Nested non-concentric microring resonators with high-Q and large fabrication tolerance

Microring resonators are one of the most sought-after optical components for realizing several on-chip functionalities that include sensing, data routing, new frequency generation, and quantum photonic applications. Many of these applications demand a high quality factor and large notch depth (high extinction ratio), which can be achieved by critical coupling. However, the critical coupling is very sensitive to fabrication accuracies and thermal drift. Geometrical parameters of the resonators are generally swept to attain critical coupling, where a few designs can pass the critical coupling condition criteria. In this work, we propose a methodology to circumvent this vital issue. The proposed technique is based on coupled-resonator systems where two different microrings are embedded into a larger microring, referred to as a nonconcentric nested microring resonator (NN-MRR). The NN-MRR configuration relaxes the requirement of the critical coupling condition by 20% when the strip optical waveguide has either smooth or rough sidewalls. Numerical simulations reveal that, unlike standard MRR, a high Q -factor ( > 1 0 5 ) and a large transmission notch depth > 10 d B can be maintained irrespective of the rings’ coupling conditions for the nested MRRs. Besides the extra degree of freedom of design provided by the inner rings, the other significant advantage of the proposed NN-MRR is its compactness. We believe that the nested MRR arrangement could be highly efficient for biosensing, nonlinear, and quantum applications within a broad ambient temperature range. We have fabricated the NN-MRRs in a silicon-on-insulator platform through electron beam lithography and experimentally demonstrated the theoretical and numerical findings.