Off-axis integrated-cavity-output spectroscopy for trace-gas concentration measurements: modeling and performance

We present a detailed study of the properties of off-axis-aligned high-finesse optical cavities under output time integration for spectroscopic applications. The dependence of the characteristics of the sample absorption spectra on a number of experimental parameters is investigated by developing a general model, where a Voigt absorption line shape is included into the cavity-transfer function. In this way, we address the issue of the measurement of trace amounts of gases in ambient air and single out the optimum conditions to give accurate estimates of the gas mixing ratio by the extraction of the integrated absorbance of the spectra. Quantitative results by the model are displayed for the detection of natural-abundance methane at 3 mu m as a function of the air-sample total pressure. Finally, we compare the model predictions with the results of a previous experiment, evidencing their consistency. (c) 2006 Optical Society of America.