Y. Mizuno, N. Hayashi and K. Nakamura
In general, the state of polarisation (SOP) should be adjusted to optimise the signal-to-noise ratio of Brillouin measurement based on
self-heterodyne detection. Although Brillouin Stokes power in silica
ﬁbres is known to be enhanced by optimising the SOP, the role ofSOP optimisation in polymer optical ﬁbres (POFs) is quite different
from that in silica ﬁbres. In this reported work, it is clariﬁed that therole of SOP optimisation in POFs lies not in enhancing the Brillouin
Stokes power but in suppressing the tail of the Rayleigh-scattered
light spectrum.

Introduction: Brillouin scattering in optical ﬁbres [1] has been applied
to a number of devices, such as lasers, microwave signal processors,
slow light generators, optical storages and strain/temperature sensors[2–4]. To improve the performance of these devices, Brillouin scattering
properties have been investigated not only in glass optical ﬁbres (including silica ﬁbres, tellurite ﬁbres, chalcogenide ﬁbres, photonic crystal
ﬁbres and rare-earth-doped ﬁbres) but also in polymer optical ﬁbres(POFs) (including perﬂuorinated graded-index (PFGI-) POFs [5, 6]
and polymethyl methacrylate (PMMA)-based step-index (SI-) POFs).
In general, self-heterodyne detection is employed to obtain Brillouingain spectra (BGS) with a high-frequency resolution [3, 5], where therelative state of polarisation (SOP) between the Stokes light and thereference light needs to be adjusted. In some applications (forexample, sensing), the measurement stability is enhanced by making
the BGS polarisation-independent based on adaptive polarisation
control [7], polarisation diversity [8], Faraday rotation [9] and polarisation scrambling [10]. However, in evaluating the Brillouin properties,
the signal-to-noise ratio (SNR) of the BGS measurement is often maximised using polarisation controllers (PCs) [5, 6]. Although it is wellknown that the role of SOP optimisation in silica ﬁbres is to simply
maximise the Brillouin Stokes power [1], the role of SOP optimisationin POFs has not been clariﬁed yet. In this Letter, we experimentallyshow that the role of SOP optimisation in POFs consists in suppressingthe tail of Rayleigh-scattered light spectrum, not in maximising theStokes power itself.

Fig. 1 shows the measured BGS in the silica SMF with optimised,anti-optimised and scrambled SOPs when the pump power was ﬁxed
at 18.7 dBm. The BFS was 10.89 GHz. As the SOP changed, theStokes power also changed drastically (by over 20 dB), but the noise
ﬂoor remained almost the same. This behaviour agrees well with thetheory [1]. Fig. 2 shows the measured BGS in the PFGI-POF with optimised, anti-optimised and scrambled SOPs. The pump power was also
18.7 dBm. The BFS was 2.77 GHz. The Rayleigh power was so muchhigher than the Stokes power that even its tail had clear inﬂuence on
the Stokes signal. When the SOP was changed, the corresponding
change was by far larger in the Rayleigh tail power (∼10 dB) than in
the Brillouin Stokes power (<2 dB). In consequence, the highest SNRof the BGS detection was obtained when the SOP was adjusted sothat the Rayleigh tail power was suppressed as much as possible; theSNR was lowest when the Rayleigh tail power was highest. Weshould also note that the apparent Stokes power was negatively correlated with the SNR, which goes against the conventional concept ofSOP adjustment in silica ﬁbres [1].
–20
silica SMF

pol. scrambled–40

Brillouin min.

–60
Ppump = 18.7 dBM

–80
10.80

Experiments: The ﬁbres under test (FUTs) employed in the experimentwere: 1. a 1 km-long silica SMF with core diameter of 9 μm, core refractive index of ∼1.47, numerical aperture (NA) of 0.13, and the propagation loss at 1.55 μm of ∼0.5 dB/km, and 2. a 1-km-long PFGI-POF
with core diameter of 50 μm, core refractive index of ∼1.35, NA of0.185, and the loss at 1.55 μm of ∼250 dB/km. The experimental
setup based on self-heterodyne detection was similar to that previously
reported in [5]. A distributed-feedback laser diode (DFB-LD) at 1547
nm was used as a light source. Index-matching oil was placed on one
end of the silica SMF to suppress the Fresnel reﬂection. One end of
the PFGI-POF was optically butt-coupled to port 2 of a circulator composed of a silica SMF, and the other end was kept open (note that theFresnel reﬂection need not be taken into consideration [6]). A polarisation scrambler with 700 kHz modulation frequency was inserted inthe pump path after an erbium-doped ﬁbre ampliﬁer (EDFA), and two
PCs were inserted in the pump and the reference paths.

10.85

10.95

10.90

frequency, GHzFig. 1 Measured BGS in silica SMF with SOP optimised, anti-optimised andscrambled

–50

Ppump

PFGI-POF= 18.7 dBM

L = 1 KM

–54

Rayleigh max.
power, dBM

Theory: In silica single-mode ﬁbres (SMFs), the Brillouin frequency
shift (BFS) is known to be approximately 10.9 GHz at 1.55 μm wavelength, and their Brillouin threshold power is as low as <1 mW (forkm-order-long SMFs) [1], leading to high Brillouin Stokes power.
Thus, the tail of the Rayleigh-scattered light spectrum has hardly any
inﬂuence on the Stokes signal, which can be directly optimised byadjusting the SOP. In contrast, in PFGI-POFs, the BFS is reported tobe as low as approximately 2.8 GHz at 1.55 μm [5], and theirBrillouin threshold power is as high as ∼13 W (for PFGI-POFs longer
than ∼50 m) [6]. Consequently, the Stokes signal is easily overlappedor buried by the Rayleigh tail, the power of which drastically changes
according to the SOP. As a result, in PFGI-POFs, the maximum SNR
is achieved not by maximising the Stokes power but by minimising
the Rayleigh tail power using PCs.

Brillouin max.

L = 1 KMpower, dBM

Polarisation state optimisation in observing
Brillouin scattering signal in polymer opticalfibres

–58

–62

pol.
scrambled–66
Rayleigh min.
–7001

23frequency, GHzFig. 2 Measured BGS in PFGI-POF with SOP optimised, anti-optimised andscrambled

It has been reported that the Brillouin Stokes power in POFs is, compared with those in glass ﬁbres, extremely low because of their multimode nature (or large core diameter) and high Brillouin threshold
power (or high propagation loss) [5, 6]. The fact that the Stokes
power itself cannot be enhanced by SOP adjustment seems to beanother reason for their small Brillouin signal. The SNR deterioration
by polarisation scrambling is also a big issue for applications that
require measurement stability [2–4]. Though some methods toenhance the Brillouin signal in POFs have been developed up to now[11], further improvement is needed, for example, by using a lower
wavelength at which the propagation loss is much lower [6].
Conclusion: We have experimentally shown that, in order to maximise
the SNR of the BGS measurement in POFs by SOP optimisation, its roleis not to enhance the Stokes power itself but to suppress the tail of theRayleigh-scattered light spectrum. We believe that this result is extremely important as technical knowledge in further investigating theELECTRONICS LETTERS 3rd January 2013 Vol. 49 No. 1

Brillouin properties in POFs and in developing relevant devices andsystems in the 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 AndoLaboratory.
© The Institution of Engineering and Technology 2013
7 November 2012
doi: 10.1049/el.2012.3918
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
1 Agrawal, G.P.: ‘Nonlinear ﬁber optics’ (Academic Press, USA, 1995)
2 Horiguchi, T., and Tateda, M.: ‘BOTDA – nondestructive measurementof single-mode optical ﬁber attenuation characteristics using Brillouininteraction: theory’, J. Lightwave Technol., 1989, 7, (8), pp. 1170–1176

3 Mizuno, Y., Zou, W., He, Z., and Hotate, K.: ‘Proposal of Brillouin
optical correlation-domain reﬂectometry (BOCDR)’, Opt. Express,2008, 16, (16), pp. 12148–12153
4 Song, K.Y., He, Z., and Hotate, K.: ‘Distributed strain measurementwith millimeter-order spatial resolution based on Brillouin optical correlation domain analysis’, Opt. Lett., 2006, 31, (17), pp. 2526–2528
5 Mizuno, Y., and Nakamura, K.: ‘Experimental study of Brillouin scattering in perﬂuorinated polymer optical ﬁber at telecommunication
wavelength’, Appl. Phys. Lett., 2010, 97, (2), p. 021103
6 Mizuno, Y., Ishigure, T., and Nakamura, K.: ‘Brillouin gain spectrum
characterization in perﬂuorinated graded-index polymer optical ﬁber
with 62.5-μm core diameter’, IEEE Photonics Technol. Lett., 2011,23, (24), pp. 1863–1865
7 Kersey, A.D., Marrone, M.J., Dandridge, A., and Tveten, A.B.:‘Optimization and stabilization of visibility in interferometric ﬁberoptic sensors using input-polarization control’, J. Lightwave Technol.,
1988, 6, (10), pp. 1599–1609
8 Uchiyama, K., Kawanishi, S., Takara, H., Morioka, T., and Saruwatari,M.: ‘100 Gbit/s to 6.3 Gbit/s demultiplexing experiment using
polarisation-independent nonlinear optical loop mirror’, Electron.
Lett., 1994, 30, (11), pp. 873–875
9 Calvani, R., Cisternino, F., Girardi, R., and Riccardi, E.: ‘Polarisation
independent all-optical demultiplexing using four wave mixing in dispersion shifted ﬁbre’, Electron. Lett., 1999, 35, (1), pp. 72–73
10 Noe, R., Rodler, H., and Ebberg, A.et al: ‘Comparison of polarization
handling methods in coherent optical systems’, J. Lightwave Technol.,
1991, 9, (10), pp. 1353–1366
11 Mizuno, Y., Hayashi, N., and Nakamura, K.: ‘Simple coupling method
for enhancing Brillouin scattering signal in polymer optical ﬁbres’,
Electron. Lett., 2012, 48, (20), pp. 1300–1301

ELECTRONICS LETTERS 3rd January 2013 Vol. 49 No. 1