Feasibility of quasicritical coupling based on LP modes and its application as a filter with tunable bandwidth and stable insertion loss.

The feasibility of the quasicritical coupling based on the high order scalar modes in tapered fiber was presented and discussed in detail theoretically. As its applications, the bandwidth evolution of the coupling process both in the Through port and the Drop port were also included in the calculations and demonstrated in the experiment. As a result, tunable bandwidth filters with stable insertion loss were realized by changing the gap between the few-mode tapered fiber and microcavity working under a state of quasicritical coupling. The bandwidth at the Through port is from 4.8 MHz to 327.1 MHz and that at the Drop port is from 15.3 MHz to 327.1 MHz, with the insertion loss fluctuation less than 10%. The combination of stable efficiency, narrow band, and bandwidth fine tunability may benefit its application in narrow-linewidth lasers, microwave photonics and nonlinear optics.