A single core-offset Mach-Zehnder interferometer (MZI) coated with polyvinyl alcohol (PVA) for simultaneous measurement of relative humidity (RH) and temperature is proposed in this paper. The sensing structure is fabricated by splicing dispersion compensating fiber (DCF) and no-core fiber (NCF) and splicing two single-mode fibers (SMF) at both ends, where the core-offset is located at the splicing of SMF and DCF. A part of the cladding of DCF is etched to excite the high-order cladding mode (LP10), and PVA is coated on the etched area. The refractive index of PVA varies due to the adsorption of water molecules. Therefore, when the ambient relative humidity and temperature change, the change of MZI phase difference causes the wavelength of the resonant dip to shift. The experimental results indicate that the proposed sensor has a sensitivity of 0.256 nm/RH% for RH range of 30%-95%, and a sensitivity of 0.153 nm/℃ for temperature range of 20℃-80℃, respectively. The simultaneous measurement of RH and temperature can be achieved by demodulating the sensitivity coefficient matrix. The proposed sensor has the characteristics of good repeatability, high sensitivity, and good stability, which make it potentially applications for the detection of RH and temperature measurement.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The real environment is complex and diverse, and a detailed understanding of the environment requires investigation and research of multiple physical quantities. Common physical quantities used to describe the external environment are relative humidity (RH) and temperature, which can profoundly affect human comfort in life. Meanwhile, RH is affected by the temperature in the environment. Therefore, it is necessary to simultaneously measure RH and temperature, which have a pivotal position in biochemistry, agriculture, electronics, instrument manufacturing, food processing, medical care and other fields. Traditional electronic RH and temperature sensors are difficult to be applied in complex environments, such as strong corrosive substances, strong electromagnetic interference and long distances. Optical fiber sensors have excellent characteristics, such as remote sensing, anti-electromagnetic interference, and high sensitivity . Optical fiber sensors are regarded as strong candidates in the field of sensing and have been widely studied and applied, such as temperature , RH , refractive index [4,5], displacement , gas , strain , etc. Nowadays, for measuring RH and temperature, optical fiber sensors are constantly being presented and manufactured, mainly including Mach-Zehnder interferometer [2–5], Sagnac interferometer , long period gratings , Michelson interferometer , fiber Bragg gratings (FBG) , Fabry-Perot interferometer (FPI) , surface plasmon resonance , etc.
The main component of optical fiber is silicon dioxide, which has poor sensitivity to RH and temperature. Therefore, coating sensitive materials can effectively improve the detection sensitivity of the optical fiber sensor, such as graphene oxide [3,14–16], polyvinyl alcohol (PVA) [17–20], GQDs- PVA [10,21], graphene oxide and polyvinyl alcohol (GO-PVA) [22,23], calcium alginate hydrogel [24,25], carbon-nanotube and polyvinyl alcohol , SnO2 , polyimide , chitosan , polymethyl methacrylate , and Norland optical adhesive (NOA) . The hygroscopic material produces a physical or chemical reaction after adsorbing water molecules, which in turn modulates the light signal. Studies have shown that PVA is a strong hydrophilic material and has good film-forming properties, which make it can be well coated on the surface of the optical fiber with good stability . Therefore, PVA and mixtures containing PVA are widely used in RH sensors [10,17–21,22,23,26].
The thermo-optical effect and thermal expansion coefficient of optical fiber is affected by temperature, which cause measurement errors of the optical fiber humidity sensor. To guarantee the accuracy of the sensor, simultaneous measurement of RH and temperature is significant. At present, the commonly used measurement method is to cascade multiple interferometers, or FBG [31,32]. This method is not conducive to distinguishing spectral information and complex production . In this paper, a sensitivity coefficient matrix is gained by tracking the shifts of two resonant dips. Through dual-wavelength matrix demodulation, the sensitivity coefficient matrix is demodulated to implement simultaneous measurement of RH and temperature. This method is always used for multi-parameter measurement [32–34].
In this paper, we propose a single core-offset structure MZI coated with PVA to implement simultaneous measurement of RH and temperature. The symmetry of the dual-core offset structure is difficult to grasp during the process of splicing, which will increase the difficulty of production. Therefore, this paper uses a single core-offset structure to avoid this problem. The MZI is fabricated by splicing dispersion compensation fiber (DCF), no-core fiber (NCF) and single-mode fiber (SMF) with the sequence of SMF-DCF-NCF-SMF. Among them, the core-offset occurred at the fusion joint of SMF-DCF. A part of the cladding layer of the DCF is corroded by the hydrofluoric acid solution, and then the higher-order cladding mode is excited. PVA is attached on the corroded cladding layer by a natural deposition method. PVA is a hydrophilic material, which means the adsorption of water molecules can change the refractive index (RI) of the PVA. As a result, the optical fiber field will be affected, and the resonance wavelengths of the MZI shifts. For studying the performance of different materials, GO is also used as a comparison. The experimental results demonstrate that the GO-coated sensor has a sensitivity of 0.122 nm/RH% in RH range of 35%-95%, and a sensitivity of 0.069 nm/℃ in temperature range of 30℃-80℃. The PVA-coated sensor has a sensitivity of 0.256 nm/RH% in RH range of 30%-95%, and a sensitivity of 0.153 nm/℃ in temperature range of 20℃-80℃. The sensitivities of the PVA-coated sensor are twice of that of the GO-coated senor. The simultaneous measurement of RH and temperature can be achieved by measuring the shifts of two resonant dips.
2. Sensing structure and principle
The structure of the sensor proposed in this paper is shown in Fig. 1(a), which is consisted of two segments of SMF, one segment of DCF, and one segment of NCF spliced together. The core-offset splice is located at the fusion joint between SMF and DCF. The core and cladding diameters of SMF and DCF are 9 μm/125 μm and 4.5 μm/110 μm, respectively. The incident light is input from the SMF and is separated by the core-offset. Part of the beam propagates in the core of the DCF, and the other part propagates in the cladding of the DCF. A part of the cladding layer of the DCF is corroded by the hydrofluoric acid solution, so higher-order cladding modes can be excited better. Generally, there is only one dominant higher-order cladding mode , which has a smaller RI and more sensitive to external environment . When the surrounding environment varies, the effective RI of core mode remains unchanged, but the effective RI of the cladding modes changes. As a result, a phase difference between core mode and cladding modes has been produced. Therefore, the interference happens when two beams of light propagate to the NCF. Based on the above analysis, COMSOL Multiphysics simulation software is used for the numerical simulation analysis of the proposed sensing structure. In the simulation, the length of NCF and DCF are 10 mm and 2 mm, respectively. As shown in Fig. 1(b), it can be found that the beam separation and coupling occur at the core-offset and at the NCF, respectively.
Based on the principle of double beam interference, the proposed sensor can be considered as MZI. As mentioned above, the interference occurs at the NCF, and the interference light intensity can be expressed as [24,36]:
λ is the center wavelength of the incident light, nco and ncl represent the effective RI of the core modes and mainly higher-order cladding modes of DCF, and Δneff represents the difference between them, L is the length of the DCF. When φ=(2k+1)π, k represents the order of resonance dip. The resonant wavelengths of dips can be expressed as:
Usually, the interference spectrum has a certain periodicity, and this characteristic is usually described by the free spectral range (FSR):
According Eq. (3) and Eq. (4), we know that Δneff and L are the main parameters that affect the FSR and the wavelengths of the resonant dips. In this paper, a PVA film was adhered to the surface of the corroded DCF. PVA molecular structure contains a large number of hydrophilic hydroxyl groups, which are extremely sensitive to water molecules, so the breaking and recombination of hydrogen bonds can quickly adsorb and desorb water molecules. The process of adsorbing water molecules is shown in the Fig. 2.
The relationship between the RI and the mass fraction of the PVA can be expressed as :
The shifts of the resonant dips can also be used to measure the relative change of ambient temperature (ΔT), which is expressed as :
So, the RH and temperature can be measured simultaneously.
3. Sensing structure manufacturing
3.1 Selection of sensor structure parameters
The sensor structure is composed of SMF, DCF, and NCF. The single core-offset is spliced by manual fusion of the fusion splicer (FITEL S178), the manufacturing process is shown in Fig. 3(a) and (b). Through Fig. 3(c), we can see that the core-offset distance is 9.4 μm.
To verify the rationality of the proposed sensor, the output sprecta of different structures are shown in Fig. 4(a). One is that the DCF is spliced between two SMFs without core-offset, the second is that the DCF is spliced between two SMFs with one core-offset, and the last is the structure proposed in this paper. The results indicate that an uniform transmission spectrum with high extinction ratio can be achieved with the structure of SMF + DCF + NCF + SMF.
Generally, the higher-order cladding modes have higher differential modal group index (Δmeff) due to the lower effective refractive index :
3.2 Chemical etching process
For optical fiber sensors, the chemical etching process can increase sensitivity, which is mainly reflected in two aspects. One is to increase the leakage of the evanescent wave, and the other is to stimulate the higher-order cladding mode. So, a part of the cladding of the DCF is corroded by hydrofluoric acid (HF, Sigma-Aldrich (Shanghai) Trading Co.Ltd.).
The DCF was fixed on the polytetrafluoroethylene plate board, and then a dropper was used to suck and drop 100 µL of 40% HF on the middle of the DCF, as shown in Fig. 6. After 10 minutes of corrosion, the DCF was rinsed with alcohol and deionized water repeatedly, and was left in air to dry for 1 h. As show in Fig. 7(a), we can see that the extinction ratio and FSR become larger, which is due to the higher order cladding modes being excited. The diameter of the corroded DCF was obtained by the scanning electron microscopy (SEM). It can be seen from Fig. 8(a) that its diameter is 96.45 µm.
Figure 7(b) shows the corresponding the FFT. The spatial frequencies of the dominant cladding mode before and after corrosion are 0.04999 nm-1 and 0.05999 nm-1, respectively. The software OptiFiber has been used to calculate the corresponding Δmeff between different LP01 and high-order cladding modes, as shown in Table 1. The wavelength λ0 is 1550 nm, L is 10 mm, by Eq. (13), it can be calculated that Δmeff is 0.0144125975. It is approximately closed to the Δmeff of LP10. The results indicate that the sensor can inspire higher-order cladding modes.
3.3 Preparation and coating of PVA or GO
For the preparation and coating of PVA (P816862, Shanghai Macklin Biochemical Co., Ltd.), 5 g PVA particles were immersed in 100 mL deionized water at 70℃ for 1 h, until the solid particles were fully dissolved. The PVA solution was centrifuged at 6800 rpm/min for 3 h until a uniform and stable 0.05 g/mL PVA solution was obtained. For the coating of PVA, the surface of DCF was cleaned with alcohol, then the DCF was suspended horizontally for about 3 mm, and the prepared PVA was evenly coated on the corrosion area of DCF. Finally, it had been left in air at 25℃ for 48 h to evaporate naturally. For the preparation and coating of GO (XF020, Nanjing XFNANO Materials Tech. Co., Ltd.), the density of GO dispersion was 0.1 mg/mL. The coating method of GO was the same as that of PVA. After standing in air for 24 h at a room temperature of 25℃, a GO film was evenly deposited on the surface of DCF.
The surface morphology of DCF coated with GO/PVA is shown in Fig. 8. It can be seen that the GO/PVA is successfully deposited uniformly on the surface of the DCF. The MZI transmission spectra coated with GO/PVA are shown in Fig. 9. The spectra changes indicate that the effective RI of cladding modes varies due to the coating material.
4. Experimental research and analysis
The experimental platform used in this paper is shown in Fig. 10, which mainly includes MZI sensor, optical spectrum analyzer (OSA, AQQ6370, YOKOGAWA), constant temperature and humidity chamber (CTHC, J-TOPH-22-B, JieXin Testing Equipment Co. Ltd.) and broadband source (BBS, ASE-C + L module, Shanghai Huiya Communication Technology Co. Ltd.). The resolution of OSA is 0.02 nm. The MZI 1 is coated with 0.1 mg/mL GO and the MZI 2 is coated with 0.05 g/mL PVA.
4.1 MZI sensor coated with GO measures RH and temperature
In the RH measurement experiment, the MZI 1 was placed in CTHC keeping constant temperature at 25℃, and the RH was increased from 35% to 95%. Figure 11(a) shows the RH measurement results of MZI 1. It can be seen that as the RH increases, the dips shift to short wavelength. This is because after adsorbing water molecules, the RI of GO decreases . The relationship between the wavelength of resonant dips and RH is shown in Fig. 11(b). The linear fitting results indicate that in the RH range of 35%-95%, the linear sensitivity of dip A, dip B and dip C are 0.072 nm/RH%, 0.095 nm/RH% and 0.122 nm/RH%, respectively. The corresponding linear fitting values are 97%, 98% and 99%, respectively.
For the tempearture characteristics measurement, the MZI 1 was placed in CTHC keeping constant RH at 60%, and the temperature changed from 30℃ to 80℃. Figure 12(a) shows that when the temperature increases the resonant dips are shifted to the long wavelength. As shown in Fig. 12 (b), the linear sensitivity of dip A, dipB and dip C are 0.052 nm/℃, 0.062 nm/℃ and 0.069 nm/℃, respectively. The corresponding linear fitting values are 93%, 95% and 94%, respectively.
Stability is also an important indicator for evaluating sensor performance. For the RH stability, keeping the temperature at 25℃ and the RH at 55% or 75% for 1 h; For the temperature stability, keeping the RH at 60% and the temperature at 30℃ or 70℃ for 1 h, we measured the spectra per 10 min to gain the dip wavelength, as shown in Fig. 13. When the RH is 55%, the maximum fluctuations of the wavelengths of resonant dip A, B and C are 0.15 nm, 0.12 nm and 0.15 nm, respectively. When the RH is 75%, the maximum fluctuations are 0.13 nm, 0.12 nm and 0.16 nm, respectively. When the temperature is 30℃, the maximum fluctuations of dip A, B and C are 0.13 nm, 0.14 nm and 0.13 nm, respectively. And when the temperature is 70℃, the maximum fluctuations are 0.14 nm, 0.15 nm and 0.14 nm, respectively.
4.2 MZI sensor coated with PVA measures RH and temperature
In the RH measurement experiment, The MZI 2 was also placed in CTHC keeping constant temperature at 25℃, and RH was increased from 30% to 95%.
PVA has a certain swelling property, so the RI of PVA will decrease as the mass fraction of PVA decreases. As a result, ncl decreases, and Δneff increases. As expected, as the RH increases, the resonant dips shift to long wavelength, the dip A shifted from 1542 nm to 1557.86 nm (15.86 nm), the dip B shifted from 1556.52 nm to 1572.18 nm (15.66 nm), as shown in Fig. 14(a). The relationship between resonant dips wavelength and RH is shown in Fig. 14(b). The linear fitting resluts indicate that within the RH range of 30%-95%, the RH linear sensitivity of dip A and dip B are 0.256 nm/RH% and 0.248 nm/RH%, and the corresponding R2 values are 99% and 99%, respectively.
The performance of the sensor coated with different concentrations of PVA was tested. We experimentally verified the sensor performance of PVA coated with 0.04 g/mL, 0.05 g/mL and 0.06 g/mL concentrations, respectively. The experimental results are shown in Fig. 15(a) and (b) and Fig. 14(a), respectively. Figure 15(a) and (b) show that the dip shifts is less than 3 nm when the RH increases from 30% to 70%, which is small than that of the sensor coated with 0.05 g/mL PVA solution (Fig. 14(a)). This is because the surface morphology of the PVA film will affect its ability to adsorb water molecules. The surface morphology of the 0.05 g/mL PVA shown in Fig. 8(d) is uniform and rough, which is benefitted for the adsorption and desorption of a large number of water molecules.
For temperature measurement, the MZI 2 was placed in the CTHC keeping constant RH at 60%. The measurement result of MZI 2 is shown in Fig. 16. The resonant dip A and dip B shift to the long wavelength when the temperature inceases from 20℃ to 80℃. The corresponding sensitivities are 0.153 nm/℃ and 0.154 nm/℃, and the R2 values are 99% and 99%, respectively.
The method used for the stability test of MZI 2 is the same as the method that of MZI 1. For RH measurement stability, keeping the temperature at 25℃ and the RH at 50% or 70% for 1 h; For temperature measurement stability, keeping the RH at 60% and the temperature at 30℃ or 70℃ for 1 h, we had also measured the spectra to gain the dips wavelength, as shown in Fig. 17. When the RHs are 55% and 75%, the maximum fluctuations of resonant dip A are 0.08 nm and 0.11 nm, respectively; for the resonant dip B, the maximum fluctuations are 0.09 nm and 0.12 nm, respectively. When the temperatures are 30℃ and 70℃, for the resonance dip A, the maximum fluctuations are 0.1 nm and 0.11 nm, respectively; for the resonance dip B, the maximum fluctuations are 0.12 nm and 0.13 nm, respectively. The wavelength fluctuation may be caused by machine vibration in the CTHC. Compared with MZI 1, the RH and temperature sensitivities of MZI 2 coated with PVA are increased by 2 times and the corresponding linearity are also relatively good. Therefore, this paper uses MZI 2 to measure RH and temperature simultaneously.
Repeatability is an important indicator for evaluating sensor performance. After one month, the same method as above has been used to test the MZI 2 sensor again. The measurement results are shown in Fig. 18. For RH measurement, the maximum error rates of dip A and dip B are 0.15% and 0.07%, respectively. For temperature measurement, the maximum error rates of dip A and dip B are 0.068% and 0.08%, respectively. The result indicate that the sensor has a good repeatability.
According to the sensitivities of RH and temperature of dip A and B, Eq. (11) can be rewritten as:
Table 2 shows the performance of sensors proposed in reported work and in this paper. By comparison, the sensor proposed in this paper has the characteristics of high sensitivity and large measurement range.
The single core-offset MZI RH and temperature sensor coated with PVA was proposed and demonstrated. The method of chemical etching and coating of PVA film enhances the sensitivity of the sensor to the external environment. In the RH range of 30%-95%, a RH sensitivity of 0.256 nm/RH% can be achieved with a linear coefficient of 99%. While in the temperature range of 20℃-80℃, the temperature sensitivity of 0.153 nm/℃ can be achieved with a linear coefficient of 99%. The sensor has characteristics such as good stability, low cost, high sensitivity and repeatability, which has potential application in the complex environment of multiple fields.
National Natural Science Foundation of China (11674109, 61774062); Natural Science Foundation of Guangdong Province (2016A030313443); Science and Technology Planning Project of Guangdong Province (2017A020219007).
The authors declare no conflicts of interest.
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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