Simple coupling method for enhancing
Brillouin scattering signal in polymer opticalfibres
Y. Mizuno, N. Hayashi and K. Nakamura
A simple, efﬁcient, and cost-effective method to enhance a Brillouinscattering signal in perﬂuorinated graded-index (GI-) polymer opticalﬁbres (POFs) is proposed and demonstrated. In this method, insteadof the conventional butt-coupling of a POF to a silica singlemodeﬁbre (SMF), a multimode ﬁbre (MMF) with a proper mode-ﬁeld diameter of the fundamental mode is additionally inserted between thetwo ﬁbres to improve the optical coupling efﬁciency from the POF
to the SMF. After theoretical analysis, it is experimentally shownthat the Brillouin signal in the POF is clearly enhanced simply byinserting standard MMFs, which are commercially available at affordable cost.

MMF [7]). The Fresnel reﬂection at the boundary between the GI-POF(n  1.35) and the silica SMF (n  1.46) is negligibly small(,,0.1 dB). We deﬁne the MFDs of the SMF, the MMF, and theGI-POF as wSMF, wMMF and wPOF, respectively. According toMarcuse’s equation [8], when the GI-POF and the SMF are simply
butt-coupled, the loss which the Brillouin-scattered light causes in theGI-POF suffers in propagating back to the SMF is
2 wPOF wSMF 2[dB]
LS-P = −10 log 2wPOF + w2SMF

In contrast, when the MMF is additionally inserted between the GI-POF
and the SMF, the total loss isLS-M-P = −10 log

Theory: Owing to their large core diameter ranging from 50 to 980 mm,
standard POFs are multimode. To simplify the analysis, however, weconsider only the Brillouin-scattered light of the fundamental mode ofa GI-POF, which is much stronger than that of other modes. This
assumption is not exactly the case, but valid for rough analysis (notethat even exciting only the fundamental mode of an MMF is experimentally feasible by a so-called centre-launching technique, i.e. by adjusting
the input beam proﬁle to match the MFD of the fundamental mode of the2 wPOF wMMF
w2POF + w2MMF

2  
2 wMMF wSMF 2+ −10 log 2[dB]wMMF + w2SMF(2)Fig. 1 shows the dependence of the calculated total loss LS-M-P on wMMF,
when wSMF ¼ 10.4 mm and wPOF ¼ 25 mm (for example). When wMMF
is the same as wSMF or wPOF, LS-M-P is  3 dB (¼LS-P). When wMMF islarger than wSMF and smaller than wPOF, there is an improvement of thetotal loss compared with LS-P; and wMMF has an optimal value wopt (inthis case  16 mm) slightly below the average of wSMF and wPOF. Thus,by inserting an MMF with a proper MFD of its fundamental mode
between the SMF and the GI-POF, the Brillouin signal in the GI-POF
should be enhanced.

5calculated loss LS-M-P, dB

Introduction: Brillouin scattering in glass optical ﬁbres (GOFs) [1] hasbeen an active ﬁeld of research for several decades, and applied tovarious devices and systems, such as lasing, microwave signal processing, slow light generation, phase conjugation, and optical storage.
Strain/temperature sensing is also one of the useful Brillouin applications, but GOFs are so fragile that they cannot be used as sensorheads to which a large strain of .10% is applied. One solution to thisproblem is to implement such Brillouin sensors using polymer opticalﬁbres (POFs), which offer not only high ﬂexibility but also easy connection, low cost, and high safety [2]. Up to now, experimental study has
been extensively performed on the Brillouin scattering properties in perﬂuorinated graded-index (GI-) POFs at 1.55 mm [3– 5]. In all our previous experiments, one end of the POFs under investigation wasdirectly butt-coupled to a silica-based singlemode ﬁbre (SMF) for
Brillouin detection. Although Brillouin scattering in POFs has beenshown to be potentially useful in developing high-precision temperaturesensing with reduced or even zero strain sensitivity [4], one of thebiggest problems is the extremely low Brillouin Stokes power due tothe following three reasons: 1. a propagation loss of GI-POFs(250 dB/km at 1.55 mm) is much higher than that of silica SMFs,
resulting in a short Brillouin effective length [1, 3]; 2. a core diameterof POFs is much larger than that of singlemode GOFs, leading to ahigh Brillouin threshold [1, 3]; and 3. only some portion of theBrillouin-scattered light caused in the POFs returns back to an SMF
for detection, and the rest is lost at the butt-coupled part because of
the difference in core diameter [3].
We have so far succeeded in drastically enhancing the Brillouin signal
in a POF by inducing stimulated Brillouin scattering (SBS) with low
threshold power using a so-called pump-probe technique [5], wheretwo light beams are injected into both ends of the POF. However, thistechnique requires some expensive devices, including a single-sideband
modulator (SSBM), a microwave generator with frequency sweep function, and a lock-in ampliﬁer. We have also developed a method toenhance the Brillouin signal using pulsed pump light and an erbiumdoped ﬁbre ampliﬁer (EDFA) [6], which also requires additional expensive devices, such as an optical pulse generator composed of an intensity
modulator (IM). Thus, a more cost-effective way of enhancing theBrillouin signal is highly demanded for a wider range of applications.
In this Letter, an extremely simple and low-cost method is developed
to enhance the Brillouin scattering signal in a POF only by additionallyinserting an MMF at the butt-coupled part. First, we theoretically provethat the MMF to be inserted should have a proper mode-ﬁeld diameter(MFD) of the fundamental mode in order to resolve the problem 3
described above and to improve the optical coupling efﬁciency fromthe POF to the SMF. Then, we experimentally show that the Brillouinsignal in the POF with 120 mm-core diameter is clearly enhanced byinserting standard MMFs with 50 mm-core diameter.

(1)

4LS-P32

18

10.4

2526
1420
MFD of MMF, µm

32Fig. 1 Calculated total loss against mode-ﬁeld diameter of fundamental
mode of MMF

Experiments: We experimentally conﬁrmed the effectiveness of thismethod. As depicted in Fig. 2, between a 1 m-long silica SMF(pigtail of an optical circulator) with 9.3 mm MFD at 1.55 mm and a
100 m-long perﬂuorinated GI-POF with 120 mm core diameter, the following three kinds of MMFs with 50 mm core diameter were employed:
a 100 m-long silica SI-MMF, a 100 m-long silica GI-MMF, and a 1.2 mlong perﬂuorinated GI-POF. Index-matching oil (n  1.46) was used toﬁll the airgap between the ﬁbre ends. The experimental setup for observing the BGS in the GI-POF in this experiment was basically the same as
our previous setup with self-heterodyne detection [3]. The wavelength ofthe light source was 1550 nm, and the optical input power to the 120 mmcore GI-POF was set to 17.6 dBm for all the measurements, which ismuch lower than the Brillouin threshold power of the MMFs. The polarisation state was optimised with a polarisation controller (PC). Whenthe silica SI- and GI-MMFs were inserted, the Brillouin signal causedin the two ﬁbres has no inﬂuence on the BGS measurement, because
the Brillouin frequency shift (BFS) in the GI-POF is 2.8 GHz [3],which is about four times lower than that in the silica ﬁbres (typically10.9 GHz) [1]. However, when the 50 mm core GI-POF was inserted,
its BFS is the same as that in the 120 mm core GI-POF under investigation; in order to separate their Brillouin signals, making use of theBFS dependence on temperature [4], we placed the 50 mm core GIPOF on the heater and raised the temperature up to 808C.

ELECTRONICS LETTERS 27th September 2012 Vol. 48 No. 20

GI-POFMMF
SMF

MFD = 10.4 µm
d = 120 µm
1. w/o MMF
2. SI-MMF (d = 50 µm)
3. GI-MMF (d = 50 µm)
4. GI-POF (d = 50 µm)

Fig. 2 Experimental conﬁguration; three kinds of MMFs inserted between
SMF and GI-POF

Fig. 3 shows the measured BGS with and without the three MMFs. Apeak at 2.6 GHz is the Brillouin signal of the heated 50 mm core GIPOF, which is rather broad probably due to its temperature non-uniformity.
We can clearly see that the Brillouin Stokes power was enhanced by inserting MMFs, especially when the SI-MMF and the GI-POF were employed.
This result can be well explained by assuming that the MFDs of the fundamental mode of the SI-MMF and the GI-POF are close to the optimal value
wopt in this experiment.
–57–58
power, dBmAcknowledgments: This work was partially supported by the Research
Fellowships for Young Scientists from the Japan Society for thePromotion of Science (JSPS), and by research grants from the Murata
Science Foundation and the Foundation of Ando Laboratory.
# The Institution of Engineering and Technology 2012
6 August 2012
doi: 10.1049/el.2012.2808
One or more of the Figures in this Letter are available in colour online.
Y. Mizuno, N. Hayashi and K. Nakamura (Precision and Intelligence
Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8503, Japan)
E-mail: ymizuno@sonic.pi.titech.ac.jp
References

GI-POF
SI-MMF–59
GI-MMF–60–61
w/o MMF–62
2.4

from the POF to the SMF. Then, we experimentally conﬁrmed the effectiveness of this method by showing that the Brillouin signal of a 120 mmcore GI-POF was efﬁciently enhanced by employing a 50 mm core silica
SI-MMF or a 50 mm core GI-POF. We should note that silica MMFs
with various core diameters (namely, various MFDs) are commerciallyavailable at relatively low cost. Thus, we believe that this method will
become a standard technique in exploiting the Brillouin scattering inPOFs owing to its simplicity and low cost.

2.6

3.0
2.8
frequency, GHz

3.2

Fig. 3 Measured BGS with and without three kinds of MMFs

Conclusions: A simple cost-effective method has been demonstrated toenhance the Brillouin scattering signal in the GI-POF by inserting an
MMF between the GI-POF and the silica SMF. First, we have theoretically shown that the MMF for this purpose should have a proper
MFD of the fundamental mode to improve the optical coupling loss

1 Agrawal, G.P.: ‘Nonlinear ﬁber optics’ (Academic Press, California,1995)
2 Liehr, S., Lenke, P., Wendt, M., Krebber, K., Seeger, M., Thiele, E.,
Metschies, H., Gebreselassie, B., and Munich, J.C.: ‘Polymer opticalﬁber sensors for distributed strain measurement and application in
structural health monitoring’, IEEE Sens. J., 2009, 9, (11), pp. 1330–1338
3 Mizuno, Y., and Nakamura, K.: ‘Experimental study of Brillouinscattering in perﬂuorinated polymer optical ﬁber at telecommunication
wavelength’, Appl. Phys. Lett., 2010, 97, (2), p. 021103
4 Mizuno, Y., and Nakamura, K.: ‘Potential of Brillouin scattering inpolymer optical ﬁber for strain-insensitive high-accuracy temperaturesensing’, Opt. Lett., 2010, 35, (23), pp. 3985– 3987
5 Mizuno, Y., Kishi, M., Hotate, K., Ishigure, T., and Nakamura, K.:‘Observation of stimulated Brillouin scattering in polymer optical ﬁber
with pump-probe technique’, Opt. Lett., 2011, 36, (12), pp. 2378– 2380
6 Mizuno, Y., and Nakamura, K.: ‘Enhancement of Brillouin scattering
signal in optical ﬁbers by use of pulsed pump light’, Appl. Phys.
Express, 2012, 5, (3), p. 032501
7 Sim, D.H., Takushima, Y., and Chung, Y.C.: ‘High-speed multimodeﬁber transmission by using mode-ﬁeld matched center-launchingtechnique’, J. Lightwave Technol., 2009, 27, (8), pp. 1018–1026
8 Marcuse, D.: ‘Loss analysis of single-mode ﬁber splices’, Bell Syst. Tech.
J., 1977, 56, (5), pp. 703–718

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