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Optica

Ultrastable optical frequency transfer and attosecond timing in deployed multicore fiber

Nazanin Hoghooghi, Mikael Mazur, Nicolas K. Fontaine, Yifan Liu, Dahyeon Lee, Charles A. McLemore, Takuma Nakamura, Tetsuya Hayashi, Giammarco Di Sciullo, Divya Ann Shaji, Antonio Mecozzi, Cristian Antonelli, Franklyn Quinlan

Published May 8, 2025• 9 citations
Publisher

Abstract

The telecommunications industry’s deployment of billions of kilometers of optical fiber has created a vast global network that can be exploited for additional applications such as environmental sensing, quantum networking, and international clock comparisons. However, for reasons such as the unidirectionality of long-haul fiber links, telecom fiber networks cannot always be adapted for important applications beyond data transmission. Fortunately, new multicore optical fibers create the opportunity for application coexistence with data traffic, creating multifunctional networks. Toward that end, we propose and demonstrate the faithful transfer of ultrastable optical signals through deployed multicore fiber in a way that is compatible with the unidirectionality of long-haul fiber optic systems, demonstrating fractional frequency instability of 3×10 −19 at 10,000 s. By supporting state-of-the-art optical atomic clocks, subsea multicore fibers can break the distance barrier for ultrastable optical frequency transfer, opening the door to intercontinental optical clock comparisons at the highest level, with applications in fundamental physics, relativistic geodesy, and the redefinition of the second.

DOI
10.1364/optica.558821

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The telecommunications industry’s deployment of billions of kilometers of optical fiber has created a vast global network that can be exploited for additional applications such as environmental sensing, quantum networking, and international clock comparisons. However, for reasons such as the unidirectionality of long-haul fiber links, telecom fiber networks cannot always be adapted for important applications beyond data transmission. Fortunately, new multicore optical fibers create the opportunity for application coexistence with data traffic, creating multifunctional networks. Toward that end, we propose and demonstrate the faithful transfer of ultrastable optical signals through deployed multicore fiber in a way that is compatible with the unidirectionality of long-haul fiber optic systems, demonstrating fractional frequency instability of 3×10 −19 at 10,000 s. By supporting state-of-the-art optical atomic clocks, subsea multicore fibers can break the distance barrier for ultrastable optical frequency transfer, opening the door to intercontinental optical clock comparisons at the highest level, with applications in fundamental physics, relativistic geodesy, and the redefinition of the second.
The telecommunications industry's deployment of billions of kilometers of optical fiber has created a vast global network that can be exploited for additional applications such as environmental sensing, quantum networking and international clock comparisons. However, for reasons such as the unidirectionality of long-haul fiber links, telecom fiber networks cannot always be adapted for important applications beyond data transmission. Fortunately, new multicore optical fibers create the opportunity for application coexistence with data traffic, creating expansive multifunctional networks. Towards that end, we propose and demonstrate the faithful transfer of ultrastable optical signals through multicore fiber in a way that is compatible with the unidirectionality of long-haul fiber optic systems, demonstrating a fractional frequency instability of 3x10-19 at 10,000 seconds. This opens the door towards intercontinental optical clock comparisons, with applications in fundamental physics and the redefinition of the second.
Experiments of transmission of sub-Hz cavity-stabilized 1542 nm laser frequency using a pair of 43 km dark fibers in urban environment are reported on successively 86 km and 172 km, with fractional frequency instability in the 10-19 range. A new approach is then introduced consisting in using part of an optical telecommunications network carrying simultaneously data traffic using a DWDM scheme to multiplex the metrological signal. This method is experimentally implemented using 22 km of fiber linking Université Paris 13 to its internet access point without degradation of the link instability. We finally present a project of large scale link between Paris and the German border using RENATER network which could constitute the first step of the building of a European optical network for ultrastable frequency dissemination and comparison.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications from fundamental metrology to high-precision measurements. For that purpose, optical fibre links have been intensively studied over the last decade. They have demonstrated impressive results far beyond the GPS capabilities on distances up to 2000 km, thanks to an active compensation of the fiber propagation noise.Up to now, optical links has been mainly implemented between two labs, when future applications will require the development of metrological fiber networks. Towards this goal, we have developed a few techniques which allow us to distribute the ultrastable optical signal to many users simultaneously. We have designed repeater laser stations, which can be used to build a cascaded link or to distribute the ultrastable signal among two optical links [1]. These stations allow us also to compare the output end signals of two optical links. These versatile stations will be a key-component of the national metrological network currently being developed in France within the Refimeve+ project. To complete the network, we have demonstrated the extraction of the ultrastable signal from a main link and its seeding to a secondary link [2].Optical links are already been successfully used for clocks comparison. Recently we were able to compare the atomic clocks of the French and German National Metrology Institutes through two connected optical links of total length 1415-km [3]. We found that the two fully independent Sr lattice optical clocks at the two sites agreed to better than 5x10-17 limited only by the clocks uncertainties. A fractional statistical uncertainty of 3x10-17 was reached after only 1000 s averaging time, which is 10 times better, and more than four orders of magnitude faster than with any other existing frequency transfer method. The atomic fountain primary frequency standards from the two institutes were also compared and they were found to agree well within their combined uncertainties of a few 10-16 [4].
Phase-coherent optical frequency transfer is essential for optical clock networking, relativistic geodesy, and distributed precision metrology. However, realizing coherent optical networks spanning thousands of kilometers in standard single-mode fiber (SMF) generally requires densely distributed amplifiers or repeater stations together with complex operational control, while long-term instability remains limited by thermally driven residual phase fluctuations. Here we show that hollow-core fiber (HCF) can simultaneously improve transfer instability and relax the reach limitation of long-span optical frequency transfer. Compared with SMF, HCF exhibits lower fiber-induced phase noise and shorter propagation delay, supporting improved short-term instability, while its much lower thermal sensitivity supports nearly one-order-of-magnitude better long-term instability. In addition, for long-haul HCF links, no observable stimulated Brillouin scattering induced saturation is found up to the maximum available injected power of 34 dBm, whereas the threshold of an equal-length SMF link remains only a few dBm. Together with the lower attenuation achievable in modern HCF, this enables ultra-long single-span optical frequency transfer. Using a 152 km HCF link with an average attenuation of 0.18 dB/km, we demonstrate single-span optical frequency transfer, achieving a fractional frequency instability of 7.3 x 10^-21 at 10,000 s and a fractional uncertainty of 1.8 x 10^-20. These results establish HCF as a transmission medium that simultaneously improves instability and extends single-span reach, opening a practical route toward future intercontinental optical frequency networks with ultrahigh precision.