Electrode optimization for efficient resonant electro-optic frequency microcombs

Resonant electro-optic comb generation in thin-film lithium niobate microrings is a powerful method for achieving large-bandwidth electro-optic combs with a small footprint. While significant progress has been made in optimizing the optical resonator, the on-chip microwave waveguide has been less explored. Here, we develop a generic analytical model that predicts the phase shift acquired by the optical wave, taking into account possible microwave losses, impedance and velocity mismatches, and different injection points. We apply this model to determine the optimal electrode length for a traveling-wave configuration and two standing-wave configurations, with microwave driving frequencies at different multiples of the optical free spectral range. We experimentally validate the model by measuring several devices fabricated at the chip and wafer levels.