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

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.