IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 24, DECEMBER 15, 2011

1863Brillouin Gain Spectrum Characterization in
Perfluorinated Graded-Index Polymer Optical FiberWith 62.5- m Core Diameter
Yosuke Mizuno, Takaaki Ishigure, Member, IEEE, and Kentaro Nakamura, Member, IEEE

Abstract—We characterize the Brillouin gain spectra (BGS) in
perfluorinated graded-index polymer optical fibers (PFGI-POFs)
with 62.5- m core diameter. First, we show that the Stokes poweris extremely high compared to that of a standard PFGI-POF with120- m core, and discuss the Brillouin threshold power. Then, wedemonstrate that the PFGI-POF length has little influence on theBGS when the length is longer than 50 m. We also predict that, at
1.55- m wavelength, it is difficult to reduce the Brillouin threshold
power of PFGI-POFs below that of long silica single-mode fibers
even if their core diameter is sufficiently reduced to satisfy thesingle-mode condition. Finally, we confirm the Brillouin linewidth
narrowing effect.
Index Terms—Brillouin gain spectrum, Brillouin linewidth, Brillouin scattering, nonlinear optics, polymer optical fiber (POF).

I. INTRODUCTION

BRILLOUIN scattering in silica-based optical fibers has
attracted a great deal of attention [1], [2] and applied
to a number of systems, such as lasers [2], signal processors[3], slow light generators [4], and strain/temperature sensors[5]–[9]. These systems have been improved in performanceby exploiting Brillouin scattering in various specialty fibers,
including tellurite glass fibers [10], chalcogenide fibers [11],bismuth-oxide fibers [12], and photonic crystal fibers [13].
Recently, we have succeeded in observing spontaneous Brillouin scattering (SpBS) at 1.55 m in polymer optical fibers(POFs) [14], which offer extremely high flexibility, low-costand easy connection, and high safety compared to otherstandard glass fibers [15]. The POFs used in the experimentwere standard perfluorinated graded-index (PFGI-) POFs with
120- m core diameter. The estimated Brillouin gain coefficientof 3.09m W was almost the same as that of a silica
single-mode fiber (SMF), which indicates that Brillouin scattering in PFGI-POFs is applicable to various practical systems
in the same way as Brillouin scattering in silica fibers. We
Manuscript received May 30, 2011; revised August 08, 2011; accepted
September 23, 2011. Date of publication September 29, 2011; date of current
version November 23, 2011. The work of Y. Mizuno was supported by theResearch Fellowships for Young Scientists from the Japan Society for thePromotion of Science (JSPS).
Y. Mizuno and K. Nakamura are with 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).
T. Ishigure is with the Faculty of Science and Technology, Keio University,
Yokohama 223-8522, Japan (e-mail: ishigure@appi.keio.ac.jp).
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.2011.2170191

have also investigated the dependences of the Brillouin frequency shift (BFS) on strain and temperature in a PFGI-POF at
1.55 m, and found that SpBS in PFGI-POFs can be potentially
utilized to develop high-accuracy temperature sensors with low
strain sensitivity [16].
However, the power of the SpBS Stokes light generated in thePFGI-POFs with 120- m core diameter was quite low due totheir multimode nature, and it needs to be enhanced for detailed
investigations of the Brillouin gain spectra (BGS) [17]–[19] including the linewidth narrowing effect. One method is to induce stimulated Brillouin scattering (SBS) with low threshold
power using so-called pump-probe technique [18], where twolight beams are launched from both ends of the PFGI-POF.
We have so far obtained extremely large Stokes signal in thisscheme [20]. However, since the BGS observed with this technique is easily influenced by the time constant of lock-in detection, detailed evaluation of its linewidth was not feasible.
Another approach to enhance the Stokes signal is to make useof PFGI-POFs with core diameters smaller than 120 m. Although PFGI-POFs with 62.5- m core diameter have becomecommercially available very recently (product ID: ID062, Sekisui Chemical Co., Ltd., 2011), no study has been reported on
their Brillouin scattering properties.
In this letter, we characterize the BGS in PFGI-POFs with
62.5- m core diameter. First, using 5-m PFGI-POFs, we showthat extremely high Stokes power can be obtained compared tothat of a PFGI-POF with 120- m core, and estimate the Brillouin threshold power to be 53.3 W. Then, we experimentallyshow that using a PFGI-POF longer thanm is not aneffective way to enhance the Stokes signal. We also theoretically predict that it is difficult to decrease the Brillouin thresholdpower of PFGI-POFs at 1.55- m wavelength down to that of
km-order-long silica SMFs even when their core diameter is sufficiently reduced to satisfy the single-mode condition. Finally,
we investigate the Brillouin linewidth as a function of pump
power, and confirm the linewidth narrowing effect.
II. EXPERIMENTS AND DISCUSSION
A. Experimental Setup
Regardless of the length and the core diameter, PFGI-POFs
used in the experiment had a numerical aperture (NA) of 0.185,, and a propagation loss ofa core refractive index ofdB km at 1.55 m. The experimental setup was basicallythe same as that previously reported in [14], where the BGS
can be observed with high resolution by heterodyne detection.
A distributed-feedback laser diode (DFB-LD) at 1547 nm was

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO. 24, DECEMBER 15, 2011

Fig. 1. (a) BGS dependence on pump power. (b) Relative Stokes power
as a function of pump power; comparison between PFGI-POFs with 62.5- and120- m core diameters.

Fig. 2. (a) BGS dependence on pump power; comparison between 80-m
PFGI-POF (solid line) and 200-m PFGI-POF (dotted line). (b) Effective fiber
length versus fiber length.

used as a light source. One end of the PFGI-POF to be measured
was optically butt-coupled to the silica SMF, and the other end
was kept open.

Another reason for the curve shift is the improvement of optical coupling efficiency at the butt coupling when the Stokes
light generated in the PFGI-POF travels back and is injected
into the SMF. Due to the unstable core alignment and the roughsurface of the PFGI-POF, it is difficult to measure the coupling loss accurately. However, we confirmed that the loss with
the PFGI-POF with 62.5- m core was several dB lower thanthat with 120- m core. This fact, along with the difference ininternal structure designed by different manufacturers, moderately explains the 10-dB curve shift.

B. Effects of Small Core Diameter
Fig. 1(a) shows the measured BGS of a 5-m PFGI-POF with
62.5- m core at pump powerof 5, 10, 15, and 20 dBm. The
polarization state optimized forof 20 dBm was employed
for all the measurements. The center frequency of the BGS,i.e., the BFS, was approximately 2.81 GHz, which is slightly
lower than previously-reported value of 2.83 GHz [14] due tothe difference in room temperature [16]. Even when
wasas low as 5 dBm, small but clear BGS was observed. Fig. 1(b)shows thedependences of the relative Stokes power, when
5-m PFGI-POFs with core diameters of 62.5 m and 120 m(Product ID: Lucina “X”, Asahi Glass Co., Ltd., 2009) were employed. The reference power was set to aboutdBm, whichis the Stokes power whenis sufficiently low. The dependence curve of the PFGI-POF with 62.5- m core was about
10 dB lower in pump power than that with 120- m core, whichindicates that, even at the same pump power, we can largely enhance the Stokes signal by using a PFGI-POF with a smaller
core diameter.
One of the reasons for the 10-dB curve shift is the differencein Brillouin threshold power. In general,is given as [2](1)where is the correction factor for multimode fibers [21],is the polarization coefficient, andis the Brillouin gain coefficient.
is the effective cross-sectional area, which is approximately in proportion to the core diameter for multimodefibers [22], [23]; andis the effective fiber length definedas( , propagation loss; , fiberlength) [2]. Using the values of[21],[2],m W [14],m(dB km),andm,of the PFGI-POF with 120- m core (m [24]) was calculated to be 97.7 W. On the other hand,of the PFGI-POF with 62.5- m core (m )was calculated to be 53.3 W, which is lower than 97.7 W by
2.63 dB. Thus, the curve shift observed in Fig. 1(b) can be partially explained by the difference in, but its amount of 10 dB
is much larger than the calculated value.

C. Effects of Long Fiber Length
According to (1), the use of long PFGI-POFs is anotherway to reduceand to enhance the Stokes signal. Fig. 2(a)shows the measured BGS of 80-m and 200-m PFGI-POFs with
62.5- m core atof 5, 10, 15, and 20 dBm. Much larger
Stokes signals (dB higher atof 20 dBm) than those ofthe 5-m PFGI-POF shown in Fig. 1(a) were observed.
However, there was hardly any difference between the BGSof the 80-m PFGI-POF and that of the 200-m PFGI-POF, witha very slight discrepancy of the BFS caused by the room- temperature difference. This means that the incident light is considerably attenuated after propagation for 80 m in the PFGIPOF. In order to estimate this effect quantitatively, the effective
PFGI-POF length
was plotted as a function of actual length
as shown in Fig. 2(b), wheregradually approaches 18 m(W) with the increasing . Thus, we proved that employing a PFGI-POF longer thanm is not an effective way
to enhance the Stokes signal.
According to (1), as the core diameter decreases, the Brillouinthresholdalso becomes lower. Whenis higher than,
SBS is induced and consequently the Stokes signal is exponentially enhanced [2]. Here, under the rough assumption that themultimode nature, NA, refractive index, and loss do not changewith core diameters, we calculatedof a 50-m PFGI-POF
with 10- m core diameter (m ) to be 2.22 W(dBm). This value is more than one order magnitude
higher than the pump power of several tens to hundreds of mW
commonly used in BGS characterization in silica SMFs [17],[19]. Even when the PFGI-POF is treated as an SMF (i.e.,[2] but
becomes larger [23] in (1)), this difference cannot
be compensated. Thus, it seems to be difficult to reduceofPFGI-POFs down to the same level of that of long silica SMFs

MIZUNO et al.: BRILLOUIN GAIN SPECTRUM CHARACTERIZATION IN PFGI-POF WITH 62.5- m CORE DIAMETER

Fig. 3. (a) Normalized BGS at pump power of 14.5, 17.5, and 23.5 dBm.
(b) Brillouin linewidth versus pump power.

by decreasing the core diameter, which is due to the limited effective length of 18 m associated with the high propagation loss
at 1.55 m.
D. Brillouin Linewidth Narrowing Effect
The Brillouin linewidth of a PFGI-POF has been reported tobe 105 MHz atof 20 dBm in [14], but detailed investigation was difficult because the Stokes power was extremely low.
Here, by making use of the enhanced Stokes power, we investigated the Brillouin linewidth dependence on.
Fig. 3(a) shows the measured BGS of the 200-m PFGI-POF
with 62.5- m core atof 14.5, 17.5, and 23.5 dBm, where
the Stokes power is normalized so that the maximum power is
0 dB. Fig. 3(b) shows the Brillouin linewidth dependence on. From these figures, we can see that, with the increasing, the 3-dB linewidth of the BGS decreases, but that its slope
gradually becomes small. This behavior agrees well with the experiment and theory of the linewidth narrowing effect in silicabased SMFs [17].
III. CONCLUSION
The BGS properties of PFGI-POFs with 62.5- m core diameter were investigated. The Stokes power was extremely highcompared to that of a PFGI-POF with 120- m core, and the Brillouin threshold power for 5-m PFGI-POF was estimated to be
53.3 W. It was also shown that employing a PFGI-POF longer
thanm is not an effective way to enhance the Stokes signal.
In addition, it was theoretically found that it is difficult to reducethe Brillouin threshold power of PFGI-POFs at 1.55- m wavelength below that of long silica SMFs even if their core diameteris sufficiently decreased to satisfy the single-mode condition. Finally, the Brillouin linewidth narrowing effect was confirmed.
We believe these results will be a good guideline for developing
practical Brillouin systems using PFGI-POFs as well as for designing new PFGI-POF structures for Brillouin applications infuture.
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