Backscattering in silicon microring resonators: a quantitative analysis

Silicon microring resonators very often exhibit resonance splitting due to backscattering. This effect is hard to quantitatively and predicatively model. This paper presents a behavioral circuit model for microrings that quantitatively explains the wide variations in resonance splitting observed in experiments. The model is based on an in-depth analysis of the contributions to backscattering by both the waveguides and couplers. Backscattering transforms unidirectional microrings into bidirectional circuits by coupling the clockwise and counterclockwise circulating modes. In high-Q microrings, visible resonance splitting will be induced, but, due to the stochastic nature of backscattering, this splitting is different for each resonance. Our model, based on temporal coupled mode theory, and the associated fitting method, are both accurate and robust, and can also explain asymmetrically split resonances. The cause of asymmetric resonance splitting is identified as the backcoupling in the couplers. This is experimentally confirmed and its dependency on gap and coupling length is further analyzed. Moreover, the wide variation in resonance splitting of one spectrum is analyzed and successfully explained by our circuit model that incorporates most linear parasitic effects in the microring. This analysis uncovers multi-cavity interference within the microring as an important source of this variation.