Wavelength modulation spectroscopy for recovering absolute absorbance

A highly sensitive, calibration-free method, wavelength modulation - direct absorption spectroscopy (WM-DAS), is proposed by extracting the characteristic frequencies of modulated laser intensity. This method combines the advantages of measuring absolute absorbance profile from calibration-free DAS with the enhanced noise rejection and high sensitivity of WMS. The preliminary validation experiment of H2O transitions shows that the WM-DAS method improves the accuracy of the absorbance profile measurement with a best-fit standard residual of 4.9 × 10-5. This new method might inspire new approaches to high-precision measurement of spectral line parameters and gas information with weak absorptions or industrial applications.