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 lineshape is included into the cavity transfer function. In this way, we address the issue of measurement of trace amounts of gases in ambient air, and single out the optimum conditions to give accurate estimates of gas mixing ratio by extraction of the integrated absorbance of the spectra. Quantitative results by the model are displayed for detection of natural-abundance methane at 3 micron 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.