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Abstract
A new tapered fiber relative humidity (RH) sensor based on polydimethylsiloxane (PDMS) and graphene oxide (GO) film coatings is proposed and demonstrated. Tapered fiber is fabricated in single mode fiber by the tapering machine. The PDMS film is coated on the surface of the fiber using the hydroxide flame sintering technique, while the GO film is deposited using the physical deposition technique. This structure can achieve strong interference effect by the smaller range of tapering process. In the RH measurement experiment, the humidity sensitivity of the sensor is measured to be as high as 0.371 dB/%RH within the RH range of 35% to 90%. The experiment investigates the impact of different numbers of PDMS coating on sensitivity. As the number of PDMS coatings increases, the RH sensitivity of the sensor also increases and reaches the highest sensitivity when coated with 10 layers of PDMS film. The sensor has high sensitivity to RH, good stability and mechanical strength, which also shows great performance in both moisture absorption and desorption. These advantages make the sensor suitable for the wide range of humidity sensing applications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Relative humidity (RH) represents the ratio of the water vapor pressure in the air to the saturated vapor pressure at the same temperature. It serves as the crucial measure for assessing atmospheric dryness [1]. RH sensing holds significant importance in various fields, including the pharmaceutical industry [2], environmental monitoring [3], agriculture [4], industrial production [5] and biochemistry [6]. Traditional techniques for RH detection mainly include capacitance [7], resistance [8], gravimetric [9] and chemical [10]. However, these types of RH sensors are not resistant to electromagnetic interference and are unsuitable for high temperature environments. In contrast, fiber humidity sensors present advantages such as immunity to electromagnetic interference, high insulation, corrosion resistance, notable sensitivity and accuracy [11]. Therefore, fiber sensors for RH sensing have garnered substantial interest among researchers. By combining various structures and diverse fiber types like side-polished structures [12], grating structures [13], photonic crystal fibers [14] and multicore fibers [15], fiber sensors for RH detection can fit more application scenes.
At present, fiber sensors for RH sensing have attracted widespread attention due to their inherent advantages. By combining various moisture-sensitive materials, a variety of humidity sensors have been manufactured. In 2016, Luo et al. proposed a novel all-fiber-optic humidity sensor composed of the tungsten disulfide (WS2) film coated on a side-polished fiber. Within the RH range of 35% to 85%, the sensor exhibited the light power variation of up to 6 dB and with the sensitivity of 0.1213 dB/%RH [16]. In 2017, Ouyang et al. proposed the use of molybdenum diselenide (MoSe2) as the humidity-sensitive material for the first time and achieved the sensitivity of 0.321 dB/%RH within the RH range of 32% to 73% [17]. In addition, RH sensitivity can be increased by coating with polymethyl methacrylate (PMMA) [18] or silica gel modified by graphene oxide (GSg) [19]. However, various reported literature indicate that graphene oxide (GO) exhibits superior humidity sensing effects compared to other materials such as MoSe2 [20]. By changing the structure of the sensor, it is also possible to achieve an enhancement in sensitivity. In 2019, Zhao et al. proposed an S-tapered fiber humidity sensor based on the graphene oxide coating. They utilized the fusion splicer to create an S-taper fiber with the taper waist diameter of 27 µm. After coating the GO film using the in-situ layer-by-layer technique, they achieved the sensor sensitivity of −0.361 dB/%RH within the RH range of 84% to 95% [21]. In 2021, Tsai et al. introduced a graphene oxide-coated Base S-type LPFG humidity sensor with the tapered waist diameter of 37 µm. Humidity experiments demonstrated that the sensor achieved the average sensitivity of −0.1824dB/%RH within the RH range of 20% to 80% [22]. In 2022, Yan et al. introduced a long-period fiber grating structure based on the GO/Co-MOF-74 film and achieved the sensitivity of 0.16 dB/%RH within the range of 50%RH to 90%RH [23]. In addition, there are methods utilizing Fabry-Perot Interferometer (FPI) microcavities for the preparation of humidity sensors [24]. Among these, using a tapered fiber to manufacture humidity sensors is also an effective method to enhance RH sensitivity. The fiber is tapered by the tapering machine to create two coupling regions. Due to the different effective refractive index of the core mode and the cladding mode, the phase difference occurs as light propagates to the waist of the taper. At the second coupling region, interference arises from the re-coupling of the light between the cladding and the core. According to this principle, humidity sensors based on tapered fibers exhibit good sensing performance and high sensitivity, which is attracting a lot of attention from researchers. However, these sensors typically use ultra-fine waist structures to increase sensitivity [25], which significantly reduces the mechanical strength of the sensors.
In this work, a tapered fiber sensor for RH sensing based on polydimethylsiloxane (PDMS) and GO films is proposed and demonstrated in order to further improve the RH sensitivity of the sensor. PDMS is a low-cost, nontoxic and versatile material that can be used to modify a wide variety of surfaces [26]. During the process of device fabrication, PDMS was first coated on the surface of a single mode fiber (SMF), which was then placed in the hydroxide flame and sintered to form the reticulated SiO2 structure [27]. This generated SiO2 structure has excellent hydrophilicity. Subsequently, the SMF was tapered using the fused biconical taper technique. After completing the tapering process, the GO film was deposited on the surface of the fiber using physical deposition techniques. GO material possesses excellent hydrophilicity due to the oxygen-containing functional groups and large specific surface area, which makes the GO suitable for humidity sensing [28,29]. In the measurement experiment for humidity, the measured sensitivity of the sensor reaches up to 0.371 dB/%RH within the RH range of 35% to 90%. As compared to similar types of RH sensors, this new sensor offers distinct advantages such as heightened sensitivity, the wide sensing range, robust mechanical strength, stability and low cost.
2. Principle and device fabrication
2.1 Principle
Tapered fiber interference sensors are based on the sensing principle of two beam interference, phase differences occur as light propagates through the waist of the cone and interference occurs at the second coupling structure. According to the principle of light interference, the interference intensity I can be expressed as [29]:
In this structure, the incident light is coupled into the tapered fiber through the SMF. As the light propagates in the tapered fiber, most of the light energy at the first coupling region is coupled from the core mode to the cladding mode. This process excites higher-order cladding mode, while the remaining light continues to propagate in the core of the fiber. Due to the different effective refractive index of the core mode and the cladding mode, the light passes through the tapered-waist portion of the light with phase difference. As a result, interference occurs when the two beams of light are re-coupled at the second coupling region. With changes in environmental RH, the effective refractive index of GO materials varies. In humidity experiments, as the RH increases, the refractive index of GO decreases, consequently causing a change in the refractive index of the tapered fiber cladding. The higher-order cladding mode excited at the first coupling structure of the fiber are highly sensitive to changes in the external refractive index. Therefore, the $\Delta {n_{eff}}$ will change as the external refractive index changes, which leads to the variation in the interference intensity.
2.2 Device fabrication
Figure 1(a), Fig. 1(b) and Fig. 1(c) shows the schematic diagram of the sensor fabrication process using SMF (Corning SMF-28e) in the experiments. During the device fabrication process, the first step involved removing the protective layer from the SMF and placing the fiber onto the fixture. The SMF was moved using the 3D displacement platform to fully immerse the sensing area in the PDMS (Dowsil Sylgard 184). The PDMS film was uniformly coated on the surface of the fiber and subjected to complete combustion through oxyhydrogen flame sintering technology. As shown in Fig. 1(a), the main chain of PDMS is Si-O bond, which will generate SiO2 and adhere to the surface of the fiber after complete combustion [30]. To ensure that the generated SiO2 layer completely and uniformly covers the fiber surface, the above process needs to be repeated several times. As shown in Fig. 2, the morphology of the tapered fiber surface is investigated through scanning electron microscope (SEM) images. The inset in the figure provides an enlarged view of the specific area on the surface. This structure significantly increases the specific surface area of the fiber, which provides additional binding sites for GO deposition [31].
Fig. 1. (a) Coated PDMS film; (b) Tapered fiber; (c) Coated GO film; (d) Transmission spectra of sensors coated with PDMS and GO films.
Fig. 2. The SEM image of tapered fiber surface coated and sintered with PDMS for 10 times. Inside the dotted box is an enlarged image of local details.
The fiber was then tapered by the oxyhydrogen flame tapering technique. Throughout the heating process, the motor-controlled displacement platform uniformly moved 1 mm to stretch the fiber. The combination of SiO2 particles created after the PDMS has completely combustion and the softened cladding material of the fiber creates defects in the cladding. This weakens the ability to confine the optical field in the fiber core and causing some light to easily leak out from the core. As a result, the fiber can achieve the strong interference effect through minor tapering processing. As shown in Fig. 1(b), the diameter of the tapered waist is 115 µm after taper processing and the coating thickness is about 10 µm. The transmission peaks shown in Fig. 1(d) are obtained, the transmission depth of dip1 is 14.1 dB. After the coating of the GO, there will be some increase in insertion loss. However, the spectrum still maintains good interference effects, with the transmission depth of dip 3 being 19.37 dB.
After completing the tapering process, GO film was deposited onto the fiber using the physical deposition method, as shown in Fig. 1(c). The sensing area of the fiber was initially immersed in acetone for 30 minutes to eliminate surface contaminants. Then, after thorough rinsing with deionized water, the fiber was sequentially immersed in NaOH and APTES solutions for 2 hours each. These pretreatments aid in modifying the chemical properties of the fiber surface, which makes them easier to deposit GO [32]. After rinsing the fiber again with deionized water, the fiber was placed in the constant temperature chamber and kept at 70 °C for 1 hour. Then, the sensing region of the fiber was immersed in the 2 mg/ml GO solution and left in the constant temperature chamber for 3 hours. The GO used is single layer, which is produced by XFNANO company, the model is XF020 and the chip diameter is about 50-200 nm. The temperature of the constant temperature chamber was set to 40 °C and GO solution was added every half hour. After completing the 3-hour deposition, the temperature of the constant temperature chamber was raised again to 70 °C for an additional 1 hour. After the 5 hours incubation at temperature, the GO film was deposited over the SiO2 layer. The GO film was characterized using a Raman spectrometer (Jobin-Yvon T64000). As shown in Fig. 3, the characteristic peak at 1347 cm−1 corresponds to the D band and the characteristic peak at 1585 cm−1 corresponds to the G band. These two peaks confirm the successful attachment of the GO material onto the fiber surface.
3. Experimental and discussion
To test the RH sensitivity of the sensor, an experimental setup for sensing tests was constructed as shown in Fig. 4. Light originated from the light source and entering the conical sensing area. The changes in the transmitted spectrum of the sensor were recorded in real-time by the OSA. The humidity inside the test chamber was controlled by the humidity generator (FD-HG), while the humidity and temperature in the sensing region were monitored in real-time by the thermohydrometer. To minimize the effect of temperature variations on humidity measurements, the experimental temperature was maintained at 23°C consistently. During the humidity testing process, the RH inside the humidity chamber gradually increased from 35% to 90% in steps of 5%, the transmission spectra at different RH were recorded.
In order to investigate whether GO has an effect on the humidity sensitivity of the sensor, a comparative experiment between coated and uncoated GO films was conducted. During the RH sensitivity testing of the sensor, the tapered fiber with a waist diameter of 115 µm was used, while the PDMS film was applied with 10 layers of coating. When the sensor surface is without the GO coating, the spectral changes of the sensor with respect to RH are depicted in Fig. 5. The transmission peak intensity shows the relatively small variation with changes in RH, which indicates lower sensitivity due to the absence of the sensitizing of GO.
In order to investigate the effect of different number of PDMS film coatings on the RH sensitivity of the sensor, three types of sensors were prepared for testing experiments. These sensors were coated with 1, 5 and 10 times PDMS films. In the experiment, the prepared sensors had the tapered waist diameter of 115 µm and the GO film was deposited on them. In exploring the variation of sensor spectra with RH, the characteristic peak dip3 between 1140 nm and 1160 nm was chosen for testing. The three sensors were then tested and the spectral test results are shown in Fig. 6. Figures 6(a) and 6(b) demonstrate that sensor sensitivities of PDMS films coated once and five times are 0.058 dB/%RH and 0.172 dB/%RH, respectively. It can be observed that as the number of PDMS film coats decreases, the sensitivity of the sensor decreases. This is due to the reduction in the SiO2 mesh structure produced, which results in the reduction in the specific surface area of the sensor. Therefore, the moisture absorption effect of the sensor is also reduced. Figure 6(c) shows the humidity test spectrum of the sensor coated with 10 times PDMS film. As seen in Fig. 6(c), the intensity of the characteristic peak increased from −41.8 dB to −21.76 dB with RH changes. Figure 6(d), Fig. 6(e) and Fig. 6(f) show the fitting results of the characteristic peak intensities of the three sensors during moisture absorption and desorption processes, respectively. Among them, the sensor coated with 10 times PDMS film has the high RH sensitivity of 0.371 dB/%RH and the linear fit is 0.9938. According to the transmission spectra of the sensors, it is possible to know that the sensitivity of the sensor increases with the number of PDMS film coating, the highest sensitivity is obtained at 10 coats. This is because as the number of coated PDMS films increases, PDMS sintering generates more SiO2 mesh structures. These structures have the larger specific surface area and the higher porosity, which increases the efficiency of the GO films in the processes of moisture absorption and desorption. When PDMS is coated and sintered for one time, the humidity-sensitive film is not uniform, which will affect the effect of moisture absorption and dehumidification and the repeatability of the experiment. When PDMS is coated and sintered for 5 times, the humidity-sensitive film is relatively uniform and stable moisture absorption and dehumidification characteristics can be obtained and the repeatability of the experiment is improved. However, due to the thin thickness of the humidity-sensitive film, high humidity sensitivity cannot be obtained. Therefore, when PDMS is coated and sintered for 10 times, the thickness of humidity-sensitive film is large and uniform and good repeatability and humidity sensitivity can be obtained. When the PDMS film is coated between 10 and 13 times, the RH sensitivity remains relatively constant. However, when the PDMS film is coated more than 15 times, there is a noticeable decreasing trend in RH sensitivity. This phenomenon occurs due to the excessive thickness of the generated SiO2 film, which hinders the penetration of water molecules. As a result, this leads to poor sensing effects and the reduction in sensing sensitivity. In addition, the maintenance of the constant sensitivity of the device during the process of moisture absorption and desorption suggests good reversibility and repeatability.
Fig. 6. (a), (b) and (c) are the transmission spectral responses at different humidity after coating and sintering PDMS for 1 time, 5 times and 10 times and coating GO, respectively. (d), (e) and (f) are linearly fitted curves for the corresponding cyclic testing at different humidity.
To further investigate the stability of the sensor, the sensor was maintained in environments with RH of 35%, 60% and 80% for 7 days, respectively. Transmission spectra were automatically recorded by the OSA every 3 hours and the daily data was averaged. The measurement results of characteristic peak intensity at different RH levels are shown in Fig. 7. It can be observed from the graph that the characteristic peak intensity exhibits minimal fluctuations at different RH levels, which is evidence of the high stability of the sensor. The sensor demodulation method is based on intensity demodulation rather than wavelength demodulation. Table 1 compares the performance of different types of fiber humidity sensors based on intensity demodulation. As shown in Table 1, the sensor exhibits relatively high sensitivity to RH. This humidity sensor can be applied to humidity monitoring in medical, agricultural and other environments, as well as humidity monitoring in electromagnetic interference and radiation environments.
Table 1. Parameter comparison for different types of fiber RH sensors based on intensity demodulation
4. Conclusion
In conclusion, the tapered fiber sensor exhibits good sensitivity and stability in humidity detection. Utilizing the fused tapering technique, tapered fiber sensors with high mechanical strength are manufactured. The sensitivity of the sensor to RH is further improved by the sensitizing effect of the moisture-sensitive material GO film. The effect of varying the number of PDMS film layers on the sensitivity of the sensor is investigated in the experiment. The results show the sensitivity of up to 0.371 dB/%RH between 35% and 90% RH with the linear fit of 0.9938. The proposed high-sensitivity RH sensor has advantages such as small size, resistance to electromagnetic interference, high mechanical strength, good stability and reversibility. The sensor has good application prospects in the fields of humidity detection.
Funding
National Natural Science Foundation of China (62090064, 62131018, 62305130, 62090063, 62075082, U20A20210, 61827821 ).
Acknowledgment
This work was supported by the National Natural Science Foundation of China (62090064, 62131018, 62305130, 62090063, 62075082, U20A20210, 61827821).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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