Photonic microcomb studies point to ultralow-noise precision sensing sources

Researchers have demonstrated two chip-scale photonic methods to generate stable microwave and millimetre-wave signals, with potential applications precision measurement

Researchers at the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, South Korea, have demonstrated a chip-scale photonic approach to generate ultralow-noise and highly stable microwave and millimetre-wave signals from optical frequency combs – also known as microcombs. The work has offered a potential route towards compact, high-performance frequency sources for technologies that require precise signals in the tens to hundreds of gigahertz range.

Such high-frequency signals are central to emerging applications in, high-resolution imaging and precision sensing, and other applications such as telecommunications. However, conventional electronic sources face a persistent challenge: as signal frequencies rise, it becomes increasingly difficult to maintain both low phase noise and long-term frequency stability. Photonics offers an alternative route because optical references can provide extremely high stability, but the difficulty has been to transfer that stability reliably into compact, chip-scale microwave and millimetre-wave devices.

In the first study, the KAIST researchers addressed the long-standing problem of how to transfer the stability of an optical reference to a microcomb. Direct stabilisation has remained difficult because high-repetition-rate microcombs do not generally allow straightforward carrier-envelope offset detection. That limitation has prevented the simple use of conventional optical frequency comb stabilisation methods in many chip-scale systems.

To overcome this barrier, the team used a mode-locked laser as a transfer oscillator and synchronised it to the microcomb through electro-optic sampling. This method allowed the researchers to transfer the stability of an optical reference directly and robustly to the repetition rate of the microcomb.

The system achieved fractional frequency stability at the 10^-18 level, alongside phase noise of -125 decibels relative to the carrier per hertz (dBc/Hz) at a 100 hertz offset from a 22 gigahertz carrier. According to the researchers, this represented state-of-the-art performance and an improvement of more than 80 decibels compared with the free-running microcomb in the low-offset-frequency regime.

In a second study, the team examined a separate but related problem: the degradation in noise performance that normally occurs when microcombs are scaled to higher repetition rates. Lower-repetition-rate microcombs, which use larger resonators, tend to show better noise characteristics. However, the move to higher repetition rates has typically led to poorer performance, which has limited the practical use of microcomb systems for millimetre-wave generation.

The researchers showed that this limitation could be overcome through the use of perfect soliton crystal states. Perfect soliton crystals allow repetition-rate multiplication while retaining the low-noise properties of the original comb. Using this approach, the team generated millimetre-wave signals at 44 gigahertz and 66 gigahertz, with timing jitter in the range of around three femtoseconds. The result demonstrated that the low-noise performance of a microwave-rate microcomb could be preserved as the system scaled into millimetre-wave frequencies.

Together, the two studies have established complementary capabilities for photonic signal generation. The first showed high-fidelity transfer of optical-reference stability to chip-scale microcombs, while the second showed that low-noise performance could be retained during frequency scaling to the millimetre-wave regime. In combination, these results point towards compact photonic signal sources that could integrate optical-level stability with high-frequency operation.

The findings could be significant for sensing systems, where stable, low-noise signals are essential but conventional electronic architectures face growing performance and integration limits. By shifting critical signal generation functions into chip-scale photonic platforms, the work has suggested a practical path towards smaller, more stable and more scalable sources for precision measurement.

For further reading please visit: 10.1002/lpor.71135