Investigation of a serially coupled double microsphere resonator to expand resonance frequency spacing of microsphere resonators

A serially coupled double microsphere resonator is investigated to reduce the nonfundamental resonances and therefore expand the resonance frequency spacing in microsphere resonator systems. Coupled-mode theory is used to model the system and investigate the resonator parameters as coupling efficiency. As a result, it is shown that the nonfundamental resonances in the output spectrum of a single 225 μm microsphere are reduced noticeably and therefore its minimum resonance frequency spacing is expanded from 5.2 pm to 0.6 nm by serially coupling to a 50 μm radius microsphere. Furthermore, it is shown that the quality factor of this resonator can reach 2.14×108, which is on the same order of a single 225 μm microsphere resonator. The model is verified by comparing the results of the model with the experimental results published by other groups to investigate induced transparency in the coupling of two microspheres.