Applied Physics Express 2 (2009) 112402

Dependence of the Brillouin Frequency Shift on Temperature
in a Tellurite Glass Fiber and a Bismuth-Oxide Highly-Nonlinear Fiber
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 September 1, 2009; accepted October 6, 2009; published online October 30, 2009
We investigate the temperature-dependences of the Brillouin frequency shift in a tellurite glass fiber and a bismuth-oxide highly-nonlinear
fiber using a Brillouin optical correlation-domain reflectometry system for sensing applications. Negative dependences of 1:14 and0:88 MHz/K are demonstrated, respectively, in contrast to the positive dependence of þ1:18 MHz/K in a standard silica fiber.
# 2009 The Japan Society of Applied Physics
DOI: 10.1143/APEX.2.112402

e have proposed Brillouin optical correlationdomain reﬂectometry (BOCDR)1–3) to measure
the distribution of strain and/or temperature along
a ﬁber under test (FUT) from a single end, and obtained
13-mm spatial resolution and 50-Hz sampling rate.2) This
resolution of 13 mm, though the best result ever reported inBrillouin scattering-based reﬂectometers, was limited by thesignal-to-noise (S/N) ratio of the system, which could beenhanced when using a specialty ﬁber with a large Brillouingain coeﬃcient as the FUT. Among various specialty ﬁbers,
tellurite glass ﬁbers4,5) and bismuth-oxide highly-nonlinear
ﬁbers (Bi-NLFs)6,7) have been widely studied due to theirlarge Brillouin gain coeﬃcients of 1:7  1010 and 6:43 1011 m/W, respectively, which are about 20 and 8 times aslarge as that of a standard silica ﬁber. Compared to thebackground loss of about 0.0005 dB/m in a silica singlemode ﬁber (SMF), those of a tellurite ﬁber and a Bi-NLF are
as high as 0.0588 and 0.8 dB/m, respectively. When a longmeasurement range is not required, however, they can beapplied, for realizing a high-resolution measurement due totheir high Brillouin gain, not only to BOCDR but also toBrillouin optical correlation-domain analysis (BOCDA).8,9)
For the implementation of such sensing systems using atellurite ﬁber or a Bi-NLF, it is imperative to clarify theirdependences of the Brillouin frequency shift (BFS) onparameters to be measured.
In this paper, we investigate the dependences of the BFS
on temperature in a tellurite ﬁber and a Bi-NLF using aBOCDR system. It is found that the BFSs in a tellurite ﬁber
and a Bi-NLF shift toward lower frequency with increasing
temperature with coeﬃcients of 1:14 and 0:88 MHz/K,respectively. These negative dependences seem to havecome from the negative dependences of elastic constants ontemperature. Furthermore, we also discuss some potential
performances of the BOCDR-based distributed temperaturesensing systems with the tellurite ﬁber or the Bi-NLF
employed.
When a light beam is injected into an optical ﬁber,backscattered light (Stokes light) is generated through theinteraction with acoustic phonons. This phenomenon iscalled the spontaneous Brillouin scattering. Due to theexponential decay of the phonons, the backscattered
Brillouin light spectrum, known as the Brillouin gain
spectrum (BGS), takes the shape of Lorentzian function.10)
The frequency at which the peak power is obtained in thespectrum is down-shifted by several GHz from the incident
light frequency, and the amount of this frequency shift isWFig. 1. Experimental setup based on BOCDR for characterizing thetemperature dependences of specialty fibers: DC, direct current; DFBLD, distributed-feedback laser diode; EDFA, erbium-doped fiber
amplifier; ESA, electrical spectrum analyzer; FUT, fiber under test;PD, photo-diode; PSCR, polarization scrambler.

known as BFS. If tensile strain or temperature change occurs
in a standard silica SMF, the BFS varies to higher frequency
in proportion to the applied strain (þ0:058 MHz/")11) or
the temperature change (þ1:18 MHz/K).12) Therefore, by
measuring the BFS in the FUT, the strain amplitude ortemperature change can be derived.
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.1) The spatialresolution z and the measurement range (i.e., interval
between correlation peaks) dm are given bycB;
2nfm fcdm ¼;
2nfm

z ¼

ð1Þ
ð2Þ

respectively, where c is the velocity of light, n the refractiveindex, fm the modulation frequency of the light source, B
the intrinsic linewidth of BGS, and f the modulation
amplitude of the light source.
The experimental setup to characterize the temperaturedependences of the specialty ﬁbers is shown in Fig. 1. Thisis basically the same as that reported previously,3) where apolarization scrambler (PSCR) was inserted in the reference
path to suppress the polarization-dependent ﬂuctuationsof the output signal observed by the electrical spectrum
analyzer (ESA). The structures of the FUTs comprising atellurite ﬁber and a Bi-NLF are shown in Figs. 2(a) and 2(b),respectively. Since it is diﬃcult to directly splice a tellurite

112402-1# 2009 The Japan Society of Applied Physics

Appl. Phys. Express 2 (2009) 112402

Y. Mizuno et al.

(a)(b)
Fig. 2. Structures of the FUTs comprising (a) the tellurite fiber and(b) the Bi-NLF.

ﬁber (5 m) and a Bi-NLF (1 m) to SMFs, a dispersion–compensation ﬁber (DCF, 30 cm)13) and a high-numericalaperture ﬁber (HNAF, 30 cm) were employed between them,respectively. Each end of the tellurite ﬁber and the Bi-NLF
was bent to suppress the Fresnel reﬂection. The measurement was conducted using this BOCDR system with themeasurement window (the correlation peak) ﬁxed at aposition within the tellurite ﬁber or the Bi-NLF to select theportion where the temperature is changed and to remove theeﬀects of the other portions.
In the measurement of the tellurite ﬁber, the power Pin of
the light beam injected into the FUT was set to 28 dBm. Theamplitude f and the frequency fm in the laser modulation
were set to 1 GHz and 7.8028 MHz, respectively, whichcorrespond to the measurement range dm of 9.47 m and thespatial resolution z of about 3 cm from eqs. (1) and (2) (thevalue of B is described shortly after). In the measurementof the Bi-NLF, with Pin set to 26 dBm, f and fm were set to2 GHz and 1.0654 MHz, respectively, corresponding to dmof 63.4 m and z of 79.9 cm. In general, the Brillouin gainbandwidth B depends on Pin ,14) and, in our experiment, it
was 10 MHz in the tellurite ﬁber and 79.2 MHz in the BiNLF as shown in Figs. 3(a) and 3(b).
Diﬀerent temperature changes were applied to a 40-cm
section of the tellurite ﬁber and the whole length of theBi-NLF placed on a hot plate. In both measurements, theposition of the correlation peak was set to the middle of thesection where temperature changes were applied, and thesampling rate of the BGS acquisition was about 2 Hz
including 10-times averaging.
The temperature-dependences of the BGS in the tellurite
ﬁber and the Bi-NLF are shown in Figs. 4(a) and 4(b),respectively. In both cases, the BGS shifted toward lower
frequency with increasing temperature. In Fig. 4(b), the BGS
peak power was so weak that it changed due to the noise ﬂoorof the ESA, which can be suppressed by a noise-ﬂoor
compensation technique.3) From these spectra, we can plot
the temperature-dependences of the BFS as shown inFigs. 5(a) and 5(b). The error bars are 2:5 MHz, corresponding to the BFS ﬂuctuations when temperature wasﬁxed. The dependences are both almost linear, and theircoeﬃcients were calculated to be 1:14 and 0:88 MHz/K,
respectively. Although the absolute values are close to that ofa standard SMF (þ1:18 MHz/K12)), their signs are opposite.
In general, the temperature coeﬃcient of the normalized
BFS is given by(a)(b)Fig. 3. Measured BGS from (a) the 5-m tellurite fiber with Pin of28 dBm, and (b) the 1-m Bi-NLF with Pin of 26 dBm.


1 @B1 @n 1 1 @C 1 @þ¼
B @Tn @T 2 C @T  @T
1 @n 1 1 @Cþþ ;¼
n @T 2 C @T

ð3Þ

where C is the eﬀective elastic constant (in optical ﬁbers, Cis almost the same as the Young’s modulus),  the density,and  the volume expansion coeﬃcient of a material. For atellurite crystal, Sonehara et al.15) have reported that theterm ð1=CÞð@C=@T Þ becomes dominant and is negative,
though the values are slightly diﬀerent depending on thecrystal orientation. From their experimental result, we canestimate the temperature coeﬃcient in the tellurite crystal
at around room temperature to be approximately 1:06
MHz/K (¼ 0:04  7950 MHz/300 K), which roughly
agrees with the experimental value (1:14 MHz/K) inthe case of the tellurite ﬁber. The negative dependenceof an elastic constant on temperature may also be the originof the negative temperature-dependence of the BFS in
the Bi-NLF, but further research is needed to clarify thispoint.
Finally, the potential performances of the distributed
temperature sensing based on BOCDR using the tellurite
ﬁber or the Bi-NLF are discussed from the aspects of thenumber of eﬀective sensing points or the ratio between themeasurement range and the spatial resolution, and themeasurement accuracy. First, the number of eﬀective
sensing points N, which can be regarded as the evaluation
parameter of the sensing system, is given by the ratio
between dm , eq. (2), and z, eq. (1), as112402-2# 2009 The Japan Society of Applied Physics

Appl. Phys. Express 2 (2009) 112402

Y. Mizuno et al.

(a)
(a)

(b)
Fig. 5. Temperature-dependences of the BFS in (a) the tellurite
fiber and (b) the Bi-NLF.

(b)Fig. 4. Temperature-dependences of the BGS in (a) the tellurite
fiber and (b) the Bi-NLF.

N¼dm
f
 BFS
¼
<:
2B
z B

ð4Þ

Here, the inequality holds true, because f must be lower
than a half of the BFS of the FUT due to the Rayleighscattering-induced noise.1) While N is about 570 in a silicaﬁber,1,2) it can be enhanced to 1250 by using the tellurite ﬁber(B ¼ 10 MHz), but is reduced to 175 by using the Bi-NLF(B ¼ 79:2 MHz). Second, the measurement accuracy ofBOCDR using the Bi-NLF is worse than that using thetellurite ﬁber or a silica ﬁber, because the absolute valueof the temperature coeﬃcient of BFS in the Bi-NLF (0.88
MHz/K) is lower than that in the tellurite ﬁber (1.14 MHz/K)
or a silica ﬁber (1.18 MHz/K). For example, the BFS
ﬂuctuations of 2:5 MHz in the Bi-NLF, the tellurite ﬁber,
and a silica ﬁber, correspond to the measurement accuracy of2:84, 2:19, and 2:12 K, respectively. Thus, from theviewpoint not only of the Brillouin gain and the optical loss,
but also of the number of eﬀective sensing points and themeasurement accuracy, the tellurite ﬁber is thought to be
more favorable than the Bi-NLF as a specialty ﬁber employed
to enhance the performance of the BOCDR system.
In conclusion, we investigated the temperature-dependences of the BFS in a tellurite glass ﬁber and a Bi-NLF for
sensing applications. In both ﬁbers, the BFS shifted toward
lower frequency with increasing temperature with coeﬃcients of 1:14 and 0:88 MHz/K, respectively. These

negative dependences seem to be caused by the negative
dependences of the elastic constants on temperature. We
consider the tellurite ﬁber is more suitable as the FUT in theBOCDR-based distributed temperature sensing systems thanthe Bi-NLF.
Acknowledgments The authors are grateful to Dr. Atsushi Mori ofNTT Photonics Laboratories, Japan, for providing the tellurite glass ﬁbersample. They also thank Professor Shinji Yamashita of the University ofTokyo, Japan, for providing the Bi-NLF sample. Y.M. thanks the Japan
Society for the Promotion of Science (JSPS) Research Fellowships forYoung Scientists. This work was supported by a Grant-in-Aid for ScientiﬁcResearch (S) and the Global Center of Excellence (G-COE) Program from
the Ministry of Education, Culture, Sports, Science and Technology, Japan(MEXT).

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