Proposal of Brillouin opticalcorrelation-domain reflectometry (BOCDR)
Yosuke Mizuno*, Weiwen Zou, Zuyuan He,and Kazuo Hotate
Department of Electronic Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan*
Corresponding author: ymizuno@sagnac.t.u-tokyo.ac.jp

Abstract: We propose a Brillouin optical correlation-domain reflectometry(BOCDR), which can measure the distribution of strain and/or temperature
along an optical fiber from a single end, by detecting spontaneous Brillouinscattering with controlling the interference of continuous lightwaves. In apulse-based conventional Brillouin optical time-domain reflectometry(BOTDR), it is difficult in principle to achieve a spatial resolution less than
1 m, and the measurement time is as long as 5-10 minutes. On the contrary,
the continuous-wave-based BOCDR can exceed the limit of 1-m resolution,and realize much faster measurement and random access to measuring
positions. Spatial resolution of 40 cm was experimentally demonstrated withsampling rate of 50 Hz.
©2008 Optical Society of America
OCIS codes: (060.2370) Fiber optics sensors; (280.4788) Optical sensing and sensors;(290.5830) Scattering, Brillouin.

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16.

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1. Introduction
Optical fiber sensors based on Brillouin scattering have been intensively studied as apromising technology for monitoring conditions in various materials and structures, due to thepossibility of distributed measurement of strain and/or temperature along an optical fiber [113]. To obtain the location information of strain/temperature along the fiber, conventionally,
pulse-based time-domain systems [5-10] have been used, which have the advantage of longmeasurement range over several tens of kilometers. Their long measurement time (~ severalminutes) and inherent limitation in spatial resolution (~ 1 m) [10], however, are notsatisfactory for practical applications.
In this paper, we propose a Brillouin optical correlation-domain reflectometry (BOCDR),
which can measure the strain distribution along the optical fiber from a single end, bydetecting spontaneous Brillouin scattering with controlling the interference of continuous
lightwaves. Spatial resolution of 40 cm was experimentally demonstrated with sampling rateof 50 Hz.
2. Principle
Brillouin scattering-based fiber sensors are classified into two types: “reflectometries”, inwhich a light beam is injected into one end of the fiber under test (FUT), and “analysissystems”, in which two light beams are injected into both ends of the FUT. Several kinds ofanalysis systems have been studied, including the Brillouin optical time-domain analysis(BOTDA) [5-7] and the Brillouin optical correlation-domain analysis (BOCDA) [11-13].
These systems can obtain relatively large signals due to utilizing the stimulated Brillouinscattering (SBS). However, these methods need two-end access, which is often not feasible inlong-range application, and cannot work completely when the FUT has even one breakage
point. Considering the wide range of application, one-end access reflectometries are morefavorable, even though their signal is not as large since they utilize the phenomenon of
spontaneous Brillouin scattering. Brillouin optical time-domain reflectometries (BOTDR) has
been proposed and studied [8, 9], as one-end access systems. However, these can’t realizeboth of fast measurement speed and high spatial resolution.
When a light beam is injected into an optical fiber, backscattered light (Stokes light) isgenerated through the interaction with acoustic phonons, and it propagates in the direction
opposite to the incident light. This phenomenon is called spontaneous Brillouin scattering.
The Brillouin scattered light spectrum, also known as the Brillouin gain spectrum (BGS),
takes the shape of Lorentzian function [14]. The frequency at which the peak power isobtained in the BGS is shifted for about 11 GHz from the incident light frequency when theincident light wavelength is 1.55 μm. This amount of frequency shift is called the Brillouinfrequency shift (BFS) fB. If tensile strain or temperature change occurs in the optical fiber, theBFS varies in proportion to the applied strain (1MHz/0.002%) or temperature change(1MHz/1K). Therefore, by measuring the distribution of the BFS along the FUT, the applied
strain amplitude or temperature change can be derived.
3. Proposal
The conceptual schematic of the proposed BOCDR system is shown in Fig. 1. A light beamfrom a laser is divided into pump and reference light beams. The pump light is injected into
the FUT, and the Stokes light is directed into a heterodyne receiver composed of two balanced
photodiodes (PDs). The reference light is used as an optical local oscillator. The electrical#96919 - $15.00 USD(C) 2008 OSA

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Fig. 1. Conceptual schematic of BOCDR system.

beat signal of the two light beams is monitored by an electrical spectrum analyzer (ESA).
Since there is a frequency difference of about 11 GHz between the Stokes light and thereference light, this configuration is called self-heterodyne scheme.
In order to resolve the position in the FUT, the optical frequency of the light beam fromthe laser is directly modulated in a sinusoidal wave by modulating the injection current to thelaser. From the viewpoint of time averaging, the correlation (or coherence) function issynthesized into a series of periodical peaks [15, 16], whose period is inversely proportional
to the frequency of the sinusoidal modulation fm. We control fm to leave only one correlation
peak within the range of the FUT, so that only the Brillouin scattering generated at theposition correspondent to the peak has high correlation with the reference light, and then giveshigh heterodyne output. The peak frequency observed in the ESA gives the BFS caused at theposition. By sweeping fm, the correlation peak is scanned along the FUT to obtain thedistribution of BGS or BFS. The spatial resolution Δz and the measurement range dm (distance
between the neighboring correlation peaks) of BOCDR are given by the same equations as
those for BOCDA [11]:V Δν
Δz = 2πg f ΔBf(1)mVg
dm = 2 fm(2)where Vg is the group velocity of light, ΔνB is the Brillouin gain bandwidth (~ 30 MHz) in
optical fibers, and Δf is the modulation amplitude of the light source.
4. Experiment
The actual experimental setup of BOCDR is depicted in Fig. 2. Before heterodyne detection,
the reference light at frequency f0 passed through a 2-km delay fiber, which is used to selectFig. 2. Experimental setup of BOCDR system.

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the order of periodical correlation peaks, and an Er-doped fiber amplifier (EDFA) forenhancing the beat signal. The pump light at frequency f0 was injected into the FUT after
being amplified by a high-power EDFA. The weak Stokes light at frequency f0-fB
backscattered from the FUT was amplified again by an EDFA. An optical filter was inserted
after the EDFA in order to suppress the Rayleigh scattering and the Fresnel reflection from theFUT at frequency f0. Before processing by an ESA, the heterodyned signal was amplified byan electrical pre-amplifier.

Fig. 3. Structure of the FUT.

The modulation frequency fm was set to 457.4 – 458.4 kHz, which corresponds to themeasurement range of 228 m according to Eq. (2). The amplitude of the frequency modulation
Δf was 5.4 GHz, and the spatial resolution Δz was calculated to be about 40 cm from Eq. (1).
Figure 3 shows the structure of the FUT comprising a 100-m conventional single-mode fiber(SMF), in which a 0.2-% strain is applied to a 50-cm section fixed on a translation stage usingepoxy glue. One end of the FUT was spliced to a circulator, and the other end was kept open.
The overall sampling rate of the BGS measurement for a single position was 50 Hz, which ismuch higher than that of the time-domain techniques (typical measurement time severalminutes). Figure 4 shows the measurement result of the distribution of the BGS along theFUT. The BGS at the strain-applied section is clearly recognized. Figure 5 shows thedistribution of the BFS. Spatial resolution better than 50 cm is achieved. The change of theBFS was about 100 MHz, which is in good agreement with the applied strain of 0.2 %.

Fig. 4. Distribution of the BGS along the FUT.

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Fig. 5. Distribution of the BFS (peak of the BGS) along the FUT.

5. Discussion
As shown in Eqs. (1) and (2), there is a trade-off between the measurement range and thespatial resolution in BOCDR, and the ratio of the two values is purely dependent on theamplitude of the modulation frequency Δf. Therefore, Δf needs to be increased in order toobtain substantial improvement in the performance of BOCDR. However, Δf cannot beincreased to the limit discussed below.
Figure 6(a) shows optical spectra of the reference light and the reflected light just before
heterodyne detection when the frequency of the light source is sinusoidally modulated withthe modulation amplitude Δf. The Rayleigh scattered light is too strong to be suppressed

(a)

(b)

(c)

Fig. 6. Schematics of (a) optical spectra of the reference light and the reflected light when thefrequency of the light source is modulated with the amplitude Δf, (b) electrical spectra with
sufficiently small Δf, and (c) electrical spectra with large Δf.

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Fig. 7. Measured BGS when Δf is 5.4 GHz, 5.5 GHz, and 5.6 GHz.

completely by an optical filter, thus it remains in the reflected light. The electrical spectra
after heterodyne detection with sufficiently small Δf is shown in Fig. 6(b). The spectrum ofthe Brillouin scattered light broadens in both directions for 2Δf with a center frequency fB. On
the other hand, the spectrum of the Rayleigh scattered light broadens from 0 Hz to 2Δf,considering that negative frequency is folded back to positive one. Figure 6(c) showselectrical spectra when 2Δf is increased to the value slightly lower than fB. Though theRayleigh spectrum is approaching the detection area, so long as it is outside the area, themeasurement is free from its influence. However, if 2Δf is further increased beyond fB, theRayleigh spectrum starts to overlap with the detection area, resulting in a considerable amountof noise. Thus, it is undesirable for Δf to be higher than half of fB.
In the experiment, Δf was set to 5.4 GHz, which is slightly lower than half of the Brillouinfrequency shift of 11 GHz in SMF. If Δf larger than 5.4 GHz was applied to the system, theS/N ratio decreased drastically, as shown in Fig. 7.
6. Conclusion
We have proposed a new type of sensing technology called BOCDR based on spontaneousBrillouin scattering with controlling the correlation of continuous lightwaves, anddemonstrated one-end-access distributed strain measurement with 40-cm spatial resolution
and 50-Hz sampling rate. We expect that the BOCDR system will become a promisingtechnology for practical use as a fiber-optic nerve system in the smart materials and structures.
Acknowledgments
The authors are grateful to Prof. Kwang Yong Song at Chung-Ang University, Korea, for
helpful discussions and comments. Yosuke Mizuno thanks the Japan Society for thePromotion of Science (JSPS) Research Fellowships for Young Scientists. This work was
supported by the “Grant-in Aid for Creative Scientific Research” from the Ministry ofEducation, Culture, Sports, Science and Technology (MEXT), Japan.

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