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Tungsten disulfide (WS2) sheet wrapped on the tapered region of micro fiber (MF) and its humidity sensing are proposed and demonstrated. WS2 coated MF (WS2CMF) is demonstrated to enhance the interaction and contact area between WS2 and the strong evanescent field of optical fiber. An enhancement in sensitivity (0.196 dB/%RH) of the WS2CMF is achieved in a RH range from 37%RH to 90%RH. Furthermore, the proposed WS2CMF shows a good repeatability from 40%RH to 75%RH and a rapid response to periodic breath stimulus. This WS2CMF holds great potential in all optical sensing networks owing to the advantages of high sensitivity, compact size and low cost.
© 2017 Optical Society of America
1. Introduction
The measurement of relative humidity (RH) sensing is of great importance in numerous industries, such as air conditioning, food process and storage, biomedical, agriculture, semiconductor industries and meteorology [1]. In recent years, optic fiber humidity sensing technology has been gaining popularity among research institutions and industries. Compared to traditional electronic methods [2], optic fiber humidity sensing offers many distinct advantages such as immunity to electromagnetic interference, remote sensing, small size, and the continuous and real time monitoring in hazardous environments [3,4]. To improve the RH sensing performance, different sensitive materials and fiber structures are used in optical fiber humidity sensing. R. Aneesh et al. [5] have demonstrated a simple titanium dioxide based optical fiber RH sensor. Ran Gao et al. [6] have proposed a reduced graphene oxide (rGO) coated hollow core fiber humidity sensor based on anti-resonant reflecting guidance with a sensitivity of 0.22 dB/%RH in a 30%RH range (from 60 to 90%RH). Haifeng Liu et al. [7] have proposed an optical fiber RH sensor based on SiO2 nanoparticles with a sensitivity of 0.441 dB/%RH in a 11.4%RH range (from 83.8 to 95.2%RH). Moreover, the two-dimensional layered transition metal dichalcogenide (TMDC) [8,9] have aroused huge interest for sensing applications due to their excellent properties. Dongquan Li et al. [10] have investigated the performance of few-layer MoS2 coated side polished fiber (SPF) to sense RH. Dattatray J. Late1 et al. [11] have described the utilization of single-layer molybdenum diselenide (MoSe2) as high-performance room temperature NH3 gas sensors. Jian Zhen Ou et al. [12] have demonstrated an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and tin disulfide (SnS2) flakes at low operating temperatures. Tungsten disulfide (WS2) [13] is a typical TMDC material with wonderful flexibility, moderate carrier mobility, and layer-dependent electronic and optical properties [14–16], making it a promising material for future use in sensing technology. However, most of the WS2-based electrical sensors [14, 17, 18] are not suitable for remote detection, working in flammable explosive environment and using in environments with strong electromagnetic-interference. We have demonstrated that WS2 coated SPF shows a sensitivity of 0.121 dB/%RH in the range from 35 to 85%RH [19], which stimulates us to further improve the performance of WS2-based all fiber sensor. To improve the sensitivity of the WS2 coated SPF, the evanescent light from the polished region of SPF and the contact area between the SPF and WS2 should be increased. Increasing the polished depth of SPF can enhance the interaction between SPF and WS2, which impairs the mechanical strength of the device. Hence, another WS2 coated fiber microstructure with high sensitivity and wide dynamic range presents tantalizing interests.
Micro fiber (MF) is one of the best candidates to be used in optic fiber sensing [1]. With high-index contrast between the MF material and the surrounding, MF guides light with low optical loss, outstanding mechanical flexibilities, tight optical confinement and large fractional evanescent fields [20]. Strong evanescent fields enables strong and rapid near-field interaction between the guided light and the surroundings, which offers MF optical sensing with high sensitivity and fast response [21]. Lei Zhang et al. [22] have reported a subwavelength-diameter tapered MF coated with gelatin layer for RH sensing. H. Ahmad et al. [23] have proposed a simple RH sensor based on a MF loop resonator with rGO. Lin Bo et al. [24] have demonstrated a novel polyethylene oxide coated MF coupler RH sensor. As a combination of fiber optics and nanotechnology, MF has been emerging as a novel platform for exploring fiber-optic sensing technology on the micro or nanoscale.
In this work, we propose a MF coated with WS2 and its humidity sensing. A WS2 film is coated onto the tapered region of the MF through random deposition using a WS2 alcohol suspension. Compared to SPF, the MF enhances the interaction and the contact area between the fiber and WS2. The enhanced interaction between strong evanescent light of the MF and WS2 results in a stronger sensing functionality. The WS2 coated MF (WS2CMF) shows a sensitivity of 0.196 dB/%RH in the RH range from 37%RH to 90%RH. The sensitivity of WS2CMF is ~1.6 times higher than that of the WS2 coated SPF. And the WS2CMF shows a good repeatability (from 40%RH to 75%RH) and fast response to breath stimulus. This WS2CMF further expands the potential of electro-optics devices, especially in sensing technology.
2. Fabrication and characterization
Figure 1(a) shows the schematic diagram of WS2CMF. The MF is fabricated from a single mode fiber (SMF, with a core diameter of 8 μm and a cladding diameter of 125 μm from Corning Inc.) by heating with a flame, then the fiber is elongated at a drawing speed of 0.1 mm/s [25]. The variation of the MF diameter is shown in Fig. 1(b). The diameter of the MF taper is ~14 μm. Then the tapered segment is immobilized onto a glass slide with the purpose of improving the mechanical strength of the device. A basin (20 mm × 5mm × 1mm) surrounding the tapered segment is constituted by using the UV adhesive (Loctite 352, Henkel Loctite Asia Pacific) as shown in Fig. 1(a). The basin is cured by an UV light source (365 nm, USHIO SP7-250DB) for 10 minutes illumination.
Fig. 1 (a) Schematic diagram of the WS2CMF; (b) Morphological characteristic of MF; (c) Scanning electron microscopy (SEM) image of the WS2CMF cross section; (d) enlarged view for the region marked by a dotted line in (c); (e) SEM image of the WS2CMNF tapered region, and enlarged view for the region; (f) Raman spectrum of the WS2 on MF.
The WS2 alcohol suspension (nanosheet concentration: 1 mg/ml, average size: 20- 200 nm) is purchased from Nanjing MKNANO Tec. Co., Ltd. The WS2 alcohol suspension is treated by ultrasonication for 25 minutes with the purpose of distributing the WS2 nanosheets to avoid agglomeration. The suspension is then dropped into the basin and the WS2CMF sample is placed in ambient surrounding for ~10 hours to evaporate the alcohol naturally. Then a WS2 film is coated onto the tapered region of the MF. The scanning electron microscopy (SEM) image of the WS2CMF cross section is shown in Fig. 1(c). Figure 1(d) shows an enlarged view with higher magnification for the region marked by a dotted line in Fig. 1(c). The WS2 film is integrated with the outer surface of MF without gaps. In the previous work, the WS2 film can only be coated in the polished region of the SPF. Compared to SPF, the MF enhances the interaction and the contact area between the fiber and WS2. The thickness of the coated WS2 is about 248 nm. Thickness of the deposited WS2 can be controlled by adjusting the concentration of WS2 sheets in the solution and the times of deposition. The tapered region profile of the WS2CMF and enlarged view with higher magnification are given in Fig. 1(e). The Raman spectrum excited by a 514.5 nm laser of WS2 film on the tapered region is measured with Raman Microscope (RENISHAW, UK) and shown in Fig. 1(f). The 2LA (M) and A1g (Γ) peak positions are at 353 cm−1 and 418.3 cm−1, respectively. By comparing the WS2 Raman modes in Fig. 1(f) with those in [26], the WS2 nanosheets wrapped on the tapered region of the MF are multilayer.
An appropriate waist of the MF taper is the key factor for fabricating a high performant WS2CMF. The transmitted optical power of the MF during the deposition process of WS2 is monitored. A 1550 nm distributed feedback (DFB) laser is used as the light source. The waist of MF taper should not be too small or too large. The over small waist of MF taper may show insufficient detected light of optical power meter. And the over large waist of MF taper shows small variation of transmitted optical power, which indicates insufficient interaction between WS2 and the evanescent wave of MF. As shown in Fig. 2, the optical transmitted power of the MF (with diameter of ~14 μm) is about −4.8 dBm at the beginning. The power remains at −33 dBm when the evaporation is finished, which denotes the completed deposition of WS2 film. The transmission loss is caused by the light scattering and absorption of the WS2. Therefore, the ~14 μm waist with ~28.6 dB transmitted optical power variation is appropriate to reach a compromise between the light interaction (WS2 and evanescent light of MF) and the loss. With the appropriate waist of the MF, the loss of the WS2CMF can be reduced by optimizing the WS2 coating technology and the thickness of coated WS2.
3. Humidity sensing experiments and discussions
As shown in Fig. 3, the experimental set-up consists of a laser source (1550 nm), an 1 × 3 coupler, a humidity chamber (BPS-100CL, Shanghai Yiheng Instruments Co., Ltd), three optical power meters (6210 optical power meter, Accelink Technologies Co., Ltd), and a personal computer. An electronic humidity/temperature meter (Testo 175H1) is placed in the chamber to monitor the actual humidity and temperature. The three output terminals of the coupler are connected to the SMF, MF, and WS2CMF.
During the humidity sensing experiments, the temperature inside the chamber is set at 25°C. The RH inside the chamber is adjusted to complete a cycle of ascending from 37%RH to 90%RH, and then descending from 90%RH to 37%RH at a step of ~5%RH. Each humidity step includes a transition time (~2 minutes) and a maintained time (~15 minutes) to establish a stable RH. The personal computer is used to record the real-time data of the optical power, RH, and temperature.
The RH monitored by the electronic humidity/temperature meter in the chamber is shown in Fig. 4(a). The optical relative power (RP) fluctuation of SMF to RH variation is shown in Fig. 4(b). The RP fluctuation of ~0.05 dB indicates that the output optical RP of the SMF cannot be influenced by the RH, and that the laser source is highly stable. The output optical RP of the MF to RH variation is shown in Fig. 4(c). The largest variation of the output optical RP in the whole cycle (37%RH - 90%RH - 37%RH) is ~0.08 dB. However, this weak variation prohibits it from being a practical RH sensing device. Figure 4(d) shows the output optical RP of the WS2CMF. The comparison between Fig. 4(a) and (d) indicates that the optical transmitted power of WS2CMF follows the change of RH in the chamber. The largest variation of output optical RP is up to 11.019 dB, which is ~137 times of the bare MF. This indicates that the WS2CMF can be used for all fiber RH sensing.
Fig. 4 (a) Variation of relative humidity in the chamber monitored by commercial humidity/temperature meter and variation of output optical RP through; (b) SMF; (c) MF and (d) WS2CMF.
The mechanism of the WS2CMF for humidity sensing may be physically understood as below: The concentration of H2O molecules increases with the increasing RH. Then the WS2 layer physically adsorbs H2O molecules with moderate adsorption energies, accompanying a moderate degree of charge transfer. Then a small amount of charge is transferred from WS2 to H2O [18, 27]. The reduction of major conduction electrons decreases the WS2 conductivity [28], resulting in decreased light absorption. The transmitted optical power of the WS2CMF is therefore increased when the RH ascends.
Figure 5 shows that both the RH and the RP of WS2CMF during 10- 60 min and 205- 255 min in detail. The RH and fluctuation of the RP keep in-phase during the ascending and descending process, which indicates that the fluctuations of the RH can be immediately followed by the RP of WS2CMF.
Extracting the data from Fig. 4, we can chalk up a relationship between the optical RP of WS2CMF and the RH as shown in Fig. 6. The solid squares (ascending) and circles (descending) represent the averaged output optical RPs at each stable humidity steps. When the RH of the chamber increases from 37%RH to 90%RH linearly, the averaged optical RP increases from −10.757 dB to −1.519 dB. Likewise, the averaged optical RP decreases from −1.519 dB to −12.538 dB as the RH descends back to 37%RH. The red line represents the linear fitting curve as the RH ascends from 37%RH to 90%RH, while the blue line indicates the descending.
According to the definition of the humidity sensitivity [29], a sensitivity of 0.165 dB/%RH with a linearity of 98.89% (ascending) is obtained as shown in Fig. 5. The experimental data give a sensitivity of 0.196 dB/%RH with a linearity of 96.66% (descending). The bare MF has low sensitivities of 0.0009 dB/%RH for the ascending process and 0.0016 dB/%RH for the descending process, respectively. The sensitivity of WS2CMF is enhanced ~122 times compared to that of the bare MF. Hence, the sensitivity of the demonstrated WS2CMF is greatly improved by the WS2 film.
Table 1 presents characteristics of various optics fibers coated with different types of material for RH sensing. Compared with other RH sensors, the sensitivity of WS2CMF is higher than that of the HEC/PVDF coated tapered fiber, WS2 coated SPF, zinc oxide coated PMMA MF, and GO/PVA composite film coated in-fiber Mach-Zehnder interferometer. The MF enhances the interaction and the contact area between the fiber and WS2, resulting in the ~1.6 times higher sensitivity than that of SPF. By optimizing the diameter of MF and improving the deposition process of WS2 film, the sensitivity and dynamic range of WS2CSPF can further be increased. The sensitivity of 0.196 dB/%RH is equal to that of the HEC/PVDF hydrogel film coated no-core fiber. In addition, the sensitivity of WS2CMF is lower than those of the rGO coated SPF, rGO coated hollow core fiber and SiO2 nanoparticles coated S-taper fiber, but the dynamic range of WS2CMF is >1.5 times wider than the three sensors mentioned above. Therefore, WS2CMF is an ideal candidate for high performance fiber RH sensing.
Table 1. Characteristics of various fiber optic humidity sensors coated with different types films
To evaluate the response and recovery time of the WS2CMF, the WS2CMF response to the periodic human breath is measured. The distance between the WS2CMF and mouth is 25 cm as shown in Fig. 7.
Figure 8 illustrates the WS2CMF response to breathing exposure. A periodic 1 s breathing is directed to the WS2CMF, the output power increases sharply for about 0.9 dB in 1 s, then after 4 s the output optical power returns to the initial value. The response of the proposed WS2CMF is faster than those reported earlier [6, 32, 33].
Another measurement is executed by adjusting the RH between 40%RH and 75%RH repeatedly for several cycles. The temperature inside the chamber is set at 25°C. Figure 9 demonstrates that the optical RP can return to the initial value even after a long time period (~230 minutes) when the RH returns to 40%RH, which proves that the WS2CMF possesses great repeatability.
Fig. 9 Variation of output optical relative power when adjusting the RH between 40%RH and 75%RH for several consecutive cycles (Temperature = 25 °C).
4. Conclusion
In summary, we have proposed and successfully demonstrated WS2 sheet wrapped on MF and its enhanced humidity sensing. The MF enhances the interaction and the contact area between the fiber and WS2. The WS2CMF is operated within a wide humidity range (37%RH - 90%RH) with high sensitivity (0.196 dB/%RH) and good reversibility. In addition, the WS2CMF shows fast response to human breath. Benefiting from its high sensitivity, fast response, and easy fabrication, the WS2CMF is suitable for applications in various fields for humidity monitoring. This work combining the WS2 and MF will benefit the applications of the novel two-dimensional material in photonic devices and all-optical fiber sensors.
Funding
This work is supported by the National Natural Science Foundation of China (Nos. 61505069, 61575084, 61177075, 61275046, 61675092, 61361166006, 61475066, 61405075, and 61401176), the Natural Science Foundation of Guangdong Province (Nos. 2016A030310098, 2014A030313377, 2014A030310205, 2015A030306046, 2015A030313320, 2016A030311019, and 2016A030313079), the Science and Technology Projects of Guangdong Province (Nos. 2012A032300016, 2014B010120002, 2014B010117002,2015A020213006, 2015B010125007, 2016B010111003, and 2016A010101017), the Science and Technology Project of Guangzhou (201607010134, 201506010046, 201605030002), and the State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University (No. PIL1406).
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