Appl. Sci. 2012, 2, 46-60; doi:10.3390/app2010046
OPEN ACCESS

applied sciences
ISSN 2076-3417
www.mdpi.com/journal/applsci
Review

Enhancement of Brillouin Scattering Signal in PerfluorinatedGraded-Index Polymer Optical Fibers
Yosuke Mizuno * and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan; E-Mail: knakamur@sonic.pi.titech.ac.jp* Author to whom correspondence should be addressed; E-Mail: ymizuno@sonic.pi.titech.ac.jp;
Tel.: +81-45-924-5052; Fax: +81-45-924-5091.
Received: 26 December 2011; in revised form: 18 January 2012 / Accepted: 20 January 2012 /Published: 31 January 2012

Abstract: Perfluorinated graded-index polymer optical fibers (PFGI-POFs), fabricated byreplacing the hydrogen atoms of standard polymethyl methacrylate-based POFs withfluorine atoms, have been extensively studied due to their relatively low propagation loss
even at telecommunication wavelength. Recently, Brillouin scattering, which is one of themost significant nonlinear effects in optical fibers, has been successfully observed in
PFGI-POFs at 1.55-μm wavelength. The Brillouin Stokes signal was, however, not large
enough for practical applications or for detailed investigations of the Brillouin properties. In
this paper, we review our recent work on Stokes signal enhancement. First, we induce
stimulated Brillouin scattering based on the so-called pump-probe technique, and discuss its
applicability to temperature sensors. Then, we investigate the influence of the core diameterand length of PFGI-POFs on Stokes signal, and observe the Brillouin linewidth narrowing
effect. We believe our work is an important technological step toward the implementation ofpractical Brillouin-based devices and systems including distributed strain and temperature
sensors.
Keywords: Brillouin scattering; Brillouin gain spectrum; perfluorinated graded-index
polymer optical fiber; pump-probe technique; fiber-optic sensing; distributed temperaturesensing; nonlinear optics

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1. Introduction
Polymer optical fibers (POFs) [1,2] have attracted considerable attention for the past several decades
due to their extremely easy and cost-effective connection, high safety, and high flexibility [3] compared
to conventional glass fibers. Therefore, in spite of their propagation loss being higher than that of silica
glass fibers, POFs have been made use of in medium-range communication applications such as home
networks and automobiles [4] as well as in high-strain monitoring applications [3,5].
On the other hand, Brillouin scattering in optical fibers [6,7], one of the most significant nonlinear
processes, has been extensively studied, and a great number of useful applications have been reported sofar, such as optical amplification [7], lasing [7,8], optical comb generation [8], microwave signal
processing [9], slow light generation [10], phase conjugation [11], tunable delay [12], opticalstorage [13], core alignment of butt-coupling [14], and strain/temperature sensing [15–18]. For the
purpose of improving the performance of these applications, Brillouin scattering has been investigated
not only in silica single-mode fibers (SMFs) but also in some specialty fibers. They include, for example,
silica multimode fibers (MMFs) [14], tellurite glass fibers [19,20], chalcogenide glass fibers [21,22],
bismuth-oxide glass fibers [23,24], and photonic crystal fibers (PCFs) [25,26].
Recently, we have succeeded in observing spontaneous Brillouin scattering (SpBS) in POFs, for the
first time to the best of our knowledge, at the telecommunication wavelength of 1.55 μm [27,28], which
will offer many advantages of POFs to the conventional Brillouin application fields. The POFs used in
the experiment were cyclic transparent optical polymer (CYTOP)-based perfluorinatedgraded-index (PFGI-) POFs, which are fabricated by replacing the hydrogen atoms of standard
polymethyl methacrylate (PMMA)-based POFs with fluorine atoms. Their detailed structure andfabrication method are well summarized in [29]. Their Brillouin gain coefficient was estimated to beapproximately 3.09 × 10−11 m/W, which was almost the same as that of silica SMFs, indicating that
Brillouin scattering in PFGI-POFs can be applied to a variety of practical devices and systems in thesame way as Brillouin scattering in silica SMFs. We have also investigated the dependences of theBrillouin frequency shift (BFS) on strain and temperature in a PFGI-POF at 1.55 μm, and found that
SpBS in PFGI-POFs can be potentially utilized to develop high-accuracy temperature sensors with
reduced strain sensitivity [30]. Small Brillouin Stokes signal was, however, a major problem to besolved for practical future applications as well as for detailed investigations of the Brillouin gain
spectrum (BGS).
In this review, we summarize two methods to enhance the Brillouin Stokes signal. First, stimulated
Brillouin scattering (SBS) is induced using the so-called pump-probe technique [31], and its
applicability to temperature sensors is discussed. Second, the influence of the core diameter and lengthof the PFGI-POFs is investigated [32], which will be a good guideline for designing PFGI-POF
structures suitable for future applications.
2. Brillouin Scattering in Optical Fibers
When a light beam is injected into a fiber under test (FUT), it interacts with acoustic phonons,
generating backscattered light called Stokes light. This phenomenon is called SpBS. Since the phonons
decay exponentially, the backscattered Brillouin light spectrum, known as BGS, takes the shape ofAppl. Sci. 2012, 2

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Lorentzian function with the bandwidth of several tens of MHz. If the power of the pump light is higher
than the Brillouin threshold power Pth, the Stokes light caused by SpBS acts as a seed of stimulated
scattering, and there occurs a transition from SpBS to SBS. As a result, the power of the Stokes light isdrastically enhanced. Pth is given as [7]Pth 

21 b Aeff
K g B Leff(1)

where b is the correction factor for multimode fibers [33], which can be treated as 2 when the numericalaperture (NA) is ~0.2; Aeff is the effective cross-sectional area, which is approximately in proportion to
the core diameter for multimode fibers [34,35]; K is the polarization coefficient, which is 1, if
polarization is maintained and 0.667 otherwise; gB is the Brillouin gain coefficient; and Leff is theeffective fiber length defined as

Leff  1  exp   L  (2)where α is the propagation loss and L is the fiber length.
Both in SpBS and in SBS, the frequency where the peak power is obtained in the BGS is
downshifted by several GHz from the incident light frequency, and the amount of this frequency shift iscalled BFS. In optical fibers, the BFS νB is given by [7]B 

2n v Ap(3)

where n is the refractive index, νA is the acoustic velocity in the fiber, λp is the wavelength of theincident pump light. If tensile strain is applied or temperature is changed in a standard silica SMF, theBFS moves to higher frequency in proportion to the applied strain (+580 MHz/%) [36] and thetemperature change (+1.18 MHz/K) [37]. In some specialty fibers, such as tellurite glass fibers, it is
known that the BFS moves to lower frequency with increasing applied strain (−230 MHz/%) [20] andtemperature (−1.14 MHz/K) [24]. In both cases, we can derive the strain amplitude and temperaturechange by measuring the BFS in the fiber.
3. Spontaneous Brillouin Scattering in PFGI-POFs
A standard POF composed of PMMA [1,2] is optimally designed for light transmission at 650 nm,
with propagation loss of ~200 dB/km. Its loss at telecommunication wavelength is, however, so high(>> 1 × 105 dB/km) that the Brillouin signal cannot be detected. In the meantime, in order to observe
Brillouin scattering in a PMMA-based POF at 650 nm, we are required to prepare all the necessary
optical devices at this wavelength, which are not easy from the viewpoint of availability and cost.
Therefore, instead of PMMA-based POFs, we used PFGI-POFs with lower loss (~250 dB/km) even at
1.55 μm, which is the optimal wavelength for long distance data transmission in silica optical fibers.
Up to now, the spontaneous Brillouin scattering properties in a PFGI-POF with 120-μm core
diameter have been investigated at 1.55 μm [27]. The BFS and the Brillouin linewidth were 2.83 GHz
and 105 MHz, respectively. Using these values, the Brillouin gain coefficient was calculated to be
3.09 × 10−11 m/W, which is almost the same as that of standard silica SMFs and higher than that of silica

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GI-MMFs. Here, we should bear in mind that the actual Brillouin gain coefficient may be higher than
this value due to the multimode nature of the PFGI-POF. The Brillouin threshold power of a
100-m PFGI-POF was estimated to be as high as 24 W, and the Stokes signal was not sufficiently large
to be applied to practical devices and systems.
The BFS dependences on strain and temperature in a 5-m PFGI-POF have also been investigated at
1.55 μm [30]. They showed negative dependences with coefficients of −121.8 MHz/% and−4.09 MHz/K, respectively, which are −0.2 and −3.5 times as large as those in silica fibers. These BFS
dependences were found to originate from the dependences of the Young’s modulus on strain andtemperature. Thus, Brillouin scattering in PFGI-POFs has a big potential for high-accuracy temperaturesensing with reduced strain sensitivity.
4. Enhancement of Brillouin Stokes Signal (I): Induction of Stimulated Scattering
In this Section, we describe the observation of SBS in a PFGI-POF with 120-μm core diameter at
1.55 μm with pump-probe technique [31]. The BGS is detected with extremely high S/N ratio, evenwith a 1-m PFGI-POF, scrambled polarization state, and no averaging. We also investigate the BGS
dependences on probe power and temperature, and confirm that SBS in a PFGI-POF measured with thistechnique can be utilized to develop high-accuracy temperature sensors as well.
4.1. Motivation and Principle
The Brillouin scattering in PFGI-POFs observed in the previous experimental setup [27,28] was notstimulated but spontaneous, because the Brillouin threshold of PFGI-POFs was estimated to be as highas 24 W, as already described in Section 3. Consequently, the power of the reflected Stokes light was so
low that we had to face the following four problems: (1) a PFGI-POF longer than several meters was
required, (2) the polarization state had to be optimized, (3) averaging of the spectral data had to be
conducted several tens of times, and (4) signal-to-noise (S/N) ratio of the BGS was quite low, evenwhen (1), (2), and (3) were cleared. In order to implement practical Brillouin sensors and other systems
using PFGI-POFs, these problems need to be resolved.
One solution is to employ so-called pump-probe technique. As described in Section 2, when thepump power is higher than the Brillouin threshold, a transition from SpBS to SBS occurs, leading todrastic enhancement of the Stokes light. On the other hand, when probe light at the same frequency as
the Stokes light is also injected into the other end of the FUT, SBS is induced even when the power ofthe pump light is much lower than the Brillouin threshold, because the probe light itself acts as a seed ofstimulated scattering [38]. This technique, called pump-probe technique, has been used to develop
Brillouin systems with high S/N ratio [17].
4.2. Experimental Setup
The FUT used in the experiment was a 1-m PFGI-POF (Asahi Glass Co., Ltd., 2009) with a core
diameter of 120 μm, NA of 0.185, a core refractive index of ~1.35, and propagation loss of~250 dB/km at 1.55 μm (manufacturer’s specifications). The experimental setup is schematically shownin Figure 1, which is similar to that of Brillouin optical correlation-domain analysis (BOCDA) [16,17].

Appl. Sci. 2012, 2

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The light beam from a 1550-nm three-electrode laser diode (LD) was divided into two. One was used as
the pump light, after being chopped with an intensity modulator (IM) for lock-in detection and being
amplified with an erbium-doped fiber amplifier (EDFA). The other was used as the probe light, after
passing two EDFAs, a single-sideband modulator (SSBM), and a polarization scrambler (PSCR). The
SSBM was employed with a microwave generator (MG) and a proper DC bias control to suppress thecarrier (pump) and the anti-Stokes component of the two first-order sidebands and to maintain a stablefrequency downshift from the pump light. This frequency downshift was swept from 2.5 GHz to
3.5 GHz with a period of 300 ms to obtain the BGS of the PFGI-POF, which is observed approximately
at 2.8 GHz. The suppression ratio of the other frequency components was kept higher than 25 dB, as
shown in Figure 2.
Figure 1. Experimental setup for observing stimulated Brillouin scattering (SBS) in
perfluorinated graded-index polymer optical fibers (PFGI-POFs) with pump-probe
technique: LD, laser diode; EDFA, erbium-doped fiber amplifier; FG, function generator;
FUT, fiber under test; IM, intensity modulator; ISO, isolator; LI-A, lock-in amplifier; MG,microwave generator; OSC, oscilloscope; PD, photo-detector; POF, polymer optical fiber;
PSCR: polarization scrambler; SSBM, single-sideband modulator; VOA, variable opticalattenuator.

Figure 2. Measured optical spectrum of the SSBM output when the frequency of the MG
was set to 2.83 GHz.

The PSCR, which can modulate the polarization state at 1 MHz, was inserted to suppress thepolarization-dependent fluctuations of the signal. The PFGI-POF and the silica SMFs were butt-coupled [27]with the gaps filled with index matching oil (n = 1.46) to minimize the Fresnel reflection. The Stokes

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light was adjusted in power with a variable optical attenuator (VOA), and converted to an electricalsignal with a 125-MHz photo-detector (PD). After passing a lock-in amplifier (LI-A) with a chopping
frequency of 5.018 MHz and a time constant of 10 ms, the electrical signal was observed as a BGS withan oscilloscope (OSC) synchronized with the frequency sweep of the SSBM.
4.3. Experimental Results
Figure 3 shows the measured BGS without averaging when the pump power and the probe power
were 23 dBm and 22 dBm, respectively. The power was normalized so that the peak power was 1.0.
Although the PFGI-POF length was only 1 m and the polarization state was scrambled, the BGS wasobserved with much higher S/N ratio than that previously reported [30]. The BFS was 2.86 GHz, whichis slightly higher than the previously reported value of 2.83 GHz [27]. This discrepancy seems tooriginate from the difference in temperature and the time constant of the LI-A which is not shortenough. The 3-dB bandwidth of the BGS measured in this experiment was about 160 MHz, but further
research is needed on the bandwidth because it is also dependent on the time constant of the LI-A (Notethat, when the time constant was shorter than 10 ms, the BGS was distorted).
Figure 3. BGS in PFGI-POF observed without averaging.

Figure 4 shows the dependence of the BGS on probe power when the pump power was fixed at
23 dBm. The probe power was reduced from 22 dBm down to 6 dBm, and averaging was conducted
30 times for the observable readout when the Stokes power was very low. As the probe power
decreased, the Stokes power also decreased, which proves that this BGS is caused by the interaction
between the pump light and the probe light, i.e., SBS.
Figure 4. Dependence of BGS on probe power in PFGI-POF.

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The dependence of the BGS on temperature was also measured as shown in Figure 5(a). The pump
power and the probe power were 23 dBm and 22 dBm, respectively, and averaging was conducted
30 times. The temperature was set to 20, 40, and 60 °C. With the increasing temperature, the BGS
shifted toward lower frequency. The temperature dependence of the BFS is shown in Figure 5(b). The
slope of −4.05 MHz/K is in good agreement with the previous report [30], which confirms that the BGS
in a PFGI-POF observed with the pump-probe technique can be applied to high-accuracy temperaturesensing.
Figure 5. Dependence of (a) BGS and (b) BFS on temperature in PFGI-POF.

(a)

(b)

4.4. Summary
The SBS in a PFGI-POF was successfully observed at 1.55 μm with pump-probe technique.
We detected the BGS with extremely high S/N ratio, even with a 1-m PFGI-POF, scrambled
polarization state, and no averaging. We also investigated the BGS dependence on probe power, which
proved that the measured BGS was caused not by SpBS but by SBS. Besides, we measured the BGS
dependence on temperature, showing that BGS in a PFGI-POF observed with this technique can beapplied to temperature sensing with high accuracy. We are sure that use of SBS with large Stokes signal
will be indispensable in developing Brillouin-based distributed temperature/strain sensing systems using
PFGI-POFs.
5. Enhancement of Brillouin Stokes Signal (II): Influence of Core Diameter and Fiber Length
In this Section, we describe the characterization on the BGS in PFGI-POFs with 62.5-μm core
diameter [32]. First, using 5-m PFGI-POFs, we show that extremely high Stokes power can be obtained
compared to that of a PFGI-POF with 120-μm core, and estimate the Brillouin threshold power to be
53.3 W. Then, we experimentally show that using a PFGI-POF longer than ~50 m is not an effective
way to enhance the Stokes signal. We also theoretically predict that it is difficult to decrease theBrillouin threshold power of PFGI-POFs at 1.55-μm 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.

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5.1. Motivation
As described in Section 3, the power of the SpBS Stokes light generated in the PFGI-POFs with
120-μm core diameter was quite low, and it needs to be enhanced for detailed investigations of the
BGS [39] including the linewidth narrowing effect. Since the BGS observed with the pump-probe
technique described in Section 4 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 makeuse of PFGI-POFs with core diameters smaller than 120 μm. Although PFGI-POFs with 62.5-μm core
diameter have become commercially available very recently (Sekisui Chemical Co., Ltd., 2011), no
study has been reported on their Brillouin scattering properties. Therefore, in this Section, wecharacterize the BGS in PFGI-POFs with 62.5-μm core diameter.
5.2. Experimental Setup
Irrespective of the length and the core diameter, PFGI-POFs used in the experiment had NA of 0.185,
a core refractive index of ~1.35, and propagation loss of ~250 dB/km at 1.55 μm. The experimental
setup was basically the same as that previously reported in [27], where the BGS can be observed withhigh resolution by heterodyne detection. A distributed-feedback laser diode (DFB-LD) at 1,547 nm wasused 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.
5.3. Effects of Small Core Diameters
Figure 6 shows the measured BGS of a 5-m PFGI-POF with 62.5-μm core at pump power Pin of 5,
10, 15, and 20 dBm. The polarization state optimized for Pin of 20 dBm was employed for all themeasurements. The center frequency of the BGS, i.e., the BFS, was approximately 2.81 GHz, which isslightly lower than previously-reported value of 2.83 GHz [27] due to the difference in room
temperature [30]. Even when Pin was as low as 5 dBm, small but clear BGS was observed.
Figure 6. BGS dependence on pump power Pin.

Figure 7 shows the Pin dependences of the relative Stokes power, when 5-m PFGI-POFs with core
diameters of 62.5 μm and 120 μm (Asahi Glass Co., Ltd., 2009) were employed. The reference power
was set to about −63 dBm, which is the Stokes power when Pin is sufficiently low. The dependenceAppl. Sci. 2012, 2

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curve of the PFGI-POF with 62.5-μm core was about 10 dB lower in pump power than that with
120-μm core, which indicates that, even at the same pump power, we can largely enhance the Stokes
signal by using a PFGI-POF with a smaller core diameter.
Figure 7. Relative Stokes power as a function of pump power; comparison between
PFGI-POFs with 62.5-μm and 120-μm core diameters.

One of the reasons for the 10-dB curve shift is the difference in Brillouin threshold power Pth. By
substituting into Equations (1) and (2) the values of b = 2 [33], K = 0.667 [7], gB = 3.09 × 10−11 m/W [27],α = 0.056 /m (= 250 dB/km), and L = 5 m, Pth of the PFGI-POF with 120-μm core (Aeff = 209 μm2[40]) was calculated to be 97.7 W. On the other hand, Pth of the PFGI-POF with 62.5-μm core
(Aeff = 108.9 μm2) was calculated to be 53.3 W, which is lower than 97.7 W by 2.63 dB. Thus, thecurve shift observed in Figure 7 can be partially explained by the difference in Pth, but its amount of
10 dB is much larger than the calculated value.
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 propagates back and is injected into the
SMF. Although, due to the unstable core alignment and the rough surface of the PFGI-POF, it isdifficult to measure the coupling loss accurately, we confirmed that the loss with the PFGI-POF with
62.5-μm core was several dB lower than that with 120-μm core. This fact, along with the difference ininternal structure designed by different manufacturers, moderately explains the 10-dB curve shift.
5.4. Effects of Long Fiber Length
According to Equations (1) and (2), to employ long PFGI-POFs is another way to reduce Pth and toenhance the Stokes signal. Figure 8 shows the measured BGS of 80-m and 200-m PFGI-POFs with
62.5-μm core at Pin of 5, 10, 15, and 20 dBm. Much larger Stokes signals (~7 dB higher at Pin of
20 dBm) than those of the 5-m PFGI-POF shown in Figure 6 were observed.
There was, however, almost no difference between the BGS of the 80-m PFGI-POF and that of the
200-m PFGI-POF, only with a 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 Leff was plotted as afunction of actual length L as shown in Figure 9, where Leff gradually approaches 18 m

Appl. Sci. 2012, 2

55(Pth ~ 13 W) with the increasing L. Thus, we proved that employing a PFGI-POF longer than ~50 m is
not an effective way to enhance the Stokes signal at 1.55 μm.
Figure 8. BGS dependence on pump power Pin; comparison between 80-m PFGI-POF(solid line) and 200-m PFGI-POF (dotted line).

Figure 9. Calculated effective fiber length vs. fiber length.

According to Equations (1) and (2), as the core diameter decreases, the Brillouin threshold Pth also
becomes lower. When Pin is higher than Pth, SBS is induced and consequently the Stokes signal isexponentially enhanced [7]. Here, under the rough assumption that the multimode nature, NA,
refractive index, and loss do not change with core diameters, we calculated Pth of a 50-m PFGI-POF
with 10-μm core diameter (hypothetical; Aeff = 17.4 μm2) to be 2.22 W (= 33.5 dBm). This value ismore than one order magnitude higher than the pump power of several tens to hundreds of mW
commonly used in BGS characterization in silica SMFs [39]. Even when the PFGI-POF is treated as an
SMF (i.e., b = 1 [7] but Aeff becomes larger [35] in Equation (1)), this difference cannot becompensated. Thus, it seems to be difficult to reduce Pth of PFGI-POFs down to the same level of thatof long silica SMFs 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 μm.

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5.5. Brillouin Linewidth Narrowing EffectAlthough the Brillouin linewidth of a PFGI-POF has been reported to be 105 MHz at Pin of 20 dBm
in [27], detailed investigations were difficult because the Stokes power was extremely low. Here, by
making use of the enhanced Stokes power, we investigated the Brillouin linewidth dependence on Pin.
Figure 10(a) shows the measured BGS of the 200-m PFGI-POF with 62.5-μm core at Pin of 14.5,
17.5, and 23.5 dBm, where the Stokes power is normalized so that the maximum power is 0 dB.
Figure 10(b) shows the Brillouin linewidth dependence on Pin. From these figures, we can see that, with
the increasing Pin, 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 insilica-based SMFs [39].
Figure 10. (a) Normalized BGS at pump power of 14.5, 17.5, and 23.5 dBm. (b) Brillouin
linewidth vs. pump power.

(a)

(b)

5.6. Summary
The BGS properties of PFGI-POFs with 62.5-μm core diameter were investigated. The Stokes
power was extremely high compared to that of a PFGI-POF with 120-μm core, and the Brillouinthreshold power for 5-m PFGI-POF was estimated to be 53.3 W. It was also shown that employing a
PFGI-POF longer than ~50 m is not an effective way to enhance the Stokes signal. In addition, it wastheoretically found that it is difficult to reduce the Brillouin threshold power of PFGI-POFs at 1.55-μm
wavelength below that of long silica SMFs even if their core diameter is sufficiently decreased to satisfy
the single-mode condition. Finally, the Brillouin linewidth narrowing effect was confirmed. These results
will be a good guideline for developing practical Brillouin systems using PFGI-POFs as well as fordesigning new PFGI-POF structures for Brillouin applications in future.
6. Conclusions
We reviewed two methods of enhancing the Brillouin Stokes signal in PFGI-POFs for future
practical applications as well as for detailed investigations of the BGS.

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In the first half of this paper, using an experimental setup similar to BOCDA system [16,17] based on
the pump-probe technique, SBS in a PFGI-POF with 120-μm core diameter was, for the first time,observed at 1.55 μm. The BGS was detected with extremely high S/N ratio, even with a 1-m
PFGI-POF, scrambled polarization state, and no averaging. We also investigated the BGS dependenceson probe power and temperature, and confirmed that SBS in a PFGI-POF measured with this technique
can also be utilized to develop high-accuracy temperature sensors.
In the latter half, the influence of the core diameter and fiber length on the Stokes signal wasinvestigated by use of PFGI-POFs with 62.5-μm core diameter. First, using 5-m PFGI-POFs, we
showed that extremely high Stokes power can be obtained compared to that of a PFGI-POF with
120-μm core, and estimated the Brillouin threshold power to be 53.3 W. Then, we experimentally
showed that using a PFGI-POF longer than ~50 m is not an effective way to enhance the Stokes signal.
We also theoretically predicted that it is difficult to decrease the Brillouin threshold power of
PFGI-POFs at 1.55-μm 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 investigated theBrillouin linewidth as a function of pump power, and confirmed the linewidth narrowing effect.
Thus, the Brillouin Stokes signal has been drastically enhanced. It is also natural to predict that the
Stokes signal will be further enhanced if we induce SBS by pump-probe technique using PFGI-POFs
with 62.5-μm core diameter or less. Therefore, we are sure that the first demonstration of distributed
temperature sensing based on Brillouin scattering in PFGI-POFs will be possibly carried out in the near
future, based on optical correlation-, time-, or frequency-domain technology. We believe our work to bea significant technological step toward the implementation of such sensing systems and other POFBrillouin devices.
Acknowledgements
We are indebted to Kazuo Hotate and Masato Kishi of the University of Tokyo, Japan, for allowing
us to use their experimental facilities. We also acknowledge Takaaki Ishigure of Keio University, Japan,
for his helpful comments and discussions. Y. Mizuno is grateful to the Research Fellowships for Young
Scientists from the Japan Society for the Promotion of Science (JSPS).
References
1.
2.
3.
4.

Kuzyk, M.G. Polymer Fiber Optics: Materials, Physics, and Applications, 1st ed.; CRC Press:
Boca Raton, FL, USA, 2006.
Koike, Y.; Ishigure, T.; Nihei, E. High-bandwidth graded-index polymer optical fiber.
J. Lightwave Technol. 1995, 13, 1475-1489.
Husdi, I.R.; Nakamura, K.; Ueha, S. Sensing characteristics of plastic optical fibres measured by
optical time-domain reflectometry. Meas. Sci. Technol. 2004, 15, 1553-1559.
Mollers, I.; Jager, D.; Gaudino, R.; Nocivelli, A.; Kragl, H.; Ziemann, O.; Weber, N.;
Koonen, T.; Lezzi, C.; Bluschke, A.; et al. Plastic optical fiber technology for reliable home
networking—Overview and results of the EU project POF-ALL. IEEE Commun. Mag. 2009, 47,
58-68.

Appl. Sci. 2012, 2
5.

58

Liehr, S.; Lenke, P.; Wendt, M.; Krebber, K.; Seeger, M.; Thiele, E.; Metschies, H.; Gebreselassie,
B.; Munich, J.C. Polymer optical fiber sensors for distributed strain measurement and application in
structural health monitoring. IEEE Sens. J. 2009, 9, 1330-1338.
6. Ippen, E.P.; Stolen, R.H. Stimulated Brillouin scattering in optical fibers. Appl. Phys. Lett. 1972,
21, 539-541.
7. Agrawal, G.P. Nonlinear Fiber Optics, 4th ed.; Academic Press: San Diego, CA, USA, 1995;
pp. 329-367.
8. Cowle, G.J.; Yu, D.; Chieng, Y.T. Brillouin/erbium fiber lasers. J. Lightwave Technol. 1997, 15,1198-1204.
9. Norcia, S.; Tonda-Goldstein, S.; Dolfi, D.; Huignard, J.P. Efficient single-mode Brillouin fiber laser
for low-noise optical carrier reduction of microwave signals. Opt. Lett. 2003, 28, 1888-1890.
10. Song, K.Y.; Herraez, M.G.; Thevenaz, L. Observation of pulse delaying and advancement in
optical fibers using stimulated Brillouin scattering. Opt. Express 2005, 13, 82-88.
11. Kuzin, E.A.; Petrov, M.P.; Davydenko, B.E. Phase conjugation in an optical fibre. Opt. Quantum
Electon. 1985, 17, 393-397.
12. Zou, W.; He, Z.; Hotate, K. Tunable fiber-optic delay line based on stimulated Brillouin scattering.
Appl. Phys. Express 2010, 3, doi:10.1143/APEX.3.012501.
13. Zhu, Z.; Gauthier, D.J.; Boyd, R.W. Stored light in an optical fiber via stimulated Brillouinscattering. Science 2007, 318, 1748-1750.
14. Mizuno, Y.; Nakamura, K. Core alignment of butt-coupling between single-mode and multi-mode
optical fibers by monitoring Brillouin scattering signal. J. Lightwave Technol. 2011, 29,
2616-2620.
15. Horiguchi, T.; Tateda, M. BOTDA—Nondestructive measurement of single-mode optical fiber
attenuation characteristics using Brillouin interaction: Theory. J. Lightwave Technol. 1989, 7,
1170-1176.
16. Hotate, K.; Hasegawa, T. Measurement of Brillouin gain spectrum distribution along an opticalfiber using a correlation-based technique—Proposal, experiment and simulation. IEICE Trans.
Electron. 2000, E83-C, 405-412.
17. Song, K.Y.; Hotate, K. Enlargement of measurement range in a Brillouin optical correlation
domain analysis system using double lock-in amplifiers and a single-sideband modulator.
IEEE Photon. Technol. Lett. 2006, 18, 499-501.
18. Mizuno, Y.; Zou, W.; He, Z.; Hotate, K. Proposal of Brillouin optical correlation-domainreflectometry (BOCDR). Opt. Express 2008, 16, 12148-12153.
19. Abedin, K.S. Stimulated Brillouin scattering in single-mode tellurite glass fiber. Opt. Express 2006,
14, 11766-11772.
20. Mizuno, Y.; He, Z.; Hotate, K. Distributed strain measurement using a tellurite glass fiber with
Brillouin optical correlation-domain reflectometry. Opt. Commun. 2010, 283, 2438-2441.
21. Abedin, K.S. Observation of strong stimulated Brillouin scattering in single-mode As2Se3
chalcogenide fiber. Opt. Express 2005, 13, 10266-10271.
22. Song, K.Y.; Abedin, K.S.; Hotate, K.; Herraez, M.G.; Thevenaz, L. Highly efficient Brillouin slow
and fast light using As2Se3 chalcogenide fiber. Opt. Express 2006, 14, 5860-5865.

Appl. Sci. 2012, 2

59

23. Lee, J.H.; Tanemura, T.; Kikuchi, K.; Nagashima, T.; Hasegawa, T.; Ohara, S.; Sugimoto, N.
Experimental comparison of a Kerr nonlinearity figure of merit including the stimulated Brillouinscattering threshold for state-of-the-art nonlinear optical fibers. Opt. Lett. 2005, 30, 1698-1700.
24. Mizuno, Y.; He, Z.; Hotate, K. Dependence of the Brillouin frequency shift on temperature in atellurite glass fiber and a bismuth-oxide highly-nonlinear fiber. Appl. Phys. Express 2009, 2,112402:1-112402:3.
25. Lee, J.H.; Yusoff, Z.; Belardi, W.; Ibsen, M.; Monro, T.M.; Richardson, D.J. Investigation of
Brillouin effects in small-core holey optical fiber: Lasing and scattering. Opt. Lett. 2002, 27,
927-929.
26. Beugnot, J.C.; Sylvestre, T.; Alasia, D.; Maillotte, H.; Laude, V.; Monteville, A.; Provino, L.;
Traynor, N.; Mafang, S.F.; Thevenaz, L. Complete experimental characterization of stimulated
Brillouin scattering in photonic crystal fiber. Opt. Express 2007, 15, 15517-15522.
27. Mizuno, Y.; Nakamura, K. Experimental study of Brillouin scattering in perfluorinated polymer
optical fiber at telecommunication wavelength. Appl. Phys. Lett. 2010, 97, doi:10.1063/1.3463038.
28. Mizuno, Y.; Nakamura, K. Brillouin scattering in polymer optical fibers: Fundamental properties
and potential use in sensors. Polymers 2011, 3, 886-898.
29. Lethien, C.; Loyez, C.; Vilcot, J.-P.; Rolland, N.; Rolland, P.A. Exploit the bandwidth capacities ofthe perfluorinated graded index polymer optical fiber for multi-services distribution. Polymers 2011,
3, 1006-1028.
30. Mizuno, Y.; Nakamura, K. Potential of Brillouin scattering in polymer optical fiber forstrain-insensitive high-accuracy temperature sensing. Opt. Lett. 2010, 35, 3985-3987.
31. Mizuno, Y.; Kishi, M.; Hotate, K.; Ishigure, T.; Nakamura, K. Observation of stimulated Brillouinscattering in polymer optical fiber with pump-probe technique. Opt. Lett. 2011, 36, 2378-2380.
32. Mizuno, Y.; Ishigure, T.; Nakamura, K. Brillouin gain spectrum characterization in perfluorinatedgraded-index polymer optical fiber with 62.5-μm core diameter. IEEE Photon. Technol. Lett. 2011,
23, 1863-1865.
33. Tei, K.; Tsuruoka, Y.; Uchiyama, T.; Fujioka, T. Critical power of stimulated Brillouin scatteringin multimode optical fibers. Jpn. J. Appl. Phys. 2001, 40, 3191-3194.
34. Marcuse, D. Loss analysis of single-mode fiber splices. Bell Sys. Tech. J. 1977, 56, 703–718.
35. Mocofanescu, A.; Wang, L.; Jain, R.; Shaw, K.D.; Gavrielides, A.; Peterson, P.; Sharma, M.P.
SBS threshold for single mode and multimode GRIN fibers in an all fiber configuration. Opt.
Express 2005, 13, 2019-2024.
36. Horiguchi, T.; Kurashima, T.; Tateda, M. Tensile strain dependence of Brillouin frequency shift insilica optical fibers. IEEE Photon. Technol. Lett. 1989, 1, 107-109.
37. Kurashima, T.; Horiguchi, T.; Tateda, M. Thermal effects on the Brillouin frequency shift injacketed optical silica fibers. Appl. Opt. 1990, 29, 2219-2222.
38. Shibata, N.; Azuma, Y.; Horiguchi, T.; Tateda, M. Identification of longitudinal acoustic mode
guided in the core region of a single-mode optical fiber by Brillouin gain spectra measurements.
Opt. Lett. 1988, 13, 595-597.
39. Yeniay, A.; Delavaux, J.M.; Toulouse, J. Spontaneous and stimulated Brillouin scattering gain
spectra in optical fibers. J. Lightwave Technol. 2002, 20, 1425-1432.

Appl. Sci. 2012, 2

60

40. Dossou, M.; Szriftgiser, P.; Goffin, A. Theoretical study of stimulated Brillouin scattering (SBS) in
polymer optical fibres. In 13th Annual Symposium IEEE/LEOS Benelux Chapter, Enschede, The
Netherlands, 2008; volume 13, 175-178.
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