Accurate resonant frequency spacing of microring filters without postfabrication trimming

Optical microring resonators are compact building blocks for many microphotonic systems, as they enable narrow-band filtering. Of particular interest are integrated banks of microphotonic filters, with accurately spaced operational frequencies, for simultaneously processing multiple channels of information. Accurate spectral spacing has not yet been reported as it requires subnanometer control of the average dimensions of the optical resonators used to build high-performance microphotonic filters. Here, the authors present a technique enabling the relative dimensional control needed to achieve accurate spectral spacing, without postfabrication trimming. The authors fabricated eight-channel second-order microring-resonator filter banks with accurate spectral spacing using scanning-electron-beam lithography. The microrings are made of silicon-rich silicon nitride, and the subnanometer relative dimensional control is achieved through carefully calibrated adjustments of the electron-beam dose. When combining the dose alternations with small changes in microring radii, the authors demonstrate filter banks with accurate channel spacings ranging from 90to180GHz. We show that we can accurately control a 2.7nm change in average width of the ring waveguide to 0.11nm, despite a 6nm step size in the scanning-electron-beam lithography system.