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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 17, SEPTEMBER 1, 2012

Characterization of Brillouin Gain Spectrain Polymer Optical Fibers Fabricated byDifferent Manufacturers at 1.32 and 1.55 μm
Yosuke Mizuno, Philipp Lenke, Katerina Krebber, and Kentaro Nakamura, Member, IEEE

Abstract— We characterize the Brillouin gain spectra (BGS)in perfluorinated graded-index polymer optical fibers (PFGIPOFs) at 1.32 µm and 1.55 µm. Three kinds of PFGI-POFs withthe same core diameter, but which are fabricated by different
manufacturers, are tested. For all the PFGI-POFs, the Stokes
power measured at 1.32 µm is higher than that at 1.55 µm dueto the lower propagation loss, but significant differences in Stokes
power are observed among the three. Based on the measurement
obtained by the optical time-domain reflectometry, we show thatthe actual propagation loss of the PFGI-POFs plays a crucialrole in observing BGS.
Index Terms— Brillouin gain spectrum (BGS), Brillouinscattering, nonlinear optics, polymer optical fiber (POF).

I. I NTRODUCTIONB

RILLOUIN scattering is one of the most significant
nonlinear processes in optical fibers, and has been thefocus of many theoretical and experimental investigations
for several decades [1], [2]. So far, a number of applications of Brillouin scattering have been reported, such as
lasing [2], signal processing [3], slow light generation [4],optical storage [5], core alignment [6], and strain/temperaturesensing [7]–[10]. In order to improve their performance, notonly Brillouin scattering in silica single-mode fibers (SMFs)
but also that in various kinds of specialty fibers has been
extensively studied. They include silica multimode fibers [11],tellurite glass fibers [12], chalcogenide glass fibers [13],bismuth-oxide glass fibers [14], and photonic crystal
fibers [15]. Recently, we successfully observed Brillouin scattering at 1.55 μm in polymer optical fibers (POFs) [16],which offer extremely high flexibility, low-cost and easy
connection, and high safety compared to other glass fibers. The
POFs used in the experiment were perfluorinated graded-index(PFGI-) POFs based on cyclic transparent optical polymer
Manuscript received March 5, 2012; revised May 24, 2012; accepted
June 21, 2012. Date of publication June 29, 2012; date of current version
July 31, 2012. The work of Y. Mizuno was supported in part by the Research
Fellowships for Young Scientists from the Japan Society for the Promotionof Science.
Y. Mizuno and K. Nakamura are with the Precision and Intelligence
Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan(e-mail: ymizuno@sonic.pi.titech.ac.jp; knakamur@sonic.pi.titech.ac.jp).
P. Lenke and K. Krebber are with the BAM Federal Institute forMaterials Research and Testing, Berlin D-12205, Germany (e-mail:philipp.lenke@bam.de; katerina.krebber@bam.de).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2012.2206803(CYTOP). Their Brillouin gain coefficient was estimated tobe approximately 3.09 × 10−11 m/W, which was almost
the same as that of silica SMFs. We also investigated thedependences of the Brillouin frequency shift (BFS) on strain
and temperature in a PFGI-POF at 1.55 μm, and found that
Brillouin scattering in PFGI-POFs can be potentially utilized
to develop high-accuracy temperature sensors with reduced
strain sensitivity [17].
In general, Brillouin scattering properties vary dependingon the structure especially of the fiber core, including
materials, diameter, and fabrication method. Up to now, BFS
in polymethyl methacrylate (PMMA)-based POFs and its
strain and temperature dependences have been estimated andcompared with those of PFGI-POFs [18], [19], which partiallyrevealed the effect of employing different core materials.
The influence of the core diameter on BGS in PFGI-POFs
has also been investigated [20]. However, no study has beenreported on how BGS in PFGI-POFs is influenced by theirfabrication method, which is of great importance in selecting
optimal POFs for future Brillouin applications.
In this letter, the BGS in PFGI-POFs fabricated with different methods is investigated. Three kinds of PFGI-POFs withthe same core diameter provided by different manufacturers
are tested at 1.32 and 1.55 μm. For all the PFGI-POFs, theStokes power measured at 1.32 μm is higher than that at1.55 μm, but large difference in Stokes power is observed
among the three PFGI-POFs. Based on the measurement by
optical time-domain reflectometry (OTDR), we conclude thatthe actual propagation loss of the PFGI-POFs plays a vitalrole in BGS observation.
II. P RINCIPLE
Spontaneous Brillouin scattering (SpBS) occurs when pumplight is Bragg-reflected by the refractive-index modulations
generated by acoustic phonons. The backscattered Stokes lightsuffers a Doppler shift called BFS, ν B , which is given by [2]2 n eff v A
νB =
(1)
λp
where n eff is the effective core refractive index, v A is theacoustic velocity, and λ p is the wavelength of the pump light.
When the power of the pump light is higher than Brillouinthreshold power Pth , stimulated Brillouin scattering (SBS) is
induced, leading to the drastic enhancement in Stokes power.
The lower Pth is, the higher the Stokes power at the same1041–1135/$31.00 © 2012 IEEEMIZUNO et al.: CHARACTERIZATION OF BGS IN POLYMER OPTICAL FIBERS

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TABLE IP HYSICAL P ROPERTIES OF THE PFGI-POFs. L , F IBER L ENGTH ;d , C ORE D IAMETER ; n eff , E FFECTIVE C ORE R EFRACTIVE I NDEX ;NA, N UMERICAL A PERTURE ; α, N OMINAL P ROPAGATION L OSS
D EDUCED FROM ATTENUATION S PECTRUM ; L eff , C ALCULATEDE FFECTIVE F IBER L ENGTH
Fiber

A

B

C

Manufacturer

Asahi glass

Sekisuichemical

Chromisfiberoptics

L (m)

103

200

200

d (μm)

62.5

62.5

62.5

n eff

∼1.35

∼1.35

∼1.35

NA

0.185

0.185

0.185

α (dB/m)gt 1.32 μm

0.025

0.025

0.025

α (dB/m)gt 1.55 μm

0.25

0.25

0.25

L eff (m)gt 1.32 μm

77.76

118.94118.94

L eff (m)gt 1.55 μm

17.82

17.87

17.87

Fig. 1. Experimental setup. DFB: distributed-feedback. EDFA: erbium-doped
fiber amplifier. ESA: electrical spectrum analyzer. FUT: fiber under test.
Nd:YAG: neodymium:yttrium aluminum garnet. PC: polarization controller.
PD: photo-detector. VOA: variable optical attenuator.

pump power is, and so Pth can be used as an indicator forroughly evaluating the Stokes power. Pth is given by [2]Pth =21 b Aeff
K g B L eff

with a photo-detector (PD), and amplified by 23 dB with
an electrical pre-amplifier. Finally, it was detected with anelectrical spectrum analyzer (ESA); and thus, the BGS canbe observed with high resolution.

(2)

where b is the correction factor for multimode fibers [21],Aeff the effective cross-sectional area, K the polarization
coefficient, gB the Brillouin gain coefficient, and L eff theeffective fiber length defined as L eff = [1−exp(−αL)]/α(α, propagation loss; L, fiber length) [2]. In evaluating the
Stokes power of the fibers having the same numerical aperture(NA) and core diameter, if we neglect the wavelength dependence on gB , L eff is the most dominant in Eq. (2), because b
and Aeff are basically functions only of NA [21] and of core
diameter [22], respectively.
III. E XPERIMENTAL S ETUP
Three kinds of PFGI-POFs, denoted by A, B, and C, wereused as the fibers under test (FUTs). Their physical properties
are summarized in Table I. Although they were fabricatedby different manufacturers, they had almost the same core
diameter d, effective core refractive index n eff , NA, andpropagation loss α both at 1.32 μm and at 1.55 μm (Notethat α was not actual loss but nominal loss deduced from theattenuation spectrum of CYTOP [23]). Due to the differencein fiber length L, the effective lengths L eff were also different.
The experimental setup is depicted in Fig. 1. As alight source, either a neodymium: yttrium aluminum garnet(Nd: YAG) laser at 1320 nm or a distributed-feedback (DFB)
laser at 1547 nm was employed. The output of the laser
was divided into two: pump light and reference light. The
pump light was injected into the FUT, one end of which
was polished and optically butt-coupled to the silica SMF.
The power of the pump light was adjusted to 20 dBm using,if necessary, an erbium-doped fiber amplifier (EDFA). The
Stokes light was mixed with the reference light, the power ofwhich was controlled with a variable optical attenuator (VOA).
The optical beat signal was converted to an electrical signal

IV. E XPERIMENTS
Figure 2 shows the measured BGS of the three FUTs at
1.32 and 1.55 μm. The pump power was 20 dBm, and thepolarization state was optimized for each measurement. At
1.55 μm, nearly identical BGS was observed for A and B,
but no BGS was obtained for C. The center frequency of
the BGS, i.e., the BFS, was ∼2.8 GHz, which agrees wellwith the previous reports at 1.55 μm [16], [17]. In contrast,
at 1.32 μm, the obtained Stokes power was much higher thanthat at 1.55 μm for all the three FUTs. Even for C, small but
clear BGS was observed. The BFS of ∼3.3 GHz is in good
agreement with the theoretical value of 3.28 GHz, which was
calculated using Eq. (1) under the assumption that n eff and v A
are independent of λ p . The Stokes power enhancement for all
the FUTs was mainly caused by the wavelength dependenceof their propagation loss [23], so we predict that, if pump lightat 1.10 or 1.20 μm (α ∼0.01 dB/m) is used, the Stokes power
will be further enhanced.
Another important feature in Fig. 2 is that, both at 1.32 μm
and at 1.55 μm, the measured Stokes power for A, B, andC was not consistent with L eff shown in Table I, which was
calculated using nominal loss α. For instance, though L eff isalmost the same for all the three FUTs at 1.55 μm, BGS was
not observed for C. Also, though L eff of A at 1.32 μm is
smaller than that of B, the measured Stokes power of A at
1.32 μm was a little higher than that of B. This inconsistency
seems to originate from the fact that the nominal loss α used
to calculate L eff does not necessarily agree with the actual
loss in the FUTs.
In order to clarify this point, Rayleigh-based photoncounting OTDR measurement was performed at 1.31 μm
for the three FUTs: A, B, and C. Measurement time was<3 minutes for each FUT. Figure 3 shows the results, which
confirm that the actual propagation loss, i.e., the slope of the1498

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 17, SEPTEMBER 1, 2012

R EFERENCES

Fig. 2.

Measured BGS for the FUT A, B, and C at 1.32 μm and 1.55 μm.

Fig. 3.

OTDR measurement results for the FUT A, B, and C at 1.31 μm.

curve, of C is over 3 times higher than those of A and B.
The actual loss seems to be nearly the same (∼α) for A andB, but B has a damaged point at ∼25-m position. These results
explain the inconsistency of the BGS measurement, showingthat undamaged PFGI-POFs with low actual propagation loss
(leading to low Pth ) should be employed to obtain as highStokes power as possible.
V. C ONCLUSION
We investigated the BGS in three PFGI-POFs fabricated bydifferent manufacturers at 1.32 and 1.55 μm. For all the PFGIPOFs, the Stokes power measured at 1.32 μm was higher thanthat at 1.55 μm due to the lower propagation loss, but striking
difference in Stokes power was observed among the three.
Then we performed OTDR measurement to clarify the reason
for the difference, and showed that the actual propagation lossof the PFGI-POFs plays an extremely important role in BGS
observation. We believe these results will be of great use in
selecting POFs suitable for Brillouin applications in future.

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