CC BY 4.0 UnportedWienold, M.Alam, T.Schrottke, L.Grahn, H.T.Hübers, H.-W.2019-03-122019-06-282018https://doi.org/10.34657/4803https://oa.tib.eu/renate/handle/123456789/1236We report on the Doppler-free saturation spectroscopy of a molecular transition at 3.3 THz based on a quantum-cascade laser and an absorption cell in a collinear pump-probe configuration. A Lamb dip with a sub-Doppler linewidth of 170 kHz is observed for a rotational transition of HDO. We found that a certain level of external optical feedback is tolerable as long as the free spectral range of the external cavity is large compared to the width of the absorption line.application/pdfenghttps://creativecommons.org/licenses/by/4.0/620Semiconductor lasersquantum cascadeOptical sensing and sensorsSpectroscopyhigh-resolutionSpectroscopyterahertzArticle