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Abstract
In this paper, a splitting ratio-adjustable Mach-Zehnder interferometer (MZI) for the measurement of relative humidity (RH) is proposed and experimentally demonstrated. The sensing head contains three sections of single mode fiber (SMF) and two sections of multimode fiber (MMF), in which the two MMFs are spliced among the three SMFs. The MMFs are corroded with hydrofluoric acid and act as mode couplers to split and recombine light owing to the core diameter mismatch with the SMF. A layer of graphene oxide (GO) is coated on the MMFs by dip-coating and natural evaporation. The effective refractive index of the GO will vary when it absorbs the water molecules. As a result, the intensity of the transmission light in the core and cladding of the single mode fiber can be adjusted. Thus, the intensity of the resonant dip will vary when the relative humidity changes. The experimental results show that a humidity sensitivity of 0.263 dB/RH% with a linear correlation coefficient of 99% can be achieved in a relative humidity range of 35% to 85%.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Relative humidity detection technology plays an important role in food processing, instrument manufacturing, structural health monitoring, and other fields [1,2]. Electric-based sensors (such as resistor and capacitive types) in the current market are the dominant products owing to their advantages such as high measurement accuracy. However, electric-based sensors also have some disadvantages under some environments, including flammable, combustible, and strong electromagnetic ambient conditions. Therefore, it is important to study sensors that are essentially safe and stable [3]. Optical fiber sensor utilizes optical elements and possesses many advantages such as high sensitivity, anti-electromagnetic interference, anti-corrosion, and a simple structure. In addition, this type of sensor is suitable to be applied in the environment mentioned above. Therefore, in-depth exploration and studies have been conducted on optical fiber sensor by many researchers. Recently, many kinds of optical fiber sensor have been proposed for the measurement of RH, such as fiber Bragg gratings coated with polymer [4,5], long-period fiber gratings coated with polychlorinated cobalt/cobalt chloride [6,7], agarose-permeated photonic crystal fiber (PCF) interferometer [8], PCF coated with Poly (vinyl alcohol) (PVA) [9], PCF coated with PVA [10], non-adiabatic tapered fiber coated with Poly (Diallylmethilammonium chloride) and polymeric Dye R-478 film [11], coreless fiber coated with hydroxyethyl cellulose and polyvinylidene fluoride hydrogel [12], U-shaped bare fiber coated with CoCl2 and doped with PVA film [13], side-polished fiber coated with WS2 [14], and U-shaped fiber doped with red phenolic and coated with polymethylmethacrylate (PMMA) film and tapered fiber coated with agarose gel [15,16]. However, in these studies, the refractive index of the coated material was varied by taking advantage of the penetration capacity of water molecules, and the poor penetrability of water molecules may lead to hysteresis. In addition, it is also difficult to obtain high sensitivity because the poor penetrability will prevent the water molecules from penetrating the film.
In recent years, graphene oxide has received much attention owing to its excellent aqueous processability, amphiphilicity, surface functionality, surface-enhanced Raman scattering and fluorescence quenching capacity [17,18]. The two-dimensional atomic structure and oxygen-containing functional groups in GO such as hydroxyl, carboxyl, epoxide, and carbonyl make GO film permeate and absorb water molecules easily [18–20]. Owing to its two-dimensional structure, the graphene-based charge carrier (electron or hole) density becomes sensitive to the environment [21,22]. This represents the main sensing mechanism of a graphene-based sensor. When the chemical gas molecules are adsorbed on the surface of graphene, its charge carrier density will change. An optical sensor based on the interaction between graphene and the evanescent wave of an optical waveguide can be manufactured by applying the sensitivity of light frequency conductivity of graphene to the environment [23–25]. After combining the graphene with an optical waveguide, the effective refractive index of the optical waveguide will be affected by the optical frequency conductivity of the graphene, and then the propagation field of the optical waveguide will also be influenced.
Generally, an MZI-based sensor uses an interferometric arm as the sensing arm, in which the phase difference will vary owing to changes in the ambient condition [26–32]. As a result, the wavelength of the resonant dip will shift. Thus, the ambient parameters can be obtained by measuring the wavelength shift of the resonant dip. We also proposed a MZI-based sensor for the simultaneous measurement of the refractive index and temperature by measuring the shift of the resonant wavelength [33]. In this paper, we propose a splitting ratio-adjustable MZI by corroding the cladding of multimode fiber with the same structure. A layer of GO is coated on the MMFs by dip-coating and natural evaporation. The effective refractive index of the GO will vary when it absorbs the water molecules. As a result, the intensity of the transmission light in the core and the cladding of the SMF will vary. Thus, the intensity of the resonant dip will vary when the relative humidity changes. So, the RH can be gained by measuring the variation of the intensity of the resonant dip. The theoretical analysis agrees with the experimental results. The results indicate that the proposed sensor features the advantages of low manufacturing cost, stability, and simple configuration, and presents attractive potential applications in the fields of biological and chemical sensing.
2. Sensor structure and principle
A schematic diagram of the sensing head is shown in Fig. 1. The sensing head contains three parts of SMF and two parts of MMF. The two ends of the SMF (SMF1 and SMF3) are used as the input and output optical fiber, respectively. All are spliced with the two MMFs, which act as mode couplers to split and recombine light owing to core diameter mismatches. In the middle, a part of SMF (SMF2) is spliced with the two MMFs to form an MZI. The diameters of the core/cladding of the SMFs and the MMFs were 9/125 μm and 105/125 μm, respectively. The built-in integration mode (FITEL S178) of the SM-MM in the fusion splicer was selected, and the arc power and time of duration were set as 140 bits and 3000 ms, respectively. The interference spectrum was affected by the length of the MMFs. On the one hand, the coupling coefficient varies with the length of the MMFs. On the other hand, the MMFs cause additional phase differences between the core mode and the cladding modes. Thus, the length of the MMFs shall be suitably selected to achieve a high coupling coefficient and low additional phase difference. Through repeated attempts and experiments, the length of the two MMFs of 2 mm were selected. Further, the length of the SMF2 will also affect the free spectral range of the interference spectrum. In general, the longer the length of SMF2, the narrower the free spectral range that can be achieved. In this paper, the length of the SMF2 of 36 mm was selected.
After the light propagates to the first splicer, one part of it is coupled to the core of SMF2, and the other part of it is coupled to the cladding of SMF2. After transmission at distance L, a phase delay occurs between the core mode and the cladding modes owing to the different propagation constants. As a result, interference occurs in the core of SMF3 when the core mode and the cladding modes are coupled to SMF3. The output intensity and phase difference can be expressed as follows:
Icore and Icladding are the light intensities of the core mode and the cladding modes, respectively. L is the length of SMF2 and Δneff is the effective refractive index difference between the core mode and the cladding modes of SMF2. λ is the wavelength of the light. When φ in Eq. (1) equals (2k + 1)π, the wavelength of the kth-order resonance dip of the interference spectrum can be shown asFrom Eqs. (1)-(3), we know that the output intensity is a function of Icore, Icladding, and Δneff. Further, the wavelength of the resonant dip will also shift when Δneff varies. In general, the MZI-based sensor obtains its ambient parameters by measuring the shift of the resonant dip wavelength as mentioned above, although the output intensity will also vary slightly. In this paper, we focus on Icore and Icladding and make them vary with the ambient humidity. Meanwhile, Δneff is almost invariable because SMF2 is not sensitive to humidity.As a humidity-sensitive material, GO is an important part in increasing the sensitivity of the sensor. Combining water molecules with GO may change the effective refractive index of the MMFs. With an increase in RH, more water molecules will be absorbed by the GO film. On the one hand, the absorbed water molecules will fill the slice of the GO layer, causing the GO film to expand directly. On the other hand, the adsorption of water molecules on the surface of GO will increase the density of carriers (holes) on the surface of GO. In other words, with an increase in humidity, more water molecules are adsorbed on the surface of the GO film, and the density of the GO carriers increases accordingly. As a result, the effective refractive index of GO film decreases with an increase in the number of water molecules that are absorbed [34,35]. This may reduce the effective refractive index of MMF and excite the eigenmodes in the MMF, thus affecting the splitting ratio and leading to an intensity change in the resonant dip.
3. Sensing head fabrication
3.1 Preparation of bare multimode optical fiber core
In order to lead-out the evanescent wave and enhance the sensitivity of the sensor, the two MMFs were corroded with hydrofluoric acid (HF). The sensor head was fixed on a Poly tetra fluoroethylene (PTFE) plate through hot melt adhesive, as shown in Fig. 2. The HF showed a hemispherical shape on the experimental plate owing to the hydrophobicity characteristic of the PTFE. The MMF should be controlled by the depth of immersion into the liquid. Through repeated tests, the sensor head was suspended 1.5 mm above the experimental plate and injected with 100 μL of HF solution. The concentration of the HF was 40%. A broadband source (BBS) and optical spectrum analyzer (OSA) were connected to monitor the corrosion of the MMF. Figure 3(a) shows that the waist diameter of the MMF varies with the etching time. The corrosion rate is 2.18 μm/min. At the 19th minute, an obvious interference spectrum can be achieved, as shown in Fig. 3(b). Here, the MMF was successively washed with deionized water, 40% sodium hydroxide solution, and deionized water. From Figs. 3(a) and 4(a), we know that the cladding of the MMF was corroded.
Fig. 3 (a) Relation between etch time and waist diameter. (b) Transmission spectrum of 19th minute of corrosion.
Fig. 4 (a) Multimode fiber coated with graphene oxide at magnification of 82. (b) Detailed drawing of part coated with graphene at magnification of 20K. (c) Enlarged drawing of cross section at multimode fiber at magnification of 30K under scanning electron microscope (SEM).
3.2 Graphene oxide film coated on multimode optical fiber core
GO sheets with a size of more than 500 nm were picked up by centrifuging 0.08 mg/mL of GO dispersion liquid (XF020, Nanjing XFNANO Materials Tech. Co., Ltd.) at 10000RPM for 15 min. Centrifugal GO solution with a larger GO sheet was deposited on side-polished fiber. Based on the platform set-up in the second step, we elevated the petri dish through the translation stage to immerse the optical fiber into the droplets. According to the droplet shape, the length of the waist area was allowed to be controlled by the immersion depth. After the droplets of GO dispersions were naturally evaporated under laboratory conditions, the GO was uniformly deposited on the surface of the MMF, as shown in Fig. 4(b). After we placed it in a laboratory environment for 24 h, the GO was fully attached to the surface of the fiber, as shown in Fig. 4(c), and the thickness of the film was about 50 nm. It should be pointed out that the thickness of the GO film can be controlled by the concentration of GO solution and the time of immersion. It is shown from the experiment that the higher the concentration of graphene, the thicker the GO film. Figure 5 shows the transmission spectra of MMF coated with/without GO. We can see that the contrast obviously increases. Meanwhile, the wavelength shifts slightly.
4. Experimental results and discussion
Figure 6 shows the experimental arrangement to investigate the response of the sensor, including a BBS, sensor head, constant temperature and humidity chamber (J-TOPH-22-B, JieXin Testing Equipment Co., Ltd.), and OSA (Yokogawa, AQ6370). The wavelength resolution of the OSA was 0.02 nm. In the experiment, the sensor head was put into the constant temperature and humidity chamber, in which the humidity increased from 35% to 85%. Figure 7(a) shows the transmission spectra of the sensor under different humidities. As expected, the intensity of the resonant dips (A, B, and C) decrease with an increasing in humidity. Meanwhile, the wavelength of the resonant dips barely shift. As we know, the interaction between the high-order modes is excited when the light is coupled from the single-mode fiber to the multimode fiber and the GO film. As the relative humidity increases, the water molecules act on the GO films to reduce the effective refractive index, which triggers a strong evanescent field. As a result, the change in effective width affects the excitation of the eigenmode in the core of the bare multimode fiber, and the base film coupled in the fiber core and the strength of the higher-order mode in the cladding layer change. This affects the splitting ratio in the interference theory and the intensity of the resonant dip. Figure 7(b) shows that the intensity of resonant dip A changes when the ambient relative humidity varies from 35% to 85%. The linear fitting results indicate a sensitivity of 0.263 dB/RH% with R2 of 99.0%.
Fig. 7 (a) Change in transmission spectrum with change in HR from 35% to 85%. (b) Relation between relative humidity and transmission peak at dip of A.
In order to further investigate the response of the sensor, we measure the relation between the intensity of resonant dips B and C and the ambient relative humidity, as shown in Fig. 8. The black and blue dots are dips B and C, respectively. For comparison, we also show dip A in the Fig. 8. We can see that the linear sensitivity of the sensor at dip B is 0.222 dB/RH% with R2 of 98.6% in a humidity range of 35% to 75%. The linear sensitivity of the sensor at dip C is 0.376 dB/RH% with R2 of 98.8% in a humidity range from 35% to 65%. The results show that high sensitivity can be achieved at long wavelengths. We think this is because the splitting ratio varies with the wavelength. As a result, the number and intensity of the cladding modes are different for different wavelengths. Thus, the sensitivity of the sensor will vary with the wavelength.
Meanwhile, the phenomenon in which the transmission peak first decreases and then increases demonstrates that a change in the transmission peak modulated by the splitting ratio will reach an extreme value. With an increase in humidity, the contrast of transmitted light decreases first and then increases. Since the evanescent field is intensified and more energy is consumed, the amplitude of the transmission peak is greatly reduced. Fourier transformation processing was carried out on the transmission spectrum in Fig. 7(a) for a better understanding of this phenomenon and the operating mode of the sensor. Figure 9(a) shows the Fourier transformation process of the transmission spectrum. With an increase in the relative humidity, the amplitude of the base mode decreases, and the amplitude of the higher-order dominant mode increases. Therefore, the interference contrast increases with an increase in RH. It is shown from Eq. (1) that the transmission peak is not only a function between the phase difference and wavelength but is also related to the light intensity of each beam involved in the interference. With a change in the external environment, the intensity of the higher-order mode is also in a dynamic state. As shown in Fig. 9(c), the amplitudes corresponding to spatial frequencies of 0.1 and 0.13 increase first and then decrease with an increase in RH. High-order modes vary with the external environment, and the transmission peaks corresponding to different wavelengths also vary. There is an extreme value for the change in the transmission peak at a certain relative humidity, as there is an extreme point of the change in the higher-order mode. This is why there is an abnormal change in the transmission peak at the wavelength of dip B and dip C.
Fig. 9 (a) The spatial frequency spectrum of the transmission spectrum at different RH values. (b) Spatial spectrum in the range of spatial frequencies from 0 to 0.002. (c) Spatial spectrum in the range of spatial frequencies from 0.05 to 0.17.
In order to investigate the stable characteristics of the sensor, the sensor head was put into the constant temperature and humidity chamber with an RH of 50% at a temperature of 25°C for 120 min. Figure 10 shows the fluctuation of the intensity of dips A, B, and C. The fluctuations were 1.23%, 1.09%, and 1.34% for dips A, B, and C in the measured time period, respectively. The fluctuation increased as the wavelength increased owing to the high sensitivity and easily affected by the ambient change. We think the fluctuation is a result of the instability of the BBS and the RH variation. A steady BBS and packaging technique of the sensor can improve the sensor’s performance.
5. Conclusion
A relative humidity sensor based on a splitting ratio-adjustable MZI was successfully developed in this paper. As far as we know, it is first time that the beam splitter /beam combiner of the MZI was used as the sensor head for the measurement of relative humidity. A sensitivity of 0.263dB/RH% and linear correlation of 99.0% were achieved in an RH range from 35% to 85%. The sensor has good stability and excellent repeatability. As the sensor features the advantages of low manufacturing cost, stability, and simple configuration, it presents attractive potential applications in the fields of biological and chemical sensing.
Funding
National Natural Science Foundation of China (11674109, 61774062); Natural Science Foundation of Guangdong Province of China (2016A030313443); Guangdong Science and Technology Department (2017A020219007).
References
1. A. Enjin, E. E. Zaharieva, D. D. Frank, S. Mansourian, G. S. Suh, M. Gallio, and M. C. Stensmyr, “Humidity sensing in drosophila,” Curr. Biol. 26(10), 1352–1358 (2016). [CrossRef] [PubMed]
2. K. O. Hill, Y. Fujii, D. C. Johnson, and B. S. Kawasaki, “Photosensitivity in optical fiber waveguides: application to reflection filter fabrication,” Appl. Phys. Lett. 32(10), 647–649 (1978). [CrossRef]
3. J. Dai, M. Yang, Y. Zhi, L. Zhi, W. Yao, G. Wang, Z. Yi, and Z. Zhi, “Performance of fiber Bragg grating hydrogen sensor coated with Pt-loaded WO3 coating,” Sensor. Actuat. Biol. Chem. 190, 657–663 (2014).
4. P. Kronenberg, P. K. Rastogi, P. Giaccari, and H. G. Limberger, “Relative humidity sensor with optical fiber Bragg gratings,” Opt. Lett. 27(16), 1385–1387 (2002). [CrossRef] [PubMed]
5. T. L. Yeo, K. T. V. Tong Sun, D. Grattan, R. Parry, Lade, and B. D. Powell, “Polymer-coated fiber Bragg grating for relative humidity sensing,” IEEE Sens. J. 5(5), 1082–1089 (2005). [CrossRef]
6. M. Konstantaki, S. Pissadakis, S. Pispas, N. Madamopoulos, and N. A. Vainos, “Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating,” Appl. Opt. 45(19), 4567–4571 (2006). [CrossRef] [PubMed]
7. T. Venugopalan, T. Sun, and K. T. V. Grattan, “Long period grating-based humidity sensor for potential structural health monitoring,” Sensor. Actuat. A-Phys. 148(1), 57–62 (2008).
8. J. Mathew, Y. Semenova, and G. Farrell, “Relative humidity sensor based on an agarose-infiltrated photonic crystal fiber interferometer,” IEEE J. Sel. Top. Quant. 18(5), 1553–1559 (2012). [CrossRef]
9. T. Li, X. Dong, C. C. Chan, K. Ni, S. Zhang, and P. P. Shum, “Humidity sensor with a PVA-coated photonic crystal fiber interferometer,” IEEE Sens. J. 13(6), 2214–2216 (2013). [CrossRef]
10. W. C. Wong, C. C. Chan, L. H. Chen, T. Li, K. X. Lee, and K. C. Leong, “Polyvinyl alcohol coated photonic crystal optical fiber sensor for humidity measurement,” Sensor. Actuat. Biol. Chem. 174, 563–569 (2012).
11. J. M. Corres, J. Bravo, I. R. Matias, and F. J. Arregui, “Nonadiabatic tapered single-mode fiber coated with humidity sensitive nanofilms,” IEEE Photonic. Tech. L. 18(8), 935–937 (2006). [CrossRef]
12. L. Xia, L. Li, W. Li, T. Kou, and D. Liu, “Novel optical fiber humidity sensor based on a no-core fiber structure,” Sensor. Actuat. A-Physical 190, 1–5 (2013).
13. S. K. Khijwania, K. L. Srinivasan, and J. P. Singh, “An evanescent-wave optical fiber relative humidity sensor with enhanced sensitivity,” Sens. Actuat. Biol. Chem. 104(2), 217–222 (2005).
14. Y. Luo, C. Chen, K. Xia, S. Peng, H. Guan, J. Tang, H. Lu, J. Yu, J. Zhang, Y. Xiao, and Z. Chen, “Tungsten disulfide (WS2-based all-fiber-optic humidity sensor,” Opt. Express 24(8), 8956–8966 (2016). [CrossRef] [PubMed]
15. C. Bariáin, I. R. MatíAs, F. J. Arregui, and M. López-Amo, “Optical fiber humidity sensor based on a tapered fiber coated with agarose gel,” Sensor. Actuat,” Biol. Chem. 69(1–2), 127–131 (2000).
16. B. D. Gupta and Ratnanjali, “A novel probe for a fiber optic humidity sensor,” Sensor. Actuat., Biol. Chem. 80(2), 132–135 (2001).
17. C. Chung, Y. K. Kim, D. Shin, S. R. Ryoo, B. H. Hong, and D. H. Min, “Biomedical applications of graphene and graphene oxide,” Acc. Chem. Res. 46(10), 2211–2224 (2013). [CrossRef] [PubMed]
18. Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, and R. S. Ruoff, “Graphene and graphene oxide: synthesis, properties, and applications,” Adv. Mater. 22(35), 3906–3924 (2010). [CrossRef] [PubMed]
19. N. V. Medhekar, A. Ramasubramaniam, R. S. Ruoff, and V. B. Shenoy, “Hydrogen bond networks in graphene oxide composite paper: structure and mechanical properties,” ACS Nano 4(4), 2300–2306 (2010). [CrossRef] [PubMed]
20. R. R. Nair, H. A. Wu, P. N. Jayaram, I. V. Grigorieva, and A. K. Geim, “Unimpeded permeation of water through helium-leak-tight graphene-based membranes,” Science 335(6067), 442–444 (2012). [CrossRef] [PubMed]
21. O. Leenaerts, B. Partoens, and F. M. Peeters, “Adsorption of H2O, NH3, CO, NO2, and NO on graphene: a first-principles study,” Phys. Rev. B Condens. Matter Mater. Phys. 77(12), 125416 (2008). [CrossRef]
22. F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, and K. S. Novoselov, “Detection of individual gas molecules adsorbed on graphene,” Nat. Mater. 6(9), 652–655 (2007). [CrossRef] [PubMed]
23. B. Yao, Y. Wu, Y. Cheng, A. Zhang, Y. Gong, Y.-J. Rao, Z. Wang, and Y. Chen, “All-optical Mach–Zehnder interferometric NH3 gas sensor based on graphene/microfiber hybrid waveguide,” Sensor. Actuat. Biol. Chem. 194, 142–148 (2014).
24. B. C. Yao, Y. Wu, A. Q. Zhang, Y. J. Rao, Z. G. Wang, Y. Cheng, Y. Gong, W. L. Zhang, Y. F. Chen, and K. S. Chiang, “Graphene enhanced evanescent field in microfiber multimode interferometer for highly sensitive gas sensing,” Opt. Express 22(23), 28154–28162 (2014). [CrossRef] [PubMed]
25. B. C. Yao, Y. Wu, C. B. Yu, J. R. He, Y. J. Rao, Y. Gong, F. Fu, Y. F. Chen, and Y. R. Li, “Partially reduced graphene oxide based FRET on fiber-optic interferometer for biochemical detection,” Sci. Rep. 6(1), 23706 (2016). [CrossRef] [PubMed]
26. H. Y. Choi, M. J. Kim, and B. H. Lee, “All-fiber Mach-Zehnder type interferometers formed in photonic crystal fiber,” Opt. Express 15(9), 5711–5720 (2007). [CrossRef] [PubMed]
27. H. Luo, Q. Sun, Z. Xu, D. Liu, and L. Zhang, “Simultaneous measurement of refractive index and temperature using multimode microfiber-based dual Mach-Zehnder interferometer,” Opt. Lett. 39(13), 4049–4052 (2014). [CrossRef] [PubMed]
28. J. Zhou, C. Liao, Y. Wang, G. Yin, X. Zhong, K. Yang, B. Sun, G. Wang, and Z. Li, “Simultaneous measurement of strain and temperature by employing fiber Mach-Zehnder interferometer,” Opt. Express 22(2), 1680–1686 (2014). [CrossRef] [PubMed]
29. X. Sun, D. Dai, L. Thylén, and L. Wosinski, “High-sensitivity liquid refractive-index sensor based on a Mach-Zehnder interferometer with a double-slot hybrid plasmonic waveguide,” Opt. Express 23(20), 25688–25699 (2015). [CrossRef] [PubMed]
30. Q. Wang, L. Kong, Y. Dang, F. Xia, Y. Zhang, Y. Zhao, H. Hu, and J. Li, “High sensitivity refractive index sensor based on splicing points tapered SMF-PCF-SMF structure Mach-Zehnder mode interferometer,” Sensor. Actuat. Biol. Chem. 225, 213–220 (2016).
31. Q. Wang, W. Wei, M. Guo, and Y. Zhao, “Optimization of cascaded fiber tapered Mach–Zehnder interferometer and refractive index sensing technology,” Sensor. Actuat. Biol. Chem. 222, 159–165 (2016).
32. Q. Yao, H. Meng, W. Wei, H. Xue, X. Rui, B. Huang, C. Tan, and X. Huang, “Simultaneous measurement of refractive index and temperature based on a core-offset Mach–Zehnder interferometer combined with a fiber Bragg grating,” Sensor. Actuat. A-Phys. 209, 73–77 (2014). [CrossRef]
33. R. Xiong, H. Meng, Q. Yao, B. Huang, Y. Liu, H. Xue, C. Tan, and X. Huang, “Simultaneous measurement of refractive index and temperature based on modal interference,” IEEE Sens. J. 14(8), 2524–2528 (2014). [CrossRef]
34. Y. Wang, C. Shen, W. Lou, and F. Shentu, “Polarization-dependent humidity sensor based on an in-fiber Mach-Zehnder interferometer coated with graphene oxide,” Sensor. Actuat. Biol. Chem. 234, 503–509 (2016).
35. 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]
