LETTERIEICE Electronics Express, Vol.11, No.18, 1–5

Brillouin signal ampliﬁcationin pumped erbium-doped
optical ﬁberMingjie Dinga), Neisei Hayashi, Yosuke Mizuno,
and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology,
Yokohama 226–8503, Japana) ding@sonic.pi.titech.ac.jp

Abstract: Ampliﬁcation of Brillouin scattering signal extends the measurement range and improves the signal-to-noise ratio of ﬁber-optic distributed
Brillouin sensors. Here, we investigate the ampliﬁcation effect of a 980-nm
pump laser on Brillouin gain spectrum in an erbium-doped optical ﬁber. The
Brillouin-scattered Stokes power increased exponentially with increasing
pump power, exhibiting approximately 87% enhancement at 252 mW compared with the Stokes power obtained without the pump. This suggests that amore signiﬁcant increase in the Stokes power is feasible with a higher-power
980-nm pump laser.
Keywords: erbium-doped optical ﬁbers, Brillouin scattering, optical ﬁber
sensors
Classiﬁcation: Fiber optics, Microwave photonics, Optical interconnection, Photonic signal processing, Photonic integration and systems
References© IEICE 2014
DOI: 10.1587/elex.11.20140627
Received June 30, 2014
Accepted August 11, 2014
Publicized August 27, 2014
Copyedited September 25, 2014

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IEICE Electronics Express, Vol.11, No.18, 1–5[12] Y. Mizuno, N. Hayashi and K. Nakamura: J. Appl. Phys. 112 (2012) 043109.
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3463038

1Introduction

Brillouin scattering in optical ﬁbers is caused by acoustic-optical interaction,
generating backscattered Stokes light, the spectrum of which is known as Brillouingain spectrum (BGS) [1]. The center frequency of BGS is shifted downward
relative to that of the incident light by the amount termed the Brillouin frequency
shift (BFS). Because BFS linearly depends on strain and temperature [2, 3],
Brillouin-distributed strain and temperature sensing has been extensively studied
for monitoring large-scale civil structures, such as buildings, bridges, dams, pipelines, and aircraft wings.
Various sensing schemes have been developed, such as time-, frequency-, andcorrelation-domain techniques [4, 5, 6, 7, 8, 9, 10, 11]. Among them, time-domain
techniques have been reported to have the widest measurement range, which islimited by optical attenuation. To date, many methods have been implemented toextend the measurement range further by using Raman-based signal ampliﬁcation[6, 7, 8]; as a result of these efforts, the measurement range is now well over
100 km and the spatial resolution is on the order of several meters. Anotherapproach utilizes pumped rare-earth-doped ﬁbers (REDFs) with ampliﬁcation
capability, such as neodymium-, thulium-, ytterbium-, and erbium-doped ﬁbers(EDFs). Although the Brillouin properties of un-pumped REDFs have been alreadyclariﬁed [12, 13], the effects of pumping have not been reported yet. Note that theﬂuorescence spectra and decay times of such REDFs are temperature-dependent[14, 15], and a novel discriminative measurement method of strain and temperature
will be feasible by combining these features with the BGS dependence.
In this paper, we investigate the effect of a 980-nm pump laser on the BGS inan EDF, which is the REDF most commonly used as a key component of opticalampliﬁers. The Brillouin-scattered Stokes power increases exponentially withincreasing 980-nm pump laser power.
2© IEICE 2014
DOI: 10.1587/elex.11.20140627
Received June 30, 2014
Accepted August 11, 2014
Publicized August 27, 2014
Copyedited September 25, 2014

Experimental setup

The EDF used as a ﬁber under test (FUT) had a length of 17.8 m, mode-ﬁelddiameter of 7.2 µm, core refractive index of ∼1.47, peak absorption of 18 dB/km at
1530 nm, and erbium concentration of 720 wtppm. Both ends of the FUT werespliced to 1-m-long silica single-mode ﬁbers (SMFs) by using a fusion splicer2

IEICE Electronics Express, Vol.11, No.18, 1–5

Fig. 1.

Experimental setup. EDFA: erbium-doped ﬁber ampliﬁer, ESA:
electrical spectrum analyzer, FUT: ﬁber under test, GPIB:
general-purpose interface bus, LD: laser-diode, PC: polarization
controller, PD: photo-diode, WDM: wavelength divisionmultiplexing.

(FSM-50S, Fujikura), where the splicing loss was suppressed to < 0:1 dB by
optimizing the arc duration.
The experimental setup for investigating the BGS in the pumped EDF isdepicted in Fig. 1, and is based on self-heterodyne detection with a high frequency
resolution [16]. The output light of a laser diode (LD) at 1550 nm was divided into
two beams by an optical coupler. One beam was used as a reference light forheterodyne detection. The other beam was ampliﬁed by using an EDF ampliﬁer(EDFA), and was injected into the FUT (incident power: 100 mW) after propagating through a circulator and a 1550/980-nm wavelength-division-multiplexing(WDM) coupler. The output of another LD at 980 nm was injected into the FUT
as a pump light, via the WDM coupler. The open end of the 1-m long SMF spliced
to the FUT was immersed into matching oil (n ¼ 1:46) to suppress the Fresnel
reﬂection. The polarization states of each beam were optimized by using polarization controllers (PCs), thus maximizing the Brillouin signal. The Stokes lightthat backscattered from the FUT was coupled with the reference light, wasconverted to an electrical signal by using a photo-diode (PD), and was observedby using an electrical spectrum analyzer (ESA).
3© IEICE 2014
DOI: 10.1587/elex.11.20140627
Received June 30, 2014
Accepted August 11, 2014
Publicized August 27, 2014
Copyedited September 25, 2014

Experimental results

Fig. 2 shows the measured BGS dependence on the power of the 980-nm pump in
the EDF. Since it is the net ampliﬁcation effect of the Stokes power that is importantin Brillouin sensing, we adjusted the noise ﬂoor of the BGS measured with pumplight (pump power = 16, 40, 100, and 252 mW) to the noise ﬂoor of the BGS
measured without pump light (∼0.8 nW); thus, the ampliﬁcation effect of the Stokes
power, i.e. the relative power between the peak and the noise ﬂoor of the BGS, canbe easily compared. The BFS obtained without the pump was 11.42 GHz, which
agrees with the previously reported result [13]. Although the BFS was not changed
by EDF pumping, the Stokes power increased with increasing pump power. Withthe maximal pump power at 252 mW, the Stokes power increased by 0.36 nW. Thisampliﬁcation is attributed not only to the ampliﬁed Brillouin-scattered light itself
but also to the ampliﬁed incident light. Note that the tail of the Rayleigh scattering

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IEICE Electronics Express, Vol.11, No.18, 1–5

Fig. 2.

Measured BGS dependence on the 980-nm pump power in theEDF.

Fig. 3.

Relative enhancement of the Stokes power vs. the 980-nm
pump power.

spectrum was also ampliﬁed and overlapped with the BGS, resulting in the elevatednoise ﬂoor.
The relative enhancement of the Stokes power was plotted as a function of the980-nm pump power, and is shown in Fig. 3. When the EDF was pumped with thepower of 252 mW, the relative enhancement reached ∼87%. Furthermore, therelative enhancement of the Stokes power dramatically increased for pump power
above ∼40 mW, indicating that, if a 980-nm pump laser with an output power
252 mW is employed, more signiﬁcant enhancement of the Stokes signal is likely
to be achieved.
4© IEICE 2014
DOI: 10.1587/elex.11.20140627
Received June 30, 2014
Accepted August 11, 2014
Publicized August 27, 2014
Copyedited September 25, 2014

Conclusion

We investigated the pumping effect of the 980-nm pump on the Brillouin Stokes
power in an EDF. As the pump power increased, the Stokes power increased aswell, even after the inﬂuence of the enhanced noise ﬂoor was subtracted. The
relative enhancement of the Stokes power reached ∼87%. These results indicatethat signiﬁcant Brillouin ampliﬁcation might be achievable by employing higherpower pumps. We believe that these results will be important in the near future for
implementing distributed strain and temperature sensors based on Brillouin scattering in EDFs.

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Acknowledgments
We are indebted to Fujikura Ltd., Japan, for providing us with the EDF sample.
This work was partially supported by Grants-in-Aid for Young Scientists (A)(no. 25709032) and for Challenging Exploratory Research (no. 26630180) fromthe Japan Society for the Promotion of Science (JSPS) and by research grants fromthe General Sekiyu Foundation, the Iwatani Naoji Foundation, and the SCAT
Foundation. N. H. acknowledges a Grant-in-Aid for JSPS Fellows (no. 25007652).

© IEICE 2014
DOI: 10.1587/elex.11.20140627
Received June 30, 2014
Accepted August 11, 2014
Publicized August 27, 2014
Copyedited September 25, 20145