“…25 The reported methods include cavity ringdown spectroscopy (CRDS), [8][9][10]12,13,15,16,20,21,26 cavity-attenuated phase shift spectroscopy (CAPS), 11,18,27 cavityenhanced spectroscopy (CEAS), 28 optical feedback CEAS, 17,29 mode-locked cavity-enhanced absorption spectroscopy, 30 and broadband CEAS (BBCEAS). 6,14,19,31 CAPS and BBCEAS methods use incoherent light sources [Xe arc lamp or light-emitting diodes (LEDs)] and do not require the light beam to be strictly matched to the spatial mode of the cavity TEM 00 mode, helping to make these instruments simple, robust, and highly suited to field applications. 6 In contrast, laser-based cavity spectroscopy techniques 32 generally require on-axis optical alignment to improve laser injection efficiency into the cavity to achieve better performance.…”