Tunable kilohertz microwave photonic bandpass filter based on backscattering in a microresonator.

A tunable microwave photonic bandpass filter (MPBPF) with a kilohertz bandwidth based on the backscattering mode of a silica microsphere resonator is proposed and experimentally demonstrated. In this work, an ultrahigh-quality-factor microsphere resonator is used to generate a radio frequency bandpass response with a bandwidth of 600 kHz. Meanwhile, scattering-induced coupling between the clockwise mode and the counterclockwise mode is introduced to reduce the number of resonance modes, and a single backscattering mode which has a high extinction ratio is obtained. Therefore, an MPBPF with a tuning range of 40 GHz and a rejection ratio of 16.9 dB is realized. This MPBPF possesses advantages such as ultranarrow bandwidth, large tuning range, and compactness, and shows great potential for microwave photonic applications.