Fluoride And Phosphate Glass Whispering Gallery Micro-resonators
Authors:   ,   Cork Institute of Technology, Ward Jonathan 
Published: August 2024

"This PhD work focuses on spherical microlasers made from commercial and experimental laser glasses. The laser light in a microsphere is confined in a region called a whispering gallery mode, which has unique properties that allow for many nonlinear effects to be observed with only sub-mW pump powers. Efficient coupling of light into the microsphere is attained by using adiabatically tapered fibres fabricated using various heating methods including a CO<sub>2</sub> laser, a sapphire tube furnace and electric heaters. However, the best result is obtained using a fibre pulling rig with an oxy-butane torch as the heat source. This system requires extreme control of the flames but can make fibres with diameters of 600 nm with low optical losses. After the coupling system was perfected, the spectral and nonlinear characteristics of the microsphere lasers were explored. Thermo-optically driven bistability in Yb<sup>3+</sup>/Er<sup>3+</sup>-doped phosphate glass microspheres at room temperature was demonstrated. The bistaility is associated with both Er<sup>3+</sup> fluorescence and lasing intensity behaviour, and chromatic switching. The influences of the host matrix on lasing and fluorescence mechanisms were determined. An Yb<sup>3+</sup>/Er<sup>3+</sup>- doped phosphate glass microsphere was heated above its glass transition temperature of 375”C purely by pumping it at 978 nm with 70 mW via a tapered optical fiber. The onset of thermal stress in the glass at a maximum pumping power results in the sphere melting and fusing to the taper coupler, without inhibition of whispering gallery mode lasing. A taper-fused microsphere laser with 4.5 μW of lasing power at 1593 nm was demonstrated. A multicolour microspherical glass light source was fabricated from an erbium doped fluoride glass called ZBLALiP. Whispering gallery mode lasing and upconversion processes give rise to laser and fluorescent emissions at multiple wavelengths, from the ultraviolet to the infrared. Thirteen discrete emissions ranging from 320 to 849 nm have been observed in the upconversion spectrum. The primary processes responsible for the generation of the observed wavelengths were identified and it was shown that this material has an improved range of emissions over other erbium doped fluoride glasses. To collect the broad range of wavelengths from a multicolour microspherical light source, a separate multimode, half-taper fibre was used to out-couple the upconversion spectrum over hundreds of nanometers, indicating the suitability of this scheme as a miniature device with a 450-nm-wide band and reasonable output coupling efficiency. Finally, a method to spatially confine or corral the movements of a micropendulum via the optical forces produced by two simultaneously excited optical modes of a photonic molecule was proposed. The photonic molecule comprises two microspherical cavities. The cavity enhanced optical force generated in the photonic molecule was numerically calculated. This force creates an optomechanical potential of approximately 5 eV deep and 10 pm wide, which can be used to trap the pendulum at any given equilibrium position by a simple choice of the frequencies of the laser light. This can lead to the development of opto-mechanically actuated systems for photonics circuits."