APPLIED PHYSICS LETTERS 102, 191906 (2013)

Brillouin gain spectrum dependences on temperature and strain
in erbium-doped optical fibers with different erbium concentrations
Mingjie Ding, Neisei Hayashi, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho,Midori-ku, Yokohama 226-8503, Japan(Received 7 February 2013; accepted 2 May 2013; published online 14 May 2013)
Brillouin Stokes power in erbium-doped optical fibers (EDFs) can be potentially controlled bypumping, but no report has been provided on its detailed characterization. In this study, as the firststep toward this goal, the Brillouin gain spectra in EDFs with three different erbium concentrations(0.72, 1.20, and 2.28 wtppt) are measured at 1.55 lm without pumping, and the Brillouin frequency
shifts (BFSs) and their dependences on strain, temperature, and erbium concentration are fully
investigated. In the EDF with 0.72-wtppt concentration, the BFS was 11.42 GHz, and its temperature
C 2013 AIP Publishing LLC.
and strain coefficients were 0.87 MHz/K and 479 MHz/%, respectively. V[http://dx.doi.org/10.1063/1.4806986]

Brillouin scattering is one of the most significant nonlinear phenomena in optical fibers,1,2 and, by exploiting thedependences on Brillouin frequency shift (BFS) on temperature and strain,3,4 it has been applied to distributed fiber-optic
temperature/strain sensing for smart materials and structures.
There are many parameters for evaluating the sensor performance, including spatial resolution, measurement range, measurement speed, and signal-to-noise ratio (SNR), which canbe, in some cases, improved by enhancing the Brillouin signalpower. For example, in the time-domain techniques,5,6 themeasurement range is limited by the optical attenuation, so it
has been elongated by using Raman amplification and/orinserting optical fiber amplifiers.7 With enhanced signal, themeasurement time, which is closely related to the time of signal integration, can also be shortened. As for the correlationdomain techniques,8–10 in Brillouin optical correlationdomain reflectometry (BOCDR),9,10 the spatial resolution waspartially limited to 13 mm by the weak Brillouin signal,10and was enhanced to 6 mm by employing a tellurite glass
fiber with a high Brillouin gain coefficient, i.e., with strong
Brillouin signal.11 Thus, Brillouin signal amplification isextremely important to improve the performance of distributed sensing systems.
Another approach to enhance the Brillouin signal is touse pumped optical fibers doped with rare-earth ions, such as
neodymium, thulium, samarium, holmium, ytterbium, anderbium ions. With their optical amplification capability,Brillouin signal is expected not only to be enhanced but also
to be controlled at high speed by adjusting the pump light,
which will be of great interest in other Brillouin application
fields. Although fundamental Brillouin properties in neodymium- and thulium-doped fibers have already been investigated,12 those in erbium-doped fibers (EDFs), which aremost commonly used as key components of optical amplifiers of all the rare-earth-doped fibers,13,14 have not beenreported yet.
In this work, we measure the Brillouin gain spectra(BGS) in 19-m-long EDFs with three different erbium concentrations (0.72, 1.20, and 2.28 wtppt) at 1.55 lm without
pumping, and investigate the BFSs and their dependences0003-6951/2013/102(19)/191906/4/$30.00

not only on temperature and strain but also on erbium
concentration. The BFS and its temperature and strain
coefficients in the EDF with 0.72-wtppt erbium concentration were 11.42 GHz, 0.87 MHz/K, and 479 MHz/%, which
decreased, increased, and decreased with increasing erbium
concentration, respectively.
A light beam injected into an optical fiber interacts withacoustic phonons and generates a backscattered light beam
called Stokes light.1,2 This phenomenon is called Brillouinscattering, and the Stokes light spectrum is called BGS. Thecenter frequency of the BGS is lower than that of the incident light by the amount called BFS. In optical fibers, theBFS  B is given as2
B ¼

2 n vA;k(1)where n is the refractive index, vA is the acoustic velocity inthe fiber, and k is the wavelength of the incident light. TheBFS is, together with vA, known to largely change depending
on temperature and strain applied to the fiber,3,4 and has
been exploited in sensors. Since the BFS and its temperatureand strain dependences drastically differ according to thefiber materials and structures, they have been investigated invarious kinds of special fibers to improve the sensor performance, which are well summarized in Ref. 12.
The physical properties of the three EDFs that wereused in the experiment as fibers under test (FUTs) are provided in Table I. They had different erbium concentrationsof 0.72, 1.20, and 2.28 wtppt, and the same cladding
TABLE I. Physical properties of EDF samples.
EDF sample No.
Length (m)Erbium concentration(wtppt)Mode-field diameter (lm)
Refractive index difference (%)
Peak absorption at 1.53 lm (dB/m)102, 191906-1

12

3

17.6
0.72
“low”
7.2
0.7
18

19
1.20
“moderate”
7.5
0.64
30

19
2.28
“high”
7.4
0.66
57

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FIG. 1. Measured BGS dependences on temperature in EDFs with erbium concentrations of (a) 0.72 wtppt (low), (b) 1.20 wtppt (moderate), and (c) 2.28 wtppt(high).

refractive indices of 1.46. Both ends of each FUT werespliced to 1-m-long silica single-mode fibers (SMFs) with afusion splicer (FSM-50S, Fujikura). By optimizing the arcduration, the optical loss at the spliced point was suppressed
to lower than 0.1 dB. The open end of the 1-m-long SMF
was immersed into matching oil (n ¼ 1.46) to suppress theFresnel reflection. The experimental setup for investigating
the BGSs in EDFs was almost the same as that reported inRef. 15 based on self-heterodyne detection. A laser diode(LD) at 1550.4 nm was used as a light source, and the opticalpower injected into the FUT was set to 20 dBm with an EDF
amplifier (EDFA). The polarization states of the incident
light and the reference light were optimized with polarization
controllers (PCs). Though the Stokes light included theBrillouin signal caused in the 1-m-long SMF between thecirculator and the FUT, it had no influence on this measurement, because the BFS in the SMF was 500 MHz lowerthan that in the EDF (see below). Different strains wereapplied to the whole length of the EDF fixed on a translation
stage using epoxy glue, while the temperature along thewhole length of the EDF was adjusted with a heater.
The temperature dependence of the BGS in the EDF
with low erbium concentration is shown in Fig. 1(a). The
temperature was controlled from 20  C up to 80  C with astep of 20  C, and we utilized a Lorentzian fitting to derive
the BFS from the BGS. The BFS at 20  C was 11.42 GHz,
which is 500 MHz higher than that of standard silica
SMFs. With increasing temperature, the BFS shifted towardhigher frequency. The same measurements were performed
for the EDFs with moderate and high erbium concentrations,as shown in Figs. 1(b) and 1(c), where similar behavior wasobserved. The optical propagation losses in these EDFs at
1.55 lm (without pumping) are higher than that in the EDF
with low erbium concentration, the Stokes power or the SNR
was lower. Figure 2 shows the BFS dependence in the EDFs
at 20  C on erbium concentration. The error bars represent
the standard errors for the measurements repeated five times.
As the erbium concentration was increased, the BFS wasdecreased. This trend is not consistent with the fact that theBFS in standard silica SMFs (without erbium doping) is10.8 GHz, which is probably caused by alumina co-doped
to suppress the clustering effect of erbium ions16,17 (note thatalumina doped to some fibers is known to raise the BFS).18
Figure 3(a) shows the measured BFS dependences in thethree EDFs on temperature, from which each slope, i.e., temperature coefficient can be plotted as a function of erbium
concentration, as shown in Fig. 3(b). With increasing erbium
concentration, the BFS temperature coefficient was linearlydecreased with a slope of 1.0 MHz/K/wtppt. The BFS
temperature coefficients of 0.8 MHz/K were lower thanthose of 1.18 MHz in silica SMFs.4 This is probably
because the temperature coefficients of the acoustic velocities (especially, Young’s moduli) in the EDFs, which
dominantly determine the BFS temperature coefficients, arelower than those in silica SMFs owing to the doped erbium
and co-doped alumina.
Next, the strain dependence of the BGS in the EDF withlow erbium concentration was measured, as shown in Fig.
4(a). As the applied strain was increased, the BGS shifted
toward higher frequency. The same measurements were performed for the other EDFs, as shown in Figs. 4(b) and 4(c),where similar behavior was observed. Figure 5(a) shows themeasured BFS dependences in the three EDFs on strain,from which the dependence of the BFS strain coefficient onerbium concentration was derived, as shown in Fig. 5(b).
With increasing erbium concentration, the strain coefficient
was slightly enhanced with a slope of 5.92 MHz/%/wtppt.
The BFS strain coefficients of 485 MHz/% were lower
than those of 580 MHz/% in silica SMFs.3 This also seems
to originate from the lower strain coefficients of the Young’s
modulus in the EDFs than those in silica SMFs.

FIG. 2. BFS in EDFs at 20  C as a function of erbium concentration.

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Appl. Phys. Lett. 102, 191906 (2013)

FIG. 3. (a) Temperature dependences of
BFS in EDFs with different erbium concentrations. (b) Temperature coefficientof BFS in EDFs as a function of erbium
concentration.

FIG. 4. Measured BGS dependences on strain in EDFs with erbium concentrations of (a) 0.72 wtppt (low), (b) 1.20 wtppt (moderate), and (c) 2.28 wtppt(high).

FIG. 5. (a) Measured BFS dependence
on strain in EDFs with different erbium
concentrations. (b) Strain coefficient of
BFS in EDFs as a function of erbium
concentration.

In conclusion, we measured the BGSs in EDFs withthree different erbium concentrations at 1.55 lm without
pumping, and fully investigated the BFSs and their dependences on strain, temperature, and erbium concentration.
In the EDF with low erbium concentration (0.72 wtppt),
the BFS and its temperature and strain coefficients were
11.42 GHz, 0.87 MHz/K, and 479 MHz/%, which were foundto be controlled to some extent by adjusting the erbium
concentration. We believe these measurement results will be
useful as a fundamental archive in developing pumped-EDFbased Brillouin sensors as well as in controlling the Brillouinproperties (at least, the Stokes power) at high speed byadjusting the pump light. To fully clarify the pumping effecton the BGS in EDFs is an urgent necessity.

We are indebted to Fujikura Ltd., Japan, for providing
us with the three EDF samples. This work was partially supported by a Grant-in-Aid for Research Activity Start-up(24860029) from the Japan Society for the Promotion ofScience (JSPS), and by research grants from the HattoriHokokai Foundation, the Mazda Foundation, and the JFE
21st Century Foundation.
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