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Abstract
We propose a dual−purpose sensor to detect ammonia besides relative humidity with tapered multimode fiber (TMMF) as the sensor structure and silica gel as the sensing layer. In achieving the desired TMMF, we heated and pulled a few pieces of multimode fibers to obtain 9-mm long tapered regions with different diameters in the range of 4−40 µm. Then synthesized silica gel by sol-gel method and passed the tapered section of the TMMFs through drops of silica gel to form the sensing layers. Using the wavelength shift and output power reduction appearing in the fiber transmission spectrum, we determine the sensor response to the alteration of ammonia concentration and relative humidity in the environment. Our experimental results reveal that a decrease in the tapered waist gives rise to an increase in the sensor sensitivity. The same results show that the highest measured sensitivity is 14.8 pm/ppm (for ammonia) and 0.435 dB/%RH (for relative humidity) when we coat the tapered region of a TMMF of 4−µm waist diameter with silica gel. Moreover, the time responses for relative humidity and ammonia sensors are 10 and 32 s, and their corresponding recovery times are 8 and 19.5 s.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Studies on fiber optics sensors (FOS) have become increasingly popular in the last decade due to advantages like compact structure, immunity to the electromagnetic field, fast response, high sensitivity, and capabilities like operating in harsh environments and remote and distributed sensing systems [1,2]. Based on those significant features, researchers have employed FOS to detect various physical parameters [3,4], chemical gases, and bio−substances [5]. Relative humidity (RH) and ammonia sensors are most suitable in the industrial field, food processing, agriculture, and environment monitoring [6]. This variety of applications has motivated researchers to develop different structures and materials to achieve state-of-the-art sensors. For example, Chen et al. [7] have exploited 3D printing of Fabry-Perot microcavity on the tip of a fiber optic for sensing of the humidity, Narasimman et al. [8] have fabricated ammonia sensors using multimode fiber coated with amine-functionalized ZnO nanoflakes, and Xu et al. [9] have employed an uncoated knot resonator to fabricate ammonia sensor. Compared to other chemical sensors in terms of usage, humidity sensors are widely used for different purposes, from air conditioning systems to high energy physics [10]. Researchers have investigated various materials to improve sensitivity. For instance, taking advantage of polyimide whose volume increases by rising the moisture quantity has been utilized as a sensing medium on a fiber probe [11]; and CaAlg hydrogel film has been used as a cladding for no core fiber [12]. Side-polished fiber (SPF) is one of the structures employed as the foundation of sensors, and covering the polished section with materials like gelatin, MoSe2, and graphene oxide (GO) offers the opportunity for humidity sensing [13–15]. Researchers, furthermore, have investigated Michelson Interferometer (MI) to monitor RH in two ways based on core offset and methylcellulose coated rounded tip [16,17]. Ultimately, the humidity sensor has various structures, materials, and a variety of applications. One of the imperative applications of this device is the breath sensor that fastens to the moisture produced by exhalation. For monitoring the human breath, Du et al. [18] and Shrivastav et al. [19] have, respectively, exploited biocompatible substances like MoS2 and chitosan polymer as the sensing materials.
Moreover, ammonia has a variety of industrial applications in everyday life. Even though ammonia naturally exists in the atmosphere at the ppb level less than 5 (NH3-con. < 5 ppb), it is not safe for a person to work in an environment with an NH3 concentration of 25 ppm more than 8 hrs. This time drops to 15 minutes by raising the ammonia concentration to 35 ppm [20]. For protecting air and water from ammonia pollution, the development of FOS has evolved towards ammonia sensing. In addition, novel materials have been used for this purpose to monitor the concentration of ammonia (NH3) and ammonium (NH4+) [21,22]. For example, depositing WO3 nanorods [23] or Fe2O3 nanotube [24] on a surface of tapered multi-core fiber provides a device to sense a high concentration of NH3. Yet, some research groups have focused on sensing lower ammonia concentrations [25–27]. In doing so, Pathak et al. [25] have exploited plasmonic phenomena by coating an Ag/SnO2 thin film on a fiber-optic ammonia sensor, Shrivastav et al. [26] have utilized PANI@SnO2 nanocomposite coated on a Mach-Zehnder interferometer structure, and López-Torres et al. [27] tunned the thickness of the SnO2 nanocoating on a microstructured optical fiber ammonia gas sensor. Some other groups have taken advantage of graphene and graphene-oxide nanocomposites like GO-Pt and GO-ZnO as the ammonia sensing layers [28–30]. Others fabricated FOS for ammonia sensing, employing compounds like polyaniline/graphite nanocomposite [31,32] and porphyrin-nanoassembled [33].
Furthermore, silica gel is sensitive to ammonia [34] and humidity [35]. Thus, the use of this material provides an opportunity to detect ammonia and moisture, both with an identical device.
There are reports on TMMF for sensing relative humidity [36,37] and ammonia [38]. Nonetheless, in this work, we have fabricated silica-gel coated, all with 9-mm long tapered regions and different waist diameters (i.e., 4−40 µm) for the sensing of humidity and ammonia concentration under 100 ppm, using the same fabrication procedure and identical characterization setup. In this experimental study, we have improved sensitivity and time characteristics for humidity sensing compared to those reported earlier [36]. In our earlier conference presentation, we have shown a silica-gel coated tapered multimode fiber (TMMF) for ammonia sensing [39]. In what follows, we demonstrate the results of our experimental investigation on the sensitivity, response time, and recovery time of identical TMMFs, in sensing humidity and ammonia. This investigation aimed to show the effects of the tapered region waist diameter. For each sensor, we finally compare our best results with those obtained by FOSs fabricated with various structures and sensing materials for humidity [9,12–17] and ammonia sensing [27–30,33].
The organization of the rest of the paper is as follows. In Section 2, we describe the fabrication process and the characterization method. Section 3 is devoted to the results and discussion where we present the sensor’s sensitivity and time characteristics for both relative humidity and ammonia. Finally, the paper is closed by conclusive remarks in Section 4.
2. Experiment
2.1 Sensor fabrication
We intend to report the fabrication of accurate and inexpensive optical sensors for ammonia and relative humidity, using the experimental setup shown in Fig. 1(a). Accordingly, we have chosen TMMF as the sensor platform and silica gel as the sensing layer. The TMMF fabrication procedure is as follows. We passed a 1-m long multimode fiber through a heater at 1300 °C and then pulled in the opposite directions along its length, using two linear motorized stages on both sides. By varying the displacement velocity of the linear motorized stages, we manipulated the waist dimension of the tapered fiber.
Fig. 1. Schematics of (a) TMMF fabrication Setup, (b) Silica gel coating, (c) and silica gel coated TMMF.
We utilized the recipe for the sol-gel technique proposed by [34] to synthesize the desired silica-gel solution with appropriate viscosity. A solution containing tetraethylorthosilicate (TEOS): ethanol (10 ml:5 ml) has been stirred for 20 minutes by a magnetic mixer. Then we added 1.5 ml of H2SO4 (0.1 mol/L) to the solution, stirring for 150 minutes at room temperature. Next, for uniformly coating the sensing region by silica-gel, we passed the TMMF through a drop of prepared silica-gel solution (Fig. 1(b)). We repeated this step four times to achieve the desired coating thickness. Then, for drying the sensing layer, we held the silica-gel coated TMMFs in the air at room temperature for three days, using a plastic holder. Figure 1(c) shows a schematic of a sol-gel-coated TMMF.
2.2 Characterization
We have employed a fiber fusion splicer, a broadband light source (BBS), and an optical spectrum analyzer (OSA) for characterizing the sensor. A sol-gel coated TMMF was fixed into a chamber of ∼2 L volume with three inputs and connected to the BBS and OSA (Fig. 2).
Fig. 2. A schematics of a dual-purpose characterization setup for relative humidity (RH) and ammonia sensors. During the RH (ammonia) sensing the mini heater (humidifier) is off.
Before each characterization, we flushed one of the inputs by argon via connecting it to an argon gas cylinder, exhausting the chamber from any undesired content. For characterizing relative humidity (RH) sensors, while keeping the third input lid sealed and the mini heater turned off, we used a humidifier to provide the required moisture through the second input. Meanwhile, we have used an SHT20 sensor to indicate the RH percentage (RH%) and the chamber temperature. For characterizing ammonia sensors, while keeping the humidifier turned off, we utilized a micropipette to inject a given volume of ammonia solution into the chamber through the third input. After sealing the top lid, we turned the mini heater on to evaporate the injected solution and provide ammonia vapors. After each injection, we waited until the sensor's response became steady. Then injected argon gas to extrude the ammonia vapor out of the chamber. We repeated this process for different ammonia vapor concentrations. For both characterizations, we maintained the room temperature at 26 °C, and the preliminary humidity for RH sensing at 20%RH.
In general, controlling gas concentration during characterizations can be accomplished by evaporating liquid analyte with a definite volume inside a test environment with specific volume, pressure, and temperature. Thus, we have gone through this procedure to indicate gas concentration by inserting and vaporizing liquid ammonia analyte in the chamber. The concentration of ammonia can be calculated by [40]:
3. Results and discussion
3.1 RH sensor
For the relative humidity, we have examined three sensors, all with the same tapered length (9-mm) and waist diameters of 4 µm (SENS1), 15 µm (SENS2), and 40 µm (SENS3). Figures 3 illustrates the measured transmission spectra through the three named RH sensors within the wavelength range of 820 nm<λ<880 nm (according to commercial detectors) for various chamber relative humidity. The spectrum deformation and a reduction in the sensor output power manifests an increase in the chamber RH%. In other words, an increase in RH% increases the sensing layer refractive index via diffusion of H2O molecules into the purse silica-gel layer. This increase in refractive index enhances the light scattering and the evanescent field interaction in the tapered region, which reduces the output power and deforms the transmission spectrum.
Using these spectra and considering the fluctuation in the transmitted power level as a measurement tolerance, we have calculated the average power loss relative to the average loss at the initial humidity of 20% versus the percentage of the relative humidity over the given wavelength range. Figure 4(a) represents the dependence of the average loss (dB) incurred on the transmitted power through SENS1 (squares), SENS2 (diamonds), SENS3 (triangles). For the sake of comparison, we repeated the test using an uncoated TMMF with a tapered region identical to that of SENS1. The solid circles represent the loss incurred on the power transmission through the uncoated TMMF (SENS0). Before going any further, we compare these results with those obtained from SENS1. This comparison reveals the significance of the sol-gel coating as the sensing layer. When no coating layer is present, an increase in the chamber humidity level increases the effective refractive index of the environment surrounding the sensing region slightly, enhancing the power loss accordingly. The refractive index of the sol-gel coating is greater than that of the air. Hence, when the coating layer is present, the higher the humidity level in the chamber, the larger the number of water molecules diffusing into the silica gel, enhancing the effective refractive index of the coating layer significantly and increasing the transmission loss accordingly. Another observation from comparing the relative power loss through the three sensors with silica gel coating is that the larger the tapered region waist diameter, the less sensitive the sensor becomes. This increase in the sensitivity occurs at the cost of losing detection range, declining from 80% for SENS2 to 72% for SENS1. Moreover, the nearly constant zero relative loss exhibited by the SENS3 with an increase in the chamber relative humidity shows that the 40-µm waist diameter in this particular sample is too large to let the sample function as an RH sensor. In other words, the intensity of the evanescent field in the sensing layer is too weak to interact with the sol-gel refractive index and sense its variation due to an increase in the diffused H2O.
Fig. 4. (a) Changes in the sensors output relative loss versus relative humidity percentage; (b) The corresponding time response for SENSE1.
To quantify the fabricated RH sensors, we use the following definition:
Applying Eq. (2) to the data for SENS1 given in Fig. 4(a), we obtain the maximum sensitivity of SRH ≅0.435 dB/RH%, in the range of 68.8%-72% relative humidity. Finally, to evaluate the sensor response time (τres) and recovery time (τrec), we have measured the wavelength shift versus time while varying the chamber relative humidity from 40% to 60%. Figure 4(b) illustrates this characteristic for the SENSE1 sensor. From this figure, we can see τres≈ 10 s and τrec≈ 8 s. To show the superiority of our fabricated sensor, we compare our results to those recently reported on RH-FOS, as shown in Table 1. This comparison shows that the two lowest sensitivities belong to the sensors of [9] and [16], made of uncoated fibers, in which the evanescent modes interact weakly with the surrounding water. Nonetheless, in the sensors made of coated fibers, the interaction of the evanescent modes with the surrounding sensing layers is strong, enhancing the sensitivity significantly.
Our fabricated sensor and the one reported by [15] are the two sensors exhibiting the highest sensitivities shown in the table. However, comparing the time responses, we can see that the silica-gel coating response to the variations in humidity is slower than the other coatings. Nevertheless, our measured time response is sufficient for monitoring the humidity of the environment in most applications. Further comparison in Table 1 demonstrates the Humidity sensor made of a TSMF coated with silica-gel, reported in [36], is structurally the closest sensor to SENSE1. Nonetheless, our champion sensor enjoys a two times larger sensitivity, seven times larger recovery time, and comparable response time.
3.2 Ammonia sensor
For the ammonia sensor, we have fabricated three TMMFs with tapered waist diameters of 4 µm (SENS1), 7.5 µm (SENS2′), and 10 µm (SENS3′), all coated with silica gel layers of thicknesses identical to the RH sensors. Figures 5 illustrates the measured transmission spectra through the three named AG sensors within the wavelength range of 686 nm < λ < 710 nm for various ammonia vapor concentrations in the chamber.
Fig. 5. The transmission spectra for (a) SENS1, similar to [39], (b) SENS2′, and (c) SENS3′ versus ammonia vapor concentration.
The results in Fig. 5 show that as the ammonia vapor concentration in the chamber increases, the transmission spectrum exhibits a redshift. As the vapor concentration in the compartment increases, more NH3 molecules diffuse into the coating layer and increase its effective refractive index, enhancing the interaction of the evanescent field with this medium and incurring the observed output power loss and redshift in the spectrum.
Next, we have extracted the redshift experienced by each of the three named AG sensors as a function of the ammonia concentration in the chamber, as shown in Fig. 6(a). A comparison of the redshift depicted by solid squares (SENS1), diamonds (SENS2′), and triangles (SENS3′) reveals that all three sensors respond linearly to the variation in ammonia vapor concentration. Moreover, the same results demonstrate that the larger the waist diameter, the less sensitive the sensor to the change in the ammonia vapor concentration in the chamber. In other words, SENS1 exhibits the highest sensitivity among the three fabricated sensors. To quantify the sensitivity of AG sensors, we use the following definition:
Fig. 6. (a) The wavelength shift experienced by SENS1 (solid squares), SENS2′ (solid diamonds), and SENS3′ (solid triangles). Open circles, upward and downward triangles, crosses represent ethanol, methanol, acetone, and propanol vapors. (b) The time response for SENS1 for ammonia is similar to [39].
To investigate how sensitive these AG sensors are to other gasses like ethanol, methanol, acetone, and propanol, we employed the sensor with utmost sensitivity (i.e., SENS1), repeating similar tests, using the latter gasses into the chamber instead of ammonia.
The results illustrated by open circles, upward and downward triangles, and crosses show zero redshifts independent of the vapor concentrations for all other gasses. These results imply an AG sensor of this type is not sensitive to the ethanol, methanol, acetone, and propanol vapors in the surrounding environment.
To investigate the time responsivity of SENS1, we changed the ammonia concentration from 0 ppm to 100 ppm and vice versa and obtained the redshift versus injection time, as illustrated in Fig. 6(b). As can be observed from this figure, the sensor has an adequate response time of 32 s and a recovery time of 19.5 s.
Finally, we have compared the specifications of SENS1 for ammonia with those of recently reported FOSs for ammonia sensing [27–30,33,38], as shown in Table 2. The comparison shown in the table indicates that the recovery time of SENS1 for ammonia is the shortest, while its response time is comparable to the shortest one reported by [29]. Albeit, its sensitivity is ∼ 48% of that of the highest reported by [30], using GO−ZnO as the sensing layer.
The comparison also reveals the closest ammonia sensor to SENS1, structurally, is the TMMF-based sensor, whose sensing layer was polyaniline nanofibers coating layer [38]. This sensor is suitable for sensing high concentrations of NH3, with significantly low sensitivity [38]. Nonetheless, our silica gel-coated TMMFs are apt for detecting NH3 concentrations of < 100 ppm, besides relative humidity being dual purpose FOSs. Moreover, SENSE1 is the only TMMF sensor with a tapered diameter below ten µm (i.e., four µm), reported so far. Hence, our experimental results can motivate other FOS researchers to endeavor for further improvement.
4. Conclusion
We have fabricated and characterized a dual−purpose sensor using tapered multimode fiber (TMMF), coating its tapered region with a layer of silica gel. We have demonstrated the effect of the size of the diameter of the tapered region waist on the device sensitivity. The experimental results show maximum sensitivities for relative humidity and ammonia detection belongs to the fiber sensor with a 9−mm tapered region and 4-µm waist diameter. Moreover, the utmost sensitivity, time response, and recovery time for humidity sensing obtained are 0.435 dB/%RH, 10 s, and 8 s, respectively. Furthermore, or ammonia sensing, the linear sensitivity, time response, and recovery time equal 14.8 pm/ppm, 32 s, and 19.5 s, respectively.
Funding
Tarbiat Modares University (IG-39703).
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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