Fast Fourier transform analysis in cavity ring-down spectroscopy: application to an optical detector for atmospheric NO_2

摘要

A fast Fourier transform (FFT) method for analysis of ring-down decays from a cavity ring-down (CRD) spectrometer has been tested and compared with alternative fitting methods. The ring-down times derived from the FFT method are obtained with a precision close to that of the Levenberg-Marquardt non-linear least-squares method, but the fitting algorithm is ～ 100 times faster, allowing real-time fitting of individual ring-down traces on a personal computer. Advantages of the FFT method are discussed, and the method is demonstrated for the measurement of NO_2 partial pressures equivalent to mixing ratios of 150pptv and above in laboratory air, using a CRD spectrometer based on an external cavity diode laser operating at wavelengths around 410 nm. The absorption by NO_2 is distinguished from other cavity losses either by using synthetic (zero) air reference samples, or by diverting the sampled laboratory airflow through an NO_2 chemical scrubber consisting of hydroxyapatite on a TiO_2 substrate. Typical mixing ratios of NO_2 in the laboratory air are ～ 25 ppbv.