[
    {
        "id": 1,
        "chunk": "# Fiber optic Bragg grating sensor based on hydrogels for measuring salinity  \n\nJun Conga, Xianmin Zhanga,\\*, Kangsheng Chena, Jian Xub  \n\naDepartment of Information and Electronic Engineering, Zhejiang University, Hangzhou 310027, China bPolymer Physics Laboratory, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China  \n\nReceived 25 April 2002; received in revised form 7 July 2002; accepted 22 July 2002",
        "category": " Abstract"
    },
    {
        "id": 2,
        "chunk": "# Abstract  \n\nWe report a new type of optical salinity sensor with a fiber Bragg grating (FBG) coated with hydrogels. The sensing mechanism in this device is based on mechanical stress that is induced in the chemically sensitive water swellable polymers (hydrogels) coating when the water escapes from it. The stress in the hydrogels coating stretches and shifts the Bragg wavelength of the FBG. Varying the composition of the gel, the sensor can be used to response to different trigger stimulus. Here the sensors for measuring salinity are demonstrated.  \n\n$\\copyright$ 2002 Elsevier Science B.V. All rights reserved.  \n\nKeywords: Optical sensor; Fiber Bragg grating; Hydrogel; Sanility",
        "category": " Abstract"
    },
    {
        "id": 3,
        "chunk": "# 1. Introduction  \n\nRecently, a considerable research effort has been focused on fiber Bragg gratings (FBGs), particularly on systems using these devices for sensing applications. Because FBG is compact, simple in fabrication, and can be operated in a wavelength-coding manner, it has become an important component for optical sensing devices in measuring temperature and strains [1–4]. Hydrogels are crosslinked polymers which swell to an appreciable extent in water, and are considered as biocompatible materials. The water content depends on the polymer structure, and can be made responsive to environmental factors, such as inorganic salt concentration and $\\mathrm{\\pH}$ value [5,6]. Many research efforts have been done to study volume changes in response to different environmental factors, and develop sensors based on hydrogels for $\\mathrm{\\DeltapH}$ , soil water potentials and other chemical parameters [7,8]. Here we report a fiber Bragg grating sensor for measuring salinity using a water swellable hydrogel as the active sensing component. By modifying the hydrogel chemistry, parameters other than salinity can be detected using the same basic approach.",
        "category": " Introduction"
    },
    {
        "id": 4,
        "chunk": "# 2. Principles of operation  \n\nThe basic principle of operation commonly used in a FBG based sensor system is to monitor the shift in wavelength of the reflected ‘‘Bragg’’ signal with the changes in the environment, e.g. strain, temperature. The Bragg wavelength, or resonance condition of a grating, is given by the expression:  \n\n$$\n\\lambda_{\\mathrm{B}}=2n_{\\mathrm{eff}}A\n$$  \n\nwhere $\\varLambda$ is the grating pitch and $n_{\\mathrm{eff}}$ is the effective refractive index of the fiber. With such a device, injecting a spectrally broadband source of light into the fiber, a narrowband spectral component at the Bragg wavelength is reflected by the grating.  \n\nHydrogels are a class of hydrophillic polymers which absorb water and swell without dissolution. They can absorb water and swell or emit water and shrink in the presence of a stimulus [5]. The swelling or shrinking actions of the hydrogels are readily converted to a mechanical response in the form of a force or pressure change which is the basis of the detection process in the present sensor.  \n\nThe basic configuration of the sensor is shown in Fig. 1. The device consists of a hydrogel-coated FBG. When the hydrogel absorbs water, mechanical expansion that occurs in the hydrogel pushes the clamps fixed on the FBG. The fiber is physically stretched and the grating period, L, expands which red shifts the Bragg wavelength. When the sensor is immersed in NaCl solution, the hydrogel will shrink and release the water. The Bragg wavelength will be decreased. Due to the fact that the residual stress that is induced depends on the NaCl concentration, the quantity of the Bragg wavelength shift can be directly related to the NaCl concentration.  \n\n![](images/8025b96dc777375253aa3734e862b253473f50b45f3dc6f94ca77d5b2019efb2.jpg)  \nFig. 1. Structure of the hydrogel-coated FBG sensor.",
        "category": " Results and discussion"
    },
    {
        "id": 5,
        "chunk": "# 3. Sensor fabrication  \n\nA single mode fiber (Corning SMF28) was stripped of the polymer jacked and soaked with hydrogen at $140\\mathrm{MPA}$ and $150^{\\circ}\\mathrm{C}$ for 2 days. FBG was formed on the fiber under $248~\\mathrm{nm}$ KrF laser exposure through a phasemask. The preradiated fiber was subsequently annealed at $150^{\\circ}\\mathrm{C}$ for $24\\mathrm{h}$ to drive out the residual hydrogen. The grating reflectivity was $10\\mathrm{dB}$ at $\\lambda_{\\mathbf{B}}$ and the grating length was $1\\mathrm{cm}$ . Two clamps was adhered on two ends of a fiber grating by the cured epoxy, as shown in Fig. 1. The polyacrylamide hydrogel was prepared by free-radical solution polymerization initiated by a photoinitiator. The monomer, acrylamide (AAm), and the cross-linker, $N\\mathcal{N}^{\\prime}$ -methylene bisacrylamide (MBAAm), were purchased from Xingyi Chemical Cooperation, Beijing, China. AAm was frozen and recrystallized from solution in a mixed solvent of benzene and chloroform three times before use. MBAAm was recrystallized from its alcohol solution twice. The photoinitiator, $\\textsf{\\textsf{q}}$ -ketoglutaric acid (KGA, reagent from Wakolure Chem. Inst. Ltd.) was used without further treatment. The deionized water was redistilled for use as solvents. Before polymerization, the fiber grating was immersed in the mixture solution. Then the solution was polymerized under UV irradiation around $\\lambda=365\\mathrm{nm}$ .  \n\n![](images/18580a9340fbd0422ba4f4360828475c8ed9fe330c0fbea04031d7d48ec31be9.jpg)  \nFig. 2. The transmission spectra of the sensor.  \n\nAfter the hydrogel was polymerized, the sensor was immersed in water. The swelling hydrogel coating pushed the FBG, the Bragg wavelength of the grating was red shifted. Transmission spectra of the sensor are shown in Fig. 2. When the sensor was dipped into a NaCl solution, the shrink of the hydrogel caused the Bragg wavelength of the FBGs to blue shift. As a result, the location of the Bragg wavelength can be used directly to determine the NaCl concentration.  \n\n![](images/b0ae78fd1ba6d5093d0700b0c7d698ebdcc8ebc56901574f11629c41b31c5a8f.jpg)  \nFig. 3. Transmission spectra of the FBG sensor as a function of NaCl concentrations: (a) sensing time is $20\\mathrm{min}$ , (b) sensing time is $40\\mathrm{min}$ .",
        "category": " Materials and methods"
    },
    {
        "id": 6,
        "chunk": "# 4. Result and discussion  \n\nThe experiments were conducted in dilute NaCl solutions. The sensor was immersed in $\\mathrm{\\DeltaNaCl}$ solution with different concentration $(0.1{-}0.7~\\mathrm{mol/l})$ ). It can be seen that the shifts of the Bragg wavelength were different for each treatment. Transmission spectra of the sensor as a function of $\\mathrm{\\DeltaNaCl}$ concentration are shown in Fig. 3. From the figure, the Bragg wavelengths of the sensors were blue shifted when the sensor was exposed to a higher NaCl concentration. The higher the $\\mathrm{\\DeltaNaCl}$ concentration of the solution is, the larger the Bragg wavelength of the sensor blue shift gets.  \n\nFig. 4 shows the NaCl concentration response of the sensor. When the sensor was immersed in the solution for a longer time $(40\\mathrm{min})$ , the response is improved. The sensitivity can be represented by the slope of the curve of Fig. 4 $:\\Delta(\\Delta\\lambda_{\\mathrm{Bragg}})/\\Delta c(\\mathrm{NaCl})$ . The sensitivity drops when the sensor was exposed to the solution with NaCl concentration higher than $0.5{-}0.7\\ \\mathrm{mol}/1$ .  \n\nResearches have shown that when certain cations, such as $\\mathrm{{Na}^{+}}$ , $\\mathrm{K^{+}}$ , ${\\mathrm{Ca}}^{2+}$ and $\\mathbf{Mg}^{2+}$ are diffused into the hydrogel matrix, they will introduced an anomalous behavior to the shrinking of the hydrogel [5]. The volume change of hydrogel is understood as a phase transition of the system consisting of the charged polymer network, counterions, and fluid [9]. The degree of volume change is based on the positive osmotic pressure of counterions, the negative pressure due to polymer–polymer affinity, and the rubber elasticity of the polymer network [10]. By modifying the hydrogel chemistry, the swelling and shrinking of hydrogel can be designed to be sensitive to other parameters, such as the pH value of aqueous solution [6]. The parameters other than salt concentration can be detected using the same basic approach.  \n\n![](images/9c1139dfe9b6b5847d1d8f3c83cef1f8176ae368fe1027280a8de673b4be7e83.jpg)  \nFig. 4. Bragg wavelengths of the FBG sensor as a function of $\\mathrm{\\DeltaNaCl}$ concentrations.  \n\nIf many Bragg gratings with different Bragg wavelength are located at intervals along a long optical fiber, and each grating coated with hydrogel, the sensor can be used as a quasi-distributed salinity detector, to monitor water quality in coastal areas or salinity level in wells and aquifers.  \n\nTo provide additional protection for glass fiber in aqueous environment, the fiber with FBGs can be coated with a thin gold or carbon film, forming a physical barrier to the diffusion of $\\mathrm{OH^{-}}$ groups to the glass structure [7]. The long-term stability of the crosslinks in the hydrogel matrix can be improved by adding $1\\%$ antioxidant inside the hydrogel matrix to reduce degradation due to radicals formation [7].  \n\nThe minimum detectable resolution of the NaCl concentration of the device is limited by two factors: the resolution of the detecting equipment and the discontinuous and slow volume changes in response to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. The latter can be reduced by using the hydrogels that can achieve large volume changes in response to the small changes of ion concentration of the solution [11].",
        "category": " Results and discussion"
    },
    {
        "id": 7,
        "chunk": "# 5. Conclusion  \n\nIn conclusion, we have demonstrated the construction and test of a sensor for detecting salt concentration in water. The technique combines fiber Bragg gratings with chemically sensitive water swellable hydrogel. The mechanism for the sensors is the continuous volume phase transition of PAAm hydrogel when salt concentration is altered. Initial experiments have shown that the sensor is an efficient means of detecting $\\mathrm{\\DeltaNaCl}$ concentration. The sensor design can be readily adapted through modification of the gel chemistry with the same sensor construction to detect other chemical species.",
        "category": " Conclusions"
    },
    {
        "id": 8,
        "chunk": "# Acknowledgements  \n\nThis research was supported by National Nature Science Foundation of China (Grant No. 29904007), Zhejiang Provincial Science Foundation of China (Grant No. 699031), and SRF for ROCS, SEM, China.",
        "category": " References"
    },
    {
        "id": 9,
        "chunk": "# References  \n\n[1] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, E.J. Friebele, Fiber grating sensors, J. Lightwave Technol. 15 (1997) 1442–1463.  \n\n[2] K.O. Hill, G. Meltz, Fiber Bragg grating technology fundamentals and overview, J. Lightwave Technol. 15 (1997) 1263–1276.   \n[3] M. Xu, H. Geiger, J. Archambault, L. Reekie, J. Pakin, Novel interrogating system for fibre Bragg grating sensors using an acousto-optic tunable filter, Electron Lett. 29 (1993) 1510– 1511.   \n[4] H. Patrick, G. Williams, A. Kersey, J. Pedrozzani, A. Vengsarkar, Hybrid fiber Bragg grating/long period fiber grating sensor for strain/ temperature discrimination, IEEE Photon Technol. Lett. 8 (1996) 1223–1225.   \n[5] T. Tanaka, I. Nishio, S.T. Sun, S.V. Nishio, Collapse of gels in electric field, Science 218 (1982) 467–469.   \n[6] J. Ricka, T. Tanaka, Swelling of ionic gels: quantitative performance of the Donnan theory, Macromolecules 17 (1984) 2916–2921.   \n[7] S. Hadjiloucas, D.A. Keating, M.J. Usher, W.C. Michie, B. Culshaw, M. Konstantaki, N.B. Graham, C.R. Moran, Hydrogel based distributed fibre optic sensor for measuring soil salinity and soil water potentials, Progress in Fibre Optic Sensors and Their Applications, IEE Colloquium on, London, UK, 1995, 9/1–9/6.   \n[8] W.C. Michie, B. Culshaw, M. Konstantaki, I. McKenzie, S. Kelly, N.B. Graham, C. Moran, Distributed pH and water detection using fiber-optic sensors and hydrogels, J. Lightwave Technol. 13 (1995) 1415–1420.   \n[9] T. Tanaka, Dynamics of critical concentration fluctuations in gels, Phys. Rev. A 17 (1978) 763–766.   \n[10] T. Tanaka, Gels, Sci. Am. 244 (1981) 124.   \n[11] E.S. Matsuo, T. Tanaka, Patterns in shrinking gels, Nature 358 (1992) 482–485.",
        "category": " References"
    },
    {
        "id": 10,
        "chunk": "# Biographies  \n\nJun Cong was born in Shandong, China, in 1977. He received his BS degree in July 2000 from Department of Information and Electronic Engineering, Zhejiang University. He is currently pursuing his MS degree. His researches are interested in the field of polymer based sensors and fiber gratings.  \n\nXianmin Zhang was born in Zhejiang, China, in 1965. He received the BS and PhD degrees in physical electronics and optoelectronics from Zhejiang University, China, in 1987 and 1992, respectively. He was appointed associate professor of information and electronic engineering at Zhejaing University in 1994 and full professor in 1999. His research interests include optic fiber sensors and optic communication.  \n\nKangsheng Chen was graduated from Zhejiang University, China, in 1962. Since 1962, he has been a member of the faculty of Zhejiang University, where he is now a professor of the Department of Electronic Engineering. He was the Chairman during 1992–1996 of the department. His current interest includes theoretical modeling and numerical simulation of guided wave structures and optical communication.  \n\nJian Xu received the BS, MS and PhD degrees in polymer materials from Department of Chemistry, Chengdu University of Science and Technology, China, in 1982, 1985, and 1994, respectively. During 1985 and 1992, he was a lecturer of Beijing University of Chemical Technology, China. Since 1995, he is working at the Institute of Chemistry, Chinese Academy of Sciences, China. He is currently a professor and deputy director of the institute. His researches are interested in the field of polymer physics.",
        "category": " References"
    }
]