Self-aligned optical microcomb emerging between octave separated lasers

Optical frequency combs (OFCs) are frequency rulers essential for precision metrology, next generation navigation, and testing of fundamental physics. Despite intense efforts, chip-integrated OFCs remain laboratory-bound, unable to fulfill their promise of compact and cost-effective deployment. While improvement in fabrication and integration are important, a conceptual limitation has fundamentally stymied progress: on-chip OFC architectures have aimed to miniaturize their table-top counterparts and relied on cascading outward from (i.e., spectrally broadening) a single pump. In integrated platforms, this approach does not readily allow for the generation of strong and low-noise octave-spaced signals that are crucially needed for robust zero-frequency offset detection. Here, we overcome this limitation via an architectural inversion where an optical microcomb forms by filling the spectrum between two octave-separated pump lasers. The two pumps generate a parametrically driven cavity soliton (PDCS) in an integrated $χ^{(3)}$ resonator, which robustly self-aligns to (i.e., synchronizes with) the pump lasers across multiple foundry-fabricated devices and operating configurations. This produces a single octave-spanning comb extending from telecom to visible wavelengths, whose zero-frequency offset is completely defined by the two harmonically-related pump lasers, and can therefore be reliably detected and stabilized. We showcase our platform’s capabilities by executing all of the three core tasks of OFC metrology: optical frequency synthesis, low-noise millimeter-wave generation, and integrated optical clock readout, using the same self-aligned microcomb with only its input locks changed.

💡 Research Summary

Optical frequency combs (OFCs) are indispensable tools for precision metrology, navigation, and fundamental physics, yet chip‑scale OFCs have remained confined to laboratories because conventional designs simply miniaturize table‑top systems that rely on a single continuous‑wave pump. This single‑pump approach inevitably yields weak, noisy octave‑spaced components at the comb edges, making carrier‑envelope offset (CEO) detection and stabilization difficult.

In this work the authors overturn that paradigm by using two pump lasers separated by an octave (ν₋ and ν₊≈2ν₋) to drive a χ³ silicon‑nitride microring resonator. The degenerate four‑wave mixing condition ν₊+ν₋=2ν₀ creates a parametric‑driven cavity soliton (PDCS) centered at ν₀=(ν₊+ν₋)/2. Crucially, Kerr nonlinearity induces a self‑alignment (synchronization) of the PDCS with both pumps: the phase‑velocity mismatch is automatically cancelled over a broad parameter range, forcing the CEO offset Δν_CEO between the PDCS and each pump to zero. Consequently the entire spectrum—from the lower pump in the telecom C‑band to the upper pump in the visible—forms a single, gap‑free frequency grid spanning one octave (≈220 THz). Because the two pumps are themselves comb lines, the CEO is predetermined by the harmonic relationship ν₊≈2ν₋ and can be measured simply by beating ν₊ against the second‑harmonic of ν₋, eliminating the need for an f‑2f interferometer.

The devices were fabricated in a commercial foundry: 660 nm‑thick Si₃N₄ microrings with a 31 µm radius and widths of 840–860 nm. Dispersion engineering minimized both the OPO frequency mismatch Δν(n₀) and the integrated dispersion D_int at the two pump frequencies, ensuring phase‑velocity matching. With on‑chip pump powers of 145 mW (ν₋) and 70 mW (ν₊), slow tuning of the C‑band pump produced a deterministic PDCS that filled the octave gap, delivering comb lines above –60 dBm across the entire span. When self‑aligned, the repetition rate ν_rep is exactly (ν₊–ν₋)/N (N=263) and follows the pump separation linearly, resulting in a dramatic reduction of repetition‑rate phase noise compared with an unsynchronized PDCS.

The authors demonstrate three core OFC applications using the same self‑aligned microcomb, merely by reconfiguring the input‑laser locks: (1) synthesis of optical frequencies from a known microwave reference, (2) generation of low‑noise millimeter‑wave signals directly from low‑phase‑noise lasers, and (3) extraction of a stable microwave timing signal from an atom‑referenced optical clock. In each case the pre‑defined CEO enables straightforward locking of the comb to external references, while the deterministic repetition rate provides a reliable microwave link.

Overall, this work introduces a scalable, self‑referencing microcomb architecture that overcomes the fundamental limitations of single‑pump Kerr combs. By exploiting octave‑separated dual pumps and Kerr‑induced synchronization, the platform delivers a robust, single‑grid, octave‑spanning comb with intrinsic two‑point stabilization. The approach promises mass‑manufacturable, low‑power, high‑performance OFCs for field‑deployable navigation, timing, and quantum‑technology systems, and opens pathways toward fully integrated photonic‑electronic comb processors.