(Washington, DC : Optical Society of America, 2018) Wienold, M.; Alam, T.; Schrottke, L.; Grahn, H.T.; Hübers, H.-W.
We 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.
