Microwave photonic technology can process radio frequency (RF) signals in the optical domain. Compared with the traditional electrical processing methods, it has the advantages of low loss, broadband, good tunability, and sound anti-electromagnetic interference. As an important component for various applications such as radar, communications, and radio astronomy, microwave photonic filter (MPF) has become a research hotspot in microwave photonics in recent years. With the development of photonic integration technology, integrated MPFs have attracted research attention. Recently, microring resonators (MRRs) have been widely employed in MPFs thanks to their compact sizes and good adjustability. The MPF should have a narrow RF bandwidth to achieve precise RF resolution. As known, typically the RF bandwidth of the MPF based on MRR is the same as the optical bandwidth of the MRR when crosstalk is ignored. Therefore, reducing the optical bandwidth of the MRR by improving its quality factor (Q factor) is the most direct and effective way to reduce the MPF bandwidth. However, the MRR loss should be reduced to increase the Q factor, which is difficult to achieve since the scattering loss caused by the waveguide sidewall roughness is usually unavoidable. Under typical silicon-on-insulator (SOI) fabrication processes, optical bandwidth of about GHz for MRR can be obtained, which cannot meet the requirements of high-precision MPF with sub-GHz frequency resolving capability. We propose and demonstrate an MPF based on three cascaded MRRs and phase modulation. With this configuration, the 3-dB RF bandwidth of the MPF can be well compressed compared with the 3-dB optical bandwidth of the MRR, and flexible tunability of the MPF is achieved.

We put forward an MPF based on cascaded three MRRs and phase modulation. By introducing two more MRRs, the phase differences between the optical carrier and the ±1 order optical sidebands can be changed much steeper from 0-π compared with the MPF constructed by a single MRR. As a result, the photocurrent obtained by beating the optical carrier and the ±1 order optical sidebands changes abruptly from constructive interference to destructive interference. Thus the slopes on both sides of the filter peak of the MPF response can be increased to achieve RF bandwidth compressing compared with that of the MPF based on a single MRR. Simulation and experimental results show that the MPF based on cascaded three MRRs and phase modulation can compress the RF bandwidth.

We simulate the phase spectra of the optical carrier and the ±1 order optical sidebands of the MPF based on cascaded three MRRs and the MPF based on single MRR. The results show that the phase difference between 8.9-9.5 GHz for the MPF based on cascaded three MRRs is 1.12π, while the phase difference for the MPF based on single MRR is only 0.83π, which means much steeper phase changing from 0-π is achieved by the MPF based on three MRRs compared with the MPF based on single MRR [Fig. 4(b)]. Additionally, the simulation results show that compared with the MPF based on single MRR, the RF bandwidth of the MPF based on cascaded three MRRs is compressed by about 52%, and the 3-dB attenuation slope is increased about 1.1 times than that of the MPF based on single MRR [Fig. 4(d)] without enhancing the Q factor . The experimental results show that the MPF based on cascaded three MRRs can compress the RF bandwidth by about 69%, and the 3-dB attenuation slope is increased about 3.6 times than that of the MPF based on single MRR (Fig. 9). Meanwhile, continuous frequency tuning in the range of 11.5-20.3 GHz [Fig. 10(b)] and RF bandwidth tuning in the range of 187.1-1597.0 MHz [Fig. 10(a)] are achieved.

We propose and demonstrate a bandwidth compressing method for the MPF based on cascaded three MRRs and phase modulation. By adopting this method, the phase differences between the optical carrier and the ±1 order optical sidebands can be changed much steeper from 0-π than that of the MPF based on single MRR to compress the RF bandwidth of the MPF. Compared with the MPF based on single MRR, the RF bandwidth of the MPF based on cascaded three MRRs is compressed by about 69% without increasing the Q factor. Additionally, the 3-dB attenuation slope is increased about 3.6 times than that of the MPF based on single MRR. Continuous frequency tuning in the range of 11.5-20.3 GHz and RF bandwidth tuning in the range of 187.1-1597.0 MHz are achieved. Furthermore, the proposed method can achieve an even narrower RF bandwidth if an MRR with a higher Q factor is adopted. Meanwhile, the proposed MPF has the potential to be fully integrated into a chip and could find extensive utilization in microwave photonic signal processing systems.