Whispering-Gallery Mode Microsphere Resonators for Applications in Environmental Sensing

Humidity, temperature and volatile organic compounds (VOCs), particularly ammonia, are key environmental conditions that have a major impact on human comfort, well-being and productivity, as well as on agriculture, food processing and storage, electronic manufacturing and many other industries. This results in the urgent need for the development of sensing technologies allowing rapid detection and accurate measurement of these environmental parameters. Over the past decades many electrical as well as optical sensors have been proposed and demonstrated for environmental applications. However, challenge always exists for these sensors in terms of sensitivity, selectivity, detection limit, speed of response and robustness, where researchers and engineers are still working continuously on improving the performance of these sensors. Whispering gallery mode (WGM) optical micro-resonators have been shown to be able of detecting minute changes in their environment. This has made them a well-established platform for highly sensitive physical, chemical and biological sensors. Silica micro-resonators with high quality factors and low absorption loss can be fabricated easily at the tip of an optical fiber, and the WGMs in such resonators can be excited by evanescent light coupling using tapered fibers. The aim of this PhD thesis is the development of novel ultra-high sensitivity sensors based on silica micro-spheres functionalized with specific coatings with a particular focus on measurement of water vapor and ammonia concentration in air. A numerical simulation model has been analysed based on perturbation theory to facilitate deep understanding of WGMs in coated micro-sphere resonators and the results of the simulations have been validated by experimental studies. Relationship between key design parameters of the sensor such as microsphere size, thickness of the coating layer, tapered fiber waist diameter, its Q factor and sensitivity has been investigated and established. A novel high sensitivity relative humidity (RH) sensor based on an agarose-coated spherical micro-resonator has been proposed and experimentally demonstrated. The sensor’s spectrum shows a wavelength shift of approximately 518 pm corresponding to a relative humidity change of 40% RH. Detailed experimental investigation of the influence of the agarose coating thickness on the sensor’s humidity response has been carried out and correlated with the analytical model results. Sensor’s performance in very low humidity environments ( A novel ultra-sensitive ammonia sensor has been proposed and developed by coating a porous silica gel on a microsphere acting as the sensing head. The sensor offers high resolution and the lowest reported to date detection limit of 0.16 ppb with response and recovery times of 1.5 s and 3.6 s respectively. Finally, a novel approach to simultaneous measurement of ammonia vapors and humidity in air with high resolution has been proposed and demonstrated experimentally. In the proposed two-parameter sensor WGMs are exited at the same time in an array of two micro-spheres coated with different polymers, namely, silica gel and agarose hydrogel, coupled to a single adiabatic fiber taper. The method can be further expanded to achieve sensing of multiple chemical and biological quantities utilizing various coatings and possibly increasing the number of sensors within the array, thus reducing the cost of sensors interrogation

Compact Humidity Sensor Based on a Multi-layer Agarose Hydrogel Coated Silica Microsphere Resonator

In this paper we report on a novel approach to implementing a compact humidity sensor that utilizes whispering gallery mode (WGM) phenomena in a silica microsphere coated with Agarose hydrogel. The spectral positions of the WGM resonances for such a sensor depend strongly on the refractive index and thickness of the coating. The WGM’s spectral shift occurs due to adsorption/desorption of the water vapor in response to changes in ambient humidity and also due to the corresponding changes of the coating thickness. We experimentally investigated the WGMs spectral shift for a 100 μm diameter silica microsphere coated with Agarose hydrogel over a wide range of relative humidity (RH) values from 30%RH to 70%RH at a constant temperature. Six dip coating cycles of 2.25% wt. /vol. Agarose hydrogel were carried out in sequence with a characterization of the sensor performed for each coating thickness. A resonance shift of 16 nm is achieved in our experiment for the six-layer Agarose hydrogel coating senso

Agarose Coated Spherical Micro Resonator for Humidity Measurements

A new type of fiber optic relative humidity (RH) sensor based on an agarose coated silica microsphere resonator is proposed and experimentally demonstrated. Whispering gallery modes (WGMs) in the micro resonator are excited by evanescent coupling using a tapered fiber with ~3.3 μm waist diameter. A change in the relative humidity of the surrounding the resonator air induces changes in the refractive index (RI) and thickness of the Agarose coating layer. These changes in turn lead to a spectral shift of the WGM resonances, which can be related to the RH value after a suitable calibration. Studies of the repeatability, long-term stability, measurement accuracy and temperature dependence of the proposed sensor are carried out. The RH sensitivity of the proposed sensor depends on the concentration of the agarose gel which determines the initial thickness of the deposited coating layer. Studies of the micro- resonators with coating layers fabricated from gels with three different Agarose concentrations of 0.5%, 1.125% and 2.25 wt./vol.% showed that an increase in the initial thickness of the coating material results in an increase in sensitivity but also leads to a decrease of quality factor (Q) of the micro resonator. The highest sensitivity achieved in our experiments was 518 pm/%RH in the RH range from 30% to 70%. The proposed sensor offers the advantages of a very compact form factor, low hysteresis, good repeatability, and low cross sensitivity to temperature

Fused silica capillary interferometer with a layer-by-layer functional coating for the analysis of chemicals content in aqueous solutions

A simple fused silica capillary interferometric (FSCI) sensor has been proposed and investigated for the detection and analysis of multiple chemical compounds content in aqueous solutions. The sensor was fabricated by splicing a commercially available fused silica capillary (FSC) with two single mode fibers to create a 0.7 cm long air cavity. The fiber surface was functionalized with two different polymers: poly (allylamine hydrochloride) (PAH) and sol-gel silica in sequence using a layer-by-layer deposition method. The operating principle of the sensor relies on light interference in the fused silica capillary cavity due to adhesion of the different chemical compounds on the functional coating surface. Studies of the sensors response to the presence of five different compounds in water solutions at different concentrations have been carried out and the results have been analyzed using the principal component analysis (PCA). This work is a preliminary investigation towards the development of a novel method for assessment of content and quality of alcoholic beverages in real time using functionalized FSCIs

Fused Silica Capillary Interferometer with a layer-by-layer Functional Coating for the Analysis of Chemicals Content in Aqueous Solutions

A simple fused silica capillary interferometric (FSCI) sensor has been proposed and investigated for the detection and analysis of multiple chemical compounds content in aqueous solutions. The sensor was fabricated by splicing a commercially available fused silica capillary (FSC) with two single mode fibers to create a 0.7 cm long air cavity. The fiber surface was functionalized with two different polymers: poly (allylamine hydrochloride) (PAH) and sol-gel silica in sequence using a layer-by-layer deposition method. The operating principle of the sensor relies on light interference in the fused silica capillary cavity due to adhesion of the different chemical compounds on the functional coating surface. Studies of the sensors response to the presence of five different compounds in water solutions at different concentrations have been carried out and the results have been analyzed using the principal component analysis (PCA). This work is a preliminary investigation towards the development of a novel method for assessment of content and quality of alcoholic beverages in real time using functionalized FSCI

Thermo-optic Tuning of a Packaged Whispering Gallery Mode Resonator Filled with Nematic Liquid Crystal

Thermo-optic tuning of whispering gallery modes (WGMs) in a nematic liquid crystal-filled thin-walled capillary tube resonator is reported. WGMs were excited by the evanescent field from a tapered optical fiber. Tapered optical fiber fabrication and reduction of wall thickness of the capillary tube was carried out by a ceramic micro-heater brushing technique. A simple and robust packaging technique is demonstrated to ensure stable and repeatable operation of the device. Tunability of WGMs with temperature was demonstrated with a sensitivity of 267.5 ± 2.5 pm/°C. The demonstrated thermo-optic method for WGMs tuning is potentially useful for many tunable photonic devices and sensors

Sensing of multiple parameters with whispering gallery mode optical fiber micro-resonators

Monitoring of multiple physical parameters, such as humidity, temperature, strain, concentrations of certain chemicals or gases in various environments is of great importance in many industrial applications both for minimizing adverse effects on human health as well as for maintaining production levels and quality of products. In this paper we demonstrate two different approaches to the design of multi-parametric sensors using coupled whispering gallery mode (WGM) optical fiber micro-resonators. In the first approach, a small array of micro-resonators is coupled to a single fiber taper, while in the second approach each of the micro-resonators within an array is coupled to a different tapered fiber section fabricated along the same fiber length. Simultaneous measurement of relative humidity and ammonia concentration in air is demonstrated with an array of two microspheres with different functional coatings coupled to a single fiber taper. Sensitivity to ammonia of 19.07 pm/ppm ammonia molecules and sensitivity to relative humidity of 1.07 pm/% RH have been demonstrated experimentally. In the second approach, an inline cascade of two cylindrical micro-resonators fabricated by coupling to multiple tapered sections along a single optical fiber is demonstrated for measurement of strain and temperature simultaneously. A strain sensitivity of 1.4 pm/με and temperature sensitivity of 330 pm/ºC have been demonstrated experimentally. Both the proposed sensing systems have the potential for increase of the number of microresonators within an array for sensing of a larger number of parameters allowing for reduction of the overall cost of sensing system

A Comprehensive Experimental Study of Whispering Gallery Modes in a Cylindrical Micro-Resonator Excited by a Tilted Fiber Taper

Whispering gallery modes (WGMs) excitation in a cylindrical microresonator formed by a section of silica optical fiber has been studied. Evanescent light coupling into the microresonator is realized using a tapered optical fiber, fabricated by a microheater brushing technique. Several types of silica fibers with different diameters are studied as microresonators, and the influence of the resonator\u27s diameter on the excitation of WGMs is investigated. The excitation of WGMs in a cylindrical fiber resonator were studied with changes to the tilt angle between the microcylinder and the fiber taper in the range of angles from a perpendicular position (0 degrees) to large tilt angles (24 degrees). The evolution of the fiber taper transmission spectrum with the change of the tilt angle results in changes in the intensity, broadening of and a blue shift in the WGM resonance spectra. Overall losses in the taper transmission spectrum decrease with the increase of the taper tilt angle from its perpendicular position, followed by a complete disappearance of the WGM resonances at large tilt angles greater than 20 degrees

A Packaged Whispering Gallery Mode Strain Sensor Based on a Polymer Wire Cylindrical Micro Resonator

We propose a whispering gallery mode (WGM) strain sensor formed by a polymer-wire cylindrical micro resonator for strain measurement applications. WGMs are generated by evanescently coupling light into the polymer-wire resonator from a silica fiber taper fabricated by the micro heater brushing technique. Accurate and repeatable measurements of a strains up to one free spectral range (FSR) shift of the WGMs (corresponding to 0.33 % of the polymer-wire elongation, 3250 μɛ) are demonstrated experimentally with the proposed sensor. Practical packaging method for the proposed strain sensor on a glass microscope slide has also been realized making the sensor portable and easy to handle. The robustness of the packaged coupling system is confirmed by vibration tests. The performance of the packaged strain sensor is evaluated and compared with that for an unpackaged sensor

Thermo-optic tuning of a packaged whispering gallery mode resonator filled with nematic liquid crystal

Thermo-optic tuning of whispering gallery modes (WGMs) in a nematic liquid crystal-filled thin-walled capillary tube resonator is reported. WGMs were excited by the evanescent field from a tapered optical fiber. Tapered optical fiber fabrication and reduction of wall thickness of the capillary tube was carried out by a ceramic micro-heater brushing technique. A simple and robust packaging technique is demonstrated to ensure stable and repeatable operation of the device. Tunability of WGMs with temperature was demonstrated with a sensitivity of 267.5 ± 2.5 pm/°C. The demonstrated thermo-optic method for WGMs tuning is potentially useful for many tunable photonic devices and sensors