Optical Whispering Gallery Mode Cylindrical Micro-Resonator Devices for Sensing Applications

Whispering gallery mode (WGM) micro-resonators are devices attractive for many practical applications including optical sensing, micro-lasers, optical switches, tunable filters and many others. Their popularity is due to the high Q-factors and the exceptional sensitivity of their optical properties to the resonator’s size, refractive index as well the properties of the surrounding medium. The main focus of this thesis is on the cylindrical WGM micro-resonators due to the simplicity of their fabrication and light coupling that they offer in comparison with other WGM devices. At first, an in-depth experimental investigation of the WGM effect in different types of cylindrical micro-resonators was carried out in order to establish the influence of the resonator’s geometry and coupling conditions on the resulting WGM spectrum. As one of the outcomes of this study, a novel method for geometrical profiling of asymmetries in thin microfiber tapers with submicron accuracy has been proposed and demonstrated. The submicron accuracy of the proposed method has been verified by SEM studies. The method can be applied as a quality control tool in fabrication of microfiber based devices and sensors or for fine-tuning of microfiber fabrication setups. The study also resulted in better understanding of the optimum conditions for excitation of WGMs in cylindrical fiber resonators, the influence of the tilt angle between the micro-cylinder and the coupling fiber taper. A novel strain sensor formed by a polymer-wire cylindrical micro-resonator has been developed. Accurate and repeatable measurements of strain have been demonstrated experimentally with the proposed sensor for the upper range of limit of detection up to 3250 με. Practical packaging method for the proposed strain sensor on a glass microscope slide has also been realized making the sensor portable and easy to use. A study of thermo-optic tuning of the WGMs in a nematic liquid crystal-filled hollow cylindrical microresonator has been carried out. A simple and robust packaging has been realized with the proposed tunable device to ensure its stable and repeatable operation. The demonstrated thermo-optic method for the WGMs tuning is potentially useful for many tunable photonic devices. Two novel all-fiber magnetic-field sensors have been designed based on photonic crystal fibers infiltrated with a magnetic fluid and a ferronematic liquid crystal utilizing the magnetic field tunability of WGM resonances. The highest experimentally demonstrated magnetic field sensitivity was 110 pm/mT in the range of magnetic fields from 0 to 40 mT. Finally, a packaged inline cascaded optical micro-resonators (ICOMRs) design is proposed for coupling multiple micro-resonators to a single fiber and simultaneous sensing of multiple parameters (strain, temperature, humidity, or refractive index) at multiple points in space has been demonstrated. The proposed design principle can find applications in quasi-distributed sensing, optical coding, optical logic gates and wavelength division multiplexed optical communications systems

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

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

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

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

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

A Coated Spherical Microresonator for Measurement of Water Vapor Concentration at PPM Levels in Very Low Humidity Environments

This paper presents a novel approach to measurements of low relative humidity (RH) values based on the whispering gallery modes (WGMs) effect in a small silica microsphere coated with a thin layer of Agarose hydrogel. The light from a narrow-linewidth tunable laser with a few GHz tuning range is launched into an adiabatic tapered fiber, which excites WGMs in the Agarose layer via evanescent coupling. Adsorption and desorption of water vapor by the Agarose layer in response to changes in ambient humidity lead to changes in the layer's refractive index, which in turn leads to a spectral shift of the WGM resonances. We experimentally demonstrate the WGMs spectral shift over a range of RH values from 1%RH to 25%RH. The proposed sensor displays linear response and is capable of measurement of low concentrations of water vapor (corresponding to about 11.7 ± 0.32 ppm water molecules in air). In addition, the proposed sensor offers the advantages of a very small form factor and good repeatability. Detailed studies of the sensor stability, cross sensitivity to temperature, response times, and hysteresis are also presented

Dynamic Micromechanical Fabry-Perot Cavity Sensors Fabricated by Multiphoton Absorption Onto Optical Fiber Tips

This research leveraged two-photon polymerization microfabrication to integrate dynamic mechanical components with Fabry-Perot resonators onto the ends of low-loss optical fibers to prototype 3 micro-optic devices. The first device featured a multi-positional mirror that enabled thin-film deposition onto cavities of any length with mirrors of significant curvature, for refractive index sensing. The second device combined an FP cavity with a spring body featuring easily scalable stiffness for pressure sensing. The third device presented a high-speed rotating micro-anemometer for measuring a wide range of gas flows. All devices represent a significant reduction in size and weight over commercially available devices

Nano-structure-based optical sensors fabrication and validation to gas sensing applications

We present three different nano-resonant structures (nanoposts, nanoholes etc.) fabricated on either bulk substrate or micron size tip of optical fiber and one graphene oxide coated glass substrate for gas detection in visible or mid-infrared region of electromagnetic spectrum. Nanostructures provide an efficient way to control and manipulate light at nanoscale paving the way for the development of reliable, sensitive, selective and miniaturized gas sensing technologies. Moreover, the inherent light guiding property of optical fiber over long distances, their microscopic cross-section, their efficient integration capabilities with gas absorption coatings and mechanical flexibility make them suitable for remote sensing applications. The three nanostructure-based gas sensing techniques are based on the detection of surface plasmon resonance (SPR) wavelength shifts, guided mode resonance (GMR) wavelength shifts, and Rayleigh anomaly (RA) mode intensity variations. The SPR and GMR based sensors operate in the visible region of light spectrum. Later, we also integrate a heater with the GMR-based fiber-tip sensor to realize a reusable gas sensor having tunable sensor recovery time. The RA-based sensor is realized by solvent-casting of chalcogenide glass to work as mid-infrared optical resonator. Further, we utilize the dynamic variations in infrared values of graphene oxide in response to gas to realize a gas sensor. First, we present a high-sensitivity gas sensor based on plasmonic crystal incorporating a thin layer of graphene oxide. The presented plasmonic crystal is formed by an array of polymeric nanoposts with gold disks at the top and perforated nanoholes in a gold thin film at the bottom. The thin coating of graphene oxide assembled on the top surface of mushroom plasmonic nanostructures works as the gas absorbent material for the sensor. The optical response of the plasmonic nanostructure is altered due to different concentrations of gas absorbed in the graphene oxide coating. By coating the surface of multiple identical plasmonic crystals with different thicknesses of graphene oxide layer, the effective refractive index of the graphene oxide layer on each plasmonic crystal will be differently modulated when responding to a specific gas. This allows identifying various gas species using the principal component analysis-based pattern recognition algorithm. The present plasmonic nanostructure offers a promising approach to detect various volatile organic compounds. Second, we report a simple yet efficient method of transferring nanopatterns to optical fiber tip. We have also demonstrated a TiO2 coated GMR structure which is sensitive to changes in surrounding refractive index and provides shifts in its resonant wavelength. The GMR sensor at the fiber tip is also demonstrated to work as a gas sensor by coating it with a thin layer of graphene oxide. This simplified and rapid nanostructuring at fiber tip can contribute to remote sensing applications through the insertion of the nanopatterned fiber tips into aqueous and gaseous analytes in regions otherwise inaccessible. Third, we present the first heater integrated nanostructured optical fiber of 200 ïÿým diameter to realize a high-sensitivity and reusable fiber-optic gas sensor. In our GMR-enabled fiber-optic gas sensor, resonance shifts upon the adsorption of the analytes on the graphene oxide (GO) coated sensor surface. For repeated use of this sensor, a regeneration of the sensor surface is required by a complete desorption of the analyte molecules from the GO layer. In our presented design, this has been achieved by the integration of a controllable heater at the fiber tip. Fourth, we present a straightforward analysis based on the maximum and minimum envelopes of the reflection spectra to dynamically investigate the changes in complex refractive index of graphene oxide in response to gases. The performance of graphene oxide -based gas sensors is strongly influenced by the variations in optical properties of graphene oxide when exposed to gas. The presented method does not require any complex dispersion model as compared to ellipsometry. Accordingly, the technique we employ can be leveraged to reliably evaluate the optical performance of any graphene oxide-based gas sensors in a simpler manner, when compared to ellipsometry. Furthermore, the accuracy of the derived values of complex refractive index of the graphene oxide layer has been confirmed by comparing with literature. Finally, we report the development of a first of a kind planar resonant structure that enhances the mid-IR absorption by the analyte adsorbed on its surface, enabling highly sensitive and selective label-free detection of gas and/or biomarkers. Chalcogenide glasses (As2S3) are promising for infrared photonics owing to their transparency in visible to far infrared, where various biomolecules and gases have their characteristic absorption lines, arising from rotational-vibrational transitions. Here we present the proposed design of a nanoscale tunable planar mid-IR optical resonator, realized by solvent-casting of As2S3. Our technique of preparing nanostructure having resonance at mid-IR enables the realization of mid-IR bio as well as gas sensors