Molecular frequency reference at 1.56  μm using a 12 C 16 O overtone transition with the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy method

We report on a molecular clock based on the interrogation of the 3ν rotational-vibrational combination band at 1563 nm of carbon monoxide C 12O. The laser stabilization scheme is based on the noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) technique in frequency modulation (FM) saturation spectroscopy. We use a high-finesse ultra-low expansion (ULE) glass optical cavity with CO as the molecular reference for long-term stabilization of the cavity resonance. We report an Allan deviation of 1.8×10 at 1 s that improves to ∼3.5×10 with 1000 s of averaging.