Discriminative strain and temperature
measurement using Brillouin scattering andfluorescence in erbium-doped optical fiber
Mingjie Ding,1,2 Yosuke Mizuno,1,* and Kentaro Nakamura1
2

1
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
School of Electrical Engineering and Telecommunications, University of New South Wales, Sydney, New South
Wales 2052, Australia*ymizuno@sonic.pi.titech.ac.jp

Abstract: We develop a novel discriminative sensing technique of strain
and temperature using Brillouin scattering and fluorescence in an erbiumdoped fiber (EDF). First, we show that the fluorescence intensity ratio(FIR), the ratio of the fluorescence intensities at two different wavelengths(1530 nm and 1565 nm in this experiment), is linearly dependent on
temperature (with a coefficient of 5.6 × 10−4 /°C) but almost independent ofstrain. Then, by combined use of the FIR and the Brillouin frequency shift
in an EDF, we experimentally demonstrate discriminative measurements ofstrain and temperature with four different sets of strain and temperaturechanges.
©2014 Optical Society of America
OCIS codes: (120.0280) Remote sensing and sensors; (280.4788) Optical sensing and sensors;(290.5830) Scattering, Brillouin.

References and links
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6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024706 | OPTICS EXPRESS 24706

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1. Introduction
As a promising technology for structural health monitoring, fiber-optic distributed strain andtemperature sensing based on Brillouin scattering has been extensively studied for several
decades [1–5], and various measurement techniques in time- [1,2], frequency- [3], andcorrelation-domains [4,5] have been developed. Their basic operating principle lies in thelinear dependence of the Brillouin frequency shift (BFS) on strain and temperature. However,
only by measuring the BFS, response to strain cannot be separated from that to temperature.
Thus, the discriminative measurement of strain and temperature has been one of the main
issues to be tackled in fiber-optic Brillouin sensing community.
One solution is to use two optical fibers: one is embedded into the structure to detect the
total effects of strain and temperature, while the other is kept loose (or packed in a thermal
insulation jacket) so that it can feel temperature (or strain) only; then these two parameters
can be numerically calculated. However, using two optical fibers increases the failure risk andinstallation difficulty in practical implementation. Some research groups [6,7] have tried toutilize multiple acoustic resonance peaks at different orders of Brillouin gain spectrum (BGS)
in a single specially-designed optical fiber, but the discrimination accuracy was not sufficientbecause all the acoustic resonance frequencies show similar behaviors in their dependenceson strain and temperature. Another method is to make use of the peak amplitude (orbandwidth) and BFS in the BGS simultaneously [8,9], but the discrimination accuracy was
not sufficient either chiefly because of the low signal-to-noise ratio in the distributed
measurement of the peak amplitude (or bandwidth). Zou et al. [10,11] have demonstrated
completely discriminative distributed strain/temperature sensing exploiting a so-called
Brillouin dynamic grating in a polarization-maintaining (PM) fiber. Although the strain andtemperature discrimination accuracies were as good as 3 με and 0.08 °C, respectively, its
experimental setup was rather complicated, containing many expensive PM devices.
In this work, we show in a proof-of-concept demonstration that a discriminative strain andtemperature measurement can be performed using the BFS and the fluorescence intensity
ratio (FIR) in an erbium-doped fiber (EDF). The FIR, determined by the fluorescenceintensities at two different wavelengths (1530 nm and 1565 nm in this experiment), shows alinear temperature dependence with a slope of 5.6 × 10−4 /°C, but scarcely depend on strain.
In combination with the previously reported results on the BFS dependences on strain and#220176 - $15.00 USD
Received 31 Jul 2014; revised 22 Sep 2014; accepted 24 Sep 2014; published 1 Oct 2014(C) 2014 OSA
6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024706 | OPTICS EXPRESS 24707

temperature in EDFs [12], we experimentally demonstrate a discriminative measurement ofstrain and temperature in four cases, where estimated values agree well with actual values.
2. Principle
Pump light injected into an optical fiber interacts with acoustic phonons, and generates
backscattered Stokes light. This phenomenon is known as spontaneous Brillouin scattering,and the Stokes light spectrum, referred to as the BGS, takes the shape of a Lorentzianfunction. The central frequency of the BGS shifts downward relative to that of the pump lightby the amount called the BFS. At 1.55-μm pumping wavelength, the BFS in glass fibers isapproximately 11 GHz, which slightly varies depending on the fiber fabrication process
and/or dopants. The BFS is generally dependent on applied strain and temperature in thefibers, and the BFS dependences have been investigated in various fibers, such as silica
single-mode fibers (SMFs) [13,14], photonic crystal fibers [15], polymer optical fibers [16],and rare-earth-doped fibers [12,17]. In an EDF with 720-wtppm erbium concentration, theproportionality constants of the BFS dependences on strain and temperature at 1.55 μm arereported to be 479 MHz/% and 0.87 MHz/°C, respectively [12]. Thus, by measuring the BFS
distribution along a fiber, the information on applied strain or temperature distribution can beobtained.
Fluorescence in an EDF occurs through the radiative transition of erbium ions between
different energy levels; specifically, from 4I13/2 to 4I15/2 in 980-nm pumping configuration [18].
The fluorescence intensity is known to be dependent on temperature, but direct use of theintensity is not suitable for practical sensor applications because of the signal fluctuations
influenced by external loss. One method of overcoming this problem is to make use of FIR,
defined as the ratio of the fluorescence intensities at two close but different energy levels (orwavelengths), which is immune to the incident power and polarization state [19–23].
According to the theory [18,21], the FIR is proportional to exp(–∆E/kT), where ∆E is anenergy difference between the two energy levels, k is a Boltzmann constant, and T is absolutetemperature. Thus, temperature information can be derived from FIR measurement.
Once BFS and FIR dependences on strain and temperature in an EDF are obtained, strain
and temperature can be calculated byT CFIR Δε 
1
=
 ε
 
T
T
ε
ε
 ΔT  CBFS ⋅ CFIR − CBFS ⋅ CFIR  −CFIRT  ΔBFS 
−CBFS

,
ε
CBFS   ΔFIR (1)where ∆ε: applied strain change, ∆T: ambient temperature change, CεBFS (CTBFS): strain(temperature) coefficient of BFS, CεFIR (CTFIR): strain (temperature) coefficient of FIR, ∆BFS:
BFS change, and ∆FIR: FIR change. As demonstrated later in this paper, the FIR in an EDF is
almost independent of strain, i.e., CεFIR ≈0. Consequently, Eq. (1) becomesT
CFIR
 Δε 
1=
 ΔT  C ε ⋅ C T  
BFS
FIR  0T
  ΔBFS 
−CBFS

,
ε
CBFS   ΔFIR (2)the simplicity of which is one of the advantages of this method.
3. Experiments
In the experiment, we used a 0.6-m-long EDF with an erbium concentration of 720 wtppm, acore refractive index of ~1.47, a mode-field diameter of 7.2 μm, and a peak absorption at 1.53
μm of 18 dB/m. Both ends of the EDF were spliced to 1-m-long silica SMFs by using a fusion
splicer, where the splicing loss was suppressed to < 0.1 dB by optimizing the arc duration.
The EDF was placed on a heater and both ends (spliced parts) were fixed on moving stages sothat strain and temperature might be independently and simultaneously applied. The
experimental setup is depicted in Fig. 1. Two laser diodes (LDs) were used; one at 1550 nm#220176 - $15.00 USD
Received 31 Jul 2014; revised 22 Sep 2014; accepted 24 Sep 2014; published 1 Oct 2014(C) 2014 OSA
6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024706 | OPTICS EXPRESS 24708

for exciting Brillouin scattering in an EDF, and the other at 980 nm for enhancing theBrillouin scattering power [24] as well as for exciting the fluorescence in the EDF. During the
FIR measurement, the LD at 1550 nm was not operated. The Brillouin Stokes light washeterodyned with the reference light, converted into an electrical signal with a photo diode,and then observed as BGS with an electrical spectrum analyzer (see [24] for details). The
fluorescence spectrum of the EDF was observed with an optical spectrum analyzer (OSA).
The polarization states were optimized for all the measurements using polarization
controllers.

Fig. 1. Experimental setup. EDF(A): erbium-doped fiber (amplifier), ESA: electrical spectrum
analyzer, GPIB: general-purpose interface bus, LD: laser diode, OSA: optical spectrum
analyzer, PC: polarization controller, PD: photo diode, WDM: wavelength divisionmultiplexing.

Figure 2 shows the measured temperature dependence of the fluorescence spectra in the
EDF pumped at 980 nm. The power of the pump light was fixed at 26 dBm (maximal output
of our LD; the pump power should be as high as possible to enhance the signal-to-noise ratioof the measurement). The temperature was increased from 30.0 to 100.0°C. A fluorescencepeak was observed at around 1530 nm. In this experiment, we define the FIR as the ratio of
the fluorescence intensity at 1565 nm to that at 1530 nm (note that the intensity ratio is equal
to the power ratio). The magnified views of the fluorescence spectra at 1530 nm and 1565 nm
are shown in the insets of Fig. 2. With increasing temperature, the optical power was
decreased at 1530 nm and increased at 1565 nm. Using these data, we calculated thetemperature dependence of the FIR, as shown in Fig. 3(a). In the temperature range from 30.0
to 100.0°C, the FIR was linearly dependent on ambient temperature with a proportionality
constant CTFIR of 5.6 × 10−4 /°C. The strain dependence of the FIR was also measured as
displayed in Fig. 3(b). The strain dependence coefficient was calculated to be 6.8 × 10−3 /%,but unstable FIR values suggest that the strain dependence is negligibly small. Thus, evenwhen strain and temperature changes are simultaneously applied to the EDF, temperature
information can be separately obtained by FIR measurement results.

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Received 31 Jul 2014; revised 22 Sep 2014; accepted 24 Sep 2014; published 1 Oct 2014(C) 2014 OSA
6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024706 | OPTICS EXPRESS 24709

Fig. 2. Fluorescence spectra in EDF. The insets show the magnified views at around (a) 1530
nm and (b) 1565 nm.

Fig. 3. FIR measured as a function of (a) temperature and (b) strain.

We then demonstrate discriminative sensing of strain and temperature in combination
with Brillouin measurement in four cases. The power of the Brillouin pump light was fixed at
28 dBm. First, without applying a strain to the EDF, the ambient temperature was raised from30.0 to 100.0°C (temperature change of 70.0°C). Correspondingly, the FIR increased from
0.641 to 0.681 (FIR change of 0.040), while the BFS change was 51.2 MHz (calculated using
Lorentzian-fitted data), as observed in Fig. 4(a). By substituting these parameters along with
CεBFS = 479 MHz/% and CTBFS = 0.87 MHz/°C [12] into Eq. (2), the strain and temperaturechanges were estimated to be 0.02% and 71.4°C, which are in good agreement with the actual
values with measurement errors of 0.02% and 1.4°C, respectively. Then, a 0.40% strain wasapplied to the EDF while the temperature was kept at 30.0°C. The FIR decreased from 0.641
to 0.639 (–0.002 change), and the BFS change was 174.4 MHz (Fig. 4(b)). Using Eq. (2), thestrain and temperature changes were estimated to be 0.37% and –3.8°C with measurement
errors of –0.03% and –3.8°C, respectively. Finally, we performed discriminative sensing in
two cases where strain and temperature changes were simultaneously applied. When thestrain increased from 0.30 to 0.40% (0.10% change) and the temperature was raised from 40.0
to 90.0°C (50.0°C change), the FIR increased from 0.648 to 0.676 (0.028 change), whereas
the BFS change was 88.0 MHz (Fig. 4(c)). Then the strain and temperature changes were#220176 - $15.00 USD
Received 31 Jul 2014; revised 22 Sep 2014; accepted 24 Sep 2014; published 1 Oct 2014(C) 2014 OSA
6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024706 | OPTICS EXPRESS 24710

estimated to be 0.09% and 51.0°C with measurement errors of –0.01% and 1.0°C,
respectively. In the same way, when the strain increased from 0.05 to 0.45% (0.40% change)
and the temperature was raised from 90.0 to 100.0°C (10.0°C change), the FIR increased
from 0.677 to 0.682 (0.005 change), and the BFS change was 196.8 MHz (Fig. 4(d)),
resulting in the strain and temperature changes of 0.39% and 8.2°C with measurement errorsof –0.01% and –1.8°C, respectively. These discriminative measurement results are
summarized in Table 1. Thus, discriminative measurability of this strain/temperature sensing
technique was experimentally proved.

Fig. 4. (a)–(d) BGS shift in the EDF measured with four different sets of strain andtemperature changes.
Table 1. Summary of discriminative measurement results.
Actual values

Estimated values

ErrorsΔstrain(%)Δtemperature(°C)

Δstrain(%)Δtemperature(°C)

Δstrain(%)Δtemperature
(°C)

0.00

70.0

0.02

71.4

0.02

1.4

0.40

0.0

0.37

−3.8

−0.03

−3.8

0.10

50.0

0.09

51.0

−0.01

1.0

0.40

10.0

0.39

8.2

−0.01

−1.8

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Received 31 Jul 2014; revised 22 Sep 2014; accepted 24 Sep 2014; published 1 Oct 2014(C) 2014 OSA
6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024706 | OPTICS EXPRESS 24711

4. Conclusion
We carried out a proof-of-concept demonstration of a discriminative strain/temperature
measurement by simultaneous use of the FIR and BGS in an EDF for the first time to the bestof our knowledge. The ratio of the fluorescence power at 1565 nm to that at 1530 nm was
employed as the FIR in this experiment, which was found to be almost independent of applied
strain. Strain and temperature changes estimated by this technique (e.g., 0.09% and 51.0°C)
agreed well with the practical values (e.g., 0.10% and 50.0°C). The measurement time is atpresent not sufficiently short because of the internal scanning function of the OSA, whichmight be replaced by optical filters and power meters.
The next important challenge is to render this scheme distributed, referring to sometechniques proposed to perform distributed Brillouin measurement [1–5]. We have to admitthat the FIR distributed measurement cannot be achieved by a simple extension of
conventional Brillouin techniques (using backscattered light) because of the following two
reasons: (1) the FIR is measured using an OSA with a relatively low sampling rate, and (2)the FIR is measured for transmitted light. To resolve (1), we should acquire the FIR data witha high sampling rate without using an OSA. One implementation would be FIR detection withan oscilloscope after frequency downshift based on optical heterodyne and filtering. To
resolve (2), the FIR measurement using not transmitted but reflected light should be explored.
The current results presented in this paper, however, indicates that, in a special case where the
EDF is uniformly heated (or cooled), strain distribution can be properly discriminated fromthe temperature information. Thus, we believe that our method is technologically attractive in
implementing fiber-optic discriminative strain/temperature sensing systems in future.
Acknowledgments
We are indebted to Fujikura Ltd., Japan, for providing us with the EDF samples. We also
appreciate Neisei Hayashi and Xinhua Guo, Tokyo Institute of Technology, Japan, for theirexperimental assistance and useful advice. This work was partially supported by Grants-inAid for Young Scientists (A) (no. 25709032) and for Challenging Exploratory Research (no.
26630180) from the Japan Society for the Promotion of Science (JSPS) and by research
grants from the General Sekiyu Foundation, the Iwatani Naoji Foundation, and the SCAT
Foundation.

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Received 31 Jul 2014; revised 22 Sep 2014; accepted 24 Sep 2014; published 1 Oct 2014(C) 2014 OSA
6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024706 | OPTICS EXPRESS 24712