Coherence-free cascade IIR microwave photonic filter with high Q factor

A new high-Q infinite impulse response (IIR) microwave photonic filter is demonstrated, in which two active recirculating delay line loops based on a semiconductor optical amplifier (SOA) are cascaded. Due to cross-gain modulation in the SOA, interferences between light beams travelling along different paths are removed, ensuring stable operation and linearity of the electrical transfer function. The whole transfer function comprises a term representing a cascade response of two IIR filters, which significantly increases the free spectral range and the Q factor. The filter exhibits a Q factor of 1870 with a rejection ratio of 25 dB.