Precise molecular spectroscopy using a stable and tuneable frequency comb

Accurate molecular spectroscopy in the mid-infrared (mid-IR) region allows precision measurements with applications in fundamental physics. We present our on-going work towards measuring absolute vibrational frequencies of various polyatomic species — in particular methanol — around 10 μm, at an unprecedented level of accuracy, using a both ultra-stable and widely tuneable near-infrared frequency comb. We have recently been able to lock mid-IR radiation to a frequency comb stabilized to a 1.54 μm near-IR reference [1, 2]. This reference, generated at the French national metrology institute (LNE-SYRTE), is monitored against atomic frequency standards [3] and transferred to LPL via a 43-km long optical fibre [4] (see Fig. 1). This provides the ultimate frequency accuracy (potentially the 3x10 ⁻¹⁶ of the Cs fountain clock) and stability (∼10 ⁻¹⁵ after 1s of integration).