Introduction

At the invitation from the founding Editor-in-Chief of Photonics Insights, Prof. Xingjun Wang and Prof. Haowen Shu from Peking University jointly authored a review article titled "Microcomb technology: from principles to applications". The article was published in Issue 4, Volume 3 of Photonics Insights in 2024. (Haowen Shu, Bitao Shen, Huajin Chang, Junhao Han, Jiong Xiao, Xingjun Wang. Microcomb technology: from principles to applications [J]. Photonics Insights, 2024, 3(4): R09)

This review comprehensively and systematically summarizes the latest research advancements in integrated microcomb technology in recent years. It highlights the physical mechanisms, generation and control methods, material platforms, and fabrication characteristics. The article also details various potential application directions, ​​with a particular focus on their recent integration with integrated photonics in information systems​​, and provides an outlook on future research directions in the field. Serving as a valuable reference, it offers critical insights for both researchers and newcomers in related disciplines.

I. Physical Mechanisms of Microcomb Generation

The concept of optical frequency combs originated from research on mode-locked lasers and was later extended to broadband light sources with equally spaced frequency components. Traditional frequency combs relied on bulky discrete mode-locked laser systems until the early 21st century when researchers proposed using on-chip microcavities to generate microcombs. A pivotal breakthrough occurred in 2014 when mode-locked microcombs were experimentally observed in microcavities, significantly advancing their development and applications.

The generation mechanism of integrated microcombs relies on ​​high-Q nonlinear optical microcavities pumped by frequency-swept lasers​​. When the pump frequency matches the cavity resonance, third-order nonlinear effects (e.g., four-wave mixing (FWM), self-phase modulation (SPM), and cross-phase modulation (XPM)) dominate parametric oscillation processes, forming coherent equidistant frequency combs in the spectral domain. By adjusting pump power and sweeping rates, microcombs exhibit multistate evolutionary dynamics: transitioning from primary modulation instability-induced continuous-wave backgrounds through spatiotemporal chaotic states, eventually stabilizing into temporal single-soliton pulses under nonlinear gain-loss balance and dispersion-nonlinear phase matching conditions. Notably, the final microcomb state depends on the cavity's dispersion design, strongly correlating with intrinsic dispersion, pump conditions, and complex nonlinear effects. For simulating microcomb evolution, primary theoretical approaches include coupled-mode equations, Ikeda maps, and Lugiato-Lefever equations (LLE), where the former requires computationally intensive mode-coupling analysis while the latter two simplify nonlinear processes via nonlinear Schrödinger equation frameworks.

In practical generation, thermal-optic effects during frequency sweeping often hinder single-soliton observation. To enhance stability, researchers have developed methods like rapid frequency sweeping, pulsed pumping, auxiliary photothermal stabilization, and self-injection locking. These approaches vary in robustness, complexity, and integration levels, collectively improving microcomb accessibility and application scope.

Fig. 1 Timeline of the microcomb technology development. The four columns from left to right are developments of the application, the design, the principle, and the fabrication, respectively

II. Material Platforms and Fabrication Innovations

To extend microcomb spectral coverage and enhance nonlinear performance, diverse material platforms have been explored (e.g., silicon nitride (Si₃N₄), lithium niobate (LiNbO₃), and photonic crystal microcavities). Key material parameters include nonlinear coefficients, optical absorption rates, and refractive index profiles. High-Q microcavity fabrication employs techniques such as crystal cutting/polishing (for MgF₂ and CaF₂ cavities), melt-solidification for surface smoothing, monolithic thin-film deposition (e.g., Si₃N₄ via CMOS processes), and bonding preformed films to target chips.

Fig. 2 Optical comb wavelength ranges and Q factors of different resonators based on various material platforms.

Hybrid and heterogeneous integration methods enable fully on-chip systems. Hybrid approaches combine microcombs with pump lasers through co-packaging, micro-lens coupling, or photonic wire bonding, albeit with high costs. Heterogeneous integration (e.g., AlGaAsOI platforms) monolithically integrates multi-material devices on a single substrate. Emerging techniques like micro-transfer printing offer higher material efficiency but face output power limitations. Quantum dot materials show potential for direct laser integration, though their application in microcomb generation remains unexplored.

III. Performance Design and Optimization

Spectral shaping and energy conversion efficiency optimization rely heavily on dispersion engineering. Early studies adjusted waveguide geometries (height/width), while recent advances utilize intermodal interactions and photonic crystal microrings for flexible dispersion control. Bright solitons typically achieve ~1% energy conversion efficiency due to low pump-pulse overlap, whereas pulsed pumping and pump recycling strategies theoretically reach 98% efficiency.

Fig. 3 Microcombs under different dispersion conditions. (a) Bright soliton under anomalous dispersion. (b) Dark pulse under normal dispersion. (c) Dispersion wave due to high-order dispersion. (d) Quartic soliton supported by fourth dispersion. (e) Bright pulse under near-zero dispersion.

Noise suppression addresses technical noise, pump laser fluctuations, and thermal/quantum noise through self-injection locking, auxiliary photothermal stabilization, and "quiet-point" operation canceling Raman-induced frequency shifts. For example, designing composite waveguide structures with positive/negative thermo-optic coefficients effectively reduces thermal noise.

IV. Chip-Scale System Applications

Microcombs enable revolutionary applications across metrology, communications, and sensing. They serve as optical frequency rulers in atomic clocks, enhance spectral resolution in dual-comb spectroscopy (DCS), and improve lidar resolution through time-of-flight (ToF) and frequency-modulated continuous-wave (FMCW) techniques. In optical coherence tomography (OCT), microcombs reduce sensitivity decay and enhance imaging resolution for portable medical devices.

Parallel optical interconnects leverage microcombs' multi-wavelength stability for terabit/s dense wavelength-division multiplexing (DWDM) systems, while microwave photonics utilize their low-noise properties for reconfigurable signal processing. Recent demonstrations of fully integrated microcomb systems with photonic-electronic co-packaging highlight progress toward chip-scale solutions in data centers and AI computing.

V. Conclusions and Outlook

This review systematically details microcomb physics, material platforms, performance optimization, and system applications. The integration of microcombs with silicon photonics enables ​​multi-functional parallelized on-chip optoelectronic systems​​, driving revolutions in data centers, 5G/6G networks, and AI computing. Future directions include expanding spectral coverage to visible/MIR bands, enhancing power efficiency through hybrid cavity designs, and achieving cost-effective mass production via advanced lithography techniques.