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We demonstrate a novel all-fiber-optic humidity sensor comprised of a WS2 film overlay on a side polished fiber (SPF). This sensor can achieve optical power variation of up to 6 dB in a relative humidity (RH) range of 35%-85%. In particular, this novel humidity fiber sensor has a linear correlation coefficient of 99.39%, sensitivity of 0.1213 dB/%RH, and a humidity resolution of 0.475%RH. Furthermore, this sensor shows good repeatability and reversibility, and fast response to breath stimulus. This WS2 based all-fiber optic humidity sensor is easy to fabricate, is compatible with pre-established fiber optic systems, and holds great potential in photonics applications such as in all-fiber optic humidity sensing networks.
© 2016 Optical Society of America
1. Introduction
Recently, atomically thin layered transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS2) and tungsten disulfide(WS2) have received increased attention due to the sizable band gaps in the visible to near-infrared spectrum exhibited by their 2D structures [1]. In a WS2 monolayer with a hexagonal configuration, each W atom is anchored by three pairs of S atoms to form alternating corners (S-W-S) in a honeycomb network, which is considered as a graphene-like material [2]. This material has many excellent characteristics, such as its wonderful flexibility, moderate carrier mobility, and layer-dependent electronic and optical properties, making it a promising material for future use in electronic devices, especially in gas or vapor sensing technology [3–6]. With these unique properties over a large surface area, WS2 could be an improvement in the field-effect transistors (FETs) used in vapor sensors. The reported WS2 sensors measure vapor by electrical signals. In [7], Huo et al have previously demonstrated the use of WS2 FETs in gas sensors, while in [6], Maria O’Brienet al have demonstrated a gas sensing response for WS2 thin films, with highly sensitive detection towards NH3. And in [4], Martin Pumerau et al have used the WS2 platform to detect methanol and water vapors by selecting specific frequencies. However, in addition to their complex and expensive fabrication, WS2 electrochemical sensors are also unsuitable for remote detection or for use in environments with strong electromagnetic-interference. Compared to the traditional electronic sensors, fiber optic sensors are compact, lightweight, and inexpensive, and are also resistant to corrosive, heat, electromagnetic and radiation effects. Making novel optical sensors through the combination of WS2 and optical fibers is potentially a way to overcome the drawbacks of electrical sensors as mentioned above [8, 9]. A WS2 film, as a sensitive material, has outstanding carrier mobility, strong photoluminescence, large photoresponsivity, and high sensitivity to humidity variation. Hence, a fiber optic sensor based on a WS2 film presents tantalizing interests, such as fast response and high sensitivity [4, 5, 10, 11].
Side-polished fibers (SPFs) are fabricated using a side-polishing technique [12], which removes a portion of the cladding to form a polished region, propagating light confined in the core can escape out to this polished surface via evanescent waves, giving rise to strong interactions between light and the external environment. Using this characteristic, many fiber optic sensors and others kinds of optical devices have been fabricated in this manner, such as all fiber integrated optical power monitors, ultraviolet (UV) power sensors, and optically-driven polarization controllers [8, 13–20].
In our work, WS2 is combined with the side-polished fibers to fabricate a fiber-optic humidity sensor. Using a WS2 alcohol suspension, a WS2 film is coated onto the polished surface of an SPF through random deposition. The simple and inexpensive fabrication, high compatibility with fiber-optic system, and its flat polished surface make SPF an ideal candidate to combine with WS2. To our knowledge, using WS2 as sensitive material in fiber-optic humidity sensor fabrication has not been reported yet. The response of the output optical power of WS2-coated SPF (WS2CSPF) as a function of humidity is determined. The experimental results demonstrate that it has high sensitivity, linearity, and repeatability. This WS2-coated SPF humidity sensor further expands the potential of electro-optics devices, especially in sensing technology.
2. Sensor fabrication and characterization
A schematic diagram of the sensor is shown in Fig. 1(a). The fabrication process can be divided into two steps: fabrication of the SPF and deposition of WS2 onto the polished region of the fiber. The SPF is fabricated from a single mode fiber (SMF) by using a wheel polishing technique [21]. The polished length of the SPF is 15mm, and the residual fiber thickness (RFT) in the polished region is measured to be ~70μm and shown in Fig. 1(b). In order to improve the mechanical strength of the device, the side-polished segment is fixed onto a glass slide (with the polished surface face-up) by a UV curing adhesive. A basin (20 mm × 5mm × 1mm) surrounding the side-polished segment is also constituted using the UV adhesive, as shown in Fig. 1(a).
Fig. 1 (a) Schematic diagram of the sensor; (b) Morphological characteristic of SPF; (c) The output optical power of the WS2CSPF during evaporating the alcohol naturally; (d) SEM image of the WS2CSPF cross section at a magnification of 846 and 19140; (e) SEM image of the WS2CSPF surface at a magnification of 259; (f) Enlarged view of the region marked by the square in (e) at a magnification of 31270.
A WS2 alcohol suspension (WS2 concentration is 1mg/ml, average WS2 nanosheet size is 20-200 nm) is purchased from MK NANO CO., LTD. The WS2 alcohol suspension is treated by ultrasonication for 25minutes in order to uniformly distribute the WS2 nanosheets in the alcohol and to avoid agglomeration. The suspension is then dropped in the basin until it is filled. The WS2-coated SPF sample is left to sit in ambient surroundings for at least 10 h to evaporate the alcohol naturally, after which a WS2 film covered the polished surface of the fiber as well as the glass surface inside the basin, to yield the final WS2CSPF humidity sensor. During the deposition process of WS2, a 1550 nm distributed feedback laser is used to monitor the optical transmitted power in the SPF. The output optical power of the WS2CSPF during evaporating the alcohol naturally is shown in Fig. 1(c). When all the ethanol in the basin is evaporated, the optical transmitted power remains at about −25.5 dBm with a 22.34 dB optical transmitted power variation, which indicates a good interaction between the WS2 film and the light of SPF. The scanning electron microscopy (SEM) image of the WS2CSPF cross section is shown in Fig. 1(d). The WS2 film shows good compactness, and it is integrated with the SPF without any gaps. The adhesion between the WS2 with fiber surface is good. The thickness of the coated WS2 is about 408 nm. According to the work of Ushma Ahuja et al. [22] and Ashok Kumar et al. [23], the perpendicular and parallel components of refractive index of WS2 is about 3.58 and 2.86. The sensing region of the sensor is shown in Figs. 1(e) and 1(f).
Raman spectra excited by a 514.5 nm laser are measured with a Raman Microscope (RENISHAW, UK). A representative spectrum is shown in Fig. 2. The 2LA(M) and A1g(Γ) peak positions are at 352 cm−1 and 420.7 cm−1, respectively. According to [24], the number of layers in WS2 nanosheet can be analyzed non-destructively by using the shifts in these peaks. By comparing the intensity ratios and peak frequencies of the WS2 Raman modes in Fig. 2 with those in [24], the WS2 nanosheets, which constitute the WS2 film coated on the polished surface of the SPF, can be estimated to be multilayer. The WS2 in the outside surface layer of the film plays a key role in eventual humidity sensing.
3. Humility sensing experiments and discussions
3.1 Experimental set-up
Figure 3 shows the experimental set-up comprised of a laser source, a 1 × 3 coupler, a humility chamber (BPS-100CL, Shanghai Yiheng Instruments Co., Ltd, Temperature adjusting range: −10°C - 100°C, Humidity adjusting range: 35%RH-95%RH), three optical power meters (optical power meter 6210, Shanghai Guangwei Communication Co., Ltd), and a personal computer. A commercial humidity/temperature meter (Testo 175H1) is inserted in the chamber to monitor the actual humidity and temperature. A 1550 nm laser (Accelink Technologies Co., Ltd) is coupled to three different fiber samples: unpolished SMF, SPF, and WS2CSPF. SMF is used to monitor the power fluctuation of the laser, and the SPF is arranged for the control measurements.
During the humidity sensing experiments, the temperature inside the chamber is set to 25°C. The relative humidity (RH) inside the chamber is adjusted to complete a cycle of ascending from 35%RH to 85%RH, and then descending from 85%RH to 35%RH in increments of 5%RH. The humidity resolution of the chamber is 3%RH. Each humidity step includes a transition time (~2 min) and a holding time to establish a stable humidity reading (~15 min). The readouts from the commercial humidity/temperature meter and the three fiber samples are recorded by a computer.
3.2 Experimental results
Relative humidity readings from the commercial humidity/temperature meter in the chamber are charted in Fig. 4(a). The fluctuations in the duration of stable humidity are caused by the fine feedback tuning of the chamber. The output optical power of the laser, monitored by the SMF and depicted in Fig. 4(b), shows a slight variation (~0.05 dB) during a measurement time of 20000 s, indicating the stability of the laser source. The output optical power of the SPF in Fig. 4(c) has a cursory dependence on the actual RH variation. The largest variation of the optical power in the whole cycle (35%RH-85%RH-35%RH) is nearly 0.38dB. This dependence indicates that the surrounding atmosphere interacts with the evanescent field that leaks from the polished region, and further affects the light guiding in the SPF. However, this weak interaction prohibits it from being a practical humidity sensing device, which can be remedied by simply using a side-polished fiber.
Fig. 4 (a) Variation of relative humidity in the chamber measured by commercial humidity/temperature meter, variation of relative output optical power through: (b) unpolished SMF, (c) SPF, (d) WS2CSPF.
The output optical power of the WS2CSPF is shown in Fig. 4(d). Two features can be easily found from this graph, the relatively large variation (~6 dB) in the output optical power and relatively fine laddering characteristics. Comparing the largest variation of the output optical power over a whole cycle for both the SPF and WS2CSPF samples, a 17 fold enhancement is found in the latter one. The similar laddering characteristic between the actual relative humidity and the relative optical power shows that the WS2CSPF sample presents a positive response to RH variation.
3.3 Analysis on sensing performance
To further investigate the performance of the tested sensor, the sensitivity, linearity, working range, response time, and repeatability of the sensor are all analyzed.
Extracting data from Figs. 4(a)-4(d) correspondingly, a relationship between the relative output optical power of WS2CSPF and RH can be obtained as shows in Fig. 5. The square markers represent the time averages of the relative output optical power at each stable humidity point in ascending order, while the circle markers correspond to the descending order. The averaged relative output optical power increases linearly from −6.176 dB to −0.890 dB as RH increases from 35%RH to 85%RH linearly in the chamber. Likewise the averaged relative output optical power decreases linearly from −0.890 dB to −6.802 dB as RH descends back to 35%RH. The red and blue lines are the linear fitting curves between the relative humidity and relative output optical power in the range of 35%RH-85%RH in ascending and descending order, respectively.
According to the definition of the humidity sensitivity [25]:
S is the sensitivity of the humidity sensor, △P is the relative variation of the optical power, and △H is the relative variation of the humidity. As depicted by the red solid square dots and line in Fig. 5, the linear fitting of the experimental data gives a sensitivity of 0.1056 dB/%RH with a 99.39% correlation (for ascending). The linear fitting of the experimental data gives a sensitivity of 0.1213 dB/%RH with a 98.27% correlation (for descending). The coincidence between the two curves confirms that the WS2CSPF humidity sensor has good reversibility. This sensitivity is higher than 0.0228 dB/%RH (0.89 dB over 50-80%RH by using a tapered fiber with an hydrogel coating) [26], 0.0103 dB/%RH (over 5-95%RH by using an in-house developed zinc oxide (ZnO) nanoparticle-immobilized bare solgel fiber) [27], its linear response area is larger than that shown in [28] (over 35-60%RH using inline Mach-Zehnder interferometer with ZnO nanowires coating) and [29] (lower than 75%RH by using no-core fiber with HEC/PVDF coating).
The measurement resolution of humidity can be calculated as [30]:
RH is the resolution of the humidity sensor and RP is the larger number between the resolution of the optical power meter and the variation of the optical source. Since the resolution of the commercial optical power meter could reach to 0.001 dB and the variation of the optical source is 0.05 dB, the corresponding humidity resolution is 0.475%RH (ascending) and 0.410%RH (descending).
Furthermore, to demonstrate the sensitivity enhancement of the WS2SPF humidity sensor, we also measure the humidity response of the bare SPF in the same manner for comparison. The experimental results are presented by circles and dotted lines in Fig. 5, with very low sensitivity of 0.0070 dB/%RH for the ascending process and 0.0051 dB/%RH for the descending process, respectively. It is clear from these data that the sensitivity of WS2SPF is enhanced about 15 times compared to that of the bare SPF. Therefore, the presence of WS2 greatly improves the sensitivity and stability of the humidity sensor.
Figure 6 illustrates the actual relative humidity and the relative output optical power of WS2CSPF during the humidity increase for 1000-3000 s. It is apparent that fine feedback tuning of the actual humidity inside the chamber can be tracked immediately by the variation of the output optical power of WS2CSPF. In the durations of 42%RH - 57%RH, the relative output optical power and the relative humidity have in-phase fluctuations. The chamber we used is the same as that in [31], and the output optical power of WS2CSPF can immediately follow the fluctuations of humidity as shown in Fig. 6, we think that the response speed (0.13%RH/s) of the sample is similar with [31].
Fig. 6 Variation of actual relative humidity and relative output optical power of WS2CSPF during1000-3000 s of increasing humidity.
In order to obtain more detailed measurements of the response time, the WS2CSPF is placed on a desk at room temperature, and a 1 s breath is directed on the sensor in 6 s cycles and the response of the WS2CSPF to this exposure is depicted in Fig. 7. The output power increases sharply for about 0.5 dB, and then decreases quickly. The rising time is about 1s and the recovery time is about 5 s. The steep rising and steep recovery curve is faster than those in [32] (a recovery time of 55 s) and in [31] (a recovery time of 29 s).
Another measurement is performed by adjusting the RH between 40%RH and 75%RH, back and forth for several cycles to test the repeatability and reversibility of the sensor. Figure 8 displays that when the RH returns to 40%RH, the output power can return to the initial value even after a long time periods (~103 s), which shows that the sensor possesses good repeatability and reversibility.
Fig. 8 Variation of relative output optical power when adjusting the relative humidity between 40%RH and 75%RH for several consecutive cycles (Temperature = 25°C).
3.4 Discussion
In order to discuss the humidity sensing mechanism for the presented WS2SPF sensor, the effects of temperature variation should be considered [33]. The relative humidity inside the chamber is set to 55%RH, and the temperature inside the chamber is adjusted to cycle from 18 °C to 52 °C and then back to 18 °C at a step of 5 °C. Each temperature step includes a transition time of approximately 2 min and a holding time of 15 min. The experimental results are presented in Fig. 9, with very low sensitivity of 0.0190 dB/°C for the ascending process and 0.0221dB/°C for the descending process, respectively. In the humidity characteristic experiment, the temperature is fixed at 25 °C with the fluctuations of only 1°C, resulting in optical power variations less than 0.02 dB.
Fig. 9 (a) Variation of temperature in the chamber measured using a commercial humidity/temperature meter; (b) Variation of relative output optical power through WS2CSPF; (c) Relative output optical power of WS2CSPF as a function of temperature at 55%RH.
The use of an SPF with a WS2 film for humidity sensing may be explained as follows. As the humidity of the chamber increases, the concentration of H2O molecules increases. Consequently, H2O molecules are physically adsorbed on the WS2 layer with moderate adsorption energies, accompanying a moderate degree of charge transfer. A small amount of charge is transferred from WS2 to H2O [34], and the acceptor character of the adsorbed H2O is consistent with the experimental results [7]. According to orbital mixing theory [35], conductivity decreases due to the reduction of major conduction electrons [34] following a humidity increase, which decreases light absorption. The transmitted loss of an SPF coated with WS2 film decreases while the transmitted optical power increases. Hence, the humidity sensing function enhancement can be achieved by using an SPF coated with a WS2 film.
4. Conclusion
In summary, we have, to the best of our knowledge, proposed and demonstrated the first WS2-based fiber-optic humidity sensor by combining WS2 with SPF. This novel humidity sensor achieves optical power output variation of up to 6 dB in 35% - 85%RH, sensitivity values of 0.1056 dB/%RH (for ascending), 0.1213 dB/%RH (for descending) and a response speed faster than 0.13%RH/s. This sensor yields a linear response with correlation coefficients of 99.39% (for ascending) and 98.27% (for descending), and good repeatability over a range of 35%RH −85%RH. Thanks to this novel sensing material, fiber based structure, and perfect sensing performance, this novel humidity sensor is not only helpful for overcoming the drawbacks of the electrical sensors, but also possesses other advantages, such as simple and inexpensive fabrication, remote sensing capabilities, immunity to electromagnetic interference, and multiplexing. Relying on the favorable chemical, electronic, and optical properties provided by WS2, WS2CSPF can be anticipated as beneficial to the existing WS2-based sensing regimes. This work will open a new gate to the employment of WS2 in various disciplines with the combination of optical fibers.
Acknowledgment
This work is supported by the National Natural Science Foundation of China (Nos. 61177075, 61275046, 61361166006, 61475066, 61405075, 61505069, 61575084 and 61401176), Guangdong Natural Science Funds for Distinguish Young Scholar (2015A030306046), Natural Science Foundation of Guangdong Province (Nos. 2014A030313377, 2014A030310205 and 2015A030313320), the Core Technology Project of Strategic Emerging Industries of Guangdong Province (Nos. 2012A032300016, 2012A080302004), Special Funds for major science and technology projects of Guangdong Province (Nos. 2014B010120002, 2014B010117002 and 2015B010125007), Special Funds for Discipline Construction of Guangdong Province (No. 2013CXZDA005), Planned Science & Technology Project of Guangzhou under Grant (No. 201506010046), Young and Innovative Talents Project of Guangdong High Education (No. 2014KQNCX025), Excellent Young Teachers Program of Guangdong High Education (No. YQ2015018) and by the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University (No. 2014H09) and the State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University (No. PIL1406).
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