This thesis presents a novel structure composed of serially coupled double microsphee resonator (SCDMR) to reduce the non-fundamental resonaces and therefore expand the resonance frequency spacing (RFS) in microsphere resonator systems. Coupled-mode theory is used to model the system and it is shown that the minimun RFS of a single 225um microsphere is expanded from 5.2pm to 0.6nm by coupling to a 50um radius microsphere while maintaining its high quality factor (2.14*10^8). it is shown that using the proposed structure in lasing application, a high-power narrow-linewidth low-threshold single-mode microsphere laser can be obtained. A model is proposed for microsphere lasers and verified by comparing its results to the experimental results published in the literature for a thulium doped tellurite glass 12.5um microsphere laser. it is illustrated that by coupling an 11.5um undoped microsphere to a multimode 50um microsphere laser, single-mode operation is obtained and an output power of 208uW pump power and the FWHM linewidth of 0.53fm are achieved. furthermore, it is shown that by using the SCDMR in sensing application for sensing rodlike becteria it is much easier to track the change in the bacteria surface density.