Ultra-Sensitive Strain and Temperature
Sensing Based on Modal Interference in
Perfluorinated Polymer Optical Fibers
Volume 6, Number 5, October 2014
Goki Numata
Neisei Hayashi
Marie Tabaru
Yosuke Mizuno, Member, IEEE
Kentaro Nakamura, Member, IEEE

DOI: 10.1109/JPHOT.2014.2352637
1943-0655 Ó 2014 IEEEIEEE Photonics Journal

Sensing in PF Polymer Optical Fibers

Ultra-Sensitive Strain and Temperature
Sensing Based on Modal Interference in
Perfluorinated Polymer Optical Fibers
Goki Numata, Neisei Hayashi, Marie Tabaru,
Yosuke Mizuno, Member, IEEE, and Kentaro Nakamura, Member, IEEE
Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
DOI: 10.1109/JPHOT.2014.2352637
1943-0655 Ó 2014 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received July 16, 2014; accepted August 4, 2014. Date of publication August 28, 2014;
date of current version September 11, 2014. This work was supported in part by Grants-in-Aid forYoung Scientists (A) under Grant 25709032 and for Challenging Exploratory Research under Grant
26630180 from the Japan Society for the Promotion of Science (JSPS) and in part by research
grants from the General Sekiyu Foundation, the Iwatani Naoji Foundation, and the SCAT Foundation.
The work of N. Hayashi was supported by a Grant-in-Aid for JSPS Fellows under Grant 25007652.
Corresponding author: Y. Mizuno (e-mail: ymizuno@sonic.pi.titech.ac.jp).

Abstract: We implement the strain and temperature sensors based on multimode interference in perfluorinated (PF) graded-index (GI) plastic optical fibers, and investigate
their sensing performance at 1300 nm. We obtain strain and temperature sensitivities of112 pm="=m and þ49:8 nm= C/m, the absolute value of which are 12.9 and over1800 times as large as those in silica GI multimode fibers, respectively. These ultra-highstrain and temperature sensitivities probably originate from the unique core material, i.e.,
PF polymer.
Index Terms: Polymer optical fibers, perfluorinated polymer, modal interference, strain
sensing, temperature sensing.

1. Introduction
Fiber-optic strain and temperature sensing has been an active research area because of lightweight, small size, and insensitivity to ambient electromagnetic fields. While a number ofmethods exploiting fiber Bragg gratings (FBGs) [1], [2], long-period gratings (LPGs) [3], [4],Raman scattering [5], [6], and Brillouin scattering [7]–[11] have been developed, one of the simple and low-cost implementations is based on the interference among guided modes of a multimode fiber (MMF). Since Mehta et al. [12] reported the first demonstration in 2003, to date,various structures have been proposed, as well summarized in Table 1 in [13]. Among them, themost typical is a so-called single-mode-multimode-single-mode (SMS) structure [14]–[16], wherean MMF is sandwiched between two single-mode fibers (SMFs).
Based on the SMS structure comprising a 1.8-m-long silica graded-index (GI-) MMF, Liu et al.
[14] have obtained a strain sensitivity of þ10:3 pm="=m and a temperature sensitivity ofþ32:5 pm= C/m at 1550 nm (corresponding to þ8:7 pm="=m and þ27:3 pm= C/m at 1300 nm,
respectively). Tripathi et al. [15] have shown that not only the absolute values but also the signsof the strain and temperature sensitivities are highly dependent on the structure (e.g., core
diameter) and the material (e.g., dopant) of silica MMFs. Recently, in order to enhance themaximal measurable strain, Huang et al. [17] have developed a large-strain sensor using aVol. 6, No. 5, October 2014

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Sensing in PF Polymer Optical Fibers

Fig. 1. Schematic of experimental setup. OSA, optical spectrum analyser; PC, polarization controller; PFGI-POF, perfluorinated graded-index plastic optical fiber; SMF, single-mode fiber; SSL,
swept-source laser.

Fig. 2. Measured optical spectra before and after transmission through the POF.

0.16-m-long polymethyl methacrylate (PMMA)-based step-index plastic optical fiber (POF)
as an MMF, and obtained strain and temperature sensitivities of 17:6 pm="=m andþ581:9 pm= C/m, respectively, at 1570 nm (corresponding to 14:6 pm="=m andþ481:8 pm= C/m at 1300 nm). However, the propagation loss of PMMA-based POFs at telecommunication wavelength is so high ð 1  105 dB/kmÞ that the maximal sensing range is inherently limited to several tens of meters. Besides, discriminative measurement of strain andtemperature is also important for practical use; it can be theoretically achieved by combined
use of multimode interference and another physical phenomenon, such as Brillouin scattering,which has, however, not been observed in PMMA-based POFs. Perfluorinated graded-index(PFGI-) POFs [18] are the only POFs that have a relatively low loss of 250 dB/km at
1550 nm (or 50 dB/km at 1300 nm) in which Brillouin scattering has been experimentally observed [19]–[22].
In this work, we implement the strain and temperature sensors based on the SMS structure
comprising a PFGI-POF and investigate their sensing performance at 1300 nm. Three 1-m-long
PFGI-POFs with core diameters of 50, 62.5, and 120 m are used. When the core diameter is62.5 m, the strain sensitivity is 112 pm="=m, the absolute value of which is 12.9 times aslarge as that in a silica GI-MMF. The temperature sensitivity at room temperature isþ49:8 nm= C/m, which is over 1800 times as large as that in a silica GI-MMF.

2. Principle
The SMS structure consists of an MMF connected at both ends to identical SMFs. At the first
SMF/MMF boundary, light is injected into the MMF through the lead-in SMF. As the spot size ofthe fundamental mode of the MMF is generally different from that of the SMF, the first few
modes of the MMF are excited and propagate through the MMF with their respective propagation constants. At the second SMF/MMF boundary, the net field coupled from these modes tothe lead-out SMF is determined by the relative phase differences among various modes of the
MMF. When the fibers are axially aligned at the first SMF/MMF boundary, only the axiallyVol. 6, No. 5, October 2014

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Fig. 3. Measurement results for the POF with 62.5-m core. (a) Spectral dependence on strain,(b) dip wavelength versus strain, (c) spectral dependence on temperature, and (d) dip wavelength
versus temperature.

symmetric modes of the MMF are excited. According to the detailed calculation [16], the opticalpower in the lead-out SMF can be expressed as
Pout ¼ a21 þ a22 exp ið1  2 ÞL þ a23 exp ið1  3 ÞL þ . . .

2(1)

where ai is the field amplitude of the i-th mode at the first SMF/MMF boundary, i is the propagation constant of the i-th mode, and L is the length of the MMF. Equation (1) indicates that the
optical power in the lead-out SMF is affected by physical changes (such as strain and temperature) influencing the propagation constants and/or the length of the MMF. By measuring thisshift either in terms of power or the spectral location of peak/dip, the changes can be quantitatively obtained.

3. Experimental Setup
Three 1-m-long PFGI-POFs with different core diameters (50, 62.5, and 120 m) were employed. The core and cladding layers are composed of doped and undoped polyperfluorobutenylvinyl ether, respectively. The refractive index at the center of the core is 1.356, whereas thatof the cladding layer is 1.342 [23]; these values do not depend strongly on the optical wavelength [24]. The polycarbonate reinforcement overcladding layer (diameter: 500 m) reduces
microbending losses and increases the load-bearing capability. The numerical aperture is
0.185, and the propagation loss is 250 dB/km at 1550 nm and 50 dB/km at 1300 nm.
Fig. 1 depicts the experimental setup. The PFGI-POF was connected to silica SMFs by buttcoupling [19], i.e., the ends of the SMFs fitted with FC connectors were connected to both endsof the POF fitted with SC connectors via FC/SC adaptors. A swept-source laser (SSL) was used

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Fig. 4. Measurement results for the POF with 50-m core. (a) Spectral dependence on strain, (b) dip
wavelength versus strain, (c) spectral dependence on temperature, and (d) dip wavelength versus
temperature.

as a light source, which can be regarded as a broadband source with a central wavelength of1320 nm and a bandwidth of 110 nm. The laser output was, after polarization adjustment,
launched into the PFGI-POF, and the transmitted light spectrum was monitored with an optical
spectrum analyzer (OSA). Strain and temperature changes were applied to the whole length of
the PFGI-POF. The room temperature was approximately 27  C.

4. Experimental Results and Discussions
Fig. 2 shows the measured optical spectra before and after transmission through the PFGI-POF
with 62.5-m core diameter. A characteristic dip was clearly observed at around 1330 nm in thelatter spectrum.
The measured dependence of this spectral dip on strain is shown in Fig. 3(a). With increasing applied strain, the spectral dip shifted to shorter wavelength. Fig. 3(b) shows the dip wavelength plotted as a function of strain. The dependence was almost linear with a coefficient of
1118 nm/%/m ð¼ 112 pm="=mÞ, the absolute value of which is approximately 12.9 and7.7 times as large as those in a silica GI-MMF [14] and a PMMA-based POF [17], respectively.
In the same way, as temperature increased, this spectral dip shifted to longer wavelength, asshown in Fig. 3(c). Fig. 3(d) shows the dip wavelength dependence on temperature, leading toa coefficient of þ49:8 nm= C/m, which is over 1800 and 100 times as large as those in a silica
GI-MMF [14] and a PMMA-based POF [17], respectively.
We also performed the same measurement using the PFGI-POFs with 50-m and 120-m
core diameters, as shown in Figs. 4(a)–(d) and 5(a)–(d), and obtained the strain and temperatureVol. 6, No. 5, October 2014

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Fig. 5. Measurement results for the POF with 120-m core. (a) Spectral dependence on strain,(b) dip wavelength versus strain, (c) spectral dependence on temperature, and (d) dip wavelength
versus temperature.

coefficients of þ3:42 pm="=m and 4:71 nm= C/m (50-m core), and 8:21 pm="=m andþ0:74 nm= C/m (120-m core), respectively. These values were much larger than those insilica GI-MMFs with the same core diameters, but not as large as that in the PFGI-POF with
62.5-m core diameter.
Considering that SMS-based sensors using silica GI-MMFs with the same core diameters do
not have large strain and temperature coefficients, these ultra-high strain and temperature sensitivities of the PFGI-POF (especially with 62.5-m core) probably originate not from its structure but from its unique material. In other words, the propagation constant of each mode andthe length in perfluorinated polymer are much more highly dependent on strain and temperaturethan those in other MMFs (either glass or polymer).

5. Conclusion
The strain and temperature sensors based on modal interference in the SMS structure comprising a PFGI-POF was implemented, and their sensing performance was investigated using an
SSL at around 1300 nm. When the core diameter was 62.5 m, the strain and temperature sensitivities were 112 pm="=m and þ49:8 nm= C/m, the absolute values of which were 12.9 and9 1800 times as large as those in a silica GI-MMF. We believe that the ultra-high temperaturesensitivity of this sensor is quite attractive for such fields as will require precise temperature
measurement. We also point out that the difference in sign between the strain and temperature
coefficients will potentially lead to extremely accurate Brillouin-combined discriminative measurement of strain and temperature [25].

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