Cavity-enhanced molecular spectroscopy: a powerful tool to detect trace gases

A novel technique to measure trace gas concentrations is reported. It is based on a high finesse Fabry-Perot cavity, used as a gas cell, thus providing very long interaction path- lengths. As in Cavity-Ring-Down Spectroscopy, light absorption inside the cavity is detected, but measurements are performed in the frequency domain instead of the commonly used time domain. Moreover, wavelength modulation spectroscopy is performed for sensitive detection of absorption losses. Monitoring of water vapor near 1400 nm is demonstrated using this method.