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Abstract
A relative humidity sensor based on a graphene oxide-coated few-mode fiber Mach-Zehnder interferometer (MZI) is proposed in this paper. The MZI was made by splicing a segment of the few-mode fiber (FMF) between two segments of a no-core fiber (NCF) and two segments of a single mode fiber (SMF) located outside the two NCFs. The core and cladding of the FMF acted as interferometric arms, while the NCFs acted as couplers for splitting and recombining light due to mismatch of mode field diameter. The cladding of the FMF was corroded with hydrofluoric acid, and a layer of graphene oxide (GO) film was coated on the corroded cladding of FMF via the natural deposition method. The refractive index of GO varied upon absorption the water molecules. As a result, the phase difference of the MZI varied and the wavelength of the resonant dip shifted with a change in the ambient relative humidity (RH). High humidity sensitivity of 0.191 and 0.061 nm/%RH in the RH range of 30-55% and 55-95%, respectively, were achieved experimentally. The high sensitivity, compact size, and simple manufacturing of the proposed sensor could offer attractive applications in fields of chemical sensors and biochemical detection.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Relative humidity (RH) sensing is critical and widely applied to many fields, such as food processing, agriculture, biopharmaceuticals, instrument manufacturing, and structural health monitoring [1,2]. Traditional electronic RH sensors offer the advantage of high measurement accuracy; however, the use of these sensors in corrosive environments, under strong electromagnetic interference, and for remote detection is challenging. Due to the characteristics of anti-electromagnetic interference and remote detection capacity [3], various fiber-optic sensors have been presented to measure RH, including fiber Bragg gratings (FBGs) [4,5], long period gratings (LPGs) [6], the Fabry-Pérot (FP) interferometer [7,8], Mach-Zehnder interferometer (MZI) [9,10], Michelson and Sagnac interferometer [11,12], side-polished fiber [13,14], microfiber or microfiber resonators [12,15–17], and surface plasmon resonance [18]. In particular, some humidity sensors consist of a hollow core fiber [19], no-core fiber [20], plastic optical fiber [21], few-mode fiber [17], polarization-maintaining fiber [10], and photonic crystal fiber (PCF) [22,23]. However, there sensors have a few limitations: (1) the microfiber structures are fragile and difficult to prepare, (2) the manufacture of FBG or FP structures is complicated, and (3) the costs associated with the PCF are high. Meanwhile, various humidity-sensitive hydroscopic materials have been used to improve the sensitivity of sensor, such as polyimide [5,6], chitosan [24,25], polymethyl methacrylate (PMMA) [26], metal oxides [9,27], agarose gel [8,22,28], polyvinyl alcohol (PVA) [7,18], and tungsten disulfide (WS2) [29].
In recent years, graphene and graphene-based materials with two-dimensional atomic structures have great attraction due to the characteristics in the fields of mechanics, electronics, and optics [30]. As one of graphene derivatives, graphene oxide (GO) not only includes many oxygen-containing groups but also has new functions/properities, such as dispersibility, hydrophobicity, and large aspect ratio [31], as the oxygen-containing functional groups are randomly distributed on the base surface and at the sheet edge. In the field of humidity sensing, GO can fully absorb ambient water molecules owing to its unique hydrophilicity and large surface area, thus making it suitable for humidity-sensitive membranes [32,33]. Various research reports [4,10,13,15,33–35] have shown that GO has a stronger moisture sensing effect than other materials such as PVA, agarose, and chitosan.
In this paper, a GO-coated fiber-optic MZI humidity sensor is presented. The MZI was fabricated by fusing a section of the few-mode fiber (FMF) between two sections of a no-core fiber (NCF) acting that acted as couplers. Compared to SMF and MMF, FMF has many special characteristics and has been used for sensing applications of bending, strain, temperature, and humidity [36]. In our opinion, the FMF can excite the core and cladding modes better. Further, compared to SMF, the higher-order modes in FMF-based fiber optic sensors are more sensitive to changes in the external environment. Compared to the MMF, the certain modes transmissions in FMF can reduce the crosstalk between the core and cladding modes or the crosstalk between different cladding modes. It benefits to get a stable interference. The cladding of the FMF was partially etched and coated with a layer of GO film via dip-coating and natural evaporation to enhance response to ambient water molecules. The refractive index of the GO film changed with a change in the ambient humidity and subsequently affected the light field in the fiber, eventually leading to a shift in the resonant wavelength of the MZI. The experimental results demonstrate that a humidity sensitivity value of 0.191 nm/RH% with a linear fitting coefficient of 99.3% in the RH range of 30-55% could be achieved. The high sensitivity performance, compact size, and simple manufacturing of the proposed sensor could have promising applications in chemical sensors and biochemical detection.
2. Sensor structure and operating principle
Figure 1 shows the schematic of the sensing head, which consists of two parts of the single mode fiber (SMF), two parts of the NCF, and a part of the FMF (four-mode step-index fiber), which are spliced in order. The core diameters of the SMFs and FMFs are 9 and 18.5 µm, and the cladding diameters of the three kinds of fiber are 125 µm, respectively. When light is introduced into NCF1 from SMF1, a part of it is coupled into the cladding of the FMF, and thus the cladding modes are excited due to the mismatch of core diameter. Meanwhile, the other part of the light enters the core of the FMF, exciting the fundamental and high order modes (includes four modes: LP01/LP11/LP21/LP02). Subsequently, the core modes and cladding modes continue to propagate in the core and cladding of the FMF, respectively, with different propagation constants. At the FMF-NCF2-SMF2 region, the core modes and cladding modes interfere and mostly couple into the SMF2. Although the number of excited modes in the FMF core is less than that in the multi-mode fiber, the above interference can be explained in terms of with the multi-mode interference occurring in the SMS (single mode fiber-multi-mode fiber-single mode fiber) structure [37], which can been considered as a multimodal MZI. The number of cladding modes and insertion loss are related to the length of the NCF. In general, a small number of excited cladding modes and lower insertion loss can be achieved for shorter NCFs.
As described previously, the proposed sensor can be regarded as a MZI. The output intensity and phase difference of the proposed MZI can be expressed as
Therefore, when the RH increases, the effective RI of the GO (nGO) will decrease due to the σ decreasing. As a result, the Δneff will decrease and the wavelength of the resonant dip shift to short wavelength. On the contrary, when the RH decreases, the wavelength of the resonant dip will move to long wavelength. Hence, the ambient relative humidity can be obtained by measuring the shift in λdip.
3. Sensing head fabrication
3.1 Splice of sensor structure
Free spectral range (FSR) is usually used to analyze the spectral characteristics of sensors based on MZI. Theoretically, the FSR (Δλ) based on MZI can be shown as,
Usually, 3-4 periods of interference mapping shown in the observed wavelength range is relative suitable [35,41]. In the experiment, the RI difference between the core and cladding of FMF is 0.005, the wavelength of the incident light is 1550 nm. Supposed the observed wavelength range is 60 nm and Δλ is 15∼20 nm, then the length of the FMF of 24∼32 mm can be got with Eq. (6). So, a length of FMF of 30 mm was selected with the FSR of 16 nm in the experiment.
We measured the transmission spectrum of each part of the sensor, including the transmission spectra of the FMF, NCF + FMF, and NCF + FMF + NCF to further analyze the sensor operation, as shown in Fig. 3(a). It can be observed that obvious interference with a high contrast ratio and small insertion loss could be achieved in the case of the NCF + FMF + NCF. This was possible because the cladding modes were effectively excited in the SMF1-NCF1-FMF region, while the core modes and cladding modes interfered and recoupled into SMF2 mostly in the FMF-NCF2-SMF2 region. In contrast, in the case of the FMF and NCF + FMF, the cladding modes were not effectively excited and interfered with the core modes, subsequently recoupling into SMF2. The fast Fourier transform (FFT) image of the transmission spectra in Fig. 3(a) is shown in Fig. 3(b). It is evident that the sensor structure proposed in this paper could excite the cladding mode strongly. This can prove very beneficial for the formation of uniform, large extinction interference fringes and humidity sensing. In addition, Fig. 3(a) shows that the FSR of the sensor with NCF + FMF + NCF structure is about 15 nm, which is agreed with the theoretical result. The experimental results show that the insertion loss of the sensor was about 17 dB.
Fig. 3. (a) Transmission spectra of each part of the MZI humidity sensor. (b) Fast Fourier transform (FFT) of the transmission spectra in (a).
3.2 Corrosion of the FMF
To lead-out the evaescent wave further, the cladding of the FMF was partly corroded using hydrofluoric acid (HF). As shown in Fig. 4, the suspended FMF was immersed in a 100 µL HF solution with a 40% concentration. After 10 min, an obvious interference pattern was gained, as shown in Fig. 5. The corrosion process was stopped, the remaining HF was sucked out using a plastic dropper, and subsequently the corroded area was repeatedly washed with deionized water. Finally, the fiber was soaked with alcohol for several minutes and cleaned again with deionized water. It can be observed from Fig. 5 that the interference contrast ratio became large and the wavelength of the interference dip shifted towards shorter wavelengths, as compared with that before the corrosion process. This is because the cladding diameter of the FMF is reduced by chemical corrosion, which caused leakage in a part of the excited cladding modes. As a result, the output transmission spectrum formed by the superposition of various interferences changed acorrordingly. The scanning electron microscopy (SEM) image shown in Fig. 6(a) indicates that the cladding diameter of the corroded FMF was about 102 µm.
Fig. 6. (a) SEM image of the fiber after corrosion and with GO coating. (b) SEM image of the GO coating on the surface of the fiber. (c) Enlarged SEM image of GO.
3.3 Coating the GO film on the FMF
To enable interaction with the ambient water molecules, a GO film should be coated on the surface of the corroded FMF. It is beneficial to use GO sheets with larger sheet diameters for coating to improve the moisture absorption capacity. First, a GO dispersion liquid was centrifuged at 10,000 RPM for 15 min to gain GO sheets with a size greater than 500 nm. The concentration of the GO dispersion liquid is 0.08 mg/mL. Second, the fiber was cleaned with alcohol several times, and the sensing area was immersed in a sufficiently sized droplet of GO dispersion and naturally evaporated for 24 h at a temperature of 25°C. And then, the GO sheets were deposited on the surface of the fiber, which can be observed in the SEM images shown in Figs. 6(b) and (c). The thickness of the GO film could be adjusted based on the concentration and volume of the GO solution. In general, thicker GO films can be acquired at higher graphene concentrations. Figure 7 shows the transmission spectra of the sensor before/after GO coating. We can see the extinction ratio and wavelength change slightly as compared with that before GO coating.
4. Experimental results and discussion
The humidity measurement system is shown in Fig. 8, which includes a broadband source, sensing head, constant temperature and humidity chamber, and optical spectrum analyser. The temperature and relative humidity of the constant temperature and humidity chamber can be adjusted independently. In the experiment, the temperature was always maintained at 25°C. When the experiment started, we increased the relative humidity in the constant temperature and humidity chamber according to the humidity gradient of 5% RH. At each humidity, the transmission spectrum was recorded when the spectrum was stabilized after 5-10 minutes. As expected, the wavelength of the resonant dip shifted to the short wavelengths when the RH increased in the measured range, as shown in Fig. 9.
Figure 10 shows the relationship between the wavelength of resonant dip and ambient RH. It can be observed that a non-linear relationship exists between the wavelength and RH in the measured range; however, a good linear relationship can be seen in the RH ranges of 30-55% and 55-95%. A sensitivity of 0.191 and 0.061 nm/%RH were achieved in the RH range of 30-55% and 55-95%, respectively. The corresponding linear fitting coefficient were 99.3 and 98.45%, respectively. The phenomenon that the sensitivity in the region of low RH was higher than that in the region of high RH could be explained: in the region of low RH, the GO sheet on the surface of the FMF was fluffy and dry with a large gap, resulting in a rapid entry of the water molecules into the GO film. This resulted in a significant change in the effective refractive index of the cladding modes and a relatively large wavelength drift. In the region of high RH, the adsorption of water molecules by the GO film gradually saturated. At this time, the effective refractive index of the GO film changed slowly with continued increase in the RH. This resulted in a relatively small wavelength drift in the region of high RH. This phenomenon was also observed in the paper [4], wherein GO was used as a moisture-sensitive material.
The rise and fall of RH were also compared to assess the performance of the proposed sensor. Figure 11 shows the relationship between the resonant dip near 1564 nm and RH when the RH decreased from 95% to 30%. The result obtained for increasing RH is also shown for the purposes of comparison. It can be seen that the wavelength of the resonant dip is little differences when the RH is rising or falling to the same value. This difference can be attributed to the small fluctuations in the RH adjustment process and incomplete adsorption and separation of the water molecules on the GO film.
Fig. 11. The relationship between the wavelength of renant dip near 1564 nm when RH increases and decreases between 30%-95%RH
In addition, the stability of the proposed sensor was studied under different RH at a temperature of 25°C. As shown in Fig. 12, the corresponding wavelength fluctuation was less than 0.306 nm and 0.07 nm over a period of 55 min at the RH of 45 and 75%, respectively. The large fluctuations in the 45% RH case might be due to the relatively higher RH sensitivity. The wavelength fluctuation maybe originate from small vibrations of the sensor or small fluctuations caused by humidity adjustment.
The performance comparison of the fiber-optic RH sensors is listed in Table 1. The results indicate that the proposed sensor has a high sensitivity in a wide range of RH. In addition, the sensor was fabricated by fiber splicing and corrosion, which has the advantages of a simple structure, easy fabrication and low cost. The high performance demostrates that the proposed sensor could potentially be used in the fields of bio-chemical and medicine.
Table 1. Performance of different types of optical-fiber RH sensors.
5. Conclusion
A fiber-optic humidity sensor with a MZI structure based on the few-mode fiber, which consists of a segment of the FMF spliced between two NCFs, was proposed and experimentally demonstrated. Based on the strong hydrophilic property of the GO coating on the etched areas of the FMF and characteristics of the MZI, the ambient RH could be obtained by measuring the shift of the resonant wavelength. A high sensitivity of 0.191 nm/%RH with a linear fitting coefficient of 99.3% was achieved in the RH range of 30-55%, while a sensitivity of 0.061 nm/%RH with a linear fitting coefficient of 98.45% was achieved in the RH range of 55-95%. The proposed sensor offers advantages such as high sensitivity, low cost, and a simple fabrication process, which render it attractive for RH monitoring in the field of bio-chemical sensing.
Funding
National Natural Science Foundation of China (11674109, 61774062); Natural Science Foundation of Guangdong Province (2016A030313443); Science and Technology Planning Project of Guangdong Province (2017A020219007).
Disclosures
The authors declare no conflicts of interest.
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