The OPUS‑2 project, funded by the German Aerospace Center (DLR) under grant 50WM2052, ran from 1 August 2020 to 31 July 2023. It was a consortium effort coordinated by Menlo Systems GmbH, with the Humboldt‑Universität zu Berlin and the Ferdinand‑Braun‑Institut (Leibniz‑Institut für Höchstfrequenztechnik) as scientific partners. Menlo Systems supplied project management, engineering integration and hardware assembly, while the university carried out laser development, spectroscopy, and data analysis. The Ferdinand‑Braun‑Institut provided the high‑finesse optical cavities, vacuum infrastructure and expertise in frequency stabilization. The goal was to deliver a flight‑ready optical frequency reference based on Ramsey‑Bordé interferometry with strontium atoms, capable of exceeding the stability of current space‑qualified clocks such as GPS and Galileo.
On the technical side, a compact high‑flux strontium oven was designed with a heating power below 10 W, enabling a continuous atomic beam suitable for Doppler‑free spectroscopy. Fluorescence detection at 689 nm was enhanced by integrating the oven, improving the signal‑to‑noise ratio and allowing long‑term observation of the Doppler‑free line. The 689 nm laser was locked to a high‑finesse cavity and its power was stabilized using an out‑of‑loop photodiode; the measured Allan deviation of the laser power was extrapolated to a clock instability that would be below the target if laser‑power fluctuations were the limiting factor. The 461 nm laser was frequency‑modulated and locked via frequency‑modulation spectroscopy, and its sideband frequency was stabilized to the cavity resonance with a variable offset. Retro‑reflectors with parallelism better than 3 arc‑seconds, improved collimators, and a redesigned RF electronics chain reduced wave‑front errors and sideband width, enabling clear Ramsey‑Bordé fringes that matched Monte‑Carlo simulations.
A key innovation was the use of a modulation technique on the clock laser frequency, combined with lock‑in detection of the 483 nm fluorescence signal. This generated an error signal largely insensitive to amplitude noise, which was fed back to the cavity‑offset control to cancel drift. The 483 nm transition, detected background‑free, was accessed with both a custom extended‑cavity diode laser and a commercial source, marking the first successful spectroscopy on this line in the project. Fundamental amplitude noise from atomic and photon shot noise was calculated to be below 2 × 10⁻¹⁵ / √τ, thus not limiting the performance.
The combined laser‑stabilization scheme achieved a fractional frequency instability of less than 1 × 10⁻¹⁴ at 1 s and below 2 × 10⁻¹⁵ at 100 s and 1000 s, meeting the project’s performance targets. A blue 3D magneto‑optical trap was also realized, capturing more than 10⁶ atoms without a repump laser, providing a benchmark for future optical lattice clocks.
The project produced several peer‑reviewed manuscripts, including a 2023 article in Quantum Science and Technology on an optical atomic clock aboard an Earth‑orbiting space station, and a master’s thesis on a cavity‑stabilized laser system for Ramsey‑Bordé spectroscopy. Presentations at international conferences such as the Les Houches School of Physics and the DPG Autumn Meeting disseminated the results. The work also informed the design of compact laser benches and oven systems for the related CAPTAIN‑QT project, extending the technology to other alkaline‑earth atoms. Overall, OPUS‑2 delivered a robust, compact optical frequency reference with performance surpassing existing space‑qualified clocks, and established a platform for future space‑borne atomic clock missions.