JOURNAL OF APPLIED PHYSICS 115, 173108 (2014)

Observation of Brillouin gain spectrum in tapered polymer optical fiber
Neisei Hayashi,a) Hideyuki Fukuda, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku,
Yokohama 226-8503, Japan(Received 27 January 2014; accepted 24 April 2014; published online 6 May 2014)
We report on the first observation of the Brillouin gain spectrum in a perfluorinated graded-index
polymer optical fiber (POF) tapered by a heat-and-pull technique. The Stokes power was slightly
enhanced by tapering probably on account of higher optical power density in the core. In addition,
the Brillouin frequency shift was decreased by 40 MHz, which was experimentally verified to bepartially caused by high heating temperature applied to the POF during the taper fabrication
process. We anticipate that our findings will provide a basic principle of temperature sensing with
C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4875102]“memory” function. VI. INTRODUCTION

Brillouin scattering, one of the most important nonlinear
effects in optical fibers, has been widely studied for several
decades, and applied to various fiber-optic devices and systems, such as lasers,1 amplifiers,1 storages,2 frequency-comb
generators,3 slow-light generators,4 and distributed strain/temperature sensors.5–9 Recently, Zou et al.10 have investigatedthe Brillouin gain spectra (BGSs) in tapered silica singlemode fibers (SMFs) at 1.55 lm and found that the Brillouinfrequency shift (BFS) in the taper waist (outer diameter:5 lm) is higher than that of the untapered silica SMF by270 MHz. This report implies that the BFS in optical fibers
can be controlled by adjusting the tapering conditions, which
will increase the freedom in designing the above-mentioned
fiber-optic Brillouin devices. Meanwhile, polymer opticalfibers (POFs) have attracted a lot of attention because of theirhigh flexibility, ease of handling, low-cost connection, andhigh safety;11 and tapered POFs have also been extensively
studied for a variety of applications.12–17 However, no reporthas been provided on the observation of Brillouin scatteringin a tapered POF yet, though the Brillouin properties in untapered POFs have already been characterized.18–20 Thus,
observing Brillouin scattering in a tapered POF is an important technological step toward the development of POF-based
Brillouin devices with higher design freedom.
In this study, we observe the BGS in a perfluorinatedgraded-index (PFGI-) POF tapered by a heat-and-pull technique for the first time. The Stokes power and the BFS in thewaist (outer diameter: 223 lm) were higher and lower (by40 MHz) than those in an untapered POF, respectively.
The Stokes power was enhanced because of higher opticalpower density in the core; whereas the BFS downshift wasexperimentally proved to be partially caused by the heatapplied to the POF during the taper fabrication.
II. PRINCIPLE

When injected into an optical fiber, light (or photons)
interacts with acoustic phonons, generating Brillouin Stokesa)

Author to whom correspondence should be addressed. Electronic mail:hayashi@sonic.pi.titech.ac.jp

0021-8979/2014/115(17)/173108/3/$30.00

light propagating in the direction opposite to the incidentlight. The Stokes light spectrum, called BGS, is of Lorentzian
shape;1 and the central frequency of the BGS is downshiftedfrom the incident light frequency. The amount of this frequency shift is called BFS, which is 10.8 GHz for a silica
SMF,1 5.4 GHz (estimated value) for a poly(methyl methacrylate)-based (PMMA-) POF,21 and 2.8 GHz for a PFGIPOF18 at 1.55 lm. If temperature change (or strain) is applied
to the fiber, the BFS shifts toward higher or lower frequency
according to the fiber core material, which is the fundamental
operating principle of fiber-optic Brillouin temperature (orstrain) sensors. The temperature dependence coefficient of theBFS has been reported to be approximately 1.2 MHz/K for asilica SMF,22 7.1 MHz/K (estimated value) for a PMMAPOF,23 and 3.2 MHz/K for a PFGI-POF24 at 1.55 lm. It isnoteworthy that, as the strain dependence coefficient of theBFS in a PFGI-POF is 121.8 MHz/%,19 the absolute valueof which is about one fifth of that of a silica SMF, Brillouinscattering in a PFGI-POF has a big potential for highprecision temperature sensing with reduced strain sensitivity.
III. EXPERIMENTAL SETUP

As a POF to be tapered in the experiment, we employed
not a standard PMMA-POF but a PFGI-POF. This is because
Brillouin scattering has been successfully observed only inPFGI-POFs,18 which have a relatively low propagation lossof 250 dB/km even at telecom wavelength,25 where mature
optical devices for Brillouin detection such as amplifiers andcirculators are available. The length of the POF sample was
1.0 m, and the other physical properties are summarized inTable I. The core/cladding layers and the overcladding layer
were, respectively, composed of doped/undoped amorphous
perfluorinated polymers (polyperfluorobutenylvinyl ether)and polycarbonate.13,25
A so-called heat-and-pull technique12–16 was used totaper the POF. A chemical etching technique17 is anotherwell-known tapering method, but was not effective in tapering this type of POF because of the high chemical toleranceof its core material.25 Although the core/cladding layers
and the overcladding layer were different in glass-transition
temperature (<108  C and 144  C, respectively), the115, 173108-1

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TABLE I. Physical properties of the POF sample.
Numerical Core diameter Outer diameter Core refractive Propagation
aperture[lm][lm]indexloss [dB/km]
0.1851204901.35

250

heat-and-pull technique was available on account of the relatively thin overcladding layer (see Table I). Figure 1
shows a schematic setup for tapering the POF, which contained an 80-mm-long furnace made of a U-shaped aluminum block fixed on a heater. One end of the POF wasconnected to a 1.55-lm light source, and the other end wasto an optical powermeter. Two regions of the POF wereclamped; one was fixed to a motorized stage, and the otherwas connected via a spring to a force gauge fixed to anotherstage, with which the pulling force was monitored. A digital
camera was mounted on the system to measure the waist diameter of the tapered POF. After the POF was tapered, its
BGS was observed based on self-heterodyne detection
detailed in Ref. 18, where the polarization state was optimized with a polarization controller.
IV. EXPERIMENTAL RESULTS

The furnace temperature was set to 105  C, which isalmost the same as the glass-transition temperature of thecore. The pulling velocity was approximately 0.2 mm/s,
which was adaptively controlled so that the pulling force
might be constantly lower than 30 mg. The tapering process
was run for 9 min. Figure 2 illustrates a schematic diagramof the tapered POF, including three microscopic images(untapered, intermediate, and waist zones). The outer diameter of the waist was 223 lm, corresponding to the core diameter of 54.6 lm, which was estimated under the assumptionthat the ratio of the core diameter to the outer diameterremains unaltered by tapering. The elongated POF length
was 102 mm (tapered length: 182 mm), and the taper ratio12
was calculated to be 0.46. The total optical loss of thetapered region (excluding the propagation loss) was
0.8 dB, which is close to the previous report.12
Next, we cut the tapered POF at two points indicated inFig. 2, and used the portion containing the waist zone as asample for Brillouin measurement. The length was 121 mm,and the distribution of the outer diameter along the length
measured with an optical microscope is shown in Fig. 3. The
outer diameter in the 60-mm-long waist zone was almost
constant (225 lm). One end of the sample (relative position ¼ 0 mm in Fig. 3) was flatly polished, and butt-coupledto a silica SMF.18
Figure 4 shows the BGS of the tapered POF measured at17  C in which that of the untapered POF (same length, i.e.,

FIG. 1. Schematic setup for POF tapering by heat-and-pull technique.

FIG. 2. Schematic structure of the tapered POF, along with the photographsof (a) untapered, (b) intermediate, and (c) waist zones. The red dotted lines
indicate the positions at which the sample was cut for Brillouinmeasurement.

121 mm) is also displayed for comparison. The incident optical power was set to as high as 30 dBm (¼1 W), because theBrillouin scattering signal from such a short POF isextremely small.18 The Stokes power of the tapered POF wasonly slightly (0.1 dB) higher than that of the untapered
POF, which does not agree with our calculation (>4-dBenhancement) taking optical power density into consideration;26,27 this is because the Stokes power is so small that theinfluence of the noise floor and the polarization-dependent
fluctuations are extremely large. The BFS of the tapered
POF was downshifted from that of the untapered POF by
40 MHz. The sign and absolute value of this BFS shift
were opposite to and smaller than that of the tapered silica
SMF (270 MHz), respectively.10
Finally, to study the origin of the BFS shift caused bytapering, we measured the BGSs in the same untapered POF
sample (used as a reference in the preceding experiment) in
the following three sequential conditions: (i) at room temperature (15  C), before heating; (ii) heated at 112  C, kept for
5 min; and (iii) cooled at room temperature (15  C), kept for
5 min. As shown in Fig. 5, the BFSs were approximately
2.84 GHz for (i), 2.53 GHz for (ii), and 2.76 GHz for (iii). As

FIG. 3. Measured outer diameter as a function of relative position along thetapered POF.

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J. Appl. Phys. 115, 173108 (2014)

originate partially from the heat applied to the POF. By using
the enhanced Brillouin signal in a much longer tapered POF,
the signal-to-noise ratio of the POF-based distributed sensing
systems could be improved. In addition, the BFS shift
induced by high heating temperature might be actively utilized as a principle of the “temperature memory effect.” We
believe that, along with the previously reported “strain memory effect,”31 this novel function could be of great use in
implementing POF-based Brillouin devices and systems
including distributed strain and temperature sensors.5–9
ACKNOWLEDGMENTS

FIG. 4. Measured BGSs in POFs: (i) tapered and (ii) untapered.

This work was partially supported by a Grant-in-Aid forYoung Scientists (A) (No. 25709032) from the Japan Society
for the Promotion of Science (JSPS), and by research grants
from the General Sekiyu Foundation, the Iwatani Naoji
Foundation, and the SCAT Foundation. N.H. acknowledges
a Grant-in-Aid for JSPS Fellows (No. 25007652).
1

FIG. 5. Measured BGSs in POF: (i) at room temperature (15  C), before
heating; (ii) heated at 112  C, kept for 5 min; and (iii) cooled at room temperature (15  C), kept for 5 min.

the POF sample was untapered, the 80-MHz downshift of
the BFS from (i) to (iii) appears to be caused not by the pulling process but by the applied high heating temperature comparable to the glass-transition temperature of the corematerial, which may induce polymer transformation28 aswell as dopant diffusion.29,30 Note that such BFS shifts havenot been observed in previous reports that investigate theBFS dependence on temperature ranging up to 80  C.19,24
However, since the BFS was downshifted in this experimentby twice the amount in the tapered POF, we speculate thatthe amount of the BFS shift after tapering is dependent not
only on the heating temperature and duration but also on mechanical deformation, on which further detailed study isrequired. We might control the BFS in POFs by properly
adjusting these conditions in future.
V. CONCLUSION

By the heat-and-pull technique, we fabricated the PFGIPOF taper with an outer diameter of the waist of 223 lm, andsuccessfully observed its BGS for the first time. The Stokes
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