Scientists at Loughborough University and an international research team have built a tiny “rainbow on a chip” that can generate multiple precisely spaced frequencies of light at the same time. The microchip-based system could eventually help future 6G networks carry more signals simultaneously by converting the optical frequencies into millimetre waves.The “rainbow” comparison comes from the way a microcomb produces many different frequencies of light. Unlike a visible rainbow, where the colours blend, the frequencies in a microcomb are separated and precisely spaced like the teeth of a comb. The light produced by the system is invisible to the human eye.
Microcomb could create multiple 6G channels
The researchers demonstrated a system that produces a series of stable, precisely spaced optical frequencies that can be converted into high-frequency electromagnetic signals known as millimetre waves.Millimetre waves are being studied for future communications because they can provide more bandwidth for transmitting data. Producing these signals with the precision and stability required for advanced systems, however, remains a challenge.“The world is becoming increasingly data hungry. We want to send and receive more information, faster and in higher resolution, and millimetre waves could help provide the capacity to do that,” said Dr Luke Peters of Loughborough University’s Emergent Photonics Research Centre.“They could ultimately contribute to faster, higher capacity 6G networks, but the potential goes far beyond communications. These frequencies could also be used in radar systems as well as spectroscopy and astronomical instruments, helping scientists study materials and make extremely precise measurements of the universe. These applications are still some way off, and there are challenges to overcome before the technology can be used in real-world systems – but our latest work has tackled a major one,” Peters said.Microcombs generate a set of optical frequencies using a device called a microresonator, a small structure built onto a chip that traps light and allows it to circulate. An antenna can then convert those optical frequencies into millimetre waves.Previous research showed that microcombs could generate a single precise millimetre-wave frequency. Producing several frequencies at once could allow each one to be used as a separate channel for transmitting information, but doing so requires the optical frequencies to remain stable and maintain high signal quality.The researchers, whose findings have been published in Nature Communications, developed a system that generates several precisely spaced millimetre-wave frequencies simultaneously.
How the 6G ‘rainbow on a chip ‘ works
Instead of relying only on a laser and microresonator, the researchers connected the chip-based microresonator to a larger loop of optical fiber. Laser light continuously travels through both components, allowing the required optical states to form and remain stable.“We’ve essentially created an incredibly precise and stable ‘rainbow on a chip’, where the loop keeps feeding the light back through the chip, allowing these states to build up efficiently, start on their own and remain stable even when the system is disturbed,” Dr Peters said.“It’s remarkably robust too. We’ve even had people jumping up and down next to the system and the microcomb remains stable,” he added.The researchers also demonstrated control over individual frequencies within the microcomb. They could increase or decrease the strength of particular frequencies while maintaining the precision and stability of the optical signal after it was converted into millimetre waves.“Being able to make individual frequencies stronger or weaker gives us much more control over the signals we produce, because different applications will need different combinations of frequencies,” Dr Peters noted.“Just as importantly, we’ve shown that the precision of the microcomb carries through to the millimetre waves. That gives us a whole set of highly controlled signals, which is exactly what you need for applications where accuracy and stability matter. That same level of precision is valuable for timing. Precision timing sits at the core of emerging quantum technologies, where extreme accuracy is a requirement,” he highlighted.
Microcomb technology could become smaller
The central microchip is about the size of a grain of rice, although the complete experimental system currently occupies a tabletop laboratory setup. The researchers are studying ways to reduce its size and energy requirements, with future versions potentially becoming small enough to fit inside a shoebox. The team is also examining whether the technology could be used on satellites, where reducing size, weight and power consumption is important.
Microcomb could support quantum timing
The researchers are testing how accurately and consistently the microcomb can operate by comparing its performance with precision clocks. The work also includes investigations into possible applications in timing, navigation and positioning through collaborations with the National Physical Laboratory and the UK Hub for Quantum Enabled Position, Navigation and Timing.“We’re really excited to see how far we can take the precision and stability of these microcombs, particularly for technologies that rely on extremely accurate timing,” said Dr Antonio Cutrona, who led the microcomb stability measurements.“We hope this study and our system open up new ways of bringing the extraordinary precision of atomic clocks into more compact technologies for timing, navigation and position, and it is particularly exciting to explore these possibilities through our wider collaborations with the National Physical Laboratory and the UK Hub for Quantum Enabled Position, Navigation and Timing,” he added.The researchers say further work is needed before the system can be incorporated into practical communications or quantum technologies. The current study establishes a way to generate and control multiple stable frequencies from a chip-based microcomb while preserving their precision after conversion to millimetre-wave signals.
