All-optical tunable microwave filter with ultra-high peak rejection and low-power consumption

We propose and experimentally demonstrate microwave photonic filters (MPFs) with high rejection ratios and large tuning ranges of the central frequency and bandwidth leveraging four cascaded opto-mechanical microring resonators (MRRs). As half waveguides of each MRR are free-hanging in the air, the nonlinear effects in the opto-mechanical MRRs could be efficiently excited. Consequently, the transmission characteristics of the cascaded MRRs could be flexibly manipulated by adjusting the input pump powers. When the resonant wavelengths of every two MRRs are tuned to be aligned, the transmission spectrum of the silicon device is a notch bimodal distribution with high extinction ratios. The optical carrier is fixed at the flat region of the bimodal distribution. Under optical double sideband (ODSB) modulation, MPFs with high rejection ratios could be achieved due to the high extinction ratio of the cascaded rings. Moreover, the central frequency and bandwidth of the MPFs could be tuned by properly adjusting the pump powers. In the experiment, with a low power of 2.56 mW, the MPF central frequency and bandwidth could be tuned from 7.12 GHz to 39.16 GHz and from 11.3 GHz to 17.6 GHz, respectively. More importantly, the MPF rejection ratios are beyond 60 dB. Furthermore, during the bandwidth tuning process, an MPF response with approximately equiripple stopband could be realized. Owing to the dominant advantages of high rejection ratios, large tuning ranges, low power consumption and compact size, the silicon device has many significant applications in on-chip microwave systems.