Sensors and Actuators A 223 (2015) 114–118
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Highly sensitive liquid-sealed multimode ﬁber interferometric temperature sensor Pengbing Hu a , Zhemin Chen a,∗ , Mei Yang a , Jingyi Yang b , Chuan Zhong c a b c
Research and Development Centre of Metrology Technology, Zhejiang Province Institute of Metrology, Hangzhou 310018, Zhejiang, China College of Optical & Electronic Technology, China Jiliang University, Hangzhou 310018, Zhejiang, China Centre for Research on Adaptive Nanostructures and Nanodevices, School of Physics, Trinity College Dublin, Dublin 2, Ireland
a r t i c l e
i n f o
Article history: Received 21 June 2014 Received in revised form 31 December 2014 Accepted 8 January 2015 Available online 17 January 2015 Keywords: Optical ﬁber Optical ﬁber measurement Temperature measurement Interferometry
a b s t r a c t A highly sensitive optical ﬁber temperature sensor based on liquid-sealed coreless multimode ﬁber (also called no core ﬁber, NCF) interferometer is proposed and experimentally demonstrated. By inserting the interferometer into a liquid-sealed capillary, a simple and highly sensitive ﬁber temperature sensor can be implemented. Owing to the high thermo-optic coefﬁcient of the liquid and thermal expansion of the sealant, the interferometric spectra of the proposed sensor are shifting obviously with the variation of temperature; temperature response of the sensor can be effectively modulated through this way. Experimental results show that the sensitivity of the temperature sensor can be improved by tuning the refractive index (RI) value of the sealed liquid; a maximum value of 5.15 nm/◦ C has been obtained when the RI value of the sealed liquid is 1.450, it is close to the RI of the NCF. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Multimode ﬁber interferometers (MMFIs) have been extensively studied and widely developed in a variety of research areas and industrial applications owing to their simple fabrication, fast response, corrosion resistance, anti-electromagnetic interference properties, etc. So far, there have been plenty of MMFI-based structures proposed to monitor environmental factors, such as temperature [1–3], refractive index (RI) [4,5], displacement [6,7], curvature [8,9], and others [10,11]. Conventional MMFI-based all optical ﬁber temperature sensors could only achieve a relatively low sensitivity (∼10 pm/◦ C), which was restricted by the relatively low thermo-optic and thermal expansion coefﬁcients of the silica basis components. Techniques which are aiming at improving the sensitivity are essential; some attempts have been made to enhance the temperature response, such as employment of specially designed multimode ﬁber and micro/macro-bending of multimode ﬁber structure in MMFI-based sensors. For instance, Li et al. presented a MMFI-based temperature sensor with a special multimode ﬁber used instead. Owing to its high thermo-optic coefﬁcient polymer cladding, the
∗ Corresponding author. E-mail addresses: [email protected]
(P. Hu), [email protected]
(Z. Chen). http://dx.doi.org/10.1016/j.sna.2015.01.009 0924-4247/© 2015 Elsevier B.V. All rights reserved.
sensitivity could reach up to 3.19 nm/◦ C, however, the measured temperature range was limited from 28 to 38 ◦ C for its broad free spectrum range . Moreover, a dual-cascaded MMFI structure temperature sensor was proposed with a sensitivity of 88 pm/◦ C , and micro/macro-bending of MMFIs were implemented and successfully improved the temperature sensitivity to 31.97 pm/◦ C and 11.6 pm/◦ C [6,14]. But all these attempts merely made a slight increase on temperature sensitivity. Recently, a new liquid-sealed packaging scheme has been designed in MMFI structures to enhance the temperature sensitivity. Fuentes et al. proposed a liquid-core multimode interference device for temperature sensing, namely, enclosure of the liquid of appropriate RI value into a capillary ﬁber, which was connected with two sections of single-mode ﬁbers (SMFs) by ﬁber ferrules. A sensitivity of 20 nm/◦ C was achieved at the cost of easy operation and broadrange measurement . Meanwhile, Lee et al. presented a simple leaky-guided multimode ﬁber interferometer by usage of a liquid material as the cladding of the multimode ﬁber, and the maximum sensitivity of about 50 nm/◦ C was achieved with the temperature range from 24 to 32 ◦ C, however, it needs special liquid material with an appropriate and ﬂat material dispersion proﬁle to act as the ﬁber cladding . In this paper, a highly sensitive optical ﬁber temperature sensor based on single-mode-no-core-single-mode (SMS) ﬁber structure which was embedded in a liquid-sealed capillary has been demonstrated. When the refractive index of the ﬁlled index matching
P. Hu et al. / Sensors and Actuators A 223 (2015) 114–118
Fig. 1. Experimental setup (inset: schematic diagram of the proposed temperature sensor).
liquid was close to but smaller than that of the no-core ﬁber (NCF), the proposed sensor exhibited high sensitivity to temperature variation. In the experiment, the refractive index matching liquids with speciﬁc RI values of 1.420, 1.430, 1.440 and 1.450 respectively were chosen to calibrate the temperature response of our sensor. 2. Sensor design and principle The proposed interferometer contains an SMS ﬁber structure, i.e., a coreless multimode ﬁber (MMF) segment sandwiched between two single-mode ﬁbers (SMFs). After inserting and fastening the structure into a liquid-sealed capillary tube, a temperature-sensitive sensor is realized, and it is interrogated in a transmission mode as schematically shown in Fig. 1. The operating mechanism of the sensor relies on multimode interference and refractometry of the interferometer. More speciﬁcally, when the light ﬁeld propagating along the lead-in SMF enters into the MMF section, a number of guided modes, including core and cladding modes, are excited in the MMF section, the excited cladding and core modes propagate further and re-couple back to the core mode of the lead-out SMF afterwards. Therefore, multimode interference can be formed by the excitation and re-coupling of the modes. Because the longitudinal propagation constants for the excited modes are associated with the cladding refractive index of the MMF, the interference spectrum will change with the refractive index. In our case as the ambient temperature rises, the refractive index matching liquid acting as the cladding of the coreless MMF (NCF) will reduce its RI value effectively as a result of the high negative thermo-optic coefﬁcient, therefore resulting in a wavelength shift in the interferometric spectrum. It exhibits a red wavelength shift of the spectrum to refractive index for such a MMI structure, as experimentally demonstrated in [4,17,19]. Moreover, thermal expansion of the stainless steel capillary tube also contributes to the temperature response of the sensor. With an increment T in the ambient temperature, the introduction of an additional axial tensile strain to the interferometer will also shift the interference pattern to short wavelengths, with the wavelength variation expressed as  = −0 (1 + 2 + pe )ˇ(T )T
where 0 is the concerned initial valley wavelength; and pe are the Poisson’s ratio and the strain-optic coefﬁcient of the NCF, respectively; ˇ(T) = L/(L·T) is the thermal expansion coefﬁcient of the tube (L is the initial length of the tube, L is the length variation of the tube under the temperature change T). Obviously, the combination of these two effects will absolutely increase the interaction between temperature and wavelength shift of the interference pattern, so the temperature sensitivity of the sensor is
Fig. 2. Experimental and simulated transmission spectra of the MMFI-based interferometer in air. (For interpretation of the references to color in the text citation of this ﬁgure, the reader is referred to the web version of this article.)
enhanced. Besides surrounding RI and strain, temperature also affects optical multimode ﬁber interferometer by itself through thermal expanding and thermo-optic effects of the ﬁber material. It has been proved that the output spectrum of the interferometer shifts to longer wavelengths as the temperature increases, which will decrease the temperature sensitivity of the proposed MMI sensor . Compared with the above two factors, however, the reduction in temperature sensitivity about 10 pm/◦ C of the sensor is relatively small and can be ignored. Herein, a NCF-based interferometer was fabricated by fusionsplicing of a 40 mm NCF with a core diameter of 125 m between two stubs of SMFs. The output spectrum of the fabricated interferometer, with two continuous periods ranging from 1450 to 1550 nm, was shown up by the red curve in Fig. 2. The ﬁgure also shows the simulated transmission spectrum in the black curve of the NCF-based interferometer by using the beam propagation method (BPM). The parameters of the NCF used in our simulation are set as follows: 125 m in diameter, 1.4570 in refractive index, and 40 mm in length. Both the spectra were normalized for better comparisons. Neglecting the optical spectral power ﬂuctuations, which may result from different coupling coefﬁcients of manually fusion-spliced SMS sections, the experimental results were well consistent with the simulation ones. After that the interferometer was inserted into a 60 mm long stainless steel capillary tube with an inner diameter of 800 m. Refractive index matching liquid of speciﬁc RI was carefully ﬁlled through one end of the tube, and then the tube was stuffed without any bubbles inside. With the interferometer kept straightforward along the tube, we sealed two ends of the tube with AB glue, and thus in this way the NCF-based temperature sensor was ﬁnally obtained. 3. Experimentations and discussions 3.1. Inﬂuence of RI on temperature response Prior to the temperature response test, the interferometer was immersed into glycerin solutions with different concentrations to ascertain the temperature dependence in a high-sensitivity RI range. Device connection for RI measurement was the same as that described in Fig. 1 with the sensor replaced by the interferometer. Light from a super-luminescent diode (SLD) propagated through the interferometer, and the transmission spectrum was measured by an optical spectrum analyzer (OSA). Fig. 3 shows
P. Hu et al. / Sensors and Actuators A 223 (2015) 114–118
Fig. 4. Spectral response of the sensor when exposed to different temperatures.
Fig. 3. Wavelength shifts of transmission spectra of the interferometer versus different RI values of glycerin solutions.
the measured wavelength shifts of the transmission spectra of the interferometer as a function of the refractive indices of the glycerin solutions. Seen from the graph, the RI sensitivity of the interferometer increases distinctly with the increment in the RI of the glycerin solutions. The average sensitivity, for example, is about 100 nm/R.I.U. (refractive index unit) in the range from 1.32 to 1.42 R.I.U., whereas it is enhanced to about 4500 nm/R.I.U. in the range from 1.440 to 1.450 R.I.U. The nonlinear sensitivity enhancement can be explained as that when the cladding refractive index gets closer to that of the NCF, the modes propagating near the NCF surface are more and more easily inﬂuenced by the cladding, leading to the RI sensitivity enhancement [4,5]. In addition, the thermo-optic coefﬁcient of the refractive index matching liquid is −3.91 × 10−4 R.I.U./◦ C and keeps almost unchanged in a certain temperature range, that is to say, temperature-induced refractive index variation of the liquid cladding is negatively linear. Hence, the experiment predicts that high refractive index liquid encapsulation can make the sensor more sensitive to temperature. In order to further study the impact of the RI of the sealed liquid on the temperature sensitivity, four sensors sealed with different refractive index matching liquids of RI values of 1.420, 1.430, 1.440 and 1.450 were prepared respectively. A calibrated commercial thermostatic bath was employed and each sensor was ﬁxed onto a glass rod to avoid any inﬂuence caused by liquid ﬂows, as shown in Fig. 1. By increasing the temperature of the thermostatic bath at intervals of about 10 ◦ C from 0 to 100 ◦ C, we conducted the temperature-response experiments for these four sensors at room temperature. Fig. 4 shows the spectral response of the sensor sealed with liquid of RI value of 1.450 when exposed to various temperatures. It can be seen that the valley wavelength in the spectrum shifted to a shorter wavelength when the temperature was changed from 0 to 100 ◦ C; and the shift distance becomes smaller, indicating that the temperature sensitivity of the sensor declines with the raised temperatures. The reason is that when the temperature increases, the RI of the sealed liquid decreases as a result of negative thermo-optic coefﬁcient of −3.91 × 10−4 R.I.U./◦ C, thus lowering the temperature sensitivity of the sensor. Fig. 5 shows the relative wavelength shifts of the transmission spectra of the sensors sealed with liquids of RI values of 1.420, 1.430, 1.440 and 1.450 to temperature, respectively. Experimental results show that the temperature sensitivity of the sensor becomes higher and higher as the RI of the sealed
liquid is increased. The maximum sensitivities of 5.15 nm/◦ C from10 to 30 ◦ C and 0.52 nm/◦ C from 30 to 100 ◦ C are hundreds of times more than that of conventional SMS temperature sensors. Qualiﬁed with a high temperature sensitivity and a wide measurement range, the proposed sensor is advantageous over other wavelength detection-based ﬁber temperature sensors [13,19,20]. 3.2. Repeatability and stability The sensor sealed with liquid of RI value of 1.450 was chosen for further investigation, such as repeatability and stability. The experiment was conducted three times to make sure the wavelength shift information acquired reliability. The wavelength shifts of the sensor under various temperature levels in both ascending and descending orders are presented in Fig. 6. It can be seen that the experimental results are in accordance with each other when the temperature was varied in ascending and descending orders. The repeatability of 0.93 ◦ C was observed at 30 ◦ C, which indicates little hysteresis and good repeatability. The stability results were obtained by testing the sensor under three constant temperature of 50 ◦ C, 70 ◦ C and 90 ◦ C, respectively, as shown in Fig. 7. The experimentations show that the sensor has a good stability with negligible variations in temperature (less than 0.095 ◦ C/min). The above measurement errors may be caused
Fig. 5. Wavelength shifts of transmission spectra of the sensors sealed with four different refractive index matching liquids at different temperatures.
P. Hu et al. / Sensors and Actuators A 223 (2015) 114–118
Inspection Administration Industry Projects, China under Grant no. 201210061, Special Major Science and Technology, Zhejiang, China under Grant no. 2012C13010-1. References
Fig. 6. Repeatability of the sensor in the temperature ranging from 10 to 100 ◦ C.
Fig. 7. Stability of the sensor under three constant temperatures of 50 ◦ C, 70 ◦ C and 90 ◦ C.
by the limitation of the thermostatic bath itself, as water ﬂows make the temperature well distributed inside the bath, it might inevitably cause any movement of the sensor and ﬂuctuation of the temperature. 4. Conclusion In conclusion, a highly sensitive no-core ﬁber interferometric temperature sensor has been proposed and experimentally demonstrated. The interferometer was ﬁrst immersed into various mixtures of glycerin to make sure the temperature dependence is in a high-sensitivity RI range. Experimental results showed the interferometer was selectively sensitive to refractive index (the RI sensitivity increased signiﬁcantly above ∼1.43 R.I.U.) and had a maximum temperature sensitivity of 5.15 nm/◦ C when packaged with the liquid of RI value of 1.450. In addition, the sensor had a good repeatability and stability response. Acknowledgements This work was supported by National Natural Science Foundation, China under Grant no. 61203205, Quality
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Pengbing Hu was born in Jiangsu, China, in January 1987 and received the B.S. and M.S. degrees in the specialty of Electronics Science and Technology, and Optical Engineering, from the College of Optical & Electronic Technology, China Jiliang University (CJLU), HangZhou, China, in 2010 and 2013, respectively. From 2013 till date, he is working at Research and Development Centre of Metrology Technology, Zhejiang Province Institute of Metrology, Hangzhou, China. His researches are mainly focused on optical ﬁber sensing and metrology.
P. Hu et al. / Sensors and Actuators A 223 (2015) 114–118
Zhemin Chen received his Ph.D. degree in Optical Engineering from Zhejiang University, Hangzhou, China, in 2008. He joined Hamamatsu Photonics K. K, Janpan, as a research assistant from 2007 to 2008. Now he is the leader of the Research and Development Centre of Metrology Technology, Zhejiang Province Institute of Metrology, Hangzhou, China. His research interests cover optical ﬁber sensing, laser spectroscopy and metrology and instrumentation.
Mei Yang received the B.S. degree in the specialty of product design from the College of Mechanical and electrical from China Jiliang University (CJLU) and now is an inservice postgraduate at Zhejiang University, Hangzhou, China. Meanwhile she is working at Research & Development Centre of Metrology Technology, Zhejiang Province Institute of Metrology, Hangzhou, China. Her research interest is optical metrology.
Jingyi Yang received her B.S. degree in optical and electronic information engineering from China Jiliang University, in 2013. She is a graduate student of the Institute of Optoelectronic Technology in China Jiliang University since 2013. Her main research interest is ﬁber optical sensors.
Chuan Zhong received M.S. degree in optical Engineering from China Jiliang University in 2013. Now he is a Ph.D. student in the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) of Trinity College Dublin, Ireland. His main research interests are ﬁber Bragg Grating sensors, plasmonic waveguide and metamaterical.