Researchers at KAIST have demonstrated a chip-scale photonic approach for generating ultralow-noise and highly stable microwave and millimeter-wave signals based on optical frequency combs (microcombs), offering a potential pathway toward compact, high-performance frequency sources for next-generation technologies.

High-frequency signals in the tens to hundreds of gigahertz range are essential for emerging applications such as 6G communications, radar, and precision sensing. However, achieving both low noise and high stability at these frequencies remains a fundamental challenge for conventional electronic signal sources.

In the first study, the researchers addressed the long-standing challenge of transferring the stability of an optical reference to a microcomb. Direct stabilization is difficult due to the lack of carrier-envelope offset detection in high-repetition-rate microcombs. To overcome this, they used a mode-locked laser as a transfer oscillator and synchronized it to the microcomb using electro-optic sampling. This approach enabled direct and robust transfer of optical-reference stability to the microcomb repetition rate, achieving fractional frequency stability at the 10^-18 level and a phase noise of -125 dBc/Hz at 100 Hz offset from a 22 GHz carrier, representing state-of-the-art performance and more than 80 dB improvement over the free-running microcomb in the low-offset-frequency regime.

In the second study, the team addressed the degradation of noise performance typically observed when scaling microcombs to higher repetition rates. While microcombs with lower repetition rates (large resonators) exhibit better noise characteristics, increasing the repetition rate generally leads to performance degradation. The researchers showed that this limitation can be overcome using perfect soliton crystal (PSC) states, which enable repetition-rate multiplication while preserving the low-noise characteristics of the original comb. As a result, they generated millimeter-wave signals at 44 GHz and 66 GHz with timing jitter on the order of 3 femtoseconds, demonstrating that the low-noise performance of a microwave-rate microcomb can be preserved during scaling to millimeter-wave frequencies.

Together, these results establish two key capabilities: (1) high-fidelity transfer of optical-reference stability to chip-scale microcombs, and (2) preservation of low-noise performance during frequency scaling to millimeter-wave regimes. This combined capability provides a practical route toward compact photonic signal sources that integrate optical-level stability with high-frequency operation.

The research was led by Dr. Changmin Ahn and Prof. Jungwon Kim at KAIST, in collaboration with Prof. Hansuek Lee. The results were published in Laser & Photonics Reviews and Optica.

· Optical-to-microcomb stability transfer for ultrastable timing and microwave/millimeter-wave generation (DOI: 10.1002/lpor.71135)

· Preserving ultralow timing jitter in microcombs with repetition-rate multiplication via perfect soliton crystal formation (DOI: 10.1364/OPTICA.581054)