- Email: [email protected]
High sensitivity fiber temperature sensor based PDMS film on Mach-Zehnder interferometer
Optical Fiber Technology 53 (2019) 102029
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
High sensitivity fiber temperature sensor based PDMS film on Mach-Zehnder interferometer
T
Jiaqi Gong, Changyu Shen, Yike Xiao, Shuyi Liu, Chong Zhang, Zeyi Ding, Huitong Deng, Jiahao Fang, Tingting Lang, Chunliu Zhao, Yi Chen Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical fiber sensor Mach-Zehnder interferometer PDMS
An optical fiber temperature sensor based on a Mach-Zehnder interferometer (MZI) coated with a film of polydimethylsiloxane (PDMS) was demonstrated. The MZI was fabricated by a fiber mismatch structure through core-offset fusion splicing method. The resonant dip wavelength of the MZI will shift with the changing of the environmental temperature own to the effects of expansion and thermo-optic of the PDMS. Experimental results showed that the proposed sensor shows the temperature sensitivity of 0.101 nm/°C under the temperature range of 20 °C to 100 °C, which is about 2 times of the existing similar temperature fiber sensors.
1. Introduction In industrial production and scientific research experiments, temperature is one of the most important physical parameter that must be strictly monitored. Temperature directly affects the performance of materials and the quality of products. With the development of science and technology, various engineering fields have put forward higher and higher requirements for the performance and efficiency of temperature measuring components. Therefore, the optimization of the temperature measurement method and the structure of the temperature measuring element face great challenges [1-4]. Optical fiber sensors are widely used in many fields, such as biomedicine, industry, chemistry, aerospace, and so on. Compared to traditional sensors, optical fiber MachZehnder interferometer (MZI) sensors have the advantages of miniaturization, high sensitivity and thermal stability, immunity to electromagnetic interference [5–9]. The typical MZI structures consist of a small stub of the hollow-core photonic crystal fiber between a lead-in and lead-out standard single mode fiber [10], a tapered hollow-core fiber (HCF) sandwiched between two single-mode fibers [11], the peanut flat structure [12], and so on. PDMS (polydimethylsiloxane) is a kind of silica gel in the solid state, which owns the advantages of non-toxic, chemically inert, hydrophobic, good light transmittance and biocompatibility. It is easy to combine with many kinds of materials at room temperature, and it has high structural flexibility due to low Young's modulus with good linear thermal expansion effect and thermo-optic effect. Therefore, it can be applied to the fiber based temperature sensing measurements [1]. In this paper, we proposed an optical fiber temperature sensor based
on Mach-Zehnder interferometer (MZI) coated with a film of PDMS. The temperature variations, which can be measured by detecting the changes of the resonant dip wavelengths of the MZI. Experimental results showed that the proposed sensor had the temperature sensitivity of 0.101 nm/°C under the temperature range from of 20 °C to 100 °C, which is about 2 times of the existing similar temperature fiber sensors. 2. Sensing principle The schematic diagram of the experimental setup is shown in Fig. 1. A mismatch structure of three single mode fibers based MZI is used as the sensor head. The sensor head is connected between a broadband source and an optical spectrum analyzer. The broadband source (BBS) with the wavelength range of 1432–1632 nm is used as the input light source. The output spectrum is detected with an optical spectrum analyzer (OSA, AQ6370, Advantest) with a wavelength resolution of 0.02 nm. The optical fiber sensor, the MZI structure is coated with the PDMS film, which is fixed on a heating plate. Fig. 2(a) shows the structure of the MZI. The MZI consists of three single mode fibers (SMFs): SMF1, SMF2 and SMF3. SMF1 and SMF2 are mismatch fusion spliced. Perpendicular to the axis of the optical fiber SMF1, SMF2 is shifted downward by 2–3 μm with the length of 4 cm. SMF2 and SMF3 are mismatch fusion spliced too, and also in the vertical direction, compared with SMF2, SMF3 is shifted upward by 2–3 μm. The mismatch structure is formed under modified parameters with the commercial electric-arc fusion splicer (Fujikura FSM-60s) by core-offset fusion splicing method. The first and the second fusion points are shown in Fig. 2(b) and Fig. 2(c), which show clearly the
E-mail addresses: [email protected] (C. Shen), [email protected] (Y. Chen). https://doi.org/10.1016/j.yofte.2019.102029 Received 14 August 2019; Received in revised form 6 October 2019; Accepted 7 October 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 53 (2019) 102029
J. Gong, et al.
Fig. 1. Experimental setup of PDMS based MZI temperature sensor, the partial enlarged shows the mismatch structure fiber MZI.
For the mismatch structures of SMF1, SMF2 and SMF3, part of the core mode light in SMF 1 was coupled into the cladding of SMF2 to excite some cladding modes light and the remaining part of core modes light enters the core of SMF2. After propagating through the SMF2, the core modes light and a part of cladding modes light of SMF2 will recoupled to the core of SMF3. Therefore, a kind of MZI system is formed. When the SMF1 core modes propagated into SMF2, the phase difference of the modes and cladding modes of ∅m can be described as, m ∅m = 2π Δneff L/λ
(1)
m Δneff
is the effective refractive index difference between the core where and the mth cladding mode. λ is the center wavelength of the input light, L is the length of the SMF2. The intensity I of the interference patterns can be described as, Fig. 2. (a) Schematic diagram of the optical fiber MZI, (b) and (c) Pictures of the two mismatch structures of the three-segment SMF showing on the splicer screen, respectively.
I = I1 + I2 + 2 I1 I2 cos(ϕm)
(2)
where I1, I2 are the intensities of the light, which are propagate along SMF2 core and cladding respectively. When the light in SMF2 propagates into SMF3, the core modes and the cladding modes would converge into the core of SMF3.
mismatch structure of the three-segment SMF. Fig. 2(b) and (c) show the pictures of the two mismatch structures of the three-segment SMF showing on the splicer screen, respectively. After that, the mismatch structure is coated with a PDMS film. Fig. 3 shows the production method of the PDMS film and the picture of the proposed sensor. The PDMS was fabricated by the following steps: firstly, the polymer precursor (Sylgard 184A) and curing agent (Sylgard 184B) was mixed with a ratio of 5:1 and stirred 10 min, and then the mix liquids were poured into a mold, in which the mismatch structure based MZI was located in its axis center as shown in Fig. 3(a). After that, the mold with the MZI was heated at 60 °C for 5 h on a heating plate. The thickness of the PDMS film is adjustable by changing the diameter of the mold, and in this paper the 5 mm-thick PDMS was obtained.
ϕm = (2k + 1) π
(3)
ϕm
is the condition of resonant dip appearing, where k is natural number. Therefore, resonant dip wavelength of λr can described as,
λr =
core clad − neff 2(neff )L
2k + 1
(4)
and are the effective refractive indices of the fiber core where and cladding, and L is the length of SMF2. When the temperature varies, the effective refractive index of the fiber cladding and the length L will change due to the linear expansion and thermo-optic of the fiber structure, then the resonant dip wavelength of the MZI will shift. core neff
clad neff
3. Result and discussion Fig. 4 shows the transmission spectra of the MZI varying with the temperature changing from 20 °C to 100 °C with the step of 5 °C. As shown in Fig. 5, the wavelength shifts to the long wavelength with the increasing of the temperature. Because with the increasing of the temperature, the PDMS expands and the L of the fiber will become large, and at the same time the effective refractive index of the fiber cladding become small. Consequently, the resonant wavelength will shift towards the larger wavelength with the increasing of the temperature. What’s more, dips of interference spectrum change regularly and are easy to analyze and distinguish. On the contrary, as the temperature decrease, as shown in Fig. 6, the dip wavelength shifts towards shorter wavelength with a good repeatability.
Fig. 3. The production method of the PDMS film and the picture of the proposed sensor. 2
Optical Fiber Technology 53 (2019) 102029
J. Gong, et al.
Table 1 Characteristics of optical fiber temperature sensors based on various structures. Sensor
Sensitivity
Measurement range
Based on peanut-shape structure [13] Based on forward core-cladding-core recoupling [14] Based on FPI [15] Based on FM-DCCF [16] Proposed sensor
0.073 nm/°C 0.0733 nm/°C
25 °C to 60 °C 25 °C to 50 °C
10.2 pm/°C 52.79 pm/°C 0.101 nm/°C
30 °C to 70 °C 30 °C to 90 °C 20 °C to 100 °C
In order to obtain the relationship between the wavelength shifts and the temperature variations, one of the resonant wavelengths dip in the transmission spectrum of the MZI (1519 nm in initial transmission spectrum) was selected as the sample wavelength. Fig. 6 shows the dip wavelength of 1519 nm shifting with the temperature increasing and decreasing. It can be seen that as the temperature increasing from 20 °C to 100 °C, the dip wavelength shifts to the longer wavelength linearly. And we also obtain a temperature sensitivity of 0.101 nm/°C with a linearly coefficient of 99.645% from about 1519 nm to 1528 nm. Conversely, when the temperature drops from 100 °C to 20 °C, the spectrum of the sensor can be changed back to its original position. The sensitivity error is about 0.14 nm on average, which shows a good repeatability on a temperature cycle of heating and cooling. Table 1 is a summary of the characteristics of optical fiber temperature sensors based on various fiber structures. Optical fiber temperature sensor that generally reported has a sensitivity of about 10 pm/°C. Compared to previous temperature sensors, the proposed sensor exhibits relatively high temperature sensitivity. In addition, the mismatch MZI structure is fabricated by normal SMFs, which is cheap and easy to obtain. What’s more, the proposed sensor is suitable for long-distance detecting of the variation of environmental temperature.
Fig. 4. Transmission spectra of the MZI varied with the changing of the environment temperature.
4. Conclusion In summary, an optical fiber temperature sensor based on MZI coated with a film of PDMS is proposed. This mismatch structure based MZI sensor was fabricated through a core-offset fusion splicing method. When the temperature changes, the effects of the linear expansion and thermo-optic effects makes the resonant dip wavelength shift and the corresponding temperature variations will be monitored. The sensitivity of 0.101 nm/°C under the temperature range of 20 °C to 100 °C was obtained, which is almost 2 times of the existing similar fiber sensors. In addition, due to its simple structural configuration, the proposed sensor provides a viable and inexpensive structure for high and long distance temperature sensitivity detections.
Fig. 5. The transmission spectrum of the MZI for temperature decreasing from 100 °C to 20 °C.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgements National Natural Science Foundation of China (11874332), National major scientific research instrument development project of Natural Science Foundation of China (61727816), National Key R&D Program of China (2017YFF0209703), National Natural Science Foundation of China (61875251, 61775202). References [1] Y. Li, Z. Liang, C. Zhao, D. Wang, High-Temperature sensor based on peanut flatend reflection structure, Optik 148 (2017) 293–299, https://doi.org/10.1016/j. optlastec.2018.08.002.
Fig. 6. The transmission spectrum on a temperature cycle of heating and cooling.
3
Optical Fiber Technology 53 (2019) 102029
J. Gong, et al. [2] Y. Ying, N. Hu, G. Si, K. Xu, N. Liu, J. Zhao, Magnetic field and temperature sensor based on D-shaped photonic crystal fiber, Optik 176 (2019) 309–314, https://doi. org/10.1016/j.ijleo.2018.09.107. [3] R. Oliveira, L. Bilro, T.H.R. Marques, C.M.B. Cordeiro, R. Nogueira, Simultaneous detection of humidity and temperature through an adhesive based Fabry-Pérot cavity combined with polymer fiber Bragg grating, Opt. Lasers Eng. 114 (2019) 37–43, https://doi.org/10.1016/j.optlaseng.2018.10.007. [4] C. Zhu, Y. Zhuang, B. Zhang, R. Muhammad, P.P. Wang, J. Huang, A miniaturized optical fiber tip high-temperature sensor based on concave-shaped fabry-perot cavity, IEEE Photonics Technol. Lett. 31 (2019) 35–38 http://ieeexplore.ieee.org/ stamp/stamp.jsp?tp=&arnumber=8538009&isnumber=8581693. [5] J. Gong, et al., An optical fiber hydrogen concentration sensor based on MachZehnder interferometer coated with a film of palladium, 2018 Asia Communications and Photonics Conference (ACP), Hangzhou, 2018, pp. 1–3. [6] C. Shen, Y. Wang, J. Chu, Y. Lu, Y. Li, X. Dong, Optical fiber axial micro-displacement sensor based on Mach-Zehnder interferometer, Opt. Express 22 (2014) 31984, https://doi.org/10.1364/OE.22.031984. [7] Y. Wang, C. Shen, W. Lou, F. Shentu, Polarization-dependent humidity sensor based on an in-fiber Mach-Zehnder interferometer coated with graphene oxide, Sens. Actuators, B 234 (2016) 503–509, https://doi.org/10.1016/j.snb.2016.05.020. [8] Y. Wang, C. Shen, W. Lou, F. Shentu, Polarization-dependent refractive index fiberoptic sensor based on the core-offset with a taper, J. Phys. Conf. Ser. 680 (2016) 12023, https://doi.org/10.1088/1742-6596/680/1/012023. [9] Z. Liu, C. Shen, Y. Xiao, J. Gong, Y. Wang, T. Lang, Y. Zhang, W. Xu, T. Zhang, Z. Jing, Y. Jin, W. Peng, Sensitivity-enhanced strain sensor based on thin-core fiber modal interferometer interacted with tilted fiber bragg grating, IEEE Sens. J. 19
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(2019) 1802–1806 http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber= 8552389&isnumber=8636211. F. Ahmed, V. Ahsani, K. Nazeri, E. Marzband, C. Bradley, E. Toyserkani, M.B.G. Jun, Monitoring of carbon dioxide using hollow-core photonic crystal fiber MachZehnder interferometer, Sensors 19 (2019) 3357, https://doi.org/10.1016/j.sna. 2018.01.051. Y. Zhao, F. Xia, M. Chen, R. Lv, Optical fiber low-frequency vibration sensor based on Butterfly-Shape Mach-Zehnder Interferometer, Sens. Actuators, A 273 (2018) 107–112, https://doi.org/10.1016/j.sna.2018.01.051. W. Lou, F. Shentu, Y. Wang, C. Shen, X. Dong, Transverse load sensor based on Mach-Zehnder interferometer constructed by a bowknot type taper, Opt. Fiber Technol. 40 (2018) 52–55, https://doi.org/10.1016/j.yofte.2017.11.013. F. Yu, P. Xue, X. Zhao, J. Zheng, Investigation of an in-line fiber Mach-Zehnder interferometer based on peanut-shape structure for refractive index sensing, Opt. Commun. 435 (2019) 173–177, https://doi.org/10.1016/j.optcom.2018.11.048. T. Jiao, H. Meng, S. Deng, S. Liu, X. Wang, Z. Wei, F. Wang, C. Tan, X. Huang, Simultaneous measurement of refractive index and temperature using a MachZehnder interferometer with forward core-cladding-core recoupling, Opt. Laser Technol. 111 (2019) 612–615, https://doi.org/10.1016/j.optlastec.2018.10.047. C. Zhao, B. Wu, F. Shi, J. Kang, D. Wang, Simultaneous measurement of trace organic vapors and temperature by use of zeolite thin film-coated fiber spherical end face and fiber Bragg grating, Opt. Eng. 56 (2017) 036117http://spie.org/ Publications/Journal/10.1117/1.OE.56.3.036117. S. Yao, Y. Shen, Y. Wu, W. Jin, S. Jian, Strain-insensitive temperature sensor based on a few-mode dual-concentric-core fiber, Opt. Laser Technol. 111 (2019) 95–99, https://doi.org/10.1016/j.optlastec.2018.09.046.
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
High sensitivity fiber temperature sensor based PDMS film on Mach-Zehnder interferometer
T
Jiaqi Gong, Changyu Shen, Yike Xiao, Shuyi Liu, Chong Zhang, Zeyi Ding, Huitong Deng, Jiahao Fang, Tingting Lang, Chunliu Zhao, Yi Chen Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Optical fiber sensor Mach-Zehnder interferometer PDMS
An optical fiber temperature sensor based on a Mach-Zehnder interferometer (MZI) coated with a film of polydimethylsiloxane (PDMS) was demonstrated. The MZI was fabricated by a fiber mismatch structure through core-offset fusion splicing method. The resonant dip wavelength of the MZI will shift with the changing of the environmental temperature own to the effects of expansion and thermo-optic of the PDMS. Experimental results showed that the proposed sensor shows the temperature sensitivity of 0.101 nm/°C under the temperature range of 20 °C to 100 °C, which is about 2 times of the existing similar temperature fiber sensors.
1. Introduction In industrial production and scientific research experiments, temperature is one of the most important physical parameter that must be strictly monitored. Temperature directly affects the performance of materials and the quality of products. With the development of science and technology, various engineering fields have put forward higher and higher requirements for the performance and efficiency of temperature measuring components. Therefore, the optimization of the temperature measurement method and the structure of the temperature measuring element face great challenges [1-4]. Optical fiber sensors are widely used in many fields, such as biomedicine, industry, chemistry, aerospace, and so on. Compared to traditional sensors, optical fiber MachZehnder interferometer (MZI) sensors have the advantages of miniaturization, high sensitivity and thermal stability, immunity to electromagnetic interference [5–9]. The typical MZI structures consist of a small stub of the hollow-core photonic crystal fiber between a lead-in and lead-out standard single mode fiber [10], a tapered hollow-core fiber (HCF) sandwiched between two single-mode fibers [11], the peanut flat structure [12], and so on. PDMS (polydimethylsiloxane) is a kind of silica gel in the solid state, which owns the advantages of non-toxic, chemically inert, hydrophobic, good light transmittance and biocompatibility. It is easy to combine with many kinds of materials at room temperature, and it has high structural flexibility due to low Young's modulus with good linear thermal expansion effect and thermo-optic effect. Therefore, it can be applied to the fiber based temperature sensing measurements [1]. In this paper, we proposed an optical fiber temperature sensor based
on Mach-Zehnder interferometer (MZI) coated with a film of PDMS. The temperature variations, which can be measured by detecting the changes of the resonant dip wavelengths of the MZI. Experimental results showed that the proposed sensor had the temperature sensitivity of 0.101 nm/°C under the temperature range from of 20 °C to 100 °C, which is about 2 times of the existing similar temperature fiber sensors. 2. Sensing principle The schematic diagram of the experimental setup is shown in Fig. 1. A mismatch structure of three single mode fibers based MZI is used as the sensor head. The sensor head is connected between a broadband source and an optical spectrum analyzer. The broadband source (BBS) with the wavelength range of 1432–1632 nm is used as the input light source. The output spectrum is detected with an optical spectrum analyzer (OSA, AQ6370, Advantest) with a wavelength resolution of 0.02 nm. The optical fiber sensor, the MZI structure is coated with the PDMS film, which is fixed on a heating plate. Fig. 2(a) shows the structure of the MZI. The MZI consists of three single mode fibers (SMFs): SMF1, SMF2 and SMF3. SMF1 and SMF2 are mismatch fusion spliced. Perpendicular to the axis of the optical fiber SMF1, SMF2 is shifted downward by 2–3 μm with the length of 4 cm. SMF2 and SMF3 are mismatch fusion spliced too, and also in the vertical direction, compared with SMF2, SMF3 is shifted upward by 2–3 μm. The mismatch structure is formed under modified parameters with the commercial electric-arc fusion splicer (Fujikura FSM-60s) by core-offset fusion splicing method. The first and the second fusion points are shown in Fig. 2(b) and Fig. 2(c), which show clearly the
E-mail addresses: [email protected] (C. Shen), [email protected] (Y. Chen). https://doi.org/10.1016/j.yofte.2019.102029 Received 14 August 2019; Received in revised form 6 October 2019; Accepted 7 October 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 53 (2019) 102029
J. Gong, et al.
Fig. 1. Experimental setup of PDMS based MZI temperature sensor, the partial enlarged shows the mismatch structure fiber MZI.
For the mismatch structures of SMF1, SMF2 and SMF3, part of the core mode light in SMF 1 was coupled into the cladding of SMF2 to excite some cladding modes light and the remaining part of core modes light enters the core of SMF2. After propagating through the SMF2, the core modes light and a part of cladding modes light of SMF2 will recoupled to the core of SMF3. Therefore, a kind of MZI system is formed. When the SMF1 core modes propagated into SMF2, the phase difference of the modes and cladding modes of ∅m can be described as, m ∅m = 2π Δneff L/λ
(1)
m Δneff
is the effective refractive index difference between the core where and the mth cladding mode. λ is the center wavelength of the input light, L is the length of the SMF2. The intensity I of the interference patterns can be described as, Fig. 2. (a) Schematic diagram of the optical fiber MZI, (b) and (c) Pictures of the two mismatch structures of the three-segment SMF showing on the splicer screen, respectively.
I = I1 + I2 + 2 I1 I2 cos(ϕm)
(2)
where I1, I2 are the intensities of the light, which are propagate along SMF2 core and cladding respectively. When the light in SMF2 propagates into SMF3, the core modes and the cladding modes would converge into the core of SMF3.
mismatch structure of the three-segment SMF. Fig. 2(b) and (c) show the pictures of the two mismatch structures of the three-segment SMF showing on the splicer screen, respectively. After that, the mismatch structure is coated with a PDMS film. Fig. 3 shows the production method of the PDMS film and the picture of the proposed sensor. The PDMS was fabricated by the following steps: firstly, the polymer precursor (Sylgard 184A) and curing agent (Sylgard 184B) was mixed with a ratio of 5:1 and stirred 10 min, and then the mix liquids were poured into a mold, in which the mismatch structure based MZI was located in its axis center as shown in Fig. 3(a). After that, the mold with the MZI was heated at 60 °C for 5 h on a heating plate. The thickness of the PDMS film is adjustable by changing the diameter of the mold, and in this paper the 5 mm-thick PDMS was obtained.
ϕm = (2k + 1) π
(3)
ϕm
is the condition of resonant dip appearing, where k is natural number. Therefore, resonant dip wavelength of λr can described as,
λr =
core clad − neff 2(neff )L
2k + 1
(4)
and are the effective refractive indices of the fiber core where and cladding, and L is the length of SMF2. When the temperature varies, the effective refractive index of the fiber cladding and the length L will change due to the linear expansion and thermo-optic of the fiber structure, then the resonant dip wavelength of the MZI will shift. core neff
clad neff
3. Result and discussion Fig. 4 shows the transmission spectra of the MZI varying with the temperature changing from 20 °C to 100 °C with the step of 5 °C. As shown in Fig. 5, the wavelength shifts to the long wavelength with the increasing of the temperature. Because with the increasing of the temperature, the PDMS expands and the L of the fiber will become large, and at the same time the effective refractive index of the fiber cladding become small. Consequently, the resonant wavelength will shift towards the larger wavelength with the increasing of the temperature. What’s more, dips of interference spectrum change regularly and are easy to analyze and distinguish. On the contrary, as the temperature decrease, as shown in Fig. 6, the dip wavelength shifts towards shorter wavelength with a good repeatability.
Fig. 3. The production method of the PDMS film and the picture of the proposed sensor. 2
Optical Fiber Technology 53 (2019) 102029
J. Gong, et al.
Table 1 Characteristics of optical fiber temperature sensors based on various structures. Sensor
Sensitivity
Measurement range
Based on peanut-shape structure [13] Based on forward core-cladding-core recoupling [14] Based on FPI [15] Based on FM-DCCF [16] Proposed sensor
0.073 nm/°C 0.0733 nm/°C
25 °C to 60 °C 25 °C to 50 °C
10.2 pm/°C 52.79 pm/°C 0.101 nm/°C
30 °C to 70 °C 30 °C to 90 °C 20 °C to 100 °C
In order to obtain the relationship between the wavelength shifts and the temperature variations, one of the resonant wavelengths dip in the transmission spectrum of the MZI (1519 nm in initial transmission spectrum) was selected as the sample wavelength. Fig. 6 shows the dip wavelength of 1519 nm shifting with the temperature increasing and decreasing. It can be seen that as the temperature increasing from 20 °C to 100 °C, the dip wavelength shifts to the longer wavelength linearly. And we also obtain a temperature sensitivity of 0.101 nm/°C with a linearly coefficient of 99.645% from about 1519 nm to 1528 nm. Conversely, when the temperature drops from 100 °C to 20 °C, the spectrum of the sensor can be changed back to its original position. The sensitivity error is about 0.14 nm on average, which shows a good repeatability on a temperature cycle of heating and cooling. Table 1 is a summary of the characteristics of optical fiber temperature sensors based on various fiber structures. Optical fiber temperature sensor that generally reported has a sensitivity of about 10 pm/°C. Compared to previous temperature sensors, the proposed sensor exhibits relatively high temperature sensitivity. In addition, the mismatch MZI structure is fabricated by normal SMFs, which is cheap and easy to obtain. What’s more, the proposed sensor is suitable for long-distance detecting of the variation of environmental temperature.
Fig. 4. Transmission spectra of the MZI varied with the changing of the environment temperature.
4. Conclusion In summary, an optical fiber temperature sensor based on MZI coated with a film of PDMS is proposed. This mismatch structure based MZI sensor was fabricated through a core-offset fusion splicing method. When the temperature changes, the effects of the linear expansion and thermo-optic effects makes the resonant dip wavelength shift and the corresponding temperature variations will be monitored. The sensitivity of 0.101 nm/°C under the temperature range of 20 °C to 100 °C was obtained, which is almost 2 times of the existing similar fiber sensors. In addition, due to its simple structural configuration, the proposed sensor provides a viable and inexpensive structure for high and long distance temperature sensitivity detections.
Fig. 5. The transmission spectrum of the MZI for temperature decreasing from 100 °C to 20 °C.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgements National Natural Science Foundation of China (11874332), National major scientific research instrument development project of Natural Science Foundation of China (61727816), National Key R&D Program of China (2017YFF0209703), National Natural Science Foundation of China (61875251, 61775202). References [1] Y. Li, Z. Liang, C. Zhao, D. Wang, High-Temperature sensor based on peanut flatend reflection structure, Optik 148 (2017) 293–299, https://doi.org/10.1016/j. optlastec.2018.08.002.
Fig. 6. The transmission spectrum on a temperature cycle of heating and cooling.
3
Optical Fiber Technology 53 (2019) 102029
J. Gong, et al. [2] Y. Ying, N. Hu, G. Si, K. Xu, N. Liu, J. Zhao, Magnetic field and temperature sensor based on D-shaped photonic crystal fiber, Optik 176 (2019) 309–314, https://doi. org/10.1016/j.ijleo.2018.09.107. [3] R. Oliveira, L. Bilro, T.H.R. Marques, C.M.B. Cordeiro, R. Nogueira, Simultaneous detection of humidity and temperature through an adhesive based Fabry-Pérot cavity combined with polymer fiber Bragg grating, Opt. Lasers Eng. 114 (2019) 37–43, https://doi.org/10.1016/j.optlaseng.2018.10.007. [4] C. Zhu, Y. Zhuang, B. Zhang, R. Muhammad, P.P. Wang, J. Huang, A miniaturized optical fiber tip high-temperature sensor based on concave-shaped fabry-perot cavity, IEEE Photonics Technol. Lett. 31 (2019) 35–38 http://ieeexplore.ieee.org/ stamp/stamp.jsp?tp=&arnumber=8538009&isnumber=8581693. [5] J. Gong, et al., An optical fiber hydrogen concentration sensor based on MachZehnder interferometer coated with a film of palladium, 2018 Asia Communications and Photonics Conference (ACP), Hangzhou, 2018, pp. 1–3. [6] C. Shen, Y. Wang, J. Chu, Y. Lu, Y. Li, X. Dong, Optical fiber axial micro-displacement sensor based on Mach-Zehnder interferometer, Opt. Express 22 (2014) 31984, https://doi.org/10.1364/OE.22.031984. [7] Y. Wang, C. Shen, W. Lou, F. Shentu, Polarization-dependent humidity sensor based on an in-fiber Mach-Zehnder interferometer coated with graphene oxide, Sens. Actuators, B 234 (2016) 503–509, https://doi.org/10.1016/j.snb.2016.05.020. [8] Y. Wang, C. Shen, W. Lou, F. Shentu, Polarization-dependent refractive index fiberoptic sensor based on the core-offset with a taper, J. Phys. Conf. Ser. 680 (2016) 12023, https://doi.org/10.1088/1742-6596/680/1/012023. [9] Z. Liu, C. Shen, Y. Xiao, J. Gong, Y. Wang, T. Lang, Y. Zhang, W. Xu, T. Zhang, Z. Jing, Y. Jin, W. Peng, Sensitivity-enhanced strain sensor based on thin-core fiber modal interferometer interacted with tilted fiber bragg grating, IEEE Sens. J. 19
[10]
[11]
[12]
[13]
[14]
[15]
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