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Abstract
The simultaneous measurement of relative humidity (RH) and temperature with an optical fiber sensor is proposed based on a microfiber coupler (MFC) coated with a single layer of molybdenum disulfide (MoS2) nanosheets. The MFC is fabricated using flame heating technology and has a waist diameter of approximately 6 µm and a waist length of approximately 3 mm. As the RH increases, the effective refractive index of MoS2 varies as a result of electric charge transfer, the spectrum dip shifts toward longer wavelengths, and the transmission intensity of the spectrum decreases. As the temperature increases, the refractive index of the cladding of the fiber in the MFC waist region increases due to the thermo-optic effect, the spectrum dip shifts toward shorter wavelengths, and the transmission intensity of the spectrum decreases. The experimental results show that the RH sensitivities are 115.3 pm/%RH and -0.058 dB/%RH in the range of 54.0 - 93.2%RH. The temperature sensitivities are -104.8 pm/°C and -0.042 dB/°C in the range of 30 - 90 °C. The proposed sensor is expected to be used for simultaneous measurement of RH and temperature in the field of biochemical analysis.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Humidity and temperature information are very important in applications such as civil engineering, the food industry, health and medical care, and air conditioning in electronic components. Studies of humidity sensors based on different sensing mechanisms have made significant progress over the past few decades [1–4]. Conventional capacitive and resistive electronic humidity sensors have been widely used [5]. However, they are susceptible to electromagnetic interference and have a long response time [5]. An optical fiber sensor can overcome these difficulties because it has the characteristics of a fast response, low cost, compact size, high precision, low susceptibility to electromagnetic interference and high sensitivity. Because of these advantages, optical fiber sensors have attracted great interest from researchers. A variety of humidity sensors based on different sensing schemes, including Mach–Zehnder interferometers (MZI) [6], fiber Bragg gratings (FBGs) [7,8], long-period gratings (LPGs) [9], etched fiber [10], and Fabry-Perot (FP) cavities [11,12], have been proposed and demonstrated. Humidity sensors based on gratings and FP cavity structures generally have low sensitivity and require a high manufacturing cost, and complex processing [5,11,12]. MZI-based-sensors usually generate periodic output spectra due to interference between light beams, which limits the measurement range [6,13]. Etched fibers are fragile during the etching process and have poor long-term stability. In addition, some researchers have used polymer fibers to develop humidity sensors [5,14–17]. Addressing the coupling loss between sensors and traditional optical fiber devices is an important problem. Microfiber sensors have the advantage of an evanescent field at the outer surface of the microfiber but usually have relatively low sensitivity [13,18]. Compared to a single microfiber, a microfiber coupler (MFC) is more sensitive to the surrounding medium because its sensing performance depends on the coupling between the two evanescent fields. A sensor with a microfiber coupler has the advantages of low fabrication difficulty, good repeatability and high sensitivity due to the strong evanescent field characteristics in the coupled region and is suitable for high-precision environmental measurements.
Molybdenum disulfide (MoS2), a typical layered transition metal two-dimensional material, has excellent characteristics, such as a unique layered structure, a large specific surface area, a high density of electronic states and a high electron mobility. In addition, MoS2 has good thermal and chemical stability. Single-layer MoS2 has a direct bandgap structure and can be applied in gas detectors and light-emitting diodes. Gas molecules, such as water molecules, can be easily adsorbed and resolved on the surface of MoS2. Researchers have taken advantage of these properties to combine MoS2 with optical fiber as a humidity sensor. Du et al. proposed a relative humidity (RH) sensor based on single-layer MoS2 nanosheets and an etched single-mode fiber for measuring human breathing gas. As the humidity increases from 20% to 80%, the transmitted power is reduced nonlinearly from 1.055 mW to 0.943 mW [19]. Li et al. reported an intensity-based RH sensor fabricated by a side-polished fiber covered with few-layer MoS2, and the optical transmitted power was reduced by ∼13.5 dB when the RH ranged from 40% to 85% [20]. The basic principle of both schemes is that the transmitted power reduces in response to an increase in the absorption of MoS2. The surface of the fiber etched by hydrofluoric acid (denoted as HF) is rough, and both etched and side-polished fibers are fragile. In addition, for practical humidity measurement, temperature cross-sensitivity issues need to be considered, which affect the accuracy of humidity measurements.
In this work, a sensor that simultaneously measures RH and temperature is proposed using a microfiber coupler that is modified by single-layer nanosheets of MoS2. The effective refractive index of MoS2 increases as a result of electric charge transfer of water molecules attached to the surface of MoS2 nanosheets. The experimental results show that the dip of the transmission spectrum shifts toward longer wavelengths and the transmission intensity of the spectrum decreases as the RH increases. Furthermore, the temperature response results indicate that the dip of the transmission spectrum shifts toward shorter wavelengths and the transmission intensity of the spectrum decreases as the temperature increases. The proposed sensor has the merits of simple structure, ultra-compactness and excellent repeatability and is expected to be used for the simultaneous measurement of RH and temperature in biochemical applications.
2. Principle and sensor fabrication
2.1 Principle
Figure 1 shows a schematic diagram of a typical microfiber coupler. The device consists of two closely spaced microfibers with two input ports (Port A and Port B), two output ports (Port C and Port D), and 3 coupling regions (2 weakly coupled regions and a strongly coupled region). When light of power ${P_1}$ is injected into one of the input ports, the output powers at the output ports are expressed by the following [21]:
As the waist region microfiber is fused by two fibers into a single fiber and the waist diameter is very thin, strong coupling theory is suitable for the sensor and plays a dominant role compared to weak coupling [25,26]. According to the analysis of Eq. (4), when strong coupling dominates in the coupling process, ${S_n} > 0$, and the interference spectrum shifts in the long-wavelength direction as the refractive index of the surrounding medium increases [23]. At the same time, the refractive index of the external environment affects the mode field distribution of the MFC. Here, we perform a three-dimensional simulation to study the beam propagation characteristics for different refractive indices of the surrounding medium using the finite-difference beam propagation method (FD-BPM; Rsoft Design Group). In the simulation, the parameters of the microfiber coupler are set according to the experimental structure. The microfiber coupler parameters produced in the experiment are as follows: the waist diameter D1 is approximately 6 µm, the waist length L2 is approximately 3 mm, and the single tapered transition lengths L1 and L3 are approximately 3.8 mm. The fiber core in the waist region is considered to be negligible due to the small cross-section of the core [25]. The fiber cladding and the external medium become the new waveguide core and cladding, respectively. The fiber used in the experiment has a core diameter of 8.2 µm. The cladding and core refractive indices are 1.4682 and 1.4628, respectively. At an incident light wavelength of 1550 nm, the simulated amplitude distribution of the light propagating in the microfiber coupler for different ambient refractive indices is presented in Fig. 3. We can clearly see that the mode field distribution is closely related to the ambient refractive index of the microfiber coupler. Furthermore, the optical transmitted power of the output port is modulated by the ambient refractive index.
Fig. 3. Simulated amplitude distribution of light propagating in the microfiber coupler for different ambient refractive index levels of (a) 1.0, (b) 1.358, and (c) 1.4182.
As the RH increases, the number of water molecules adsorbed on the MoS2 nanosheets increases, and the effective refractive index of MoS2 also increases. Thus, the RH sensor becomes more sensitive to variations in the RH. From Eq. (4), the RH sensitivity can be expressed as follows:
The temperature variation affects the physical parameters of the fiber itself, including the refractive index, diameter and length of the fiber. The values of these parameters depend on the thermo-optic effect and thermal expansion effect of the fiber material. For the presented structure, changes in the refractive index of the fiber cladding ${n_2}$ and the radius r and length L of the fiber due to temperature variations can be expressed as follows [27]:
Fig. 4. Simulated transmission of MFC for different refractive index values of the fiber cladding of ${n_2}$.
2.2 Sensor fabrication
In the experiment, the microfiber coupler was fabricated by heating and melting two sections of single-mode fiber. First, the two sections of fiber were entangled with each other, and their two ends were fixed on the translation stage. Then, a hydrogen flame was used to heat the entangled region of the fiber to a molten state while controlling the translation stage to stretch the fiber. The distance over which the translation stage moved depended on the requirements of the microfiber coupler. The parameters that determine the device performance, such as the diameter and length of the waist region, were precisely adjusted by controlling the hydrogen gas flow and stretching length of the translation stage. Thus, this structure could be fabricated with good repeatability. Lastly, a UV glue was dropped onto the interface between the slide and both ends of the microfiber coupler to decrease its fragility. A single layer of MoS2 nanosheet dispersion (Nanjing MKNANO Technology Co., Ltd.) at a concentration of 2 mg/ml was prepared. The dispersion was then diluted 50 times with absolute ethanol and treated in an ultrasonic cleaner at a power of 100 W for 10 minutes to uniformly disperse the MoS2 nanosheets. A small amount of the ultrasonically treated MoS2 dispersion was placed on the waist region of the MFC. The fabricated sensor was air-dried for 1 hour in the air. Since the concentration of the MoS2 nanosheet dispersion used was constant and the volume of each drop on the surface of the fiber was similar, the number of MoS2 nanoparticles finally adsorbed on the fiber was similar. Therefore, the sensor had good repeatability. An optical microscope image of the microfiber coupler with MoS2 nanosheets is shown in Fig. 5.
3. Results and discussion
3.1 Experimental setup
A schematic diagram of the RH measurement experimental setup is shown in Fig. 6. Port B of the microfiber coupler is connected to a supercontinuum broadband source (SBS) that provides input light from 600 nm to 1700nm. Port D is connected to an output spectrum analyzer with 0.2 nm resolution (OSA, Yokogawa AQ6370C) to detect the spectrum transmitted through the sensing unit. An electronic humidity sensor (Testo 175H1) is placed in the humidity chamber to detect RH in ambient conditions. The sensing unit consists of a microfiber coupler and dried MoS2 nanosheets. The humidity chamber and humidifier provide a variable humidity environment for the experiment.
3.2 RH measurement
Figure 7 shows the transmission spectra with an RH ranging from 54% to 93.2% at a room temperature of 28 °C. As the RH increases, the transmission dip shifts toward longer wavelengths, and the transmission intensity decreases. To better study the relationship between spectral characteristics and RH, we selected the dip with a wavelength of 1622.4 nm, as shown in Fig. 8. The wavelength dip moves from 1622.4 nm to1627.12 nm as the RH increases from 54.0% to 93.2%. An increase in RH causes more water molecules to attach to the surface of the MoS2 nanosheets. Accordingly, part of the electric charge on the MoS2 nanosheets is transferred to the water molecules [28–30]. Consequently, the movement of electric charge results in an increase in the effective refractive index of MoS2, which indicates that the refractive index ${n_3}$ of the surrounding environment increases. Therefore, it is easy to obtain $\frac{{d{n_3}}}{{dRH}} > 0$ in Eq. (5). According to Eq. (3), as the effective refractive index ${n_3}$ increases, the strong coupling coefficient ${C_1}$ decreases, and the weak coupling coefficient ${C_2}$ increases. Since the waist diameter of the MFC is very thin, ${C_1} > > {C_2}$, and strong coupling dominates in the coupling process. Thus, $\frac{{\partial {{\varphi} _{SC}}}}{{\partial {n_3}}} + \frac{{\partial {{\varphi} _{WC}}}}{{\partial {n_3}}} < 0$. Considering $\frac{{\partial {{\varphi} _{SC}}}}{{\partial \lambda }} > 0,\ \frac{{\partial {{\varphi} _{WC}}}}{{\partial \lambda }} > 0$ and $\frac{{d{n_3}}}{{dRH}} > 0$, it is easy to obtain ${S_{RH}} > 0$. Thus, as the RH increases, the transmission interference spectrum shifts to longer wavelengths, which is consistent with our theoretical analysis.
Fig. 7. Transmission characteristics of the microfiber coupler coated with a single layer of MoS2 nanosheets at different RHs.
Simultaneously, in the tapered waist region of the MFC, due to the fine diameter of the fiber, the microfiber cannot completely confine the light within the fiber, and a large portion of the light energy extends into the external environment in the form of an evanescent field. Thus, the strength of the interaction between the light and the coated MoS2 is significantly enhanced, and the sensor is more sensitive to changes in the effective refractive index of MoS2. Thus, a variation in the effective refractive index of MoS2 causes a corresponding variation in the interference spectrum at the MFC output port. In addition, Fig. 8 shows that the transmission intensity of the dip decreases as the RH increases. This decrease may occur because as the RH increases, the increase in the imaginary part of the refractive index of MoS2 enhances the absorption of light by MoS2 [19]. The detailed mechanism explaining the changes in the real part of the refractive index of MoS2 remains to be established in future work [19,20].
The response of the wavelength and transmission intensity of the dip with increasing RH is shown in Fig. 9. As the RH increases from 54.0% to 93.2%, the wavelength dip shifts nearly linearly by 4.72 nm, and the transmission intensity decreases by 2.996 dB. The fitted curve shows that the sensitivities of the RH sensor are 115.3 pm/%RH with an R2 value of 0.989 and -0.058 dB/%RH with an R2 value of 0.909, respectively. The highest resolution obtained is 1.73%RH. Here, we compare the performance parameters of the RH sensors based on the coupler structure, as shown in Table 1. Compared with other sensors [31,32], the measurement range is expanded. Table 2 presents a comparison of some fiber-optic RH sensors based on two-dimensional materials. The values of the coupling coefficients of the MFC depend on the waist diameter, the degree of fusion of the two fibers and the effective refractive index of MoS2. Reducing the waist diameter can enhance the interaction between the evanescent field of the fiber surface and MoS2, so further reducing the diameter of the microfiber coupler can significantly increase the sensitivity of the sensor. We believe that the sensitivity of the sensor can be increased by optimizing the parameters of the MFC and using an appropriate concentration of the MoS2 nanosheet dispersion.
Table 1. RH sensors based on the coupler structure.
Table 2. Comparison of RH sensors based on two-dimensional materials.
3.3 Temperature measurement
The effect of temperature changes on the performance of the sensor is not negligible. As illustrated in Fig. 10, when the temperature increases from 30 °C to 90 °C with a step of 10 °C, the wavelength dip shifts in the short-wavelength direction. This shift can be attributed primarily to the combination of the thermo-optic and thermal expansion effects of the optical fibers and MoS2 nanosheets [21,27]. The effect of temperature on the fiber itself is greater than that of MoS2 due to its good thermal stability. For the presented structure, the sensing unit is mainly in the waist area of the MFC. Since the fiber diameter is very thin in the waist region of the MFC, the fiber core can be neglected. Therefore, the thermo-optic effect of the fiber cladding in the waist region is a major consideration. As the temperature increases, from Eq. (6), the refractive index ${n_2}$ of the fiber cladding and the size of the fiber increase. However, the predominant effect on the interference spectral shift is an increase in ${n_2}$ due to the thermo-optic effect. Thus, it is easy to obtain $\frac{{d{n_2}}}{{dT}} > 0$ in Eq. (7). At the same time, according to Eq. (3), the strong coupling coefficient ${C_1}$ increases and the weak coupling coefficient ${C_2}$ decreases with an increase in the refractive index ${n_2}$. For the presented structure, strong coupling dominates in the coupling process, and $\frac{{\partial {{\varphi} _{SC}}}}{{\partial {n_2}}} + \frac{{\partial {{\varphi} _{WC}}}}{{\partial {n_2}}} > 0$. Similarly, it is known that ${S_T} < 0$, so the wavelength dip shifts in the short-wavelength direction. Figure 11 shows a linear fitting analysis of the relationship between the wavelength and transmission intensity of the dip and temperature. The fitted curve shows that the temperature sensitivities of the dip are approximately -104.8 pm/°C and -0.042 dB/%RH, respectively. Table 3 compares the performance parameters of fiber sensors for the simultaneous measurement of RH and temperature.
Table 3. Comparison of fiber-optic sensors for simultaneous measurement of RH and temperature.
3.4 Dual-parameter simultaneous measurement of RH and temperature
The above results show that the wavelength shift and transmission intensity of the dip depend on the RH and temperature. We developed a second-order matrix to describe the dependence of the wavelength shift and intensity variation on the RH and temperature:
From the previous experimental results, the sensitivity values are known as ${K_{\lambda ,RH}} = 115.3$ pm/%RH, ${K_{\lambda ,T}} = - 104.8$ pm/°C, ${K_{t,RH}} = - 0.058$ dB/%RH, and ${K_{t,T}} = - 0.042$ dB/°C. The ratios ${K_{\lambda ,RH}}/{K_{t,RH}} \approx - 1988$ and ${K_{\lambda ,T}}/{K_{t,T}} \approx 2495$ are different. Thus, this matrix can be used to achieve simultaneous measurement of RH and temperature. From Eq. (8), the variations of RH and temperature can be calculated as follows:
4. Conclusion
In conclusion, an optical fiber sensor with the ability to simultaneously measure RH and temperature has been proposed and experimentally demonstrated. The sensor is based on a microfiber coupler coated with a single layer of MoS2 nanosheets. The RH and temperature affect the effective refractive index of MoS2 and the refractive index of the cladding of the fiber, respectively. When the RH increases, water molecules adsorb onto the surface of the MoS2 nanosheets and increase the effective refractive index of MoS2 as a result of electric charge transfer. When the temperature increases, the refractive index of the fiber cladding in the MFC waist region increases. Both the effective refractive index of MoS2 and the refractive index of the fiber cladding affect the coupling conditions of the interference spectrum. The relationship between the spectral response of the dip in terms of wavelength and transmission intensity and the RH and temperature has been investigated. The RH sensitivities reach 115.3 pm/%RH with a resolution of 1.73%RH and -0.058 dB/RH, and the temperature sensitivities are -104.8 pm/°C with a resolution of 1.91 °C and -0.042 dB/°C. We describe the simultaneous measurements of RH and temperature through a matrix. The proposed sensor is expected to be used for simultaneous measurement of RH and temperature in biological and chemical applications.
Funding
National Natural Science Foundation of China (NSFC) (11874281); Natural Science Foundation of Tianjin City (17JCZDJC31700, 18JCTPJC49200, 18JCTPJC60100); Opening Foundation of State Key Laboratory of Integrated Optoelectronics (IOSKL2017KF15); the Basic Research Project in Higher Education Institutions of Tianjin (2018KJ213).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. C. Chen, X. Wang, M. Li, Y. Fan, and R. Sun, “Humidity sensor based on reduced graphene oxide/lignosulfonate composite thin-film,” Sens. Actuators, B 255(2), 1569–1576 (2018). [CrossRef]
2. Z. Yuan, H. Tai, Z. Ye, C. Liu, G. Xie, X. Du, and Y. Jiang, “Novel highly sensitive QCM humidity sensor with low hysteresis based on graphene oxide (GO)/poly(ethyleneimine) layered film,” Sens. Actuators, B 234, 145–154 (2016). [CrossRef]
3. A. Kafy, A. Akther, M. I. R. Shishir, H. C. Kim, Y. Yun, and J. Kim, “Cellulose nanocrystal/graphene oxide composite film as humidity sensor,” Sens. Actuators, A 247, 221–226 (2016). [CrossRef]
4. Y. Zhao, Y. Peng, M. qing Chen, and R. J. Tong, “Humidity sensor based on unsymmetrical U-shaped microfiber with a polyvinyl alcohol overlay,” Sens. Actuators, B 263, 312–318 (2018). [CrossRef]
5. A. Leal-Junior, A. Frizera-Neto, C. Marques, and M. J. Pontes, “Measurement of temperature and relative humidity with polymer optical fiber sensors based on the induced stress-optic effect,” Sensors 18(3), 916 (2018). [CrossRef]
6. Y. Wang, C. Shen, W. Lou, and F. Shentu, “Fiber optic humidity sensor based on the graphene oxide/PVA composite film,” Opt. Commun. 372, 229–234 (2016). [CrossRef]
7. G. Berruti, M. Consales, M. Giordano, L. Sansone, P. Petagna, S. Buontempo, G. Breglio, and A. Cusano, “Radiation hard humidity sensors for high energy physics applications using polyimide-coated fiber Bragg gratings sensors,” Sens. Actuators, B 177, 94–102 (2013). [CrossRef]
8. B. N. Shivananju, S. Yamdagni, R. Fazuldeen, A. K. S. Kumar, S. P. Nithin, M. M. Varma, and S. Asokan, “Highly sensitive carbon nanotubes coated etched fiber Bragg grating sensor for humidity sensing,” IEEE Sens. J. 14(8), 2615–2619 (2014). [CrossRef]
9. T. Venugopalan, T. Sun, and K. T. V. Grattan, “Long period grating-based humidity sensor for potential structural health monitoring,” Sens. Actuators, A 148(1), 57–62 (2008). [CrossRef]
10. J. Ascorbe, J. M. Corres, I. R. Matias, and F. J. Arregui, “High sensitivity humidity sensor based on cladding-etched optical fiber and lossy mode resonances,” Sens. Actuators, B 233, 7–16 (2016). [CrossRef]
11. S. Wu, G. Yan, Z. Lian, X. Chen, B. Zhou, and S. He, “An open-cavity Fabry-Perot interferometer with PVA coating for simultaneous measurement of relative humidity and temperature,” Sens. Actuators, B 225, 50–56 (2016). [CrossRef]
12. C. Li, X. Yu, W. Zhou, Y. Cui, J. Liu, and S. Fan, “Ultrafast miniature fiber-tip Fabry–Perot humidity sensor with thin graphene oxide diaphragm,” Opt. Lett. 43(19), 4719 (2018). [CrossRef]
13. Y. Peng, Y. Zhao, M. Q. Chen, and F. Xia, “Research Advances in Microfiber Humidity Sensors,” Small 14(29), 1800524 (2018). [CrossRef]
14. N. Irawati, H. A. Rahman, H. Ahmad, and S. W. Harun, “A PMMA microfiber loop resonator based humidity sensor with ZnO nanorods coating,” Measurement 99, 128–133 (2017). [CrossRef]
15. H. A. Rahman, N. Irawati, T. N. R. Abdullah, and S. W. Harun, “PMMA microfiber coated with ZnO nanostructure for the measurement of relative humidity,” in IOP Conference Series: Materials Science and Engineering (2015).
16. G. Woyessa, K. Nielsen, A. Stefani, C. Markos, and O. Bang, “Temperature insensitive hysteresis free highly sensitive polymer optical fiber Bragg grating humidity sensor,” Opt. Express 24(2), 1206–1213 (2016). [CrossRef]
17. G. Woyessa, J. K. M. Pedersen, A. Fasano, K. Nielsen, C. Markos, H. K. Rasmussen, and O. Bang, “Zeonex-PMMA microstructured polymer optical FBGs for simultaneous humidity and temperature sensing,” Opt. Lett. 42(6), 1161–1164 (2017). [CrossRef]
18. A. Lokman, S. Nodehi, M. Batumalay, H. Arof, H. Ahmad, and S. W. Harun, “Optical fiber humidity sensor based on a tapered fiber with hydroxyethylcellulose/polyvinylidenefluoride composite,” Microw. Opt. Technol. Lett. 56(2), 380–382 (2014). [CrossRef]
19. B. Du, D. Yang, X. She, Y. Yuan, D. Mao, Y. Jiang, and F. Lu, “MoS2-based all-fiber humidity sensor for monitoring human breath with fast response and recovery,” Sens. Actuators, B 251, 180–184 (2017). [CrossRef]
20. D. Li, H. Lu, W. Qiu, J. Dong, H. Guan, W. Zhu, J. Yu, Y. Luo, J. Zhang, and Z. Chen, “Molybdenum disulfide nanosheets deposited on polished optical fiber for humidity sensing and human breath monitoring,” Opt. Express 25(23), 28407 (2017). [CrossRef]
21. K. Li, N. M. Y. Zhang, N. Zheng, T. Zhang, G. Liu, and L. Wei, “Spectral Characteristics and Ultrahigh Sensitivities Near the Dispersion Turning Point of Optical Microfiber Couplers,” J. Lightwave Technol. 36(12), 2409–2415 (2018). [CrossRef]
22. K. Li, T. Zhang, G. Liu, N. Zhang, M. Zhang, and L. Wei, “Ultrasensitive optical microfiber coupler based sensors operating near the turning point of effective group index difference,” Appl. Phys. Lett. 109(10), 101101 (2016). [CrossRef]
23. W. Lin, H. Zhang, B. Song, Y. Miao, B. Liu, D. Yan, and Y. Liu, “Magnetically controllable wavelength-division-multiplexing fiber coupler,” Opt. Express 23(9), 11123 (2015). [CrossRef]
24. J. Teng, J. Yang, C. Lv, T. Chen, J. Guo, J. Feng, and P. Wu, “Guidelines for design and fabrication of fused fiber coupler based wavelength division multiplexings,” Opt. Fiber Technol. 20(3), 239–244 (2014). [CrossRef]
25. F. P. Payne, C. D. Hussey, and M. S. Yataki, “Polarisation analysis of strongly fused and weakly fused tapered couplers,” Electron. Lett. 21(13), 561–563 (1985). [CrossRef]
26. F. P. Payne, C. D. Hussey, and M. S. Yataki, “Modelling fused single-mode-fibre couplers,” Electron. Lett. 21(11), 461–462 (1985). [CrossRef]
27. D. Liu, Q. Wu, C. Mei, J. Yuan, X. Xin, A. K. Mallik, F. Wei, W. Han, R. Kumar, C. Yu, S. Wan, X. He, B. Liu, G.-D. Peng, Y. Semenova, and G. Farrell, “Hollow Core Fiber Based Interferometer for High-Temperature (1000 °C) Measurement,” J. Lightwave Technol. 36(9), 1583–1590 (2018). [CrossRef]
28. M. Donarelli, S. Prezioso, F. Perrozzi, F. Bisti, M. Nardone, L. Giancaterini, C. Cantalini, and L. Ottaviano, “Response to NO2 and other gases of resistive chemically exfoliated MoS2-based gas sensors,” Sens. Actuators, B 207, 602–613 (2015). [CrossRef]
29. H. Li, S. H. Ahn, S. Park, L. Cai, J. Zhao, J. He, M. Zhou, J. Park, and X. Zheng, “Molybdenum disulfide catalyzed tungsten oxide for on-chip acetone sensing,” Appl. Phys. Lett. 109(13), 133103 (2016). [CrossRef]
30. Y. Wang, C. Shen, W. Lou, F. Shentu, C. Zhong, X. Dong, and L. Tong, “Fiber optic relative humidity sensor based on the tilted fiber Bragg grating coated with graphene oxide,” Appl. Phys. Lett. 109(3), 031107 (2016). [CrossRef]
31. L. Bo, P. Wang, Y. Semenova, and G. Farrell, “Optical microfiber coupler based humidity sensor with a polyethylene oxide coating,” Microw. Opt. Technol. Lett. 57(2), 457–460 (2015). [CrossRef]
32. L. Sun, Y. Semenova, Q. Wu, D. Liu, J. Yuan, X. Sang, B. Yan, K. Wang, C. Yu, and G. Farrell, “Investigation of humidity and temperature response of a silica gel coated microfiber coupler,” IEEE Photonics J. 8(6), 1–7 (2016). [CrossRef]
33. T. Ouyang, L. Lin, K. Xia, M. Jiang, Y. Lang, H. Guan, J. Yu, D. Li, G. Chen, W. Zhu, Y. Zhong, J. Tang, J. Dong, H. Lu, Y. Luo, J. Zhang, and Z. Chen, “Enhanced optical sensitivity of molybdenum diselenide (MoSe2) coated side polished fiber for humidity sensing,” Opt. Express 25(9), 9823–9833 (2017). [CrossRef]
34. Y. D. Chiu, C. W. Wu, and C. C. Chiang, “Tilted fiber Bragg grating sensor with graphene oxide coating for humidity sensing,” Sensors 17(9), 2129 (2017). [CrossRef]
35. Y. Huang, W. Zhu, Z. Li, G. Chen, L. Chen, J. Zhou, H. Lin, J. Guan, W. Fang, X. Liu, H. Dong, J. Tang, H. Guan, H. Lu, Y. Xiao, J. Zhang, H. Wang, Z. Chen, and J. Yu, “High-performance fibre-optic humidity sensor based on a side-polished fibre wavelength selectively coupled with graphene oxide film,” Sens. Actuators, B 255, 57–69 (2018). [CrossRef]
36. Y. Wang, Q. Huang, W. Zhu, and M. Yang, “Simultaneous Measurement of Temperature and Relative Humidity Based on FBG and FP Interferometer,” IEEE Photonics Technol. Lett. 30(9), 833–836 (2018). [CrossRef]
37. S. xi Jiao, Y. Zhao, and J. jin Gu, “Simultaneous measurement of humidity and temperature using a polyvinyl alcohol tapered fiber Bragg grating,” Instrum. Sci. Technol. 46(5), 463–474 (2018). [CrossRef]
38. A. Urrutia, J. Goicoechea, A. L. Ricchiuti, D. Barrera, S. Sales, and F. J. Arregui, “Simultaneous measurement of humidity and temperature based on a partially coated optical fiber long period grating,” Sens. Actuators, B 227, 135–141 (2016). [CrossRef]
