ECAMP 15

The generation of ultra-stable microwave signals with low phase noise is a fundamental requirement for high-precision atomic clocks. In particular, state-of-the-art microwave fountains, such as those based on cesium (Cs) and rubidium (Rb) atoms, operate at the quantum projection noise (QPN) limit, where the stability of the interrogation signal plays a critical role. At LNE-OP, we have developed an optically derived microwave source that leverages the superior stability of optical oscillators. This technique relies on the frequency division of a 1542 nm ultrastable laser using an optical frequency comb (OFC), enabling the generation of a spectrally pure 11.98 GHz signal. This microwave signal is essential for probing atomic transitions in Cs and Rb fountains, maintaining high short-term stability.

To achieve long-term stability, the optical reference is initially frequency-locked to a hydrogen maser, ensuring proper tracking of long-term drifts. An additional phase-lock is implemented to eliminate residual long-term offsets, maintaining strict phase coherence necessary for precision timekeeping. The laser, originally stabilized to an ultrastable cavity, exhibits a frequency stability of 5×10⁻¹⁶ at 1 s. The optical frequency comb, based on an erbium-doped fiber laser, ensures precise frequency division from the optical to the microwave domain. A digital phase-lock system further compensates for residual frequency drifts, maintaining an output stability at the 10⁻¹⁵ level.

This new system effectively replaces the previous cryogenic sapphire oscillator (CSO), which, due to helium shortages, is now operational only at critical times. Our approach offers a viable and sustainable alternative while maintaining comparable or superior performance across key stability metrics. Furthermore, it provides LNE-OP with a complete microwave architecture to fully exploit the PHARAO-ACES space clock ensemble (launched in April 2025). The full implementation relies on a stabilized fiber-optic link that distributes the 1542 nm signal with minimal added phase noise.

Beyond the controlled laboratory environment, we extend this technology towards field-deployable optical clocks, where maintaining an accurate RF reference under remote conditions becomes critical. These transportable systems require a robust local RF reference to enable precise frequency measurements outside laboratory infrastructure. To address this challenge, we demonstrate the bootstrapping of an OFC to an accurate 1542 nm reference signal disseminated via the REFIMEVE+ fiber network. This network delivers an ultrastable optical carrier across laboratories, and our approach ensures the local generation of low-noise RF signals at 10 MHz and 1 GHz in remote environments. By leveraging this infrastructure, we enable high-precision frequency synthesis, facilitate remote clock comparisons between metrology institutes, and strengthen the robustness of optical clock networks.

In addition to fiber-based dissemination, we investigate an alternative approach that eliminates the need for external optical or RF references. This method involves directly bootstrapping the frequency comb to an optical clock laser, a strategy particularly suited for autonomous or mobile clock applications. The clock laser inherently carries the high accuracy and stability of the atomic transition it probes. Through frequency division by the OFC, we can extract a local RF reference at 10 MHz or 1 GHz, essential for clock operation and electronic synchronization. This technique ensures that optical clocks can operate independently in environments where fiber-based references are unavailable, thereby expanding the reach of high-precision timekeeping beyond traditional infrastructure.

Preliminary experimental results validate the feasibility of these approaches, demonstrating excellent stability and phase noise performance both in laboratory settings and in remote configurations. Our findings indicate that optically derived microwaves can serve as robust replacements for traditional RF sources, opening new pathways for advanced time and frequency metrology. Future work will focus on optimizing phase noise characteristics, integrating these systems into international clock comparison networks, and further developing the standalone optical clock bootstrapping technique for real-world field applications.