A team of researchers from Chalmers University of Technology (Gothenburg, Sweden), alongside researchers from Purdue University (West Lafayette, IN), has developed a system that uses microcomb chips and chip-based integrated photonics technology for more accurate time indications and navigation (see video).
“Optical atomic clocks provide the most accurate timing system,” says Kaiyi Wu, a postdoctoral researcher at Purdue University. “However, such systems are built on bulk optics that are limited to the laboratory.”
The new technology is significantly smaller, thanks to micron- to millimeter-sized chip-based integrated photonics components the team is working to develop. This includes on-chip frequency combs (also called microcombs) that can transfer the high-frequency stability from a ytterbium (stable material that ticks trillions of times per second) ion clock transition found in optical frequencies down to an electronically detectable radio frequency (RF).
Optical frequency combs possess optical lines with equidistance, says Wu, so “by stabilizing one of the optical frequencies from the comb to the atomic transition frequency, and at the same time stabilizing the offset frequency of the overall comb lines (known as the carrier-envelope offset frequency),” the team can obtain a stable RF signal from the heterodyne (a signal frequency created by combining or mixing two other frequencies) beat between the optical lines.
Micron-size offers significant advantages
These optical frequency combs can be generated by an integrated silicon nitride (SiN) microring device, which scales them down into microcombs. More specifically, the system uses two micron-sized integrated photonic devices. And since both combs are generated from a SiN platform, there is potential to integrate dual-combs onto the same chip in a compact size.
The team’s development also involves an octave-spanning microcomb that can be directly generated, which Wu says is a requirement for getting the offset frequency. However, she explains “such microcombs will have a coarse comb line spacing (i.e., the repetition rate) in the ~1-THz range. This is too high to be electronically detectable.”
To build an optical clock system with microcombs, the researchers used a Vernier dual-microcomb approach, in which two microcombs have slightly different comb line spacings. The heterodyne beats between these two sets of comb lines are electronically detectable, which allows the team to lock its dual-comb setup to an ultranarrow linewidth laser at 871 nm.
“It’s designed to be locked to the ytterbium ion clock transition when frequency-doubled,” Wu says. “The stability of this 871-nm laser is successfully transferred to that of the RF clock signal.”
Microcomb goals
These optical frequency combs are an important component for optical clocks, Wu says, adding that the team’s work toward miniaturizing components should allow the technology to advance portable optical clocks outside of laboratories and into a variety of settings and be used in our daily life.
The researchers’ ultimate goal is to create a completely integrated microcomb system using photonic integration technology. Their next step will involve incorporating more components, along with the microcombs, onto a single chip. For example, integrating the dual-comb on the same chip, using on-chip heaters for tuning the microcomb, and exploring spectral filters for separating different wavelength components.
“There is continuous development of photonic integration technology for optical atomic clock systems,” says researcher Victor Torres-Company, a photonics professor at Chalmers University, where he leads the Ultrafast Photonics group. “In the future, we hope to use microcombs to deploy optical clocks that enable an improvement in the accuracy of time dissemination, with implications in more secure transactions and faster operations, and navigation and positioning that would be enabled by improving the accuracy of GPS.”
FURTHER READING
K. Wu et al., Nat. Photon. (2025); https://doi.org/10.1038/s41566-025-01617-0.
Justine Murphy | Multimedia Director, Digital Infrastructure
Justine Murphy is the multimedia director for Endeavor Business Media's Digital Infrastructure Group. She is a multiple award-winning writer and editor with more 20 years of experience in newspaper publishing as well as public relations, marketing, and communications. For nearly 10 years, she has covered all facets of the optics and photonics industry as an editor, writer, web news anchor, and podcast host for an internationally reaching magazine publishing company. Her work has earned accolades from the New England Press Association as well as the SIIA/Jesse H. Neal Awards. She received a B.A. from the Massachusetts College of Liberal Arts.