Theoretical and experimental investigation of novel passive and active optical microresonators

The scope of this thesis is to develop and investigate novel “fiberised” microresonator lasers suitable for side/remote pumping and signal collection. Several resonator configurations (stub, bottle, rod and sphere) made from normal optical fibres are studied in detail towards development of all-in-fibre microlasers.A theoretical model for tapered fibre-coupled microspheres based on general coupled mode theory is developed, describing the excitation of multimode WGMs. By precisely designing the taper coupling condition, excitation of specific mode or group of modes is possible. Simulations show coupling induced cross-coupling between frequency degenerate WGMs in a perfect microsphere, and provide their intra-cavity intensity distribution which is important in lasing and nonlinear applications. Lasing characteristics of Yb3+-doped microbottle lasers are theoretically investigated. Along with the experiment, it is demonstrated that desired lasing characteristics for a specific WGM is achievable by precisely setting the taper-resonator coupling condition, intrinsic Q factors and dopant concentration. Yb3+-doped MBLs are experimentally demonstrated to be promising devices in realization of selective WGM microlasers. Such cavities naturally exhibit multimode lasing. Precise, fast and low cost spectral cleaning methods are employed towards developing robust, stable and single mode lasers. It is observed that by further improving the quality factor of such resonators, cascaded Stimulated Raman Scattering (SRS) lasing is enabled, that together with Yb3+-lasing, extend the spectra beyond the emission band of Yb ions, well in the O-band of the telecom spectrum. It is also demonstrated that pumping and signal collection in such laser cavities can be achieved without the complex and sensitive tapered fibre coupling mechanism. In this study, microtaper-free side-pumped laser-milled microrod and microstub lasers are developed as robust, stand-alone and compact devices towards the realization of ultra-short all-in-fibre microlasers