Design considerations in high-sensitivity off-axis integrated cavity output spectroscopy

E.J. Moyer, D.S. Sayres, G.S. Engel, J.M.St. Clair, F.N. Keutsch, N.T. Allen, J.H. Kroll, J.G. Anderson, Design considerations in high-sensitivity off-axis integrated cavity output spectroscopy. Phys. B 92, 467 (2008)

CT Off-axis integrated cavity output spectroscopy(OA-ICOS) has generated much interest because it potentially allows highly sensitive field measurements with robustoptical alignment. We discuss here design choices involvedin design of an OA-ICOS instrument and how these choicesimpact instrument sensitivity, using as our example the design of the Harvard ICOS isotope instrument, which demonstrates the highest reported sensitivity for mid-IR OA-ICOS(2.4×10−11cm−1Hz−1/2at 6.7 µm, obtained during measurements of water vapor isotopologues H2O, HDO, and H218Oin the laboratory and onboard NASA’s WB-57 high-altituderesearch aircraft). We compare the sensitivity of several OAICOS instruments with differing design parameters, show howcomparisons are hindered by differing definitions of instrument performance metrics, and suggest a common metric ofMDAmeas, the fractional absorption equivalent to 1σ uncertaintyin an actual measurement, normalized to 1 s integration. Wealso note that despite an emphasis on sensitivity in the literature, in the Harvard ICOS isotope instrument and likely alsosimilar instruments, systematic errors associated with fittingof the baseline laser power a