Trace gas detection with CW cavity ring-down laser absorption spectroscopy

This paper describes the development of a device to measure trace concentrations of impurities in corrosive gases using continuous wave excitation of a Cavity Ring-Down Spectroscopy cell (CW-CRDS). CRDS is an absorption technique, based on Beer's law that determines a sample's concentration from the rate of decay of light coupled into a high-finesse optical resonator containing the sample. It provides absolute and highly accurate measurement of molecular concentrations. The technique requires only modest laser power that single mode diode lasers provide, making the system easily transportable. The near-IR region that is easily accessible with ambient temperature diodes contains both vibrational overtone transitions and forbidden electronic transitions of many important species. Our research focuses on investigating the properties of CRDS to increase detection sensitivity of these species. Specifically, we are studying moisture detection at the parts-per-billion to the parts-per-trillion range. Recently, we obtained an absorption equivalent detection limit of better than 1/spl times/10/sup -11/ cm/sup -1/ Hz/sup -1/2/, which allows us to detect water at less than one ppb. We discuss the device and its applications, and present performance data.