  Integrated frequency comb generators based on Kerr parametric oscillation
have led to chip-scale, gigahertz-spaced combs with new applications spanning
hyperscale telecommunications, low-noise microwave synthesis, LiDAR, and
astrophysical spectrometer calibration. Recent progress in lithium niobate (LN)
photonic integrated circuits (PICs) has resulted in chip-scale electro-optic
(EO) frequency combs, 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 Lithium Niobate. Here, we overcome
both challenges with an integrated triply resonant architecture, combining
monolithic microwave integrated circuits (MMICs) with PICs based on the
recently emerged thin-film lithium tantalate. With resonantly enhanced EO
interaction and reduced birefringence in Lithium Tantalate, we achieve a
four-fold 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 450nm (60THz) with >2000 lines, and the generator
fits within a compact 1cm^2 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 millimeter wave synthesis and unlock
octave-spanning EO combs. The methodology of co-designing microwave and optical
resonators can be extended to a wide range of integrated electro-optics
applications.
