Innovative concepts for a compact Cs clock based on a metrological dual-frequency laser – CHoCoLa

Information and communication technology (ICT) has revolutionized major industries, from media and entertainment to financial services, from software to pharmaceuticals. In this context, future ICTs challenges will include massive data processing, high performance computing and security. All these challenges depend critically on frequency and time standards. Accurate clocks are needed for authentication of financial transactions, for network management and synchronization of frequency references in wireless telecommunication, for fast recovery of GPS signals in dense situations, and for synchronization of radar systems.

Atomic frequency references are now the best commercially available time and frequency references for timescales longer than few seconds. Future atomic clocks will rely on lasers for their operation, to improve the performances and to reduce the size and consumption. Among them, pulsed coherent population trapping (CPT) clocks, based on the use of two, frequency and phase-locked, lasers are ideal to reach the best performances. Unfortunately, this two-laser scheme limits its practical implementation, in terms of compactness and cost. In this context, the availability, in a compact form, of a single laser delivering two frequencies (a “dual-frequency laser”) would pave the way to a compact high-performance CPT clock.

In this project, we propose to realize a compact and high stability pulsed CPT Cesium clock demonstrator based on the original use of a metrological dual-frequency (DF) dual-polarization (DP) laser and to assess unprecedented frequency stabilities, below 5x10-13 at 1 s and 2x10-14 at 1 h, in a volume below 2 Litres, electronic control unit being in an external 19”, 5U case. The dual-frequency dual-polarization laser will consist of a Vertical External Cavity Surface Emitting Laser (VECSEL) in an innovative architecture and will be based on an optimized semiconductor gain structure at 852 nm addressing Cesium D2 line (WP2). An in depth understanding of all limiting noise issues, critical to achieve the high-class stability of final clock demonstrator, will be theoretically and experimentally investigated in WP3. The realization of a dedicated Cesium resonator and an innovative compact and robust electro-optical bench will allow for the final demonstration of clock stability below 5x10-13 at 1 s and 2x10-14 at 1 h, in a target volume below 2 Litres (WP4).

This project, led by Thales Research and Technology (TRT), gathers specific and complementary skills of six partners whose expertise are well recognized. Systèmes de Référence Temps-Espace (SYRTE) will bring its unique understanding and skills on ultra-stable atomic clocks and Thales Electron devices (TED) will bring its extensive commercial and technological expertise in the field of atomic clocks. Laser expertise will be covered by TRT for its longstanding background in the field of DF and DP lasers, and Laboratoire Charles Fabry (LCF) for its thorough expertise on optically-pumped VECSEL architecture. Laboratoire Aimé Cotton (LAC) will bring its in-depth understanding of noise and laser dynamics, and Laboratoire de Photonique et Nanostructures (LPN) its expertise on high performance ½-VCSELs design and growth.

The objective of CHOCOLA answers perfectly to the societal challenge “Information and communication Society” as it will demonstrate a compact and highly precise clock, one of the key elements of future digital technologies. Innovative concepts investigated within this project will be an important step towards the development of atomic clocks with performance close to a passive hydrogen maser but in a much more compact form. At the end of CHOCOLA, the clock demonstrator will have reached a sufficiently high maturity level (TRL 3) to enter a derisking phase, which will be initiated by TED all along the project through its participation to the clock design and test, as well as its deep knowledge of atomic clock technologies and markets.