Wideband-Tunable on-Chip Microwave Photonic Filter with Ultrahigh-Q U-Bend-Mach–Zehnder-Interferometer-Coupled Microring Resonators

Abstract

Microwave photonic filters are a key fundamental subsystem in microwave photonics. A wideband-tunable on-chip microwave photonic filter (MPF), which monolithically integrates a dual-drive Mach-Zehnder modulator (DDMZM), is proposed with ultrahigh-Q Mach–Zehnder-interferometer (MZI)-coupled microring resonators (MRRs) in cascade and a high-speed photodetector (PD). It is the first time an MPF has been developed by introducing U-bend-MZI-coupled MRRs whose resonance wavelength and 3 dB bandwidth are tunable. In particular, the ultrahigh-Q U-bend-MZI-coupled MRRs are realized with ultralow loss broadened silicon waveguides based on a modified Euler curve. For the fabricated U-bend-MZI-coupled MRRs, which have an effective radius of 29 µm, the Q-factor is as high as 1.3 × 106, while the free spectral range (FSR) is as large as 113 GHz. Furthermore, the introduction of two cascaded U-bend-MZI-coupled MRRs enables the MPF to have flat-top bandstop (or bandpass) responses with a high extinction ratio (ER) of 20–35 dB as well as a tunable 3 dB bandwidth of 0.15–3 GHz. Meanwhile, the central frequency can be tuned thermally from 2 to 40 GHz. The work provides a feasible route to realize a compact, high spectral resolution, and wideband-tunable MPF on silicon, enabling a range of RF applications from wireless communication to flexible high-frequency microwave photonic signal processing.