Highly Flexible Microwave Photonic Single-Passband Filter

A highly flexible microwave photonic single-passband filter is proposed and experimentally demonstrated. Two different light waves with their polarizations aligned with two principal axes of a polarization modulator are modulated, and two phase-modulated signals with opposite phase modulation indices are generated. Two different lower parts of the lower sidebands are removed by a fiber Bragg grating (FBG) to convert two phase-modulated signals into two intensity-modulated signals. Two high-pass frequency responses with different cut-off frequencies are realized. Since the two converted intensity-modulated signals are out of phase, the overlapped frequency responses are cancelled, and a single-passband filter is realized. The bandwidth and the central frequency can be tuned independently by properly adjusting the two wavelengths simultaneously. Furthermore, both the bandwidth and the central frequency can be simultaneously tuned by adjusting one of the two wavelengths. A theoretical analysis is performed, and an apodized FBG is fabricated and incorporated into the proposed single-passband filter. The simulated results and the experimental results agree very well.