On quantitative measurements in phase-shift off-axis cavity-enhanced absorption spectroscopy

The contribution of amplified spontaneous emission (ASE) from a fibre-coupled DFB near-infrared diode laser has been analysed for phase-shift off-axis cavity-enhanced absorption spectroscopy (PS-OA-CEAS). A new phasor representation of the laser coherent component and the ASE components, with both long and short ring-down times, has been proposed for calculating absorption spectra from the phase and amplitude of the cavity output intensity. The line strength of the R(4) CO2 line at 1604.7 nm has been estimated for the case when the intensity of the ASE component, passing through the cavity without long-term ringing, contributes up to ∼60% of the total light intensity observed at the cavity output. It has been shown that neglecting the ASE contribution can lead to an overestimation of the line strength by a factor of ∼4.8, whereas the line strength, calculated in accordance with the phasor model introduced here, is close to the actual line strength from Hitran.