 May 2018
Published: xx xx xxxx

Bending-loss-independent
operation of slope-assisted
Brillouin optical correlation-domainreflectometry
Heeyoung Lee , Tianyi Ma, Yosuke Mizuno & Kentaro Nakamura
We demonstrate loss-insensitive operation of slope-assisted Brillouin optical correlationdomain reflectometry by employing a special silica fiber with low bending loss. As fundamental
characterization, we measure the coefficients of the power-change dependencies on strain andtemperature to be 1.42 × 10−4 dB/µε and 3.28 × 10−3 dB/K, respectively. Subsequently, by comparing
the distributed strain and temperature measurement results using a standard silica fiber and the special
fiber, we show that this configuration offers highly stable loss-insensitive operation for practical use in
the future.
Distributed strain and temperature sensing based on Brillouin scattering 1 in optical fibers has been oneof the major targets of research in the community of smart materials and structures. Various configurations
have been reported thus far, including Brillouin optical time-domain analysis (BOTDA)2–7, Brillouin opticalfrequency-domain analysis (BOFDA)8,9, Brillouin optical correlation-domain analysis (BOCDA)10–15, Brillouin
optical time-domain reflectometry (BOTDR)16–20, Brillouin optical frequency-domain reflectometry (BOFDR)21,
and Brillouin optical correlation-domain reflectometry (BOCDR)22–30. Each configuration has different merits
and demerits; for instance, the time-domain techniques generally have long measurement ranges, but their spatial
resolutions are limited to sub-meter order31, though some recent papers have reported significantly enhanced
resolutions32,33. In an analogous fashion, the correlation-domain techniques have random accessibility12 andextremely high spatial resolutions of up to millimeter-order14, but they suffer from limited measurement ranges23,
although some enhancements have been reported24. Here, let us focus on BOCDR.
Since the basic idea was first implemented22, BOCDR has been extensively studied for more than ten years,
leading to substantial improvement of its performance24–30. Some of the recent advances include the development of simplified configurations25, polymer-fiber-based configurations27, and high-speed configurations, suchas phase-detected BOCDR28,29 and slope-assisted (SA-) BOCDR30. In SA-BOCDR, the Brillouin frequency shift(BFS), which corresponds to the information on the applied strain and temperature, is converted into the spectral
power on the slope of the Brillouin gain spectrum (BGS). In addition to the high-speed operation30, SA-BOCDR
has some unique features, such as the detection capability of the shortest-ever strain (<2 mm) below the nominal
spatial resolution34,35 and the sensitivity on not only strain and temperature but also locally applied loss30. The
latter feature is sometimes beneficial36, but from the viewpoint of stable measurement of strain and temperature,
the final output of SA-BOCDR should not be influenced by unintended loss applied to the sensing fiber.
In this work, we demonstrate loss-insensitive operation of SA-BOCDR by employing a special silica fiber withlow bending loss. We characterize the sensing performance of the loss-insensitive SA-BOCDR and find that thecoefficients of the power-change dependencies on strain and temperature are 1.42 × 10−4 dB/µε and 3.28 × 10−3
dB/K, respectively (when the spatial resolution is 144 mm and the measurement range is 14.7 m). Subsequently,
by comparing the distributed strain and temperature measurement result using the special fiber with that using astandard silica single-mode fiber (SMF), we prove the effectiveness of this loss-insensitive configuration.

Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, 2268503, Japan. Correspondence and requests for materials should be addressed to Y.M. (email: ymizuno@sonic.
pi.titech.ac.jp)
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Figure 1. Schematic of the SA-BOCDR setup. AMP: electrical amplifier, EDFA: erbium-doped fiber
amplifier, ESA: electrical spectrum analyzer, FUT: fiber under test, OSC: oscilloscope, PD: photodiode, PSCR:
polarization scrambler.

Results

Principle.

Brillouin-based distributed sensors including BOCDR basically utilize the BFS dependencies
on strain and temperature. The strain- and temperature-dependence coefficients in silica SMFs at 1550 nm arereported to be approximately 500 MHz/% and 1.1 MHz/K, respectively37,38. To perform a distributed measurement, BOCDR exploits what we call a correlation peak, at which the Brillouin-scattered light is selectively
observed. The correlation peak is generated by sinusoidal modulation of the laser output frequency, and its position can be arbitrarily set along the fiber under test (FUT) of the system10,22. If the modulation frequency fm isswept, the correlation peak is scanned along the FUT, which provides distributedly measured BFSs. According to
the theory23, the measurement range of the basic scheme is limited to
dm =

c
2nfm

(1)

where c is the velocity of light in vacuum and n is the refractive index of the fiber core; the spatial resolution is
given byΔz =c∆vB
2πnfm ∆f(2)

where ΔvB is the Brillouin bandwidth (= ~30 MHz in silica SMFs) and Δf is the modulation amplitude.
In standard BOCDR schemes, to obtain the strain or temperature information at one sensing position along
the FUT, the whole BGS needs to be acquired and then the BFS, at which the spectral power becomes the highest,
is derived, resulting in a relatively time-consuming process. To solve this problem, in SA-BOCDR30, instead of
the BFS itself, we use the change in the spectral power on the BGS slope (at a constant frequency vB0), which is in
one-to-one correspondence to the BFS. This configuration is free from the time-consuming frequency-sweeping
process, leading to much higher-speed operation. The optimal value of vB0, which is determined by differentiating
the BGS30, is 10.85 GHz for standard silica SMFs. The coefficients of the power-change dependencies on strain
and temperature in standard silica SMFs are reported to be 1.95 × 10−4 dB/µε and 4.42 × 10−3 dB/K, respectively30(when the spatial resolution is 88 mm and the measurement range is 12.9 m), which are in moderate agreementwith the values theoretically calculated from the BFS dependencies and the BGS shape30.

Experimental setup. Figure 1 shows the experimental setup of SA-BOCDR, which is basically the same asthat previously reported30. The pump light was amplified to ~26 dBm using an erbium-doped fiber amplifier andinjected into the FUT. The reference light was passed through a ~1-km-long delay fiber, amplified to ~2 dBm, andcoupled with the Brillouin-scattered light (amplified to ~1 dBm) for heterodyne detection. By inserting a polarization scrambler in the pump path, the polarization-dependent signal fluctuations were suppressed to improve thesignal-to-noise ratio (SNR). The heterodyned optical signal was converted into an electrical signal with a photodiode, amplified by 23 dB using an electrical amplifier, and observed with an electrical spectrum analyzer (ESA)(video bandwidth: 3 kHz, resolution bandwidth: 10 MHz) as a BGS. Exploiting the narrow band-pass filteringfunction of the ESA, the power change at a fixed frequency (=10.81 GHz; calculated by analyzing the measured
BGS, as mentioned below) on the BGS slope was sequentially output to an oscilloscope, on which averaging was
performed 1024 times. The modulation amplitude Δf was set to 1.3 GHz. For distributed measurements, themodulation frequency fm was swept from 6.91 to 7.10 MHz (note that, in a single-point measurement for characterizing the system performance, fm was fixed at 7.04 MHz, which corresponds to the location of the correlation
peak 7.9 m away from the circulator). According to Eqs (1and 2), these conditions resulted in the nominal spatial
resolution of 144 mm and the measurement range of 14.7 m. The room temperature was 21 °C.
The low-bending-loss silica fiber used in the experiment was a 12.0-m-long trench-index-type special
single-mode fiber (SMF; FutureGuide-BIS-B, Fujikura)39,40 with a low bending loss (e.g., 0.018 dB/turn at 1550 nm
for a bending radius of 7.5 mm). One end of this special fiber was connected to the 1.0-m-long pigtail of the second port of an optical circulator via an “FC/APC” adaptor, and the other end was cut with an angle and immersed
into index matching oil to suppress the Fresnel-reflection-dependent signal fluctuations.
Fundamental characterization. First, without modulating the optical frequency of the laser output(i.e., using the whole length of the FUT), the strain and temperature dependencies of the BGS and BFS in the
low-bending-loss fiber were investigated. As shown in Fig. 2(a), with increasing strain, the BGS shifted to higher
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Figure 2. Measured dependencies of the BGS (insets) and the BFS on (a) strain and (b) temperature. Each BGS
was normalized so that its maximal power became 1. The BFS was extracted from the BGS using Lorentzian
fitting. The dotted lines are linear fits.

Figure 3. BGS measured when the spatial resolution was 144 mm and the measurement range of 14.7 m. The
linear range on the lower-frequency side is indicated in red.

frequency (see the inset), and the BFS linearly depended on strain with a coefficient of 465 MHz/%. In the samemanner (Fig. 2(b)), with increasing temperature, the BGS also shifted to higher frequency (see the inset), and the
BFS dependence coefficient on temperature was 1.2 MHz/K. The strain and temperature dependence coefficientsare 0.9 and 1.1 times the values in standard silica fibers reported in refs37,38; the small discrepancies are naturalconsidering that the fiber cores are both composed of germanium-doped silica. Note that Brillouin bandwidth

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Figure 4. Measured strain dependence of the power-change distributions. Each distribution is shifted by 0.2 dB.

Figure 5. Measured power-change dependencies on (a) strain and (b) temperature. The dotted lines are linear fits.

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Figure 6. Structure of the FUT, where heat, strain, and bending loss were applied.

Figure 7. Normalized power-change distributions with rod radii of 8 and 10 mm when the FUT was (a) the
standard silica SMF and (b) the low-bending-loss silica SMF. Each distribution is shifted by 2.

ΔvB of this special fiber (non-strained, at room temperature) was approximately 40 MHz, which was used to calculate the nominal spatial resolution of the system using this fiber (see Eq. (2)).
Next, we performed a single-point measurement of strain and temperature to evaluate the sensing performanceof SA-BOCDR using the low-bending-loss fiber. The measured BGS is shown in Fig. 3. By differentiating the BGS
with respect to frequency, the optimal vB0 value was calculated to be 10.81 GHz, and the linear range (defined as
the range where the change in the BGS slope is suppressed within 20% compared to its maximum)30 was calculated
to be ~70 MHz. We then applied strains of up to 1200 με to the 0.15-m-long section (at the midpoint of which thecorrelation peak was located) and measured the dependencies of the power-change distributions (Fig. 4). Each distribution was displayed with an artificial shift of 0.2 dB for clear comparison. At the strained position, a clear peak
was observed, and its peak amplitude increased with increasing strain. Refer to Methods for the reason why thepower-change distributions do not form rectangular shapes at the strained position. Figure 5(a) shows the maximal
power change plotted as a function of the strain. The dependence was almost linear with a coefficient of 1.42 × 10−4
dB/µε; the slight discrepancy from the linear trend seems to have been caused by the imperfectly straight BGS slope
even within the linear range. The power-change dependence on temperature (<50 °C) was investigated under thesame conditions, and as shown in Fig. 5(b), an almost linear behavior with a coefficient of 3.28 × 10−3 dB/K was
obtained. These values were in moderate agreement with the theoretical values calculated using the BGS shape andthe strain and temperature dependence coefficients of the BFS.

Demonstration of bending-insensitive distributed measurement. Finally, the loss-insensitive oper-

ation of SA-BOCDR was demonstrated using the FUTs depicted in Fig. 6. Two different types of 12.0-m-longsilica SMFs (a standard SMF (FPC-SM20, Alnair Labs) and the aforementioned low-bending-loss SMF) were
employed. Heat (50 °C) and strain (600 µε) were applied to 0.15-m-long sections, and bending losses were applied
between the heated and strained sections. The bending loss was applied by winding the fibers around rods for one
turn without strain. The radii of the rods were 8 and 10 mm, which theoretically induce bending losses of ~3.1
and ~0.8 dB for the standard SMF, respectively40. The bending losses induced to the special SMF were relatively
low (<0.02 dB).
Figure 7(a) shows the power-change distributions measured along the standard SMF with 8- and 10-mm rod
radii. The vertical axis was normalized so that the maximal power change at the heated section became 1 (when

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the rod radius was 10 mm). Note that the two distributions were displayed with an artificial shift of 2 for clear
comparison. Regardless of the rod radii, the heated sections were correctly detected, and the rough locations of
the applied bending losses were also detected (Refer to Methods for the reason why the power-change distributions do not form rectangular shapes at the heated/strained positions). However, when the rod radius was 10 mm,
the location of the strained section was correctly detected, but the strain magnitude appeared to be much smaller
than the actual value (which is approximately 0.6). This is because the BGS power reduced by bending loss leadto the reduction in the spectral slope, resulting in the reduced sensitivity to strain. When the rod radius was evensmaller (8 mm), the loss was so high that the strained section was not detected at all. This is because the considerable loss almost completely diminished the BGS (i.e., the spectral slope became almost flat), and consequently,
the strain-induced spectral shift posed only negligible influence on the final output. In contrast, Fig. 7(b) shows
the normalized power-change distributions measured along the low-bending-loss SMF with 8- and 10-mm rod
radii. The two distributions were similar to each other irrespective of the rod radii, and the heated and strained
sections were correctly detected in both measurements. The applied bending losses did not affect the measured
results, indicating that, by SA-BOCDR using this special fiber, a stable distributed measurement of strain andtemperature can be performed with no influence of unintended local loss.

Conclusion

In this work, we verified the loss-insensitive operation of SA-BOCDR using the special SMF. First, we measured the coefficients of the power-change dependencies on strain and temperature to be 1.42 × 10−4 dB/µε and3.28 × 10−3 dB/K, respectively. Then, by comparing the distributed strain and temperature measurement results
using the standard SMF and the special SMF, we proved the effectiveness of this loss-insensitively configured
SA-BOCDR. To enhance the measurement stability further, the use of low-bending-loss polarization-maintaining
fibers may be an option. We anticipate this configuration with high stability is of significant use in practically
employing SA-BOCDR for structural health monitoring in the future.

Methods

Unique final output of SA-BOCDR.

The final output of SA-BOCDR is a power-change distribution along
an FUT, where the power change gives information of strain, temperature, and loss30. However, the power-change
distribution obtained using SA-BOCDR is not in one-to-one correspondence to the BFS distribution (the correct
BFS information can be derived, provided the SNR is sufficiently high)34. When strain or temperature change isuniformly applied to a section, the BFS distribution should form a rectangular shape, while the power-change
distribution in SA-BOCDR basically exhibits a trapezoidal shape. In particular, when the length of the strainedor heated section is equal to the nominal spatial resolution, it shows a triangular shape. Even when the strainedor heated section is shorter than the nominal resolution, a trapezoidal-shaped shift in the power change can beobserved, which is called the beyond-nominal-resolution effect34. This effect, which neither standard BOCDR
systems nor SA-BOTDR/BOTDA systems41–43 can offer, is unique to SA-BOCDR and will be of great use in practical applications30. See ref.34 for the physical origin of this unique final output of SA-BOCDR.

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Acknowledgements

This work was supported by JSPS KAKENHI Grant Numbers 17H04930 and 17J07226, and by research grants
from the Japan Gas Association, the ESPEC Foundation for Global Environment Research and Technology, theAssociation for Disaster Prevention Research, the Fujikura Foundation, and the Japan Association for Chemical
Innovation.

Author Contributions

H.L., T.M., and Y.M. designed and performed the experiments. H.L. and T.M. performed the experimental data
analysis. H.L., Y.M., and K.N. performed the theoretical analysis. H.L. and Y.M. wrote the manuscript with input
from all co-authors.

Additional Information

Competing Interests: The authors declare no competing interests.
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