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Abstract
We have manufactured an intensity modulated optical fiber SMDMS sensor with hydroxyethyl cellulose (HEC) hydrogel coating for simultaneous measurement of RH and temperature. The SMDMS sensor was manufactured by splicing single-mode fiber (SMF), multi-mode fiber (MMF), dispersion compensation fiber (DCF), MMF, and SMF in sequence to form a structure of SMF + MMF + DCF + MMF + SMF (SMDMS). The cladding of MMFs and DCF were corroded by hydrofluoric acid (HF) and coated with HEC hydrogel to excite a strong evanescent field and increase the sensitivity of the SMDMS sensor. The adsorption of water molecules by HEC will cause a change in the effective refractive index of cladding mode, which will eventually change the intensity of the transmission spectrum. The experimental results indicate that the sensitivities are 0.507 dB/%RH and 0.345 dB/°C in the RH range of 30%−80% and temperature range of 10°C−50°C, respectively. At last, a dual-parameter measurement matrix is constructed based on the experimental results to achieve the simultaneous measurement of RH and temperature. The SMDMS sensor has the advantages of high sensitivity and good robustness, and has potential application prospects in daily life and other fields.
© 2022 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Accurately monitor relative humidity (RH) and temperature play a very important role in industrial production, health care, food packaging, scientific research, weather monitoring and other fields. Optical fiber sensors have incomparable advantages over traditional electronic humidity and temperature sensors, mainly including small size, light weight, high accuracy, corrosion resistance, and good electromagnetic tolerance [1,2]. Based on the above advantages, the researchers invented and manufactured a variety of optical fiber sensors. There are mainly surface plasmon resonance [3], long period gratings [4], Mach-Zehnder interferometer (MZI) [5], fiber Bragg gratings (FBG) [6], Sagnac interferometer [7], Michelson interferometer (MI) [8], Fabry-Perot interferometer (FPI) [9], etc.
Studies have shown that humidity sensitive materials coated on sensing area of optical fiber sensor can effectively improve the sensitivity of sensor. Common humidity-sensitive materials include graphene oxide (GO) [5], polyvinyl alcohol (PVA) [10–12], GQDs- PVA [13,14], GO-PVA [15,16], calcium alginate hydrogel [17,18], SnO2 [19], polyimide [20], chitosan [21], polymethyl methacrylate [22], hydroxyethyl cellulose/polyvinylidenefluoride [23], etc. Among them, the humidity-sensitive properties of hydrogels are relatively excellent [10–12,17,18,20,23]. Hydroxyethyl cellulose (HEC) hydrogel is a gel formed by polymer polymerization using water as a dispersion medium, and a cross-linked three-dimensional network structure is formed in water. HEC contains a large number of hydrophilic groups (carboxyl groups, hydroxyl groups, etc.) and hydrophobic groups (alkyl groups, etc.), which can fast adsorb and release water molecules, and are very sensitive to changes in moisture. Meanwhile, HEC hydrogel has cohesive properties and is easy to form a film to wrap the optical fiber. The HEC film can increase the sensor's ability to absorb water molecules, and can also protect the optical fiber.
In this article, a SMDMS optical fiber RH and temperature measurement sensor coated with HEC hydrogel was manufactured, and the sensitization performance of HEC hydrogel was verified. From theoretical research, the intensity modulation of the transmission spectrum can be achieved by adjusting the split ratio between the cladding and the core. The adjustment of the splitting ratio is mainly determined by the refractive index (RI) of the cladding. For this reason, all the cladding of MMF and part of the cladding of DCF are corroded by hydrofluoric acid (HF, Sigma-Aldrich (Shanghai) Trading Co. Ltd). At the same time, HEC (H810927, Shanghai Macklin Biochemical Co., Ltd.) is coated on the corroded optical fiber area to further promote the sensing performance of the SMDMS sensor. The experimental results show that the RH detection range of the SMDMS sensor is 30%−80%, and the highest detection sensitivity is 0.507 dB/%RH. The temperature detection range of the SMDMS sensor is 10°C−50°C, and the highest detection sensitivity is 0.346 dB/°C. At last, a dual-parameter measurement matrix was constructed based on the experimental results to realize the simultaneous measurement of RH and temperature.
2. SMDMS sensor structure and principle
The structure of SMDMS sensor diagram is shown in Fig. 1(a), which is manufactured by splicing single-mode fiber (SMF), multi-mode fiber (MMF), dispersion compensation fiber (DCF), MMF and SMF in sequence. The core and cladding diameters of MMFs and DCF are 105 /125 µm and 4.5 /110 µm, respectively. All the cladding of MMFs and part of the cladding of DCF are corroded by HF acid. This method can excite multiple higher-order cladding modes, but usually only one cladding mode is dominant. [24]. HEC hydrogel is coated on the corroded optical fiber area to improve the moisture absorption capacity of the sensor.
Fig. 1. (a) SMDMS structure diagram. (b) SMDMS structured light field propagation simulation diagram.
COMSOL Multiphysics software was used to simulate the propagation of the beam in the SMDMS structure to verify whether the structure can form interference. The length of each part of the optical fiber set in the simulation is the same as the actual one, the DCF and MMFs are 10 mm and 8 mm, respectively. The simulation result is shown in Fig. 1(b). Since the core diameters of SMF and MMFs do not match, the incident light will be separated at the MMF and then transmitted to the cladding and core of the DCF respectively. After the two beams of light are transmitted through the DCF, the fundamental core mode and the dominant cladding mode are coupled at the MMF. The transmission spectrum can be expressed as [25]:
Ico and Icl represent the core mode intensity and the mainly cladding mode intensity in the interference arm, respectively. The phase difference between the two modes is represented by φ, which is mainly determined by the incident light input wavelength λ, Δneff, and the interference arm length L. So φ can be expressed as: when cosφ=−1, the intensity of the transmission spectrum has a minimum value, which can be expressed as:From Eq. (1–3), the intensity of the transmission spectrum can be controlled by Icl and Ico.
Studies have shown that the moisture-sensitive material layer can act as a new optical fiber cladding when it is coated on the fiber with the cladding being moved. The RI of the moisture-sensitive material will change due to the adsorption of water molecules, which will causes the leak of light. As a result, the ratio between Icl and Ico change, and the intensity of the transmission spectrum can be modulated [17,25]. In general, interference arm is only treated in the wavelength modulation type optical fiber RH sensor, but it can be found that the intensity of the resonant dips also changes when the wavelength of the resonant dips changes regularly [5,11–14,26–28]. Therefore, COMSOL Multiphysics software was used to simulate the intensity of the optical fiber core mode and cladding modes under different cladding RI. In the simulation process, the cladding RI of the optical fiber at the interference arm was only changed. As shown in Fig. 2, the intensities of the optical fiber cladding modes and core mode decrease when the RI of the cladding decreases. The resutls indicate that a special processing for sensitivity enhancement and etching of the optical fiber in the optical coupling area and the interference arm area can further improve the intensity response of the sensor.
HEC is a modified cellulose with a large number of oxygen-containing functional groups. It has good thickening, bonding, film forming, and moisture retention characteristics, and it is sensitive to changes in water molecules. An energy dispersive spectrometer (EDS) is used to analyze the composition of elements in HEC, as shown in Fig. 3. As we can see that C and O are the main components of HEC with proportion of 49.46% and 37.94%, respectively. The model diagram of HEC adsorption of water molecules is shown in Fig. 4. When HEC adsorbs water molecules, the water molecules connect to the HEC tightly due to the breakage and recombination of hydrogen bonds. So, the RI of the HEC film coated on the fiber changes when it adsorbs water molecules. As a result, the adjacent optical fiber cladding is affected and the intensity of the transmission spectrum is changed.
Due to the thermal expansion effect and the thermo-optical effect, the exact measurement of the optical fiber RH sensor is affectted by temperature. Therefore, simultaneous detection of temperature and RH is very necessary. Research has shown that it is a feasible method to demodulate the sensitivity coefficient matrix of the double resonant dips [14,29,30]. The intensities of the resonant dips are affectted by RH and temperature, and the relationship can be expressed as:
here, the dip 1 and 2 sensitivities of RH and temperature are represented by A and B, and C and D, respectively. ΔIdip1 and ΔIdip2 are the intensity changes of the resonant dip 1 and dip 2, respectively. The amount of change in RH and temperature are represented by ΔRH and ΔT. Based on matrix theory, Eq. (4) and (5) can be rewritten as:Inverting the matrix of Eq. (6), Eq. (7) can be obtained, in which RH and temperature can be gotton easily with the experimental results.
3. SMDMS sensor production process
3.1 Fabrication of SMDMS structure
A fusion splicer (FITEL S178) is used to splice the SMF, MMF, DCF, MMF and SMF in sequence to form a SMDMS structure, as shown in Fig. 5(a-b). In order to stimulate the strong evanescent field and high-order cladding mode and enhance the SMDMS sensor's response to environmental changes, the SMDMS sensor is fixed on the Poly tetra fluoroethylene (PTFE) board and used 40% HF acid to corrode the cladding of MMFs and DCF, as shown in Fig. 5(c-d). Through repeated experiments, the transmission spectrum with obvious extinction ratio (ER) can be obtained by etching for 12 minutes, as shown in the Fig. 6(a). In order to further analyze the changes brought about by the corrosion treatment of the optical fiber, Fast Fourier transform (FFT) is performed on the spectra obtained before and after corrosion, as shown in Fig. 6(b). As we can see that the amplitude and spatial frequency of the FFT spectrum after corrosion have both increased. This is because the evanescent field is enhanced and the higher-order cladding mode is excited after the etching treatment [31]. The diameter of the MMFs and DCF after corrosion can be observed through scanning electron microscope (SEM), as shown in Fig. 7. At this time, the diameters of MMFs and DCF were 101.8 µm and 99.59 µm, respectively. It indicates that the cladding of MMF is absolutely corroded, while the cladding of DCF is partly corroded.
Fig. 5. SMDMS sensor production process: (a-b) SMDMS splicing, (c-d) SMDMS corrosion, (e-f) HEC coating.
Fig. 6. (a) SMDMS structure before and after corrosion transmission spectrum. (b) FFT after SMDMS structure corrosion.
To improve the length of the interference arm, the output spectra with different DCF lengths were measured, as shown in Fig. 8. In general, it is better to contain 3–5 cycles of resonant dips, which is beneficial of analyze the spectral changes obtained in the experiment [5]. The wavelength used in this experiment is 1530 nm-1600 nm, so the wavelength range of each cycle should be 14 nm-23 nm. As we can see that the wavelength interval between the dips is 15.32 nm when the length of the DCF is 10 mm, which is agreed with theoretical result.
3.2 Preparation and coating of HEC
To improve the sensitivity of the sensor, the experiments with different concentrations of HEC were carried out. At first, 1 g, 2 g and 3 g HEC particles were immersed in 100 mL of deionized water, respectively. Then, the HEC was quickly placed on a centrifugal mixer and stirred at a speed of 5800 rpm/min for 1 h until the HEC was completely dissolved and dispersed evenly. In order to avoid condensation of the HEC hydrogel, the temperature of the preparation process was controlled at 25°C.
The process of coating HEC hydrogel on sensitive areas of MMF and DCF is mainly divided into four steps. Firstly, before coating the HEC hydrogel, the corroded areas of MMFs and DCF was cleaned repeatedly with deionized water and absolute ethanol. Second, in order to make the HEC hydrogel completely wrap the optical fiber, the cleaned MMFs and DCF was placed on the PTFE and suspended it 3 mm high. Third, a dropper was used to evenly coat the prepared HEC hydrogel on the surface of MMFs and DCF until the MMFs and DCF were completely covered by the HEC hydrogel, as shown in Fig. 5(e-f). Finally, the SMDMS sensor covered with HEC hydrogel was placed in drying box at 25 °C for 12 h and a uniform and stable hydroxyethyl cellulose film was formed in the sensitive area of MMFs and DCF. The transmission spectra before and after HEC hydrogel coating are shown in Fig. 9. It can be seen that the intensity and wavelength of the resonant dips changes. This is because the RI of HEC is bigger than that of air, and the evanescent field is affected. Figure 10 shows the morphology of different concentrations of HEC hydrogel coated on MMFs and DCF. As we can see, the surface morphology formed by different concentrations of HEC hydrogel is different, and the uneven morphology is more conducive to the adsorption and maintenance of water molecules.
Fig. 10. The morphology of MMFs (a, c, e) and DCF (b, d, f) surface layer coated with different concentration HEC. (a) and (b) 0.01 g/mL, (c) and (d) 0.02 g/mL, (e) and (f) 0.03 g/mL.
4. Experimental analysis and discussion
In this experiment, optical spectrum analyzer (OSA, AQ6370, YOKOGAWA.) with a resolution of 0.02 nm, broadband source (BBS, ASE-C + L module, Shanghai Huiya Communication Technology Co. Ltd.), and a constant temperature and humidity chamber (CTHC, J-TOPH-22-B, JieXin Testing Equipment Co. Ltd.) were used to study the characristics of the sensor, as shown in Fig. 11.
4.1 RH response of SMDMS sensor
For RH measurement, the SMDMS sensor was placed in the CTHC, and the temperature was kept at 25°C. The RH was increased from 30% to 80%, and the transmission spectrum was recorded every 10% change in RH.
In order to determine whether HEC can play a sensitization effect, the SMDMS sensor without the HEC hydrogel coating was tested first. Figure 12 shows the experimental results. As we can see that the intensity and wavelength of the dip A and B varies small when the RH increases. Figure 12(b) indicates that the sensitivities of resonant dip A and B are 0.009 dB/%RH and 0.066 dB/%RH, respectively.
Fig. 12. (a) RH response of SMDMS sensor without HEC coating. (b) The intensity of the resonant dips are linearly fitted to RH.
To verify the sensitization effect of HEC hydrogel, it is necessary to compare different concentrations of HEC. Figure 13–15 shows the RH experimental results of the SMDMS sensor when it was coated with different concentrations of HEC hydrogel, in which obvious changes of the intensities of dips can be seen. The close combination of HEC hydrogel and optical fiber can be regarded as a hybrid waveguide. As the RH increases, HEC hydrogel absorbs more water molecules. As a result, the RI of HEC decreases and a strong evanescent field is excited [32]. The mode field excitation of the MMF will be affected, and the intensity of the core mode and cladding modes leaked into the DCF will be reduced. Meanwhile, the effective RI of the DCF cladding decreases as the RI of the HEC decreases, so that the light transmitted in the DCF leaks further. Eventually, the intensity of the spectrum decreases. The change in the RI of the DCF cladding made Δneff change, which is why the wavelength of the resonant dips shifted.
Fig. 13. (a) RH response of SMDMS sensor coated with 0.01 g/mL HEC. (b) The intensity of the resonant dips are linearly fitted to RH.
Fig. 14. (a) RH response of SMDMS sensor coated with 0.02 g/mL HEC. (b) The intensity of the resonant dips are linearly fitted to RH.
Fig. 15. (a) RH response of SMDMS sensor coated with 0.03 g/mL HEC. (b) The intensity of the resonant dips are linearly fitted to RH.
The experimental results of different HEC hydrogel concentrations in the RH range of 30%-80% are listed in Table 1. As we can see that the sensitivity of the sensor will become better as the concentration of HEC hydrogel increases. This is because the surface of the coating becomes rougher and uneven when the concentration of HEC increases. The uneven and rough topography makes it easier to store and release water molecules. Compared with Fig. 12, we can find that the sensitivity of the sensor is significantly improved after it is coated by HEC hydrogel. It should be noted that the sensitivity of different resonant dips is different, because the effective RI of the cladding mode of the same order is different at different wavelengths [33,34 ]. We also found that the too high-concentration HEC coating will have a smoother morphology, which is not conducive to the adsorption of water molecules. In Table 1, the sensitivity and linearity of the experimental results coated with 0.03 g/mL HEC are the most excellent. In the RH range of 30%−80%, the sensitivities of resonant dip A and B are 0.437 dB/%RH and 0.507 dB/%RH, and the correspongding R2 are 98% and 99%, respectively. Therefore, we believe that 0.03 g/mL concentration of HEC is the most suitable for SMDMS sensors.
Table 1. Comparison of measurement results of SMDMS sensors coated with different HEC concentrations
4.2 Temperature response of SMDMS sensor coated with 0.03 g/mL HEC
The SMDMS sensor was placed in the CTHC, and the RH was kept at 50%. The temperature was increased from 10°C to 50°C, and the transmission spectrum was recorded every 10°C. Due to the thermal expansion effect and the thermo-optical effect, the intensities of the core mode and the cladding moes are affectted and the intensity of the transmission spectrum changes. The experimental result is shown in Fig. 16. In the temperature range of 10°C-50°C, the sensitivities of resonant dip A and B are 0.237 dB/°C and 0.346 dB/°C, and the corresponding R2 are 99 and 99%, respectively.
Fig. 16. (a) Temperature response of SMDMS sensor. (b) The intensity of the resonant dips are linearly fitted to different temperature.
4.3 Simultaneous measurement of RH and temperature
Repeatability and stability are important indicators to evaluate the performance of the SMDMS sensor. The SMDMS sensor was placed for a week, and used the above method to retest RH and temperature. The results of repeated experiment are shown in Fig. 17. For dip A, the maximum error rates for RH and temperature repeatability are 1.2% and 1.6%, respectively. For dip B, the maximum error rates for RH and temperature repeatability are 1.9% and 1.4%, respectively. It can be seen that the repeatability of the sensor is excellent.
For the RH stability measurement of the SMDMS sensor, the SMDMS sensor was placed in the CTHC for 1 h, and the RH was kept constant at 50% and the temperature was constant at 25°C. For the temperature stability measurement of the SMDMS sensor, the SMDMS sensor was placed in the CTHC for 1 h, and the RH was kept constant at 50% and the temperature was constant at 30°C. The intensity values of dip A and dip B were recorded every 10 minutes. Figure 18 shows the measurement results of SMDMS sensor stability. For dip A, the standard deviations of RH and temperature stability are 0.08861 and 0.14593, respectively. For dip B, the standard deviations of RH and temperature stability are 0.14125 and 0.15781, respectively. It can be seen that the SMDMS sensor has small fluctuations, which is mostly caused by the jitter of the instrument.
According to the experimental results, the RH and temperature dual parameter measurement matrix can be expressed as:
As the environment change, the values of the intensity change are recorded and substituted into Eq. (8), and then the changes of RH and temperature can be gotten simultaneously.
For matrix feasibility test, the SMDMS sensor was placed in the CTHC and the initial RH and temperature were set to 70% and 25°C, respectively. Then the RH was decreased to 60% and the temperature was increased to 40°C. The transmission spectra of the sensor response in both environments were recorded, as shown in Fig. 19. It can be concluded that ΔIdipA and ΔIdipB are 1.599 dB and 0.759 dB, respectively. ΔIdipA and ΔIdipB are substituted into Eq. (8), and the calculation results of ΔRH and ΔT are −12.0275% and 15.43054°C, respectively. The error rate of the measured value and the theoretical value are 2.0275%/RH and 0.43054°C, respectively. We think it is due to two factors. One is the sensor structure itself or the RH sensitive material. The other one is the instability of the CTHC.
In order to evaluate the sensors proposed in this article, the same type of fiber optic RH and temperature sensors are listed in Table 2. It can be seen that the SMDMS sensor proposed in this article has higher sensitivity.
Table 2. Performance comparison of optical fiber RH and temperature sensors.
5. Conclusion
The SMDMS structure optical fiber RH and temperature sensor coated with HEC was manufactured and studied. An etching and sensitization processing were performed to improve the intensity response of the sensor to RH and temperature. Meanwhile, the sensing performance of different concentrations of HEC hydrogels has been verified. For SMDMS sensor, the measuring ranges of RH and temperature are 30%−80% and 10°C−50°C, respectively, and the corresponding measuring sensitivities are 0.507 dB/%RH and 0.346 dB/°C, respectively. The SMDMS sensor has the advantages of high sensitivity, good robustness, and low manufacturing cost, and has potentialapplication prospects in daily life and other fields.
Funding
National Natural Science Foundation of China (11674109, 61774062); Science and Technology Planning Project of Guangdong Province (2017A020219007); Natural Science Foundation of Guangdong Province (2016A030313443); South China Normal University "Challenge Cup" Gold Seeds Cultivation Project (21GDKC05).
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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