Terahertz gas phase spectroscopy using a high-finesse Fabry-Perot cavity

The achievable instrument sensitivity is a critical parameter for the continued development of terahertz (THz) applications. Techniques such as cavity-enhanced techniques and cavity ring-down spectroscopy have not yet been employed at THz frequency due to the difficulties to construct a high-finesse Fabry-Perot cavity. Here, we describe such a THz resonator based on a low-loss oversized corrugated waveguide with highly reflective photonic mirrors obtaining a finesse above 3000 around 620 GHz. These components enable a Fabry-Perot THz absorption spectrometer with an equivalent interaction length of 1 km giving access to line intensities as low as 10(-27) cm(-1)/(molecule/cm(2)) with a S/N ratio of 3. In addition, the intracavity optical power has allowed the Lamb dip effect to be studied with a low-power emitter; an absolute frequency accuracy better than 5 kHz can be easily obtained providing an additional solution for rotational spectroscopy. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement