At the heart of the advance is a technique the authors call multimodal locking, a scheme that stabilizes not just a single resonance of the optical microresonator but multiple modes simultaneously, locking the comb dynamics to a regime in which dissipative Kerr solitons can form and persist without delicate manual tuning. In conventional microcomb systems, soliton formation typically requires a precise and rapid traversal of a “chaotic” modulation instability regime, a step that is notoriously sensitive to thermal transients in the resonator chip. Laser frequency must be swept with exquisite control while the soliton’s own heat dissipation shifts the resonator’s resonances, creating a feedback loop that can destroy the soliton state within microseconds. The multimodal locking approach circumvents this fragility by engaging several mode families of the resonator at once, so that the interplay among them pins the comb to a stable soliton state even as environmental and thermal conditions fluctuate.

The significance of achieving a low repetition rate in the same device cannot be overstated. A frequency comb’s repetition rate is the spacing between its individual comb teeth, and for many applications, coarser spacing is a decisive advantage. In laser ranging and distance metrology, for example, the ambiguity range of an interferometric measurement is set by the inverse of the comb’s free spectral range. Microcombs, generated in whispering-gallery-mode or ring resonators only a few millimeters across, usually exhibit repetition rates of tens to hundreds of gigahertz, which severely limits the unambiguous measurement range available to a comb-tooth-resolved distance measurement. By engineering a low-repetition-rate soliton state, the research team extends this ambiguity range dramatically, making the microcomb directly useful for absolute distance measurement, multi-wavelength interferometry and coherent communications, where each comb line must be spectrally resolvable and addressable.

Achieving both low repetition rate and robust soliton generation in a single microresonator is a formidable engineering challenge. Low repetition rates demand large resonator diameters, which in turn produce dense spectra of resonances and increased susceptibility to mode crossings with higher-order transverse and polarization mode families. Such crossings historically destabilize soliton formation, causing comb states to collapse or preventing single-soliton operation altogether. The multimodal locking strategy turns this liability into an asset: rather than engineering mode crossings away, the researchers harness the coupling among mode families to lock the comb dynamics into a favorable basin of attraction. The result is a soliton microcomb whose low repetition rate is preserved over day-scale timeframes, a duration that places the device among the most operationally stable microcomb demonstrations reported to date.

Day-scale stability matters because it transforms the microcomb from a laboratory demonstration into an instrument. Many high-precision experiments, from astronomical spectrograph calibration to the accumulation of statistical confidence in atomic spectroscopy, require comb sources that remain locked and stable across many hours or even days of continuous operation. Conventional fiber-laser-based frequency combs achieve this, but at the cost of table-top size, high power consumption and substantial maintenance overhead. Microcombs promised to shrink the technology onto a photonic chip, yet until now their operational fragility has confined most demonstrations to short observation windows. The robust low-repetition-rate soliton microcomb described in the new work demonstrates that chip-scale frequency combs can, with appropriate locking architecture, sustain the kind of long-duration, hands-off operation that real metrology campaigns demand.

The physics underlying soliton microcombs rests on the Kerr nonlinearity of the resonator material, typically silicon nitride, magnesium fluoride or fused silica. When continuous-wave pump light resonantly builds up inside the ring, the Kerr effect produces a four-wave-mixing process that seeds sidebands spaced by the resonator’s free spectral range. Under the right conditions of pump detuning and intracavity power, these sidebands evolve into a coherent train of ultrashort optical pulses, dissipative Kerr solitons, that circulate around the resonator and emit a broadband comb spectrum with tooth spacing equal to the repetition rate. The comb’s output frequencies are determined by two radio-frequency quantities, the pump laser frequency and the repetition rate, which is why the combs can serve as rulers for measuring optical frequencies against a reference. Any drift in either quantity propagates directly into measurement error, and the multimodal locking mechanism suppresses precisely this drift by anchoring the comb’s underlying mode structure to the resonator’s intrinsic stability.

Applications for such a device span an unusually broad swath of science and engineering. In high-precision laser ranging, low-repetition-rate combs with tooth spacings in the low-gigahertz or even megahertz regime allow interferometric distance measurements over ranges of meters to kilometers without the need for auxiliary synthetic-wavelength techniques. In astronomical spectroscopy, comb calibration sources with well-controlled line spacing enable the detection of Earth-like exoplanets through radial velocity shifts measured in centimeters per second. In telecommunications, low-repetition-rate combs align naturally with standard channel grids used in wavelength-division multiplexed fiber networks, making them attractive as multi-carrier sources. In spectroscopy, dual-comb techniques benefit from repetition rates that are low enough to permit resolution of closely spaced molecular absorption features. A microcomb that is simultaneously robust, low-repetition-rate and day-scale stable therefore functions as a versatile enabling component across these domains rather than a single-purpose device.

The demonstration also speaks to a broader trend in photonics: the migration from delicate, expert-operated laboratory setups to self-contained, self-stabilizing systems. The self-starting character implied by multimodal locking is particularly important for this transition. A comb that requires a skilled operator to initiate soliton formation, sweeping the laser with practiced timing past the chaotic regime, cannot be deployed in an observatory dome, on a satellite platform or inside an industrial instrument. A comb that engages its soliton state reliably on demand and then holds it through thermal cycling, laser drift and mechanical perturbation removes the last practical barrier to integration. The authors’ demonstration of day-scale continuous operation is, in effect, a stress test passed by a technology that researchers have been attempting to harden for more than a decade.

The implications extend to metrology standards and timing as well. Optical atomic clocks, which now achieve fractional frequency uncertainties below one part in 10^18, rely on frequency combs to divide down their optical oscillations to countable microwave rates. Miniaturizing the comb stage of such clocks is a prerequisite for portable optical timekeeping, which would benefit navigation in GPS-denied environments, geodesy that monitors sea level and crustal motion with millimeter precision, and fundamental physics tests that track the constancy of natural constants over months and years. A robust chip-scale soliton microcomb with day-scale stability contributes a key subsystem to this vision, and the multimodal locking principle may generalize to other resonator platforms, materials and spectral regions, including the mid-infrared where molecular fingerprinting awaits compact comb sources.

Looking forward, the study suggests several directions that the photonics community is likely to pursue. Combining multimodal locking with on-chip pump lasers and integrated electronics would complete the transformation toward fully self-contained comb modules with no free-space optics. Extending the approach to lower repetition rates still, into the megahertz regime where fiber-laser combs operate, would bring microcombs into direct competition with their bulkier predecessors for the most demanding ranging and spectroscopy tasks. Improvements in resonator fabrication, reducing mode-volume disorder and engineering dispersion with greater precision, will further expand the parameter space in which robust soliton states can be locked. The present work demonstrates that the fragility once considered intrinsic to soliton microcombs is not a fundamental limit but an engineering problem, and one that can be solved by embracing, rather than avoiding, the multimodal structure of real optical resonators.

The research, published in Light: Science & Applications, marks a milestone in the maturation of microcomb technology. By uniting three properties that had previously been achieved only separately, self-starting robustness, low repetition rate and day-scale operational stability, the work delivers a soliton microcomb genuinely suited to high-precision metrology. As the demand for compact frequency references accelerates across navigation, astronomy, communications and fundamental science, the multimodal locking strategy reported by Tang, Yang, Yan and colleagues offers a clear and technically grounded route from chip-scale optical chaos to dependable, precision measurement light.

Optical frequency combs—laser sources whose output resembles a spectrum of equidistant frequencies—have revolutionized myriad domains including metrology and communications. The advent of microresonator-based frequency combs, or microcombs, has further turbocharged this revolution by enabling compact, chip-scale comb sources with impressively high repetition rates spanning GHz to THz bands. Silicon nitride photonics has emerged as the premier platform for these devices, offering ultralow optical loss, CMOS compatibility, and versatile integration possibilities. However, harnessing dissipative Kerr solitons in Si₃N₄ microresonators, a nonlinear optical phenomenon vital for stable and broadband comb generation, has proven delicate and fraught with reproducibility challenges mainly traced back to thermal instabilities.

Thermal effects manifest as sudden resonance shifts in the microresonator cavities, primarily stemming from light-induced heating. These refractive index variations render the soliton formation process highly unpredictable and transient—preventing deterministic access to stable soliton states. Prior approaches to initiate soliton states often employed fast laser frequency scanning, pulsed pumping schemes, or auxiliary lasers to counteract thermal dynamics. While partially successful, such methods introduce experimental complexity and compromise performance by narrowing the accessible soliton existence range, hindering the transfer of lab prototypes to robust commercial platforms.

The pioneering study dives deep into the root cause of thermal instability. Through rigorous compositional analyses and spectroscopy, the researchers identified trace copper impurities embedded within the Si₃N₄ waveguides—an element previously unsuspected in this context. These copper ions, originating from residual contaminants in the silicon wafer substrates, become unintentionally gettered during the high-temperature fabrication process of the photonic circuits. Their presence augments optical absorption and introduces thermal nonlinearities that translate into refractive index fluctuations destabilizing the soliton states.

Armed with this insight, the team devised specialized chemical treatments and fabrication protocol modifications targeting copper impurity removal. This copper extraction dramatically diminishes absorption-induced heating and mitigates the associated thermal drift in the microresonators. The carefully optimized copper removal achieves a regime where dissipative Kerr soliton formation is no longer limited by thermal constraints, granting reliable access to stable soliton combs using conventional slow laser scanning techniques. This contrasts starkly with prior reliance on complex fast frequency sweeps or auxiliary fields.

Demonstrations verified that these Cu-free Si₃N₄ microresonators consistently yield deterministic soliton formation across a broad range of laser tuning profiles. The results showcase clear soliton steps persisting over extensive temporal windows without requiring special scan speed calibrations. Notably, soliton microcombs produced exhibit high spectral purity, broad bandwidths, and stability compatible with prevailing integrated photonics applications. This leap forward eliminates a major practical bottleneck for on-chip frequency comb deployment, reducing system complexity while enhancing robustness.

The implications ripple across multiple frontiers of science and technology. Optical communication systems seeking high-capacity data transmission can now more reliably implement microcomb sources for wavelength division multiplexing without onerous temperature management. Concurrently, remote sensing and lidar platforms gain a pathway to compact, energy-efficient comb generators vital for velocity and distance measurements. Moreover, quantum photonics and frequency metrology stand to benefit from enhanced comb coherence and direct soliton state access, empowering advanced timekeeping and spectroscopy.

Importantly, the copper elimination techniques integrate seamlessly into front-end-of-line CMOS-compatible foundry processes, ensuring immediate relevance for industrial-scale wafer fabrication. This compatibility paves the way for wafer-scale manufacturing of thermal-stable Si₃N₄ microcomb chips, bridging the gap from experimental setups to widespread commercial adoption. The new fabrication paradigm could spearhead mass production of advanced photonic integrated circuits that leverage soliton microcombs as foundational components.

From a broader perspective, this breakthrough underscores the intricate interplay of materials science and nonlinear optics in modern photonics. Residual metal impurities, often regarded as innocuous, emerge as critical factors shaping device performance at the nanoscale. The findings call for renewed attention to contamination control and purification protocols in photonic device fabrication, especially as integration densities and complexity continue to escalate. Similar impurity-induced effects could conceivably affect other nonlinear or passive photonic elements, warranting comprehensive material characterization in future research.

Looking ahead, the authors emphasize that their copper management strategies unlock a host of new possibilities for exploring soliton dynamics in integrated photonics. With thermal noise effectively suppressed, investigations into multi-soliton states, complex soliton interactions, and long-term stability can proceed with greater fidelity. This foundation also sets the stage for integrating microcombs with active components such as modulators and detectors on a single chip, moving toward fully integrated photonic systems.

In summary, the identification and elimination of copper impurities represent a transformative step forward for microcomb research and industry. By solving the thermal instability puzzle in Si₃N₄ photonic integrated circuits, Ji and colleagues have unlocked practical, deterministic soliton generation with minimal complexity. Their contribution heralds a new era of reliable, manufacturable, and high-performance microcombs poised to drive innovations in metrology, communications, sensing, and beyond. As chip-scale photonics increasingly infiltrates cutting-edge technologies, such materials-driven breakthroughs will remain paramount for future advancement.

This work, published in Nature, epitomizes the profound impact that meticulous materials analysis combined with precision engineering can have on sophisticated optoelectronic platforms. It not only addresses a foundational challenge but also catalyzes progress toward ubiquitous soliton-enabled photonic integrated circuits, bringing the promise of microcomb technology to everyday devices globally.

Article Title: Deterministic soliton microcombs in Cu-free photonic integrated circuits

Article References:
Ji, X., Li, X., Qiu, Z. et al. Deterministic soliton microcombs in Cu-free photonic integrated circuits. Nature 646, 843–849 (2025). https://doi.org/10.1038/s41586-025-09598-4

DOI: https://doi.org/10.1038/s41586-025-09598-4