Reported tunable optical filters based on MRR can be produced by using various physical effects, including thermo-optic effect [
6], electro-optic effect [
7], and free carrier plasma dispersion effect [
8]. A 10-μm-diameter SOI-based ring resonator with the FSR of 18 nm and the tunable range of 6.4 nm is demonstrated using thermo-optic effect [
9], however they suffer from large power dissipation, thermal crosstalk and low integration. Alternatively, both approaches using electro-optic effect of LiNbO
3 [
10] and free carrier plasma dispersion effect of Si [
11] enable fast modulation, but their tuning ranges are small due to the small effective index change. Another method is to combine strip waveguides with nematic LC cladding [
12-
14], LC molecules are highly birefringent, and orientation can be electrically controlled by external electric fields. The tuning ranges of 31 and 4.5 nm are demonstrated for strip-waveguide MRRs guiding the TM mode and TE mode, but at the expense of smaller FSRs of 4.27 and 4.0 nm respectively [
15], and these conventional LC MRRs usually have big sizes (large radii). Moreover, a rather high drive voltage of 100 V is required, which seriously limits the application of such LC MRRs.