This work encompasses a study of fibre optic devices and micro-resonator cavities and their use as strain sensors, interferometers, and eventually as light sources. We demonstrate the use of a pi-shifted fibre Bragg grating as a strain sensor and quantify its performance through several metrics relevant to its operation in a frequency-locked system. We then discuss the use of waveguide devices as refractive index sensors in a detailed and critical analysis of most of the literature on fibre optic-based interferometry where we define new metrics of assessment that are aimed to be device-independent. We then categorize and study the performance of the literature on fiber-optic refractive index sensors and highlight few key devices of each category based on their performance or design. We relate the performance of these interferometers to various design parameters to outline a way for future researchers to create high-performance devices. Finally, we study light emission in dye-loaded droplet micro-resonators, which are a promising new platform to generate low-threshold lasing action excited by lasers coupled to the whispering gallery modes of the resonator. We characterize multiple types of emissions in these cavities and pave the way for future microdroplet-based lasers. / Graduate / 2022-08-10