JOURNAL OF APPLIED PHYSICS 112, 043109 (2012)

Dependences of Brillouin frequency shift on strain and temperaturein optical fibers doped with rare-earth ions
Yosuke Mizuno,a) Neisei Hayashi, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku,
Yokohama 226-8503, Japan(Received 13 May 2012; accepted 20 July 2012; published online 29 August 2012)
Brillouin scattering properties in rare-earth-doped fibers, including Nd3þ-doped, Tm3þ-doped,
Sm3þ-doped, and Ho3þ/Tm3þ co-doped fibers, can potentially be controlled at high speed bypumping, but there has been no report on their detailed investigations. In this study, as the first step
toward this goal, the Brillouin gain spectra (BGS) in such rare-earth-doped single-mode fibers aremeasured, for the first time, at 1.55 lm without pumping, and the Brillouin frequency shift (BFS)
and its dependences on strain and temperature are investigated. Clear BGS was observed for the
Nd3þ-doped and Tm3þ-doped fibers, but BGS was not detected for the Sm3þ-doped and Ho3þ/
Tm3þ co-doped fibers probably because of their extremely high propagation losses at 1.55 lm
and small Brillouin gain coefficients. The BFS of the Nd3þ-doped fiber was 10.82 GHz, and its
strain and temperature coefficients were 466 MHz/% and 0.726 MHz/K, respectively. As for the
Tm3þ-doped fiber, the BFS was 10.90 GHz, and its strain and temperature coefficients were
433 MHz/% and 0.903 MHz/K, respectively. These measurement results are compared with thoseC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4747926]
of silica fibers. V

I. INTRODUCTION

For the past several decades, optical fibers doped withrare-earth ions, such as Er3þ, Nd3þ, Tm3þ, Sm3þ, Ho3þ,
Yb3þ, and Pr3þ, have been extensively studied and applied
to optical fiber amplifiers and fiber lasers.1–3 In the meantime, Brillouin scattering in silica single-mode fibers (SMFs)
has drawn considerable interest4–7 and been applied to various devices and systems, such as lasers, tunable delay lines,
phase conjugators, signal processors, slow light generators,
optical storages, and strain/temperature sensors.8–12 In orderto improve their performance, Brillouin scattering properties
in various non-silica fibers have been investigated, and someof them have already been practically applied. Such nonsilica fibers include tellurite glass fibers,13,14 chalcogenide
glass fibers,15,16 bismuth-oxide glass fibers,14,17 photonic
crystal fibers (PCFs),18 polymethyl methacrylate (PMMA)based polymer optical fibers (POFs), and perfluorinatedgraded-index (PFGI-) POFs.19,20 Brillouin scattering in rareearth-doped fibers has a big potential to offer high-speed signal controllability by pumping; and besides, with its amplification function, the measurement range of the Brillouin
optical time-domain analysis or reflectometry (BOTDA/R)8,11 will be elongated. Since its properties have not been
investigated yet, detailed study on the Brillouin properties inrare-earth-doped fibers is the first step to exploit its potential
in practical systems.
In this work, we measure the spontaneous Brillouin gain
spectra (BGS) in four kinds of 2-m-long rare-earth-doped
SMFs (Nd3þ-doped, Tm3þ-doped, Sm3þ-doped, and Ho3þ/
Tm3þ co-doped fibers) at 1.55 lm without pumping, andinvestigate the Brillouin frequency shift (BFS) and itsa)Author to whom correspondence should be addressed. Electronic mail:ymizuno@sonic.pi.titech.ac.jp.

0021-8979/2012/112(4)/043109/5/$30.00

dependences on strain and temperature. While clear BGS
was observed for the Nd3þ-doped and Tm3þ-doped fibers,
no BGS was detected for the Sm3þ-doped and Ho3þ/Tm3þ
co-doped fibers, which seems to be caused by their extremely
high propagation losses at 1.55 lm and small Brillouingain coefficients. The BFS of the Nd3þ-doped fiber was10.82 GHz, and its strain and temperature coefficients were
466 MHz/% and 0.726 MHz/K, respectively. As for the
Tm3þ-doped fiber, the BFS was 10.90 GHz, and its
strain and temperature coefficients were 433 MHz/% and0.903 MHz/K, respectively. We believe that these results
will be an important archive in developing practical devices
and systems based on Brillouin scattering in pumped rareearth-doped fibers.
II. PRINCIPLE

When a light beam is injected into an optical fiber, itinteracts with acoustic phonons and generates a backscattered light beam called the Stokes light.5 This phenomenon
is called Brillouin scattering, and the backscattered Stokes
light spectrum is called the BGS. The center frequency of
the BGS is known to be lower than that of the incident light,
and the amount of this frequency downshift is called the
BFS. In optical fibers, the BFS  B is given as
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. One
parameter for evaluating the Stokes power is the Brillouinthreshold power Pth, which indicates the incident opticalpower required to induce the transition from spontaneous to
stimulated scattering. Pth is given as5112, 043109-1

C 2012 American Institute of Physics
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Mizuno, Hayashi, and Nakamura

J. Appl. Phys. 112, 043109 (2012)

TABLE I. BFS at room temperature and its strain and temperature coefficients in silica SMF, tellurite fibers, bismuth-oxide fibers, Ge-doped PCFs,
and PFGI-POFs at 1550 nm.

Fiber
Silica SMFa
Telluriteb
Bismuth-oxidec
Ge-doped PCFd
PFGI-POFe

BFS (GHz)

Strain coefficient(MHz/%)

Temperature coefficient(MHz/K)

10.85
7.95
8.83
10.29
2.83

þ580
230
––
þ409
122

þ1.18
1.14
0.88
þ0.82
4.09

a

References 6 and 7.
References 13 and 14.
c
Reference 14.
d
Reference 18.
eReference 20.
b

Pth ¼

21 Aeff;
K gB Leff(2)

where Aeff is the effective area, gB is the Brillouin gain coefficient, K is the polarization parameter (¼ 0.667 when polarization state is not maintained), and Leff is the effective
length defined as
Leff ¼

1e
a

a L

;

(3)

where a is the propagation loss and L is the fiber length. As
Pth becomes lower, the Stokes power for the same incident
power grows higher.

TABLE II. Physical properties of four FUTs used in experiments.

Product ID
Length (m)Core diameter (lm)
NA

Nd3þ

Tm3þ

Sm3þ

Ho3þ/Tm3þ

Nd009
2
4
0.18

Tm134
2
3
0.16

Sm633
2
3
0.14

TH530
2
6
0.18

Since one of the useful applications of Brillouin scattering is strain/temperature sensing, the BFS dependences on
strain and temperature in various fibers have been investigated so far. Table I summarizes the BFS and its dependences on strain and temperature in silica SMFs,6,7 tellurite
fibers,13,14 bismuth-oxide fibers,14 Ge-doped PCFs (mainpeak only),18 and PFGI-POFs.20 Supposing that the wavelength of the incident light is 1550 nm and that n is not dependent on the wavelength, all the values in Table I havebeen recalculated using Eq. (1). As can be seen, the BFS andits strain and temperature dependences drastically change
according to the fiber materials and structures.
III. EXPERIMENTAL SETUP

We employed as fibers under test (FUT) four kinds ofrare-earth-doped SMFs manufactured by CorActive Inc.:
Nd3þ-doped, Tm3þ-doped, Sm3þ-doped, and Ho3þ/Tm3þ
co-doped fibers. Their lengths, core diameters, and numericalapertures (NAs), provided by the manufacturer, are summarized in Table II. One end of each FUT was spliced to a silica

FIG. 1. Measured Brillouin gain spectrafor (a) Nd3þ-doped, (b) Tm3þ-doped, (c)Sm3þ-doped, and (d) Ho3þ/Tm3þ codoped fibers.

043109-3

Mizuno, Hayashi, and Nakamura

SMF connected to a circulator, and the other end was
immersed into matching oil (n ¼ 1.46) to suppress the Fresnel reflection.
The experimental setup for investigating the BFS
dependences on strain and temperature in the FUTs was thesame as that previously reported in Ref. 19, where selfheterodyne detection was used to observe the BGS with ahigh resolution. The wavelength of the light source was1550 nm, and the polarization state was adjusted with apolarization controller (PC). The Brillouin signal generated
in the silica SMF placed between the circulator and the
FUTs was removed from the measurement results in the following procedure: (1) obtain the BGS when bending wasapplied to the region around the spliced point to induce considerable loss of >50 dB, (2) obtain the BGS after the bending was released, and (3) subtract the BGS obtained in (1)from that obtained in (2). Note that the bending-applied
region was so short (<1 cm) that its influence on the measurement results was negligible. For each BGS measurement,
20-times averaging was performed.
IV. EXPERIMENTAL RESULTS

Figures 1(a)–1(d) show the measured BGS for the four
FUTs. The incident optical power Pin was 19 dBm for the
Nd3þ-doped fiber, and 21 dBm for the other fibers. Thoughdistorted by the noise floor of the electrical spectrum analyzer (ESA), clear BGS was observed for the Nd3þ-doped
and Tm3þ-doped fibers at 10.82 GHz and at 10.90 GHz,
respectively, but no signal was detected for the Sm3þ-doped

J. Appl. Phys. 112, 043109 (2012)and Ho3þ/Tm3þ co-doped fibers both within and out of therange of Figs. 1(a) and 1(b).
In order to clarify the reason, the loss spectra of the four
FUTs were measured using a white light source with a wavelength range of 350 to 1600 nm, as shown in Figs. 2(a)–2(d).
We can see that the loss at 1550 nm is extremely high for theSm3þ-doped and Ho3þ/Tm3þ co-doped fibers. Under theassumption that the loss in silica SMF and the spliced pointis negligibly low (according to the manufacturer, splicing
loss is as low as several dB for all the four fibers), the effective length Leff at 1550 nm can be calculated to be 1.83, 1.33,
0.87, and 0.87 m for the Nd3þ-doped, Tm3þ-doped, Sm3þdoped, and Ho3þ/Tm3þ co-doped fibers, respectively. (Theloss of the Sm3þ-doped fiber is not exactly presented in Fig.
2(c), but the Leff becomes almost constant when the loss ishigher than 25 dB.) Although Leff is relatively short for theSm3þ-doped and Ho3þ/Tm3þ co-doped fibers, it does notappear to be the only cause for their weak Brillouin signals.
Considering Eq. (2), since the effective areas of the four
FUTs are in the same order judging from the core diameters
given in Table II,21 the main cause may be the difference in
Brillouin gain coefficient, which needs to be further studied.
It is worthwhile, however, to try to observe the Brillouin signal in the Sm3þ-doped and Ho3þ/Tm3þ co-doped fibers
using a high-power light source at another wavelength suchas 980 nm, which is one of the typical wavelengths used forpumping Er3þ-doped fibers.
Next, the BFS dependences on strain and temperature in
the Nd3þ-doped and Tm3þ-doped fibers were investigated.
The measured strain dependence of the BGS in the Nd3þ-FIG. 2. Measured loss spectra of (a)Nd3þ-doped, (b) Tm3þ-doped, (c) Sm3þdoped, and (d) Ho3þ/Tm3þ co-doped
fibers. Note that the unit of the verticalaxis is not dB/m but dB/(fiber length).

043109-4

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J. Appl. Phys. 112, 043109 (2012)

FIG. 3. Measured strain dependences of(a) BGS and (b) BFS, and temperaturedependences of (c) BGS and (d) BFS in
Nd3þ-doped fiber.

doped fiber is shown in Fig. 3(a). As the applied strain
increased, the BGS shifted toward higher frequency. Thechange in Stokes power seems to be mainly due to thechange in polarization state (note that the polarization state

was not optimized for all the BGS measurements).22 From
Fig. 3(a), using a Lorentzian fitting of each BGS, the strain
dependence of the BFS can be plotted as shown in Fig. 3(b).
The strain dependence was almost linear with a coefficient

FIG. 4. Measured strain dependences of(a) BGS and (b) BFS, and temperaturedependences of (c) BGS and (d) BFS in
Tm3þ-doped fiber.

043109-5

Mizuno, Hayashi, and Nakamuraof 466 MHz/%. The BGS and BFS dependences on temperature were also investigated as shown in Figs. 3(c) and 3(d).
The temperature coefficient of the BFS dependence wasfound to be 0.726 MHz/K.
In the same way, the BGS and BFS dependences on
strain and temperature in the Tm3þ-doped fiber were investigated. The dependences on strain are shown in Figs. 4(a) and
4(b), and those on temperature are shown in Figs. 4(c) and
4(d). Although the Brillouin signal was so small that it waslargely influenced by the change in polarization state as well
as by the noise floor of the ESA, the BFS itself was properly
measured. The strain and temperature coefficients of the
BFS dependences in the Tm3þ-doped fiber were 433 MHz/%and 0.903 MHz/K, respectively. Thus, the strain and temperature coefficients of the BFS dependences were, in both theNd3þ-doped and Tm3þ-doped fibers, lower than those ofsilica SMFs,6,7 which indicates that these coefficients, alongwith the BFS itself, can be moderately controlled by adjusting the rare-earth doping concentration.
V. CONCLUSION

The BGS in four kinds of 2-m-long rare-earth-doped
SMFs (Nd3þ-doped, Tm3þ-doped, Sm3þ-doped, and Ho3þ/
Tm3þ co-doped fibers) was measured at 1.55 lm without
pumping, and the BFS and its dependences on strain andtemperature were investigated. The BFS at room temperaturewas 10.82 GHz for the Nd3þ-doped fiber and 10.90 GHz
for the Tm3þ-doped fibers, but no BGS was detected for theSm3þ-doped and Ho3þ/Tm3þ co-doped fibers probably due
to their extremely high propagation losses at 1.55 lm andsmall Brillouin gain coefficients. The strain and temperature
coefficients of the BFS dependences in the Nd3þ-doped fiber
were 466 MHz/% and 0.726 MHz/K, respectively. As for the
Tm3þ-doped fiber, the strain and temperature coefficientswere 433 MHz/% and 0.903 MHz/K, respectively. We
believe that these results will be an important archive indeveloping practical devices and systems based on pumped
rare-earth-doped fibers with the high-speed controllability of
their Brillouin signals.
ACKNOWLEDGMENTS

This work was supported by the Research Fellowships
for Young Scientists from the Japan Society for the Promotionof Science (JSPS), and by research grants from the Foundation of Ando Laboratory and the Murata Science Foundation.
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