Simplified Configuration of Brillouin Optical
Correlation-Domain Reflectometry
Volume 6, Number 5, October 2014
Neisei Hayashi
Yosuke Mizuno, Member, IEEE
Kentaro Nakamura, Member, IEEE

DOI: 10.1109/JPHOT.2014.2361638
1943-0655 Ó 2014 IEEEIEEE Photonics Journal

Simplified Configuration of BOCDR

Simplified Configuration of Brillouin Optical
Correlation-Domain Reflectometry
Neisei Hayashi, Yosuke Mizuno, Member, IEEE, andKentaro Nakamura, Member, IEEE
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama, 226-8503, Japan
DOI: 10.1109/JPHOT.2014.2361638
1943-0655 Ó 2014 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received September 1, 2014; revised September 22, 2014; accepted September 23,
2014. Date of publication October 8, 2014; date of current version October 15, 2014. This work was
supported in part by Grants-in-Aid for Young Scientists (A) under Grant 25709032 and for Challenging Exploratory Research under Grant 26630180 from the Japan Society for the Promotion of
Science (JSPS) and in part by research grants from the Iwatani Naoji Foundation and the SCAT
Foundation. The work of N. Hayashi was supported by a Grant-in-Aid for JSPS Fellows under Grant
25007652. Corresponding author: N. Hayashi (e-mail: hayashi@sonic.pi.titech.ac.jp).

Abstract: We develop a simple and cost-efficient configuration of Brillouin opticalcorrelation-domain reflectometry (BOCDR), the setup of which does not include an additional reference path used in standard BOCDR systems. The Fresnel-reflected light froman open end of a sensing fiber is used as reference light. The limitations of spatial resolution, measurement range, and their ratio are theoretically clarified, and then, a distributed strain measurement with a G 100-mm spatial resolution and a 4.1-m measurement
range is demonstrated with a high signal-to-noise ratio.
Index Terms: Brillouin scattering, optical fiber sensors, distributed measurement, strain
sensing, temperature sensing.

1. Introduction
Optical fiber sensing based on Brillouin scattering has emerged as a promising technique formonitoring conditions in diverse materials and structures, owing to its measurability of strain
and/or temperature distribution along a fiber. To date, a variety of distributed sensing systems
have been implemented, which can be basically categorized into five techniques: Brillouin optical time-domain reflectometry (BOTDR) [1]–[4], Brillouin optical time-domain analysis (BOTDA)[5]–[11], Brillouin optical frequency-domain analysis (BOFDA) [12]–[15], Brillouin optical correlation-domain analysis (BOCDA) [16]–[21], and Brillouin optical correlation-domain reflectometry(BOCDR) [22]–[28]. BOTDR and BOTDA have the measurement range as long as several tensof kilometers or longer, but the spatial resolution of basic time-domain techniques is inherently
limited to 1 m [29], [30], though several methods have recently been developed to enhance
the resolution [2]–[4], [7]–[11]. BOFDA is free from the Brillouin linewidth broadening at high spatial resolution (3 cm [14]), and BOCDA has an extremely high spatial resolution (1.6 mm [17]).
However, in standard BOFDA and BOCDA, two light beams (pump and probe) need to be injected into both ends of the sensing fiber, resulting in their complicated setup and/or high cost.
Even their linear configurations [15], [19]–[21] do not ameliorate the situation. To resolve thisproblem, BOCDR was proposed in 2008.

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BOCDR [22]–[28], which operates based on the correlation control of continuous waves(Brillouin Stokes light and reference light) [31], has a substantial one-end accessibility, a highspatial resolution (13 mm with a silica fiber [23] and 6 mm with a tellurite fiber [25]), and ahigh sampling rate. Although the spatial resolution and the measurement range are in the
trade-off relation, temporal gating [26] and dual modulation [27] techniques have been developed to mitigate this drawback. Another practical advantage of BOCDR is a system simplicity
and cost efficiency; the experimental setup of its basic configuration does not include relatively
expensive devices, such as electro-optic modulators (optical-pulse generators, single-sideband
modulators, etc.) and vector network analyzers (cf. BOFDA). Further simplification of the
BOCDR system will greatly enhance its utility.
In this work, we develop an even simpler experimental setup of BOCDR, named S-BOCDR,
which exploits as reference light the Fresnel-reflected light from an open end of a sensing fiberand does not include an additional reference path used in standard implementations. After clarifying the theoretical limitations of spatial resolution, measurement range, and their ratio, we
demonstrate a distributed strain measurement with a G 100-mm spatial resolution and a 4.1-m
measurement range.

2. Principle and Theory
When pump light is launched into an optical fiber, it interacts with acoustic phonons, generatingbackscattered Stokes light. This phenomenon is known as spontaneous Brillouin scattering [32],and the Stokes light spectrum, referred to as the Brillouin gain spectrum (BGS), takes the shapeof a Lorentzian function. The central frequency of the BGS is known to shift downward relative
to that of the pump light by the amount called the Brillouin frequency shift (BFS). When thepump wavelength is 1.55 m, the BFS in silica single-mode fibers (SMFs) is approximately
10.8 GHz, which slightly varies depending on the fiber fabrication process. If a strain (or temperature change) is applied to a silica SMF, the BFS shifts toward a higher frequency withþ580 MHz/% [33] (or þ1.18 MHz/K [34]) at 1.32 m, corresponding to þ493 MHz/% (orþ1.00 MHz/K) at 1.55 m. Thus, by measuring the BFS distribution along a fiber, the information on applied strain or temperature distribution can be obtained.
BOCDR is one of the Brillouin-based distributed sensing techniques. In a basic BOCDR system (see Fig. 1 in Ref. [22]), the Stokes light returned from a fiber under test (FUT) is mixed
with the reference light, and their beat signal is converted into an electrical signal with a photo
diode (PD) and is detected as a BGS with an electrical spectrum analyzer (ESA). By modulating
the laser frequency with a sinusoidal waveform, correlation peaks are periodically synthesizedalong the FUT [31]. The FUT length is often set to be shorter than the correlation peak intervalso that only one correlation peak is in the FUT. By controlling the optical path difference (e.g.,
by changing the reference path length), a correlation peak of any order can be located in the
FUT. The position of the correlation peak can be scanned along the FUT by sweeping the frequency of the sinusoidal modulation, and thus, a distributed BGS measurement is feasible. The
measurement range of the basic BOCDR, corresponding to the interval of the two neighboringcorrelation peaks, is expressed as [24]dBOCDR ¼

c
2nfm

(1)

where c is the light velocity in vacuum, n is the core refractive index, and fm is the modulation
frequency of the laser frequency. In the meantime, the spatial resolution is given by [24]zBOCDR ¼

cB2nfm f(2)

where B is the Brillouin linewidth and f is the modulation amplitude of the laser frequency.
As the measurement range and the spatial resolution are in the trade-off relation, their ratio isVol. 6, No. 5, October 2014

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Fig. 1. Conceptual setup of S-BOCDR.

often used as a system evaluation parameter, which is given by
NBOCDR ¼

f
:
B

(3)

Here, it is reported that fm higher than B does not contribute to the enhancement of zBOCDR[24]. In addition, f needs to be lower than a half of the BFS of the FUT because of theRayleigh noise [22]. Therefore, the theoretical limitations of the spatial resolution and therange-to-resolution ratio can be expressed as
c
nBFS
BFS
max
NBOCDR
¼
:
2B

min
¼
zBOCDR(4)(5)

If a silica SMF ðn ¼ 1:46; BFS ¼ 10:8 GHz; B ¼ 30 MHzÞ is used, the theoretically highest
resolution and ratio are calculated to be approximately 6.0 mm and 565, respectively. The ratio
can be improved by employing special schemes based on temporal gating [26] and/or dualfrequency modulation [27].
Next, a schematic of a conceptual S-BOCDR setup is shown in Fig. 1. The laser output is directly launched into an FUT, and the reflected light is guided to a PD followed by an ESA. The
reflected light consists not only of the Brillouin Stokes light but also of the Fresnel-reflected light
generated at the open end of the FUT, which is exploited as the reference light. When the laser
frequency is sinusoidally modulated, correlation peaks are synthesized along the FUT. The 0th
correlation peak is generally fixed at a zero-optical-path-difference point, which corresponds tothe open end in this case (note that neither a total reflection mirror [19] nor a loop path [20] is required at the end). To suppress the influence of the 0th peak, some loss is artificially applied
near the open end. Although the Fresnel reflection power at the end is simultaneously reduced,
it does not necessarily deteriorate the signal-to-noise ratio (SNR) of the Brillouin measurement[35]. By sweeping the modulation frequency upward, the 1st correlation peak gradually approaches the 0th peak. As long as only the 1st peak (except the 0th peak) is in the FUT, a distributed measurement can be correctly performed. When the 1st peak has come to the middle pointof the FUT, the 2nd peak starts to appear in the FUT, resulting in the erroneous measurement.
Thus, the measurement range of S-BOCDR is limited todS-BOCDR ¼

L
2

(6)

where L is the FUT length. The spatial resolution is given, as a function of the sensing position lin the FUT (see Fig. 2 for the definition of l), by
zS-BOCDR ¼

Vol. 6, No. 5, October 2014

B ðL  lÞ
:
f

(7)

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Fig. 2. Definitions of L and l.

Fig. 3. Experimental setup of S-BOCDR.

Although the resolution drastically changes depending on the sensing position, we define therange-to-resolution ratio of S-BOCDR as that calculated using the lowest resolution ðl ¼ 0Þ. The
ratio is then given by
NS-BOCDR ¼

f
:
2B

(8)

Consequently, the theoretical limitations of the resolution and the ratio are given by
2B ðL  lÞ
BFS
BFS
max
NS-BOCDR ¼:
4BzSmin-BOCDR ¼(9)(10)

As the 1st correlation peak is used in S-BOCDR, the ratio cannot be improved in principle by thedual-modulation technique [27]. When a 100-m-long silica SMF ðBFS ¼ 10:8 GHz; B ¼
30 MHzÞ is used, the highest resolution is 180 mm at l ¼ 0 m and 90 mm at l ¼ 50 m. Notethat these theoretically highest performance has been derived without taking into consideration
the degradation of SNR, and that it is thus difficult to achieve the highest performanceexperimentally.

3. Experiments
The S-BOCDR setup used in the experiment is depicted in Fig. 3. The pump light was amplified
up to 30 dBm with an erbium-doped fiber amplifier (EDFA), and a 10-dB bending loss (experimentally optimized) was applied to the section near the open end. Considering that the Fresnel
reflectivity at the open end which is cut vertically to the fiber axis is  4% ð¼ 14 dBÞ, the reference power at the PD was roughly calculated to be 4 dBm. The sinusoidal modulation of thelaser frequency was performed by directly modulating the laser driving current.
We employed a silica SMF as an FUT, which had a numerical aperture of 0.13, a core refractive index of 1.46, a core diameter of 9 m, a cladding diameter of 125 m, and a propagation

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Fig. 4. Structure of FUT.

Fig. 5. Measured BGS distribution when a strain of 0.56% was applied.

Fig. 6. Measured BFS distribution when strains of 0.00, 0.27, 0.40, and 0.56% were applied. The inset shows the BFS dependence on applied strain.

loss of 0.5 dB/km at 1.55 m. The FUT length was 8.2 m, leading to the measurement rangeof 4.1 m. In this paper, to clearly show a distributed measurability with a high SNR, we swept
the modulation frequency fm from 12.370 MHz to 24.445 MHz and fixed the modulation amplitude f at 0.82 GHz, corresponding to the spatial resolution of 95 mm at l ¼ 0 m and48 mm at l ¼ 4:1 m. Using two translation stages, different strains of G 0.6% were applied
to a 0.6-m-long section of the FUT (see Fig. 4 for the detailed FUT structure). The sampling
rate at a single sensing point was 3.3 Hz, limited by the data acquisition from the ESA. The
number of the sensing points was set to 41, and consequently, the measurement time was12 s. The operating temperature was kept at 27  C.
The measured BGS distribution with a 0.56% strain applied is shown in Fig. 5. A BFS upshift
was observed at the strain-applied section with a high SNR. The measured BFS distributions
with strains of 0.00, 0.27, 0.40, and 0.56% applied (Fig. 6) shows that the 0.6-m-longVol. 6, No. 5, October 2014

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strain-applied section was successfully detected. As shown in the inset of Fig. 6, the BFS
shifted to higher frequencies with increasing strain with a proportionality constant of 491 MHz%(calculated using the BFS values at the middle point of the strain-applied section). This value isin good agreement with the previously reported value [33]. The BFS changed even along thestrain-free sections by about 2.1 MHz (standard deviation), which indicates that the strain andtemperature measurement errors are  0.004% and  1.8  C, respectively. Thus, a distributed measurability based on S-BOCDR was demonstrated.

4. Conclusion
We developed a simple configuration of BOCDRVS-BOCDRVwhere the Fresnel-reflected lightfrom an open end of a sensing fiber serves as reference light and no additional reference path
is required. First, the theoretical limitations of spatial resolution, measurement range, and theirratio were clarified. Then, a distributed strain measurement with a spatial resolution of G 100 mm(varied depending on the sensing position) and a measurement range of 4.1 m (half the FUTlength) was demonstrated with a high SNR. Here, we have to admit that S-BOCDR has the following drawbacks: 1) an FUT with a length of double the measurement range needs to be employed, 2) the range-to-resolution ratio is reduced by half, 3) if the sensing fiber is cut during theoperation, the measurement cannot be continued, 4) the polarization state cannot be fully optimized using standard polarization controllers (which does not matter if polarization scrambling [36] is employed to stabilize the measurement), and 5) the system simplification does notnecessarily lead to a significant reduction in cost. However, it is of considerable physical interest
that a high-resolution distributed measurement can be performed by such a simple setup. Moreover, the successful demonstration of the S-BOCDR operation implies that this principle couldin turn be a possible noise source for standard BOCDR systems, providing a useful guideline for
improving their SNR. Thus, we believe that the development of S-BOCDR is an important technological step toward the implementation of higher-performance distributed sensors based on
BOCDR.

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