WASHINGTON — Researchers have shown, for the first time, that transmission of ultrastable optical signals from optical clocks across tens of kilometers of deployed multicore fiber is compatible with simultaneous transmission of telecommunications data. The achievement demonstrates that these emerging high-capacity fiber optic networks could be used to connect optical clocks at various locations, enabling new scientific applications.
As global data demands continue to surge, multicore fiber is being installed to help overcome the limits of existing networks. These fibers pack multiple light-guiding cores into a single strand, greatly increasing capacity for applications like streaming, finance and artificial intelligence.
"We wanted to see if multicore fiber, which is beginning to be deployed commercially, could support the transmission of optical clock signals over long distances in a way that would be compatible with telecommunications networks," said research team co-leader Frank Quinlan, from the National Institute of Standards and Technology (NIST). "The impending redefinition of the second based on optical clocks and important quantum networking protocols both rely on this type of ultrastable signal transmission."
In Optica , Optica Publishing Group 's flagship journal for high-impact research, a multi-institutional group of researchers demonstrates reliable transfer of ultrastable optical signals through deployed multicore fiber alongside simulated telecom traffic. They report a fractional frequency instability of just 3 × 10⁻¹⁹ over a period of nearly 3 hours — a level of stability that is suitable for the most demanding timekeeping and scientific measurements.
"The ability to carry signals from cutting-edge optical atomic clocks across oceans could make it possible to link and compare the world's most accurate clocks between continents," said NIST's Nazanin Hoghooghi, research team co-leader. "Initially, this could help support the high precision measurements for fundamental physics investigations and the work needed to redefine the second. Eventually, it could enable an international, potentially even intercontinental, network of connected clocks that could push the limits of accuracy even further."
Collaborating across continents
The new research grew out of an international collaboration between NIST, Nokia Bell Labs, Sumitomo Electric Industries Ltd. in Japan, the University of Colorado – Boulder and the University of L'Aquila in Italy. While the unique collaboration originally focused on using low-noise lasers for fiber sensing, as conversations evolved, the team began to think about whether multicore fiber — originally designed for high-capacity data transmission — might also support the precise transmission of signals from optical atomic clocks.
"Optical atomic clocks are now so precise that 18 digits are necessary to represent their frequency," said Quinlan. "However, maintaining this precision while transmitting the frequencies is challenging because optical fibers are extremely sensitive to environmental changes like temperature and strain, which adds a lot of noise to the signal."
Transmitting ultrastable light over long distances requires bidirectional travel to sense and correct frequency instability added by the fiber. However, in long-haul fiber networks, data typically flows one way per fiber — one for outbound and another for inbound traffic — because bidirectional use of a single fiber introduces noise and reflections that cause errors.
Previous demonstrations of stabilized fiber links over long distances have either relied on dark fiber, where no telecom signals are present, or used specialized equipment to enable bidirectional transmission of the ultrastable frequency signal. The researchers developed a new method that avoids the need for either dark fiber or specialized equipment.
The new method takes advantage of the fact that in a multicore fiber, each core experiences similar environmental effects. This means that outbound light can be sent through one core and return light through another, avoiding bidirectional errors while still allowing the use of noise-correcting techniques. The same approach can be applied to the data traffic in the fiber, supporting ultrastable frequency transfer without sacrificing data integrity.
One fiber, many paths
The investigators used a multicore fiber testbed in L'Aquila, Italy, to test their method's ability to cancel the noise of deployed multicore fiber and to compare its performance to standard fiber as well as the requirements for transferring the stability of state-of-the-art optical atomic clocks.
The researchers point out that this work wouldn't have been possible without the telecommunications industry's significant investment in multicore optical fiber technology, including multicore fiber design and construction, cabling for deployment and technology for input and output light coupling. Additionally, the multicore fiber testbed established at the University of L'Aquila was key to demonstrating performance on deployed fiber.
The experiments showed that the frequency noise added by approximately 25 km of multicore fiber was consistent with the theoretical limits observed in standard single-core fiber, indicating that using multicore fiber does not degrade performance. Also, the long-term fiber-induced instability was low enough to support state-of-the-art optical clocks.
They also found that the fiber-induced instability was limited by the short fiber paths used in the laboratory and not by properties of the multicore fiber itself. This suggests that with a better lab setup, performance could be further improved and that longer stretches of multicore fiber should work just as well for transmitting signals from optical clocks.
"It is important to emphasize that this work is only the first step," said Hoghooghi. "To truly show compatibility with long-haul fiber networks, we must show compatibility with optical amplification, which is required for fiber telecommunications links over about 80 km long. We are currently working on this."
Paper: N. Hoghooghi, M. Mazur, N. Fontaine, Y. Liu, D. Lee, C. McLemore, T. Nakamura, T. Hayashi, G. Di Sciullo, D. Shaji, A. Mecozzi, C. Antonelli, F. Quinlan, "Ultrastable optical frequency transfer and attosecond timing in deployed multicore fiber," 12, 894-899 (2025).
DOI: 10.1364/OPTICA.558821 .
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Optica Publishing Group is a division of the society, Optica , Advancing Optics and Photonics Worldwide. It publishes the largest collection of peer-reviewed and most-cited content in optics and photonics, including 18 prestigious journals, the society's flagship member magazine, and papers and videos from more than 835 conferences. With over 400,000 journal articles, conference papers and videos to search, discover and access, our publications portfolio represents the full range of research in the field from around the globe.
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