LETTERIEICE Electronics Express, Vol.14, No.21, 1–7

Pilot demonstration ofrefractive index sensing
using polymer optical ﬁber
crushed with slottedscrewdriver
Yosuke Mizuno1a), Heeyoung Lee1, Shumpei Shimada1,
Yukihiro Matsumoto2, Yosuke Tanaka3, Hitoshi Nakamura4,
and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology,

Yokohama 226–8503, Japan2

Department of Architecture and Civil Engineering,

Toyohashi University of Technology, Aichi 441–8580, Japan
3

Institute of Engineering, Tokyo University of Agriculture and Technology,

Tokyo 184–8588, Japan
4

Department of Civil and Environmental Engineering,Tokyo Metropolitan University, Tokyo 192–0397, Japana) ymizuno@sonic.pi.titech.ac.jp

Abstract: We demonstrate power-based refractive index (RI) sensing usinga polymer optical ﬁber (POF) crushed with a slotted screwdriver. This
structure can be easily fabricated by simply pressing the end of a slottedscrewdriver against part of the POF. Neither external heat sources, chemicals,
nor ultrasonic transducers, which have been conventionally used, are necessary. The transmitted power has a linear RI dependence in the RI range from∼1.32 to ∼1.43 (coefﬁcient: 173 dB/RIU (RI unit)). The temperature dependence of the transmitted power is experimentally shown to be negligible,
which indicates the potential of this structure as a temperature-independent
RI sensor.
Keywords: optical ﬁber sensors, refractive index sensing, polymer opticalﬁbers
Classiﬁcation: Optical systems
References© IEICE 2017
DOI: 10.1587/elex.14.20170962
Received September 19, 2017
Accepted October 3, 2017
Publicized October 20, 2017
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DOI: 10.1587/elex.14.20170962
Received September 19, 2017
Accepted October 3, 2017
Publicized October 20, 2017
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1© IEICE 2017
DOI: 10.1587/elex.14.20170962
Received September 19, 2017Accepted October 3, 2017
Publicized October 20, 2017
Copyedited November 10, 2017

Introduction

Optical ﬁber sensing techniques have been intensively explored because of theircapability to measure a variety of physical parameters, such as strain [1, 2],temperature [1, 2], pressure [3], acoustic impedance [4, 5], humidity [6], reﬂectivity[7, 8], nuclear radiation [9], etc. Fiber-optic refractive index (RI) sensing has also
been receiving increasing attention in chemical, biomedical, and pharmaceuticalresearch areas, and a number of techniques have been reported for several decades[10, 11, 12, 13]. One of the most cost-efﬁcient RI sensing techniques with highsensitivity is based on evanescent waves generated at the tapered region of glass
optical ﬁbers [12, 13]. Glass optical ﬁber tapers are, however, so fragile that weneed to take special care in fabricating and handling them. One solution for thisproblem is the use of polymer optical ﬁber (POF) tapers [14, 15, 16, 17, 18, 19, 20],
which are much more ﬂexible than glass ﬁbers.
To date, many methods of tapering POFs have been reported. Two well-known
methods are based on a heat-and-pull technique with an external heat source[14, 15, 16, 17] and a chemical etching technique [18], but the use of external
heat sources and chemicals is neither safe nor convenient. To overcome this issue,
a POF tapering technique of converting the propagating light energy into heatwithout the use of an external heat source has been developed [19]. However,high-power light injection is required, which sometimes causes burning at thePOF ends [21] and/or a so-called fuse phenomenon [22, 23]. An RI sensor using
V-shaped POFs has also been developed [20], but with relatively low measurement
accuracy because of the structural instability. Furthermore, RI sensing using an
ultrasonically crushed POF has recently been reported [24], but it requires anultrasonic transducer, which is inconvenient in some applications.
In this study, we demonstrate RI sensing using a POF crushed with a slottedscrewdriver. The end of a slotted screwdriver is simply pressed against part of the3

IEICE Electronics Express, Vol.14, No.21, 1–7

POF with a constant load. This structure can be easily fabricated without the use of
external heat sources, chemicals, or ultrasonic transducers. First, the transmitted
power is found to show a linear RI dependence in the RI range from ∼1.32 to ∼1.43[coefﬁcient: 173 dB/RIU (RI unit)]. The transmitted power is then found to exhibit
almost no temperature dependence at around room temperature, which indicates thefeasibility of temperature-independent RI sensing.
2Experimental setup

The POF used here was a perﬂuorinated graded-index (PFGI-) POF [25, 26] with acore diameter of 50 µm, a cladding diameter of 70 µm, an overcladding diameter of490 µm, and a propagation loss of ∼0.25 dB/m at 1550 nm. The RIs of the corecenter, cladding layer, and overcladding layer at 1550 nm were 1.356, 1.348, and1.590, respectively. The core and cladding layer comprised polyperﬂuorobutenylvinyl ether with different dopant concentrations, and their boundary was invisible.
In contrast, the overcladding layer was made up of polycarbonate (mechanicallymuch stronger than the core and cladding), of which the boundary with the cladding
layer was observable with a microscope. The water absorption of PFGI-POFs isreported to be much lower than that of standard poly(methyl methacrylate)-based
POFs [14, 16]; this is preferable for liquid RI sensing [27].
Fig. 1 depicts the POF processing procedure. The end of a slotted screwdriver(tip thickness: 0.3 mm; tip angle: 17°) was pressed with a 20-N load for 60 s against
the midpoint of the 30-cm-long POF on an anvil composed of stainless steel. The
crushed region of the POF was directly (i.e., not bent as in Refs. 20 and 24) used asa sensing head.

Fig. 1.

© IEICE 2017
DOI: 10.1587/elex.14.20170962
Received September 19, 2017
Accepted October 3, 2017
Publicized October 20, 2017
Copyedited November 10, 2017

Processing procedure of the crushed polymer optical ﬁber(POF).

Fig. 2 is an experimental setup for RI sensing using the crushed POF, which
was straightly aligned. The output light from a laser at 1550 nm (power: 10 dBm;
bandwidth: ∼1 MHz) was passed through a 1-m-long silica single-mode ﬁber(SMF) and injected into the POF. The transmitted light was subsequently directed
to an optical spectrum analyzer (OSA) via a 3-m-long silica multimode ﬁber
(MMF; core diameter: 50 µm) to mitigate the optical coupling loss at the POF
output end. The power of its spectral peak was measured with 10 times averaging.
Both POF ends were connected to the silica ﬁbers by butt-coupling using “ferruleconnector/subscriber connector (FC/SC)” adaptors [26].

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Fig. 2.

3

Experimental setup for the refractive index sensing using thecrushed polymer optical ﬁber (POF). MMF: multimode ﬁber,
OSA: optical spectrum analyzer, and SMF: single-mode ﬁber.

Experimental results

First, we investigated the RI dependence of the transmitted power when the crushed
region was surrounded by a sucrose solution (∼0.1 ml) at 25°C. By controlling theconcentration in the range from 0% to 60%, its RI was varied from 1.318 to 1.426(with the RI dependence on optical wavelength taken into consideration) [28]. The
temperature dependence of the transmitted power was then measured in the range
from 10°C to 35°C when the ambient RI of the crushed region was 1.366(concentration: 30%).
Figs. 3(a) and (b) show the side-view and top-view micrographs of the crushed
POF, respectively. The ∼600-µm-long region was crushed with a maximal depth of∼210 µm. The crushed region had a height of ∼260 µm, which was approximately
half of the initial outer diameter (490 µm) of the uncrushed region. The width ofthe crushed region was ∼610 µm, which was ∼1.2 times larger than the initial outerdiameter. The boundary between cladding and overcladding layers is clearlyobserved in Fig. 3(a); at the crushed region, the core and cladding layer (notexposed to the atmosphere) are pressed and curved in the same way as theovercladding layer. The optical propagation loss at the crushed region was approximately 26 dB at 1550 nm.

(a)Fig. 3.

© IEICE 2017
DOI: 10.1587/elex.14.20170962
Received September 19, 2017
Accepted October 3, 2017
Publicized October 20, 2017
Copyedited November 10, 2017(b)

Micrographs of the crushed polymer optical ﬁber: (a) side viewand (b) top view.

Fig. 4(a) shows the measured RI dependence of the transmitted power at 25°C.
The vertical axis was normalized so that the power at RI = 1.318 (sucrose concentration = 0%) became 0 dB. With increasing RI, the transmitted power increased(the loss decreased) with a dependence coefﬁcient of 173 dB/RIU (To verify thereproducibility of this sensor, we tested three more samples, all of which showedsimilar behaviors with dependence coefﬁcients ranging from 153 to 175 dB/RIU).

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IEICE Electronics Express, Vol.14, No.21, 1–7(a)Fig. 4.

(b)(a) Measured relative transmitted power dependence on therefractive index (RI). (b) Relative transmitted power dependence on temperature when the refractive index was 1.366.

Simple comparison of the absolute value with those in previous reports measured
under different conditions and POF structures cannot avoid being unfair. However,the positive coefﬁcient of the transmitted power against RI was in agreement withsome of the previous POF-based results [20, 29], and can be explained as follows.
Namely, as the RI of the liquid approaches that of the overcladding layer (∼1.59),
the light leaking at the crushed region re-enters the POF [29]. Note that this positive
dependence is opposite to those of an ultrasonically crushed POF [24] and a
D-shaped POF [30], which have much longer crushed/D-shaped regions (6 mm and10 mm, respectively). When the RI was 1.366 (sucrose concentration: 30%; nearly
located at the midpoint of the linear region), the measurement error, deﬁned as thestandard deviation of the power ﬂuctuations (the power was measured every 1minute for 1 hour) was þ=0:26 dB. Note that the transmitted power was notinﬂuenced when part of the uncrushed regions of the POF was touched by thesucrose solutions.
Finally, we measured the temperature dependence of the transmitted power inthe temperature range from 10°C to 35°C, as shown in Fig. 4(b). The RI was set to1.366. The vertical axis was normalized so that the power measured at 25°C
became 0 dB; the range of the vertical axis (from −12 to +12 dB) was set to almost
the same as that of Fig. 4(a) for easy comparison. In order to evaluate the genuine
characteristics of the sensor, the inﬂuence of the RI dependence of the sucrose
solution on temperature was compensated in advance [31]. The transmitted power
was almost constant irrespective of temperature with a ﬂuctuation amplitude of
þ=0:24 dB, which is comparable to the measurement error of this sensor. This
result indicates that temperature-independent RI sensing is potentially feasible byuse of this method.
4© IEICE 2017
DOI: 10.1587/elex.14.20170962
Received September 19, 2017
Accepted October 3, 2017
Publicized October 20, 2017
Copyedited November 10, 2017Conclusion

We demonstrated RI sensing using a POF crushed with a slotted screwdriver. This
structure can be easily fabricated with no need to employ external heat sources,chemicals, or ultrasonic transducers. The only requirement is to simply press
the end of a slotted screwdriver against part of the POF. The transmitted power

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was experimentally shown to have a linear RI dependence with a coefﬁcient of
173 dB/RIU in the RI range from ∼1.32 to ∼1.43. When the RI was 1.366, thetemperature dependence of the transmitted power was found to be negligible ataround room temperature, which leads to the possibility of temperature-independent RI sensing. Thus, we believe that this simple RI sensing technique using aPOF crushed with a slotted screwdriver will be of great convenience in achievingtemperature-independent ﬁber-optic RI sensing for chemical, biomedical, pharmaceutical, and process-control applications in the future.
Acknowledgments
This work was supported by MLIT Construction Technology Research andDevelopment Subsidy Program, by JSPS KAKENHI Grant Numbers 17H04930
and 17J07226, and by research grants from the Japan Gas Association, the ESPECFoundation for Global Environment Research and Technology, the Association for
Disaster Prevention Research, the Fujikura Foundation, and the Japan Association
for Chemical Innovation.

© IEICE 2017
DOI: 10.1587/elex.14.20170962
Received September 19, 2017
Accepted October 3, 2017
Publicized October 20, 2017Copyedited November 10, 20177