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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 13, JULY 1, 2015

Thermal Memory Effect in Polymer Optical Fibers
Kazunari Minakawa, Neisei Hayashi, Yosuke Mizuno, Member, IEEE, and Kentaro Nakamura, Member, IEEE

Abstract— The basic properties of a thermal memory effectof polymer optical fibers (POFs) are experimentally investigated.
We measure the thermally induced loss as a function of timeat several high temperatures, and find that the loss becomesalmost constant after heating for ∼200 s. The loss remains
unchanged even after the heated section is cooled to room
temperature. We subsequently measure the optical time-domainreflectometry traces under three different conditions: 1) before a
POF section is heated; 2) shortly after the POF section is heatedat high temperature; and 3) after the heated section is cooled
to room temperature. The traces measured under 2) and 3) aremoderately identical, which indicates that the thermal memory
effect can be exploited in developing excess-heat detecting systemin future.
Index Terms— Polymer optical fibers, temperature sensing,
optical time-domain reflectometry, thermal memory effect.

I. I NTRODUCTION
IBER-OPTIC strain and temperature sensors have been
extensively studied as promising tools for structural healthmonitoring [1]–[10]. Conventionally, their sensing heads weremainly composed of glass optical fibers, which cannot withstand strain of over several percent. To tackle this problem,
strain and temperature sensors using polymer opticalfibers (POFs) [11] have attracted a lot of attention, because
POFs are so flexible that they can withstand larger strain of
several tens of percent (even 100% [12]). To date, various
strain and temperature sensors using POFs have been developed, including those based on fiber Bragg gratings [13], [14],modal interference [15]–[17], Brillouin scattering [18]–[25],and Rayleigh scattering [26]–[29].
One of the unique features of POF-based strain sensors
is what we call a “strain memory effect” [12], [19], [20],[26], [27], with which POFs themselves store the information(magnitude and location) of applied large strain even after thestrain is released. This nature is caused by plastic deformationof the polymer materials. We expect that, by exploiting thiseffect, we need not always put expensive analyzers at theends of the sensing fibers; after earthquakes etc, an officer has

F

Manuscript received February 19, 2015; revised March 25, 2015; accepted
April 6, 2015. Date of publication April 10, 2015; date of current version
June 9, 2015. This work was supported in part by the Grants-in-Aid for Young
Scientists (A) under Grant 25709032, in part for the Challenging Exploratory
Research under Grant 26630180 through the Japan Society for the Promotionof Science (JSPS), and in part by the Research Grants through the General
Sekiyu Foundation, Iwatani Naoji Foundation, and SCAT Foundation. The
work of N. Hayashi was supported by the Grant-in-Aid for JSPS Fellows
under Grant 25007652.
The authors are with the Precision and Intelligence Laboratory, Tokyo
Institute of Technology, Yokohama 226-8503, Japan (e-mail: kminakawa@sonic.pi.titech.ac.jp; hayashi@sonic.pi.titech.ac.jp; ymizuno@sonic.pi.titech.
ac.jp; knakamur@sonic.pi.titech.ac.jp).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LPT.2015.2421950

only to go round with a single analyzer. Then, the application
range of fiber-optic sensing technology, which has been limited
only to large-scale civil structures owing to its high cost,
can be extended to smaller-scale multi-family residences andindividual houses.
In the meantime, the optical loss of POFs is dependent on
temperature. The loss is reported to increase with increasing
temperature [25], [26], and especially at high temperatureof >100 °C, irreversible loss has been observed [25], [26].
In addition, we have recently found that, when a POF sample
is heated at such high temperature and then cooled to room
temperature, its Brillouin frequency shift (BFS) does not returnto the initial value at room temperature [22]. These results
suggest the existence of a useful “thermal memory effect”
of POFs, on the detail of which no report has been provided
yet. This effect might be exploited to detect the excess heat
only by interrogating the POFs later, for instance, at regular(periodic) inspections.
In this work, we experimentally investigate the “thermal
memory effect” of POFs. First, thermally induced loss isplotted as a function of time at several high temperatures.
The loss becomes almost constant after heating of the POFs
for ∼200 s. We then measured the optical time-domainreflectometry (OTDR) traces under three different conditions:(1) before a POF section is heated, (2) after the POF sectionis heated at high temperature for a sufficiently long time, and(3) after the heated section is cooled to room temperature.
By comparing these results, we discuss the future prospectsof the thermal memory effect.
II. M EASUREMENT A PPARATUS
As POFs to be measured, we used perfluorinatedgraded-index (PFGI-) POFs [30] with three-layered
structure (Ginover; Sekisui Chemical Co., Japan): core
(50 μm diameter), cladding (100 μm diameter), andovercladding (750 μm diameter). The core and cladding
layersarecomposed
of
doped
and
undopedpolyperfluorobutenylvinyl ether, respectively. The refractive
index at the center of the core is 1.356, while that of thecladding is 1.342. The protecting overcladding layer consistsof polycarbonate. The propagation loss is relatively low in awide wavelength range from ∼600 nm to 1500 nm [30].
One of the most popular distributed loss sensing techniques
is OTDR based on Rayleigh scattering [31], [32]. In general,
short optical pulses (∼10 ns) are launched into a fiber under
test (FUT), and the Rayleigh-backscattered signal is detectedon the same end. The events that increase the optical loss
(bending, connection, etc) are spatially resolved with a timeof-flight technique, i.e., using a famous equation: L = ct/2n,

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MINAKAWA et al.: THERMAL MEMORY EFFECT IN POFs

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Fig. 1.
Schematic of OTDR-2100GOF system. DAQ, data acquisition;
LD, laser diode; MMF, multimode fiber; PC, personal computer; PD, photodetector.

where L is the relative position along the FUT, c is thelight velocity in vacuum, t is the time delay between pulse
injection and detection, and n is the refractive index of the
FUT core.
In order to detect the weak backscattered light in
PFGI-POFs with high sensitivity, a commercial photoncounting OTDR system (OTDR-2100GOF; Scientex Inc.,
Japan) was employed in this experiment. The schematic of
the OTDR system is depicted in Fig. 1. A semiconductor laser
diode (LD) outputs an optical pulse at 650 nm with a peak
power of several tens of milliwatts, the width of which canbe set to either 2 or 10 ns by direct modulation of the driving
current (via a personal computer (PC)). The optical pulse isdirected to the FUT via a silica-based multimode fiber (MMF)
coupler. The backscattered light is then input to a photoncounting photo detector (PD), with which the optical intensity
is detected as pulse density of photons, generating a densitymodulated pulse train. By repeatedly recording many results
for the same inputs, a histogram plotted as a function of thelocation along the FUT is obtained, which corresponds to theconventional OTDR trace (a figure intelligibly explaining thismechanism is found in Ref [26]). Note that, according to the
manufacturer, the vertical axis of the OTDR trace measured
using this system is not the backscattered power itself but an
attenuation rate to an internally defined reference value (notdescribed in the manual).
III. E XPERIMENTSFirst, we clarify the heating duration required for thethermally induced loss in POFs to be constant. Subsequently,
we verify and discuss the thermal memory effect of POFs bycomparing the measured OTDR traces.
A. Temporal Dependence of Thermally Induced Loss
The experimental setup for measuring the temporaldependence of the thermally induced loss in POFs is shownin Fig. 2. The continuous wave (CW) output from an LD
at 641 nm with a power of ∼1 mW was directly coupled
to a PFGI-POF and injected into a silica GI-MMF via
butt-coupling using an “SC” adaptor [18], and the transmitted
power was monitored using an optical power meter (PM).
A 150-mm-long section of the POF was guided in a furnace
placed on a heater; the ambient temperature of the POF in
the furnace was precisely controlled using a thermocouple.
The measured temporal variations of the thermally induced
loss at 115, 120, 125, 129, and 133 °C are shown in Fig. 3(a).

Fig. 2. Experimental setup for measuring temporal variations of the loss
thermally induced in the PFGI-POF. The top-left inset shows the crosssectional view of the furnace; all the values are in the unit of millimeters.

Fig. 3.
(a) Temporal variations of the loss thermally induced in
the PFGI-POF. (b) Thermally induced loss measured at t = 400 s, plotted
as a function of heating temperature.

Fig. 4. Experimental setup for evaluating the thermal memory effect in
PFGI-POFs using OTDR.

At 115 and 120 °C, the loss was within the range of measurement error (+/−2 dB; caused by the fluctuations of the
LD output and by the slight tension applied to the heatedsection), while at 125, 129, and 133 °C, the loss clearly
increased with time t. Here, t = 0 is defined as the moment
at which the POF section was vertically introduced into thefurnace using motorized stages. When the heated section wasabove ∼120 °C, we could observe the strong scattering of
the incident light to the FUT along the section. This result
indicates that the thermally induced loss was generated as
the radiation loss along the heated section. The induced loss
increased during t =∼100–200 s, and then became almost
constant.
Figure 3(b) shows the heating temperature dependenceof the induced loss measured at t = 400 s. The loss was
negligible below ∼120 °C, but above ∼120 °C, it became
higher with increasing temperature. The amount of this loss

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Fig. 5.

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 13, JULY 1, 2015

Measured OTDR traces before heating, after heating (at (a) 125 °C, (b) 129 °C, and (c) 133 °C), and after cooling.

remained almost identical even after the heated POF section
was cooled to room temperature. This threshold temperature
is higher than the glass transition temperature of thecore/cladding material by ∼15 °C. Considering that the phasetransition of polymer materials to rubbery state generally
occurs in the range ∼10–20 °C around the glass transition
temperature [33] and that the core temperature should be lower
than the ambient temperature measured using a thermocouple,
it is strongly implied that the thermal induction of the loss in
POFs originates from the fluctuation of the interfacial surface
between the core and cladding which was induced by thephase transition of the core and cladding to rubbery state.
B. Evaluation of Thermal Memory EffectWe evaluate the thermal memory effect in POFs in
distributed sensing using the experimental setup shownin Fig. 4. The OTDR system was connected to a PFGI-POF
with a length of roughly 30 m, and its 150-mm-long section
was heated in the furnace, the temperature of which wascontrolled in the same manner as in Section III. A. The opticalpulse width was set to 2 ns, corresponding to ∼220-mm
spatial resolution in the POF. When the POF section washeated, high temperature environment was maintained for atleast 600 s (sufficiently longer than 200 s; see Fig. 3(a)) toensure that the thermally induced loss grew constant.
Figs. 5(a)–(c) show the measured OTDR traces under
three conditions: (1) at room temperature, before heating the
POF section; (2) shortly after heating the section for a sufficiently long time at high temperatures (125, 129, and 133 °C
in Figs. 5 (a), (b), and (c), respectively); and (3) at room
temperature, after cooling the heated section. The reflectedsignal was integrated for 600,000 optical pulses. The dataat positions of <8 m were not presented due to the signal
fluctuations inevitably caused by the system. The signal levels
at the position of 8 m were not the same for the three cases
because of the unstable butt-coupling of the PFGI-POF andthe silica GI-MMF [18].
In the case of 125 °C (Fig. 5(a)), the POF length was 29 m;
at the open end of the POF, Fresnel reflection signal wasobserved. Before heating the POF section, no drastic drop
was observed in the OTDR trace. After the POF section
was heated, a drastic drop of ∼3.5 dB was observed at theposition of ∼15 m, where the heated section was located.
Here, the amount of the drop (3.5 dB) did not agree withthe loss observed in Fig. 3(b), which seems to be caused by the
specification of the OTDR system (see Section II) and by thedifference of the LD outputs, i.e., CW or pulses. After coolingthe heated section, the drop became slightly larger (∼4.4 dB)
probably because the POF was damaged when it was verticallytaken out of the furnace using the stages. Even if so, this result
proves that, though detailed quantitative evaluation is difficultat present, the OTDR trace when the POF was locally heatedat high temperature in the past can be obtained at a later time,i.e., the operation of the thermal memory effect of the POF.
It is also notable that no quantifiable reflection was induced at
the heated section, which might be a cost-effective method(with no need for additional devices) for suppressing theFresnel reflection to improve the signal-to-noise ratio in
POF-based Brillouin optical correlation-domain reflectometry [10], [34] (a drastic increase in loss simply by bending
is difficult for POFs with large cores [30]).
In the case of 129 °C (Fig. 5(b)), the POF lengthwas ∼30 m. Similar behaviors were observed; the drop of
the OTDR trace at the heated section (position = ∼20 m)
was ∼5.5 dB after heating and ∼6.0 dB after cooling, whichwere higher than those at 125 °C, as is expected in Fig. 3(b).
In the case of 133 °C (Fig. 5(c)), the POF length was ∼31 m.
Basically, similar behaviors were observed, but the drop at theheated section (position = ∼25 m) was so large (8.0 dB)
that the trace was buried by the noise floor.
Thus, the thermal memory effect in POFs was confirmed,
but the temperature range at which this effect can be
exploited is limited to ∼120–129 °C in this experiment. Suchtemperature range could be set somewhat arbitrarily in future,
because the glass transition temperature in polymer materials,
which probably affects this effect, can be controlled by adding
plasticizers and/or by copolymerizing different materials [35].
This will be a useful feature in designing practical excess-heat
detectors based on this thermal effect of POFs.
IV. C ONCLUSION
The fundamental properties of the “thermal memory
effect”—-an attractive function of POF-based temperature
sensors—-was experimentally investigated. First, we measured

MINAKAWA et al.: THERMAL MEMORY EFFECT IN POFs

the loss thermally induced at several high temperatures asa function of time. When the temperature was higher than∼120 °C, the loss grew almost constant after heating the
POFs for ∼200 s. This loss was maintained even after the
POFs were cooled to room temperature. Next, we measured
the OTDR traces under three different conditions: (1) before a
POF section was heated, (2) after the POF section was heatedat high temperature for a time much longer than 200 s, and(3) after the heated section was cooled to room temperature.
The traces obtained under (2) and (3) did not exhibit
large change irrespective of the heating temperature, whichindicated that the thermal memory effect will be useful
in developing excess-heat detecting systems in the near
future. What remain to be solved include the improvement of
quantitative evaluation of temperature, the broadening of thetemperature range in which this effect can be exploited, andthe discrimination of strain and thermal memory effects.
ACKNOWLEDGMENT
The authors are indebted to Scientex Inc., Japan, andHiroki Tanaka, Tokyo Institute of Technology, Japan, for theirexperimental assistance. They also acknowledge the staff ofthe Technical Department, Tokyo Institute of Technology,
Japan, for fabricating the furnace used in the experiments.
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