Delivering industry-grade solutions for ultra-stable optical frequency transfer over the Internet

For over a decade, Exail has been the industrial partner of REFIMEVE, the French national Metrological Fiber Network with European Vocation. Selected to industrialize a laboratory prototype, Exail developed a turnkey, industry-grade solution for transferring optical frequency reference over long-distances. The REFIMEVE network now spans from Paris to more than 30 sites across France and neighboring countries. One of the latest links was deployed at CERN, enabling the transmission of an optical clock signal with greater accuracy than what can be achieved using GNSS.

Transferring ultra-precise frequency for clock comparisons and dissemination to end-users

Today’s most accurate clocks are room-sized optical systems, located in the main metrology research laboratories around the world. They provide a precise absolute reference that is crucial for fundamental research, such as optical frequency metrology, high resolution spectroscopy, radioastronomy, or even matter-antimatter experiments...

Transmitting ultra-precise frequencies over hundreds or even thousands of kilometers without degradation is extremely difficult. While GNSS signals from telecommunications satellites can be used for frequency transfer, their relative precision is limited to around 10^-16, insufficient for comparing the best optical clocks, which now reach stabilities at the 10^-18 level. Optical frequency transfer is the only technology capable of maintaining this extreme level of precision over long distances.

Initiated in 2012 by LPL and LTE laboratories (Paris region), the REFIMEVE project set out to transfer ultra-precise time and frequency references on the Internet over long-distances without disrupting regular traffic. The transfer is done through Telecom optical fibers (in the C-band, at around 1542 nm) thanks to a partnership with RENATER, the French National Telecommunications Network for Technology, Education and Research

Exail has been a partner of REFIMEVE since 2014, benefiting from technology and knowledge transfer to develop the Regeneration Laser Station (RLS). The first RLS was deployed in 2017. As the network expanded, Exail developed the Multi-branch Laser Station (MLS) in 2024, once again building on a laboratory prototype. Incorporating new technologies, the MLS offers the highest long-term performance recorded for this type of solution. Positioned at the strategical network nodes, each MLS can distribute a frequency reference to up to six different links. Over time, MLS is expected to gradually replace the RLS across the entire network.

Designed with a European scope in mind, REFIMEVE is actively extending its reach beyond France. Cross-border links are being established with research networks in neighboring countries—including the UK, Germany, and Italy—with future extensions planned to Belgium and the Netherlands for international clock comparisons

REFIMEVE in 2025, 30 labs connected across France, and growing European reach

More than 8000 km of fiber are already connected to the REFIMEVE network, linking its initial hub in Paris (at LTE lab) to physics laboratories in most main French cities. To date, Exail has deployed over 30 RLS and MLS stations, with more units planned as the infrastructure approaches full deployment.

Today, REFIMEVE stands as a national French research infrastructure, playing a pivotal role in the field of optical frequency metrology. It enables different research laboratories to compare “their own reference” with unprecedented precision. This capability benefits a broad range of scientific experiments and end-users by providing access to time and frequency references at the best level possible. Looking ahead, sharing such precise references could enable new applications, including for example secured communications (sharing of quantum key distribution - QKD), but also geophysics and seismology (with distributed sensing capabilities) or astrophysics, including the detection of dark matter and gravitational waves.

“Clock comparisons between neighbor countries are primarily made using Exail’s regeneration stations.” Etienne Cantin, CNRS research engineer on the REFIMEVE infrastructure. “We provide a RLS to our counterpart to make the comparison measurement with our French reference. There is currently no equivalent equipment available to create a network for the sharing of optical frequency reference over fiber at this extreme level of performance.”

Metrology system turned into certified industry-grade turnkey devices

In the REFIMEVE network, the initial time and frequency references are given by the LTE laboratory at Observatoire de Paris, which provides France’s official Coordinated Universal Time (UTC). An ultra-stable laser referenced to atomic clocks through a frequency comb generates a laser oscillation with a frequency stable and accurate at the level of 10^-15 which is the reference signal to be disseminated and is injected into existing telecom networks.

Exail oversees deployment of all the hardware (RLS and MLS) throughout the network and ensures signal quality via a dedicated remote supervision platform. All stations are connected to this platform and continuously stream real-time data for centralized monitoring and diagnostics.

The regeneration of the reference optical signal in the RLS stations is achieved thanks to the heterodyne phase-locking of an integrated low-noise laser diode on the reference signal. The regeneration station actively compensates for phase noise accumulated during propagation along the fiber—mainly from acoustic and thermal fluctuations—making it an ideal solution for high-precision remote comparisons of optical clocks.

“The RLS developed by Exail incorporates advanced components for laser frequency control, similar to those used in quantum technologies, but also many custom electronic components developed at LPL and LTE labs to enable the extreme precision required.” explains Étienne Cantin. “Signal processing is critical in our application, especially due to signal degradation and losses along the fiber. The RLS also includes all components necessary for remote supervision, which is essential to monitor the REFIMEVE network.”

Thanks to continuous joint development efforts, Exail and the REFIMEVE research teams have achieved cutting-edge long-term performance (E. Cantin et al, New J. Phys. 2021) with the new MLS systems. These units benefit from Exail’s expertise in micro-optic free-space optical design, which significantly reduces sensitivity to temperature variations compared to fully fiber-based technologies.

“Exail’s micro-optic expertise has been a key factor. REFIMEVE has relied on the quality of Exail’s products and remote supervision capabilities for more than a decade. This long-term partnership has been a major contributor to the project’s success.” adds Etienne Cantin

Enhancing precision at CERN: accurate frequency reference for matter–antimatter experiments

In 2023, Exail established a new optical fiber link to CERN as part of the REFIMEVE network. This link between CERN and Paris will deliver an accurate frequency reference, significantly improving the precision of timekeeping across the laboratory. It will also support high-sensitivity experiments such as ALPHA, which investigates fundamental differences between matter and antimatter.

CERN states that “it has the potential to improve the precision of clocks across CERN and could provide a new way for the Laboratory to access Coordinated Universal Time (UTC) – the global standard for timekeeping. The optical signal from the link can synchronize with UTC more precisely than via GPS satellite, which is currently used across CERN.”

Turning telecom networks into large-scale distributed environmental sensors through advanced fiber stabilization

In March 2025, Exail announced its participation in SENSEI, a European project that aims to develop new photonic devices and technologies to enhance the security, resilience, and efficiency of telecommunication fiber networks through improved supervision capabilities, while exploiting them as large-scale distributed environmental sensors.

The project will introduce novel functionalities enabling telecom fibers to perform acoustic sensing, potentially unlocking cost-effective, high-resolution seismic mapping on a continental scale. For Exail, SENSEI represents an opportunity to advance its RLS systems by integrating sensing capabilities—developed in collaboration with REFIMEVE, LPL, and LTE—adding a new dimension to Exail’s metrology-grade optical frequency transfer solutions.