LETTERIEICE Electronics Express, Vol.12, No.2, 1–6

Strain and temperaturesensing based on multimodeinterference in partiallychlorinated polymer opticalﬁbers
Goki Numata, Neisei Hayashi, Marie Tabaru, Yosuke Mizunoa),and Kentaro Nakamura
Precision and Intelligence Laboratory, Tokyo Institute of Technology,
Yokohama 226–8503, Japana) ymizuno@sonic.pi.titech.ac.jp

Abstract: We develop strain and temperature sensors based on multimodeinterference in a partially chlorinated graded-index polymer optical ﬁber
(PCGI-POF) and experimentally investigate their sensing performance atroom temperature using incident light of ∼1300 nm wavelength. The lengthof the PCGI-POF was 0.7 m. The measured strain and temperature sensitivities were −4.47 pm/µε and +9.66 nm/°C/m, respectively, the absolutevalues of which were 0.29 times and over 350 times the values in a silica
GI-MMF. This result suggests that the modal interference in PCGI-POFs ispotentially applicable to high-sensitivity temperature measurement with low
strain sensitivity.
Keywords: polymer optical ﬁbers, strain sensors, temperature sensors
Classiﬁcation: Fiber optics, Microwave photonics, Optical interconnection, Photonic signal processing, Photonic integration and systems
References© IEICE 2015
DOI: 10.1587/elex.12.20141173
Received December 12, 2014
Accepted December 16, 2014
Publicized January 7, 2015
Copyedited January 25, 2015

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001467

1© IEICE 2015
DOI: 10.1587/elex.12.20141173
Received December 12, 2014
Accepted December 16, 2014
Publicized January 7, 2015
Copyedited January 25, 2015

Introduction

Strain and temperature sensing using optical ﬁbers has been extensively studied
owing to its advantages such as small size, light weight, and immunity againstelectromagnetic noise. A number of methods based on ﬁber Bragg gratings (FBGs)[1, 2], long-period gratings (LPGs) [3, 4], Rayleigh scattering [5, 6], Raman
scattering [7, 8], and Brillouin scattering [9, 10, 11, 12, 13] have been developed,and one of the most simple and cost-efﬁcient techniques involves the exploitationof modal interference in a multimode ﬁber (MMF) [14, 15, 16, 17, 18, 19]. To date,various structural implementations have been proposed (see Table 1 in Ref [14]).
Among them, a structure comprising an MMF sandwiched between two singlemode ﬁbers (SMFs) — often referred to as a single-mode-multimode-single-mode(SMS) structure — has been most widely studied because of its simplicity and easeof fabrication.

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IEICE Electronics Express, Vol.12, No.2, 1–6

A strain sensitivity of +18.6 pm/µε and a temperature sensitivity of +32.5 pm/°C/m have been reported at 1550 nm (corresponding to +15.6 pm/µε and+27.3 pm/°C/m at 1300 nm, respectively) by using the SMS structure with a
1.8-m-long silica graded-index (GI) MMF [15]. Subsequently, it was proven thatthe ﬁber structure (e.g., core diameter) and core material (e.g., dopant) of silica
MMFs signiﬁcantly inﬂuence not only the absolute values but also the signs of thestrain and temperature sensitivities [17]. However, glass ﬁbers are generally sofragile that they cannot be utilized as sensor heads for monitoring large strains ofover ∼3%.
One method for extending the maximal measurable strain involves the use of apolymer optical ﬁber (POF) as an MMF; a POF can endure large strains of over
several tens of percent. Based on the SMS structure comprising a 0.16-m-longpolymethyl methacrylate (PMMA)-based step-index POF, strain and temperaturesensitivities of −2.82 pm/µε and +581.9 pm/°C/m, respectively, have been obtained at 1570 nm (corresponding to −2.34 pm/µε and +481.8 pm/°C/m at1300 nm) [19]. Subsequently, in order to lower the propagation loss at telecom
wavelengths, SMS-based sensors have been developed using perﬂuorinated (PF)GI-POFs [20], and ultra-high strain and temperature sensitivities of −112 pm/µε
and +49.8 nm/°C/m, respectively, have been achieved at 1300 nm with a core
diameter of 62.5 µm [21]. The absolute values of these sensitivities are 12.9 and>1800 times those in a silica GI-MMF [15]. However, PFGI-POFs, composed of
cyclic transparency optical polymer (CYTOP®), have two drawbacks: high fabrication cost and low thermal resistance (<70°C). Recently, partially chlorinated(PC) GI-POFs have attracted considerable attention because of their low propagation loss at the 650–690/760–790 nm region, relative cost efﬁciency, and highthermal resistance of >100°C [22, 23], which may enable higher-temperaturesensing.
In this study, we implement and characterize strain and temperature sensors
based on the SMS structure comprising a 0.7-m-long PCGI-POF of core diameter
120 µm at a wavelength of ∼1300 nm. The measured strain sensitivity is −4.47 pm/µε, the absolute value of which is 0.29 times the value in a silica GI-MMF. In
contrast, the measured temperature sensitivity at room temperature is +9.66 nm/°C/m, which is over 350 times that in a silica GI-MMF. This result indicates thatthe multimodal interference in PCGI-POFs is potentially applicable to highly
sensitive temperature sensing with low strain sensitivity.
2© IEICE 2015
DOI: 10.1587/elex.12.20141173
Received December 12, 2014
Accepted December 16, 2014
Publicized January 7, 2015
Copyedited January 25, 2015

Principle

The SMS structure consists of an MMF connected at both ends to identical SMFs.
At the ﬁrst SMF/MMF interface, light enters the MMF through the lead-in SMF.
Because the spot size of the fundamental mode of the MMF is generally different
from that of the SMF, a few lower modes are excited. The excited modes propagate
through the MMF with their respective propagation constants. At the second SMF/
MMF interface, the relative phase differences among the different modes of theMMF determine the net ﬁeld coupled from these modes to the lead-out SMF. Whenthe ﬁbers are axially aligned at the ﬁrst SMF/MMF interface, only the axially3

IEICE Electronics Express, Vol.12, No.2, 1–6

Fig. 1.

Measured optical spectra before and after transmission throughthe PCGI-POF.

symmetric modes are excited in the MMF. According to a detailed calculation [16],the optical power in the lead-out SMF can be expressed as
Pout ¼ ja0 2 þ a1 2 exp ið0  1 ÞL þ a2 2 exp ið0  2 ÞL þ    j2 ;

ð1Þwhere ai is the ﬁeld amplitude of the i-th mode at the ﬁrst SMF/MMF interface, iis the propagation constant of the i-th mode, and L is the MMF length. Equation (1)indicates that the optical power in the lead-out SMF is affected by changes inphysical properties (such as strain and temperature) inﬂuencing the propagation
constants and/or length of the MMF. Therefore, such changes can be quantiﬁed bymeasuring the shift in either the power or the spectral location of a peak/dip.
3

Experimental setup

We employed a PCGI-POF (VT120, Sekisui Chemical) with an outer diameter of
750 µm, a core diameter of 120 µm, and a core refractive index of ∼1.52. This is theonly commercially available speciﬁcation. Considering the high propagation loss attelecom wavelengths (approximately 47 dB/m, measured by a simple cut-backmethod), the length of the PCGI-POF was set as 0.7 m.
The experimental setup was essentially the same as that used in Ref [21]. The
PCGI-POF was connected to silica SMFs by butt-coupling [24, 25], i.e., the ends ofthe SMFs ﬁtted with ‘FC’ connectors were connected to both ends of the PCGIPOF ﬁtted with ‘SC’ connectors via FC/SC adaptors. A swept-source laser with a20-kHz sweep rate was used as a broadband light source with a central wavelengthof 1320 nm and a bandwidth of 110 nm. The laser output was passed into the PCGIPOF, and the spectrum of transmitted light was monitored using an optical spectrum analyzer. The polarization state was optimized using a polarization controller.
The entire length of the PCGI-POF was strained or heated. The room temperature
was approximately 27°C.
4© IEICE 2015
DOI: 10.1587/elex.12.20141173
Received December 12, 2014
Accepted December 16, 2014
Publicized January 7, 2015
Copyedited January 25, 2015

Experimental results

Fig. 1 shows the measured optical spectra before and after transmission through thePCGI-POF. As the propagation loss of the PCGI-POF at 1300 nm is extremely
high, the spectrum after transmission was drastically diminished in intensity, but4

IEICE Electronics Express, Vol.12, No.2, 1–6(a)Fig. 2.

Measured strain dependence of (a) spectral dip and (b) dip
wavelength.

(a)Fig. 3.

© IEICE 2015
DOI: 10.1587/elex.12.20141173
Received December 12, 2014
Accepted December 16, 2014
Publicized January 7, 2015
Copyedited January 25, 2015

(b)

(b)

Measured temperature dependence of (a) spectral dip and (b)dip wavelength.

several characteristic dips were still observed. Among them, the spectral dip at∼1290 nm was the most stable with respect to changes in the polarization state;
therefore, the sensing properties were investigated using this dip. Note that thespectral distortion was signiﬁcant in PCGI-POFs because of their imperfect end
faces, asymmetric coupling to the SMFs, and longitudinal non-uniformity of theﬁber structure (fabrication technology of special POFs has not been satisfactorilyestablished yet).
The measured dependence of the spectral dip on strain is shown in Fig. 2(a).
The dip shifted to shorter wavelengths with increasing applied strain. Fig. 2(b)shows the dip wavelength plotted as a function of strain. The dependence wasalmost linear with a slope of −44.7 nm/% (¼ 4:47 pm/µε), the absolute value of
which is approximately 0.29, 1.9, and 0.040 times the value in a silica GI-MMF[15], a PMMA-based POF [19], and a PFGI-POF [21], respectively. The dependence of the dip on temperature was also measured (Fig. 3(a)). The dip shiftedto longer wavelengths as temperature increased. Fig. 3(b) shows the dip wavelength plotted as a function of temperature; the dependence was almost linear with aslope of +6.76 nm/°C, which corresponds to 9.66 nm/°C/m. Although this (abso-5IEICE Electronics Express, Vol.12, No.2, 1–6lute) value is ∼0.2 times that in a PFGI-POF [21], it is still over 350 and ∼20 times
the value in a silica GI-MMF [15] and a PMMA-based POF [19], respectively. The
large slope of the dip-wavelength-temperature plot indicates highly sensitive
temperature measurability, while the small slope of the dip-wavelength-strain plot
indicates that PCGI-POF-based SMS sensors are less sensitive to applied strain.
Thus, the modal interference in PCGI-POFs, which have high thermal resistanceand cost efﬁciency compared to PFGI-POFs, can be potentially exploited todevelop high-sensitivity temperature sensors with reduced strain sensitivity.
In this experiment, the core diameter of the PCGI-POF was 120 µm. It is worth
noting that if PCGI-POFs with different core diameters are available in the future,
their sensing characteristics may differ substantially from that observed in thisstudy [17, 21].
5Conclusion

Strain and temperature sensors based on multimode interference in an SMS
structure composed of a 0.7-m-long PCGI-POF were developed, and their sensing
properties were experimentally investigated at room temperature with incident lightof ∼1300 nm wavelength. Strain and temperature sensitivities of −4.47 pm/µε and+9.66 nm/°C/m were obtained, the absolute values of which are 0.29 times
and over 350 times the values in a silica GI-MMF, respectively. This result suggeststhat the heat-resistant and cost-effective PCGI-POF-based SMS sensor is a goodcandidate for performing high-sensitivity temperature measurement with low strain
sensitivity.
Acknowledgments
This work was partially supported by Grants-in-Aid for Young Scientists (A)(no. 25709032) and for Challenging Exploratory Research (no. 26630180) fromthe Japan Society for the Promotion of Science (JSPS) and by research grants fromthe General Sekiyu Foundation, the Iwatani Naoji Foundation, and the SCAT
Foundation. N. H. acknowledges a Grant-in-Aid for JSPS Fellows (no. 25007652).

© IEICE 2015
DOI: 10.1587/elex.12.20141173
Received December 12, 2014
Accepted December 16, 2014
Publicized January 7, 2015
Copyedited January 25, 20156