Compact relative humidity sensor based on an

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Compact relative humidity sensor based on an Agarose hydrogel coated silica microsphere resonator

Arun Kumar Mallik*a, Dejun Liua, Vishnu Kavungala, Xiaokang Liana, Gerald Farrella, Qiang Wub and Yuliya Semenovaa a

Photonics Research Center, Dublin Institute of Technology, Kevin Street, Dublin 8, Ireland Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle Upon Tyne, NE1 8ST, United Kingdom

b

ABSTRACT In this paper, we propose a novel approach to measurements of low relative humidity (RH) levels based on the whispering gallery modes (WGMs) phenomenon in a silica microsphere resonator coated with Agarose. The spectral dips of the WGM resonances excited in the proposed sensor depend strongly on the changes in the refractive index (RI) of the coating material as well as the surrounding RI. A study of the humidity-induced RI changes in a thin Agarose layer, applied to the surface of a 162 µm-diameter silica microsphere was carried out by correlating the experimental results and numerical simulations performed using the perturbation theory. We experimentally demonstrate a linear sensing characteristic in a low-humidity range from 10% to 45% RH. The estimated quality factor of the micro-resonator is 2.82×106 and detection limit for the sensor is 0.057 %RH, corresponding to the RI resolution of 8.4×10-7 RIU. Keywords: whispering gallery mode micro-resonator; relative humidity sensing; fiber optic sensors 1.

INTRODUCTION

Relative humidity (RH) measurements play a significant role in many industries such as agriculture, food, clinical medicine, manufacturing, civil engineering, textile, semiconductors and many other fields. Such measurements are often critical because humidity may affect the quality of a product, for example, the shelf life of foodstuffs. Humidity sensing is also very important in industrial control systems and for human comfort. Fiber-optic humidity sensors have some notable advantages over conventional electronic sensors, such as compact dimensions, lighter weight, multi-parametric sensing capability, immunity to electromagnetic interference, water and corrosion resistance and radiation tolerance. One of the popular structures that forms the basis of many fiber optic sensors utilizes the whispering gallery mode (WGM) phenomenon in spherical micro-resonators. Spherical micro-resonators can be fabricated easily at the tip of an optical fiber, and the WGMs in such a resonator can be excited by evanescent light coupling. Similar sensors have found many applications in measurements of molecular adsorption [1], refractive index (RI) [2], temperature [3] and gas concentration [4] due to their high sensitivity to the surrounding conditions and simple fabrication. The high Q factors of WGMs in fiber optic micro-resonators lead to a potentially higher humidity resolution compared with other fiber optic humidity sensors. For example, when sensing humidity with a long-period fiber grating device coated with SiO2 nanoparticles, it is estimated that the minimum detectable humidity change is of the order of 1% RH, whereas for a sensor based on a microsphere coated with similar SiO2 nanoparticles, the minimum detectable change is estimated to be circa 0.003 %RH [3]. Due to a large RI contrast between the silica microsphere and its surrounding medium (air) the radiation loss is typically very small, resulting in very high Q factors (~1010) [5]. Teraoka et al. presented a perturbation theory of the WGMs shifts for sensing applications of micro-spherical resonators coated with high RI materials [6]. Later, Nai Lin et al. also used this approximate model to calculate the thickness-dependent sensitivity of a zeolite coated microsphere to ammonia gas [7]. Here we present for the first time numerical and experimental studies for a humidity sensor based on a silica fiber microsphere coated with a thin layer of Agarose hydrogel prepared from a 2.25% wt./vol. Agarose solution. Agarose gel is a hygroscopic material, which has proved to be highly stable, not soluble in water at room temperature. In addition, it can be easily applied to the surface of the device by dip coating. Changes in the surrounding RH induce changes in the *[email protected]

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RI of the Agarose coating layer and thus cause a spectral shift in the WGM resonant wavelengths, which with a suitable calibration can be applied for the measurements of humidity. WGMs in the micro resonator are excited by evanescent light coupling from a tapered fiber with a ~3-4 µm waist diameter. Sensor’s humidity response is demonstrated experimentally, and the experimental sensitivity is compared with that calculated using the perturbation theory, discussed by Teraoka and Arnold in [6]. 2.

THEORY

Multiple WGMs can exist in a mircoresonator and each WGM can be characterized by a set of integers: n, l, and m, which represent the radial, azimuthal and polar mode numbers respectively. When l = m and, n = 1, the mode is referred to as a fundamental. A WGM can have either a TE or TM polarization, which can be selectively excited by controlling the polarization of the coupling light. For simplicity, we only consider the TE polarization. Fig. 1 shows a schematic diagram of the microsphere of radius a0 with a surface coating layer of thickness t. The RIs of the microsphere and the Agarose coating layer are n1 and n2 and the RI of the surrounding medium is n3.

Fig. 1 Schematic diagram of the Agarose coated microsphere.

The characteristic equation for the resonant wavelength λR of WGM with the mode number l is given by [7]: ( (



) = )

where =

( (

)

( ( ) ( (

)+ )+ )− )−

( (

) (1) )

( (

) ( ) (

) )

=



=

where k = 2π/λR is the wave vector, a1 = a0 + t is the total radius of the microsphere including the Agarose layer. ψl and χl are the lth order of spherical Ricatti-Bessel and spherical Ricatti-Neumann functions, respectively. For a given l value there are multiple values of λR that satisfy the Eq. (1). The RI sensitivity for the TE polarization (STE) of the coated

microsphere is given by [7]:

δ (n22 )

STE

δλ λ δk δk = R =− R . = , where k δ n2 δ n2 k 2

∫

∞

0

∫

a1

a0

E02 (r )dr (2) 2

n(r ) Eo (r ) dr

where E0(r) is the radial field distribution of the coated sphere and n(r) is the RI distribution along the radial direction. 3.

EXPERIMENTAL IVESTIGATION AND DISCUSSION

The microsphere for our experiments was fabricated at the tip of a standard SMF28 single mode fiber by applying a series of electric arc discharges. As a result, the tip of the fiber is gradually softened and becomes spherical in shape due to surface tension. The sphere diameter in our experiment was measured using an optical microscope as 162 µm. Agarose hydrogel was fabricated using commercially available white Agarose powder from Sigma Aldrich (A6013). The hydrogel solution was prepared by adding 2.25% wt./vol. of the Agarose powder into deionized (DI) water followed by

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stirring at 80 °C temperature until the Agarose powder completely dissolved in the DI water. The RI of the Agarose gel was measured as 1.3385 with the help of Abbe refractometer. The microsphere sample was mounted on a computercontrolled translation stage. The pulling speed of the microsphere from a hot Agarose gel was set to 1 mm/sec. The coated microsphere was kept at room temperature for one day before use to allow for drying. For evanescent light coupling to and from the microsphere, a tapered fiber was fabricated with a waist diameter of approximately ~3-4 microns using a customized micro-heater brushing technique [8]. The fiber taper then was placed in a close proximity with the microsphere inside a chamber in which both humidity and temperature could be controlled. The WGM resonance near 1550 nm was selectively excited and detected using a narrow linewidth tunable laser. The laser was finely swept at a very low frequency around the WGM resonance within the range of a few GHz. The light transmitted through the fiber taper was monitored by a photodetector connected to an oscilloscope as shown in Fig. 2 (a).

Fig. 2 (a) Experimental setup for RH measurements; transmission spectrum with the selected WGM resonance and Q factor estimate for the 162 µm diameter silica sphere before (b) and after (c) its coating with Agarose.

The quality factor of microsphere before coating was estimated from the experimental spectrum as Q = λR/ΔλFWHM =1.06×107, where λR is the resonance wavelength and ΔλFWHM is the full width at half-maximum of the resonant lobe calculated by fitting the resonant dip with the Lorentz function as illustrated in Fig. 2 (b). After coating the microsphere with the Agarose layer the Q of microsphere decreases to 2.82×106 due to the increase in loss due to absorption within the Agarose layer. The Agarose coating thickness was estimated by a comparative means. A single mode fiber was coated with the Agarose layer using the same hydrogel solution and dip-coating technique as the microsphere. The coated fiber then was cleaved to improve the visibility of the coating layer and a SEM image of the cross-section was taken. Fig. 3 (a) illustrates an SEM image of the cleaved fiber end with an estimated thickness of t = 3.55 µm. This estimated value for the Agarose layer thickness was subsequently used in the numerical simulation. Fig. 3 (b) shows an experimentally measured dependence of the WGM resonance spectral shift with an increase of RH. Any increase in humidity inside the chamber gives rise to the adsorption of water molecules on the surface of the microsphere. Some of the water molecules replace the air inside the micro pores of the coating layer due to the hygroscopic nature of the material and capillary forces, which in turn gives rise to an increase in the effective RI of the Agarose layer. Experimentally the RH was increased from 10% to 45% at a constant temperature of 19±0.2°C, and a redshift in the WGM spectrum is observed. A commercial electronic hygrometer was used as a reference for the RH measurements inside the chamber. The linear regression value R2 = 0.99936 illustrates the high linearity of the sensor’s response within 10-45% RH humidity range. The sensitivity of the sensor within the studied RH range was 0.5268 pm/%RH.

Fig. 3. (a) SEM image of the cross section of an Agarose coated fiber; (b) WGM resonance wavelength shift vs RH inside the chamber; c) experimental WGM spectra fitted by the theoretical model for the mode n = 1, l = m =470, a0 = 81.05 µm, t = 3.55 µm; (d) change of the Agarose layer RI corresponding to the RH changes (calculated using the theoretical model).

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For the numerical simulation with Eq. (1) we assumed the following parameters, microsphere radius a0 = 81.05 µm, RI of silica n1 = 1.4682, the RI of the Agarose coating n2 = 1.3385 and the RI of the top Agarose coating layer whose pores are infiltrated with water n3 = 1.33 and Agarose layer thickness t = 3.55 µm. The fundamental WGM mode chosen near the wavelength of 1550 nm was n = 1, l = m =470. The value of the spectral shift due to humidity was correlated to the numerically calculated change in the RI of the coating without the change in the azimuthal mode number l. Using the power transfer characteristics theory proposed by Yariv [9], the theoretical normalized data was well matched with the experimental spectra as shown in Fig 3 (c). We obtained a total change in the Agarose layer RI ΔnAgarose = 4.7×10-4 RIU within the same humidity range. The plot shown in Fig 3 (d) shows the calculated dependence of the Agarose layer RI versus RH. The best match between the experimental data and numerical simulations was achieved assuming the physical diameter of the sphere of 162.1µm, very close to the value determined from the optical microscope. The detection limit (DL) for a sensor represents the smallest measurable physical parameter change and it can be expressed as DL = R/S, where R is the resolution of the sensor and S is the sensor’s sensitivity. R can be calculated as 2 R = 3 σ N2 + σ T2 + σ SR , where

,

,

represent the standard deviations associated with amplitude noise,

temperature and detector spectral resolution respectively [10]. We assumed the SNR (Signal to Noise Ratio) of the system is approximately 60 dB, then σN is calculated as 1 fm. The standard deviation due to temperature stabilization is taken as σT = 10 fm [10]. The standard deviation associated with the spectral resolution of the tunable laser whose linewidth is less than 1MHz is σSR = 1.1 fm [10]. The overall sensor resolution is calculated as R = 30.3 fm. Hence, the DL for the microsphere with a 3.55 µm thick Agarose layer is calculated as 0.057% RH within the 10-45 %RH humidity range. The RI sensitivity of the Agarose layer coated sphere estimated from Eq. (2) was STE = 36.01 nm/RIU, which corresponds to an RI resolution of 8.4 x 10-7 RIU. 4.

CONCLUSION

A novel and compact low level RH sensor based on a whispering gallery mode microresonator has been proposed and experimentally demonstrated. A change in the RI of the Agarose coating arising due to changes in the surrounding RH leads to a spectral shift of the WGM resonances, which can be related to the RH value after a suitable sensor calibration. The experimental results and numerical simulations for the sensor’s response to relative humidity are in a good agreement, indicating that adopted theoretical model can be used to effectively improve the sensitivity of the sensor.

REFERENCES [1]. Arnold, S., Khoshsima, M., Teraoka, I., Holler, S., and Vollmer, F., “Shift of whispering gallery modes in microspheres by protein adsorption,” Opt. Lett. 28, 272–274 (2003). [2]. Zijlstra, P., Van der Molen, K. L. and Mosk, A. P. “Spatial refractive index sensor using whispering gallery modes in an optically trapped microsphere,” Appl. Phys. Lett., 90, 161101 (2007). [3]. Ma, Q., Huang, L., Guo, Z., and Rossmann, T. “Whispering- Gallery Mode Silica Micro-Sensors for Temperature and Gas-Phase Concentration Measurements,” 27th AIAA Aerodynamic Measurement Technology and Ground Testing Conference (2010). [4]. Farca, G., Shopova, I. and Rosenberger, A. T. “Cavity-enhanced laser absorption spectroscopy using microresonator whispering-gallery modes,” Opt. Express, 15, 17443–17448 (2007). [5]. Gorodetsky, M. L., Savchenkov, A. A. and Ilchenko, V. S. "Ultimate Q of optical microsphere resonators,” Opt. Lett. 21, 453-455 (1996). [6]. Teraoka, I. and Arnold, S. “Theory of resonance shifts in TE and TM whispering gallery modes by non-radial perturbations for sensing applications,” J. Opt. Soc. Am. B, 23, 1381–1389 (2006). [7]. Lin, N., Jiang, L., Wang, S., Chen, Q., Xiao, H., Lu, Y. and Tsai, H. “Simulation and optimization of polymercoated microsphere resonators in chemical vapor sensing,” Appl. Opt., 50, 5465-5472 (2011). [8]. Brambilla, G., Finazzi, V., Richardson, D. “Ultra-low-loss optical fiber nanotapers,” Opt. Express, 12, 2258– 2263 (2004). [9]. Yariv, A. “Universal relations for coupling of optical power between microresonators and dielectric waveguides,” Electron. Lett. 36, 321-322 (2000). [10]. White, I. M. and Fan, X. “On the performance quantification of resonant refractive index sensors,” Opt. Express, 16, 1020–1028 (2008).

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