Continuous-wave cavity ringdown absorption spectroscopy with a swept-frequency laser: Rapid spectral sensing of gas-phase molecules

A cavity ringdown spectrometer, based on a continuous-wave swept-frequeney laser, enables efficient, rapid recording of wide-ranging absorption spectra as characteristic spectral signatures of airborne molecules. The rapidly swept laser frequency resonates with the longitudinal modes of the ringdown cavity, effectively sampling the absorption spectrum of an intracavity gas at intervals defined by the cavity's free spectral range and generating a full absorption spectrum within a single rapid sweep of the widely tunable laser frequency. We report a new analog detection scheme that registers a single data point for each buildup and ringdown decay event without logging details of the full signal waveform; this minimizes demand on digitizer speed and memory depth, reducing the time scale of data processing. This results in a compact, robust, easy-to-use instrument that offers fresh prospects for spectroscopic sensing of trace species in the atmosphere.