Narrow Passband Tunable Optical Filter Based on Silicon High-Q Rings Assisted MZI Structure

A picometer-level narrowband optical bandpass filter with adjustable bandwidths and high extinction ratios (ERs) is proposed and demonstrated based on silicon high quality (Q) rings in a Mach-Zehnder interferometer (MZI) structure. For each high-Q ring, the resonance bandwidth could be effectively tuned owing to the add-drop bending coupled structure. The drop transmissions of the two rings are combined in the MZI structure to construct bandpass optical filter responses. By manipulating the phase difference between the two arms of the MZI, the ER and the shape factor of the filter could be significantly improved. More importantly, due to the simultaneously adjustable ring bandwidths and resonance offsets, the bandwidth tuning ranges of the narrowband optical filter would be dramatically increased. The experimental results show that the 3 dB-bandwidth of the optical filter could be tuned from 1.2 pm to 41.5 pm (150 MHz∼5.1875 GHz) with maintaining the ERs beyond 30 dB and the in-band ripples below 1 dB. Furthermore, owing to the advantage of the MZI structure, the optical filter could obtain high shape factors (SFs). To the best of our knowledge, it is the first time to realize a silicon-based picometer-bandwidth bandpass optical filter with a bandwidth tuning range beyond 40 pm, ERs beyond 30 dB and high SFs. The proposed narrow passband optical filters with tunable bandwidths, high ERs, good SFs and low ripples have various important applications in on-chip high-precision optical signal processing.