Microresonator optical frequency combs utilizing ultra-low loss and wafer-scale manufacturable photonic integrated circuits (PICs)—especially those based on foundry-available silicon nitride—have been pivotal in advancing their fibre-based laboratory counterparts to chip-scale system-level applications in science and technology. Their versatility has been demonstrated in hyperscale data communication, parallel light detection and ranging6, neuromorphic computing, ultra-low-noise microwave synthesis, broadband spectroscopy and astrophysical spectrometer calibration. The availability of thin-film lithium niobate (LiNbO3) using smart-cut has triggered the development of EO photonic integrated circuits with a large Pockels coefficient. This platform has re-ignited interest in electro-optic (EO) frequency combs. The recently emerged integrated EO combs complement soliton microcombs, and exhibit similar compactness while offering innate stability of the repetition rates set by the microwave modulation frequency. Coherent sideband generation mediated by the EO Pockels effect does not have a minimum optical threshold, unlike parametric oscillations in Kerr comb formation. Additionally, it does not require the complex laser tuning mechanisms needed for dissipative Kerr soliton initiation. Microcombs based on dissipative Kerr solitons also suffer from reduced conversion efficiency at lower repetition rates, particularly in the sub-100-GHz frequencies (for example, X band for radar, K band for 5G).

Despite these advantages, EO combs still face outstanding challenges. The achieved frequency comb span, along with the line count, has been limited compared to soliton microcombs, which have attained octave-spanning operation and the generation of more than 2,000 comb lines. This is due to the insufficient EO coupling rate for generating thousands of sidebands. As a result, large microwave pump power is needed to attain the requisite modulation depth. State-of-the-art integrated EO combs therefore require specialized power microwave circuits and bulk protective circulators, which remain challenging to integrate into chip-scale systems. A further limitation stems from the intrinsic birefringence of LiNbO, which imposes a span limit due to mode mixing. Together, these factors have limited the bandwidth of state-of-the-art integrated EO combs below 140 nm and exacerbated their microwave power requirements.

Here we overcome these challenges by bringing coplanar waveguide resonators from monolithic microwave integrated circuits (MMICs) into photonic integrated circuits and implementing an integrated triply resonant EO comb generator. In this triply resonant scheme, all three fields involved in the EO three-wave mixing process—two nearby optical modes and one microwave mode—are resonant, resulting in enhanced interaction. The tight field confinement offered by microwave photonic co-integration enhances the single-photon EO coupling rate by more than 300 times compared to bulk implementations24. Combined with dispersion-engineered lithium tantalate (LiTaO3) photonic integrated circuits that exhibit 17 times lower intrinsic birefringence than the workhorse EO material LiNbO3, and driven by a hybrid integrated semiconductor laser diode, the device is capable of generating over 2,000 sidebands (a 450-nm span) while consuming under 7 W of on-chip power. We attain an 80-nm span, comparable to current state-of-the-art non-resonant integrated EO comb generators7, with only 13 dBm of microwave power (peak-voltage less than 1.5Vp, readily achievable with low-voltage complementary metal oxide semiconductor (LVCMOS) or GaAs MMIC low-phase-noise amplifiers), representing an over 16-fold power reduction compared to a conventional non-resonant electrode design. Moreover, the enhanced EO coupling rate in our triply resonant scheme is shown to lead to unprecedented detuning-agnostic operation, with a comb existence range exceeding 90% of the free spectral range (FSR) despite using a microresonator, enabling robust turnkey operation with full FSR sweeping of the comb lines free of spectral holes.