Applied Physics Express 10, 062502 (2017)https://doi.org/10.7567/APEX.10.062502

Polymer optical ﬁber tapering using hot water
Yosuke Mizuno1*, Hiroki Ujihara1, Heeyoung Lee1, Neisei Hayashi2, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Tokyo 153-8904, Japan2*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received February 21, 2017; accepted May 3, 2017; published online May 19, 2017
We perform a pilot trial of highly convenient taper fabrication for polymer optical ﬁbers (POFs) using hot water. A >380-mm-long POF taper issuccessfully fabricated, and its >150-mm-long waist has a uniform outer diameter of >230 µm. The shape is in good agreement with the theoreticalprediction. The optical loss dependence on the strain applied to the waist shows an interesting behavior exhibiting three regimes, the origins ofwhich are inferred by microscopic observations. We then discuss the controllability of the taper length.
© 2017 The Japan Society of Applied Physics

rillouin scattering in optical ﬁbers1) has been exploited to develop various devices such as strain andtemperature sensors.2–8) To improve their performance, the Brillouin scattering properties in some special glassﬁbers have been investigated.9,10) However, as glass ﬁbers are
fragile and break at relatively small strains [∼3% for silica
single-mode ﬁbers (SMFs)], they cannot be directly used tomeasure larger strains. One method of tackling this problem
is to use polymer optical ﬁbers (POFs), which are so ﬂexiblethat they can withstand even over 100% strain.11)
Several types of POFs are currently commerciallyavailable. The most widely used type is poly(methyl
methacrylate)-based (PMMA-) step-index POFs,12) whichmainly transmit visible light. However, Brillouin scattering in
PMMA-POFs has not yet been experimentally observed
because some of the optical devices necessary for Brillouinmeasurement are not commercially available at visible
wavelengths. Another type widely used in laboratories is
perﬂuorinated graded-index (PFGI-) POFs13) that are composed of cyclic transparent optical polymer (CYTOP) andtransmit not only visible light but also telecom wavelength
light. As many kinds of optical devices can be used attelecom wavelengths, Brillouin scattering in PFGI-POFs has
already been experimentally observed.14) To date, we haveinvestigated its physical properties at 1.55 µm, including thegain coeﬃcient,14) threshold power,14) Brillouin frequency
shift (BFS),14) and the BFS dependences on strain15,16) andtemperature,15) proving its applicability to high-sensitivity
temperature sensing15) and large-strain sensing.16) Meanwhile, one serious demerit of PFGI-POFs in Brillouin sensing
applications is that the Brillouin Stokes power is relatively
low because of their large core diameters (50–120 µm).14) The
Stokes power in PFGI-POFs should be enhanced to implement sensors with a high signal-to-noise ratio.17)
One simple method of enhancing the Stokes power in
PFGI-POFs is to increase the incident light power, themaximal limit of which is, however, much lower than that ofsilica SMFs. This is because the threshold power of the opticalﬁber fuse in PFGI-POFs is relatively low.18,19) Anothermethod is to induce stimulated Brillouin scattering by apump–probe technique,20) which is suitable for two-endaccess sensing systems2,4,6,21,22) but cannot be directly employed in single-end-access systems.3,5,7,8,23) The combined
use of pulsed pump light and a low-power optical ampliﬁer isalso known to be an eﬀective method of enhancing the Stokes
power,24) but it is not compatible with continuous wave (CW)based sensing systems.4–8,23) Thus, to enhance the Stokes

Bpower in CW-based one-end-access sensing systems, such as
Brillouin optical correlation-domain reﬂectometry,7,8,23) yet
another method is desirable. For this purpose, here, we focus
on the use of long tapered PFGI-POFs, which can enhance the
Stokes power (generated at the tapered section), because theoptical power density at the core center increases.25)
To date, many kinds of tapering techniques have been
demonstrated.26–41) Among them, what we call a heat-andpull technique has been most widely used to taper opticalﬁbers, including glass ﬁbers,26–32) PMMA-POFs,33,34) and
PFGI-POFs.25,35–37) Many kinds of heat sources have beenused for heating ﬁbers, including a ﬂame,25–29) a CO2 laser,30)
a compact furnace,31,33–36) a fusion splicer,32) and a solder
gun,37) but these are sometimes not easy to handle, prepare,or control. To overcome this inconvenience in tapering POFs,
hot water appears to be a good candidate as a heat source,
because the glass-transition temperature of POFs is relatively
low (∼100 °C).42) Besides, as heat is applied to the POFs
almost uniformly (both radially and longitudinally) in hot
water, its higher controllability might facilitate the fabricationof long tapered POFs. However, no papers regarding thetapering of POFs using hot water have yet been published.
In this paper, as a pilot trial, we demonstrate the heat-andpull fabrication of PFGI-POF tapers using hot water as a heatsource. In an experiment, a ∼380-mm-long PFGI-POF taperwith a ∼150-mm-long waist is successfully fabricated. The
waist has an almost uniform outer diameter of ∼230 µm, andthis geometry agrees well with the theoretical prediction.
During fabrication, the optical propagation loss is measured
as a function of strain applied to the waist. It exhibits aninteresting behavior containing three regimes, the origins ofwhich are experimentally investigated.
The PFGI-POFs (Asahi Glass Fontex) tapered in thisexperiment have a three-layered structure: core (diameter:
50 µm; refractive index: ∼1.36), cladding (diameter: 70 µm),
and overcladding (diameter: 490 µm). The core and cladding
consist of doped and undoped CYTOP, respectively, thewater absorption of which is negligibly small,43) and theovercladding is composed of polycarbonate. The opticalpropagation loss is relatively low (∼0.25 dB=m) even at
1.55 µm. The length of the PFGI-POF samples was 800 mm.
The experimental setup is depicted in Fig. 1. The output ofa semiconductor laser at 1550 nm (power: 10 dBm) was
injected into one end of a PFGI-POF via a silica SMF (buttcoupled),14) and the transmitted light was directly guided toan optical power meter. The room temperature was 18 °C.
First, a 150-mm-long section around the midpoint of the

062502-1© 2017 The Japan Society of Applied Physics

Appl. Phys. Express 10, 062502 (2017)

Y. Mizuno et al.

Fig. 3. Propagation loss of the tapered POF plotted as a function of thestrain applied to the waist.
Fig. 1. Schematic of the POF tapering setup using hot water.

(a)Fig. 2. Temporal variation in the propagation loss after the POF waspartially immersed in hot water.

PFGI-POF was immersed in hot water (maintained at 97 °C)
in a round vessel (diameter: 300 mm), and the temporalvariation in the transmitted power was monitored until thepower became almost constant. Subsequently, the PFGI-POF
was pulled (or in other words, strained) in both directions via
pulleys (diameter: 95 mm) at a rate of 0.50 mm=s (0.25 mm=s
for each direction) until the PFGI-POF was completely cut.
The transmitted power was also monitored during the pulling
process. The same experiments were performed using other
PFGI-POF samples with diﬀerent pulling periods (pulling
stopped before the samples were cut), and the tapered
sections were observed using an optical microscope.
Figure 2 shows the temporal variation in the total opticalloss after the 150-mm-long section of the PFGI-POF
was immersed in hot water. The loss increased with time
(probably caused by thermal shrinkage),44) and approximately 80 min later, it became almost constant at ∼3.6 dB.
Figure 3 shows the optical loss plotted as a function of thestrain ε applied to the taper waist, which can be calculated
as follows. Under the assumption that the volume of a ﬁber
is maintained before and after the tapering process, it follows
by simple calculation (see the online supplementary data
at http://stacks.iop.org/APEX/10/062502/mmedia) that thestrain ε applied to the taper waist is given by" ¼ eD=Lw  1;

ð1Þ

where D is the total pulled length in both directions, and Lw is
the length of the ﬁber section immersed in hot water (150 mm
in this experiment). As seen in Fig. 3, the loss remained
unchanged for strains of less than ∼100%. For larger strainsof up to ∼400%, the loss increased with a coeﬃcient of∼0.010 dB=%. For even larger strains, the loss increased
markedly with a coeﬃcient of ∼0.048 dB=%, and the PFGIPOF was completely cut at its midpoint when the strain

(b)

(c)

Fig. 4. Micrographs of the tapered POFs: (a) untapered section, (b) waist(strain: 360%), and (c) waist (strain: 500%).

reached ∼560%. Thus, the dependence of loss on the strain
exhibited three regimes, the origin of which is discussed in
the following paragraph.
Another PFGI-POF sample was tapered in the samemanner, and when the strain was 360%, it was removed fromthe hot water. The loss was 7.2 dB. Figures 4(a) and 4(b)show the micrographs of the untapered section and the taper
waist, respectively. In Fig. 4(a), the core and cladding (theirboundary is not visible) as well as the overcladding weretransparent, whereas in Fig. 4(b), the core and cladding
showed white turbidity caused by a phenomenon calledcrazing (i.e., the stress concentration causes nanoscale voids
to form in the polymer, whereby light is scattered, resulting inwhitening).45) The distribution of the outer diameter along thelength measured with a microscope is shown in Fig. 5.
A ∼380-mm-long section was tapered, and the outer diameterof the ∼150-mm-long waist around its midpoint wasapproximately 230 µm, which was relatively uniform with astandard deviation of ∼4 µm. This shape is in good agreementwith the theoretical trend indicated by the solid curve[calculated using Eq. (1) (see the online supplementary data
at http://stacks.iop.org/APEX/10/062502/mmedia); theoretical outer diameter of the waist: 233 µm]. The sections thatwere not immersed in hot water at all should be nontapered,
but actually, they were slightly tapered, because the vapor
from the hot water served as a heat source. This could be
mitigated by thermally shielding part of the PFGI-POF.
Subsequently, yet another PFGI-POF sample was also
tapered at 500% strain. The micrograph of the midpoint of062502-2© 2017 The Japan Society of Applied Physics

Appl. Phys. Express 10, 062502 (2017)

Y. Mizuno et al.

Fig. 5. Outer diameter distribution of the tapered POF (strain: 360%).
The solid curve shows the theoretical trend.

the waist is shown in Fig. 4(c). The turbid core and cladding
were cut, and only the overcladding maintained the ﬁber
shape. On the basis of the aforementioned experimental
observations, we can infer the origins of the three regimes ofthe dependence of loss on strain. Namely, the change at∼100% strain corresponds to the start of the white turbidity of
the core and cladding, that at ∼400% strain to the breakage ofthe turbid core and cladding, and that at ∼560% strain to thebreakage of the overcladding.
Finally, we discuss the length of the PFGI-POF tapers
fabricated by this technique. As mentioned above, longer
PFGI-POF tapers are desirable for Brillouin sensing applications. If the PFGI-POFs are pulled equally in both directions, the resultant tapered length is determined by the initiallength of the PFGI-POF section immersed in hot water.
Therefore, if a larger (or longer) vessel is used, longer PFGIPOF tapers can be easily fabricated. Even when the size ofthe vessel is ﬁxed, we expect that much longer PFGI-POF
tapers can be obtained if we give a new twist to the pulling
structure. For instance, if one end of the PFGI-POF is pulled
at 0.75 mm=s and the other end is pushed at 0.25 mm=s, the
PFGI-POF is actually pulled at a rate of 0.50 mm=s while thepulled section is scanned along the PFGI-POF. This scheme
may result in an extremely long PFGI-POF taper with highuniformity. Further study is required on this point.
In conclusion, we performed a pilot trial of taper fabrication for PFGI-POFs using hot water. We experimentally
fabricated a ∼380-mm-long PFGI-POF taper with a ∼150mm-long waist, the outer diameter of which was uniform(∼230 µm). The geometry was in good agreement with thetheory. The optical loss had an interesting dependence on
the strain applied to the waist, containing three regimes, themechanism of which was elucidated by microscopic observations. We also discussed the method of elongating the
PFGI-POF tapers. Compared with other tapering techniques,
our technique using hot water potentially has the advantagesof ease of handling, cost eﬃciency, safety (no chemicals
used), and high controllability of the length; thus, we consider that long PFGI-POF tapers fabricated by our techniquewill be a powerful tool for enhancing the signal-to-noise ratioof POF-based distributed Brillouin sensors in the future.
Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 25709032, 26630180, 25007652, and 17H04930 and by research grants
from the Japan Gas Association, the ESPEC Foundation for Global Environment
Research and Technology, and the Association for Disaster Prevention Research.

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