Bandwidth tunable lter based on ideal quasi-critical coupling state in WGM cavity

A tunable bandwidth add-drop lter in ideal quasi-critical coupling (QCC) state was analyzed in theory in detail and demonstrated in experiment, with two vector modes HE $^X_{11}$ and HE $^Y_{11}$ coupled single whispering gallery mode (WGM) as its basic conguration. In the experiment, the bandwidth tuning range at the Drop port was from 54.0 MHz to 775.8 MHz with the conversion efciency maintained above 95%, and bandwidth tuning range at the Through port was from 6.6 MHz to 720.6 MHz with the insertion loss uctuation less than 5%. The demonstrated lter gains advantages of stable efciency, narrow band, and broad bandwidth tuning range, which will be helpful for its application in microwave photonics, narrow linewidth lasers and nonlinear optics.