Measurement range enlargement in Brillouin
optical correlation-domain reflectometry basedon temporal gating scheme
Yosuke Mizuno, Zuyuan He, and Kazuo Hotate
Department of Electrical Engineering and Information Systems, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanymizuno@sagnac.t.u-tokyo.ac.jp; ka@sagnac.t.u-tokyo.ac.jp; hotate@sagnac.t.u-tokyo.ac.jp

Abstract: We newly develop a temporal gating scheme to enlarge themeasurement range of Brillouin optical correlation-domain reflectometry(BOCDR) for a fiber-optic distributed strain measurement. In this scheme,
the trade-off problem between the measurement range and the spatial
resolution can be dissolved. 66-cm spatial resolution and 1-km measurement
range were simultaneously obtained with 50-Hz sampling rate.
2009 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (280.4788) Optical sensing and sensors;(290.5830) Scattering, Brillouin.

References and links
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Commun. E90-B, 1810-1815 (2007).
6. K. Hotate, and T. Hasegawa, “Measurement of Brillouin gain spectrum distribution along an optical fiberusing a correlation-based technique – proposal, experiment and simulation,” IEICE Trans. Electron. E83-C,
405-412 (2000).
7. K. Hotate, and M. Tanaka, “Distributed fiber Brillouin strain sensing with 1-cm spatial resolution bycorrelation-based continuous-wave technique,” IEEE Photon. Technol. Lett. 14, 197-199 (2002).
8. K. Y. Song, Z. He, and K. Hotate, “Distributed strain measurement with millimeter-order spatial resolution
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21, 474-476 (2009).
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Brillouin scattering: towards ultimate resolution,” Proc. 12th Intern. Conf. Optical Fiber Sensors, 324-327(1997).
13. Y. Koyamada, Y. Sakairi, N. Takeuchi, and S. Adachi, “Novel technique to improve spatial resolution in
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14. G. P. Agrawal, Nonlinear Fiber Optics (Academic Press, California, 1995).
15. K. Hotate and Z. He, “Synthesis of optical-coherence function and its applications in distributed andmultiplexed optical sensing,” IEEE J. Lightwave Technol. 24, 2541-2557 (2006).
16. M. Kannou, S. Adachi, and K. Hotate, “Temporal gating scheme for enlargement of measurement range ofBrillouin optical correlation domain analysis for optical fiber distributed strain measurement,” Proc. 16th
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17. K. Hotate, H. Arai, and K. Y. Song, “Range-enlargement of simplified Brillouin optical correlation domain
analysis based on a temporal gating scheme,” SICE J. Cont. Meas. Syst. Int. 1, 271-274 (2008).

1. Introduction
Many fiber-optic strain/temperature sensors have been proposed based on various phenomena
including scattering, optical loss, and polarization. In particular, much research interest has
been focused on sensors utilizing Brillouin scattering, in which the Brillouin frequency
depends on the longitudinal strain and the temperature change in the optical fiber, thus
allowing the distributed measurement of strain and/or temperature along the fiber [1]-[11].
Brillouin scattering-based distributed fiber-optic sensors are classified into two types:“reflectometries”, in which a light beam is injected into one end of the fiber under test (FUT),and “analysis systems”, in which two light beams are injected into both ends of the FUT.
Several kinds of analysis systems have been studied, including the Brillouin optical timedomain analysis (BOTDA) [3]-[5] and the Brillouin optical correlation-domain analysis(BOCDA) [6]-[8]. Although those systems utilizing the stimulated Brillouin scattering (SBS)
can obtain relatively large signals, they need two-end access, which is often not feasible in along-range measurement, and cannot work completely when the FUT has even one breakage
point. Thus, for some practical applications, one-end access reflectometries are morefavorable, even though their signal is not as large because they utilize the spontaneous
Brillouin scattering.
Recently, we have proposed Brillouin optical correlation-domain reflectometry (BOCDR)[9]-[10] to measure the strain distribution along an FUT from a single end of the fiber. In apulse-based conventional Brillouin optical time-domain reflectometry (BOTDR) [11], thespatial resolution is generally limited by a combination of linewidth broadening and signal
weakening associated with the use of short optical pulses. The best spatial resolution for basic
systems is typically 1 m [12], though some progress has been made to enhance the resolution
further [13]. On the contrary, by controlling the interference of continuous lightwaves,
BOCDR shows such unique features as random access to measuring positions, high spatial
resolution, and high sampling rate. In our previous experiments, 13-mm spatial resolution has
been obtained with 50-Hz sampling rate, while the total measurement time depends on thenumber of sensing points over the FUT [10].
However, BOCDR suffers from a trade-off between the measurement range and the spatial
resolution. Their ratio NR was fixed at approximately 580 due to the limitation of Rayleighscattering [9]. This means that, for example, when the spatial resolution was 40 cm or 13 mm,
the measurement range was 224 m [9] or 7.6 m [10], respectively. Thus, in order to achieve
kilometer-order measurement range, the spatial resolution must be about 2 m or larger, whichis even worse than that of BOTDR.
In this paper, we newly implement a temporal gating scheme to obtain higher NR, namely,
to enlarge the measurement range of BOCDR while maintaining the spatial resolution. In thisscheme, any correlation peak within the FUT can be arbitrarily selected, so that multiple
correlation peaks can be utilized. 66-cm spatial resolution and 1-km measurement range (NR =1515) were simultaneously achieved with 50-Hz sampling rate.
2. Principle
When a light beam is injected into an optical fiber, backscattered light (Stokes light) is
generated through the interaction with acoustic phonons. This phenomenon is called
spontaneous Brillouin scattering. The Brillouin-scattered light spectrum, also known as theBrillouin gain spectrum (BGS), takes the shape of Lorentzian function [14]. The frequency atwhich the peak power is obtained in the BGS is shifted for about 11 GHz from the incident
light frequency when the incident light wavelength is 1.55 µm. This amount of frequency shift
is called the Brillouin frequency shift (BFS) fB. If tensile strain or temperature change occurs
in the fiber, the BFS varies in proportion to the applied strain (1 MHz / 0.002 %) or#108831 - $15.00 USD(C) 2009 OSA

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temperature change (1 MHz / 1 K). Therefore, by measuring the distribution of the BFS along
the FUT, the applied strain amplitude or temperature change can be derived.
In the basic BOCDR, the Stokes light due to the spontaneous Brillouin scattering of thepump light in the FUT is heterodyned with the reference light (self-heterodyne scheme) [9]. In
order to resolve the strain-applied position, the pump light and the reference light aresinusoidally frequency-modulated, producing periodical correlation peaks along the FUT [15].
The measurement range of conventional BOCDR, that is, the interval of the correlation peaks
dm is given by:Vg(1)dm = 2 f ,mwhere Vg is the group velocity of light and fm the modulation frequency. If we utilize a non0th correlation peak (where the 0th peak is defined at the zero optical path-difference
position), the position of the correlation peak can be moved along the FUT by changing fm.
Hence, distributed strain measurement can be performed by controlling fm. The spatial
resolution ∆z is given by:
Vg ∆νB(2)∆z = 2πf ∆f ,mwhere ∆νB is the Brillouin gain bandwidth (~ 30 MHz) in optical fibers, and ∆f the modulation
amplitude of the light source [6]. The number of effective sensing points NR, which can be
regarded as the evaluation parameter, is given by the ratio between dm and ∆z, as:dm π∆f
NR = ∆z = ∆ν .
B

(3)

From Eq. (3), in order to enhance the measurement range substantially, ∆f needs to be
increased. However, it is difficult to increase ∆f higher than a half of the BFS of the fiber(about 5.4 GHz for standard SMFs) [9]. Another way to enhance the measurement range is toutilize multiple intervals of the correlation peaks. In this case, we select only one correlation
peak for distributed measurement, but suppress other peaks to avoid cross-talk. Therefore, themeasurement range is multiplied. The temporal gating scheme is an effective method for thispurpose, and has been applied to BOCDA [16, 17].
The concept of the temporal gating scheme in BOCDR is explained, referring to an
experimental setup depicted in Fig. 1(a). The pump light is converted into optical pulses by anintensity modulator (IM), as shown in Fig. 1(b). By setting the pulse width, tW, to be the sameas the interval of correlation peaks:
1(4)tW = 2 f ,m
the Stokes light from each correlation peak can be temporally resolved. The interval of opticalpulses is set to double the length of the FUT to avoid the overlap of reflected signals. In
addition, by adjusting the relative phase, at the right time when the Stokes light from aparticular correlation peak returns, it is heterodyned with the reference light, which is also
converted into optical pulses with the same width tW. Thus, it becomes possible to arbitrarily
select any particular correlation peak within the FUT.

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Fig. 1. (a). Experimental setup of the BOCDR with a temporal gating scheme. (b) Pulse shapesof the pump light and the reference light.

3. Experiments
The experimental setup is depicted in Fig. 1(a). A distributed-feedback laser diode (DFB-LD)
at frequency f0 = 193.15 THz (1552 nm) was used as a light source, and a sinusoidal
frequency modulation was applied to generate correlation peaks within an FUT. The output
from the LD was divided into two light beams by a coupler. One of the beams was directly
used as the reference light of self-heterodyne detection, after passing a 5-km delay fiber forcontrolling the order of periodical correlation peaks and an Er-doped fiber amplifier (EDFA)
for enhancing the heterodyned beat signal. The other beam was injected into the FUT as thepump light, after being amplified by a high-power EDFA to 28 dBm. The weak Stokes light at
frequency f0 - fB, backscattered from the FUT, was amplified again by an EDFA. An opticalfilter composed of fiber Bragg grating (FBG) with a 3-dB bandwidth of about 10 GHz wasinserted after the EDFA in order to suppress the Rayleigh scattering and the Fresnel reflectionin the FUT at frequency f0. The optical beat signal of the reference light and the Stokes light
was detected by balanced photo-diodes (PDs) and converted to an electrical signal. The
relative polarization state of the two light beams was optimized by adjusting polarization
controllers manually. After 15-dB amplification from an electrical pre-amplifier, the signal
was monitored by an electrical spectrum analyzer (ESA). The measurement data weretransferred to a personal computer (PC). Two LiNbO3 IMs were set in the pump and reference
paths for the temporal gating scheme.
The structure of the FUT is shown in Fig. 2. It was composed of a 1-km standard singlemode fiber (SMF), in which about 0.1-% strain was applied to a 3-m section (990 - 993 m)
fixed on a translation stage using epoxy glue (It is difficult to apply accurately 0.1-% strain toa section as long as 3 m due to the FUT’s own weight). One end of the FUT was spliced to acirculator, and the other end was kept open. The overall sampling rate of the measurement fora single position was 50 Hz.

Fig. 2. Structure of the FUT.

In order to experimentally confirm that, without the temporal gating scheme, themeasurement cannot be performed correctly if there are multiple correlation peaks within the
FUT, first, we consider the case where there is no undesired correlation peak (N = 0). The
modulation frequency fm was 68.40 – 68.84 kHz, which corresponds to a measurement range#108831 - $15.00 USD(C) 2009 OSA

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dm of 1.5 km according to Eq. (2). The amplitude of the frequency modulation ∆f was 5.4
GHz, and the nominal spatial resolution ∆z was calculated to be about 2.6 m from Eq. (1).
Figure 3(a) shows the measurement result of the distribution of the BGS along the FUT.
The BGS at the strain-applied section is recognized. Figure 3(b) shows the distribution of the
BFS. We can see that 3-m strain was successfully detected. The change of the BFS was about
60 MHz, which agrees with the applied strain of about 0.1 %. The accuracy of themeasurement at a single position was about +/– 5 MHz, which corresponds to the strain of +/–0.01 % (+/– 100 µε) in this experiment.

Fig. 3. Distribution of (a) BGS, and (b) BFS.

Next, we consider the effect of multiple correlation peaks within the FUT. By adjusting
the modulation frequency fm and the amplitude of the frequency modulation ∆f, the spatial
resolution was constantly kept to 2.6 m, with one of the correlation peaks within the strainapplied section. The interval between the correlation peaks was set to 1.5 km, 753 m, 377 m,
188 m, and 94 m, which correspond to the cases where there are 0, 1, 2, 4, and 9 undesired
correlation peaks in the 1-km FUT, respectively.
As shown in Fig. 4, when there is no undesired correlation peak (N = 0), the BGS has its
peak at about 10.9 GHz corresponding to the applied strain. However, when there is one
undesired correlation peak (N = 1), the peak at 10.9 GHz is overlapped with that at 10.85 GHz
corresponding to zero strain, which makes distributed strain measurement impossible. The
further the number of the undesired peaks is increased, the larger the peak at 10.85 GHz
becomes, and so the peak at 10.9 GHz becomes unobservable. Thus, it was confirmed that thebasic BOCDR measurement without the temporal gating scheme cannot be performed
correctly if there are multiple correlation peaks within the FUT.

Fig. 4. Measured BGS without the temporal gating scheme applied, when there are multiple
correlation peaks within the FUT. The length of the FUT is 1 km. dm represents the interval
between the correlation peaks, and N the number of undesired peaks within the FUT.
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Then, we confirmed the operation of the temporal gating scheme, using the two LiNbO3
IMs inserted before the EDFAs in the pump and reference paths in Fig. 1(a). The same 1-km
SMF was used as the FUT, in which strain was applied to the same 3-m section (990 - 993 m).
The modulation frequency fm was 276.1 kHz, which corresponds to a 377-m interval ofcorrelation peaks according to Eq. (1). This means that there are three correlation peaks (twoundesired peaks) within the FUT. The amplitude of the frequency modulation ∆f was set to
1.4 GHz, and the spatial resolution ∆z was calculated to be 2.6 m from Eq. (2). These
conditions are almost the same as those for the experiment already described above (See Fig.
4, N = 2). According to Eq. (4), the width and the period of the optical pulses were set to 1.81
µs and 11.50 µs, respectively (duty cycle = 15.7 %). The overall sampling rate of the BGS
measurement for a single position was 50 Hz.
The changes of the BGS with and without strain applied when each peak was selected areshown in Figs. 5(a)-5(c). The relative phase differences of optical pulses were 226, 113, and 0
degrees for Figs. 5(a), 5(b), and 5(c), respectively, where a positive phase difference meansthat the phase of the reference pulses is faster than that of the pump pulses. In Figs. 5(a) and
5(b), the BGS shows hardly any change because the selected correlation peaks are outside thestrain-applied section. On the contrary, in Fig. 5(c), the peak of the BGS shifts because theselected correlation peak is within the strain-applied section. Thus, the temporal gating
scheme was confirmed to be effective.

Fig. 5. Measured BGS when (a) the first, (b) the second, and (c) the third correlation peaks
were selected, with (blue) and without (orange) strain applied to the section corresponding to
the third correlation peak.

Finally, a distributed strain sensing based on the temporal gating scheme was performed.
Figure 6 shows the structure of the FUT. 0.3-% strain was applied to a 90-cm section (990 –#108831 - $15.00 USD(C) 2009 OSA

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990.9 m). The modulation frequency fm was 275.65 – 276.25 kHz, and the amplitude of thefrequency modulation ∆f was set to 5.4 GHz, and so the spatial resolution ∆z was calculated to
be 66 cm. Figures 7(a) and 7(b) show the measurement results of the distribution of the BGS
and the BFS, respectively. The strain-applied 90-cm section was clearly detected. The changeof the BFS was about 150 MHz, which is in good agreement with the applied strain of 0.3 %.
The accuracy of the measurement at a single position was about +/– 20 MHz, whichcorresponds to a strain resolution of +/– 0.04 % (+/– 400 µε) in this experiment.

Fig. 6. Structure of the FUT.

Fig. 7. Distribution of (a) BGS, and (b) BFS.

Thus, we succeeded in achieving 1-km measurement range and 66-cm spatial resolution
simultaneously. Their ratio NR is calculated to be 1515, which is about three times as large asthat of the basic BOCDR system (about 580). In order to enhance NR further, the S/N ratio of
the system must be improved. We think further research is needed on this point.
4. Conclusion
In this paper, a temporal gating scheme was newly developed to enlarge the measurement
range of the BOCDR system while maintaining the high spatial resolution. 66-cm spatial
resolution and 1-km measurement range were simultaneously achieved with 50-Hz samplingrate. This means that the ratio between the measurement range and the spatial resolution wasenhanced from the conventional value 580 to 1515. We expect that the BOCDR system will
become a promising technology for practical use as a fiber-optic nerve system in the smart
materials and structures.
Acknowledgments
The authors are grateful to Dr. Weiwen Zou at the University of Tokyo, Japan, for helpful
discussions and comments. Yosuke Mizuno thanks the Japan Society for the Promotion of
Science (JSPS) Research Fellowships for Young Scientists. This work was supported by the“Grant-in Aid for Creative Scientific Research” and the “Global Center of Excellence (GCOE) Program” from the Ministry of Education, Culture, Sports, Science and Technology(MEXT), Japan.

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Received 16 Mar 2009; revised 9 May 2009; accepted 9 May 2009; published 14 May 2009

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