MEMS-actuated wavelength drop filter based on microsphere whispering gallery modes

MEMS-enabled tuneable optical coupling between optical microsphere resonators and optical fibre waveguides is reported. We describe the design, fabrication and experimental characterization of a MEMS platform, based on electrothermal actuators, which controls the resonator-to-waveguide separation. We compare the simulated and experimental displacements of the actuators in an unloaded and loaded state, where the load is a 1 mm optical spherical resonator. We then demonstrate the proof of concept application of selective wavelength dropping using the MEMS platform by modulating the coupling between the spherical resonator and a tapered optical fibre waveguide

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

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

Glassy Materials Based Microdevices

Microtechnology has changed our world since the last century, when silicon microelectronics revolutionized sensor, control and communication areas, with applications extending from domotics to automotive, and from security to biomedicine. The present century, however, is also seeing an accelerating pace of innovation in glassy materials; as an example, glass-ceramics, which successfully combine the properties of an amorphous matrix with those of micro- or nano-crystals, offer a very high flexibility of design to chemists, physicists and engineers, who can conceive and implement advanced microdevices. In a very similar way, the synthesis of glassy polymers in a very wide range of chemical structures offers unprecedented potential of applications. The contemporary availability of microfabrication technologies, such as direct laser writing or 3D printing, which add to the most common processes (deposition, lithography and etching), facilitates the development of novel or advanced microdevices based on glassy materials. Biochemical and biomedical sensors, especially with the lab-on-a-chip target, are one of the most evident proofs of the success of this material platform. Other applications have also emerged in environment, food, and chemical industries. The present Special Issue of Micromachines aims at reviewing the current state-of-the-art and presenting perspectives of further development. Contributions related to the technologies, glassy materials, design and fabrication processes, characterization, and, eventually, applications are welcome

Novel Sensing Mechanisms for Chemical and Bio-sensing Using Whispering Gallery Mode Microresonators

Due to their ultra-high quality factor and small mode volume, whispering gallery mode (WGM) microresonators have proven to have exceptional sensing capabilities, with single particle level sensitivity to virions, proteins, and nucleic acids. Current sensing mechanisms rely on measuring the changes in the transmission spectrum of the resonator upon adsorption of the analyte on the surface of the resonator, appearing as either shift, splitting, or broadening of the resonance mode, all of which measure the polarizability of adsorbed analytes. In this dissertation, we present two new sensing mechanisms for WGM microresonators: the measurement of a dynamic chemical reaction around the resonator, exemplified by the polymerization of hydrogel, and the Raman spectroscopy of molecules on the surface of WGM microresonator through WGM-based surface-enhanced Raman scattering. Further, an on-going work on sensing using mesoporous silica micro-bottle resonator is described in the last chapter. Our work on the measurement of gelation of polyacrylamide hydrogel using WGM resonators is the first report of using WGM resonators to continuously monitor a chemical reaction (i.e. gelation) in situ. The results from WGM sensing is compared with rheology, a well-established technique for hydrogel characterization. From the similarities and differences in the measured results from WGM and rheology, we suggest that whereas rheology measures the viscoelastic properties of the hydrogel, WGM resonators measure the hydrogel density indirectly through its refractive index. The two techniques provide data that complement each other, which can be used to study the gelation reaction in more details. Raman spectroscopy is a powerful technique for molecular fingerprinting, but the weak Raman signal often requires enhancement from techniques such as surface-enhanced Raman scattering (SERS). Conventionally, metallic nanostructures are used for SERS, but recently there has been increasing interest in the enhancement of Raman scattering from dielectric substrates due to their improved stability and biocompatibility compared with metallic substrates. The combination of WGM resonator and Raman spectroscopy can be a promising sensing platform with both high sensitivity and specificity. Here, we demonstrate the enhanced Raman scattering from rhodamine 6G molecules coated on silica microspheres, excited through WGMs. A total Raman enhancement factor of 1.4 × 104 is observed

Liquid-core-microtubule-enhanced Laser sensor for high resolution temperature measurement

We present a fiber laser temperature sensor based on temperature dependence whisper gallery mode-based microcavity. Three different thermo-optic coefficient liquids are injected into a thin wall thickness microtubule for different temperature sensitivity of whisper gallery mode shift. The sensor temperature sensitivity increases with the absolute value of the liquid core thermo-optic coefficient. The theoretical model of whisper gallery mode temperature response is analyzed. A liquid core microtubule is inserted into the erbium-doped fiber ring laser cavity as the optical filter and sensing unit simultaneously, so the lasing wavelength can be used to detect the temperature change of the medium. Due to the narrow spectral width of sensing laser, a high sensing resolution system is experimentally confirmed. With the combination of liquid core to increase the sensitivity and fiber ring laser cavity to increase sensing resolution, the high sensitivity 112.8 pm/°C of sensing system is demonstrated and the thermal resolution is 8.16 × 10 -3 °C

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

NOVEL OPTICAL MICRORESONATORS FOR SENSING APPLICATIONS

Optical microresonators have been proven as an effective means for sensing applications. The high quality (Q) optical whispering gallery modes (WGMs) circulating around the rotationally symmetric structures can interact with the local environment through the evanescent field. The high sensitivity in detection was achieved by the long photon lifetime of the high-Q resonator (thus the long light-environment interaction path). The environmental variation near the resonator surface leads to the effective refractive index change and thus a shift at the resonance wavelength. In this Dissertation, we present our recent research on the development of new optical microresonators for sensing applications. Different structures and materials are used to develop optical resonator for broad sensing applications. Specifically, a new coupling method is designed and demonstrated for efficient excitation of microsphere resonators. The new coupler is made by fusion splicing an optical fiber with a capillary tube and consequently etching the capillary wall to a thickness of a few microns. Light is coupled through the peripheral contact between inserted microsphere and the etched capillary wall. Operating in the reflection mode and providing a robust mechanical support to the microresonator, the integrated structure has been experimentally proven as a convenient probe for sensing applications. Microspheres made of different materials (e.g., PMMA, porous glass, hollow core porous, and glass solid borosilicate glass) were successfully demonstrated for different sensing purposes, including temperature, chemical vapor concentration, and glucose concentration in aqueous solutions. In addition, the alignment free, integrated microresonator structure may also find other applications such as optical filters and microcavity lasers

Dual sensitization enhancement in cavity optomechanics for ultra-high resolution temperature sensing

As a basic physical parameter, temperature plays an important role in science and industry areas. The cavity optomechanics especially the spring effect provide an ideal platform for precision measurement. Here, we bridge between optical sensitization and optomechanical transduction by fabricating a liquid-core microbubble resonator to realize dual sensitization enhancement. The high thermo-optic coefficient liquid is injected into the microbubble to increase the temperature sensitivity of optical resonant peak shift. The optomechanical spring effect is used to transduce the amplified optical shift to mechanical frequency change and further enhance the temperature response. Through the enhancement combination of optical and mechanical methods, we have achieved a sensitivity of 8.1 MHz/°C, which is at least two orders of magnitude higher than traditional optomechanical approaches. The temperature resolution is estimated as high as 5.3×10 -5 °C with mechanical frequency linewidth 8.6 kHz. A capillary ethanol evaporation experiment is constructed to demonstrate capability of the tiny temperature fluctuations measurement. The novel dual approach greatly enhanced the ultra-high resolution sensing capability and have a flexible sensitivity adjust potential with simply injecting different liquids