Optics Communications 283 (2010) 2438–2441

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Optics Communications
journal homepage: www.elsevier.com/locate/optcom

Distributed strain measurement using a tellurite glass ﬁber with Brillouin
optical correlation-domain reﬂectometry
Yosuke Mizuno *, Zuyuan He, Kazuo Hotate
Department of Electrical Engineering and Information Systems, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japana r t i c l ei n f oArticle history:
Received 19 November 2009
Received in revised form 1 February 2010
Accepted 1 February 2010

Keywords:
Tellurite glass ﬁber
Brillouin scattering
Brillouin frequency shift
Distributed strain measurement
Nonlinear optics

a b s t r a c t
A tellurite glass ﬁber with a high Brillouin gain was employed for distributed strain measurement with
Brillouin optical correlation-domain reﬂectometry (BOCDR). First, the spatial resolution of BOCDR wasevaluated using the tellurite ﬁber. With the high Brillouin gain of the ﬁber, it was conﬁrmed clearly inthe experiment that the spatial resolution is limited by the Rayleigh scattering-induced noise. Then,the dependence of the Brillouin frequency shift (BFS) on strain in the tellurite ﬁber was investigated,showing a negative dependence with a coefﬁcient of 0.023 MHz/le. Using this tellurite ﬁber, the distribution of the BFS around a 1-cm strain-applied section was successfully measured with BOCDR of a nominal spatial resolution of 6 mm.
Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction
Brillouin optical correlation-domain reﬂectometry (BOCDR) [1]
can measure the distribution of strain and/or temperature alonga ﬁber under test (FUT) from a single end. So far, 13-mm spatial
resolution has been achieved [2]. Although this resolution is thebest ever reported in spontaneous Brillouin scattering-based
reﬂectometers, it was limited by the signal-to-noise (S/N) ratio in
the conventional silica ﬁber-based Brillouin gain spectrum (BGS)
measurement. It is expected that the spatial resolution can be improved by using a specialty ﬁber with a large Brillouin gain coefﬁcient as the FUT to enhance the S/N ratio.
Among various specialty ﬁbers, tellurite glass ﬁbers [3,4] havebeen vigorously studied due to their high refractive index(n = 2.03), high nonlinearity, and broad Raman gain bandwidth,
and thus far been applied to developing optical devices, such as
ultra-wide-band Raman ﬁber ampliﬁers (RFAs) [5], efﬁcient Tm3+doped tellurite ﬁber lasers [6], and carrier-envelope offset (CEO)locked frequency combs [7]. In addition to these applications, they
can also be used as the gain medium for Brillouin scattering. Abedin[4] investigated the properties of stimulated Brillouin scattering(SBS) and the performance of slow light generation in an Er3+-doped
tellurite ﬁber, and showed that a tellurite ﬁber has a large Brillouingain coefﬁcient (gB  1.7  1010 m/W) and is suitable for slow light
generation.

* Corresponding author. Tel.: +81 3 5841 6768; fax: +81 3 5841 6025.
E-mail addresses: ymizuno@sagnac.t.u-tokyo.ac.jp (Y. Mizuno), ka@sagnac.t.utokyo.ac.jp, hotate@sagnac.t.u-tokyo.ac.jp (K. Hotate).
0030-4018/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.optcom.2010.02.002

In this paper, a tellurite ﬁber is employed in BOCDR to realize
high-spatial-resolution distributed strain sensing. Although thebackground loss (0.02 dB/m at 1550 nm) of the tellurite ﬁbers
is much higher than that of silica ﬁbers (0.0005 dB/m), making
use of their high Brillouin gain, we believe tellurite ﬁbers can beapplied to developing short-range but high-spatial-resolution Brillouin sensors. First, the spatial resolution of BOCDR using the tellurite ﬁber is evaluated. With the high Brillouin gain of the ﬁber, it isconﬁrmed clearly in the experiment that the spatial resolution islimited by the Rayleigh scattering-related noise. Next, the straindependence of the Brillouin frequency shift (BFS) in a tellurite ﬁber
is investigated. It is found that, with increasing strain, the BFS in
the tellurite ﬁber shifts toward lower frequency with a coefﬁcientof 0.023 MHz/le. This negative dependence seems to be causedby the negative strain-dependence of the Young’s modulus of thetellurite ﬁber. Then, a distributed strain measurement is performedby using this tellurite ﬁber as the FUT in BOCDR with a nominal
spatial resolution of 6 mm, which is the inherent limitation as described later.
2. Principles
Brillouin scattering occurs when light is Bragg-reﬂected by therefractive index modulations produced by acoustic phonons. The
backscattered light (Stokes light) suffers a Doppler shift called
BFS, which depends on tensile strain applied to the optical ﬁber.
For example, in a standard silica single-mode optical ﬁber (SMF),
the BFS of about 11 GHz slightly varies to higher frequency in proportion to the applied strain with a coefﬁcient of +0.058 MHz/leY. Mizuno et al. / Optics Communications 283 (2010) 2438–2441[8]. Hence, the BFS can provide the information on the strain magnitude in the ﬁber.
BOCDR is based on spontaneous Brillouin scattering. As described in [1], its basic principle is to generate a correlation peakwithin the FUT by applying the same frequency modulation tothe pump and the reference light. The experimental setup is thesame as that reported previously [9], where a polarization scrambler was inserted in the reference path to suppress the polarization-dependent ﬂuctuations of the BGS. The measurement range
dm (distance between the correlation peaks) is given bydm ¼c;
2n f m2439

Fig. 1. Structure of the FUT.

ð1Þ

where c is the velocity of light in vacuum, n the refractive index, andfm the modulation frequency of the light source. The following
expression of the spatial resolution Dz was derived for BOCDA[10,11], but it also holds well for BOCDR according to our previousexperimental results [1,2,9]:Dz ¼

c DmB;
2 p n f m Df

ð2Þ

where DmB is the Brillouin gain bandwidth in optical ﬁbers, and Df isthe modulation amplitude of the light source. Here, it is known thatfm higher than DmB does not contribute to the enhancement of Dz in
Eq. (2) [10]. In addition, Df must be lower than a half of the BFS of
the FUT because of the behavior of Rayleigh scattering-related noisein BOCDR as described schematically in [1]. Therefore, under thecondition of fm = DmB, the limitation of the spatial resolution Dzmin
is given, by the BFS, asDzmin ¼c

ð3Þ

pnBFS

In Table 1, n, BFS, and Dzmin of silica, bismuth oxide [12], As2Se3
chalcogenide [13], and tellurite ﬁbers are summarized. About 6mm spatial resolution is expected by using a tellurite ﬁber.
3. Experiments
First, we evaluated experimentally the spatial resolution ofBOCDR using the tellurite ﬁber. The structure of the FUT is depicted
in Fig. 1. Since it is difﬁcult to directly splice a tellurite ﬁber (5 m)to an SMF with a good mode matching, a dispersion-compensation
ﬁber (DCF) (30 cm) was inserted between them. One end of the tellurite ﬁber was connected to the DCF with a tilted V-groove connection [14], and the other end was bent to suppress the Fresnel
reﬂection. The light-source modulation frequency fm was set to7.8028 MHz to place a correlation peak within the tellurite ﬁber.
Fig. 2 shows the whole spectra of the electrical output of theBOCDR system when Df was increased from 0.1 to 4.0 GHz. Dueto the large Brillouin gain coefﬁcient of the tellurite ﬁber, alongwith its strong Rayleigh scattering, the evolution of the spectra
was observed much more clearly than that in our previous experiment [1]. As Df became larger, the spectrum of the Brillouin-scattered light broadened in both directions for 2Df around the BGS
peak at about 8 GHz. The BGS peak frequency gives the BFS at aspeciﬁc position in the FUT selected by the light-source frequency

Fig. 2. Electrical spectra when the modulation amplitude Df was increased from 0.1
to 4.0 GHz. Each spectrum is shifted by 35 dB. The values on the ordinate are valid
only for Df = 0.1 GHz.

modulation. This is the mechanism of the distributed measurement of BOCDR [1]. In Fig. 2, a large amount of noise spreadingfrom 0 Hz to 2Df is also clearly shown, which is induced by theRayleigh scattering. When Df was 4.0 GHz, the noise began to overlap the BGS peak. This result conﬁrms experimentally that the limitation of Df is a half of the BFS of the ﬁber, and thus the spatialresolution must be larger than the value given in Eq. (3).
Next, the strain-dependence of the BFS in the tellurite ﬁber wasinvestigated using the same FUT. Since the tellurite ﬁber was short,the measurement was conducted using BOCDR to place the measurement window (the correlation peak) within the tellurite ﬁber
to remove the effects of other ﬁbers. Df and fm were set to 1 GHz
and 7.8028 MHz, respectively, which correspond to the measurement range dm of 9.47 m and the spatial resolution Dz of about
3 cm according to Eqs. (1) and (2). As shown in Fig. 3, DmB was
10 MHz, which is much narrower than those of more than
23 MHz reported so far [4,15] probably due to the high-gain condition and the difference in glass purity. In the tellurite ﬁber, different strains were applied to a 40-cm section (5.7–6.1 m from theTable 1n, BFS, and Dzmin of silica, bismuth-oxide [12], As2Se3 chalcogenide [13], and telluriteﬁbers.
FibernBFS (GHz)

Dzmin (mm)SilicaBismuth oxide
Chalcogenide
Tellurite

1.46
2.22
2.8
2.03

10.86
8.83
7.97
7.95

6.0
4.9
4.3
5.9

Fig. 3. Measured BGS from 5-m tellurite ﬁber with no strain applied.

2440

Y. Mizuno et al. / Optics Communications 283 (2010) 2438–2441

circulator) ﬁxed on a translation stage using epoxy glue. The position of the correlation peak was set to the middle of the strain-applied section, and the sampling rate of the BGS measurement wasabout 2 Hz including 10-times averaging.
The strain-dependence of the BGS in the tellurite ﬁber is shownin Fig. 4a. With increasing the applied strain, the BGS shifted toward lower frequency. From these spectra, we can plot thestrain-dependence of the BFS as shown in Fig. 4b. The error bars
are ±2.5 MHz, corresponding to the BFS ﬂuctuations when strain
was ﬁxed. The slope of the straight-line approximation was calculated to be 0.023 MHz/le. It is notable that the sign of the straindependence is opposite to that of a silica SMF (+0.058 MHz/le [8]).
In general, the strain coefﬁcient of the normalized BFS in opticalﬁbers is given by:

1 @ mB 1 @n 1 @E1 @q¼þ;
mB @ e n @ e 2E @ e 2q @ eð4Þ

where E is the Young’s modulus, and q the density. The values ofeach term have been determined as 0.23, 2.88, and 0.36 by Horiguchi et al. [8] for silica ﬁbers with n = 1.46. Since the BFS is about
10.86 GHz in SMFs, the strain coefﬁcient is calculated to be+0.033 MHz/le, which roughly matches the measured value(+0.058 MHz/le). In order to estimate the case of tellurite ﬁbers
with n = 2.03, we make two assumptions as follows: (i) the ﬁrstterm is 0.445 (=0.23  2.032/1.462) because it is proportional
to the square of n [8], and (ii) the third term is 0.16 (=0.36  2.2/5.1) because q of silica crystals is 2.2 g/cm3, while that of tellurite
crystals is 5.1 g/cm3 [16]. Using the measured strain coefﬁcient(0.023 MHz/le), the second term can be calculated back to be2.61. This negative dependence of the Young’s modulus on strain

Fig. 5. (a) Structure of the FUT for a distributed measurement and (b) photo of thestrain-applied section.

Fig. 6. (a) Measured distribution of the BGS along the FUT and (b) that of the BFS.

Fig. 4. (a) Strain-dependence of the BGS in the tellurite ﬁber and (b) that of the BFS.

has been observed also in soda glass ﬁbers (silica + Na2O + CaO) byMallinder et al. [17], with a second term of 2.555. Thus, we presume that the negative strain-dependence of the BFS is due to thenegative dependence of the Young’s modulus on strain in the tellurite ﬁber.
Finally, this tellurite ﬁber was employed as the FUT in distributed strain measurement by BOCDR to improve the S/N ratio andthus to achieve the inherent limitation of the spatial resolution given by Eq. (3). The modulation frequency fm was set to 9.972599–
9.975924 MHz (fm 6 DmB of the tellurite ﬁber), which corresponds

Y. Mizuno et al. / Optics Communications 283 (2010) 2438–2441

to a measurement range dm of 7.4 m according to Eq. (1). The
amplitude of the frequency modulation Df was 3.94 GHz(Df 6 BFS/2), and the nominal spatial resolution Dz was calculated
to be about 6 mm from Eqs. (2) and (3). The structure of the FUT isshown in Fig. 5a and b. The length of the tellurite ﬁber was 3.7 m,and 0.15% strain was applied to a 1-cm section. Other conditions
concerning the FUT were the same as those for the preceding measurement. Polarization scrambling was employed, and the sampling rate of the measurement for a single position was 10 Hz
including 5-times averaging. The measurement was performed at
50 points sequentially for 5 s.
Fig. 6a shows the measured distribution of the BGS along theFUT. The BGS at the strain-applied section was recognized withmuch higher S/N ratio than that in our previous experiment withan SMF as the FUT [2]. Fig. 6b shows the distribution of the BFS.
The 1-cm strain-applied section was detected with a nominal spatial resolution of 6 mm, which is the inherent limitation given by
Eq. (3). The gradual variation of the BFS seems to have come fromthe strain distribution inside the epoxy glue. The change of the BFS
was about 34 MHz, which corresponds to 0.148% strain. This value is in good agreement with the applied strain of 0.15%. The accuracy of the measurement at a single position was about ±2.5 MHz,
which corresponds to a strain accuracy of ±0.01% (±100 le in thisexperiment.
4. Conclusions
We employed a tellurite ﬁber as the FUT in BOCDR to realize ashort-range but high-spatial-resolution distributed strain measurement. The spatial resolution of BOCDR was evaluated experimentally using the tellurite ﬁber. Thanks to the high Brillouingain of the ﬁber, the behavior of Rayleigh scattering-induced noisein BOCDR was observed clearly. This experiment conﬁrmed thatthe modulation amplitude Df of the light source is inherently limited to below a half of the BFS of the ﬁber, and thus the spatial resolution is correspondingly limited. The investigation on thedependence of the BFS on strain in the tellurite ﬁber shows thatthe BFS in this kind of ﬁber shifted toward lower frequency withincreasing strain, and its coefﬁcient was found to be0.023 MHz/le. This negative dependence seems to originate fromthe negative dependence of the Young’s modulus of the tellurite ﬁber on strain. A distributed strain measurement with BOCDR was2441

demonstrated using the tellurite ﬁber. With the high Brillouin gainof the tellurite ﬁber and thus the enhanced S/N ratio in BGS measurement, a 1-cm strain-applied section was successfully detectedwith 6-mm nominal spatial resolution, which is the inherent limitation of the ﬁber. This is the ﬁrst demonstration of a millimeterorder spatial resolution among one-end-access strain-sensing
reﬂectometers. We expect the BOCDR system to be useful not onlyin strain monitoring of civil structures but also in the investigationof internal structures of optical components because of its highresolution measurability.
Acknowledgements
The authors are grateful to Dr. Atsushi Mori of NTT Photonics
Laboratories, Japan, for providing the tellurite glass ﬁber sample.
One of the authors (Y.M.) 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 Scientiﬁc Research (S) and the Global Center of Excellence (G-COE) Programfrom the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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