We report on the realization of a Doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) instrumentation based on a high-finesse (50,000) cavity with a detection sensitivity of 4×10−13 cm−1 Hz−1/2. For the 푃푒(11) transition targeted at 1.5316 μm, this corresponds to a C2H2 concentration of 240 ppq (parts-per-quadrillion) detected at 100 Torr. The setup was originally affected by recurrent dips in the cavity transmission, which were attributed to excitation of high-order transverse mode by scattering from the mirrors. The effect of these was reduced by insertion of a small pinhole in the cavity.
Silander, I., Hausmaninger, T., Ma, W., Ehlers, P., & Axner, O. (2015). Doppler-Broadened noise-immune cavity-Enhanced optical heterodyne molecular spectrometry down to 4 × 10-13 cm-1 Hz-1/2: Implementation of a 50,000 finesse cavity. Optics Letters, 40(9), 2004-2007. https://doi.org/10.1364/OL.40.002004