  Applicability and accuracy of traditional scanned-wavelength direct
absorption spectroscopy (SDAS) diagnostic method is largely affected by
combustion state such as temperature and pressure. To resolve this problem, an
innovative baseline-free scanned-wavelength direct absorption spectroscopy
(BF-SDAS) method is proposed in this work. This method does not need the
zero-absorption regions for baseline fitting and can exclude the influence of
baseline fitting error on the final result. It is especially suitable for the
cases when the scanning range is too narrow or the absorption linewidth is too
large to acquire the baseline. In this paper, the BF-SDAS is first derived and
theoretically analyzed based on the Fourier analysis, and then the accuracy of
this method is validated by measuring the concentration of H2O molecules in a
static gas absorption cell at room temperature.
