Silicon Photonic Vernier Cascaded Microring Filter for Broadband Tunability

We report a 4
^th
-order Vernier cascaded microring (VCMR) filter on Silicon-on-Insulator that can be continuously tuned over an effective free spectral range of 32 nm using the thermo-optic effect. The filter consists of two cascaded double-microring filter stages with free spectral ranges of 7.8 nm and 6.5 nm. The cascaded architecture allows the two stages to be thermally isolated and individually calibrated, which enables accurate tuning of the VCMR filter without further spectral optimization at each set wavelength. We achieved a wavelength tuning accuracy of ±15 pm over 32 nm and a 3-dB bandwidth variation of ±6.5%, which represent the most accurate tuning performance reported for silicon Vernier microring filters to date. In addition, integrated on-chip thermistors allow for precise tracking of the local temperatures of the microrings, thereby providing a mechanism for monitoring the filter performance.