New form of cavity enhanced absorption spectroscopy

Some of the most sensitive absorption measurements to date are based on schemes that use a resonant cavity to dramatically increase the path length of a light beam interacting with an absorbing analyte, such as a trace gas. In cavityenhanced absorption measurements one can either measure the absorption directly by observing a change in the magnitude of the transmitted light, or one can determine the change in the rate of absorption. The latter has the advantage that it is somewhat immune to intensity fluctuations of the laser source. Cavity ringdown spectroscopy (CRDS) can be implemented with pulsed lasers or with continuous wave (cw) lasers, where it is sometimes referred to as cavity-leakout spectroscopy. After coupling a pulse into the resonator or after switching a cw laser off, the light remains in the cavity for some time, but slowly loses in intensity due to the finite reflectivity of the cavity mirrors. Observing this loss in time is a measure of the total loss of the resonator. In the presence of an analyte the losses are increased which results in a faster decay. The change in the decay times is a direct measure of the concentration of the analyte. A successful implementation of CRDS with pulsed lasers requires pulses of high peak power and fast sensitive detectors. In cw CRDS the cavity has to be stabilized to permit efficient coupling of the cw laser into the resonators and fast sensitive detectors are also required. Here we present a novel implementation of cavity ringdown spectroscopy that does not require high peak power pulsed lasers, or a stabilize d cavity, or fast detectors.