Applied Physics Express 8, 072501 (2015)
http://dx.doi.org/10.7567/APEX.8.072501

Polymer optical ﬁber tapering without the use of external heat source
and its application to refractive index sensing
Hiroki Ujihara, Neisei Hayashi, Kazunari Minakawa, Marie Tabaru, Yosuke Mizuno, and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Received March 17, 2015; accepted May 26, 2015; published online June 10, 2015
We perform a pilot trial of the highly convenient taper fabrication of perﬂuorinated graded-index polymer optical ﬁbers. Instead of conventional
external heating, we utilize internal heating caused by high-power propagating light (500 mW in this experiment). An approximately 4-mm-longsection of a polymer ﬁber is tapered, and the outer diameter of the >2-mm-long waist around its midpoint is approximately 200 µm, which is quite
uniform with a standard deviation of 4.3 µm. The polymer ﬁber taper fabricated by this technique is shown to be capable of generating evanescentwaves and thus measuring the refractive indices of liquids from 1.333 to 1.410. © 2015 The Japan Society of Applied Physics

he use of Brillouin scattering in optical ﬁbers1) has
unleashed a wide range of applications, such as lasers,
signal processors, optical storages, phase conjugators,
slow light generators, and strain=temperature sensors.1–6)
To improve their performance, the Brillouin properties ofvarious special glass ﬁbers, which are well summarized inRef. 7, have been investigated. However, these glass ﬁbers
are fragile and must be treated with care; besides, they cannot
be used to measure large strains of >3%. One method forovercoming these problems is to employ polymer opticalﬁbers (POFs) with considerable ﬂexibility. POFs canreportedly withstand up to ∼100% strain.8)
Commercially available POFs are classiﬁed into two
types: poly(methyl methacrylate)-based (PMMA-) step-indexPOFs9) and perﬂuorinated graded-index (PFGI-) POFs10)
based on a cyclic transparent optical polymer (CYTOP ®).
The former mainly transmits visible light at around 650 nm,
whereas the latter transmits not only visible light but also
telecom-wavelength light at up to 1.55 µm. Brillouin scattering in PMMA-POFs has not been experimentally observed
yet because some of the optical devices required for themeasurement are extremely diﬃcult to prepare at visible
wavelengths. On the other hand, Brillouin scattering inPFGI-POFs has already been experimentally observed, sincevarious optical devices are available at telecom wavelengths.11) Their fundamental Brillouin properties, including
the Brillouin gain coeﬃcient,11) Brillouin threshold power,11)
Brillouin frequency shift (BFS),11) and BFS dependenceson strain (relatively small strain12) and large strain13)) andtemperature,12) have been investigated at 1.55 µm, especiallyfor sensing applications. These results indicate that Brillouinscattering in PFGI-POFs could be used to develop highsensitivity temperature sensors12) and large-strain sensors.13)
The Brillouin Stokes power of PFGI-POFs is, however,typically low because of their large core diameters (50, 62.5,and 120 µm),11) and must be enhanced both for a moredetailed investigation of their Brillouin gain spectra14–16) andfor sensor implementation with a high signal-to-noiseratio.2–6,17)
The highest optical power that can be injected into POFs ismuch lower than that of silica single-mode ﬁbers (SMFs)
because of the low threshold power of the optical ﬁber
fuse in PFGI-POFs (several hundreds of milliwatts forfuse propagation; much higher power is generally required
for fuse ignition).18,19) Therefore, several methods havebeen developed to enhance the Stokes signal in PFGI-POFs.
One method is to induce stimulated Brillouin scattering using

Ta pump–probe technique, with which an extremely largeStokes signal was obtained in PFGI-POFs.20) Anothermethod is to use pulsed pump light and a low-power
erbium-doped ﬁber ampliﬁer (EDFA), exploiting the nonlinearity of Brillouin scattering.21) We anticipate that the useof tapered PFGI-POFs is also one of the promising methods
for enhancing the Brillouin signal (generated at the taperedsection), as the optical power is more concentrated at thecore center.22)
The so-called heat-and-pull technique is most commonlyused to taper optical ﬁbers, and has been applied not onlyto glass ﬁbers23–29) but also to PMMA-POFs30,31) and PFGIPOFs.22,32–34) However, external heat sources, such as aﬂame,22–26) a CO2 laser,27) a compact furnace,28,30–33) a soldergun,34) and a fusion splicer,29) must be prepared, leading tohandling diﬃculty and=or inconvenience. A chemical etching
technique is also used to taper optical ﬁbers,35) but the use ofchemicals impairs convenience. In addition, it is not eﬀective
in tapering PFGI-POFs because of the high chemical tolerance of their core material (CYTOP).10) A more convenienttaper fabrication technique of PFGI-POFs is thus required.
In this study, as a pilot trial, we fabricate a PFGI-POF taper
by exploiting the internal heating caused by high-power
propagating light without the use of an external heat source.
We also experimentally show that this PFGI-POF taper canbe used to measure the refractive index of liquids.
The PFGI-POF (Asahi Glass Fontex) tapered in this experiment has a three-layered structure consisting of the core
(50 µm diameter), and cladding (70 µm diameter) and overcladding (490 µm diameter) layers. The core and cladding
layer are composed of doped and undoped polyperﬂuorobutenylvinyl ether, respectively. The refractive index at thecenter of the core is 1.356, and that of the cladding layer is1.342. The polycarbonate overcladding layer reduces bending
losses and enhances the load-bearing capacity. The propagation loss is relatively low (∼250 dB=km), even at 1.55 µm.
The experimental setup used is depicted in Fig. 1. The
output of a laser diode at 1552 nm was ampliﬁed withan EDFA and injected into a 200-mm-long PFGI-POF.
The transmitted power was monitored using an optical power
meter. Both ends of the PFGI-POF were butt-coupled tosilica SMFs.11) First, by pressing one point of the PFGI-POF
using a nipper, a loss was artiﬁcially induced, which wasmeasured to be ∼23 dB [optimal value for the incident powerof 27 dBm (= 500 mW)]. Subsequently, using motorizedstages, the whole length of the PFGI-POF was stretched at arate of 500 µm=s. The room temperature was 18 °C.

072501-1© 2015 The Japan Society of Applied PhysicsAppl. Phys. Express 8, 072501 (2015)

H. Ujihara et al.

Fig. 1. Schematic of the experimental setup for PFGI-POF tapering.
EDFA, erbium-doped ﬁber ampliﬁer; LD, laser diode; PFGI-POF,perﬂuorinated graded-index polymer optical ﬁber; SMF, single-mode ﬁber.

Fig. 3. Micrograph of the tapered PFGI-POF.

Fig. 2. Thermograph around the loss-induced point in the PFGI-POF.

First, the PFGI-POF section around the artiﬁcially induced
loss was monitored using an infrared thermometer,36) asshown in Fig. 2. The incident light, the power of which
was set to 27 dBm, was injected from the left-hand side. Thispower is higher than the threshold for ﬁber fuse propagation,18,19) but generally lower than the threshold for fuseignition; the fuse was never ignited in this tapering experiment (in Refs. 18 and 19, a POF end was brought into
contact with an absorbent material to ignite the fuse). The
high-temperature region had a tail extending for ∼5 cm
toward the right. This seems to originate from the fact that theoptical paths of the propagating rays were partially disturbed
at the loss, and that the optical energy was then absorbed by
the overcladding layer located slightly away from the loss,generating heat as a result. The maximal temperature of
the color bar in Fig. 2 is 36 °C, which is much lower than
the glass transition temperature of the core material (approximately 100 °C) for the following two reasons: (1) 36 °C is
not the temperature at the hottest point that is initially tapered
but that at the center of the infrared image (owing to thespeciﬁcations of the camera) and (2) the temperature measured by this method is the surface temperature of the overcladding layer of the POF, which is lower than the coretemperature.
Next, while maintaining the incident light power (27 dBm),
the loss-induced PFGI-POF was stretched for 4 mm. Then, as
shown in Fig. 3, taper deformation started rightward from thepoint ∼1 mm away from the induced loss. The distribution ofthe outer diameter along the length measured with an optical
microscope is shown in Fig. 4. An approximately 4-mm-longsection was tapered, and the outer diameter of the ∼2-mmlong waist around its midpoint was approximately 200 µm,
which was quite uniform with a standard deviation of 4.3 µm.
Such a high uniformity seems to have been achieved by theyielding of the overcladding layer made of polycarbonate(see Refs. 37 and 38). The outer diameters at the relative
positions of 0 and 6 mm (see Fig. 4) were ∼40 and ∼90 µm
smaller than the initial outer diameter (490 µm), respectively.
This is because the nontapered regions were also slightly
stretched, which is clear if we consider that the fabricatedFig. 4. Outer diameter distribution around the waist of the PFGI-POFtaper.

taper length is almost the same as the stretched length(∼4 mm). The deformation on the right (relative position of
6 mm in Fig. 4) was larger probably because the temperature
was higher (see Fig. 2). Thus, the structural asymmetry in theaxial direction is inevitable in this method, and may present
a problem with some applications. The propagation loss of
this taper, excluding the initially induced ∼23 dB loss, wasmeasured to be ∼25 dB.
To verify the refractive index sensing capability on thebasis of evanescent wave generation,32–34,39–41) the whole
length (∼4 mm) of the tapered PFGI-POF section was placedin a V-groove (machined ﬂat to minimize ﬁber bending) and
immersed in sucrose solutions with diﬀerent concentrations.
Sucrose concentrations of 0.0, 12.0, 24.0, 35.0, and 45.0%
were employed, corresponding to the refractive indices of1.333, 1.351, 1.371, 1.390, and 1.410, respectively.42) The
PFGI-POF taper and the V-groove were carefully cleaned,
washed with ethanol and deionized water, and then dried
before the immersion of the taper into the next sucrose
solution. In this experiment, light at a power of 10 dBm wasinjected into the PFGI-POF taper from the same side as in thecase of taper fabrication. The transmitted light power wasmeasured in all cases at a constant temperature of 18 °C.
The measured loss dependence on the refractive indexof the sucrose solution is shown in Fig. 5. The loss inthe vertical axis, calculated relative to the incident power(10 dBm), does not include the initially induced ∼23-dB loss.
As the refractive index increased, the loss almost linearly(though the vertical axis is in the unit of dB) increased inthis range with a dependence coeﬃcient of 107 dB=RIU(refractive index unit). Although it is diﬃcult to compare
the absolute value directly with those in previous reports
measured under diﬀerent experimental conditions and POF
structures,31,32,34) the positive coeﬃcient of the loss (i.e.,negative coeﬃcient of the transmitted power) agrees with that072501-2© 2015 The Japan Society of Applied Physics

Appl. Phys. Express 8, 072501 (2015)

H. Ujihara et al.

Fig. 5. Optical loss at the taper (excluding the initially induced loss) as afunction of the refractive index of liquids. The measured data are shown as
green circles, and the black line shows a linear ﬁt.

in the case where the PMMA-POF tapers fabricated using
a heat-and-pull technique were tested with a similar setup.31)
When the nontapered regions were immersed into the sucrose
solutions, no quantiﬁable inﬂuence was observed on thetransmitted power. Thus, we conﬁrmed that the PFGI-POF
taper fabricated using internal heating generates the evanescent waves, which is one of the fundamental functions ofﬁber optic tapers.
In conclusion, we demonstrated PFGI-POF taper fabrication. Without the use of an external heat source, internal
heating caused by high-power propagating light wasexploited. We showed that the PFGI-POF taper generates
evanescent waves that can be used to measure the refractive
index of liquids. The initially induced loss, which remains
after taper fabrication, is an important issue to be resolved;
therefore, the PFGI-POF tapers fabricated by this technique
may be more suitable for refractive index sensing with areﬂectometry conﬁguration, such as that exploiting tapered
ﬁber tips.43) The clariﬁcation of the controllability of thetaper shape characteristics, such as outer diameter and waist
length, will certainly be the subject of future work. If long
PFGI-POF tapers can be relatively easily fabricated by thistechnique, we anticipate that it will exert a powerful impact
on POF-based Brillouin sensing.
Acknowledgments The authors are grateful to Hiroki Tanaka, Tokyo
Institute of Technology, Japan, for his experimental assistance. This work waspartially supported by Grants-in-Aid for Young Scientists (A) (No. 25709032)
and for Challenging Exploratory Research (No. 26630180) from the Japan Society
for the Promotion of Science (JSPS), and by research grants from the Iwatani
Naoji Foundation, the SCAT Foundation, and the Konica Minolta Science andTechnology Foundation. N.H. acknowledges a Grant-in-Aid for JSPS Fellows(No. 25007652).

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072501-3© 2015 The Japan Society of Applied Physics