We have investigated the implications of using a pulsed, nearly Fourie
r-transform-limited, single-mode light source for cavity ring-down spe
ctroscopy (CRDS) in the mid-infrared spectral range. We show that in t
he case where the coherence time and duration of the light pulse excee
ds the cavity roundtrip time, mode beating generates oscillations in t
he ring-down waveform. When the period of the oscillations is comparab
le to the ring-down time, it becomes difficult to obtain meaningful de
cay constants. This situation can be avoided by careful choice of cavi
ty geometry and mode matching conditions together with suitable electr
onic filtering.