Ultra-flat electro-optic frequency comb based on a chirp-modulated lithium niobate resonator.

Chirp modulation can generate a relatively flat electro-optic frequency comb (EO comb) and offers the advantage of frequency reconfigurability, demonstrating significant potential in high-precision sensing and absorption spectroscopy measurements. However, nonresonant devices such as waveguides are susceptible to limitations in modulation efficiency and bandwidth during electro-optic modulation. In this paper, by utilizing chirp modulation resonance mode, we have realized an EO comb based on a lithium niobate resonator with small tooth spacing and high flatness. Theoretically, the chirp modulation of phase is achieved by modulating the dispersion coupling term in the resonant mode transmission equation. Compared with conventional waveguide-based EO combs, the resonant mode chirp modulation is capable of generating a multistage flat comb, and thus the bandwidth of the comb is significantly expanded. In the experiment, with a repetition rate as low as 20 kHz and a bias voltage of 1 V, the comb bandwidth extended to over 150 MHz, where the number of 3 dB flat comb teeth for a single stage exceeds 2,000. Finally, we evaluated the measurement capability of the frequency comb at different temperatures by utilizing the transmission spectrum of the germanium-doped silica waveguide cavity as the absorption spectrum, measuring a temperature sensitivity of 1505.00 MHz/K and a temperature instability of 1.13 mK/Hz.