Loughborough University physicists and an international team have demonstrated that a grain-of-rice-sized microchip can be used to produce a spectrum of precisely spaced frequencies of light, which is then converted into multiple high-frequency electromagnetic signals known as millimetre waves.
Millimetre waves are of growing interest for future communications because they offer much more bandwidth – essentially more space for transmitting data – but generating them with precision and stability needed for advanced technologies remains challenging.
“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.”
One way to generate millimetre waves is using a microcomb – a highly precise spectrum of light frequencies, arranged similar to the colours of a rainbow but invisible to the human eye. These light frequencies can be converted into millimetre waves using a specialised antenna.
Why the “rainbow” comparison? Like a rainbow, a microcomb is made up of many different frequencies of light. In a rainbow, these frequencies blend together. In a microcomb, the frequencies are separated and precisely spaced – like the teeth of a comb. The light produced by the microcomb in this study is invisible to the human eye. Illustration generated using artificial intelligence.
In a new Nature Communications paper, the Loughborough-led team has demonstrated just that. They have created a system that produces a stable, high-quality microcomb that can be converted into multiple precisely spaced millimetre-wave frequencies at once.
The key is the way the team generates its microcomb. Microcombs are typically created by shining laser light into a microresonator – a tiny structure on a microchip that traps and circulates light.
The Loughborough design works differently by combining the chip-based microresonator with a larger loop of optical fibre, creating a system in which the laser light continually circulates through both.
The ‘rainbow on a chip’: a tiny microchip containing a microresonator that produces a precisely organised ‘rainbow’ of light frequencies, known as a microcomb. The chip is the size of a grain of rice, with a one euro coin pictured for scale.
“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”, said Dr Peters.
“It’s remarkably robust too. We’ve even had people jumping up and down next to the system and the microcomb remains stable.”
Researchers from the Emergent Photonics Research Centre put their microcomb system’s stability to the test by jumping next to it.
In their latest study, the team also demonstrated that they could control the microcomb “rainbow”, making some frequencies stronger or weaker. Crucially, they showed that its precision and stability was carried through to the millimetre-wave signals it produced.
“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,” said Dr Peters.
“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.”
What’s next
The researchers are now exploring how their microcomb technology could be taken beyond the laboratory.
While the microchip at its heart is the size of a grain of rice, the complete system is currently a tabletop laboratory set-up. Future versions could be even more compact and energy efficient – potentially small enough to fit inside a shoebox.
The team is particularly interested in exploring whether the technology could one day be used on satellites, where size, weight and power are important.
Precision timing for quantum technologies
The researchers are also exploring just how accurate their microcomb set-up could be. The team is testing it against precision clocks and exploring potential applications in timing, navigation and position through collaborations with the National Physical Laboratory and is an integral part of the efforts of the National Physical Laboratory and the UK Hub for Quantum Enabled Position, Navigation and Timing (QEPNT).
“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.”
The study brings together expertise from Loughborough University’s Emergent Photonics Research Centre, the University of Sussex, City University of Hong Kong, QXP Technologies, INRS-EMT, and Swinburne University of Technology and ARC-COMBS.