Long-term stability in continuous wave cavity ringdown spectroscopy experiments.

Allan variance has been used to characterize the slow drift of a near-IR distributed feedback laser-based continuous wave cavity ringdown spectroscopy (CW-CRDS) system. Long-term drift in the cavity loss rate, highly correlated with changes in ambient pressure but not temperature, is observed. With differential measurement of on- and off-peak decay rates, the drift between them largely cancels out, but some residual drift remains if the lasers are detuned more than a few hundred megahertz from each other. A sensitivity to bulk cavity loss (1sigma) of 4.4 x 10(-12) cm(-1) has been obtained during an optimum integration time of approximately 30 min with our CW-CRDS setup, which corresponds to the methane detection limit (3sigma) in N(2) of 0.24 parts in 10(9) by volume (ppbv) at 20 Torr or 29 parts in 10(12) by volume (pptv) at 760 Torr pressure. The stability of our system is demonstrated by measuring sub-ppbv methane in N(2) at 760 Torr through recording the spectrum of methane lines with our setup.