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

The contribution of amplified spontaneous emission (ASE) from a DFB diode laser has been analysed using a new phasor model for phase-shift off-axis cavity-enhanced absorption spectroscopy. Simulated phases φout at cavity output are shown for different phase shifts of narrow-band ASE for Lorentzian line absorption, A0 = 5 × 10−5, w = 0.04 cm−1. Trace 1 is typically observed in phase-shift cavity ring-down spectroscopy (PS-CRDS) without ASE components, whilst traces 2-4 are observed in phase-shift off-axis cavity-enhanced absorption spectroscopy with ASE components. It has been shown that neglecting the ASE contribution can lead to an overestimation of the line strength.