Bandwidth Tunable Filter Based on Ideal Quasi-Critical Coupling State in WGM Cavity

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