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

Vishnu Kavungal, Dublin Institute of Technology

Document Type Theses, Ph.D

Thesis submitted for the degree of Doctor of Philosophy, to Dublin Institute of Technology, 2018.


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.