Researchers from Peking University Yangtze Delta Institute of Optoelectronics, have proposed a universal design scheme for discrete whispering gallery mode (WGM) microcavity devices. The developed devices achieve an exceptional quality factor (Q) of up to 1 billion while maintaining remarkable robustness. On this platform, the team has successfully demonstrated low-threshold (4mW) and low-noise (-103dBc/Hz10kHz) soliton microcombs. This groundbreaking research provides an innovative technological pathway for advancing both fundamental research and engineering applications in microcavity-based precision measurement and microwave photonics.

Optical frequency combs (OFCs) have emerged as a transformative technology, finding extensive applications in advanced scientific research and engineering applications, such as atomic clocks, coherent communications, spectral analysis, and LiDAR. Microcombs, leveraging the nonlinear four-wave mixing effect in high-Q optical microcavities, exhibit substantial advantages, including compact size, low power consumption, high integrability, and broad repetition frequency range. These distinctive characteristics position microcombs as a promising solution to address the existing limitations in conventional OFC systems, potentially heralding a new paradigm in frequency comb technology. 

Currently, researchers have successfully implemented low-noise soliton microcombs based on on-chip and discrete optical microcavities. Although discrete microcavities lack high integration, they exhibit unique advantages such as ultra-high Q, low thermal noise, wide repetition frequency range, and excellent fiber compatibility. Nevertheless, their packaging technology still faces significant challenges——insufficient stability and hermeticity can lead to substantial degradation of the quality factor, while the coupling state is susceptible to environmental vibrations, temperature and humidity fluctuations, particularly in demanding scenarios such as vehicular, outdoor, and aerospace applications. Therefore, the development of integrated packaging technologies for discrete microcavities with superior environmental isolation and coupling stability has become a crucial breakthrough to advance the field, as well as a technological barrier that demands urgent resolution.

Building upon the aforementioned research background, researchers from Peking University Yangtze Delta Institute of Optoelectronics have proposed an integrated universal design scheme for ultra-high Q microcavity devices. The packaged devices exhibit exceptional thermodynamic properties and environmental robustness, capable of withstanding rigorous durability, vibration, and temperature shock tests. Furthermore, researchers have achieved the generation of low-threshold, low-phase-noise soliton microcombs on this platform, establishing a viable technical pathway for advancing research in microcavity nonlinear effects and narrow-linewidth laser technologies, with the potential to drive innovation in related fields. The research results are published entitled "High robustness, billion Q packaged microcavity devices for soliton microcombs" in Chinese Optics Letters, Vol. 23, Issue 2, 2025.

Through advanced laser machining technology, the silica microrod with Q factors greater than 10⁹ was fabricated. By optimizing the geometric morphology of microcavity, the mode volume was moderately increased to suppress cavity thermal noise. Based on this, a high-performance microcavity coupling system was constructed by integrating low insertion loss tapered fibers. Regarding the packaging, a nested structure of alloy and PEEK materials was employed for the housing, utilizing adhesives with graded Young's modulus gradients and low thermal expansion coefficients to secure the cavity and the coupling waveguide. A built-in TEC temperature control device and external standardized FC/APC polarization-maintaining fiber connectors were incorporated to further enhance spectral stability. Comprehensive environmental testing demonstrates that the packaged module exhibits exceptional resistance to thermal deformation and vibration, making it suitable for complex environments. The packaged microcavity device was capable of generating a soliton microcomb with a repetition frequency of 24.98 GHz at wavelengths near 1,550 nanometres with 4 milliwatts threshold power, showcasing superior phase noise performance and operational stability compared to unpackaged Kerr frequency combs.

Fig. 1 (a) Microcavity image and mode profile. (b) Schematic diagram of the microrod-taper coupling packaged structure. (c) is the comparison diagram of the same typical resonant mode in the Q for unpackaged (left) and packaged (right) microcavities. (d) is the variation of the Q factors of packaged microcavities placed outdoors, the red and black dashed lines represent the maximum and minimum temperature respectively, and the time points with high humidity have been marked in the figure. (e) Optical spectrum of the soliton microcomb.

The proposed integrated general design scheme for discrete optical microcavities in this study demonstrates vast potential applications in soliton light sources and other fields including external-cavity narrow linewidth lasers. This scheme exhibits significant scalability, with the research team successfully transferring packaging technology to both crystal microcavities and on-chip microcavities. Looking ahead, the team will continue to delve into the engineering packaging processes of microcavities, leveraging the unique advantages of WGM microcavities to drive innovative developments in novel light sources and sensing technologies, thereby paving the way for new technological breakthroughs in the field of optics.