Applied Physics Express 2 (2009) 062403

Stable Entire-Length Measurement of Fiber Strain Distribution
by Brillouin Optical Correlation-Domain Reflectometry
with Polarization Scrambling and Noise-Floor Compensation
Yosuke Mizuno, Zuyuan He, and Kazuo Hotate
Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan
Received April 20, 2009; accepted May 12, 2009; published online June 5, 2009
We performed a stable high-speed entire-length measurement of strain distribution by suppressing the signal fluctuations automaticallyusing polarization scrambling. Strain distribution along the entire length of a 100-m fiber was successfully measured with 40-cm spatial
resolution and 19-Hz sampling rate. We also demonstrated a signal-to-noise ratio improvement by compensating the noise-floor of theelectrical spectrum analyzer. # 2009 The Japan Society of Applied Physics
DOI: 10.1143/APEX.2.062403

rillouin scattering-based ﬁber-optic sensors candetermine the distribution of strain and/or temperature along an optical ﬁber, and have been vigorously
studied as a promising technology for monitoring conditions
in various materials and structures.1–4) Recently, we haveproposed Brillouin optical correlation-domain reﬂectometry(BOCDR)5) to measure the strain distribution along a ﬁber
under test (FUT) from a single end of the ﬁber. Compared topulse-based conventional Brillouin optical time-domain
reﬂectometry (BOTDR)6) with the typical spatial resolutionof 1 m7) and the measurement time of several minutes,
BOCDR shows unique features by controlling the interference of continuous lightwaves, such as random access tomeasuring positions, high spatial resolution, and high-speed
measurement. In our previous experiment, 13-mm spatial
resolution was obtained with 50-Hz sampling rate (excluding
data processing time).8)
It has been also demonstrated that BOCDR can be applied
to a distributed measurement of polarization beat length.9)
This measurement is based on the fact that the Brillouinscattered signal obtained in BOCDR is sensitive to the stateof polarization (SOP). However, due to this polarization
dependence, frequent SOP optimization by manually
adjusting polarization controllers (PCs) was needed for a
distributed strain/temperature measurement along a long
FUT, and consequently, the measurement time became longin practice. In order to accomplish a stable high-speed
distributed measurement along a long FUT, the polarizationeﬀect needs to be suppressed automatically. Several
techniques have thus far been proposed to mitigate thepolarization dependence, including adaptive polarization
controllers,10) polarization diversity,11,12) Faraday rotation,13)
and polarization scrambling.14,15) Among them, polarization
scrambling is advantageous over the others on the point thatit can avoid the need of continuous actuation and the use of
extra devices, and doesn’t deteriorate the sampling rate.
Polarization scrambling has already been used to solve
polarization-originated problems, such as polarization mismatch in coherent detection14) or polarization-dependent gain
in optically-ampliﬁed ﬁber communication systems.15) In
BOCDR, the scrambler changes the SOP of the light much
faster than the sampling rate, and therefore the measurement
becomes independent of the instantaneous SOP mismatch
between the reference light and the Stokes light.
In this paper, we perform a stable high-speed entire-length
measurement of strain distribution by suppressing the signal

B

ﬂuctuations with polarization scrambling. In the experiment,
we demonstrate a stable distributed strain measurement
along the entire length of a 100-m FUT with 40-cm spatial
resolution and 19-Hz sampling rate. In addition, we develop
a new method to improve the signal-to-noise (S/N) ratio of
the system by compensating the noise-ﬂoor of the electrical
spectrum analyzer (ESA).
The basic principle of BOCDR is to select the state of
spontaneous Brillouin scattering over one speciﬁc point fromthe FUT by applying the same frequency modulation to thereference and the back-scattered Stokes light.5) The spatial
resolution z and the measurement range (i.e., interval
between correlation peaks) dm are given as
Vg B;2fm f
Vg
dm ¼
;
2fm

z ¼

ð1Þ
ð2Þ

where Vg is the group velocity of light in the ﬁber, B the
intrinsic linewidth of Brillouin gain spectrum (BGS), fm thelaser modulation frequency, and f the modulation
amplitude, which is limited to a half of the Brillouinfrequency shift (BFS), i.e., about 5.4 GHz when standard
single-mode ﬁbers (SMFs) are used at 1550-nm wavelength.
The experimental setup of BOCDR with polarization
scrambling is depicted in Fig. 1. A distributed-feedback
laser diode (DFB-LD) at frequency f0 (1552 nm) was used asa light source, and a sinusoidal frequency modulation wasapplied to generate a correlation peak within an FUT. The
output from the LD was divided into two light beams by acoupler. One of the beams was directly used as the reference
light of self-heterodyne detection, after passing a 5-km delay
ﬁber for controlling the order of the correlation peak, apolarization scrambler (PSCR) which can modulate the SOP
at 1 MHz14,15) for suppressing the BGS ﬂuctuations, an Erdoped ﬁber ampliﬁer (EDFA) for enhancing the heterodyne
beat signal, and an optical ﬁlter composed of ﬁber Bragg
grating (FBG) with a 3-dB bandwidth of about 10 GHz forsuppressing the ampliﬁed spontaneous emission (ASE)noise. 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 optical ﬁlter with a 3-dB bandwidth of about
10 GHz was inserted after the EDFA in order to suppress
the eﬀects due to the Rayleigh scattering and the Fresnel

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Appl. Phys. Express 2 (2009) 062403

Y. Mizuno et al.

Fig. 1. Experimental setup of BOCDR with polarization scrambling:
AC, alternating current; DC, direct current; DFB-LD, distributedfeedback laser diode; EDFA, erbium-doped fiber amplifier; ESA,
electrical spectrum analyzer; FUT, fiber under test; GPIB, generalpurpose interface bus; PD, photo-diode; PSCR, polarization
scrambler.

Fig. 2. Structure of the FUT.

reﬂection from the FUT at frequency f0 . The optical beatsignal of the reference light and the Stokes light wasconverted by balanced photodiodes (PDs) to an electricalsignal. After 15-dB ampliﬁcation from an electrical preampliﬁer, the signal was monitored by an electrical spectrum
analyzer (ESA). The measurement data were transferred to apersonal computer.
The structure of the FUT is shown in Fig. 2. It wascomposed of a standard SMF with a length of about 100 m,
in which 0.3% strains were applied to a 1-m section (33.8 –34.8 m) and a 50-cm section (76.8 – 77.3 m) ﬁxed ontranslation stages using epoxy glue. One end of the FUT
was spliced to a circulator, and the other end was kept open.
The modulation amplitude f was set to 3.3 GHz and themodulation frequency fm was changed from 735.86 to
766.28 kHz, which corresponds to the measurement range
dm of about 140 m and the spatial resolution z of about
40 cm from eqs. (1) and (2). The span of the ESA was set to(a)10.763 –11.063 GHz. The overall sampling rate of the BGS
measurement for a single position was 19 Hz (including dataprocessing time), and 1001 data were acquired for one BGS.
Therefore, the sampling rate of one datum is 19 kHz, whichis over 50 times lower than the polarization modulation
frequency of 1 MHz. The total measurement time was 40 s,
which can be set even shorter if the number of sensing points
is reduced (currently, 760 points).
Figure 3(a) shows the measurement results of the
distribution of the BFS along the entire length of the FUT,with and without the polarization scrambling. Whenpolarization was not scrambled, at some positions where
the SOP was not optimized, the signal level largely
ﬂuctuated within the range of the span of the ESA as themeasuring position shifted, and consequently, the frequency
at which the power became maximal was unstable. Meanwhile, when polarization was scrambled, the ﬂuctuations
were suppressed, and the large changes of the BFS werestably detected only at the strain-applied positions. The
magniﬁed views around the strain-applied sections areshown in Figs. 3(b) and 3(c). Thus, 1-m and 50-cm strains
were successfully detected with 40-s measurement time. The
accuracy of the measurement at a single position was about10 MHz, which corresponds to the strain of 0:02%(200 ") in this experiment.
Next, we developed a new method to improve the S/Nratio of the system by compensating the noise-ﬂoor of the
ESA. Since BOCDR is based on weak spontaneous Brillouinscattering, its S/N ratio is low especially when the spatial
resolution is high.8) Noise-ﬂoor compensation was performed according to the following procedures: (1) suppress
the ﬂuctuations of the BGS by polarization scrambling, (2)acquire the BGS with suﬃciently large strain applied, as
noise-ﬂoor, and (3) after a conventional distributed strain
measurement, use the diﬀerence calculated by subtracting
the noise-ﬂoor from the observed BGS, as ‘‘net BGS’’.
First, the eﬀect of the compensation method wasexperimentally evaluated when the measuring position was
ﬁxed. Figures 4(a) and 4(b) show the measured BGS without
and with the noise-ﬂoor compensation, respectively, when
the correlation peak was located at the center of a 1-m strainapplied section. The resolution was set to 80 cm, and themagnitudes of the applied strains were 0, 0.08, 0.16, 0.24,
and 0.32%. Averaging was carried out 10 times for one
BGS, and the sampling ratio was about 2 Hz. The large strain
used for acquiring the noise-ﬂoor was 0.5%. When the noiseﬂoor was not compensated, as shown in Fig. 4(a), the width

(b)

(c)

Fig. 3. (a) Measured distribution of the Brillouin frequency shift (BFS) along the entire length of the 100-m FUT with (blue) and without (red) apolarization scrambling, (b) its magnified view with the range of 30 – 40 m, and (c) that with the range of 70 – 80 m.

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Appl. Phys. Express 2 (2009) 062403

Y. Mizuno et al.

(b)

(a)

Fig. 4. Measured BGS (a) without and (b) with noise-floor
compensation. Strains of 0, 0.08, 0.16, 0.24, and 0.32% were applied.

(a)Then, a distributed strain measurement was performed
using this method. The FUT was composed of a standard
SMF with a length of about 100 m, in which 0.2 and 0.14%
strains were applied to a 1-m section (33.8 – 34.8 m) and a
50-cm section (76.8 – 77.3 m), respectively. One end of the
FUT was spliced to a circulator, and the other end was kept
open. The modulation amplitude f was set to 5.4 GHz andthe modulation frequency fm was about 750 kHz, whichcorresponds to the measurement range dm of about 140 m
and the spatial resolution z of about 24 cm from eqs. (1)and (2). Averaging was carried out 5 times for one BGS, andthe sampling rate was about 4 Hz. The measured distributions of BGS and BFS are shown in Figs. 5(a) and 5(b),respectively. The S/N ratio was drastically improved
compared to that of conventional systems,5,8) and themeasurement accuracy at a single position was enhanced
to about 5 MHz, which corresponds to the strain of0:01% (100 "). This result indicates that the inﬂuenceof the reﬂected light from the correlation peak sidelobes ismuch smaller than the noise-ﬂoor of the ESA in BOCDR.
In conclusion, we performed a stable entire-length
measurement of strain distribution by suppressing the BGS
ﬂuctuations with polarization scrambling. In the experiment,
we demonstrated a distributed strain measurement along theentire length of a 100-m FUT with 40-cm spatial resolution
and 19-Hz sampling rate. In addition, we developed a new
method of compensating the noise-ﬂoor of the ESA to
improve the S/N ratio of the system.
Acknowledgments The authors are grateful to Dr. Weiwen Zou of
the University of Tokyo, Japan, for helpful discussions and comments. Y.M.
thanks the Japan Society for the Promotion of Science (JSPS) Research
Fellowships for Young Scientists. This work was supported by a Grant-inAid for Creative Scientiﬁc Research and the Global Center of Excellence(G-COE) Program from the Ministry of Education, Culture, Sports, Scienceand Technology (MEXT), Japan.

(b)Fig. 5. Measured distributions of (a) BGS and (b) BFS. The insets in(b) show the magnified views around strain-applied positions.

of the BGS changed according to the BFS, i.e., themagnitude of the applied strain. It became large near thefrequencies at which the noise-ﬂoor had relatively highpower (e.g., when 0.24% strain was applied), which leads tothe deterioration of the measurement accuracy. Meanwhile,when the noise-ﬂoor was compensated, as shown in
Fig. 4(b), the width of the BGS was stable, resulting inhigher measurement accuracy. The BGS power ﬂuctuationsin Fig. 4(a) were larger than that in Fig. 4(b), not only
because the noise-ﬂoor shape was involved in the power
ﬂuctuations but because the SOP was optimized. In
Fig. 4(b), the signal power decreased around 11.04 GHz,
which can be removed by employing a strain of more than
0.5% as the large strain used to acquire the noise-ﬂoor. Thus,
this compensation was conﬁrmed to improve the S/N ratioof the system when the measuring position was ﬁxed.

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062403-3# 2009 The Japan Society of Applied Physics