Pulsed, single-mode cavity ringdown spectroscopy

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Abstract
We discuss the use of single-mode cavity ringdown spectroscopy with pulsed lasers for quantitative gas density and line strength measurements. The single-mode approach to cavity ringdown spectroscopy gives single exponential decay signals without mode beating, which allows measurements with uncertainties near the shot-noise limit. The technique is demonstrated with a 10-cm-long ringdown cavity and a pulsed, frequency-stabilized optical parametric oscillator as the light source. A noise-equivalent absorption coefficient of 5 × 10-10 cm-1 Hz-1/2 is demonstrated, and the relative standard deviation in the ringdown time (στ/τ) extracted from a fit to an individual ringdown curve is found to be the same as that for an ensemble of hundreds of independent measurements. Repeated measurement of a line strength is shown to have a standard deviation <0.3%. The effects of normally distributed noise on quantities measured using cavity ringdown spectroscopy are discussed, formulas for the relative standard deviation in the ringdown time are given in the shot- and technical-noise limits, and the noise-equivalent absorption coefficient in these limits are compared for pulsed and continuous-wave light sources.