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 × 10 6 , 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.