Narrowing of the linewidth of an optical parametric oscillator by an acousto-optic modulator for the realization of mid-IR noise-immune cavity-enhanced optical heterodyne molecular spectrometry down to 10^-10 cm^-1 Hz^-1/2

The linewidth of a singly resonant optical parametric oscillator (OPO) has been narrowed with respect to an external cavity by the use of an acousto-optic modulator (AOM). This made possible an improvement of the sensitivity of a previously realized OPO-based noise-immune cavity-enhanced optical heterodyne molecular spectrometry instrument for the 3.2 - 3.9 µm mid-infrared region by one order of magnitude. The resulting system shows a detection sensitivity for methane of 2.4 × 10−10 cm−1 Hz−1∕2 and 1.3 × 10−10 cm−1 at 20 s, which allows for detection of both the environmentally important 13CH4 and CH3D isotopologues in atmospheric samples.