Ultrabroadband integrated electro-optic frequency comb in lithium tantalate

The integrated frequency comb generator based on Kerr parametric oscillation1 has led to chip-scale, gigahertz-spaced combs with new applications spanning hyperscale telecommunications, low-noise microwave synthesis, light detection and ranging, and astrophysical spectrometer calibration2, 3, 4, 5-6. Recent progress in lithium niobate (LiNbO3) photonic integrated circuits (PICs) has resulted in chip-scale, electro-optic (EO) frequency combs7,8, offering precise comb-line positioning and simple operation without relying on the formation of dissipative Kerr solitons. However, current integrated EO combs face limited spectral coverage due to the large microwave power required to drive the non-resonant capacitive electrodes and the strong intrinsic birefringence of LiNbO3. Here we overcome both challenges with an integrated triply resonant architecture, combining monolithic microwave integrated circuits with PICs based on the recently emerged thin-film lithium tantalate (LiTaO3)9. With resonantly enhanced EO interaction and reduced birefringence in LiTaO3, we achieve a fourfold comb span extension and a 16-fold power reduction compared to the conventional, non-resonant microwave design. Driven by a hybrid integrated laser diode, the comb spans over 450 nm (more than 60 THz) with more than 2,000 lines, and the generator fits within a compact 1-cm2 footprint. We additionally observe that the strong EO coupling leads to an increased comb existence range approaching the full free spectral range of the optical microresonator. The ultra-broadband comb generator, combined with detuning-agnostic operation, could advance chip-scale spectrometry and ultra-low-noise millimetre wave synthesis10, 11, 12-13 and unlock octave-spanning EO combs. The methodology of co-designing microwave and photonics can be extended to a wide range of integrated EOs applications14, 15-16.