IEICE TRANS. ELECTRON., VOL.E101–C, NO.7 JULY 2018

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INVITED PAPER

Special Section on Distinguished Papers in Photonics

Multimodal Interference in Perfluorinated Polymer Optical Fibers:
Application to Ultrasensitive Strain and Temperature Sensing∗
Yosuke MIZUNO†a) , Member, Goki NUMATA† , Tomohito KAWA† , Heeyoung LEE† ,Neisei HAYASHI†† , Nonmembers, and Kentaro NAKAMURA† , Member

SUMMARY
We review the recent advances on strain and temperaturesensing techniques based on multimodal interference in perfluorinated (PF)graded-index (GI) polymer optical fibers (POFs). First, we investigate
their fundamental characteristics at 1300 nm. When the core diameter is62.5 μm, we obtain strain and temperature sensitivities of −112 pm/με and+49.8 nm/◦ C, the absolute values of which are, by simple calculation, approximately 13 and over 1800 times as large as those in silica GI multimode fibers, respectively. These ultra-high strain and temperature sensitivities probably originate from the unique PF polymer used as core material.
Subsequently, we show that the temperature sensitivity (absolute value) issignificantly enhanced with increasing temperature toward ∼70◦ C, which
is close to the glass-transition temperature of the core polymer. When thecore diameter is 62.5 μm, the sensitivity at 72◦ C at 1300 nm is 202 nm/◦ C,
which is approximately 26 times the value obtained at room temperatureand >7000 times the highest value previously reported using a silica multimode fiber. Then, we develop a single-end-access configuration of thisstrain and temperature sensing system, which enhances the degree of freedom in embedding the sensors into structures. The light Fresnel-reflected
at the distal open end of the POF is exploited. The obtained strain andtemperature sensitivities are shown to be comparable to those in two-endaccess configurations. Finally, we discuss the future prospects and giveconcluding remarks.
key words: polymer optical fibers, perfluorinated polymer, modal interference, strain sensing, temperature sensing

1.

Introduction

Strain and temperature sensing using optical fibers has
been an active research area because of light weight, small
size, and insensitivity to ambient electromagnetic fields.
While a number of methods exploiting fiber Bragg gratings(FBGs) [1], [2], long-period gratings (LPGs) [3], [4], Raman
scattering [5], [6], and Brillouin scattering [7]–[11] havebeen developed, one of the simple and low-cost implementations is based on the interference among guided modes of amultimode fiber (MMF). Since Mehta et al [12] reported thefirst demonstration in 2003, to date, various structures havebeen proposed, as well summarized in Table 1 in Ref. [13].
Among them, the most typical is a so-called single-modemultimode-single-mode (SMS) structure [14]–[16], wherean MMF is sandwiched between two single-mode fibers(SMFs).
Manuscript received September 27, 2017.
Manuscript revised December 22, 2017.
∗
This is a review article.
†
The authors are with Institute of Innovative Research, Tokyo
Institute of Technology, Yokohama-shi, 226–8503 Japan.
††
The author is with Research Center for Advanced Science andTechnology, The University of Tokyo, Tokyo, 153–8904 Japan.
a) E-mail: ymizuno@sonic.pi.titech.ac.jp
DOI: 10.1587/transele.E101.C.602

Based on the SMS structure comprising a 1.8-m-longsilica graded-index (GI-) MMF, Liu et al [14] have obtained
a strain sensitivity of +18.6 pm/με and a temperature sensitivity of +58.5 pm/◦ C at 1550 nm. Tripathi et al [15] haveshown 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 the maximal measurable strain, Huang et al [17] havedeveloped a large-strain sensor using a 0.16-m-long polymethyl methacrylate (PMMA)-based step-index plastic optical fiber (POF) as an MMF, and obtained strain and temperature sensitivities of −1.72 pm/με and +56.8 pm/◦ C, respectively, at 1570 nm. However, the propagation loss of
PMMA-based POFs at telecommunication wavelength is sohigh ( 1 × 100 dB/m) that the maximal sensing range is inherently limited to several tens of meters. Besides, discriminative measurement of strain and temperature 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 ∼0.25 dB/m at 1550 nm (or∼0.05 dB/m at 1300 nm) in which Brillouin scattering has
been experimentally observed [19]–[23].
In this work, first, we implement the strain and temperature sensors based on the SMS structure comprisinga PFGI-POF and investigate their sensing performance at
1300 nm [24]. Three 1-m-long PFGI-POFs with core diameters of 50, 62.5, and 120 μm are used. When the core diameter is 62.5 μm, the strain sensitivity is −112 pm/με, theabsolute value of which is, by simple calculation, approximately 13 times as large as that in a silica GI-MMF (thiscalculation is based on the assumption that the sensitivity islinearly dependent on optical wavelength and MMF length,
which has now been clarified to be incorrect [15], [25]; however, as no fair method of comparison has not been established yet, we will use this simple evaluation in this paper). The temperature sensitivity at room temperature is+49.8 nm/◦ C, which is over 1800 times as large as that ina silica GI-MMF. Next, we investigate the performance oftemperature sensors composed of 1-m-long PFGI-POFs in awide temperature range from room temperature to high temperature around the glass-transition temperature [26]. Whenthe core diameter is 62.5 μm, the temperature sensitivity (ab-c 2018 The Institute of Electronics, Information and Communication Engineers
Copyright MIZUNO et al.: MULTIMODAL INTERFERENCE IN PERFLUORINATED POLYMER OPTICAL FIBERS: APPLICATION TO ULTRASENSITIVE STRAIN

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solute value) increases with increasing temperature, reaching +202 nm/◦ C at ∼72◦ C, which is 26 times as high asthat at room temperature. This behavior is confirmed to be
unique to POFs with relatively low glass-transition temperature. Then, by using Fresnel reflection at the distal open
end of the PFGI-POF, we develop a single-end-access configuration of this strain and temperature sensors [25]. We
experimentally show that when the length of the PFGI-POF
is the same, the strain and temperature sensitivities do notlargely change regardless of the configurations (i.e., either
single- or two-end access). This single-end-access configuration will enhance the degree of freedom in embedding thesensors into structures. Finally, we discuss future prospects
and present concluding remarks.
2.

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. Reproduced with permission from Ref. [24]; copyright (2014) IEEE.

Principle

The SMS structure consists of an MMF connected at bothends to identical SMFs. At the first SMF/MMF boundary,light is injected into the MMF through the lead-in SMF. As
the spot size of the 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 theirrespective propagation constants. At the second SMF/MMF
boundary, the net field coupled from these modes to thelead-out SMF is determined by the relative phase diﬀerencesamong various modes of the MMF. When the fibers are axially aligned at the first SMF/MMF boundary, only the axially symmetric modes of the MMF are excited. According
to the detailed calculation [16], the optical power in the leadout SMF can be expressed as
Pout = |a20 + a21 exp i(β0 − β1 )L+ a22 exp i(β0 − β2 )L + · · · |2 ,(1)

where ai is the field amplitude of the i-th mode at the first
SMF/MMF boundary, βi is the propagation constant of thei-th mode, and L is the length of the MMF. Equation (1) indicates that the optical power in the lead-out SMF is aﬀectedby physical changes (such as strain and temperature) influencing the propagation constants and the length of the MMF.
By measuring this shift either in terms of power or the spectral location of peak/dip, the changes can be quantitatively
obtained.
3.

Fundamental Characterization

3.1

Experimental Setup

Three 1-m-long PFGI-POFs with diﬀerent core diameters of
50, 62.5, and 120 μm 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 that of thecladding layer is 1.342 [27]. The polycarbonate reinforcement overcladding layer (diameter: 500 μm) reduces microbending losses and increases the load-bearing capability.

Fig. 2 Measured optical spectra before and after transmission throughthe POF. Reproduced with permission from Ref. [24]; copyright (2014)IEEE.

The numerical aperture is 0.185, and the propagation loss is∼0.25 dB/m at 1550 nm and ∼0.05 dB/m at 1300 nm.
Figure 1 depicts the experimental setup. The PFGIPOF was connected to silica SMFs by butt-coupling [19]. A
swept-source laser (SSL) was used as a light source, which
can be regarded as a broadband source with a central wavelength of 1320 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 andtemperature change were applied to the whole length of the
PFGI-POF. The room temperature was ∼27◦ C.
3.2 Experimental Results
Figure 2 shows the measured optical spectra before and after
transmission through the PFGI-POF with 62.5-μm core diameter. A characteristic dip was clearly observed at around1330 nm in the latter spectrum.
The measured dependence of this spectral dip on strain
is shown in Fig. 3 (a). With increasing applied strain, thespectral dip shifted to shorter wavelength. Figure 3 (b)shows the dip wavelength plotted as a function of strain.
The dependence was almost linear with a coeﬃcient of−1118 nm/% (= −112 pm/με), the absolute value of which
is approximately 13 and 7.7 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, as shown in Fig. 3 (c). Figure 3 (d) shows the dip wavelength dependence on temperature, leading to a coeﬃcient of +49.8 nm/◦ C, which is over1800 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

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the PFGI-POFs with 50-μm and 120-μm core diameters, and obtained the strain and temperature coeﬃcients of +3.42 pm/με and −4.71 nm/◦ C (50-μm core), and−8.21 pm/με and +0.74 nm/◦ C (120-μm core), respectively.
These values were much larger than those in silica GIMMFs with the same core diameters, but not as large asthat in the PFGI-POF with 62.5-μm core diameter. Thisis partially because the critical wavelength [15] with 62.5μm core diameter is located closer to ∼1300 nm (measured
wavelength region) than those with 50-μm and 120-μm core
diameters.
Considering that SMS-based sensors using silica GIMMFs with the same core diameters do not have large
strain and temperature coeﬃcients, these ultra-high strain
and temperature sensitivities of the PFGI-POF (especiallywith 62.5-μm core) probably originate not from its structure but from its unique material (PF polymer). In otherwords, the propagation constant of each mode and the lengthin perfluorinated polymer are much more highly dependent
on strain and temperature than those in other MMFs (either
glass or polymer).
Thus, in this Section, the strain and temperature sensors based on modal interference in the SMS structure
comprising a PFGI-POF was implemented, and their basic sensing performance was investigated using an SSL ataround 1300 nm. When the core diameter was 62.5 μm, thestrain and temperature sensitivities were −112 pm/με and+49.8 nm/◦ C, the absolute values of which were approximately 13 and > 1800 times as large as those in a silica GIMMF. We believe that the ultra-high temperature sensitivityof this sensor is attractive for such fields as will require precise temperature measurement. We also point out that thediﬀerence in sign between the strain and temperature coeﬃ-

Fig. 3 Measurement results for the POF with 62.5-μm core: (a) spectral
dependence on strain, (b) dip wavelength vs strain, (c) spectral dependence
on temperature, and (d) dip wavelength vs temperature. Reproduced withpermission from Ref. [24]; copyright (2014) IEEE.

cients will potentially lead to extremely accurate Brillouincombined discriminative (not distributed) measurement ofstrain and temperature [28].
4.

Temperature Sensitivity Enhancement

4.1 Experimental Setup
The same 1-m-long PFGI-POFs with three diﬀerent core diameters (50, 62.5, and 120 μm) were used as the MMFs.
The experimental setup for characterizing the PFGI-POFbased temperature sensors in a wide temperature range wasbasically the same as that shown in Fig. 1. Both ends of the
PFGI-POF were butt-coupled to silica SMFs [19], and thewhole length of the PFGI-POF was placed in a thermostaticchamber to control the ambient temperature. One importantdiﬀerence from Fig. 1 is the light source. As spectral measurement with as wide a span as possible is preferable in thisexperiment, we employed an ultra-wideband source (UWS1000, Santec) that emits super-continuum light with an output spectrum from 1100 to 1760 nm (pumped at 1550 nm).
Its spectral power density was higher than −30 dBm/nm overthe full range; this value is much larger than that of a standard white-light source, which cannot be used because of
the high propagation loss of the PFGI-POF. The UWS output was injected into the PFGI-POF, and the transmission
spectrum was measured using an OSA. The room temperature was 20◦ C.
4.2 Experimental Results
The measured optical spectra before and after transmissionof the PFGI-POF with 50-μm core diameter at room temperature are shown in Fig. 4. Before transmission, a relatively flat spectrum ranging from 1100 to 1760 nm was observed. The peak at 1550 nm corresponds to the pump frequency of the super-continuum generation. After transmission, not only ∼10–20-dB relatively uniform loss but also
wavelength-dependent loss was observed in the spectrum,
resulting in the several characteristic dips and peaks. These
dips and peaks originate from the modal interference, andtheir wavelengths are dependent on the ambient temperatureof the POF.
We then investigated the wide-range temperature de-Fig. 4 Measured optical spectra before and after transmission throughthe PFGI-POF. Copyright (2015) The Japan Society of Applied Physics(JSAP).

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Fig. 5 Dip-wavelength dependence on wide-range temperature in (a) the
PFGI-POF and (b) the silica GI-MMF with 50-μm core diameter. Diﬀerent colors were used to clarify diﬀerent dips. (c) Temperature sensitivity(absolute value of the temperature-dependence coeﬃcients) at 1300 nm in
the PFGI-POF and the silica GI-MMF plotted as functions of temperature.
Copyright (2015) JSAP.

pendence of the wavelengths of some relatively clear spectral dips in the PFGI-POF with 50-μm core diameter(Fig. 5 (a)). The measured data are discontinuously plotted because of the two reasons: (1) measurement at one
fixed temperature took ∼10 min since we needed to wait
for the temperature to be stable. Therefore, the measurements were performed in every 10◦ C range separately (i.e.,
20–30◦ C, 30–40◦ C, . . . ), with their data merged later (somepeaks appeared or disappeared with time on account of thepolarization fluctuations [14]; this instability could be mitigated by employing polarization diversity or polarization
scrambling); (2) when the dips shifted close to the 1550nm pump peak, they were buried by the spectral floor (thisproblem could be resolved by use of a properly designed
optical filter at 1550 nm). As shown in Fig. 5 (a), with increasing temperature, all the dip wavelengths decreased.
The dependence coeﬃcient at room temperature at 1300 nm
was approximately −5.3 nm/◦ C (calculated by interpolation/extrapolation), which moderately agrees with the previously reported value (−4.7 nm/◦ C) [24]. The absolute values of the coeﬃcients at shorter wavelengths were slightly
smaller than those at longer wavelengths in this wavelength
range, which indicates that the critical wavelength [15] islonger than ∼1700 nm. In the temperature range from 20 to∼50◦ C, the dependence coeﬃcient of any of the dips was
kept almost constant, but over ∼50◦ C, their absolute values
gradually became larger. The coeﬃcient at 67◦ C at 1300 nm
reached −85 nm/◦ C, which was ∼16 times the value at room
temperature. Over ∼67◦ C, the spectral change was so drastic that correct measurement was no longer performed.
Subsequently, we confirmed that this peculiar behavior is a unique characteristic to POFs by performing the

Fig. 6 Temperature dependences of (a) the dip wavelengths and (b) thecoeﬃcient in the PFGI-POF with 62.5-μm core diameter, and (c,d) those in
the PFGI-POFs with 120-μm core diameter. Copyright (2015) JSAP.

same measurement using a silica GI-MMF with the same
core diameter (50 μm). Figure 5 (b) shows the dip wavelengths measured as functions of temperature ranging from
20 to 130◦ C. The dependence coeﬃcient at room temperature at 1300 nm (−2.2 nm/◦ C) was close to that at∼130◦ C (−2.3 nm/◦ C), which indicates that the temperaturedependent gradual change in the coeﬃcients (in this temperature range) is unique to POFs. This behavior, similar to
the nonlinear temperature dependence of the Brillouin frequency shift in POFs [29], seems to have been caused by
the partial phase transition of the polymer material. The
temperature of ∼67◦ C, at which the largest coeﬃcient (absolute value) was obtained in the PFGI-POF, probably corresponds to the glass transition temperature of the polyperfluorobutenylvinyl ether (∼108◦ C; note that the phase transition, which has some influence on the thermal expansion
coeﬃcient and each modal index, generally occurs in thetemperature range of roughly 50◦ C in polymers [27] and thatthe starting temperature of phase transition is lower than theglass transition temperature by several tens of degrees). Figure 5 (c) shows the temperature sensitivities, i.e., the absolute values of the temperature-dependence coeﬃcients of thedip wavelength, at 1300 nm in the PFGI-POF and the silica
GI-MMF plotted as functions of temperature. It is clear thatthe temperature sensitivity in the PFGI-POF abruptly grows
large at 67◦ C, while that in the silica GI-MMF remains almost constant.
We also performed the same measurements using
the PFGI-POFs with 62.5-μm and 120-μm core diameters(Fig. 6 (a)–(d)). In both cases, with an increase in temperature, the dip wavelengths increased. The diﬀerence not onlyin the absolute value but also in the sign has been already reported [24] and explained by the structural influence on thecritical wavelengths [15] (which are shorter than 1100 nm

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for the 62.5-μm-core PFGI-POF and longer than 1700 nm
for the 120-μm-core PFGI-POF). The coeﬃcients at room
temperature at 1300 nm were +7.7 nm/◦ C for the 62.5-μmcore PFGI-POF and +1.1 nm/◦ C for the 120-μm-core PFGIPOF. The diﬀerence between these values and those previously reported [24] is probably caused by the manually prepared end-surfaces and lot-to-lot non-uniformity, leading to
the diﬀerent excited modes. The maximal values obtained athigh temperature at 1300 nm were +202 nm/◦ C for the 62.5μm-core PFGI-POF and +85.6 nm/◦ C for the 120-μm-core
PFGI-POF, which are ∼26 and ∼78 times larger than thoseat room temperature, respectively. The temperature sensitivity at 1300 nm of +202 nm/◦ C, obtained at ∼72◦ C using the62.5-μm-core PFGI-POF, is well over 7000 times the largest
value previously reported using a silica GI-MMF [14] andis, to the best of our knowledge, the highest value everreported in modal-interference-based temperature sensors.
Note that, although the dependence is nonlinear, the wavelength of a certain dip is in one-to-one correspondence withtemperature, which can be exploited for temperature sensing(we need to be careful in designing the system because themeasurement error is expected to vary with temperature).
The temperatures at which the largest temperature sensitivities (absolute values of the temperature-dependence coefficients) were experimentally obtained were ∼69, ∼72, and∼82◦ C in the PFGI-POFs with 50-, 62.5-, and 120-μm core
diameters, respectively, suggesting a positive correlation between the highest operable temperature and the core diameter of the POF.
Thus, in this Section, we investigated the performanceof SMS-based temperature sensors comprising 1-m-long
PFGI-POFs with 50-, 62.5-, and 120-μm core diameters in awide temperature range. When the core diameter is 62.5 μm,
with an increase in temperature, the temperature sensitivity,
i.e., the temperature-dependence coeﬃcients of the spectral
dips, increased, reaching +202 nm/◦ C at ∼72◦ C at 1300 nm,
which was over 26 times the value obtained at room temperature. Over ∼72◦ C, correct measurement was diﬃcult
due to the spectral instability. We verified that this behavior is unique to POFs with relatively low glass-transition
temperature by demonstrating that the temperature sensitivity of a silica-MMF-based sensor shows almost no changewith increasing temperature up to 130◦ C. Besides, considering that the phase-transition temperature of polymers canbe controlled by adding plasticizers and by copolymerizing/blending diﬀerent materials [30], we expect that such
an ultra-high temperature sensitivity will be achievable not
only around 70–80◦ C but also around somewhat arbitrary
temperature, opening up the way for more useful and practical applications.
5.

Single-End-Access Configuration

5.1

Motivation

Substantial eﬀorts have been made to improve the performance of SMS-based strain and temperature sensors, but al-

Fig. 7 Conceptual setups of (a) transmissive configuration and (b) reflectometric configuration. MMF, multimode fiber; SMF, single-mode fiber.

Fig. 8 Actual experimental setups of (a) transmissive configuration and(b) reflectometric configuration. OSA, optical spectrum analyzer; PFGIPOF, perfluorinated graded-index polymer optical fiber; PSCR, polarization scrambler; SSL, swept-source laser. The yellow lines indicate silica
single-mode fibers.

most all the SMS-based sensors reported thus far are twoend-access systems, in which both ends of the MMF are
physically connected to SMFs (Fig. 7 (a)). From a practical
viewpoint, single-end-access configurations (Fig. 7 (b)) withan enhanced degree of freedom in embedding the sensors
into structures are preferable. There are only three examples
of such single-end-access configurations; the first is a displacement sensor reported by Mehta et al [12], which comprises an SMF-MMF-airgap-mirror structure. It is, however,diﬃcult to directly employ this structure to measure strain
and temperature; in addition, use of the mirror reduces theease of handling. The second is a temperature sensor reported by Li et al [31], in which no descriptions on strain
sensing capability have been provided at all. The third is astrain sensor reported by Ribeiro et al [32], in which strain
is measured based on modulated power; its operating principle is quite diﬀerent from that of other frequency-based
SMS sensors.
Here, by using Fresnel reflection at the distal open endof the MMF, we develop a single-end-access configurationof SMS-based strain and temperature sensing systems, inwhich PFGI-POFs were employed as MMFs to obtain highsensitivities. We experimentally show that when the MMF
length is the same, the strain and temperature sensitivities
do not largely change regardless of the configurations (i.e.,either single- or two-end access).
5.2 Experimental Setup
The same type of PFGI-POF with a core diameter of
62.5 μm was used as the MMF. The POF length is 0.7 m.
The actual experimental setups of transmissive (for comparison) and reflectometric configurations are shown in
Fig. 8 (a) and 8 (b), respectively. In both configurations, an
SSL with a 20-kHz sweep rate was employed as a broadband light source with a central wavelength of 1320 nm
and a bandwidth of 110 nm. The laser output was first input to a polarization controller to suppress the polarizationdependent spectral fluctuations, and then injected into the

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Fig. 9 Measured spectral dependences on strain in (a) transmissive configuration and (b) reflectometric configuration. The dip wavelength dependences on strain in (c) transmissive configuration and (d) reflectometricconfiguration.

Fig. 10 Measured spectral dependences on temperature in (a) transmissive configuration and (b) reflectometric configuration. The dip wavelength
dependences on temperature in (c) transmissive configuration and (d) reflectometric configuration.

POF. In Fig. 8 (a), both ends of the POF were butt-coupled
to silica SMFs [19], and the spectrum of the transmitted light
was monitored using an OSA. In contrast, in Fig. 8 (b), one
end of the POF was butt-coupled to a silica SMF in the sameway, but the other end was kept open (flatly polished); thespectrum of the reflected light was monitored using an OSA.
Note that the optical circulator used in Fig. 8 (b) had an operating wavelength of 1280 to 1340 nm and insertion losses
of approximately 0.5 dB for both directions (i.e., port 1 toport 2 and port 2 to port 3). In both cases, the entire lengthof the POF was strained or heated. The room temperature
was approximately 28◦ C.

ously reported value [24]; the discrepancy probably originates from the imperfect end faces, asymmetric coupling to
the SMFs, and longitudinal structural non-uniformity of the
POF.
Subsequently, the dependences of the dip on temperature were also measured (Figs. 10 (a) (transmissive) and10 (b) (reflectometric)). The dips shifted to shorter wavelengths as temperature increased. The dip wavelengths plotted as functions of temperature (Figs. 10 (c) and 10 (d)) indicate that the temperature-dependence coeﬃcients were9.63 nm/◦ C (transmissive) and 10.1 nm/◦ C (reflectometric);
these values are also consistent with those previously reported [24]. Thus, the reflectometric configuration was experimentally proved to possess almost the same strain andtemperature sensitivities as those of transmissive configuration with an advantage of single-end accessibility. One
concern is that when the POF length is longer than several
dozen meters, the additional propagation loss may result in
the reflected spectrum buried by the noise floor. This problem could be mitigated by attaching a mirror at the end of
the POF.
Thus, in this Section, a single-end-access configuration of SMS-based strain and temperature sensing systems
exploiting Fresnel reflection at the distal open end of the
MMF (PFGI-POF) was developed. High strain and temperature sensitivities of −122.2 pm/με and 10.1 nm/◦ C, respectively, were obtained. These values were comparable
to those in the two-end-access configuration. This result
implied that, in analyzing the performance of SMS-based
sensors, we should take into consideration not only the (relatively fundamental) two modes but also many other modes
simultaneously (see Appendix of Ref. [25]). We anticipate
that our method will drastically enhance the ease of handlingof SMS-based fiber-optic sensors.

5.3

Experimental Results

The measured dependences of the spectral dips on strain intransmissive and reflectometric configurations are shown in
Fig. 9 (a) and 9 (b), respectively. In both cases, a relatively
broad single dip was observed in this range, which shiftedto shorter wavelengths with increasing applied strain. The
spectral power in Fig. 9 (b) was approximately 20 dB lower
than that in Fig. 9 (a); this is valid considering the loss of
Fresnel reflection at the POF end (∼2.2% reflection, corresponding to a 16.5 dB loss; by a simple calculation based
only on the core refractive index diﬀerence) and the additional POF propagation loss (if compared with the Fresnel
loss, negligibly low with this short POF (0.7 m)). In thereflectometric configuration, if necessary, we could form amirror at the open end of the POF to increase the spectral
power. Figures 9 (c) and 9 (d) show the dip wavelengths
plotted as functions of strain. The dependences were bothalmost linear with slopes of −1218 nm/% (= −121.8 pm/με;transmissive) and −1222 nm/% (= −122.2 pm/με; reflectometric). These values moderately agree with the previ-

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6.

Conclusion

Recent advances on the strain and temperature sensing
based on the SMS structure comprising a PFGI-POF were
presented. First, the fundamental sensing characteristics
were investigated at 1300 nm. Three 1-m-long PFGI-POFs
with core diameters of 50, 62.5, and 120 μm were tested.
When the core diameter was 62.5 μm, the strain sensitivity was −112 pm/με, the absolute value of which wasapproximately 13 times as large as that in a silica GIMMF. The temperature sensitivity at room temperature was+49.8 nm/◦ C, which was over 1800 times as large as that ina silica GI-MMF. Subsequently, the performance of temperature sensors composed of 1-m-long PFGI-POFs in a wide
temperature range from room temperature to high temperature (around the glass-transition temperature) was investigated. When the core diameter was 62.5 μm, the temperature sensitivity (absolute value) increased with increasing
temperature, reaching +202 nm/◦ C at ∼72◦ C, which was 26
times as high as that at room temperature. By showing thatthe temperature sensitivity of a silica MMF-based sensor
does not change with increasing temperature to 130◦ C, thisbehavior was verified to be unique to POFs with relatively
low glass-transition temperature. Finally, by using Fresnel
reflection at the distal open end of the PFGI-POF, a singleend-access configuration of this strain and temperature sensors was implemented. We experimentally proved that when
the length of the PFGI-POF is the same, the strain and temperature sensitivities do not largely change regardless of theconfigurations (i.e., either single- or two-end access).
In addition to these achievements, we have found thatthe strain and temperature sensitivities do not largely depend on the length of the MMF [25], that the temperaturesensitivity at room temperature can be enhanced by annealing [33] and that the modal interference in partially chlorinated GI POFs is potentially applicable to high-sensitivity
temperature measurement with low strain sensitivity [34].
Future tasks include clarification of the critical wavelength
dependence on fiber properties [15] (which will be a useful
guideline for the researchers who search for new materials
with high sensitivities), extremely large strain sensing withplastic deformation [35], clarification of the shortest lengthof the MMF, clarification of cross eﬀect of strain and temperature [36], and discriminative (not distributed) sensing ofstrain, temperature, and some other parameters by combined
use of FBG and/or Brillouin scattering. We anticipate that
this review paper will be a good basis for researchers working on POF-based SMS sensing technology.
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Yosuke Mizuno
received B.E., M.E., andDr.Eng. degrees in electronic engineering fromthe University of Tokyo, Japan, in 2005, 2007,
and 2010, respectively. From 2010 to 2012, asa Research Fellow (PD) of the Japan Society for
the Promotion of Science (JSPS), he worked onpolymer optics at Tokyo Institute of Technology,
Japan. In 2011, he stayed at BAM Federal Institute for Materials Research and Testing, Germany, as a Visiting Research Associate. Since
2012, he has been an Assistant Professor at thePrecision and Intelligence Laboratory (presently, Institute of Innovative Research), Tokyo Institute of Technology, where he is active in fiber-optic
sensing, polymer optics, and ultrasonics. He is a senior member of theIEEE Photonics Society, and a member of the Japan Society of Applied
Physics (JSAP), the Optical Society of Japan (OSJ), and the Institute ofElectronics, Information, and Communication Engineers (IEICE) of Japan.

Goki Numata
received a B.E. degree from
Shibaura Institute of Technology, Tokyo, Japan,
in 2014, and an M.E. degree from Tokyo Institute of Technology, Tokyo, Japan, in 2016. He
is the winner of the Outstanding Student Award
2014, Opto-Electronics Conference (OPE), theInstitute of Electronics, Information, and Communication Engineers (IEICE) of Japan. He iscurrently working at Olympus Corporation as anelectronics engineer.

Tomohito Kawa
received a B.E. degreefrom Tokyo Institute of Technology, Tokyo,
Japan, in 2016. He is currently studying polymer fiber optics for his M.E. degree at Tokyo
Institute of Technology. His research interests
include fiber-optic sensors and polymer optics.
He is a member of the Japan Society of Applied
Physics (JSAP).

Heeyoung Lee
received a B.E. degree in mechanical engineering from Kyungpook National
University, Korea, in 2014, and an M.E. degree
in information processing from Tokyo Instituteof Technology, Japan, in 2017. Since 2017,she has been continuing to study fiber-optic sensors for her Dr.Eng. degree at Tokyo Institute ofTechnology. She is a Research Fellow (DC1) of
the Japan Society for the Promotion of Science(JSPS) and a member of the Japan Society ofApplied Physics (JSAP).

IEICE TRANS. ELECTRON., VOL.E101–C, NO.7 JULY 2018610

Neisei Hayashi
received a B.E. degreein advanced production from the Gunma National College of Technology, Japan, in 2011,
and M.E. and Dr.Eng degrees in electronic engineering from Tokyo Institute of Technology,
Japan, in 2013 and 2015, respectively. Since
2016, as a Research Fellow (PD) of the Japan
Society for the Promotion of Science (JSPS), hehas been continuing to study fiber sensing at the
University of Tokyo, Japan. His research interests include fiber-optic sensing, polymer optics,
and nonlinear optics. He is the winner of the Telecom System Technology Award for Student 2012 and the NF Foundation R&D Encouragement
Award 2016. He is a member of the Japan Society of Applied Physics(JSAP), and the Institute of Electronics, Information, and Communication
Engineers (IEICE) of Japan.

Kentaro Nakamura
received B.E., M.E.,
and Dr.Eng. degrees from Tokyo Institute ofTechnology, Japan, in 1987, 1989, and 1992, respectively. Since 2010, he has been a Professor at the Precision and Intelligence Laboratory(presently, Institute of Innovative Research), Tokyo Institute of Technology. His research field isthe applications of ultrasonic waves, measurement of vibration and sound using optical methods, and fiber-optic sensing. He is the winner ofthe Awaya Kiyoshi Award for Encouragement ofResearch from the Acoustical Society of Japan (ASJ) in 1996, and the Best
Paper Awards from the Institute of Electronics, Information and Communication Engineers (IEICE) in 1998 and from the Symposium on UltrasonicElectronics (USE) in 2007 and 2011. He is a member of the IEEE, the ASJ,
the Japan Society of Applied Physics (JSAP), the IEICE, and the Instituteof Electrical Engineers of Japan (IEEJ).