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 P(e)(11) transition targeted at 1.5316 μm, this corresponds to a C(2)H(2) 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.