Intracavity laser absorption measurements at ultrahigh spectral resolution

Intracavity laser absorption spectroscopy with ultrahigh sensitivity and enhanced spectral resolution is demonstrated. It entails use of an intracavity étalon that selects equally spaced modes of the cavity. These modes are reduced in intensity when they occur at spectral locations where absorption that is due to intracavity species also occurs. We obtained absorption spectra by tuning the étalon in small steps across its free spectral range, recording intracavity spectra at each step, and summing the result. The maximum resolution is determined by the width of the étalon peaks, which was ~0.01 cm(-1). When the finesse of the étalon is increased, the resolution equal to the width of a single laser mode can be achieved. With this technique, spectra at Doppler-free resolution such as that required for studies of high vibrational-overtone transitions of molecules in supersonic jets are possible.