Relative Humidity Sensor Based on S-Taper Fiber ... - IEEE Xplore

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IEEE SENSORS JOURNAL, VOL. 15, NO. 6, JUNE 2015

Relative Humidity Sensor Based on S-Taper Fiber Coated With SiO2 Nanoparticles Haifeng Liu, Yinping Miao, Bo Liu, Wei Lin, Hao Zhang, Binbin Song, Mengdi Huang, and Lie Lin

Abstract— A simple and compact optical fiber relative humidity (RH) sensor based on SiO2 nanoparticles has been proposed and experimentally demonstrated in this paper. S-taper fiber is fabricated as the sensitive element using simple fusion spicing and the S-tapered region is coated with a layer of hydrophilic material by direct immersion into the SiO2 nanoparticle solution. The resonance wavelength as well as peak transmission are both sensitive to environmental humidity due to the change of effective cladding refractive index caused by the strong surface absorption of the porous SiO2 nanoparticle coating with hydrate activity. Experimental results show that this humidity sensor has a good reversibility from 26.5%RH to 95.2%RH and a good linearity from 83.8%RH to 95.2%RH. The maximum sensitivities of 1.1718 nm/%RH and 0.441 dB/%RH have been achieved for a high humidity range of 83.8%RH to 95.2%RH. The proposed humidity sensor has such distinguished features as compact size, low cost, and ease of fabrication, and it has potential applications for high humidity environments. Index Terms— Optical fiber sensors, S-taper fiber, humidity measurement, nanomaterial.

I. I NTRODUCTION

R

ELATIVE Humidity (RH) is the ratio of water vapor to the saturation value in the air at a certain temperature. Relative humidity monitoring is very important in many application fields. A large quantity of humidity sensors have been proposed for environmental monitoring, weather forecasting, chemical applications, and seed cultivation, etc. Most of these

Manuscript received August 30, 2014; revised December 30, 2014; accepted December 30, 2014. Date of publication January 8, 2015; date of current version April 22, 2015. This work was supported in part by the China Post-Doctoral Science Foundation Funded Project under Grant 2012M520024, in part by the Key Natural Science Foundation Project of Tianjin under Grant 13JCZDJC26100, in part by the National Undergraduates Innovating Experimentation Project 201410055070, in part by the National Natural Science Foundation of China under Grant 11004110, Grant 11204212, Grant 11274182, and Grant 61377095, and in part by the National Key Basic Research and Development Program of China under Grant 2010CB327605. The associate editor coordinating the review of this paper and approving it for publication was Dr. Anna G. Mignani. H. Liu, B. Liu, W. Lin, H. Zhang, B. Song, M. Huang, and L. Lin are with the Key Laboratory of Optical Information Science and Technology, Ministry of Education, Institute of Modern Optics, Nankai University, Tianjin 300071, China (e-mail: [email protected]; liubo@ nankai.edu.cn; [email protected]; [email protected]; [email protected]; [email protected]; linlie@ nankai.edu.cn). Y. Miao is with the Tianjin Key Laboratory of Film Electronic and Communicate Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, China (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2015.2389519

sensors are traditional electric humidity sensors which usually employ two types of sensing elements: electric capacity and electric resistance. When these sensors are exposed in high humidity environment for a long time, they are easy to be contaminated to affect the long-term measurement accuracy. In recent years, fiber-optic fiber humidity sensors (OFHSs) have attracted great research interests due to their distinguished features such as compactness, immunity to electromagnetic interference, and remote operation mode. And numerous fiber structures highly sensitive to the environmental refractive index, including fiber gratings [1]–[5], evanescent field sensors [6]–[10], and fiber-optic interferometers [11]–[15], have been proposed for humidity measurement. Among these structures, A long-period grating (LPG) coated with gelatin exhibits a relative humidity sensitivity of 0.833%RH/dB for the RH range of 90%RH to 99%RH [2]. A tapered fiber with a hydrogel coating has been proposed and achieves a sensitivity of 0.0228 dB/%RH when RH increases from 50% to 80%RH [10]. A multilayer-based Fabry-Pérot (F-P) optical fiber sensor constructed by TiO2 and SiO2 films has been proposed and shows a RH sensitivity of 0.43nm/%RH within a RH range from 1.8%RH to 74.7%RH [13]. An in-fiber Mach-Zehnder interferometer (MZI) based on a single-mode multimode single-mode (SMS) fiber configuration fulfills a linear spectral response to humidity with enhanced sensitivity of 0.119 dB/%RH for the range of 35–90%RH [14]. Particularly, an S-tapered fiber modal interferometer with high RI sensitivity but temperature insensitivity has been demonstrated [16]–[18]. In addition, S-taper fiber possesses several advantages such as simple structure, compactness, ease of fabrication. Therefore, S-taper fiber interferometer is a good candidate for functional-material-assisted humidity sensing. To our knowledge, humidity sensor based on this S-taper fiber structure has not been reported in previous work. In this paper, an optical fiber humidity sensor is presented based on the S-taper fiber coated with SiO2 nanoparticles. SiO2 nanoparticles have been demonstrated to enhance the hydrate sensitivity of OFHSs due to the large surface area and porosity that enables quick water adsorption and evaporation, and this material has been used in optical fiber humidity sensors based on its desirable advantages and superhydrophilic nature [19]–[21]. In our work, The S-tapered fiber is simply fabricated by applying off-axis pull while tapering the fiber with a fusion splicer and then the tapered region is coated with SiO2 nanoparticles by immersing the tapered fiber into the SiO2 nanoparticle solution. The sensing principle of the proposed device has been theoretically analyzed and the

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Fig. 1. (a) Schematic diagram of the experimental setup. (b) Microscopic image of the S-tapered fiber humidity sensor.

transmission characteristics of the proposed humidity sensor have been experimentally investigated. Experimental results show that both of the transmission spectrum and interference dips are sensitive to humidity, and especially for the high region humidity range of 83.8%RH to 95.2%RH. It would find potential applications in geotechnical and agricultural monitoring for which typical humidity values range from 90% to 100%RH. II. P RINCIPLE AND D EVICES FABRICATION Fig. 1(a) shows the experimental setup for humidity measurement test, which consists of a super-continuum broadband source (SBS, 600nm to 1700nm), an optical spectrum analyzer (OSA: Yokogawa AQ6370C), a humidity chamber, two fiber holders and a hygrometer (Aosong AH8002, simultaneous measurement temperature and relative humidity). The lead-in and lead-out ports of the S-tapered fiber humidity sensor are connected with the SBS and OSA, respectively. The S-tapered fiber is fabricated by applying off-axis pull during the fiber tapering procedure (SMF-28e, Corning, Inc.) with a fiber fusion splicer (FiTel S178A, Japan). The relative axial off-set is adjusted through the manual re-positioning of the fusion splicer clamps. The length and waist diameter of the S-tapered fiber are controlled by the discharge current and the splicing time, respectively. Then S-tapered fiber is straightly held by two translation stages so that a layer of SiO2 nanoparticle coating could be deposited onto the S-tapered segment to prevent the bending effect on the sensing region, and the interferometric transmission spectral fringes are monitored by the OSA. The film is fabricated by directly immersing the cleaned S-taper segment into the SiO2 nanoparticle solution for at least 2 minutes and thereafter being dried in air for 2 minutes till the transmission spectrum is stable under a room temperature of 22 °C. This procedure has been repeated three times to roughly acquire a thickness of ∼7 μm. To ensure the film is coated over the S-tapered segment, the fiber immersing length should be long enough. During the film fabrication process, an adjustable platform with a glass substrate on it is employed to load the SiO2 nanoparticle solution. Here, the SiO2 nanoparticles of 30.5nm in diameter are mixed with

Fig. 2. (a) Schematic diagram of the S-tapered fiber modal interferometer. (b) Interference spectra of before and after SiO2 nanoparticle coating.

water to form a 40%wt solution. The microscopic image and geometry of optical fiber humidity sensor are shown in Fig. 1(b). In order to clearly observe the coated SiO2 texture, a part of the coating film is stripped off. Fig. 2(a) presents a schematic diagram of the S-tapered fiber humidity sensor, which is based on a compact fiber modal interferometer structure. When the incident light propagates into the tapered region, high-order modes are excited due to the perturbation in the first coupling region in the S-tapered fiber. The fundamental core mode and high-order cladding modes propagate through the sensing segment in the meanwhile, and when the light propagates to the second coupling region, the high-order cladding modes re-couple back and interfere with the fundamental core mode [16]. The nth interference spectrum attenuation peak wavelength λn of the S-tapered fiber can be expressed as: λn = 2n e f f L/(2n + 1)

(1)

where, n e f f is the effective refractive index difference between the effective refractive indices of the core and the cladding modes, n is integer, L is the effective length of the S-tapered region . The output light intensity of the interference spectrum could be expressed as: I O (λ) = I F (λ) + I H (λ) + 2 I F (λ)I H (λ)cos(2πn e f f L/λ) (2) Where, λ refers to optical wavelength, and I F and I H are the light intensities of the fundamental mode and high-order modes, respectively. The transmission spectra of the S-tapered fiber before and after SiO2 nanoparticle coating processing are given in Fig. 2(b). For this optical fiber humidity sensor, from Eqs. (1) and (2), we can see that both the attenuation peak wavelengths and intensity are dependent on n e f f which is directly related to the effective refractive indices of the

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Fig. 3.


Transmission spectral responses under different relative humidities.

cladding mode. When the S-tapered region is coated with a layer of SiO2 nanoparticles, the effective refractive indices of cladding modes decrease, which leads to the increment of n e f f . Therefore, as shown in Fig. 2(b), the attenuation peak wavelength exhibits a red shift and the intensity is tunable as well. After coating a layer of SiO2 nanoparticles onto the sensing segment, the OFHS is located in a humidity-controlled chamber. Owing to the super-hydrophilic of SiO2 nanoparticles, n e f f has a positive response as the effective cladding mode RI decreases with the increment of water content in SiO2 nanoparticles. Due to the coupling between the core mode and high-order cladding modes, the attenuation peaks would experience red shift, and in the meanwhile, the output spectral intensity Io (λ) will change with the variation of n e f f . III. E XPERIMENT R ESULTS AND D ISCUSSION In order to investigate the transmission spectral responses and performances of the S-tapered-based humidity sensor, our proposed sensor element is exposed in a humidity-controlled chamber to produce a cycled RH variation. The two ends of the sensor head are both held straightly by two fiber holders to prevent the influence of strain. The experimental test begins with 26.5% RH and the humidity is slowly increased up to 95.2%. Meanwhile, a hygrometer is used for monitoring temperature and humidity of chamber. The experiment is carried out under room temperature and atmospheric pressure. Fig. 3 shows part of the spectral responses of the optical fiber RH sensor as the humidity increases from 26.5%RH to 95.2%RH. The transmission dips show red shift and the peak transmission increases with the increment of humidity, especially for the RH range of 83.8%RH∼95.2%RH, as shown in Fig. 3. This is attributed to the super-hydrophilic of the SiO2 nanoparticle layer. One reason is that SiO2 nanoparticles have a strong surface activity induced by the Si-OH bonds. Here, -OH is easily synthesized with water to form hydrogen bonds, and present as “adsorbed water”. In addition, the other reason is that the large surface area and porosity of SiO2 nanoparticle coating also enable effective water absorption and evaporation. With the

Fig. 4. Resonance wavelength shift of the transmission spectrum in response to the relative humidity: (a) dip A (b) dip C.

water content increasing, the effective refractive index of the cladding modes would decrease, and thus n e f f increases with the decrease of effective cladding mode refractive index. From Eq.(1), the attenuation peak wavelength is in proportional to n e f f , and therefore, the transmission spectral peaks move toward longer wavelength region. Meanwhile, from Eq. (2), the transmission losses also change gradually with the humidity variation. Here we take dip A and C, for instance, to analyze the performance and the spectral response of the proposed RH sensor. Fig. 4(a) and (b) illustrate the peak wavelength as functions of environmental RH for dip A and C, respectively. As shown in Fig. 4, it can be found that dip A and C experience a red shift by about 15nm when humidity increases from 26.5% to 95.2% RH. The wavelength response of this sensor shows a high repeatability and low hysteresis. It can be classified into two sensitive regions according to the RH sensitivity. The two sensitive regions may be caused by the above-mentioned two factors affecting the SiO2 layer absorption. When the RH is less than 83.8%, the SiO2 nanoparticle coating of the humidity sensor adsorbs water mainly caused by the presence of Si-OH in cooperation with water to form hydrogen bonds, and thus the RH sensitivity is gradually increased. When the RH is larger than 83.8%, the

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employed SBS. In all, the performances of the S-tapered fiber humidity sensor show good reversibility and low hysteresis. Furthermore, the sensitivity could be further optimized by choosing appropriate SiO2 nanoparticle size, coating thickness and the S-tapered fiber geometry. IV. C ONCLUSION In this paper, a compact and simple structure sensor has been proposed and experimentally demonstrated using the S-taper fiber coated with SiO2 nanoparticles for relative humidity measurement. Experimental results show that both of the transmission loss and interference peak wavelength gradually change with the humidity variation, especially for the high region humidity beyond 83.8%RH. Due to the high sensitivity of S-taper fiber to ambient refractive index and the super-hydrophilic of SiO2 nanoparticles, the humidity sensitivities up to 1.1718nm/%RH and 0.441dB/%RH have been achieved, respectively. Spectral monitoring for the RH increasing and decreasing tests shows that the transmission spectrum of this RH sensor has good reversibility. Owing to advantages of compact size, low cost, high sensitivity, acceptable integration, flexible maneuverability, and immunity to electromagnetic interference; the SiO2 -coated S-tapered fiber RH sensor is expected to find potential applications in high humidity as well as harsh electromagnetic environments. R EFERENCES Fig. 5.

Transmission responses of (a) dip A and (b) dip C.

resonance wavelength becomes more sensitive to the increment of RH due to the dominance of water absorption. The RH sensitivities reach about 1.0928nm/%RH and 1.1718nm/%RH with R2 =0.98756, 0.99203, respectively. Therefore, this sensor is particularly suitable for applications in high humidity environments. Within the high RH region, the optical fiber humidity sensor shows humidity sensitivity beyond 1nm/%RH. The transmission loss responses of dip A and C to humidity are presented in Fig. 5, which are similar to their respective wavelength responses. With increment of humidity, the transmission losses increase from −40.54dB to −36.44dB and from −39.17dB to −33.20dB for dip A and dip C, respectively. Similar to the wavelength responses, the transmission losses have a positive RH response below 83.8%RH, and for the humidity above 83.8%RH, experiment results are linearly fitted with the R2 = 0.99175, 0.99495 for dip A and dip C, exhibiting the RH sensitivities of 0.271dB/%RH and 0.441dB/%RH, respectively. Since reversibility plays an important role for practical applications of a sensing system, we have experimentally investigated this issue. During our experiment, the results are recorded and compared for RH increasing and RH decreasing runs. As can be seen from Fig. 4 and Fig. 5, the RH responses of wavelength and transmission loss are well repeatable for these different runs. Some fluctuations are present in the transmission spectra, which may be caused by the power instability of the experimentally

[1] P. Kronenberg, P. K. Rastogi, P. Giaccari, and H. G. Limberger, “Relative humidity sensor with optical fiber Bragg gratings,” Opt. Lett., vol. 27, no. 16, pp. 1385–1387, Aug. 2002. [2] K. M. Tan, C. M. Tay, S. C. Tjin, C. C. Chan, and H. Rahardjo, “High relative humidity measurements using gelatin coated long-period grating sensors,” Sens. Actuators B, Chem., vol. 110, no. 2, pp. 335–341, Oct. 2005. [3] J. M. Corres, I. del Villar, I. R. Matias, and F. J. Arregui, “Enhanced sensitivity in humidity sensors based on long period fiber gratings,” in Proc. 5th IEEE Conf. Sensors, Oct. 2006, pp. 193–196. [4] D. Viegas et al., “Simultaneous measurement of humidity and temperature based on an SiO2 -nanospheres film deposited on a long-period grating in-line with a fiber Bragg grating,” IEEE Sensors J., vol. 11, no. 1, pp. 162–166, Jan. 2011. [5] H. Chen, Z. Gu, and K. Gao, “Humidity sensor based on cascaded chirped long-period fiber gratings coated with TiO2 /SnO2 composite films,” Sens. Actuators B, Chem., vol. 196, pp. 18–22, Jun. 2014. [6] C. Bariáin, I. R. Matías, F. J. Arregui, and M. López-Amo, “Optical fiber humidity sensor based on a tapered fiber coated with agarose gel,” Sens. Actuators B, Chem., vol. 69, nos. 1–2, pp. 127–131, May 2000. [7] I. R. Matias, F. J. Arregui, J. M. Corres, and J. Bravo, “Evanescent field fiber-optic sensors for humidity monitoring based on nanocoatings,” IEEE Sensors J., vol. 7, no. 1, pp. 89–95, Jan. 2007. [8] L. Zhang, F. Gu, J. Lou, X. Yin, and L. Tong, “Fast detection of humidity with a subwavelength-diameter fiber taper coated with gelatin film,” Opt. Exp., vol. 16, no. 17, pp. 13349–13353, Aug. 2008. [9] T. Li, X. Dong, C. C. Chan, C.-L. Zhao, and P. Zu, “Humidity sensor based on a multimode-fiber taper coated with polyvinyl alcohol interacting with a fiber Bragg grating,” IEEE Sensors J., vol. 12, no. 6, pp. 2205–2208, Jun. 2012. [10] A. Lokman, S. Nodehi, M. Batumalay, H. Arof, H. Ahmad, and S. W. Harun, “Optical fiber humidity sensor based on a tapered fiber with hydroxyethylcellulose/polyvinylidenefluoride composite,” Microw. Opt. Technol. Lett., vol. 56, no. 2, pp. 380–382, 2014. [11] W. Xie, M. Yang, Y. Cheng, D. Li, Y. Zhang, and Z. Zhuang, “Optical fiber relative-humidity sensor with evaporated dielectric coatings on fiber end-face,” Opt. Fiber Technol., vol. 20, no. 4, pp. 314–319, Aug. 2014. [12] M. Yang et al., “Dielectric multilayer-based fiber optic sensor enabling simultaneous measurement of humidity and temperature,” Opt. Exp., vol. 22, no. 10, pp. 11892–11899, May 2014.

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[13] P. Hu, X. Dong, K. Ni, L. H. Chen, W. C. Wong, and C. C. Chan, “Sensitivity-enhanced Michelson interferometric humidity sensor with waist-enlarged fiber bitaper,” Sens. Actuators B, Chem., vol. 194, pp. 180–184, Apr. 2014. [14] M. Shao, X. Qiao, H. Fu, H. Li, J. Zhao, and Y. Li, “A Mach–Zehnder interferometric humidity sensor based on waist-enlarged tapers,” Opt. Laser Eng., vol. 52, pp. 86–90, Jan. 2014. [15] B. Gu, M. Yin, A. P. Zhang, J. Qian, and S. He, “Optical fiber relative humidity sensor based on FBG incorporated thin-core fiber modal interferometer,” Opt. Exp., vol. 19, no. 5, pp. 4140–4146, Feb. 2011. [16] R. Yang, Y.-S. Yu, Y. Xue, C. Chen, Q.-D. Chen, and H.-B. Sun, “Single S-tapered fiber Mach–Zehnder interferometers,” Opt. Lett., vol. 36, no. 23, pp. 4482–4484, Dec. 2011. [17] Y. Miao et al., “Magnetic field tunability of optical microfiber taper integrated with ferrofluid,” Opt. Exp., vol. 21, no. 24, pp. 29914–29920, Dec. 2013. [18] R. Yang et al., “S-tapered fiber sensors for highly sensitive measurement of refractive index and axial strain,” J. Lightw. Technol., vol. 30, no. 19, pp. 3126–3132, Oct. 1, 2012. [19] K. Ogawa, S. Tsuchiya, H. Kawakami, and T. Tsutsui, “Humiditysensing effects of optical fibres with microporous SiO2 cladding,” Electron. Lett., vol. 24, no. 1, pp. 42–43, Jan. 1988. [20] J. Bravo, L. Zhai, Z. Wu, R. E. Cohen, and M. F. Rubner, “Transparent superhydrophobic films based on silica nanoparticles,” Langmuir, vol. 23, no. 13, pp. 7293–7298, May 2007. [21] J. M. Corres, I. R. Matias, M. Hernaez, J. Bravo, and F. J. Arregui, “Optical fiber humidity sensors using nanostructured coatings of SiO2 nanoparticles,” IEEE Sensors J., vol. 8, no. 3, pp. 281–285, Mar. 2008.

Haifeng Liu was born in Shanxi, China, in 1986. He received the master’s degree from Nankai University, Tianjin, China, in 2013. He is recommended for admission to Nankai University, where he is currently pursuing the Ph.D. degree. His major research focuses on fiber sensor and photonic devices based on mode coupling.

Yinping Miao was born in Shanxi, China, in 1980. She received the Ph.D. degree from Nankai University, Tianjin, China, in 2009. She is currently an Associate Professor with the School of Electronics Information Engineering, Tianjin University of Technology, Tianjin. Her research interests are mainly focused on optical fiber communication and sensing technology and their real applications.


Bo Liu was born in Shandong, China, in 1975. He received the Ph.D. degree in optics from Nankai University, Tianjin, China, in 2004, where he is currently a Professor with the Institute of Modern Optics. His research interests are in fiber sensors, fiber Bragg grating sensors, and interrogation systems.

Wei Lin was born in Fujian, China, in 1988. He received the bachelor’s degree from Nankai University, Tianjin, China, in 2011. He is currently recommended for admission to Nankai University, where he is currently pursuing the Ph.D. degree. His major research focuses on photonic devices based on mode coupling.

Hao Zhang was born in Tianjin, China, in 1978. He received the Ph.D. degree in optics from Nankai University, Tianjin, in 2005, where he is currently an Associate Professor with the Institute of Modern Optics. His research interests include micro/nanostructured fiber devices, novel fiber sensors, and fiber lasers.

Binbin Song was born in Shandong, China, in 1988. She received the bachelor’s degree from Shandong Jianzhu University, Shandong, in 2012. She is currently recommended for admission to Nankai University, where she is currently pursuing the Ph.D degree. Her major research focuses on photonic devices based on mode coupling.

Mengdi Huang was born in Fujian, China, in 1994. She is recommended for admission to Nankai University, Tianjin, China, where she is currently pursuing the bachelor’s degree.

Lie Lin was born in Beijing, China, in 1963. He received the Ph.D. degree in optics from Nankai University, Tianjin, China, where he is currently a Professor with the Institute of Modern Optics. His research interests are in biomedical photonics and optoelectronic materials.