Independent tuning of free spectral range in thin-film lithium niobate microring resonators.

In conventional traveling-wave optical cavities, the free spectral range (FSR) and the resonance frequencies are inherently linked, limiting the ability to control the FSR independently. This work presents a systematic investigation of techniques to tune the waveguide dispersion in thin-film lithium niobate (TFLN) microring resonators (MRRs), enabling independent control of the FSR without affecting a specific resonance. We experimentally characterize and compare the effects of electro-optic and thermo-optic tuning on the FSR and resonance wavelengths, revealing the difference in tuning waveguide mode dispersion between distinct mechanisms. Furthermore, we demonstrate that the FSR tuning behavior can be engineered by varying the waveguide width, offering an additional degree of freedom for dispersion control arising from the interplay between geometry and material dispersion. These findings provide new insights into the control of integrated photonic devices and can contribute to applications requiring independent control of FSR and resonance wavelengths.