Team Makes Significant Progress in the Manipulation and Application of Chip-Integrated Dark-Pulse Microcombs

Content Overview 

The invention of optical frequency comb (OFC) technology has brought revolutionary advancements to fields such as timing, spectroscopy, precision measurement, and fundamental physics verification. As a miniaturized OFC technology, microresonator-based frequency combs ("microcombs") have achieved major progress over the past decade. Microcomb generation systems can now be fully integrated on-chip, and their system-level applications have been widely validated in laboratories, including coherent communications, astronomical spectrograph calibration, ranging and imaging, frequency synthesis, optical atomic clocks, and photonic neural network computing.

However, translating microcomb technology from the laboratory to practical applications still faces many challenges. While advancements in heterogeneous and hybrid integration technologies, along with the discovery of laser self-injection locking, have enabled chip-scale microcombs compatible with CMOS manufacturing processes, specific microcomb states with excellent coherence remain difficult to reproduce and control. The root of this challenge lies in the extremely complex interactions within the laser-microresonator coupled system, including mutual coupling and mode competition between the laser and the microresonator, as well as the synergistic effects of Kerr nonlinearity and photothermal effects operating on different time scales. Existing linear self-injection locking theory can describe the linear coupling between the laser and the external microresonator but fails to capture and explain these complex nonlinear dynamics. The vast application potential of microcomb technology can only be fully realized in scenarios such as data centers, space platforms, and mobile terminals through a deep understanding and precise control of these complex nonlinear physical processes.

To address this challenge, the research team utilized a hybrid integration of a semiconductor laser and a silicon nitride (Si₃N₄) microresonator to investigate the nonlinear dynamics of dark pulses under the self-injection locking effect (Fig.1a, b). The distributed feedback (DFB) laser chip, driven by a small printed circuit board (PCB), emits continuous-wave laser light at 1550 nm, which is coupled into the silicon nitride chip waveguide via edge coupling. When the DFB laser frequency is tuned to a resonance mode of the silicon nitride microresonator, the laser light enters the microresonator and, via Rayleigh scattering, returns into the DFB laser cavity, triggering self-injection locking of the laser. At this point, the laser frequency autonomously locks to the resonance frequency of the microresonator, and the laser frequency noise is significantly suppressed. Simultaneously, due to the photothermal effect, heating of the microresonator by the laser causes a collective shift of its resonance modes. When the intracavity power is sufficient, Kerr nonlinearity triggers four-wave mixing, facilitating the transfer of photon energy to other resonance modes. In the microresonator with normal group velocity dispersion (GVD), the synergy between self-injection locking and the Kerr effect ultimately leads to the generation of a dark-pulse microcomb (Fig. 1c).

     By precisely controlling the nonlinear interaction between the laser and the silicon nitride microresonator, this work has for the first time observed the universal phenomena of dark-pulse formation and its discrete state-switching behavior within the system (Fig.1c). To this end, a comprehensive theoretical model was established, systematically analyzing the synergistic mechanisms of laser-microresonator mutual coupling, Kerr nonlinearity, and the photothermal effect. Numerical simulations based on this model revealed discrete dark-pulse states consistent with experiments. Furthermore, experiments discovered a universal "noise-quenching" effect in the dark pulse's repetition rate, where within each discrete dark-pulse state, the repetition rate shows insensitivity to laser noise, exhibiting phase noise suppression exceeding 23.5 dB. By directly detecting the dark-pulse microcomb with a high-speed photodetector, the system can directly output a low-phase-noise microwave signal.

Fig. 1: (a) Principle and schematic of self-injection locking, Kerr nonlinearity, and photothermal effect in the laser-microresonator coupled system. (b) Experimental setup, including a printed circuit board, a DFB laser chip, a Si₃N₄microresonator chip, and a lensed fiber. (c) Four discrete dark-pulse states with distinct spectral features and pulse widths.

This work investigates the complex Kerr-thermal dynamics of dark-pulse microcomb formation in integrated photonic chips from theoretical, numerical, and experimental perspectives. It reveals for the first time the universal phenomena of dark-pulse formation and state switching, as well as the unique "noise-quenching" effect in laser-microresonator hybrid systems. This work provides critical insights and control methods for understanding the formation mechanisms of dark pulses in such hybrid systems. Simultaneously, it offers a concise and effective solution for chip-based low-phase-noise microwave oscillators, providing theoretical and technical support for advanced applications in fields such as next-generation wireless communications, precision timing, deep-space navigation, high-precision radar, and high-sensitivity astronomical detection.

Publication Information 

The co-first authors of this work are Shichang Li (a joint Ph.D. student from the Shenzhen International Quantum Academy and Southern University of Science and Technology), Kunpeng Yu (a joint Ph.D. student from the Shenzhen International Quantum Academy and University of Science and Technology of China), and Dr. Dmitry A. Chermoshentsev (Assistant Professor at the Russian Quantum Center). The corresponding authors are Associate Researcher Wei Sun and Researcher Junqiu Liu. The team led by Professor Igor A. Bilenko at the Russian Quantum Center also made significant contributions to this work. The research was supported by the National Natural Science Foundation of China International Cooperation and Exchange Program, the Ministry of Science and Technology's 2030 Project, Guangdong Province, and Shenzhen Municipality.

Universal Kerr-thermal dynamics of self-injection-locked microresonator dark pulses, Physical Review Letters 135, 133803 (2025). Original link: https://www.nature.com/nature-index/article/10.1103/zlqs-yc51