CAVITY RING-DOWN SPECTROSCOPY WITH FOURIER-TRANSFORM-LIMITED LIGHT-PULSES

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.