Optical Microring Resonance Split Removal via Localized Photolytic Refractive Index Modifications

Abstract

Photonic microring filters are twofold degenerate and the clockwise (CW) and counter-clockwise (CCW) propagating optical modes share the same resonance wavelength. If unperturbed, a wave that is coupled to a microring resonator (MRR) travels unidirectional along the MRR and results in a distinct and sharp peak at resonance condition. However, in practical applications particularly high refractive index photonic channel waveguides based on silicon with Δn=2 between waveguide core and cladding are prone to unwanted back-scattering phenomena [1] - [3]. This inherent property arises from the quality of the etched waveguide sidewalls, where even nm-sized ripples and roughness caused by lithography and etching substantially increase both propagation loss and back-reflections of the traveling wave. Such back-scattering phenomena degrade the quality of resonant structures and can result in resonance splitting which can impair the overall photonic system functionality. The phenomena of mode splitting in silicon photonic resonators has been observed and systematically studied in a number of research works [2] - [3]. Adapted component designs with tuneable integrated reflectors inside the ring cavity have been demonstrated in order to overcome these parasitic effects [3]. Although the proposed devices add further beneficial functionality for certain applications, the technique requires a complete new design of the microring with drawbacks of extended footprints and additional phase shifters.