APPLIED PHYSICS LETTERS 97, 021103 共2010兲

Experimental study of Brillouin scattering in perfluorinated polymer opticalfiber at telecommunication wavelength
Yosuke Mizunoa兲 and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan

共Received 4 June 2010; accepted 20 June 2010; published online 13 July 2010兲
Brillouin scattering properties in a perfluorinated graded-index polymer optical fiber 共POF兲 with120 ␮m core diameter were experimentally investigated using a laser with an operating wavelengthof 1.55 ␮m. The Brillouin frequency shift and the Brillouin bandwidth were 2.83 GHz and 105
MHz, respectively. The calculated Brillouin gain coefficient of 3.09⫻ 10−11 m / W was comparable
to that of fused silica fibers. The Brillouin threshold power of the 100 m POF was estimated to be
24 W, which we believe can be reduced by employing POFs with smaller cores. © 2010 American
Institute of Physics. 关doi:10.1063/1.3463038兴
Polymer optical fibers 共POFs兲 共Refs. 1 and 2兲 offer extremely easy and low-cost connection compared to otherstandard glass fibers. They are also so flexible that strainof over 40% can be applied.3 Therefore, in spite of theirhigher loss than that of silica fibers, POFs have been usedboth in medium-range communication applications suchas home networks and automobiles,4 and in high-strain
monitoring applications.3,5 On the other hand, Brillouinscattering6,7 is one of the most important nonlinear effects in
optical fibers, and has been extensively studied. It has many
useful applications, such as optical amplification,7 lasing,7,8
optical comb generation,8 microwave signal processing,9slow light generation,10 phase conjugation,11 tunable delay
generation,12 and strain/temperature sensing.13–15 So far,
Brillouin scattering has been investigated not only for silica
fibers but also for some specialty fibers including tellurite
glass fibers,16,17 As2Se3 chalcogenide fibers,18,19 bismuthoxide fibers,20,21 and photonic crystal fibers.22,23 However,
the observation of Brillouin scattering in POFs has not beenreported yet, which is expected to add various advantages ofPOFs to the conventional Brillouin application field.
In this letter, we report on the observation of Brillouinscattering in a POF in the 1.55 ␮m wavelength region.
The Brillouin frequency shift 共BFS兲 of 2.83 GHz and theBrillouin linewidth of 105 MHz were measured. TheBrillouin gain coefficient gB of the POF was calculated to be
3.09⫻ 10−11 m / W, which is close to that of fused silica fibers. The Brillouin threshold power was estimated to be as
high as 24 W, which is in agreement with the theoreticalpredictions reported so far.
A standard POF based on polymethyl methacrylate
共PMMA兲 共Refs. 1 and 2兲 is optimized for transmission at650 nm, with a propagation loss of ⬃200 dB/ km. Its loss
at telecommunication wavelength, however, is so high共Ⰷ1 ⫻ 105 dB/ km兲 that the Brillouin signal cannot be detected. Meanwhile, in order to observe Brillouin scattering ina PMMA-based POF at 650 nm, we need to prepare all thenecessary optical devices at this wavelength, which are quitedifficult or expensive to prepare. Therefore, we employed
a 100 m perfluorinated graded-index POF 共PFGI-POF兲
共Ref. 24兲 instead of a PMMA-based POF. It has a numericala兲Electronic mail: ymizuno@sonic.pi.titech.ac.jp.

0003-6951/2010/97共2兲/021103/3/$30.00

aperture 共NA兲 of 0.185, a core diameter of 120 ␮m, acore refractive index of 1.35, and a relatively low loss
共⬃150 dB/ km兲 even at 1.55 ␮m.
The experimental setup to study the Brillouin scatteringin the PFGI-POF is depicted in Fig. 1. In order to measure
the BFS with a high resolution, self-heterodyne detection15
was used. All the optical paths except the POF were composed of silica single-mode fibers 共SMFs兲. A distributedfeedback laser diode 共DFB-LD兲 at 1552 nm with 10 MHz
linewidth was used as a light source, and its output wasdivided into two light beams with a coupler. One of thebeams was directly used as the reference light of the heterodyne detection, after passing a polarization controller 共PC兲.
The other beam was amplified with an erbium-doped fiber
amplifier 共EDFA兲 and injected into the POF as the pumplight. Then, the optical beat signal between the backscattered
Stokes light and the reference light was converted to an electrical signal with a photodetector 共PD兲. Finally, the signal
was amplified by 23 dB with an electrical preamplifier, andmonitored with an electrical spectrum analyzer 共ESA兲. The
measurement data were transferred to a personal computer.
In this setup, the coupling between the silica SMF and thePOF was implemented using so-called butt coupling.25 Considering the difference between the core diameters 共8 ␮m ofSMF versus 120 ␮m of POF兲, a large optical loss is expected when light travels from the POF into the SMF, which
contributes only to the attenuation of the Stokes light once

FIG. 1. 共Color online兲 Experimental setup for observing Brillouin scatteringin the PFGI-POF: DAQ, data acquisition; dc, direct current; DFB-LD,distributed-feedback laser diode; EDFA, erbium-doped fiber amplifier; ESA,electrical spectrum analyzer; PC, polarization controller; and PD,photodetector.

97, 021103-1© 2010 American Institute of Physics

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021103-2

Appl. Phys. Lett. 97, 021103 共2010兲

Y. Mizuno and K. Nakamura

reaches saturation, which indicates the transition from spontaneous to stimulated Brillouin scattering. Though a roughestimation of Pth is often performed using this kind offigure,7,16,18,27,28 the saturation of the Stokes gain was notobserved in Fig. 3. Therefore, Pth of this POF seems to behigher than 30 dBm 共=1 W兲. The detailed estimation of Pthwill be given later in this paper.
Next, we estimate the Brillouin gain coefficient gB. Using the acoustic velocity vA and the Brillouin linewidth ⌬␯B,
gB is given by28
gB =FIG. 2. 共Color online兲 Measured BGS in the PFGI-POF at the pump powerof 20 dBm. The inset shows the magnified view around the BGS peak.

generated in the POF. It was measured to be approximately
12 dB, which will be improved by developing an optical lenssystem. On the other hand, the loss was less than 0.2 dB
when light travels from the SMF into the POF. This is sufficiently low to characterize the Brillouin scattering in thePOF.
Figure 2 shows the measured Brillouin gain spectrum
共BGS兲 when the 100 m PFGI-POF was pumped with 20
dBm light. The peak corresponding to the BFS was observed
at 2.83 GHz, about four times lower than that of standard
silica fibers, which allows the use of a PD and an ESA thatare cheaper with lower bandwidth. The acoustic velocity vAis given by the BFS ␯B as7vA =␯ B␭ p,2n共1兲where n is the refractive index 共1.35兲 and ␭p is the wavelength of the pump light 共1552 nm兲. So vA in this POF can becalculated to be 1627 m/s, which is much lower than that ofstandard bulk PMMA, ⬃2700 m / s.26 By fitting the BGS
with a Lorentzian curve, the 3 dB Brillouin linewidth ⌬␯B
was measured to be 105 MHz, which is three to five times
broader than that of silica fibers.27
Figure 3 shows the relative Stokes gain as a function ofpump power. In general, the Stokes gain begins to grow exponentially at the Brillouin threshold power Pth and then22␲n7 p12c␭2p␳vA⌬␯B,

共2兲

where p12 is the longitudinal elasto-optic coefficient, c thelight velocity, and ␳ the density. Since the accurate valuesof p12 and ␳ are not known for perfluorinated PMMA,
we used the values of standard PMMA 共Ref. 29兲 in thiscalculation. We think further research is needed on thispoint, but using the measured values of vA = 1627 m / sand ⌬␯B = 105 MHz, along with n = 1.35, p12 = 0.297, ␭p= 1552 nm, and ␳ = 1187.5 kg/ m3, gB was calculated to be3.09⫻ 10−11 m / W, which is almost the same as that of fusedsilica fibers 共3 – 5 ⫻ 10−11 m / W兲.7Then, we estimate the Brillouin threshold power Pth of
the PFGI-POF. An alternative way to calculate gB is to use
the following equation:30
gB =21bAeff,KPthLeff

共3兲

where Aeff is the effective cross-sectional area and Leff is theeffective length defined as Leff = 关1 − exp共−␣L兲兴 / ␣ 共␣, background loss; L, fiber length兲. For multimode fibers, a correction factor b is needed,31 which can be treated as 2 when theNA of the fiber is nearly 0.2. K is a constant that depends on
the polarization properties of the fiber,28,32 which is 1 if thepolarization is maintained and 0.667 otherwise. Then,using the values of gB = 3.09⫻ 10−11 m / W, b = 2 ,Aeff = 209 ␮m2,33 K = 0.667, ␣ = 0.056/ m, and L = 100 m, Pthcan be calculated to be 24 W. This is quite high but valid
compared to the theoretically predicted values of ⬃10 W
共L = 300 m兲 共Ref. 33兲 or ⬃100 W 共L = 100 m兲 共Ref. 34兲
reported by other groups. As Pth is in proportion to bAeff in
Eq. 共3兲, we think Pth can be reduced to a moderate power
level by employing POFs with smaller cores.
In conclusion, we investigated the Brillouin scattering in
the PFGI-POF at 1.55 ␮m wavelength. The BFS and theBrillouin bandwidth were 2.83 GHz and 105 MHz, respectively. Using these values, the Brillouin coefficient was calculated to be 3.09⫻ 10−11 m / W, which is close to that offused silica fibers. This indicates that, as Brillouin scatteringin silica fibers has a wide field of application, Brillouin scattering in POFs can also be utilized to develop various practical devices and systems with their low cost, ease of installation, and high flexibility.
This work was partly supported by the Research Fellowships for Young Scientists from the Japan Society for thePromotion of Science 共JSPS兲.
1

FIG. 3. 共Color online兲 Relative gain of the Stokes light backscattered from
100 m PFGI-POF as a function of pump power.

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