Tunable Microwave Photonic Filter With a Narrow and Flat-Top Passband

A frequency-tunable microwave photonic filter (MPF) with a narrow and flat-top passband implemented using a phase modulator (PM) and a superstructured fiber Bragg grating (SFBG) is proposed and experimentally demonstrated. The key component in the MPF is the SFBG, which is designed and fabricated based on the equivalent phase shift (EPS) technique. Two phase shifts are introduced, and the combination of the two phase shifts would lead to a flat-bottom notch in the reflection band. By incorporating the SFBG into the MPF, a frequency response with a narrow and flat-top passband is achieved. The reflection bandwidth and the notch width of the SFBG, which determine the frequency tunable range and passband width, can be controlled by the length and maximum index modulation (MIM) of the SFBG. An experiment is performed. An MPF with a 3 dB bandwidth of 143 MHz, a 20 dB bandwidth of 370 MHz, and a tunable range from 0.4 to 6.4 GHz is demonstrated.