Optical frequency combs consist of equally spaced discrete optical frequency components and are essential tools for optical communications, precision metrology, timing and spectroscopy. To date, wide-spanning combs are most often generated by mode-locked lasers or dispersion-engineered resonators with third-order Kerr nonlinearity. An alternative comb generation method uses electrooptic (EO) phase modulation in a resonator with strong second-order nonlinearity, resulting in combs with excellent stability and controllability. Previous EO combs, however, have been limited to narrow widths by a weak EO interaction strength and a lack of dispersion engineering in free-space systems. In this work, we overcome these limitations by realizing an integrated EO comb generator in a thin-film lithium niobate photonic platform that features a large electro-optic response, ultralow optical loss and highly co-localized microwave and optical fields, while enabling dispersion engineering. Our measured EO frequency comb spans more than the entire telecommunications L-band (over 900 comb lines spaced at ∼ 10 GHz), and we show that future dispersion engineering can enable octave-spanning combs. Furthermore, we demonstrate the high tolerance of our comb generator to modulation frequency detuning, with frequency spacing finely controllable over seven orders of magnitude (10 Hz to 100 MHz), and utilize this feature to generate dual frequency combs in a single resonator. Our results show that integrated EO comb generators, capable of generating wide and stable comb spectra, are a powerful complement to integrated Kerr combs, enabling applications ranging from spectroscopy to optical communications.The migration of optical frequency comb generators to integrated devices is motivated by a desire for efficient, compact, robust, and high repetition-rate combs [1,2]. At present, almost all on-chip frequency comb generators rely on the Kerr (third-order, χ (3) ) nonlinear optical process, where a continuous wave (CW) laser source excites a low-loss optical microresonator having a large Kerr nonlinear coefficient. This approach has enabled demonstration of wide-spanning Kerr frequency combs from the near-to mid-infrared in many material platforms [3][4][5][6][7]. Owing to the complex nature of the parametric oscillation process, however, the formation dynamics and noise properties of the Kerr combs are not yet fully understood and are still under active investigation [8,9]. Sophisticated control protocols are typically required to keep Kerr combs stabilized.An alternative frequency comb-generation method uses the electro-optic (EO) effect in materials with secondorder (χ (2) ) nonlinearity. Conventionally, EO frequency comb generators pass a CW laser through a sequence of discrete phase and amplitude modulators [10][11][12]. Such EO comb generators can feature remarkable comb power and flat spectra, and can support flexible frequency spacing. They usually have narrow bandwidth, however, comprising only tens of lines and spanning only a few nan...
