Based on the pump-probe technique (PPT), stimulated Brillouinscattering (SBS) was observed in a 100 m-long silica graded-index
multimode optical ﬁbre with the same core diameter as that of a
perﬂuorinated graded-index polymer optical ﬁbre (PFGI-POF)(=50 µm). The SBS threshold was measured to be ∼50 mW, which
was much lower than 5.08 W, the SBS threshold obtained for thesame ﬁbre without the PPT. This result indicates that the PPT is
useful even under multimode conditions and that the difﬁculty inobserving SBS in PFGI-POFs consists not in their physical problems
but in their material problems (high propagation loss etc.).

Introduction: Among various kinds of sensors, optical ﬁbre sensors
based on Brillouin scattering have especially attracted considerable
attention in the area of smart materials and structures [1–5], because
distributed strain and temperature sensing along an optical ﬁbre canbe performed. Up to now, glass optical ﬁbres (GOFs) have been
mainly used as the sensing heads of the Brillouin sensors. However,GOFs are so fragile that they cannot withstand strains of over several
per cent. One method of overcoming this disadvantage is to employ
plastic optical ﬁbres (POFs) as the sensing heads, which have many
advantages such as extremely high ﬂexibility (more than 50% strain
can be applied), low-cost connection, and ease of handling. Besides,
POFs have a ‘memory effect’: they can memorise the information on
applied large strains due to their plastic deformation [6]. Among
several types of POFs, perﬂuorinated graded-index (PFGI-) POFs
have been mainly used because their propagation loss is not too higheven at telecom wavelengths.
There are two major techniques for achieving distributed strain andtemperature sensing based on Brillouin scattering: time domain andcorrelation domain. The time-domain techniques are further divided
into two types: ‘Brillouin optical time-domain reﬂectometry(BOTDR)’ [1], in which incident light is injected into only one end of
a sensing ﬁbre, and ‘Brillouin optical time-domain analysis(BOTDA)’ [2], in which incident light is injected into both ends of asensing ﬁbre to induce stimulated Brillouin scattering (SBS). The
correlation-domain techniques are also divided into two: ‘Brillouin
optical correlation-domain reﬂectometry (BOCDR)’ [4] and ‘Brillouin
optical correlation-domain analysis (BOCDA)’ [5].
One of the most serious problems in implementing such distributed
Brillouin sensors using PFGI-POFs is their weak Brillouin signals [7].
As a result, especially when high spatial resolution is required, abetter candidate is not ‘reﬂectometry’ but ‘analysis’, where SBS isinduced by the pump-probe technique, leading to drastic enhancementof the Brillouin signals. Although SBS in the PFGI-POFs has alreadybeen observed by the pump-probe technique using an experimental
setup similar to that of BOCDA [8], the setup including lock-in detection was rather complicated. In order to develop POF-based BOTDA
systems, SBS in the PFGI-POFs should be observed with a much
simpler setup, but this trial has not been successful [9]. So far wehave had no idea whether this difﬁculty is due to their structural ormaterial problems, which needs to be clariﬁed to proceed to the nextstep.
In the work reported in this Letter, based on the pump-probe technique without lock-in detection, we observed SBS in a 100 m-long silica
graded-index multimode optical ﬁbre (GI-MMF) which has the same
core diameter as that of a PFGI-POF, and measured its SBS thresholdto be approximately 17 dBm (= ∼ 50 mW). This value was much
lower than 5.08 W obtained for the same silica GI-MMF without
using the pump-probe technique, which indicates that this technique is
useful even with multimode nature. Then we conclude that the difﬁcultyin observing SBS in PFGI-POFs consists not in their structural problems(multimode nature, small core diameter etc.) but in their materialproblems (high propagation loss, small Brillouin gain coefﬁcient etc.).
Theory: When pump light is injected into an optical ﬁbre, backscattered
Stokes light is generated due to spontaneous Brillouin scattering (SpBS)[10]. The centre frequency of the Stokes light is known to be lower thanExperiments: As a ﬁbre under test (FUT), we employed a 100 m-longsilica GI-MMF with a numerical aperture (NA) of 0.2, a core diameterof 50 µm, a cladding diameter of 125 µm, a core refractive indexof ∼1.46, and a propagation loss of about 1.0 dB/km at 1.55 µm. Bothends of this silica GI-MMF were coupled to silica SMFs using so-called
butt-coupling with air gaps ﬁlled with index-matching oil (n ∼ 1.46) tosuppress the Fresnel reﬂection.
The experimental setup for observing SBS with the pump-probe technique is schematically shown in Fig. 1. Two laser diodes (LDs) wereused; one was for providing pump light, and the other was for probelight. The pump light at frequency ν0 was ampliﬁed with an erbiumdoped ﬁbre ampliﬁer (EDFA) and injected into the silica GI-MMF.
The probe light was, after being ampliﬁed also with an EDFA, injected
into the other end of the silica GI-MMF, the frequency of which wasswept around ν0 – BFS by controlling the driving temperature of theLD. Part of the Stokes light was converted into an electrical signal
with a photodetector (PD) and monitored with an electrical spectrum
analyser (ESA); by comparing 1. the beat signal between the Stokes
light and the Rayleigh-scattered pump light with 2. the beat signal
between the probe light and the Rayleigh-scattered pump light, thefrequency of the probe light was accurately adjusted. The rest of theStokes light was directed to an optical powermeter (OPM), the outputof which was acquired as Brillouin gain spectrum (BGS), i.e. as a function of the frequency difference between the pump light and the probelight.
probe

V0 – ~BFSLDEDFA
pump

V0silica GI-MMFLDEDFAESAPDStokesOPM

Fig. 1 Experimental setup for observing SBS in silica GI-MMF withpump-probe technique
EDFA: erbium-doped ﬁbre ampliﬁer; ESA: electrical spectrum analyser; LD:laser diode; OPM: optical powermeter; PD: photodetector
43silica GI-MMF
L = 100 m
d = 50 µm

2
10–1

Ppump = 22.1 dBm
Pprobe = 15.3 dBm–2
10.2 10.3 10.4 10.5 10.6frequency, GHzarelative Stokes power, dBN. Hayashi, Y. Mizuno and K. Nakamurathat of the pump light. The amount of this frequency shift is called
Brillouin frequency shift (BFS), which is, at 1.55 µm, typically∼10.8 GHz for silica singlemode ﬁbres (SMFs) and ∼2.8 GHz forPFGI-POFs. In general, the Stokes power grows higher with increasing
pump power. When the pump power becomes higher than a certain
power called Brillouin threshold Pth, the Stokes power begins todrastically increase due to the transition from SpBS to SBS.
For sensing applications, the Brillouin Stokes power should be as
high as possible, which leads to high spatial resolution, long measurement range, short measurement time etc. The so-called pump-probe
technique is one of the effective techniques to enhance the Stokespower. With this technique, by injecting probe light at the same
frequency as the Stokes light additionally into the other end of theﬁbre, SBS can be induced at much lower pump power, i.e. theBrillouin threshold can be largely reduced. This scheme has been
fully exploited in developing SMF-based BOTDA [2] and BOCDA
systems [4].

relative power, dB

Observation of stimulated Brillouinscattering in silica graded-index multimodeoptical fibre based on pump-probe
technique4silica GI-MMF
L = 100 m
d = 50 µm

3Pprobe = 15.3 dBm

2100

5 10 15 20pump power, dBm

25bFig. 2 Experimental resultsa Relative BGS in silica GI-MMF with pump-probe techniqueb Relative Stokes power dependence on pump power in silica GI-MMF withpump-probe technique

ELECTRONICS LETTERS 28th February 2013 Vol. 49 No. 5

The BGS in the silica GI-MMF measured with the pump-probe technique is shown in Fig. 2a. The pump power and the probe power wereset to 22.1 dBm and 15.3 dBm, respectively. A clear peak was observed
at 10.40 GHz (which differs from 10.43 GHz in our previous experiment [11] probably due to the difference in room temperature [12]).
Here we should note that the BFS of the silica SMFs connected to theGI-MMF was 10.80 GHz, which is outside the range of Fig. 2a.
Fig. 2b shows the relative Stokes power against pump power when theprobe power was ﬁxed at 15.3 dBm. The Stokes power was drasticallyenhanced when the pump power was higher than the Brillouin threshold
Pth of ∼17 dBm (= ∼50 mW). This value is much lower than 5.08 W(obtained without the pump-probe technique) [11], which indicatesthat the pump-probe technique is effective even when FUTs are
multimode with large core diameters.
The experimental results above imply that the difﬁculty in observing
SBS in PFGI-POFs with the pump-probe technique does not consist in
their structural problems, because SBS was successfully induced for thesilica GI-MMF with the same length and core diameter. Thus we speculate that the difﬁculty lies in the material problems of the PFGI-POFs,
including their high propagation loss, small Brillouin gain coefﬁcientetc. (Note that the previously reported value of gB for PFGI-POFs(3.09 × 10−11m/W) [7] was estimated partially based on the physicalparameters for bulk poly(methyl methacrylate) (PMMA), on which
point further study is needed.)Conclusion: By using the pump-probe technique without lock-in detection, SBS in a 100 m-long silica GI-MMF was observed. Compared tothe Brillouin threshold of 5.08 W theoretically obtained when thepump-probe technique was not used [11], with this technique it wasexperimentally found to be as low as ∼17 dBm ( = ∼50 mW), indicatingthat the pump-probe technique is effective even with multimode nature.
Based on the results above, the difﬁculty in observing SBS inPFGI-POFs appears to consist not in their structural problems but intheir material problems, such as the high propagation loss and thesmall Brillouin gain coefﬁcient. We believe that this Letter provides
signiﬁcant information for developing POF-based BOTDA systems in
the near future.
Acknowledgments: This work was partially supported by aGrant-in-Aid for Research Activity Start-up (24860029) from theJapan Society for the Promotion of Science (JSPS), and by research
grants from the Murata Science Foundation, the Hattori-Hokokai
Foundation, the Mazda Foundation, and the Foundation of Ando
Laboratory.

© The Institution of Engineering and Technology 201318 November 2012
doi: 10.1049/el.2012.4017
One or more of the Figures in this Letter are available in colour online.
N. Hayashi, Y. Mizuno and K. Nakamura (Precision and Intelligence
Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8503, Japan)
E-mail: hayashi@sonic.pi.titech.ac.jp
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