Optical micro-resonators in chalcogenide glass

Abstract

This thesis focuses on the production of gallium lanthanum sulphide (GLS) microspheres and their potential uses. Microspheres, and micro-resonators in general, have attracted considerable attention because of their promise in all-optical-switching, micro-lasers, multiplexers and many other applications. In this thesis, several applications have been investigated, culminating in the demonstration of laser action in a neodymium doped GLS microsphere.The method of microsphere production used here was to drop crushed glass through a vertical furnace purged with an inert gas, typically argon. Bulk glass was crushed to a suitable size and uniformity before microsphere production. The glass was crushed with a pestle and mortar, and sieved to achieve glass particles of the required size. Systems have been designed and developed for the production of microspheres and these have resulted in microspheres with diameters from 0.5μm up to 580μm being achieved. The failure to confirm spheres of a smaller size than this owes more to the equipment used to analyze the samples rather than a lack of spheres themselves. The material that has passed through the furnace will typically contain a mixture of particles that have melted into spheres and particles that have not. A system has been put in place to separate spherical particles from non-spherical ones. Together these production and separation systems have resulted in the reliable and reproducible production of microspheres in GLS as well as other types of glass.The quality of a resonator is quantified in its Q-factor. This can be measured by coupling light into a microsphere and observing the spectral intensity of scattered light. Using this method GLS microspheres were shown experimentally to have a Q-factor of up to 1.2x10⁵ at a wavelength of 1.5μm. It is also possible to calculate a maximum theoretical Q-factor for a given material, from its attenuation. By using data for the theoretical minimum attenuation of GLS /GLSO a maximum possible Q-factor has been calculated as over 4x10¹⁰ in GLSO at 3μm and 10⁹ at 1μm. Similarly by using attenuation measurements made on state of the art GLSO a maximum Q factor of 10⁷ at 1μm was calculated.The possibility of using microspheres as ball lenses has been investigated and shown to be possible. Laser action was observed in a neodymium doped GLS microsphere. The laser had a threshold of 83mW of incident pump power, with a peak at 1082nm and a line width of <0.05nm.

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ORCID for Daniel Hewak: orcid.org/0000-0002-2093-5773

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