Recent developments in cavity ring-down spectroscopy with tunable cw lasers in the mid-infrared

Abstract

We report on our recent advances with cavity ring-down spectroscopy using mid-infrared cw lasers. An external high- finesse cavity is excited on a single fundamental mode with a tunable laser operating in the 3 micrometers region. After excitation the laser power is turned off for a short time and the subsequent decay of the field stored inside the cavity is observed. The effective pathlength covered by the laser light inside the cavity during the decay amounts to several km, depending on the mirror reflectivity. Measurement of the decay time gives the photon losses and thus enables the detection of weakly absorbing species inside the cavity. This approach is closely related to cavity ring-down spectroscopy with pulsed lasers. However the cw approach exhibits several advantages concerning spectral resolution and detection sensitivity. Application of this method to online monitoring of trace gases seems to be very promising. We demonstrate detection of hydrocarbons, like membrane and ethylene on the ppb level.