BRIEF NOTEJapanese Journal of Applied Physics 54, 118001 (2015)
http://dx.doi.org/10.7567/JJAP.54.118001

Refractive index sensing using V-shaped polymer optical ﬁbers
Heeyoung Lee, Neisei Hayashi, Yosuke Mizuno, and Kentaro Nakamura*
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
E-mail: hylee@sonic.pi.titech.ac.jp; knakamur@sonic.pi.titech.ac.jp
Received August 7, 2015; accepted August 25, 2015; published online September 30, 2015
Although polymer optical ﬁber (POF) tapers with high ﬂexibility have been used to measure the refractive indices (RIs) of liquids, their fabrication
have caused some inconvenience including the need to use external heat sources or chemicals. Here, as an alternative, we develop a simple,secure, and low-cost method of measuring RIs of liquids using V-shaped bent POFs. When the bending angle is 120° (experimentally optimized),with increasing RI, the transmitted power increases almost linearly with a dependence coefﬁcient of approximately 210 dB/RI unit.
© 2015 The Japan Society of Applied Physics

Fiber-optic refractive index (RI) sensing has been attracting agreat deal of attention in biological and chemical researchﬁelds, and various techniques have been developed.1–4)
Among them, RI sensing using evanescent waves generated
at the tapered region of glass optical ﬁbers3,4) is one of themost cost-eﬃcient and sensitive techniques. Glass opticalﬁber tapers are, however, fragile and require careful fabrication and handling. One strategy of tackling this problem is
to exploit polymer optical ﬁber (POF) tapers,5–10) which
provide extremely high ﬂexibility.
Conventionally, POF tapers are fabricated by a heat-andpull technique with an external heat source5–8) or a chemical
etching technique.9) However, employing external heatsources (ﬂame,5) a compact furnace,6,7) a solder gun,8) etc)
and chemicals is neither convenient nor suﬃciently secure.
Although a POF tapering technique of converting propagating light energy into heat without the use of an external heatsource has also been developed,10) an artiﬁcial optical lossof 23 dB needs to be initially induced, resulting in its hightotal optical loss (48 dB). The requirement of high-power
light injection also poses a serious issue, because it may
cause burning at the POF ends11) or a so-called fuse eﬀect.12)
In this work, we demonstrate RI sensing using a V-shaped
bent POF. To fabricate the sensing part, it is necessary to
locally cut the overcladding layer of a POF sample and thenbend that region. With no need to employ heat sources or
chemicals, this technique is extremely simple, secure, andlow in cost. We investigate the optimal bending angle atwhich the transmitted power increases almost linearly with adependence coeﬃcient of ∼210 dB=RI unit (RIU).
The POFs employed in the experiment were perﬂuorinatedgraded-index (PFGI-) POFs13,14) with a core diameter of 62.5µm, a cladding diameter of 70 µm, an overcladding diameterof 750 µm, and a propagation loss of ∼250 dB=km at 1550
nm. The RIs of the core center, cladding, and overcladding
were 1.356, 1.348, and 1.590, respectively. The core andcladding layers were composed of the same material (i.e.,
polyperﬂuorobutenylvinyl ether; with diﬀerent dopant concentrations), and their boundaries were not visible; the overcladding layer was composed of polycarbonate. Comparedwith standard poly(methyl methacrylate)-based POFs,5,7) one
important advantage of PFGI-POFs in RI sensing of liquids isthat their water absorption is extremely low.15)
The POF processing procedure is depicted in Fig. 1(a).
The midpoint of the overcladding layer of a ∼25-cm-long
POF was cut (notched) for ∼300 µm using a ﬁber cutter andwas bent into a V-shaped manner. Here, the bending angle(a)(b)
Fig. 1. (Color online) V-shaped POF: (a) processing procedure, and(b) example micrograph (bending angle = 120°).

Fig. 2. (Color online) Schematic of the experimental setup for refractive
index sensing. EDFA, erbium-doped ﬁber ampliﬁer; LD, laser diode; OSA,optical spectrum analyzer.

was deﬁned to be 0° when the POF was straight (not bent).
An example micrograph of the V-shaped POF is shown inFig. 1(b). The core=cladding layer was bared to air.
The experimental setup for RI sensing using the V-shaped
POF is schematically shown in Fig. 2. The output light froma laser diode (LD) at 1550 nm was ampliﬁed to 10 dBm (ﬁxedpower) with an erbium-doped ﬁber ampliﬁer (EDFA) andinjected into the POF. The transmitted light was ampliﬁed by
35 dB (ﬁxed gain), and its spectral peak power was measured
using an optical spectrum analyzer (OSA) with 100-times
averaging. Both ends of the POF were connected to silica

118001-1© 2015 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 54, 118001 (2015)

H. Lee et al.

Fig. 3. Dependence of relative power reduction on bending angle.

single-mode ﬁbers (SMFs) by butt-coupling via FC=SCadaptors.14)
First, we measured the reduction in the transmitted power
as a function of the bending angle when the V-shaped region
was immersed into water at 25 °C (with an RI of 1.318 at1550 nm). On the basis of this result, the bending angle usedfor RI sensing was determined. Then, we investigated theRI dependence of the transmitted power when the V-shaped
region was immersed into sucrose solution at 25 °C, the RIof which was varied from 1.318 to 1.348 by controlling theconcentration in the range from 0 to 20%.16)
Figure 3 shows the bending angle dependence (every 10°)of the reduction in the transmitted power when the V-shaped
region was immersed into water. The vertical axis is the value
relative to the transmitted power when the V-shaped region
was placed in air. At angles lower than 110°, the power
reduction was less than 2 dB, i.e., the transmitted power wasalmost insusceptible to the ambient material properties. Atthe angle of 120°, the power reduction abruptly increased
to approximately 11 dB. In the range from 120 to 160°, thepower reduction gradually became small with increasing
angle, probably because of the multimodal eﬀect (a nonmonotonic dependence of optical loss on bending radius isreported in SMFs).17) Thus, the bending angle used for
RI sensing, at which the highest sensitivity is expected, wasdetermined to be 120°. The bending loss at this angle measured in air, i.e., the diﬀerence in transmitted light power
before and after POF processing, was approximately 7 dB,
which was ∼18 dB lower than that10) of the aforementioned
POF tapering method without external heating (excluding theinitial artiﬁcial loss).
Subsequently, the RI dependence of the transmitted power
was investigated at the bending angle of 120° (Fig. 4). The
vertical axis is the value relative to the transmitted power
when the V-shaped region was immersed into water. With
increasing RI, the transmitted power increased monotonically, and the dependence was almost linear in this RI range(though on a log scale) with a coeﬃcient of approximately
210 dB=RIU. This positive RI dependence of the transmitted
power is the same as that in one of the POF-taper-based

Fig. 4. (Color online) Relative transmitted power plotted as a function of
the refractive index of liquid at the bending angle of 120°. Measured data areshown as red circles, and the blue line is a linear ﬁt.

sensors,9) and can be explained by the theory of evanescentwave absorption of cladded multimode ﬁber tapers.18)
In conclusion, a simple, secure, and low-cost method
for measuring RIs of liquids by using V-shaped POFs wasdemonstrated. When the bending angle was 120°, the transmitted power increased almost linearly with increasing RI ofliquids, and the dependence coeﬃcient was ∼210 dB=RIU.
We anticipate that this method will be a promising alternative
to POF-taper-based RI sensing.
Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 25709032, 26630180, and 25007652, and by research grants from theIwatani Naoji Foundation, the SCAT Foundation, and the Konica Minolta Scienceand Technology Foundation.

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