Low-loss Silicon Optical Filter with Narrow Bandwidth and Steep Roll-off Based on Cascaded Feed-forward Microring Resonators

We propose two kinds of architectures based on cascaded feed-forward microring resonators (MRRs) for integrated optical bandpass filtering. We compare the performance of the two proposed filters with traditional high-order MRRs composed of the same number of ring cavities using the scattering matrix method. For the same target 3 dB bandwidth of 15 GHz or 5 GHz, the proposed optical filters shows different line shape with lower insertion loss and different roll-off feature. As a proof of concept, we fabricate the device with three MRRs on a silicon-on-insulator (SOI) substrate using deep ultraviolet photolithography. The device shows a 3 dB bandwidth of ~25 GHz and an on-chip insertion loss of -2.5 dB. The roll-off rates are around 45 dB/nm at the sideband points -10 dB and -20 dB lower than the passband center. In contrast, the transmission's roll-off rates of a single MRR in the device are ~22 dB/nm and ~6 dB/nm at the -10 dB and -20 dB sideband points, respectively. Compared with traditional high-order MRRs, our schemes have lower thermal crosstalk because the resonators are not tightly coupled and thus can be placed far apart. The performance of the proposed structures can be further improved by replacing the MRRs with micro-disk resonators or high-order MRRs.