Ultrahigh-Q microwave photonic filter with Vernier effect and wavelength conversion in a cascaded pair of active loops

A new cascaded microwave photonic filter that can realize a high Q value is presented. It consists of two infinite impulse response filters based on two active loops. Owing to wavelength conversion employing cross-gain modulation of the amplified spontaneous emission spectrum of the semiconductor optical amplifier in one loop, the interference between the modulated optical signals of different taps from two loops can be avoided, and stable transmission characteristics of the filter can then be achieved. Using this cascaded structure, the free spectral range and Q value can be increased significantly, and tunability can also be realized. Measured results of a high Q of 3338 and a rejection ratio of about 40 dB are achieved.