Open Access
Add to BibSonomy
Abstract
A relative humidity (RH) sensor based on helix structure-assisted Mach-Zehnder interferometer (HSA-MZI) with graphene oxide (GO) coated was proposed and experimentally demonstrated in this research. The sensor was composed of a lead-in single mode fiber (SMF), a lead-in multimode fiber (MMF), a triangular four core fiber (TFCF) with a twisted structure, a lead-out MMF, and a lead-out SMF. The twisted structure was prepared by twisting under continuous CO2 laser heating. The TFCF was coated with graphene oxide (GO) via natural evaporation. The torsional region in the TFCF provided an effective means for exciting the higher order cladding modes, and strong evanescent fields were introduced from the higher order cladding modes to improve their sensitivity to the changes of the refractive index (RI) in the surrounding environment. The effective RI of the GO film changes was caused by the GO absorption of water molecules, so the intensity of the transmission signal could be adjusted accordingly. The proposed RH sensor had a high RH sensitivity of -0.885 dB/%RH in the RH ranges of 70% and 80%. Additionally, the respiratory rise and fall time were 0.42 s and 6.54 s, respectively.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Humidity, as an important parameter in food processing, biology, chemistry, and other fields [1,2], has attracted wide attention. Electric-based sensors (such as resistive-type and capacitive-type humidity sensors) occupy a dominant position in the current market because of their high measurement accuracy. However, electric sensors have some disadvantages because they are flammable, explosive, and susceptible to strong electromagnetic interference. Therefore, it is necessary to accelerate the research and application of the effective tools that are inherently safe and stable while monitoring the RH.
In recent years, RH sensors based on fiber-optic technology have been widely studied due to their many advantages. Fiber-optic humidity sensors can be classified into two categories according to whether sensitizing materials are used. Most of them use hygroscopic materials such as humidity sensitive coatings [3–6] to increase the humidity response of the sensors. However, those sensors have the disadvantages of high cost and/or complex manufacturing processes. In order to solve these problems, fiber-optic RH sensors without hygroscopic materials have been presented and demonstrated [7–10]. However, those structures are fragile, they have low sensitivity, and the manufacturing equipment is expensive.
Graphene has attracted much attention due to its unique electronic and photonic properties [11–14]. Graphene oxide (GO), one of the most important derivatives of graphene, is considered to be a promising biological and chemical sensing material. The 2-D atomic structure and the oxygen-containing functional groups of graphene such as hydroxyl, carboxyl, epoxide, and carbonyl make the GO film easy to penetrate. It can also absorb water molecules easily [15–17]. The main sensing mechanism of the graphene-based sensor is the graphene-based carriers (electrons or holes) that make the sensors environmentally sensitive. Applying the optical frequency conductivity sensitivity of GO to the environment, a fiber-optic sensor based on the interaction of GO and optical waveguide evanescent waves can be fabricated. However, most of the previous fiber structures have had low sensitivity to RH because the fiber cladding prevents the interaction between the evanescent waves and the GO film. In order to improve the sensitivity of the humidity sensors, several structures of the fiber-optic GO-based humidity sensors have been studied and applied, such as tapering [18–20], misalignment [21], hydrofluoric acid etching [22,23], and side polishing [24,25]. However, they are either more or less fragile in structure, low in sensitivity, and complex in operation. Therefore, considering the factors of the manufacturing cost, the safety in the manufacturing process, and the mechanical strength of the sensor, a feasible and effective way to enhance the RH sensitivity is extremely necessary.
In this study, an effective method for improving humidity sensitivity based on a single-mode-multimode-triangular four core-multimode-single-mode fiber (SMTMS) structure with a helix-distorted region and GO coating is proposed, and the experimental demonstration is described. We used the finite difference beam propagation method (FD-BPM) to simulate the influence of the twisted TFCF structure on the light field intensity distribution in the process of the optical transmission. The preparation method of the proposed RH sensor is described. The Raman spectroscopic characteristic of the GO sheets was measured. The longitudinal section of the TFCF with GO coated was characterized using a scanning electron microscope (SEM). The RH sensitivity, repeatability, stability, and response time of the proposed sensor were investigated experimentally. Compared with other fiber-optic humidity sensors, the sensor proposed in this study overcomes the shortcomings of expensive lasers and a complex fabrication process for grating-based sensors, the low mechanical strength and fragility of a sensor for mechanical cutting and geometric change sensors, and the long production cycle and complex fabrication process for coated sensors. The proposed RH sensor has great potential and broad application prospects in industrial production, food processing, and environmental monitoring.
2. Configuration and working principle
A lead-in MMF, TFCF, and the lead-out MMF were inserted into the SMF in a certain order for connection, as shown in Fig. 1(a). The lead-in and lead-out MMF were mainly used to construct the splitter and combiner of the MZI. The helix structure excited high-order cladding modes from the side cores and coupled into the cladding of the TFCF. The length of the torsion zone need to be sufficient to allow for useful evanescent field interaction with the GO film. The microscopic images of the joints of the SMF-MMF and MMF-TFCF are shown in Fig. 1(b) and 1(c). A spiral deformation zone was fabricated on TFCF using a heating source composed of a CO2 laser. The microscopic image of the helix structure is shown in Fig. 1(d). The TFCF was coated with a GO film by natural evaporation. The microscopic image of the GO film is shown in Fig. 1(e).
Fig. 1. (a) Schematic of the proposed RH sensor. Optical microscopy image of the joint of (b) the SMF-MMF and (c) the MMF-TFCF. Optical microscopy image of (d) the helix structure and (e) the GO film.
We assumed that the propagating light interference between the central core and the side cores could be neglected. The inference intensity of the HSA-MZI IHSA-MZI can be expressed as
In order to demonstrate that the helix structure provided an efficient means to excite higher order cladding modes, the intensity distribution of the light field before and after the preparation of the helix structure were simulated by the FD-BPM. To reduce the computational complexity, there was no material outside the SMF, MMF, and TFCF cladding. The calculating parameters of the central/side cores diameter and the SMF/MMF/TFCF core/cladding RI used in this research were the same as those in Ref. [33]. The distance between the central and side core d was set to 35 µm. The lengths of the lead-in SMF, lead-in MMF, TFCF, lead-out MMF, and lead-out SMF were set to 1 mm, 1 mm, 20 mm, 1 mm, and 1 mm, respectively. The length, twisted turns, and pitch of helix structure were set to 6 mm, 30, and 200 µm, respectively. The twisted region was located at the center of the TFCF. Figures 2(a) and 2(b) shows the intensity distribution of light field before and after the preparation of the helix structure along the Y-Z axis. By comparing Figs. 2(a) and 2(b), we found that when the TFCF was twisted, the propagation mode of the light changed significantly. The intensity distributions of X-Y axis at the fiber length of about 22 mm are shown in Figs. 2(c) and 2(d). Compared with the absence of the helix structure, we could predict that more higher order cladding modes could be excited by the helix structure. Reference [34] showed that the interaction between the evanescent field increased with the modal order. The interactions between the evanescent fields and the different cladding modes had different degrees. With the increase of the modal order, the interaction of the evanescent field increases, so higher order cladding modes led to greater light-matter interaction in the environment around the TFCF.
Fig. 2. Normalized optical field intensity distribution along the Y-Z axis of the SMTMS structure (a) without and (b) with helix structure. The field intensity distribution along the X-Y axis (c) without and (d) with helix structure at the length of 22 mm.
3. Experiments and discussion
3.1 Fabrication of the proposed sensor
The length of the lead-in, lead-out MMF, and the TFCF were chosen to be approximately 1 mm, 1 mm, and 20 mm, respectively. The helical regions were fabricated using a CO2 laser splicing system (Fujikura, LZM-110P+). The duration was set to 300 s. The sweep speed was set to 0.020 µm/ms. The left rotator was set to activate, the rotation direction was set to reverse, and the rotation speed was set to 0.036 °/ms. The right rotator was set to be inactive. Hence LP was 200 µm, and LHS was approximately 6 mm. The twisted region was located in the center of the TFCF, i.e., the distance between the starting point of the twisted region and the joint of the lead-in MMF and the TFCF was 7 mm.
The GO sheets were dispersed in an ethanol solution using ultrasonic oscillation equipment for 30 min. The concentration of the GO solution was 2 mg/mL. The Raman spectrum of the GO flakes is shown in Fig. 3(a). Because of the planar internal bond stretching of the sp2 hybridized carbons along with the overtones and combined modes, the Raman shifts of the D and G peaks were approximately 1360 cm−1 and 1599 cm−1, respectively. The position of the 2D band was approximately 2945 cm−1. The HSA-MZI was immersed into the droplets of the GO dispersion solution. Under laboratory conditions, the GO droplets evaporated naturally, and the GO sheets were deposited on the surface of the TFCF. To prove that the GO flakes had been deposited on the surface of the TFCF, the scanning electron microscopy (SEM) image of the TFCF is shown in Fig. 3(b). As can be seen from Fig. 3(b), the GO film adhered to the lateral surface of the TFCF. The thickness of the GO film was approximately 3.4 µm. The thickness of the GO film could be controlled by the concentration of the GO solution and the immersion time.
Fig. 3. (a)Raman spectrum of the GO flakes. (b) Lateral surface SEM image of the TFCF with the GO film.
The transmission spectra of the MZI samples based on the SMTMS structure before and after the helix structure/GO coating were measured, as shown in Fig. 4(a). By comparing the spectra of the MZI and the HSA-MZI, it can be seen that the addition of the torsional structure in the MZI led to the increase of the insertion loss. When the GO film was deposited on the surface of the TFCF, the transmission loss increased and the wavelength drifted slightly compared with the values before the GO deposited. The reason for this was that a loss in the transmission power was likely to be transmitted to the GO film through the evanescent field coupling. In order to determine the power distribution of the modes, the spatial frequency spectra of the samples mentioned above were analyzed via fast Fourier transform (FFT), as depicted in Fig. 4(b). As can be seen from Fig. 4(b), the spatial frequency spectra of the samples mainly consisted of the fundamental mode and the cladding mode cluster. The multi-peaks of HSA-MZI frequency spectrum verified that more cladding modes were stimulated and they participated in the prediction of the interference. After the GO flakes were deposited on the TFCF, the center frequency of the two main components of the cladding mode cluster shifted slightly and the normalized power increased.
Fig. 4. (a) Interference patterns of the MZI samples. (b) FFT analysis of the MZI samples shown in part (a).
3.2 Experimental results and analysis
The experimental set-up of the proposed sensor for RH measurement is shown in Fig. 5. A prototype of the GO coated HSA-MZI sensor was connected to a supercontinuum broadband source (YSL photonics, SC-5) and an optical spectrum analyzer (Yokogawa, AQ6370C). A commercial RH meter (Mastech, MS6503) was placed in a chamber to monitor the real-time humidity.
The transmission spectra of the prototype of the proposed sensor versus different RH levels are shown in Fig. 6(a). In order to obtain the relationship between the RH and the intensity of the transmission notch, the resonant dips of approximately 1403.0 nm and 1540.0 nm were selected as the dips A and B. Figure 6(b) shows that the measurement values of dip A varied with different RH values. It can be seen from Fig. 6(b) that the relationship between the RH and the amplitude was approximately linear and monotonically increasing. The linear sensitivity of the proposed sensor at dip A was 0.190 dB/%RH over an RH range of 10%-80%. The RH characteristic of dip B was measured and the results are shown in Fig. 6(c). It can be observed from Fig. 6(c) that dip B showed a higher RH sensitivity (-0.885 dB/%RH) at the RH range of 70% to 80%. The high humidity sensitivity of dip B may have been due to the high RI sensitivity of the main component and the intensity change caused by micro-bending produced due to the strain of the accumulation of water drops on the sensor’s surface. The RH characteristics of fiber-optic humidity sensors that were coated with GO film for different structures are shown in Table 1. Compared with six other humidity sensors coated with GO film [18–21,23,32], the proposed RH sensor in this research had relatively high sensitivity. Compared with the sensor proposed in Ref. [24], the measured value was lower. However, it had a narrow monitoring range and a larger sensor size.
Fig. 6. (a) Spectral variation of the proposed sensor in the RH range from 10% to 80% with a step size of 5%. Relationship between RH and transmission peak at the dips of (b) A and (c) B.
Table 1. Characteristics of different structures for humidity sensors coated with GO film.
The reason for the difference in RH sensitivity between dips A and B needed to be analyzed. To further study the RH response of the prototype of the proposed sensor, we measured the relationship between the RH of the environment and the different dips. The resonant dips (1, 2, 3, 4, 5, and 6) were defined as shown in Fig. 7(a). The intensities of these dips versus the RH are shown in Fig. 7(b). It can be seen from Fig. 7(b) that the RH sensitivities of the prototype of the proposed sensor in dips A, 1, and 2 were opposite to those in the dips B, 3, 4, 5, and 6. We think that this phenomenon may have been due to the negative RI sensitivity of the interference between the core and the lower order cladding modes, as well as the positive RI sensitivity of the interference between the core and the higher order cladding modes or between two different cladding modes [35].
Fig. 7. (a) Sketch map of the resonance dips. (b) The relationship between the RH and the different transmission peaks.
At the same time, it can be seen from Fig. 7(b) that the transmission peak intensity approximately first decreased and then increased in dips 2, 3, and 4, and it first smoothed out and then decreased in dips B, 5, and 6. The results show that the transmission peak modulated by the environmental RH reached the extreme value. In order to better understand this phenomenon and the working mode of the proposed RH sensor, the transmission spectra in Fig. 7(a) were processed by FFT, as shown in Fig. 8(a). As the RH increased, the amplitude of the fundamental mode and the cladding modes 1 and 2 first increased and then decreased, as shown in Figs. 8(b)–8(d). Equation (1) showed that the transmission peak was not only a function between the phase difference and the wavelength, but also related to the intensity of each light beam involved in the HSA-MZI. The spectrum of the interference was the superposition of multiple resonant wavelengths. With the change of the external RH, the strengths of the cladding modes were also in dynamic states. The cladding modes varied with the change of the external RH, and the transmission peaks corresponding to different wavelengths were different. At a certain RH, the change of the transmission peak had an extreme value, because there was an extreme value change in the cladding modes. This is why the transmission peaks of those dips varied abnormally in intensity.
Fig. 8. (a) Spatial frequency spectra of the transmission spectrum with different RH levels. The RH versus the spatial frequency peak of (b) the fundamental mode, (c) cladding mode 1, and (d) cladding mode 2.
Repeatability was an important parameter for the RH sensor. The repetitive results of the proposed RH sensor in dips A and B during the ascent are shown in Figs. 9(a) and 9(b). As can be seen in Figs. 9(a) and 9(b), the repetitive experimental results of dips A and B had consistency for each RH detection. The relationships of dips A and B for the repetition times and the RH sensitivity error are shown in Figs. 9(a) and 9(b) inset, respectively. The reason for the larger RH sensitivity error in dip B may have been due to fine measurement errors and the narrow linear interval of the RH.
Stability was one of the main indexes used to evaluate the performance of the RH sensor. The stability of the proposed sensor was tested at three fixed RH levels of 11.1%, 60.0%, and 76.6%. The results of dip A, which were recorded for the three fixed RH values within 180 min with a step of 5 min, are shown in Fig. 10. The stand deviations of dip A with the RH values of 11.1%, 60.0%, and 76.6% were determined to be 0.1155 dB, 0.0250 dB, and 0.1081 dB, respectively.
The respiratory response was another main index that was used to evaluate the performance of the RH sensor. The response characteristics of the proposed RH sensor were measured. The optical power interrogation system was used to measure the respiratory rise/fall time, including a narrow line width laser (HOYATEK, HY-DFB-1550) for which the peak wavelength was 1550.12 nm, a photoelectric detector module, and a commercial oscilloscope (Tektronix, TDS1012-SC), as shown in Fig. 11(a). The voltage of the transmitted light with deep breathing from an average-sized adult at room humidity as a function of time is shown in Fig. 11(b). The respiratory rise and fall times were 0.42 s and 6.54 s, respectively. Three consecutive breathing cycles of different depths are shown in Fig. 11(b) inset. The respiratory-voltage response curve shows that the sensor could be completely restored to the baseline, as expected. The proposed sensor has the potential for the near-real-time analysis of dynamic humidity changes.
Fig. 11. (a) Experimental set-up for the humidity response time measurement. (b) Dynamic performance of the proposed sensor.
4. Conclusion
A sensitive-enhanced fiber-optic RH sensor based on a helix structure-assisted SMTMS structure with a GO coating was successfully fabricated and characterized. A sensitivity of -0.885 dB/%RH was achieved in an RH range from 70% to 80%. The reasons why different dips had different RH sensitivities and the transmission peak modulated by the environmental RH reached an extreme value were explained. The proposed RH sensor was repeatable, and it had good stability. More prominently, the respiratory rise and fall times of 0.42 s and 6.54 s were achieved. Smaller hysteresis could be used to measure breathing. Such experimental results validate the fact that the proposed RH sensor has a high potential for biological, chemical, and medical applications.
Funding
National Natural Science Foundation of China (61535004, 61735009, 61827819); Special Fund for Guangxi Bagui Scholars, Guangxi project (AA18242043, AD17195074); National Defense Pre-Research Foundation of China (6140414030102); Ph.D. Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities (3072019GIP2519).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
References
1. T. Yeo, T. Sun, and K. Grattan, “Fibre-optic sensor technologies for humidity and moisture measurement,” Sens. Actuators, A 144(2), 280–295 (2008). [CrossRef]
2. J. M. López-Higuera, L. R. Cobo, A. Q. Incera, and A. Cobo, “Fiber optic sensors in structural health monitoring,” J. Lightwave Technol. 29(4), 587–608 (2011). [CrossRef]
3. X. Wang, G. Farrell, E. Lewis, K. Tian, L. Yuan, and P. Wang, “A humidity sensor based on a singlemode-side polished multimode-singlemode (SSPMS) optical fibre structure coated with gelatin,” J. Lightwave Technol. 35(18), 4087–4094 (2017). [CrossRef]
4. Y. Liu, H. Deng, and L. Yuan, “A novel polyvinyl alcohol and hypromellose gap-coated humidity sensor based on a Mach–Zehnder interferometer with off-axis spiral deformation,” Sens. Actuators, B 284, 323–329 (2019). [CrossRef]
5. S. Zheng, “Long-period fiber grating moisture sensor with nano-structured coatings for structural health monitoring,” Struct Health Monit. 14(2), 148–157 (2015). [CrossRef]
6. K. Ni, C. C. Chan, L. Chen, X. Dong, R. Huang, and Q. Ma, “A chitosan-coated humidity sensor based on Mach-Zehnder interferometer with waist-enlarged fusion bitapers,” Opt. Fiber Technol. 33, 56–59 (2017). [CrossRef]
7. Y. Wu, T. H. Zhang, Y. J. Rao, and Y. Gong, “Miniature interferometric humidity sensors based on silica/polymer microfiber knot resonators,” Sens. Actuators, B 155(1), 258–263 (2011). [CrossRef]
8. M. Y. M. Noor, N. Khalili, I. Skinner, and G. D. Peng, “Optical relative humidity sensor based on a hollow core-photonic bandgap fiber,” Meas. Sci. Technol. 23(8), 085103 (2012). [CrossRef]
9. L.-P. Sun, J. Li, L. Jin, Y. Ran, and B.-O. Guan, “High-birefringence microfiber Sagnac interferometer based humidity sensor,” Sens. Actuators, B 231, 696–700 (2016). [CrossRef]
10. Y. Liu, H. Lin, Y. Dai, A. Zhou, and L. Yuan, “Humidity sensor based on an in-fiber integrated Mach-Zehnder interferometer,” IEEE Photonics Technol. Lett. 31(5), 393–396 (2019). [CrossRef]
11. F. Bonaccorso, Z. Sun, T. Hasan, and A. Ferrari, “Graphene photonics and optoelectronics,” Nat. Photonics 4(9), 611–622 (2010). [CrossRef]
12. S. Morozov, K. Novoselov, M. Katsnelson, F. Schedin, D. Elias, J. A. Jaszczak, and A. Geim, “Giant intrinsic carrier mobilities in graphene and its bilayer,” Phys. Rev. Lett. 100(1), 016602 (2008). [CrossRef]
13. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbon films,” Science 306(5696), 666–669 (2004). [CrossRef]
14. Y. Zhang, Y.-W. Tan, H. L. Stormer, and P. Kim, “Experimental observation of the quantum Hall effect and Berry's phase in graphene,” Nature 438(7065), 201–204 (2005). [CrossRef]
15. 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]
16. 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]
17. R. Nair, H. Wu, P. Jayaram, I. Grigorieva, and A. Geim, “Unimpeded permeation of water through helium-leak–tight graphene-based membranes,” Science 335(6067), 442–444 (2012). [CrossRef]
18. 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,” Sens. Actuators, B 234, 503–509 (2016). [CrossRef]
19. M. Shao, Y. Zang, X. Qiao, H. Fu, and Z.-a. Jia, “Humidity sensor based on hybrid fiber Bragg grating/abrupt fiber taper,” IEEE Sens. J. 17(5), 1302–1305 (2017). [CrossRef]
20. Y. Zhao, A.-W. Li, Q. Guo, X.-Y. Ming, Y.-Q. Zhu, X.-C. Sun, P. Li, and Y.-S. Yu, “Relative humidity sensor of S fiber taper based on graphene oxide film,” Opt. Commun. 450, 147–154 (2019). [CrossRef]
21. S. Liu, H. Meng, S. Deng, Z. Wei, F. Wang, and C. Tan, “Fiber Humidity Sensor Based on a Graphene-Coated Core-Offset MachZehnder Interferometer,” IEEE Sens. Lett. 2(3), 1–4 (2018). [CrossRef]
22. 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]
23. S. Deng, H. Meng, X. Wang, X. Fan, Q. Wang, M. Zhou, X. Guo, Z. Wei, F. Wang, and C. Tan, “Graphene oxide-film-coated splitting ratio-adjustable Mach-Zehnder interferometer for relative humidity sensing,” Opt. Express 27(6), 9232–9240 (2019). [CrossRef]
24. R. Chu, C. Guan, Y. Bo, J. Shi, Z. Zhu, P. Li, J. Yang, and L. Yuan, “All-optical graphene-oxide humidity sensor based on a side-polished symmetrical twin-core fiber Michelson interferometer,” Sens. Actuators, B 284, 623–627 (2019). [CrossRef]
25. 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]
26. M. La Notte and V. M. Passaro, “Ultra high sensitivity chemical photonic sensing by Mach–Zehnder interferometer enhanced Vernier-effect,” Sens. Actuators, B 176, 994–1007 (2013). [CrossRef]
27. O. Leenaerts, B. Partoens, and F. Peeters, “Adsorption of H2O, NH3, CO, NO2, and NO on graphene: A first-principles study,” Phys. Rev. B 77(12), 125416 (2008). [CrossRef]
28. F. Schedin, A. Geim, S. Morozov, E. Hill, P. Blake, M. Katsnelson, and K. Novoselov, “Detection of individual gas molecules adsorbed on graphene,” Nat. Mater. 6(9), 652–655 (2007). [CrossRef]
29. G. W. Hanson, “Dyadic Green’s functions and guided surface waves for a surface conductivity model of graphene,” J. Appl. Phys. 103(6), 064302 (2008). [CrossRef]
30. Z. Li, E. A. Henriksen, Z. Jiang, Z. Hao, M. C. Martin, P. Kim, H. Stormer, and D. N. Basov, “Dirac charge dynamics in graphene by infrared spectroscopy,” Nat. Phys. 4(7), 532–535 (2008). [CrossRef]
31. 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]
32. R. Gao, D.-f. Lu, J. Cheng, Y. Jiang, L. Jiang, and Z.-m. Qi, “Humidity sensor based on power leakage at resonance wavelengths of a hollow core fiber coated with reduced graphene oxide,” Sens. Actuators, B 222, 618–624 (2016). [CrossRef]
33. Y. Liu, A. Zhou, and L. Yuan, “Gelatin coated Michelson interferometric Humidity sensor based on a multicore fiber with helical structure,” J. Lightwave Technol. 37(10), 2452–2457 (2019). [CrossRef]
34. A. Leung, P. M. Shankar, and R. Mutharasan, “A review of fiber-optic biosensors,” Sens. Actuators, B 125(2), 688–703 (2007). [CrossRef]
35. Y. Zhang, A. Zhou, B. Qin, H. Deng, Z. Liu, J. Yang, and L. Yuan, “Refractive index sensing characteristics of single-mode fiber-based modal interferometers,” J. Lightwave Technol. 32(9), 1734–1740 (2014). [CrossRef]
