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Abstract
In this paper, a fiber optic Fabry–Perot (FP) for relative humidity (RH) sensing is presented. The proposed FP cavity is constructed by splicing a 50-mm length of no-core fiber (NCF) in a single mode fiber. Then, the end side of the NCF is coated with a polyvinyl alcohol (PVA) thin film membrane with different thicknesses (1, 2, 3, 4 µm respectively) to work as a mirror. The fringes pattern of the FP undergoes a spectral shift owing to the alteration in the PVA refractive index with the ambient RH alternative. The highest obtainable sensitivity was observed at thickness of 3 µm, which is about 0.866 nm/RH%. After that, the diameter of the NCF is tuned from 125 to 65 µm using hydrofluoric acid (HF40%) to maximize the evanescent field and thus improved sensitivity to about 0.908 nm/RH% at the diameter of 95 µm for 30% to 90% RH range. The sensor shows good stability, and easy fabricated.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Traditional humidity detection methods comprise the infrared optical absorption hygrometer, mechanical hygrometer, electronic sensors, chilled mirror hygrometer, and wet/dry bulb psychomotor [1–4]. With the fiber optics progresses and fiber sensing technique, optical fiber relative humidity (RH) sensors have been turned into a novel and promising humidity detection method. Contrasted with the traditional humidity sensors, RH optical fiber sensors have many excellent benefits, such as great sensitivity, size compactness, extreme flexibility, suitable immunity to electromagnetic interference, which make them appropriate for utilizing in harsh and extreme surroundings [1,5–8]. The basic principle for sensing is certain physical/optical parameters of fiber including spectral, output power, polarization, or reflectivity will effected by the ambient RH alternative. Therefore, numerous optical fiber humidity sensors with a diverse construction or sensitive coating substances have been established. Example of general optical fiber RH sensors include Fabry-Perot (F-P) fiber sensors [9–12], fiber Bragg gratings (FBGs) [13,14], long period gratings (LPGs) [15,16], Mach-Zehnder interferometer (MZI) [5,6,17,18].
Numerous types of optical fiber humidity sensors have been made using diverse sensitive substances that are coated at the tip side of the fiber, such as CuO [19], gold [5], polymers [6,20–23] graphene oxide (GO) [24], graphene quantum dot [12], silica gel [25], MoS2 [26], chitosan [27], and agarose [27,28]. Where the variation of humidity maintains the effects of the spectral, intensity, or phase of light signal in the optical fiber. The signal can be further directly influenced by the humidity sensitive substance. As a result, this needs the humidity sensitive substance to be adhesive and transparent [12].
In this work, used polyvinyl alcohol (PVA) which is biocompatible, water-soluble, a frequently used hygroscopic material with excellent optical properties for the construction of wavelength demodulated optical fiber RH sensors. In addition, it optically transparent that contain R-OH (R= -CH2-CH-) as monomer units, which own homogeneous dispersion and good environmental stabilities. It can be easily dissolved in hot water, and coated onto a fiber surface by a dip-coating technique. Where the refractive index (RI) of PVA can be changed dramatically by the surrounding RH [29].Thus, its utilized as a sensitive coating substance since it adhesive on the no-core fiber (NCF's) flat end using the dip coating. [30]. Therefore, effective refractive index (RI) of PVA will be altered when the RH changes, and consequently affecting the resonance wavelength. Accordingly, a simply constructed optical fiber Fabry-Perot humidity sensor combined of a traditional single mode optical fiber and PVA is proposed and constructed. The interference spectral fringes will be altered when the PVA will straight influence the light signal, by changing the refractive index and volume of the PVA caused by the humidity. The proposed sensor has a straightforward construction method and excellent performance. The humidity spectral response, linearity, and the repeatability have been investigated in the work, in detail. The experimental results displayed its excellent performance for relative humidity detections.
2. Materials and methods
2.1. Sensor fabrication
The schematic of the humidity sensor is based on polyvinyl alcohol Fabry–Perot (HS-PVA) and it is shown in Fig. 1. The construction process consists of two steps: The first step includes the, construction of the SMF-etched NCF and the second step deposition of PVA onto the end side of the PVA. The transmission spectrum via the SMF-etched NCF fiber structure with different etched NCF was first studied. The SMF-etched NCF structure is made through splicing a section standard SMFs (Corning SMF-28) to a section of 50-mm length of NCF (FG125LA from Thorlabs) with a flat end of NCF. The core/cladding diameters of the standard SMF are 9∕125 µm. The NCF has the similar cladding diameter of 125 µm.
The construction process of SMF-etched NCF structure starts with striping acrylate coating from NCF and SMF, then the NCF is spliced from one sides with SMF by utilizing a fusion splicer (Fujikura FSM-60S), while the cleaver is used to get a flat end of the other side of the NCF. After that, the fabricator spliced SMF-NCF structure is set in the quartz substance U-shaped groove. This groove is utilized to hold the etching acidic solution (HF 40%- from Himedialabs Company). Both sides of the U-shaped groove are tightly sealed. This causes a reduction in NCF diameter from 125 to 65 µm via adjusting the etching period. Different diameters of NCF of 110, 95, 80, and 65 µm were attained via the chemical etching method. To control the diameters of the NCF, there are two important factors will be carefully adjusted, which are etching time, and an average etching rate. The NCF diameter has been decreased from 125 µm to 65 µm, with increased etching time in the period of 7.5, 15, 22.5, and 30 min, respectively. After examining each diameter, it is observed that the optimum NCF diameter was 95 µm as it will be explained in Sec. 3.
2.2 PVA coat preparation
PVA thin film membrane has been made by several steps. Firstly, PVA aqueous solution was made by adding 1 g of PVA powder to 100 ml of deionized water that is resulting 10 mg/ml concentration of PVA solution. Dissolution achieved by forcibly stirring the mixture for 45 min at 90°С using magnetic stirrer until the solution was clear and homogenous. A length of 50 mm and diameter 125µm of NCF has been dip coated through submerge the end of NCF in the PVA solution at a speed of 0.5 mm/s. The thickness of PVA as a dip coat will be depended on the viscosity of the liquid and the drawing speed of the coating process. The sensor was dried for 10 minutes at 55 °С after being immersed in the PVA solution for various times (10, 20, 30, 40 minutes) to achieve deposition on the tip with thickness 1, 2, 3, 4 µm, respectively. Then, the sensors tested after 24 hours. Since the PVA effective refractive index changes with ambient RH, the propagation constants for each guided mode within the NCFs will change as the environmental refractive index changes, causing shifts in the output spectra, as shown in Fig. 2.
3. Experimental setup
To characterize the sensor response to RH%, it placed in a closed chamber with RH% ranging from 30% to 90%, where the increase in RH% was 10% and steadied for 10 seconds. The operating systems of the controlled RH chamber consist of three fans as shown in the Fig. 3.
Fig. 2. The SMF-etched NCF structure under microscope imaging. This figure shows that the surface of the etched fiber is quite smooth with flat cleave end leading in a reduce in scattering loss.
One fan was used to supply the chamber with the water vapor from a humidifier at a constant temperature of 27.7 ◦C. The function of the two other fans was to accelerate the humidity distribution inside the chamber as well as controlling the humid air inside the chamber. The RH was measured by utilizing an electronic hygrometer embedded inside the chamber. The proportional flowed of humid and dry air into the chamber was used to modulate the RH within the chamber. In this work, the effect of temperature was neglected during the experimentations.
4. Sensing principles and operation
The broadband light source with a wavelength range of 1550–1650 nm (model SLD1550S-A1 from Thorlabs) was guided to the sensing region by used circulator, then the reflected signal guided from the sensor to OSA optical spectrum analyzer (Yokokawa, Ando AQ6370) with resolution 0.02 nm. A hygrometer was used to calibrate the humidity levels.
As seen in Fig. 1, light travels through the splicing region, where the light modes will split (along the NCF) to high order modes (cladding modes) and fundamental modes. The modes will travel a distance before reflected back at the ends (mirror) of NCF. After that, the modes will be reassembled at the same splicing region after traveling 2L along the NCF. Because the fundamental mode and high-order modes have different trajectories, interference will occur in the fusion region, resulting in multi-mode interference (MMI) [31].
When the high-order and fundamental modes are excited and propagated within the NCF, they will interfere and cause multi-mode interference (MMI). The reflected wavelength, $\mathrm{\lambda}_{\circ}$ can be computed using Eq. (1) [32]:
The refractive index, diameter, length, and the interference order number of NCFs are represented by n, D, L, and p, respectively. When considering the evanescent field created at the NCF/external medium interface, the diameter can be considering an effective value of D + 2Z, where Z is the penetration depth. This parameter can be obtaining by using Eq. (2). [33]:
where n2 denotes the refractive index of the surrounding medium, and θ is the incidence angle at the NCF/surrounding medium interface.5. Result and discussion
The effect of RH monitoring has been studied using an planned fiber sensor coated with varied PVA thickness layers, with the slowly increasing in RH over the range from 30% to 90% RH in 10% steps at a temperature of 27.7°C, as shown in Fig 4. The spectra shifted toward longer wavelengths (redshift) with an increase in RH, which corresponds to an increase in external refractive index around the sensor. This can be explained by: as the RH increases, the PVA coating absorbing more water molecules from the atmosphere. The refractive index would decrease as water molecules are absorbed into the PVA film. The obtainable sensitivity at different coating thicknesses which is shown in Table 1. From these results, it can be observed also, the highest RH sensitivity obtained with 3 µm coating thickness, which represents the optimum thickness layer. Some papers show that the sensitivity of RH increases with increasing the coating thickness. This behavior was also observed and described in other papers that were publish [23,34–36].
Fig. 4. Reflection spectrum of planned sensor with various PVA thickness coated (a) 1 µm,(b) 2 µm,(c) 3 µm,(d) 4 µm.
Table 1. Sensitivity of the planned RH fiber sensor with a length of 5cm and a diameter of 125µm at various coating thicknesses
The linear fitting for length 50 mm immersed in PVA with different times, which shown in Fig. 5. The sensitivity increased as the time-immersed increases when the thickness of PVA increases from 1 µm to 3 µm. However, PVA thickness at 4 µm possesses the lowest sensitivity; this might be attributed to increase in losses at the reflective coated. When the RH increase, the PVA expands as it absorbs moisture, leads to lowering its refractive index progressively. As a result, the RI decreases as the coating swelling increases, lead to reducing the wavelength of the reflection dip. In other side, the planned sensor with 3 µm thickness coating possesses a good polynomial fitting coefficient (R2) of about 97.5%.
Fig. 5. Linear fitting of planned sensor for length 5cm, diameter 125 µm with various PVA thickness coated (a)1 µm,(b) 2 µm,(c) 3 µm,(d) 4 µm.
After that, the diameter of NCF decreased by using an etching technique, in which its end cleaved has been coated with 3µm thickness of PVA thin film, which represents the highest sensitivity thickness as shown by the above results. The values of the sensitivity with different NCF diameter are shown in Table 2. The spectral response and the linear fitting are displayed in Figs. 6 and 7.
Fig. 6. Reflection spectra of NCF for length 5cm,thickness 3 µm and different diameters (a) 110 µm,(b) 95 µm,(c) 80 µm,(d) 65 µm.
Fig. 7. Linear fitting for length 5cm of the planned sensor with different NCF diameter (a) 110µm, (b) 95 µm,(c) 80µm and (d) 65µm with constant PVA coating thickness with 3 µm.
Table 2. The values of sensitivity with different NCF diameter with constant coating thickness with 3µm.
Table 3. Comparison with other optical fiber humidity sensors.
From the results in Table 3, it can be observed that the wavelength shifted toward longer wavelengths as the diameter decreases (red shift) under the condition of the RH changed from 30% to 90%RH. It is clear that the NCF diameter has a direct effect on sensitivity. As compared to other diameters, the NCF with a diameter 95µm has the highest sensitivity. In addition to increasing of the sensitivity, the NCF with etched diameter 95 µm also assists to enhance the polynomial fitting coefficient (R2) to more than 98.86%. In this work, the NCF length of 50 mm, diameter of 95 µm and PVA coating thickness of 3µm has represented the highest sensitivity, as shown in Fig. 7.
The repeatability and stability are an essential property needed to determine for practical applications. To check the repeatability, the same experiment was performed in the same order after some period of time. To confirm the stability and the repeatability of the planned sensor, the sensor tested again after one week. The proposed sensor with diameter of 95 µm and PVA coating thickness of 3µm showed a sensitivity of 891 pm/RH%, which this confirms the good stability, the spectral response and the linear fitting as shown in Fig. 8.
Fig. 8. (a) Spectral response, and (b) linear fitting for planned sensor of NCF for length 50 mm, 95 µm diameter, and PVA coating thickness of 3µm, after one weak.
The planned sensor based polyvinyl alcohol Fabry–Perot (FP) shows the small fluctuation in output spectra with maximum hysteresis of about 1 nm. In addition, the average error bar in the figure shows the maximum fluctuations during the examination. Error bar (standard deviation) for both experiments has been evaluated; as shown in Figs. 9. The results indicate that both sensing measurements are almost similar with standard deviations of 19.72% and 19.38%, respectively.
6. Conclusion
For RH measurement, a reflection optical fiber Fabry–Perot (FP) sensor based on NCF coated with PVA has been proposed and construction. This work consists of two steps, the first one, a NCF with length 50 mm, diameter 125 µm immersed its end cleaved side in PVA solution for different times (10, 20, 30, 40 min) to attained different coating thicknesses (1, 2, 3, 4 µm) using dip coating technique. As the PVA coating thicknesses varied from 1-4 µm, the sensitivity varied as 678 pm/RH %, 852 pm/RH %, 866 pm/RH %, and 419 pm/RH %, respectively. So, it is clear that the PVA thickness of 3 µm shows the highest sensitivity. After that, the NCF has been etched to reduce its diameter from 125 µm to 65 µm in steps of 15 µm to maximize the evanescent field. The planned sensor with etched NCF of 95 µm shows the highest sensitivity. The proposed sensor has a great deal of potential for real-time RH monitoring, especially in humid environments with high humidity percentages. The proposed sensor has good stability, a low manufacturing cost, and is simple in design, making it a great alternative to conventional sensors.
Funding
Ministry of Higher Education and Scientific Research (MOHESR); University of Baghdad (UoB).
Acknowledgments
This work was supported by Ministry of Higher Education and Scientific Research (MOHESR); University of Baghdad (UoB).
Disclosures
There are no conflicts of interest to disclose.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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