Optical clocks based on trapped laser-cooled atoms and single ions have demonstrated frequency stability and estimated systematic frequency uncertainty far surpassing the current generation of caesium microwave frequency standards, with the result that a future optical redefinition of the SI second is anticipated. However before this can happen several key challenges remain to be addressed.

Firstly, the uncertainty budgets of the optical clocks need to be validated through a programme of international comparisons between systems developed independently by different research groups around the world. Continuity with the current caesium-based definition must also be ensured, by performing absolute frequency measurements with as low an uncertainty as possible. And finally, we need to improve the robustness of optical clocks and automate their operation. This will enable them to be operated routinely as secondary representations of the second, regularly contributing to International Atomic Time (TAI) via reporting to the International Bureau of Weights and Measures, and being used to steer the local UTC(k) time scales maintained by national timing laboratories.

I will discuss recent progress towards addressing these challenges, in particular drawing on examples of work performed in the European collaborative project Robust Optical Clocks for International Timescales (ROCIT).

Using near-infrared light, we tightly-lock a mid-infrared quantum cascade laser frequency comb to another laser, achieving a residual integrated phase noise of 200 mrad. This high coherence is pertinent for highly-sensitive dual-comb spectroscopy and metrology.

We will report our recent progress toward H2+ spectroscopy by use of a SI-referenced Mid-IR source laser. H2+ molecular ions are very interesting candidates to improve the determination of fundamental constants, such as the proton to electron mass ratio mp/me and search for new physics beyond the standard model. At LKB, an erbium fibered frequency comb is phase locked to the LNE-SYRTE frequency standards thanks to the T-REFIMEVE network. By sum frequency generation in a AgGaSe2 crystal between a CO2 laser and an output of the comb at 1895 nm, a shifted frequency comb centered at 1560 nm is generated. The latter is then mixed with the original one to generate a beatnote used to stabilise the Mid-IR laser. As a first application, a narrow saturated absorption line in formic acid has been extensively studied. Pressure, power and modulation depth shifts and broadenings have been evaluated, leading to a determination of its central frequency at a sub ppt (10^-12) resolution, high enough for H2+ spectroscopy and fundamental constant determination.

Dual-comb systems have demonstrated their potential for metrology, e.g. spectroscopy vibrometry or ranging. However, the implementation of dual-combs is often complex and generally requires substantial optical and optoelectronic hardware. Here, we propose a simple and compact architecture based on a bidirectional frequency shifting loop, that provides more than 100 mutually coherent comb lines. The system makes use of a CW laser and a slow electronic detection chain (10 MSa/s). We have implemented two configurations enabling dynamic multi-heterodyne interferometry at 20kHz: the first one makes use of acousto-optic frequency shifters, and allows highly sensitive distributed acoustic sensing along a fiber. The second one involves electro-optic frequency shifters, and enables ranging with a sub-mm resolution.

We show that our developed free-running, bidirectional ring Ti:Sa laser cavity meets the requirements for Dual Comb Spectroscopy in the UV range (UV-DCS). Two counter-propagative frequency combs with slightly different repetition rate are generated in such a cavity and we show quantitatively that this repetition rate difference can be explained by the self-steepening effect. Molecular absorption lines of the O2 A-band centered around 760~nm are measured with a 1,5 GHz spectral resolution, demonstrating that the mutual coherence of the two combs allows GHz-resolution DCS measurements. Moreover, we demonstrate that the generated output peak power allows for efficient second harmonic generation (SHG), in the scope of developing laboratory and open-path UV-DCS experiments.