Ultrahigh-Q microwave photonic filter with tunable Q value utilizing cascaded optical-electrical feedback loops

A microwave photonic filter with the highest reported quality factor (Q) value of 4895.31 is proposed and experimentally demonstrated by using two cascaded infinite impulse response (IIR) filters. Each IIR filter comprises both optical and electronic signals in a feedback loop and thus the loop length can be reduced without the need to consider the light coherence length. The Vernier effect enables a significant improvement of the free spectral ranges and Q values of the cascaded filter. The Q value of the proposed microwave photonic filter can be changed when the loop lengths of two cascaded filters are carefully adjusted. In addition, for a fixed Q, the frequency response of the filter can also be tuned by adjusting the bias of the Mach-Zehnder modulator in each loop.