Cavity-enhanced saturation spectroscopy of molecules with sub-kHz accuracy ^†

Saturation spectroscopy is frequently used to obtain sub-Doppler measurement of atomic and molecular transitions. Optical resonant cavities can be used to enhance the effective absorption path length, and the laser power inside the cavity as well to saturate very weak ro-vibrational transitions of molecules. Three different cavity-enhanced methods, cavity enhanced absorption spectroscopy, cavity ring-down spectroscopy, and noise-immune cavity enhanced optical heterodyne molecular spectroscopy (NICE-OHMS), were compared by measuring the Lamb dip of a C₂H₂ line at 1.4 µm using a cavity with a finesse of 120000. The center of the line was determined by different cavity-enhanced methods, each giving a sub-kHz (δν/ν≈10⁻¹²) statistical uncertainty. The sensitivity and precision of different methods were analyzed and compared. As demonstrated in this study, the NICE-OHMS method is the most sensitive one, but more investigation on the systematic uncertainty is necessary before its application in metrology studies toward a sub-kHz accuracy.