Pulsed Cavity Ringdown Laser Absorption Spectroscopy in a Hollow Waveguide

Abstract

Cavity ringdown laser absorption spectroscopy (CRDS) is a modified version of standard absorption spectroscopy (AS) for providing extremely sensitive measurements of gas species absorbing at a particular wavelength in a gas cell volume. Typically, the enhancement in sensitivity with CRDS is a 10/sup 2/-10/sup 3/ improvement over AS. Herein, we analyze incorporating pulsed CRDS into a hollow-waveguide (HWG) both for reducing the sample volume as well as enhancing the signal-to-noise ratio (SNR) by up to /spl sim/10/sup 4/ by injecting light into the HWG cavity through a small aperture in one of the cell mirrors. For low power instrument applications (i.e. planetary science), the enhancement in SNR results in a potential /spl sim/10/sup 4/ reduction in laser power for a comparable CRDS terrestrial laboratory measurement at one extreme, or a potential /spl sim/10/sup 8/ improvement in CRDS temporal resolution through reduced sample averaging with a fixed low-power laser source. A pulsed (vs. CW) laser source is employed to remove the requirement for a precision tuned laser cavity at /spl sim/wavelength spatial resolutions.