Comb-linked, cavity ring-down spectroscopy for measurements of molecular transition frequencies at the kHz-level.

We present a low uncertainty measurement technique for determining molecular transition frequencies. This approach is complementary to sub-Doppler saturation spectroscopies and is expected to enable new frequency measurements for a wide variety of molecular species with uncertainties at the kHz-level. The technique involves measurements of Doppler broadened lines using cavity ring-down spectroscopy whereby the probe laser is actively locked to the ring-down cavity and the spectrum frequencies are linked directly to an optical frequency comb that is referenced to an atomic frequency standard. As a demonstration we have measured the transition frequency of the (30012) ← (00001) P14e line of CO2 near 1.57 μm with a combined standard uncertainty of ~9 kHz. This technique exhibits exceptional promise for measurements of transition frequencies and pressure shifting parameters of many weak absorbers, and indicates the potential for substantially improved measurements when compared to those obtained with conventional spectroscopic methods.