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Simultaneous measurement of temperature and relative humidity by compact Mach-Zehnder interferometer and Fabry-Perot interferometer
Journal Pre-proofs Simultaneous measurement of temperature and relative humidity by compact Mach-Zehnder interferometer and Fabry-Perot interferometer Rui-jie Tong, Yong Zhao, Hong-kun Zheng, Feng Xia PII: DOI: Reference:
S0263-2241(20)30036-1 https://doi.org/10.1016/j.measurement.2020.107499 MEASUR 107499
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
12 September 2019 1 January 2020 10 January 2020
Please cite this article as: R-j. Tong, Y. Zhao, H-k. Zheng, F. Xia, Simultaneous measurement of temperature and relative humidity by compact Mach-Zehnder interferometer and Fabry-Perot interferometer, Measurement (2020), doi: https://doi.org/10.1016/j.measurement.2020.107499
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Elsevier Ltd. All rights reserved.
Simultaneous measurement of temperature and relative humidity by compact Mach-Zehnder interferometer and Fabry-Perot interferometer Rui-jie Tong, Yong Zhao, Hong-kun Zheng, Feng Xia College of Information Science and Engineering, Northeastern University, Shenyang 110819, China
Corresponding Author: [email protected]
Abstract: A novel and compact temperature and relative humidity (RH) simultaneously measurement sensor based on compact Mach-Zehnder interferometer (MZI) and Fabry-Perot interferometer (FPI) is proposed and experimental implemented. The sensing structure is made by splicing a 200 μm single mode fiber (SMF) between two 10 cm SMFs with large core-offset. The core-offset distance is 62.8 μm coated with RH sensitive material graphene quantum dots and polyvinyl alcohol (GQDs-PVA). The transmitted structure acts as MZI, and the reflected structure acts as FPI, then the simultaneous detection of temperature and RH can be achieved by monitoring the spectrums of FPI and MZI, respectively. The sensitivity to RH is -0.132 nm/%RH with resolution of 0.15 %RH, and the sensitivity to temperature is 0.37 nm/°C with the resolution of 0.05°C. The proposed sensor is compact, high sensitivity, simple making and low cost. Key words: relative humidity sensor; temperature measurement; Fabry-Perot interferometer; Mach-Zehnder interferometer. 1.
Introduction With the development of agriculture, biopharmaceutical, aerospace and precision instrument
manufacturing industries, the importance of relative humidity (RH) accurate measurement is more and more obvious, such as soil humidity affects the yield of crops, environmental humidity affects the quality of biopharmaceuticals and the service life of precision instruments [1-4]. Compared with traditional RH prospecting instrument based on polymer, semiconductor ceramic and alumina, RH sensors based on optical fiber are compact size, anti-electromagnetic interference, remote sensing, and so on [5-7], and have attracted intensive attention. Up to now, optical fiber prospecting instrument based on different sensing methods fiber grating [8, 9], Fabry-Perot interferometer (FPI) [10, 11], Mach-Zehnder interferometer (MZI) [12, 13], surface plasmon resonance [14]) have been put forward for RH detection, however, most of RH sensors are affected by temperature, which could introduce measurement error [15]. Therefore, the simultaneous detection of temperature and RH need cause concern. Although RH and temperature simultaneous detection is difficult, plenty of simultaneous measurement methods have been proposed [15-17], and the existing optical fiber sensors for RH and temperature simultaneously detection can be divided into three categories: The first category adopts two sections of optical fiber grating with different center wavelengths for temperature and RH measurement. At present, the most adopted method is cascading two long period grating (LPG) or cascade LPG and fiber Brag grating (FBG) together for RH and temperature simultaneous detection. However, the two LPG cascaded will make the optical spectrum chaos, which is not conducive to the reading of data [18, 19]. The second category is cascading fiber Brag grating with different optical fiber interferometer RH sensors. FBG is insensitive to refractive index but to temperature [20], therefore, the RH and temperature can be simultaneous measured. Although FBG is insensitive to RH,
the temperature sensitivity is only a few dozen picometers per degree Celsius [21]. The third category for temperature and RH simultaneously detection is achieved by cascading different optical fiber interferometer together, tracking the shift of interference spectrum, and then obtaining RH and temperature sensitivity by matrix calculation. Compared with the proposed two methods, the third method has domainant advantages: high sensitivity, structural diversity, simple making and so on. Yuting Bai and his groups proposed a microfiber coupler for RH and temperature sensing, the proposed sensor is covered with molybdenum disulfide nanosheets. Although the sensitivities of RH and temperature are 115.3 pm/%RH with RH increasing from 54.0 %RH to 93.2 %RH and -104.8 pm/°C with temperature increasing from 30°C to 90°C, respectively [22]. While, the ultrafine diameter (about 6μm) makes the sensor fragile. In this paper, a novel composite interference sensor for RH and temperature simultaneously measurement is proposed and experimentally implemented. The sensing structure is made by inserting 200 μm single mode fiber (SMF) between two 10 cm SMFs, and 62.8 μm core-offset welding is performed, and the core offset region is covered with RH sensing material GQDs-PVA. For the compact sensing structure, the transmitted structure acts as a transmitted MZI, the reflected structure acts as FPI. By monitoring the amount movement of transmitted spectrum and reflected spectrum, the RH and temperature can be simultaneously measured. What’s more, transmitted spectrum and reflected spectrum are mutually independent which is easier for demodulation. 2.
Sensing principle The cross-section drawn of proposed compact sensing structure is presented in Fig. 1, where the
proposed sensor is made by 200 μm SMF between two 10 μm SMFs with large core-offset splicing.
Fig. 1. The cross-section drawn of sensing structure.
For the proposed sensor, when the incident light propagates to the first fusion point, light will be divided into two part: two reflected light (RA and reflected light RB) and three transmitted light (TA, TB, TC). RA is generated by Fresnel reflection caused by the different refractive index between optical fiber core and clad, RB is generated by Fresnel reflection caused by the different refractive index between optical fiber core and RH sensitive material film. Apart for reflected light at the first splicing fusion, part of incident light continue forward which includes TA, TB and TC. TA transmits along the air between SMF1 and SMF3, TB transmits along RH sensitive material film on the surface of SMF2, and TC. transmits along the clad of SMF2, therefore, the MZ interference occurs. At the same time, part of transmitted light A reflects due to the different refractive index between RH sensitive material film and air at the second splicing fusion, and TC appears and then part of TC transmits into the core of SMF1, therefore, the FP interference occurs. For FPI and MZI, interferometer model forms an interferometer and can be described as [23-24]: i
i 1
i
I I n 2 I n I m cos(n,m ) n 1
For MZI, i 3
n 1 m 2
nm
(1)
I I1 I 2 I 3 2 I1 I 2 cos(1,2 ) 2 I1 I 3 cos(1,3 ) 2 I 2 I 3 cos(2,3 )
(2)
Where I n and I m present the intensity of any two part of light in one interferometer, and n,m presents the phase difference between them
n,m 2 L neffn,m /
(3)
n, m In Eq. 3, L is the optical path difference of any two part of light, and neff presents the effective
refractive index difference between corresponding two part of light. Therefore, the dip wavelength can be derived as
MZI
n,m 2 Lneff
(4)
2k 1
For FPI, i 4
I I1 I 2 I 3 I 4 2 I1 I 2 cos(1,2 ) 2 I1 I 3 cos(1,3 ) 2 I1 I 4 cos(1,4 ) 2 I 2 I 3 cos(2,3 ) 2 I 2 I 4 cos(2,4 ) 2 I 3 I 4 cos(3,4 )
(5)
Therefore, FPI can be described as:
FPI
n,m 2 Lneff
(6)
2k 1
GQDs-PVA, as a kind of RH sensitive material, is the mixture of Polyvinyl Alcohol (PVA) and graphene quantum dots (GQDs), and has been adopted for temperature and RH sensing [7, 11]. In this paper, GQDs is 4.5 mg, and PVA is 80.5 mg that is determined by our previous work. The refractive index of GQDs-PVA increases with the decrease of RH values, which results in the increase of the effective refractive index between two interference light, and then causes the interference spectrum shifts to left. As to temperature, the increase of temperature will causes GQDs-PVA film lose water molecules, and refractive index of GQDs-PVA decreases. The effect is opposite to the process of RH increasing, so the interference spectrum shifts to right direction. By observing the spectral movements of FPI and MZI at different temperatures and RH, the corresponding temperature and RH sensitivity are obtained, and after matrix demodulation, the temperature and RH sensitivities of proposed sensor can be obtained. 3.
Making process of proposed sensing structure To fabricate the proposed compact sensor, the ordinary SMF (diameter of core and cladding are 9
μm and 125 μm, respectively) is adopted, and the making manufacturing flow is presented in Fig. 2 (a)-(e). In Fig. 2(a), two 10 μm SMFs are cut flat by optical fiber cutter, which is presented in Fig. 2 (a). And then transfer SMF with flat end face on the holder of fusion splitter (FITEL S178), chose semi-automatic welding mode, and adjust the size of fiber dislocation, according to our preliminary work, core-offset value 62.8 μm is a good choice [25], and the one splicing between two sections of SMF with large core-offset was finished shown in Fig. 2 (c). Considering the measurement range, free spectrum range (FSR) is an important parameter to be considered, FSR is defined as FSR
2
2nL
,
where L is the extent of core-offset region. The FSR is inversely proportional to the length of core-offset region, given the actual operation, 200 μm is chosen as the length of the core-offset region.
In addition, the length of core-offset region is controlled by a 3-D micro-displacement platform adjustment frame. Repeat the above welding process, the proposed sensing structure is completed and shown in Fig. 2 (f). And then GQDs-PVA is deposited on the sensing region by a dip coater (YZ-4200), and then the proposed sensor is put at 28°C for 5h. The proposed sensing structure with GQDs-PVA coating is shown in Fig. 2 (g), where the thickness of coating film is 4.7 μm according to our previous work [7], and the length of sensing region changes to 187.87 μm, therefore, the FSR of proposed sensor will increase. The spectra of the sensing structure before and after coating are shown in Fig. 3, the FSR of spectrum before and after GQDs-PVA coating are 5.875 nm and 6.108 nm, respectively. In addition, the contrast and loss of spectrum before and after GQDs-PVA coating decrease, which is mainly introduced by the absorbance of GQDs-PVA.
Fig. 2. Making manufacturing flow of proposed sensing structure.
Fig. 3. The spectrum of proposed sensing structure before and after coating.
4.
Experiment and discussion The overall system diagram of proposed sensor is presented in Fig. 4, the incident light is
provided by amplified spontaneous emission (ASE), which is made by CONOUER, and working band is 1520-1620 nm. Interference spectrum is recorded by optical spectrum analyzer (OSA), which is made by YOKOGAWA, and the band is AQ6370D. What’s more, the resolution of OSA is 0.02 nm during the whole experiment, temperature is controlled by a thermostat, different RH values is generated by different saturated salt solutions, and RH calibration is realized by hygrometer (Rotronic HC2-S(3)). The path of light transmission is following: the light from the ASE launches into SMF1 and
the circulator, and then reaches to the sensing region, the spectrums of FPI and MZI are recorded by the OSA.
Fig. 4. Diagram of experimental system; (a) broadband amplified spontaneous emission (ASE); (b) optical spectrum analyzer (OSA); (c) thermostat; (c) RH generator and RH calibration.
Firstly, the RH characteristic of compact sensing structure is experimental researched, and the temperature remains at 25°C during the whole process. The spectrums of FPI and MZI under different RH values are presented in Fig. 5, where the spectrums of the FPI and the MZI shift to left with RH values increasing. In Fig. 5 (a), at 27.83 %RH, one of the valleys occurred at 1570.1 nm is selected as the characteristic wavelength. When the RH value is increased to 76.17 %RH, 3.448 nm wavelength shift of the valley is detected. Fig. 6 (a) reveals RH values and wavelength spectrums of FPI has a linear relationship, and the slope of the linear fitting line is 0.072. In Fig. 5 (b), at 27.92 %RH, one of the valleys occurring at 1598.7 nm is selected as the characteristic wavelength. When the RH value is increased to 76.17 %RH, 6.384 nm wavelength shift of the valley is detected.
Fig. 5. (a) the spectrum of FPI with RH increasing from 27.83 %RH to 76.17 %RH, (b) the spectrum of MZI RH increasing from 27.9 %RH to 76.1 %RH.
Fig. 6. (a) The fitting line between RH and interference spectrum of FPI, (b) the fitting line between RH and interference spectrum of MZI.
Fig. 6 (b) reveals RH values and wavelength spectrums of MZI has a linear relationship, and the slope of the linear fitting line is 0.132. The linear fitting slopes in Fig. 6 (a) and Fig. 6 (b) reveal that the resolution to RH of proposed FPI and MZI are 0.27 %RH and 0.15 %RH, respectively. While, the error bars in Fig. 6 (a), the maximum wavelength deviation between the measurement value and fitted value, is 0.103 nm, while that in Fig. 6 (b) is 0.34 nm which is almost triple over that in Fig. 6 (a). Therefore, the proposed FPI is more suitable for RH measurement. In addition, temperature characteristic of proposed compact interferometer is also
experimentally verified. The temperature is adjusted by thermostat (Boxun), and the temperature changes from 22.8°C to 32.8°C. The spectrums of FPI and MZI under different RH are shown in Fig. 7 (a) and (b), respectively, where the spectrums of FPI and MZI shift to right with temperature increasing. And the relationship between spectrum and temperature are shown in Fig. 8.
Fig. 7. the spectrum of FPI (a) and MZI (b) with temperature increasing from 22.8°C to 32.8°C.
Fig. 8 (a) the fitting line between temperature and interference spectrum of FPI, (b) the fitting line between temperature and interference spectrum of MZI.
Fig. 8 (a) reveals the RH values and interference spectrum of proposed FPI has a linear relationship, and the slope of the linear fitting line is 0.172 with temperature changing from 22.8°C to 32.8°C. Fig. 8 (b) reveals the RH values and interference spectrum of proposed MZI has a linear relationship, and the slope of the linear fitting line is 0.37 with temperature changing from 22.8°C to 32.8°C. The linear fitting slopes in Fig. 8 (a) and Fig. 8 (b) reveal that the resolution to temperature of proposed FPI and MZI are 0.12°C and 0.05°C, respectively. The error bars in Fig. 8 (a), the maximum wavelength deviation between the measurement value and fitted value is 0.086 nm, while that in Fig. 8 (b) is 0.11nm. Therefore, the proposed MZI is more suitable for RH measurement. The measurement of RH and temperature have been hereto theoretical analyzed and experimental implemented. And expectation of simultaneously measuring RH and temperature is also proved viable. For FPI, the valley near 1570.1 nm is chosen to demodulated RH and temperature, and the sensitivities
of RH and temperature are 0.072 nm/%RH and 0.172 nm/°C, respectively. For MZI, the valley near 1598.7nm is chosen to demodulated RH and temperature, and the sensitivities of RH and temperature are 0.132 nm/%RH and 0.37 nm/°C, respectively. Therefore, the variation of RH and temperature can be demodulated from the matrix build as following: FPI -0.072 0.172 RH (7) = -0.132 0.37 T MZI Where FPI and MZI are wavelength shifts, as well as RH and T are change of RH and temperature, respectively. In order to verify the feasibility of the sensing matrix, we have done the
following experiments: keeping the temperature unchanged, and the relative humidity value changes from 27.9 %RH to 68.5 %RH. Due to different data acquisition sequence, the range of relative humidity value of FPI is 28.73-63.43%RH. From the calculation of sensitivity matrix, the spectrum shift of MZI and FPI are 5.36 nm and 2.49 nm, respectively. While the experimental results show that the movement of MZI and FPI are 5.45 nm and 2.58 nm, respectively, which is shown in Fig 9. It can be seen that both MZI and FPI have measurement errors. Although there are errors, it is also verified that the sensor matrix is feasible as the later data demodulation. Therefore, how to reduce the error will be the focus of future work.
Fig. 9. (a) the spectrum of FPI with RH increasing from 28.73 %RH to 63.43 %RH, (b) the spectrum of MZI RH increasing from 27.9 %RH to 68.5 %RH.
The performance comparison of fiber sensors for the simultaneous measurement of RH and temperature with other sensors is shown in Table 1. Although the measurement range is not the widest, the sensitivity is the best among that in Table 1. What’s more, compared with other RH and temperature simultaneous measurement configuration, the sensing structure proposed in the manuscript is compared size and low cost. And the high sensitivity means the sensor has the potential of practical application. Table 1. The comparison with other similar researches RH range
RH sensitivity
Temperature range
Temperature sensitivity
(%RH)
(nm/%RH)
(°C)
(nm/°C)
FPI+FBG
20-90
0.0545
10-50
0.0195
[20]
FBG+FBG
35-95
0.006
10-90
0.0104
[8]
54.0-93.2
0.1153
30-90
0.1048
[22]
-0.132
22.8-32.8
0.37
Configuration
microfiber coupler
5.
Our
27.83-76.
manuscript
17
Results
reference
In conclusion, a composite sensor consisting of FPI and MZI is proposed and experimentally implemented for RH and temperature simultaneous measurement. GQDs-PVA, as a kind of RH and temperature sensing material, is covered on the surface of core-offset region, and the refractive index of GQDs-PVA changes with the change of temperature and RH, which causes the interference spectrum shift. The spectrum of FPI and MZI have a blue shift with the increase of RH and a red shift with the increase of temperature. The sensitivities of FPI and MZI to RH are -0.072 nm/%RH and -0.132 nm/%RH, respectively, and the corresponding resolution are 0.27 %RH and 0.15 %RH, respectively. In addition, the sensitivities of FPI and MZI to temperature are 0.172 nm/°C and 0.37 nm/°C, respectively, and the correspond resolution are 0.12°C and 0.05°C, respectively, the high resolution to RH and temperature makes the proposed sensor have potential in practical application. The spectra of proposed FPI and proposed MZI compact in larger core-offset structure are independent that avoids the complicated demodulation process of fast Fourier transform, which made the proposed sensor can realize the simultaneous measurement of RH and temperature. It worthy to mention that the proposed sensing structure is made by precision instrument, such as micro displacement platform, fusion splitter, and so on, which makes the sensor making process is repeatable, in addition, the sensing structure also can be made by multimode fiber. Acknowledgement This work was supported in part by the National Natural Science Foundation of China under Grant 61933004 and 61773102, the Fundamental Research Funds for the Central Universities under Grant N160408001, N150401001 and part by the State Key Laboratory of Synthetical Automation for Process Industries under Grant 2013ZCX09.
Reference: [1] A. Alvarez-Herrero, H. Guerrero, D. Levy, High-sensitivity sensor of low relative humidity based on overlay on side-polished fibers, Sens. J. IEEE 4 (2004) 52-56. [2] C. Wang, G. Yan, Z. Lian, et al., Hybrid-cavity Fabry-Perot interferometer for multipoint relative humidity and temperature sensing, Sens. Actuators B 255 (2017) 1937-1938. [3] Rui-jie Tong, Yong Zhao, et al., Multimode interferometer based on no-core fiber with GQDs-PVA composite coating for relative humidity sensing, Optical Fiber Technology 48 (2019) 242-247. [4] Y. Wang, C. Shen, W. Lou, F. Shentu, C. Zhong, X. Dong, L. Tong, Fiber optic relative humidity sensor based on the tilted fiber bragg grating coated with graphene oxide, Appl. Phys. Lett. 109 (2016) 10990-1536. [5] Zhao Y, Tong R J, Xia F, et al. Current status of optical fiber biosensor based on surface plasmon resonance[J]. Biosensors & bioelectronics, 2019, 142:111505. [6] A.D.D. Le, Y.G. Han, Relative humidity sensor based on a few-mode microfiber knot resonator by mitigating the group index difference of a few-mode microfiber, J. Light wave Technol. 36 (2018) 904-909. [7] Rui-jie Tong, Yong Zhao, et al., Relative humidity sensor based on small up tapered photonic crystal fiber Mach-Zehnder interferometer, Sens. Actuators A. 280 (2018) 24-30. [8] Feilin Zhang, Ming Li, et al., A method for standardizing the manufacturing process of integrated temperature and humidity sensor based on fiber Bragg grating, Optical Fiber Technology 46 (2018) 275-281. [9] Yinping Miao, Bo Liu, et al., Relative Humidity Sensor Based on Tilted Fiber Bragg Grating With Polyvinyl Alcohol Coating, IEEE Photonics Technology Letters, 21(2009) 441-443. [10] Huang C , Yang M , Xie W , et al. Optical fiber Fabry-Perot refractive index sensor based on porous Al2O3 film[J]. IEEE Photonics Technology Letters, 27(2015) 2127-2130. [11] Yong Zhao, Rui-jie Tong, et al., Relative humidity sensor based on hollow core fiber filled with GQDs-PVA, Sensors & Actuators: B. Chemical 284 (2019) 96-102. [12] Sun H, Yang Z, Zhou L, et al. A relative humidity sensing probe based on etched thin-core fiber coated with polyvinyl alcohol [J]. Optics Communications, 356(2015) 556-559. [13] Liu Y, Lin H, Dai Y, et al. Humidity Sensor Based on an In-Fiber Integrated Mach-Zehnder Interferometer[J]. IEEE Photonics Technology Letters, 31(2019) 393-396. [14] Shinbo, K.; Ohdaira, Y.; Baba, A.; Kato, K.; Kaneko, F. Vapor sorption to polyvinyl alcohol thin film observed using a hybrid sensor of quartz-crystal-microbalance and surface-plasmon-resonance. Mol. Cryst. Liq. Cryst. 622(2015), 67-73. [15] Hao L, Lei L, Ma X, et al. Relative Humidity and Temperature Sensing Characteristics of an Optical Fiber Probe Based on Hollow-Core Fiber and Calcium Alginate Hydrogel Film[J]. IEEE Sensors Journal, 18(2018) 8022-8027. [16] Weijuan Chen, Zhihao Chen, et al., Agarose coated macro-bend fiber sensor for relative humidity and temperature measurement at 2 μm, Optical Fiber Technology 50 (2019) 118-124. [17] Jiri Hromadka, Nurul N. Mohd Hazlan, et al., Simultaneous in situ temperature and relative humidity monitoring in mechanical ventilators using an array of functionalised optical fibre long period grating sensors, Sensors & Actuators: B. Chemical 286 (2019) 306-314. [18] D. Viegas, M. Hernaez, et al., Simultaneous Measurement of Humidity and Temperature Based on an SiO-Nanospheres Film Deposited on a Long-Period Grating In-Line With a Fiber Bragg Grating,
IEEE Sensors Journal, 11(2011) 162-166. [19] Aitor Urrutia, Javier Goicoechea, et al., Simultaneous measurement of humidity and temperature based on a partially coated optical fiber long period grating, Sensors and Actuators B, 227 (2016) 135-141. [20] Chengling Lee, Yanwun You, et al., Hygroscopic polymer microcavity fiber Fizeau interferometer incorporating a fiber Bragg grating for simultaneously sensing humidity and temperature, Sensors and Actuators B, 222 (2016) 339-346. [21] Shuqin Zhang, Xinyong Dong, et al., Simultaneous measurement of relative humidity and temperature with PCF-MZI cascaded by fiber Bragg grating, Optics Communications 303 (2013) 42-45. [22] Yuting Bai, Ying Miao, et al., Simultaneous measurement of relative humidity and temperature using a microfiber coupler coated with molybdenum disulfide nanosheets, Optical Materials Express, 9(2019) 2846-2858. [23] B. Dong, L. Wei, and D.-P. Zhou, “Coupling between the smallcore-diameter dispersion compensation fiber and single-mode fiber and its applications in fiber lasers,” J. Light. Technol., 28 (2010) 1363-1367. [24] Wu Y, Zhang Y, et al., “Fiber-optic hybrid-structured fabry-perot interferometer based on large lateral offset splicing for simultaneous measurement of strain and temperature,” J. Lightw. Technol., 35 (2017) 4311-4315. [25] Shuna Wang, Riqing Lv, et al., A Mach-Zehnder interferometer-based High Sensitivity Temperature sensor for human body monitoring, Optical Fiber Technology 45 (2018) 93-97.
Author Contributions Section Rui-jie Tong: Methodology, Writing - Original Draft, Validation Yong Zhao：Conceptualization, Methodology, Funding acquisition Hong-kun Zheng：Formal analysis, Validation Feng Xia：Writing - Original Draft, Software
Declaration of Interest statement There are no interests to declare.
Yong Zhao
Highlights
A novel and compact temperature and relative humidity sensor was proposed and implemented. The sensor was based on compact Mach-Zehnder interferometer and Fabry-Perot interferometer. Detection of temperature and RH can be achieved by monitoring the spectrums of FPI and MZI. The sensitivity to RH is -0.132 nm/%RH with resolution of 0.15 %RH. The sensitivity to temperature is 0.37 nm/°C with the resolution of 0.05°C.
S0263-2241(20)30036-1 https://doi.org/10.1016/j.measurement.2020.107499 MEASUR 107499
To appear in:
Measurement
Received Date: Revised Date: Accepted Date:
12 September 2019 1 January 2020 10 January 2020
Please cite this article as: R-j. Tong, Y. Zhao, H-k. Zheng, F. Xia, Simultaneous measurement of temperature and relative humidity by compact Mach-Zehnder interferometer and Fabry-Perot interferometer, Measurement (2020), doi: https://doi.org/10.1016/j.measurement.2020.107499
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Elsevier Ltd. All rights reserved.
Simultaneous measurement of temperature and relative humidity by compact Mach-Zehnder interferometer and Fabry-Perot interferometer Rui-jie Tong, Yong Zhao, Hong-kun Zheng, Feng Xia College of Information Science and Engineering, Northeastern University, Shenyang 110819, China
Corresponding Author: [email protected]
Abstract: A novel and compact temperature and relative humidity (RH) simultaneously measurement sensor based on compact Mach-Zehnder interferometer (MZI) and Fabry-Perot interferometer (FPI) is proposed and experimental implemented. The sensing structure is made by splicing a 200 μm single mode fiber (SMF) between two 10 cm SMFs with large core-offset. The core-offset distance is 62.8 μm coated with RH sensitive material graphene quantum dots and polyvinyl alcohol (GQDs-PVA). The transmitted structure acts as MZI, and the reflected structure acts as FPI, then the simultaneous detection of temperature and RH can be achieved by monitoring the spectrums of FPI and MZI, respectively. The sensitivity to RH is -0.132 nm/%RH with resolution of 0.15 %RH, and the sensitivity to temperature is 0.37 nm/°C with the resolution of 0.05°C. The proposed sensor is compact, high sensitivity, simple making and low cost. Key words: relative humidity sensor; temperature measurement; Fabry-Perot interferometer; Mach-Zehnder interferometer. 1.
Introduction With the development of agriculture, biopharmaceutical, aerospace and precision instrument
manufacturing industries, the importance of relative humidity (RH) accurate measurement is more and more obvious, such as soil humidity affects the yield of crops, environmental humidity affects the quality of biopharmaceuticals and the service life of precision instruments [1-4]. Compared with traditional RH prospecting instrument based on polymer, semiconductor ceramic and alumina, RH sensors based on optical fiber are compact size, anti-electromagnetic interference, remote sensing, and so on [5-7], and have attracted intensive attention. Up to now, optical fiber prospecting instrument based on different sensing methods fiber grating [8, 9], Fabry-Perot interferometer (FPI) [10, 11], Mach-Zehnder interferometer (MZI) [12, 13], surface plasmon resonance [14]) have been put forward for RH detection, however, most of RH sensors are affected by temperature, which could introduce measurement error [15]. Therefore, the simultaneous detection of temperature and RH need cause concern. Although RH and temperature simultaneous detection is difficult, plenty of simultaneous measurement methods have been proposed [15-17], and the existing optical fiber sensors for RH and temperature simultaneously detection can be divided into three categories: The first category adopts two sections of optical fiber grating with different center wavelengths for temperature and RH measurement. At present, the most adopted method is cascading two long period grating (LPG) or cascade LPG and fiber Brag grating (FBG) together for RH and temperature simultaneous detection. However, the two LPG cascaded will make the optical spectrum chaos, which is not conducive to the reading of data [18, 19]. The second category is cascading fiber Brag grating with different optical fiber interferometer RH sensors. FBG is insensitive to refractive index but to temperature [20], therefore, the RH and temperature can be simultaneous measured. Although FBG is insensitive to RH,
the temperature sensitivity is only a few dozen picometers per degree Celsius [21]. The third category for temperature and RH simultaneously detection is achieved by cascading different optical fiber interferometer together, tracking the shift of interference spectrum, and then obtaining RH and temperature sensitivity by matrix calculation. Compared with the proposed two methods, the third method has domainant advantages: high sensitivity, structural diversity, simple making and so on. Yuting Bai and his groups proposed a microfiber coupler for RH and temperature sensing, the proposed sensor is covered with molybdenum disulfide nanosheets. Although the sensitivities of RH and temperature are 115.3 pm/%RH with RH increasing from 54.0 %RH to 93.2 %RH and -104.8 pm/°C with temperature increasing from 30°C to 90°C, respectively [22]. While, the ultrafine diameter (about 6μm) makes the sensor fragile. In this paper, a novel composite interference sensor for RH and temperature simultaneously measurement is proposed and experimentally implemented. The sensing structure is made by inserting 200 μm single mode fiber (SMF) between two 10 cm SMFs, and 62.8 μm core-offset welding is performed, and the core offset region is covered with RH sensing material GQDs-PVA. For the compact sensing structure, the transmitted structure acts as a transmitted MZI, the reflected structure acts as FPI. By monitoring the amount movement of transmitted spectrum and reflected spectrum, the RH and temperature can be simultaneously measured. What’s more, transmitted spectrum and reflected spectrum are mutually independent which is easier for demodulation. 2.
Sensing principle The cross-section drawn of proposed compact sensing structure is presented in Fig. 1, where the
proposed sensor is made by 200 μm SMF between two 10 μm SMFs with large core-offset splicing.
Fig. 1. The cross-section drawn of sensing structure.
For the proposed sensor, when the incident light propagates to the first fusion point, light will be divided into two part: two reflected light (RA and reflected light RB) and three transmitted light (TA, TB, TC). RA is generated by Fresnel reflection caused by the different refractive index between optical fiber core and clad, RB is generated by Fresnel reflection caused by the different refractive index between optical fiber core and RH sensitive material film. Apart for reflected light at the first splicing fusion, part of incident light continue forward which includes TA, TB and TC. TA transmits along the air between SMF1 and SMF3, TB transmits along RH sensitive material film on the surface of SMF2, and TC. transmits along the clad of SMF2, therefore, the MZ interference occurs. At the same time, part of transmitted light A reflects due to the different refractive index between RH sensitive material film and air at the second splicing fusion, and TC appears and then part of TC transmits into the core of SMF1, therefore, the FP interference occurs. For FPI and MZI, interferometer model forms an interferometer and can be described as [23-24]: i
i 1
i
I I n 2 I n I m cos(n,m ) n 1
For MZI, i 3
n 1 m 2
nm
(1)
I I1 I 2 I 3 2 I1 I 2 cos(1,2 ) 2 I1 I 3 cos(1,3 ) 2 I 2 I 3 cos(2,3 )
(2)
Where I n and I m present the intensity of any two part of light in one interferometer, and n,m presents the phase difference between them
n,m 2 L neffn,m /
(3)
n, m In Eq. 3, L is the optical path difference of any two part of light, and neff presents the effective
refractive index difference between corresponding two part of light. Therefore, the dip wavelength can be derived as
MZI
n,m 2 Lneff
(4)
2k 1
For FPI, i 4
I I1 I 2 I 3 I 4 2 I1 I 2 cos(1,2 ) 2 I1 I 3 cos(1,3 ) 2 I1 I 4 cos(1,4 ) 2 I 2 I 3 cos(2,3 ) 2 I 2 I 4 cos(2,4 ) 2 I 3 I 4 cos(3,4 )
(5)
Therefore, FPI can be described as:
FPI
n,m 2 Lneff
(6)
2k 1
GQDs-PVA, as a kind of RH sensitive material, is the mixture of Polyvinyl Alcohol (PVA) and graphene quantum dots (GQDs), and has been adopted for temperature and RH sensing [7, 11]. In this paper, GQDs is 4.5 mg, and PVA is 80.5 mg that is determined by our previous work. The refractive index of GQDs-PVA increases with the decrease of RH values, which results in the increase of the effective refractive index between two interference light, and then causes the interference spectrum shifts to left. As to temperature, the increase of temperature will causes GQDs-PVA film lose water molecules, and refractive index of GQDs-PVA decreases. The effect is opposite to the process of RH increasing, so the interference spectrum shifts to right direction. By observing the spectral movements of FPI and MZI at different temperatures and RH, the corresponding temperature and RH sensitivity are obtained, and after matrix demodulation, the temperature and RH sensitivities of proposed sensor can be obtained. 3.
Making process of proposed sensing structure To fabricate the proposed compact sensor, the ordinary SMF (diameter of core and cladding are 9
μm and 125 μm, respectively) is adopted, and the making manufacturing flow is presented in Fig. 2 (a)-(e). In Fig. 2(a), two 10 μm SMFs are cut flat by optical fiber cutter, which is presented in Fig. 2 (a). And then transfer SMF with flat end face on the holder of fusion splitter (FITEL S178), chose semi-automatic welding mode, and adjust the size of fiber dislocation, according to our preliminary work, core-offset value 62.8 μm is a good choice [25], and the one splicing between two sections of SMF with large core-offset was finished shown in Fig. 2 (c). Considering the measurement range, free spectrum range (FSR) is an important parameter to be considered, FSR is defined as FSR
2
2nL
,
where L is the extent of core-offset region. The FSR is inversely proportional to the length of core-offset region, given the actual operation, 200 μm is chosen as the length of the core-offset region.
In addition, the length of core-offset region is controlled by a 3-D micro-displacement platform adjustment frame. Repeat the above welding process, the proposed sensing structure is completed and shown in Fig. 2 (f). And then GQDs-PVA is deposited on the sensing region by a dip coater (YZ-4200), and then the proposed sensor is put at 28°C for 5h. The proposed sensing structure with GQDs-PVA coating is shown in Fig. 2 (g), where the thickness of coating film is 4.7 μm according to our previous work [7], and the length of sensing region changes to 187.87 μm, therefore, the FSR of proposed sensor will increase. The spectra of the sensing structure before and after coating are shown in Fig. 3, the FSR of spectrum before and after GQDs-PVA coating are 5.875 nm and 6.108 nm, respectively. In addition, the contrast and loss of spectrum before and after GQDs-PVA coating decrease, which is mainly introduced by the absorbance of GQDs-PVA.
Fig. 2. Making manufacturing flow of proposed sensing structure.
Fig. 3. The spectrum of proposed sensing structure before and after coating.
4.
Experiment and discussion The overall system diagram of proposed sensor is presented in Fig. 4, the incident light is
provided by amplified spontaneous emission (ASE), which is made by CONOUER, and working band is 1520-1620 nm. Interference spectrum is recorded by optical spectrum analyzer (OSA), which is made by YOKOGAWA, and the band is AQ6370D. What’s more, the resolution of OSA is 0.02 nm during the whole experiment, temperature is controlled by a thermostat, different RH values is generated by different saturated salt solutions, and RH calibration is realized by hygrometer (Rotronic HC2-S(3)). The path of light transmission is following: the light from the ASE launches into SMF1 and
the circulator, and then reaches to the sensing region, the spectrums of FPI and MZI are recorded by the OSA.
Fig. 4. Diagram of experimental system; (a) broadband amplified spontaneous emission (ASE); (b) optical spectrum analyzer (OSA); (c) thermostat; (c) RH generator and RH calibration.
Firstly, the RH characteristic of compact sensing structure is experimental researched, and the temperature remains at 25°C during the whole process. The spectrums of FPI and MZI under different RH values are presented in Fig. 5, where the spectrums of the FPI and the MZI shift to left with RH values increasing. In Fig. 5 (a), at 27.83 %RH, one of the valleys occurred at 1570.1 nm is selected as the characteristic wavelength. When the RH value is increased to 76.17 %RH, 3.448 nm wavelength shift of the valley is detected. Fig. 6 (a) reveals RH values and wavelength spectrums of FPI has a linear relationship, and the slope of the linear fitting line is 0.072. In Fig. 5 (b), at 27.92 %RH, one of the valleys occurring at 1598.7 nm is selected as the characteristic wavelength. When the RH value is increased to 76.17 %RH, 6.384 nm wavelength shift of the valley is detected.
Fig. 5. (a) the spectrum of FPI with RH increasing from 27.83 %RH to 76.17 %RH, (b) the spectrum of MZI RH increasing from 27.9 %RH to 76.1 %RH.
Fig. 6. (a) The fitting line between RH and interference spectrum of FPI, (b) the fitting line between RH and interference spectrum of MZI.
Fig. 6 (b) reveals RH values and wavelength spectrums of MZI has a linear relationship, and the slope of the linear fitting line is 0.132. The linear fitting slopes in Fig. 6 (a) and Fig. 6 (b) reveal that the resolution to RH of proposed FPI and MZI are 0.27 %RH and 0.15 %RH, respectively. While, the error bars in Fig. 6 (a), the maximum wavelength deviation between the measurement value and fitted value, is 0.103 nm, while that in Fig. 6 (b) is 0.34 nm which is almost triple over that in Fig. 6 (a). Therefore, the proposed FPI is more suitable for RH measurement. In addition, temperature characteristic of proposed compact interferometer is also
experimentally verified. The temperature is adjusted by thermostat (Boxun), and the temperature changes from 22.8°C to 32.8°C. The spectrums of FPI and MZI under different RH are shown in Fig. 7 (a) and (b), respectively, where the spectrums of FPI and MZI shift to right with temperature increasing. And the relationship between spectrum and temperature are shown in Fig. 8.
Fig. 7. the spectrum of FPI (a) and MZI (b) with temperature increasing from 22.8°C to 32.8°C.
Fig. 8 (a) the fitting line between temperature and interference spectrum of FPI, (b) the fitting line between temperature and interference spectrum of MZI.
Fig. 8 (a) reveals the RH values and interference spectrum of proposed FPI has a linear relationship, and the slope of the linear fitting line is 0.172 with temperature changing from 22.8°C to 32.8°C. Fig. 8 (b) reveals the RH values and interference spectrum of proposed MZI has a linear relationship, and the slope of the linear fitting line is 0.37 with temperature changing from 22.8°C to 32.8°C. The linear fitting slopes in Fig. 8 (a) and Fig. 8 (b) reveal that the resolution to temperature of proposed FPI and MZI are 0.12°C and 0.05°C, respectively. The error bars in Fig. 8 (a), the maximum wavelength deviation between the measurement value and fitted value is 0.086 nm, while that in Fig. 8 (b) is 0.11nm. Therefore, the proposed MZI is more suitable for RH measurement. The measurement of RH and temperature have been hereto theoretical analyzed and experimental implemented. And expectation of simultaneously measuring RH and temperature is also proved viable. For FPI, the valley near 1570.1 nm is chosen to demodulated RH and temperature, and the sensitivities
of RH and temperature are 0.072 nm/%RH and 0.172 nm/°C, respectively. For MZI, the valley near 1598.7nm is chosen to demodulated RH and temperature, and the sensitivities of RH and temperature are 0.132 nm/%RH and 0.37 nm/°C, respectively. Therefore, the variation of RH and temperature can be demodulated from the matrix build as following: FPI -0.072 0.172 RH (7) = -0.132 0.37 T MZI Where FPI and MZI are wavelength shifts, as well as RH and T are change of RH and temperature, respectively. In order to verify the feasibility of the sensing matrix, we have done the
following experiments: keeping the temperature unchanged, and the relative humidity value changes from 27.9 %RH to 68.5 %RH. Due to different data acquisition sequence, the range of relative humidity value of FPI is 28.73-63.43%RH. From the calculation of sensitivity matrix, the spectrum shift of MZI and FPI are 5.36 nm and 2.49 nm, respectively. While the experimental results show that the movement of MZI and FPI are 5.45 nm and 2.58 nm, respectively, which is shown in Fig 9. It can be seen that both MZI and FPI have measurement errors. Although there are errors, it is also verified that the sensor matrix is feasible as the later data demodulation. Therefore, how to reduce the error will be the focus of future work.
Fig. 9. (a) the spectrum of FPI with RH increasing from 28.73 %RH to 63.43 %RH, (b) the spectrum of MZI RH increasing from 27.9 %RH to 68.5 %RH.
The performance comparison of fiber sensors for the simultaneous measurement of RH and temperature with other sensors is shown in Table 1. Although the measurement range is not the widest, the sensitivity is the best among that in Table 1. What’s more, compared with other RH and temperature simultaneous measurement configuration, the sensing structure proposed in the manuscript is compared size and low cost. And the high sensitivity means the sensor has the potential of practical application. Table 1. The comparison with other similar researches RH range
RH sensitivity
Temperature range
Temperature sensitivity
(%RH)
(nm/%RH)
(°C)
(nm/°C)
FPI+FBG
20-90
0.0545
10-50
0.0195
[20]
FBG+FBG
35-95
0.006
10-90
0.0104
[8]
54.0-93.2
0.1153
30-90
0.1048
[22]
-0.132
22.8-32.8
0.37
Configuration
microfiber coupler
5.
Our
27.83-76.
manuscript
17
Results
reference
In conclusion, a composite sensor consisting of FPI and MZI is proposed and experimentally implemented for RH and temperature simultaneous measurement. GQDs-PVA, as a kind of RH and temperature sensing material, is covered on the surface of core-offset region, and the refractive index of GQDs-PVA changes with the change of temperature and RH, which causes the interference spectrum shift. The spectrum of FPI and MZI have a blue shift with the increase of RH and a red shift with the increase of temperature. The sensitivities of FPI and MZI to RH are -0.072 nm/%RH and -0.132 nm/%RH, respectively, and the corresponding resolution are 0.27 %RH and 0.15 %RH, respectively. In addition, the sensitivities of FPI and MZI to temperature are 0.172 nm/°C and 0.37 nm/°C, respectively, and the correspond resolution are 0.12°C and 0.05°C, respectively, the high resolution to RH and temperature makes the proposed sensor have potential in practical application. The spectra of proposed FPI and proposed MZI compact in larger core-offset structure are independent that avoids the complicated demodulation process of fast Fourier transform, which made the proposed sensor can realize the simultaneous measurement of RH and temperature. It worthy to mention that the proposed sensing structure is made by precision instrument, such as micro displacement platform, fusion splitter, and so on, which makes the sensor making process is repeatable, in addition, the sensing structure also can be made by multimode fiber. Acknowledgement This work was supported in part by the National Natural Science Foundation of China under Grant 61933004 and 61773102, the Fundamental Research Funds for the Central Universities under Grant N160408001, N150401001 and part by the State Key Laboratory of Synthetical Automation for Process Industries under Grant 2013ZCX09.
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Author Contributions Section Rui-jie Tong: Methodology, Writing - Original Draft, Validation Yong Zhao：Conceptualization, Methodology, Funding acquisition Hong-kun Zheng：Formal analysis, Validation Feng Xia：Writing - Original Draft, Software
Declaration of Interest statement There are no interests to declare.
Yong Zhao
Highlights
A novel and compact temperature and relative humidity sensor was proposed and implemented. The sensor was based on compact Mach-Zehnder interferometer and Fabry-Perot interferometer. Detection of temperature and RH can be achieved by monitoring the spectrums of FPI and MZI. The sensitivity to RH is -0.132 nm/%RH with resolution of 0.15 %RH. The sensitivity to temperature is 0.37 nm/°C with the resolution of 0.05°C.