Absorption spectroscopy in general consists in measuring the absorption of light of different wavelengths by molecules. We then obtain "molecular spectra" which describe the absorption of molecules as a function of wavelength. The applications are very vast and diverse, here we consider mainly the instrumental tools that we have developed. These techniques have all been designed to allow the measurement of very small absorptions in the wavelength range from visible to mid-infrared.
Our innovations concern the coupling of laser sources with optical cavities which, in turn, allow us to push back the limits of the sensitivity of absorption measurements. The developed techniques allow to measure in laboratory, or in the field, extremely low levels of absorption, our record is equivalent to the detection of a decrease of the intensity of a light beam of 2% after an absorption path corresponding to the distance earth-moon (385000 km). Such performances open new applications in particular in the fields of gas analysis (measurement of low concentrations or "traces") and molecular spectroscopy of weak transitions for the study of molecules present in the terrestrial or planetary atmosphere. We are currently working to increase the accuracy and precision in frequency (or wavelength) of our ultrasensitive spectral measurements.