Abstract
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) CO 2 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.
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