- Email: [email protected]
SnO2 composite films
Sensors and Actuators B 196 (2014) 18–22
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Humidity sensor based on cascaded chirped long-period fiber gratings coated with TiO2 /SnO2 composite films Haiyun Chen a,b,1,2 , Zhengtian Gu c,∗ , Kan Gao d,3 a School of Optical-Electric and Computer Engineering, University of Shanghai for Science and Technology, PO Box 249, 516 Jungong Road, Shanghai 200093, PR China b Institute of Information Optics, Zhejiang Normal University, 688 Yingbin Road, Jinhua 321004, Zhejiang Province, PR China c Laboratory of Opto-electric Functional Films, College of Science, University of Shanghai for Science and Technology, PO Box 249, 516 Jungong Road, Shanghai 200093, PR China d No. 23 Research Institute of China Electronic Technology Corporation Group, 230 Tieshan Road, Baoshan district, Shanghai 201900, PR China
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 15 January 2014 Accepted 23 January 2014 Available online 31 January 2014 Keywords: Fiber optics Relative humidity (RH) Cascaded chirped long-period fiber grating (CCLPFG) Composite film Sensitivity Resolution
a b s t r a c t We propose a novel scheme of high-sensitivity relative humidity (RH) sensor based on cascaded chirped long-period fiber gratings (CCLPFG) and sol–gel derived TiO2 /SnO2 composite films. Two identical linearly chirped LPFGs without a separation were inscribed into a single-mode fiber core sequentially, which produces interference patterns in the transmission spectrum. The mole ratio of SnO2 is 4% in the sol–gel derived TiO2 /SnO2 composite films that were deposited on the cladding surface. The improved adsorption ability of water molecules of the composite films, combined with fine fringe patterns of CCLPFG, enables this sensor to be very sensitive to humidity. In experiment, a wavelength shift of 12.37 nm was achieved for RH varying from 40% to 95% at 15 ◦ C, which exhibits an approximately linear response. The RH sensitivity of this sensor will slightly decrease for increasing temperature. The sensitivities for 15 ◦ C, 20 ◦ C, 30 ◦ C and 40 ◦ C are 0.221 nm/%, 0.218 nm/%, 0.214 nm/% and 0.211 nm/%, respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Relative humidity (RH) measuring is important in numerous areas such as health monitoring, food processing, air conditioning and weather observations [1]. In recent years, great efforts have been dedicated to developing optical-fiber-based RH sensing techniques owing to the advantages such as small size, low weight and immunity to radio interference [1–9]. So far, a wide range of materials such as SiO2 [2], hydrogel [3], gelatin [4], agarose gel [5], PVA [6], PDDA/Poly R-48 [7], PEO [8] and TiO2 [9] have been developed for fiber-based RH sensing. In these sensors, the materials are present in form of films that are deposited onto the cladding surface. The films coated on fiber serve as transducers that adsorb water molecules, which in turn changes the film refractive index (RI) or thickness and thus changes the output of fiber. Among these film materials, sol–gel derived TiO2 has a high adsorption of water
∗ Corresponding author. Tel.: +86 021 65693858; fax: +86 021 55274813. E-mail addresses: [email protected], [email protected] (Z. Gu), [email protected] (K. Gao). 1 Tel.: +86 021 65666454; fax: +86 021 55274813. 2 Tel.: +86 0579 82410523; fax: +86 0579 82298863. 3 Tel.: +86 021 33792800; fax: +86 021 33792777. 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.090
molecules. When scaled to the nanosize, it will have a high surfaceto-volume ratio and, hence, more surface area to interact with the water molecules [10]. Aneesh et al. reported a TiO2 -based optical fiber RH sensor [9]. When the central decladded fiber core that carries TiO2 nanoparticle immobilized nanostructured sol–gel thin film is exposed to humidity environment, an adsorption of the optical power carried by the evanescent field is induced. This adsorption of the optical power varies with humidity and finally results in a modulated fiber output, which serves as a criterion for determining humidity. Nevertheless, the adsorption ability of TiO2 films can be further improved by doping with Sn2+ at a suitable ratio [11], which is expected to further improve the sensitivity of TiO2 -based RH sensor. Long-period fiber gratings (LPFGs) play significant roles in the fiber-based RH sensing technique [2–6]. An LPFG can couple the core mode to codirectional cladding modes and produce discrete attenuation bands in the transmission spectra. Especially, LPFGs coated with sensitive thin film can act as chemical sensors [12]. By cascading two identical 3-dB LPFGs, an in-fiber Mach–Zenhder interferometer (MZI) is formed, whose transmission spectra are characterized as sinusoidal channeled patterns. It is stressed that a separation fiber between two LPFGs is indispensable in this uniform LPFG based MZI scheme to introduce phase difference and thus, to form interference patterns. Generally, MZI-based sensors possess
H. Chen et al. / Sensors and Actuators B 196 (2014) 18–22
a
TiO2/SnO2 composite film cladding
λ1
n3
n n1
λ2
core CCLPFG
b
19
L
n2 n4 a1 a2 a3
Fig. 1. Schematic of humidity sensor: (a) structural diagram; (b) refractive index profile.
higher sensitivity to RI than that is based on a single LPFG, which depends on the fact that an extra phase difference is introduced by the separation fiber in addition to the envelope shift [13]. Furthermore, the finer-scale interference fringes offer potentially higher resolution of the MZI-based sensors. For the chemical sensing applications, however, the presence of this separation fiber is apt to introduce extra interference of bending or temperature. In fact, MZI can also be obtained by cascading two identical linearly chirped LPFGs even in absence of a separation fiber [14], which offers an alternate in-fiber interfering scheme for sensing applications. In this paper, we present a compact and high-sensitivity relative humidity sensor based on zero-interval cascaded chirped longperiod fiber gratings (CCLPFG) coated with TiO2 /SnO2 composite films for the first time, to the best of our knowledge. This sensor synthesizes the interfering fringe characteristics of CCLPFG and the high adsorption ability of water molecules of TiO2 films doped with SnO2 , which ensures a high sensitivity to humidity. Furthermore, the elimination of separation fiber from the MZI scheme improves the sensor compactness and reduces the interference of bending and temperature. Moreover, the sensor designer has another parameter (grating chirp) at his disposal to tune the sensitivity, in addition to the grating length and period as well as the film parameters. 2. Sensing principle The RH sensor structure diagram and radial profile of refractive index are illustrated in Fig. 1. As shown in Fig. 1(a), two identical linearly chirped LPFGs are inscribed in the fiber core without a separation and the sensitive film whose refractive index is higher than that of silica is deposited directly onto the cladding. The structure of a single linearly chirped LPFG is described as [14]:
(z) = 0 + C z −
L 2
(1)
where z is along the fiber axis and the origin is at the grating edge; L is the grating length, 0 is the grating period at grating center (z = L/2), namely, the average period; C (m/cm) denotes the grating chirp. Therefore, the phase-matching condition of a chirped LPFG can be given as [14]: res (z) = (nco − ncl,m )(z) eff eff
grating. Therefore, a separation fiber which acts to introduce phase difference in cascaded uniform LPFGs is no longer indispensable in CCLPFG. In the coated CCLPFG, the sensitive film acts not only to adsorb water molecules, but also to adjust the sensitivity. By depositing the films with suitable parameters (refractive index n3 and thickness h3 ), the sensitivity can be significantly improved. This improvement depends on the mechanism of mode transition and reorganization induced by the growth of films [15], which helps the designer to determine the optimum film thickness since the film refractive index is approximately known when film material is selected. Based on the design methodology for coated LPFGs in transition mode [16], the calculated range of film thickness for high sensitivity is approximately 66–78 nm for n3 = 2.38, which serves as a reference for depositing films onto the cladding surface with optimum thicknesses in the follow-up experiments. For analyzing the transmission characteristics of this nonuniform grating structure, transfer-matrix method [17] is a powerful tool where the grating is divided into N sections and each section is considered as a uniform grating. The simulated transmission spectra of bare chirped LPFG and CCLPFG are shown in Fig. 2; the grating structural parameters in the simulation are: 0 = 305 m, L = 2 cm, C = 6 m/cm and = 4 × 10−4 , where denotes the average modulation of core index. As a result of chirping effect, the spectrum of a chirped LPFG features broadened attenuated peaks [14]. When two identical chirped LPFGs are concatenated, however, high-contrast interfering fringe is evident in Fig. 2, which affords potential application as a sensor. According to the simulation, the two fringe patterns in Fig. 2 around 1310 nm and 1650 nm correspond to the couplings from core mode LP01 to cladding mode LP06 and LP07 , respectively. Since the higher-order cladding mode coupling is more sensitive to the environmental refractive index [18], the fringe corresponding to LP07 is interrogated in the remainder of this paper. The solid line in Fig. 3 illustrates the fringe of a CCLPFG coated with a film (n3 = 2.38, h3 = 70 nm), whereas the dashdotted line is the fringe when n3 increases to 2.3805. A comparison between Figs. 2 and 3 shows that the presence of film induces a
(2)
where nco and ncl,m denote the effective refractive indexes (ERIs) eff eff of core mode and mth cladding mode, respectively. From Eq. (2), it is known that the core mode of different wavelengths, e.g. 1 and 2 in Fig. 1, is coupled to cladding modes at different positions in a chirped LPFG; the excited cladding modes will be recoupled into fiber core by the second grating at a corresponding position. For a given wavelength of 1 , for example, the distance from coupling to recoupling position is equal to the grating length L. Across this length, the residual core mode after the first grating and the excited cladding modes propagate through the core and cladding respectively. This introduces phase differences between core mode and cladding modes, which results in interference effect when the cladding modes are recoupled into the core by the second
Fig. 2. Simulated transmission spectrum of a bare CCLPFG.
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H. Chen et al. / Sensors and Actuators B 196 (2014) 18–22
Fig. 3. Simulated transmission spectra of a coated CCLPFG for different film refractive indexes.
shift of fringe towards shorter wavelength by 50 nm, which is dominantly caused by the increase of the cladding mode ERI as the film is deposited onto the fiber surface. For the designed humidity sensor in this paper, the adsorption of water molecules by the porous film results in an increase of film refractive index and thus gives rise to a blue shift of the fringe pattern. As is depicted in Fig. 3, the shift of the central peak is evaluated to be 11.8 nm when the film refractive increases from 2.38 to 2.3805.This implies a resolution of film refractive index is available to be the order of 10−7 since the wavelength resolution of a commonly used optical spectrum analyzer (OSA) reaches 10−2 nm. 3. Fabrication In experiment, the used fiber was Corning SMF-28 which was hydrogen loaded in advance for a week to enhance its photosensitivity. The gratings were fabricated by illuminating the fiber core with 248 nm KrF excimer laser. The pulse energy, frequency and exposure time are 24 mJ, 150 Hz and 13 s, respectively. The fabrication of a single chirped LPFG was performed by writing the grating with period from 298 m to 310 m and the step was 1 m. For a given grating period value, 5 periods were inscribed as a section of uniform grating. Consequently, the length of this chirped LPFG is estimated to be 1.98 cm and the equivalent chirp is 6.06 m/cm. Likewise, another identical grating was fabricated directly adjacent to the first one. All of these procedures were automatically performed under the control of computer programs.
Fig. 5. Surface morphology of TiO2 /SnO2 composite film under atomic force microscope.
The fabricated CCLPFG was then annealed at 120 ◦ C for 12 h to improve thermal stability. The dash-dotted line in Fig. 4 illustrates the measured transmission spectrum of fabricated CCLPFG, where two high-contrast fringe patterns are observed within the wavelength range from 1250 nm to 1650 nm. The measured spectrum is accordant with the simulated one in Fig. 2, despite that a part of the right pattern was invisible due to the wavelength limit of the used OSA. The SnO2 solution was obtained by dissolving 1.9 g of SnCl2 ·2H2 O in 50 ml absolute ethanol and was refluxed and stirred at 80 ◦ C for 1 h, whereas the TiO2 solution was prepared by dissolving 15 ml of C16 H36 O4 Ti in 200 ml absolute ethanol. This solution was stirred at room temperature for 2 h, during which the pH value of solution was adjusted to 4.5 by adding drops of diluted HNO3 . Afterward, five drops of polyethylene glycol (PEG) were added in the solution which was stirred for 30 min. Then the SnO2 and TiO2 solution were mixed where the molar ratio of SnO2 was 4%. Finally, the mixed solution was refluxed and stirred at 80 ◦ C for 2 h and aged at 30 ◦ C for 24 h to get sol. Doping with SnO2 at a suitable ratio was expected to enhance the surface roughness of TiO2 films, whereas the adding of PEG enables the film not to be apt to crack. Thus, this composite film possesses better hydrophilic and physical properties than pure TiO2 film. The precleaned grating was drawn from the sol smoothly at a speed of 5 cm/min. Then it was put into a tube-type resistance furnace for thermal treatment. It was heated from room temperature to 450 ◦ C at a rate of 3 ◦ C/min and kept at 450 ◦ C for 60 min, and then cooled down unaffectedly. Fig. 5 gives the surface morphology of TiO2 /SnO2 composite film under atomic force microscope (PSIA XE-100) where the surface roughness is evident. This porous structure is much suitable for humidity sensing. The film refractive index and thickness were measured to be 2.3789 and 72.9 nm, respectively, which fall in the high-sensitivity region according to the design methodology for coated LPFGs in transition mode [16]. 4. Results and discussion
Fig. 4. Measured transmission spectra of bare and coated CCLPFG.
The solid line in Fig. 4 is the spectrum of coated CCLPFG. Compared with the transmission spectrum of bare CCLPFG, a shift of the fringe patterns towards shorter wavelength is evident, which depends on the fact that the deposition of film increases the ERI of cladding modes and thus, results in the decrease of resonant wavelength according to the phase-matching condition of LPFG [17]. Another conspicuous change is the decrease of peak losses, which reduced the contrast between maximum and minimum transmission, i.e. the fringe contrast. This change mainly stems from the
H. Chen et al. / Sensors and Actuators B 196 (2014) 18–22
influence of film on the coupling strength between core mode and cladding modes. The peak loss of a single chirped LPFG in fabrication is controlled to be about 3 dB; thus, the fringe contrast of bare CCLPFG reaches maximum according to the MZI mechanism [13]. The deposition of TiO2 /SnO2 composite film changes the coupling strength from core mode to cladding modes, which causes the peak loss of a single chirped LPFG to deviate from 3 dB and thus results in the great degradation of the fringe contrast of CCLPFG. High-temperature treatment may also play a role in the decrease of fringe contrast since the grating, combined with the dip-coated film, was put in the furnace for thermal treating at high temperature around 450 ◦ C to form porous film structure. As mentioned above, the higher-order cladding mode coupling is more sensitive to the environmental refractive index, so the right pattern of coated CCLPFG in Fig. 4 is interrogated in the following RH sensing test. In addition, the peak marked with a letter “P” is rather smooth, which means relative ease of searching the peak wavelength. Therefore, the wavelength shift of the marked peak serves as a measure to characterize the humidity variation. The humidity sensing was carried out in a humidity controllable room. Light from broadband optical source was injected into the sensor and the output spectrum was monitored using an OSA (YOKOGAWA, AQ6370). The environmental temperature was kept at 15 ± 0.5 ◦ C so as to minimize the interference of temperature fluctuation. The humidity was controlled to vary from 40% to 95% gradually and the ensuing change of spectrum was monitored and recorded. For reference, a commercial hydrometer (TES 1360A) was employed to measure the humidity synchronously. The recorded peak shifts are shown in Fig. 6, where a blue shift of wavelength is evident as the RH increases. For the humidity from 40% to 95%, a wavelength shift of 12.37 nm is achieved. Therefore, the average sensitivity of this sensor is evaluated to be 0.221 nm/%. The dependence of
21
Fig. 6. Peak shift of the sensor for RH from 40% to 95%.
peak wavelength on RH increasing from 40% to 95% is illustrated in Fig. 7(a) (filled square), which shows an approximately linear response of this sensor to RH across the interrogated range. The dependence of peak wavelength on RH decreasing from 95% to 40% is also demonstrated in Fig. 7 (circle), which implies that the RH response of this sensor is reversible and stable for the interrogated RH range. The resultant data shown in Fig. 7 indicate that the largest RH difference for the same peak wavelength for RH increase and decrease is about 3.8%, whereas the greatest wavelength difference for the same RH is about 0.8 nm. In order to inspect the influence of temperature on the humidity sensing ability of this sensor, we also implemented the humidity sensing tests at different environmental temperatures.
Fig. 7. Response of peak wavelength to RH at temperature of (a) 15 ◦ C, (b) 20 ◦ C, (c) 30 ◦ C and (d) 40 ◦ C.
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H. Chen et al. / Sensors and Actuators B 196 (2014) 18–22
The results illustrated in Fig. 7 show that the sensor has similar linear response to the humidity from 40% to 95% when the temperature rises to 20 ◦ C, 30 ◦ C, and 40 ◦ C. An increase of peak wavelength for a given relative humidity value is evident when the temperature increases, which relies on the intrinsic sensitivity of LPFG-based devices to temperature [19]. Furthermore, the fitting results show that the increase of temperature degrades the humidity sensitivity of this sensor. The humidity sensitivities for 15 ◦ C, 20 ◦ C, 30 ◦ C and 40 ◦ C are 0.221 nm/%, 0.218 nm/%, 0.214 nm/% and 0.211 nm/%, respectively. This sensitivity degradation stems from the fact that the water vapor is increasingly difficult to be adsorbed by the porous film when the environmental temperature increases [9]. 5. Conclusions In summary, we have demonstrated a novel scheme of humidity sensor based on cascaded chirped long-period fiber gratings coated with sol–gel derived TiO2 /SnO2 composite films. This is an MZI-based LPFG sensor whose fine fringe pattern ensures higher sensitivity than the common LPFGs. Meanwhile, a separation fiber between two gratings is no longer necessary in this scheme, which improves the device compactness and reduces the possible external interference. Incorporated with the porous TiO2 /SnO2 composite films, this sensor exhibits high sensitivity and linear response to the humidity. The peak wavelength shift of 12.37 nm is achieved for RH 40% ∼ 95% at 15 ◦ C and the average sensitivity is evaluated to be 0.221 nm/%. Further humidity sensing tests at different temperatures show that increasing temperature causes the increase of peak wavelength for a given humidity value and degrades the humidity sensitivity of this sensor. It is stressed that the sensitivity may be further enhanced by optimizing the grating structural parameters such as grating chirp and length as well as the core index modulation. Owing to the advantages of high-sensitivity and compactness, our proposed RH sensor will likely be developed for the practical applications.
[6] L. Alwis, T. Sun, K.T.V. Grattan, Fibre optic long period grating-based humidity sensor probe using a Michelson interferometric arrangement, Sens. Actuators B: Chem. 178 (2013) 694–699. [7] J.M. Corres, F.J. Arregui, I.R. Matias, Sensitivity optimization of tapered optical fiber humidity sensors by means of tuning the thickness of a nanostructured sensitive coatings, Sens. Actuators B: Chem. 122 (2007) 442–449. [8] Q. Wu, Y. Semenova, J. Mathew, P.F. Wang, G. Farrel, Humidity sensor based on a single-mode hetero-core fiber structure, Opt. Lett. 36 (10) (2011) 1752–1754. [9] R. Aneesh, S.K. Khijwana, Titanium dioxide nanoparticle based optical fiber humidity sensor with linear response and enhanced sensitivity, Appl. Opt. 51 (12) (2012) 2164–2171. [10] N. Bonini, M.C. Carotta, A. Chiorino, V. Guidi, C. Malagu, G. Martinelli, L. Paglialonga, M. Sacerdoti, Doping of a nanostructured titania thick film: structural and electrical investigations, Sens. Actuators B: Chem. 68 (2000) 274–280. [11] Z.M. Shi, L. Yan, L.N. Jin, X.M. Lu, G. Zhao, The phase transformation behaviors of Sn2+ -doped Titania gels, J. Non-Cryst. Solids 353 (2007) 2171–2178. [12] A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, M. Giordano, G. Guerra, Coated long-period fiber gratings as high-sensitivity optochemical sensors, J. Lightw. Technol. 24 (4) (2006) 1776–1786. [13] B.H. Lee, J. Nishii, Bending sensitivity of in-series long-period fiber gratings, Opt. Lett. 23 (20) (1998) 1624–1626. [14] M. Das, K. Thyagarajan, Wavelength-division multiplexing isolation filter using concatenated chirped long period gratings, Opt. Commun. 197 (2001) 67–71. [15] A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, Cladding mode reorganization in high-refractive-index-coated long-period grating: effects on the refractive-index sensitivity, Opt. Lett. 30 (19) (2005) 2536–2538. [16] P. Pilla, C., Trono, F., Baldini, F., Chiavaioli, M., Giordano, A. Cusano, Giant sensitivity of long period gratings in transition mode near the dispersion turning point: an integrated design approach, Opt. Lett. 37(19), 2012, 4152-4154. [17] T. Erdogan, Fiber grating spectra, J. Lightw. Technol. 15 (8) (1997) 1277–1294. [18] H.J. Patrick, A.D. Kersey, F. Bucholtz, Analysis of the response of long period fiber gratings to external index of refraction, J. Lightw. Technol. 16 (9) (1998) 1606–1612. [19] X.W. Shu, L. Zhang, I. Bennion, Sensitivity characteristics of long-period fiber gratings, J. Lightw. Technol. 20 (2) (2002) 255–266.
Biographies Haiyun Chen was born in Shaoxing, China, in 1977. He received the B.S. degree and M.S. degree from Zhejiang Normal University, Jinhua, China, in 2001 and 2007, respectively. He has worked in Department of Physics, Zhejiang Normal University since 2001. He is currently working toward the Ph.D. degree at University of Shanghai for Science and Technology. His current research interests include fiber grating, opto-electronic sensing.
Acknowledgments This work is jointly supported by the National Science Foundation of China (60777035), the Scientific Research Key Project Fund(20840), the Research Project (11ZZ131) of Education Committee of Shanghai, and Shanghai Leading Academic Discipline Project(S30502).
Zhengtian Gu was born in Shanghai, China, in 1965. He received the Ph.D. degree from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, China, in 2000. He is a professor at University of Shanghai for Science and Technology. His current research interests include fiber grating sensors, opto-electronic engineering and opto-electric functional films
References [1] T.L. Yeo, T. Sun, K.T.V. Grattan, Fibre-optic technologies for humidity and moisture measurement, Sens. Actuators A: Phys. 44 (2008) 280–295. [2] D. Viegas, J. Goicoechea, J.M. Corres, J.L. Santos, L.A. Ferreira, F.M. Araujo, I.R. Matias, A fibre optic humidity sensor based on a long-period fibre grating coated with a thin film of SiO2 nanospheres, Meas. Sci. Technol. 20 (2009) 031002 (4pp). [3] L.W. Wang, Y. Liu, M. Zhang, D.S. Tu, X.H. Mao, Y.B. Liao, A relative humidity sensor using a hydrogel-coated long period grating, Meas. Sci. Technol. 18 (2007) 3131–3134. [4] K.M. Tan, C.M. Tay, S.C. Tjin, C.C. Chan, H. Rahardjo, High relative humidity measurements using gelatin coated long-period grating sensors, Sens. Actuators B: Chem. 110 (2005) 335–341. [5] Y.P. Miao, K.K. Zhang, Y.J. Yuam, B. Liu, H. Zhang, Y. Liu, J.Q. Yao, Agarose gel-coated LPG based on two sensing mechanisms for relative humidity measurement, Appl. Opt. 52 (1) (2013) 91–95.
Kan Gao was born in Jiangsu, China, in 1975. He received his Ph.D. degree from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences in 2004. Now he is a senior engineer of China Electronics Technology Group Corporation No.23 Research Institute. His main research interests are optical fiber gratings, optical fiber sensing and optical fiber laser.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Humidity sensor based on cascaded chirped long-period fiber gratings coated with TiO2 /SnO2 composite films Haiyun Chen a,b,1,2 , Zhengtian Gu c,∗ , Kan Gao d,3 a School of Optical-Electric and Computer Engineering, University of Shanghai for Science and Technology, PO Box 249, 516 Jungong Road, Shanghai 200093, PR China b Institute of Information Optics, Zhejiang Normal University, 688 Yingbin Road, Jinhua 321004, Zhejiang Province, PR China c Laboratory of Opto-electric Functional Films, College of Science, University of Shanghai for Science and Technology, PO Box 249, 516 Jungong Road, Shanghai 200093, PR China d No. 23 Research Institute of China Electronic Technology Corporation Group, 230 Tieshan Road, Baoshan district, Shanghai 201900, PR China
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 15 January 2014 Accepted 23 January 2014 Available online 31 January 2014 Keywords: Fiber optics Relative humidity (RH) Cascaded chirped long-period fiber grating (CCLPFG) Composite film Sensitivity Resolution
a b s t r a c t We propose a novel scheme of high-sensitivity relative humidity (RH) sensor based on cascaded chirped long-period fiber gratings (CCLPFG) and sol–gel derived TiO2 /SnO2 composite films. Two identical linearly chirped LPFGs without a separation were inscribed into a single-mode fiber core sequentially, which produces interference patterns in the transmission spectrum. The mole ratio of SnO2 is 4% in the sol–gel derived TiO2 /SnO2 composite films that were deposited on the cladding surface. The improved adsorption ability of water molecules of the composite films, combined with fine fringe patterns of CCLPFG, enables this sensor to be very sensitive to humidity. In experiment, a wavelength shift of 12.37 nm was achieved for RH varying from 40% to 95% at 15 ◦ C, which exhibits an approximately linear response. The RH sensitivity of this sensor will slightly decrease for increasing temperature. The sensitivities for 15 ◦ C, 20 ◦ C, 30 ◦ C and 40 ◦ C are 0.221 nm/%, 0.218 nm/%, 0.214 nm/% and 0.211 nm/%, respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Relative humidity (RH) measuring is important in numerous areas such as health monitoring, food processing, air conditioning and weather observations [1]. In recent years, great efforts have been dedicated to developing optical-fiber-based RH sensing techniques owing to the advantages such as small size, low weight and immunity to radio interference [1–9]. So far, a wide range of materials such as SiO2 [2], hydrogel [3], gelatin [4], agarose gel [5], PVA [6], PDDA/Poly R-48 [7], PEO [8] and TiO2 [9] have been developed for fiber-based RH sensing. In these sensors, the materials are present in form of films that are deposited onto the cladding surface. The films coated on fiber serve as transducers that adsorb water molecules, which in turn changes the film refractive index (RI) or thickness and thus changes the output of fiber. Among these film materials, sol–gel derived TiO2 has a high adsorption of water
∗ Corresponding author. Tel.: +86 021 65693858; fax: +86 021 55274813. E-mail addresses: [email protected], [email protected] (Z. Gu), [email protected] (K. Gao). 1 Tel.: +86 021 65666454; fax: +86 021 55274813. 2 Tel.: +86 0579 82410523; fax: +86 0579 82298863. 3 Tel.: +86 021 33792800; fax: +86 021 33792777. 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.090
molecules. When scaled to the nanosize, it will have a high surfaceto-volume ratio and, hence, more surface area to interact with the water molecules [10]. Aneesh et al. reported a TiO2 -based optical fiber RH sensor [9]. When the central decladded fiber core that carries TiO2 nanoparticle immobilized nanostructured sol–gel thin film is exposed to humidity environment, an adsorption of the optical power carried by the evanescent field is induced. This adsorption of the optical power varies with humidity and finally results in a modulated fiber output, which serves as a criterion for determining humidity. Nevertheless, the adsorption ability of TiO2 films can be further improved by doping with Sn2+ at a suitable ratio [11], which is expected to further improve the sensitivity of TiO2 -based RH sensor. Long-period fiber gratings (LPFGs) play significant roles in the fiber-based RH sensing technique [2–6]. An LPFG can couple the core mode to codirectional cladding modes and produce discrete attenuation bands in the transmission spectra. Especially, LPFGs coated with sensitive thin film can act as chemical sensors [12]. By cascading two identical 3-dB LPFGs, an in-fiber Mach–Zenhder interferometer (MZI) is formed, whose transmission spectra are characterized as sinusoidal channeled patterns. It is stressed that a separation fiber between two LPFGs is indispensable in this uniform LPFG based MZI scheme to introduce phase difference and thus, to form interference patterns. Generally, MZI-based sensors possess
H. Chen et al. / Sensors and Actuators B 196 (2014) 18–22
a
TiO2/SnO2 composite film cladding
λ1
n3
n n1
λ2
core CCLPFG
b
19
L
n2 n4 a1 a2 a3
Fig. 1. Schematic of humidity sensor: (a) structural diagram; (b) refractive index profile.
higher sensitivity to RI than that is based on a single LPFG, which depends on the fact that an extra phase difference is introduced by the separation fiber in addition to the envelope shift [13]. Furthermore, the finer-scale interference fringes offer potentially higher resolution of the MZI-based sensors. For the chemical sensing applications, however, the presence of this separation fiber is apt to introduce extra interference of bending or temperature. In fact, MZI can also be obtained by cascading two identical linearly chirped LPFGs even in absence of a separation fiber [14], which offers an alternate in-fiber interfering scheme for sensing applications. In this paper, we present a compact and high-sensitivity relative humidity sensor based on zero-interval cascaded chirped longperiod fiber gratings (CCLPFG) coated with TiO2 /SnO2 composite films for the first time, to the best of our knowledge. This sensor synthesizes the interfering fringe characteristics of CCLPFG and the high adsorption ability of water molecules of TiO2 films doped with SnO2 , which ensures a high sensitivity to humidity. Furthermore, the elimination of separation fiber from the MZI scheme improves the sensor compactness and reduces the interference of bending and temperature. Moreover, the sensor designer has another parameter (grating chirp) at his disposal to tune the sensitivity, in addition to the grating length and period as well as the film parameters. 2. Sensing principle The RH sensor structure diagram and radial profile of refractive index are illustrated in Fig. 1. As shown in Fig. 1(a), two identical linearly chirped LPFGs are inscribed in the fiber core without a separation and the sensitive film whose refractive index is higher than that of silica is deposited directly onto the cladding. The structure of a single linearly chirped LPFG is described as [14]:
(z) = 0 + C z −
L 2
(1)
where z is along the fiber axis and the origin is at the grating edge; L is the grating length, 0 is the grating period at grating center (z = L/2), namely, the average period; C (m/cm) denotes the grating chirp. Therefore, the phase-matching condition of a chirped LPFG can be given as [14]: res (z) = (nco − ncl,m )(z) eff eff
grating. Therefore, a separation fiber which acts to introduce phase difference in cascaded uniform LPFGs is no longer indispensable in CCLPFG. In the coated CCLPFG, the sensitive film acts not only to adsorb water molecules, but also to adjust the sensitivity. By depositing the films with suitable parameters (refractive index n3 and thickness h3 ), the sensitivity can be significantly improved. This improvement depends on the mechanism of mode transition and reorganization induced by the growth of films [15], which helps the designer to determine the optimum film thickness since the film refractive index is approximately known when film material is selected. Based on the design methodology for coated LPFGs in transition mode [16], the calculated range of film thickness for high sensitivity is approximately 66–78 nm for n3 = 2.38, which serves as a reference for depositing films onto the cladding surface with optimum thicknesses in the follow-up experiments. For analyzing the transmission characteristics of this nonuniform grating structure, transfer-matrix method [17] is a powerful tool where the grating is divided into N sections and each section is considered as a uniform grating. The simulated transmission spectra of bare chirped LPFG and CCLPFG are shown in Fig. 2; the grating structural parameters in the simulation are: 0 = 305 m, L = 2 cm, C = 6 m/cm and = 4 × 10−4 , where denotes the average modulation of core index. As a result of chirping effect, the spectrum of a chirped LPFG features broadened attenuated peaks [14]. When two identical chirped LPFGs are concatenated, however, high-contrast interfering fringe is evident in Fig. 2, which affords potential application as a sensor. According to the simulation, the two fringe patterns in Fig. 2 around 1310 nm and 1650 nm correspond to the couplings from core mode LP01 to cladding mode LP06 and LP07 , respectively. Since the higher-order cladding mode coupling is more sensitive to the environmental refractive index [18], the fringe corresponding to LP07 is interrogated in the remainder of this paper. The solid line in Fig. 3 illustrates the fringe of a CCLPFG coated with a film (n3 = 2.38, h3 = 70 nm), whereas the dashdotted line is the fringe when n3 increases to 2.3805. A comparison between Figs. 2 and 3 shows that the presence of film induces a
(2)
where nco and ncl,m denote the effective refractive indexes (ERIs) eff eff of core mode and mth cladding mode, respectively. From Eq. (2), it is known that the core mode of different wavelengths, e.g. 1 and 2 in Fig. 1, is coupled to cladding modes at different positions in a chirped LPFG; the excited cladding modes will be recoupled into fiber core by the second grating at a corresponding position. For a given wavelength of 1 , for example, the distance from coupling to recoupling position is equal to the grating length L. Across this length, the residual core mode after the first grating and the excited cladding modes propagate through the core and cladding respectively. This introduces phase differences between core mode and cladding modes, which results in interference effect when the cladding modes are recoupled into the core by the second
Fig. 2. Simulated transmission spectrum of a bare CCLPFG.
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Fig. 3. Simulated transmission spectra of a coated CCLPFG for different film refractive indexes.
shift of fringe towards shorter wavelength by 50 nm, which is dominantly caused by the increase of the cladding mode ERI as the film is deposited onto the fiber surface. For the designed humidity sensor in this paper, the adsorption of water molecules by the porous film results in an increase of film refractive index and thus gives rise to a blue shift of the fringe pattern. As is depicted in Fig. 3, the shift of the central peak is evaluated to be 11.8 nm when the film refractive increases from 2.38 to 2.3805.This implies a resolution of film refractive index is available to be the order of 10−7 since the wavelength resolution of a commonly used optical spectrum analyzer (OSA) reaches 10−2 nm. 3. Fabrication In experiment, the used fiber was Corning SMF-28 which was hydrogen loaded in advance for a week to enhance its photosensitivity. The gratings were fabricated by illuminating the fiber core with 248 nm KrF excimer laser. The pulse energy, frequency and exposure time are 24 mJ, 150 Hz and 13 s, respectively. The fabrication of a single chirped LPFG was performed by writing the grating with period from 298 m to 310 m and the step was 1 m. For a given grating period value, 5 periods were inscribed as a section of uniform grating. Consequently, the length of this chirped LPFG is estimated to be 1.98 cm and the equivalent chirp is 6.06 m/cm. Likewise, another identical grating was fabricated directly adjacent to the first one. All of these procedures were automatically performed under the control of computer programs.
Fig. 5. Surface morphology of TiO2 /SnO2 composite film under atomic force microscope.
The fabricated CCLPFG was then annealed at 120 ◦ C for 12 h to improve thermal stability. The dash-dotted line in Fig. 4 illustrates the measured transmission spectrum of fabricated CCLPFG, where two high-contrast fringe patterns are observed within the wavelength range from 1250 nm to 1650 nm. The measured spectrum is accordant with the simulated one in Fig. 2, despite that a part of the right pattern was invisible due to the wavelength limit of the used OSA. The SnO2 solution was obtained by dissolving 1.9 g of SnCl2 ·2H2 O in 50 ml absolute ethanol and was refluxed and stirred at 80 ◦ C for 1 h, whereas the TiO2 solution was prepared by dissolving 15 ml of C16 H36 O4 Ti in 200 ml absolute ethanol. This solution was stirred at room temperature for 2 h, during which the pH value of solution was adjusted to 4.5 by adding drops of diluted HNO3 . Afterward, five drops of polyethylene glycol (PEG) were added in the solution which was stirred for 30 min. Then the SnO2 and TiO2 solution were mixed where the molar ratio of SnO2 was 4%. Finally, the mixed solution was refluxed and stirred at 80 ◦ C for 2 h and aged at 30 ◦ C for 24 h to get sol. Doping with SnO2 at a suitable ratio was expected to enhance the surface roughness of TiO2 films, whereas the adding of PEG enables the film not to be apt to crack. Thus, this composite film possesses better hydrophilic and physical properties than pure TiO2 film. The precleaned grating was drawn from the sol smoothly at a speed of 5 cm/min. Then it was put into a tube-type resistance furnace for thermal treatment. It was heated from room temperature to 450 ◦ C at a rate of 3 ◦ C/min and kept at 450 ◦ C for 60 min, and then cooled down unaffectedly. Fig. 5 gives the surface morphology of TiO2 /SnO2 composite film under atomic force microscope (PSIA XE-100) where the surface roughness is evident. This porous structure is much suitable for humidity sensing. The film refractive index and thickness were measured to be 2.3789 and 72.9 nm, respectively, which fall in the high-sensitivity region according to the design methodology for coated LPFGs in transition mode [16]. 4. Results and discussion
Fig. 4. Measured transmission spectra of bare and coated CCLPFG.
The solid line in Fig. 4 is the spectrum of coated CCLPFG. Compared with the transmission spectrum of bare CCLPFG, a shift of the fringe patterns towards shorter wavelength is evident, which depends on the fact that the deposition of film increases the ERI of cladding modes and thus, results in the decrease of resonant wavelength according to the phase-matching condition of LPFG [17]. Another conspicuous change is the decrease of peak losses, which reduced the contrast between maximum and minimum transmission, i.e. the fringe contrast. This change mainly stems from the
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influence of film on the coupling strength between core mode and cladding modes. The peak loss of a single chirped LPFG in fabrication is controlled to be about 3 dB; thus, the fringe contrast of bare CCLPFG reaches maximum according to the MZI mechanism [13]. The deposition of TiO2 /SnO2 composite film changes the coupling strength from core mode to cladding modes, which causes the peak loss of a single chirped LPFG to deviate from 3 dB and thus results in the great degradation of the fringe contrast of CCLPFG. High-temperature treatment may also play a role in the decrease of fringe contrast since the grating, combined with the dip-coated film, was put in the furnace for thermal treating at high temperature around 450 ◦ C to form porous film structure. As mentioned above, the higher-order cladding mode coupling is more sensitive to the environmental refractive index, so the right pattern of coated CCLPFG in Fig. 4 is interrogated in the following RH sensing test. In addition, the peak marked with a letter “P” is rather smooth, which means relative ease of searching the peak wavelength. Therefore, the wavelength shift of the marked peak serves as a measure to characterize the humidity variation. The humidity sensing was carried out in a humidity controllable room. Light from broadband optical source was injected into the sensor and the output spectrum was monitored using an OSA (YOKOGAWA, AQ6370). The environmental temperature was kept at 15 ± 0.5 ◦ C so as to minimize the interference of temperature fluctuation. The humidity was controlled to vary from 40% to 95% gradually and the ensuing change of spectrum was monitored and recorded. For reference, a commercial hydrometer (TES 1360A) was employed to measure the humidity synchronously. The recorded peak shifts are shown in Fig. 6, where a blue shift of wavelength is evident as the RH increases. For the humidity from 40% to 95%, a wavelength shift of 12.37 nm is achieved. Therefore, the average sensitivity of this sensor is evaluated to be 0.221 nm/%. The dependence of
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Fig. 6. Peak shift of the sensor for RH from 40% to 95%.
peak wavelength on RH increasing from 40% to 95% is illustrated in Fig. 7(a) (filled square), which shows an approximately linear response of this sensor to RH across the interrogated range. The dependence of peak wavelength on RH decreasing from 95% to 40% is also demonstrated in Fig. 7 (circle), which implies that the RH response of this sensor is reversible and stable for the interrogated RH range. The resultant data shown in Fig. 7 indicate that the largest RH difference for the same peak wavelength for RH increase and decrease is about 3.8%, whereas the greatest wavelength difference for the same RH is about 0.8 nm. In order to inspect the influence of temperature on the humidity sensing ability of this sensor, we also implemented the humidity sensing tests at different environmental temperatures.
Fig. 7. Response of peak wavelength to RH at temperature of (a) 15 ◦ C, (b) 20 ◦ C, (c) 30 ◦ C and (d) 40 ◦ C.
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The results illustrated in Fig. 7 show that the sensor has similar linear response to the humidity from 40% to 95% when the temperature rises to 20 ◦ C, 30 ◦ C, and 40 ◦ C. An increase of peak wavelength for a given relative humidity value is evident when the temperature increases, which relies on the intrinsic sensitivity of LPFG-based devices to temperature [19]. Furthermore, the fitting results show that the increase of temperature degrades the humidity sensitivity of this sensor. The humidity sensitivities for 15 ◦ C, 20 ◦ C, 30 ◦ C and 40 ◦ C are 0.221 nm/%, 0.218 nm/%, 0.214 nm/% and 0.211 nm/%, respectively. This sensitivity degradation stems from the fact that the water vapor is increasingly difficult to be adsorbed by the porous film when the environmental temperature increases [9]. 5. Conclusions In summary, we have demonstrated a novel scheme of humidity sensor based on cascaded chirped long-period fiber gratings coated with sol–gel derived TiO2 /SnO2 composite films. This is an MZI-based LPFG sensor whose fine fringe pattern ensures higher sensitivity than the common LPFGs. Meanwhile, a separation fiber between two gratings is no longer necessary in this scheme, which improves the device compactness and reduces the possible external interference. Incorporated with the porous TiO2 /SnO2 composite films, this sensor exhibits high sensitivity and linear response to the humidity. The peak wavelength shift of 12.37 nm is achieved for RH 40% ∼ 95% at 15 ◦ C and the average sensitivity is evaluated to be 0.221 nm/%. Further humidity sensing tests at different temperatures show that increasing temperature causes the increase of peak wavelength for a given humidity value and degrades the humidity sensitivity of this sensor. It is stressed that the sensitivity may be further enhanced by optimizing the grating structural parameters such as grating chirp and length as well as the core index modulation. Owing to the advantages of high-sensitivity and compactness, our proposed RH sensor will likely be developed for the practical applications.
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Biographies Haiyun Chen was born in Shaoxing, China, in 1977. He received the B.S. degree and M.S. degree from Zhejiang Normal University, Jinhua, China, in 2001 and 2007, respectively. He has worked in Department of Physics, Zhejiang Normal University since 2001. He is currently working toward the Ph.D. degree at University of Shanghai for Science and Technology. His current research interests include fiber grating, opto-electronic sensing.
Acknowledgments This work is jointly supported by the National Science Foundation of China (60777035), the Scientific Research Key Project Fund(20840), the Research Project (11ZZ131) of Education Committee of Shanghai, and Shanghai Leading Academic Discipline Project(S30502).
Zhengtian Gu was born in Shanghai, China, in 1965. He received the Ph.D. degree from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, China, in 2000. He is a professor at University of Shanghai for Science and Technology. His current research interests include fiber grating sensors, opto-electronic engineering and opto-electric functional films
References [1] T.L. Yeo, T. Sun, K.T.V. Grattan, Fibre-optic technologies for humidity and moisture measurement, Sens. Actuators A: Phys. 44 (2008) 280–295. [2] D. Viegas, J. Goicoechea, J.M. Corres, J.L. Santos, L.A. Ferreira, F.M. Araujo, I.R. Matias, A fibre optic humidity sensor based on a long-period fibre grating coated with a thin film of SiO2 nanospheres, Meas. Sci. Technol. 20 (2009) 031002 (4pp). [3] L.W. Wang, Y. Liu, M. Zhang, D.S. Tu, X.H. Mao, Y.B. Liao, A relative humidity sensor using a hydrogel-coated long period grating, Meas. Sci. Technol. 18 (2007) 3131–3134. [4] K.M. Tan, C.M. Tay, S.C. Tjin, C.C. Chan, H. Rahardjo, High relative humidity measurements using gelatin coated long-period grating sensors, Sens. Actuators B: Chem. 110 (2005) 335–341. [5] Y.P. Miao, K.K. Zhang, Y.J. Yuam, B. Liu, H. Zhang, Y. Liu, J.Q. Yao, Agarose gel-coated LPG based on two sensing mechanisms for relative humidity measurement, Appl. Opt. 52 (1) (2013) 91–95.
Kan Gao was born in Jiangsu, China, in 1975. He received his Ph.D. degree from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences in 2004. Now he is a senior engineer of China Electronics Technology Group Corporation No.23 Research Institute. His main research interests are optical fiber gratings, optical fiber sensing and optical fiber laser.