BRIEF NOTEJapanese Journal of Applied Physics 56, 078002 (2017)https://doi.org/10.7567/JJAP.56.078002

Temperature sensing based on multimodal interference in polymer optical ﬁbers:
Room-temperature sensitivity enhancement by annealing
Tomohito Kawa1, Goki Numata1, Heeyoung Lee1, Neisei Hayashi2, Yosuke Mizuno1*, and Kentaro Nakamura1
1

Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan
Research Center for Advanced Science and Technology, The University of Tokyo, Meguro, Tokyo 153-8904, Japan2*E-mail: ymizuno@sonic.pi.titech.ac.jp
Received February 15, 2017; accepted May 19, 2017; published online June 13, 2017
To date, we have developed a temperature sensor based on multimodal interference in a polymer optical ﬁber (POF) with an extremely highsensitivity. Here, we experimentally evaluate the inﬂuence of annealing (heat treatment) of the POF on the temperature sensitivity at room
temperature. We show that the temperature sensitivity is enhanced with increasing annealing temperature, and that, by annealing the POF at90 °C, we can achieve a temperature sensitivity of +2.17 nm/°C, which is 2.9 times larger than that without annealing (+0.75 nm/°C).
© 2017 The Japan Society of Applied Physics

Researchers have developed various types of ﬁber-optic
interference-based temperature sensors,1–3) among whichthose based on multimodal interference in multimode ﬁbers(MMFs) have gained considerable attention for two decadeson account of their advantages, such as system simplicity,
cost eﬃciency, and high sensitivity.4–22) A lot of conﬁgurations have been reported so far (summarized in Ref. 7),
and one of the types that can be most easily implemented iscategorized as “single-mode-multimode-single-mode” (SMS)
type,7–24) in which an MMF is simply sandwiched between
two single-mode ﬁbers (SMFs).
Let us review the recent developments of SMS-based temperature sensors. Ten years ago, using a 1.8-m-long silica
graded-index (GI-) MMF, a temperature sensitivity of +58.5
pm=°C was obtained at 1550 nm.8) Two years later, the relationship between the temperature sensitivity and the optical
wavelength was partially clariﬁed, and both the absolute value
and the sign of the sensitivity were found to depend on socalled “critical wavelengths”,9) which are inﬂuenced by thecore diameter and=or the dopant of silica MMFs. In 2012, apolymer optical ﬁber (POF) was ﬁrst employed as the MMF
to be sandwiched between two SMFs; when a 0.16-m-longpoly(methyl methacrylate) (PMMA)-based step-index POF
was used, a temperature sensitivity of −56.8 pm=°C was obtained at 1570 nm.21) However, the extremely high propagation loss of PMMA-based POFs at ∼1300 and ∼1550 nm(≫100 dB=m)25) limited the transmitted optical power,
resulting in a lower signal-to-noise ratio of the measurement.
To overcome this problem, since 2014, we have been
studying SMS-based temperature sensing using perﬂuorinated (PF) GI-POFs,22–24) which are the only POFs with arelatively low loss even at 1300 nm (∼0.05 dB=km) and at
1550 nm (∼0.25 dB=m).26) Using a PFGI-POF with a lengthof 1.0 m and a core diameter of 120 µm, we obtained atemperature sensitivity of +0.74 nm=°C at room temperature
at 1300 nm,22) the absolute value of which is more than 15times the value reported using silica MMFs.
We also found that the absolute value of the temperaturesensitivity is signiﬁcantly enhanced with increasing temperature toward ∼80 °C,23) which is close to the glass-transition
temperature (∼100 °C) of the core polymer.27) For instance,
when the core diameter was 120 µm, the sensitivity at 82 °C
at 1300 nm was 85.6 nm=°C, which is ∼78 times the value
obtained at room temperature and more than 1500 times thevalue previously reported using a silica MMF. This dramaticrise in the temperature sensitivity appears to be caused by thefact that the physical properties (core diameter, ﬁber length,
refractive index, etc.) of the POF are altered at the temperature several tens of degrees lower than the glass-transition
temperature.28) This physical change is probably irreversible
and may have some inﬂuence on the temperature sensitivity
at room temperature, but no report has been provided thus
far. Note that the sensitivity of POF-based pressure sensors
has been reported to be improved by annealing of the POF
in advance.29)
In this work, we experimentally evaluate the inﬂuence ofannealing (heat treatment) of the POF on the temperaturesensitivity at room temperature. The temperature sensitivity isfound to be enhanced with increasing annealing temperature.
We obtain a temperature sensitivity of +2.17 nm=°C byannealing the POF at 90 °C, which is 2.9 times the value
without annealing (+0.75 nm=°C).
In an SMS structure, the light incident from an SMF toan MMF excites a few lower modes in the MMF, because
the spot-sizes are diﬀerent between the fundamental or the0th modes in the SMF and the MMF. The excited modes
propagate along the MMF, tracing their respective paths, andreach the other MMF-to-SMF boundary. Here, the net ﬁeld
coupled to the SMF depends on the relative phase diﬀerencesamong the multiple modes that have propagated along theMMF. If we assume that the MMF and the two SMFs are
all axially aligned, the modes excited in the MMF can beconsidered to be axially symmetric, and the optical power
Pout output from the SMS structure is given as10)
Pout ¼ ja20 þ a21 exp ið 0  1 ÞLð1Þþ a22 exp ið 0  2 ÞL þ    j2 ;where i denotes an integer, ai the ﬁeld amplitudes of thei-th modes at the ﬁrst SMF-to-MMF boundary, βi the propagation constants of the i-th modes, and L the MMF length.
According to Eq. (1), it is clear that the optical output power
depends on the physical changes caused by temperature;and consequently, temperature sensing turns out feasible bymeasuring the shift of spectral dips or peaks.
In the experiment, as an MMF in the SMS structure,
we employed 0.3-m-long PFGI-POFs with a core diameterof 120 µm. The experimental setup is schematically shownin Fig. 1. A swept-source laser (SSL; sweep rate: 20 kHz;
central wavelength: 1320 nm; bandwidth: 110 nm) was used
as a broadband light source. Before injection to the POF, the078002-1© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 078002 (2017)

T. Kawa et al.

Fig. 1. (Color online) Schematic of experimental setup. OSA, opticalspectrum analyzer; PC, polarization controller; PFGI-POF, perﬂuorinatedgraded-index polymer optical ﬁber; SSL, swept-source laser.

Fig. 3. (Color online) Dip wavelength measured with respect totemperature before and after 90 °C annealing. The solid lines are linear ﬁts.

(a)

(b)

Fig. 2. (Color online) Temperature dependences of the spectral dip
measured at room temperature (a) before and (b) after 90 °C annealing.

laser output was input to a polarization controller (PC) so thatthe polarization-dependent spectral ﬂuctuations8) were suppressed. The laser output was then injected to the POF via an“SC=PC-FC=PC” adaptor, at which the ends of the POF andthe SMF were butt-coupled to each other.30) The other POFto-SMF boundary was also butt-coupled in the same manner.
The spectrum of the transmitted light was ﬁnally monitored
using an optical spectrum analyzer (OSA). The entire lengthof the POF was placed in a thermostatic chamber; each
POF sample was annealed at 60, 70, 80, 90, 100 °C for
1 h [excluding the temperature rising and falling durations(approximately 30 and 10 min, respectively); suﬃciently
long for measured data saturation], and then the temperaturesensitivity around room temperature (27 °C) was measured.
Figures 2(a) and 2(b) show the temperature dependencesof the spectral dip at room temperature before and after
annealing at 90 °C, respectively. The clearest dips thatappeared around the central wavelength of the light source
were selected. In both the ﬁgures, the dips shifted to longer
wavelength with increasing temperature. The temperaturedependences of the central wavelengths of the dips are shownin Fig. 3. The dependences were both almost linear before
and after annealing, however the temperature sensitivity
before annealing was +0.75 nm=°C (which agrees well withthe previously obtained value),22) while that after annealing
was +2.17 nm=°C; 2.9 times sensitivity enhancement wasobtained by annealing at 90 °C.
We also attempted to perform the same measurement when
the annealing temperature was 60, 70, 80, and 100 °C. The
measurement was successful in the cases of 60, 70, and 80 °C;
but the POF was melted during annealing at 100 °C, inducing
such a high loss that no light was able to propagate throughthe POF. The annealing temperature dependence of thetemperature sensitivity at room temperature is shown in
Fig. 4. With increasing annealing temperature, the temperature sensitivity at room temperature was enhanced, probably
because the change in the physical properties of the polymer

Fig. 4. (Color online) Temperature sensitivity at room temperature plotted
as a function of annealing temperature.

becomes striking as the annealing temperature is close to theglass-transition temperature. If users pursue a higher sensitivity, an annealing temperature of 90 °C is the most suitableof all the ﬁve temperatures tested in our experiment, because
the measurement was still performed with a moderate signalto-noise ratio only with ∼10-dB loss [Fig. 2(b)]. Even at
60 °C annealing, we observed slight increase in the sensitivity,
which is within the inevitable measurement error caused by
the SMF-to-POF coupling conditions diﬀerent sample bysample.22–24) Though we have to admit that this measurement
is only for PFGI-POFs with particular physical and structuralproperties, we can still draw a more general conclusion thatthe temperature sensing performance of POF-based SMS
sensors is to some extent controllable by annealing. It shouldbe also noted that our measurement result does not necessarily
indicate that the measurable temperature range of SMS-based
sensors using POFs is limited up to ∼60 °C, because once
POFs are annealed, their sensing properties may not be
aﬀected in the temperature range below the annealing temperature. In this meaning, annealing could be regarded asa method of increasing the upper limit of the measurable
temperature range.
In summary, regarding the SMS-based temperature sensors
using POFs, we evaluated the inﬂuence of annealing of thePOF on the temperature sensitivity at room temperature.
When a 0.3-m-long PFGI-POF with a core diameter of
120 µm was used, the temperature sensitivity was enhancedwith increasing annealing temperature. A temperature sensitivity of +2.17 nm=°C was achieved by annealing the POF at
90 °C. This value was 2.9 times larger than that without
annealing (+0.75 nm=°C). Thus, we believe that annealing
is a useful technique for improving (or controlling) thetemperature sensitivity at room temperature of POF-based
SMS sensors.

078002-2© 2017 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 56, 078002 (2017)

T. Kawa et al.

Acknowledgments This work was supported by JSPS KAKENHI Grant
Numbers 25709032, 26630180, 25007652, 17J07226, and 17H04930, and byresearch grants from the Japan Gas Association, the ESPEC Foundation for
Global Environment Research and Technology, and the Association for Disaster
Prevention Research.

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