Advances in CW cavity ring-down spectroscopy

Summary form only given. Cavity ring-down spectroscopy (CRDS) is a very sensitive, quantitative technique for measuring concentrations of weakly absorbing gas species. In CRDS, a high-finesse cavity is filled with radiation; the exciting radiation is then extinguished. The exponential decay time of the light leaking out of the cavity is measured, revealing the resonator losses, including those caused by absorbing species inside the cavity. Single-mode CW lasers can be efficiently coupled into such cavities. Measuring the ring-down time as the laser is tuned reveals a high-resolution spectrum of any absorbing species. Typical absorption sensitivities are below 10/sup -9/ cm/sup -1/. Recently, we servo-locked the laser and cavity frequencies to maximize the coupling efficiency. The difference in the mode structures for s- and p-polarized light in a three-mirror ring resonator (TMRR) facilitates CW locking without interfering with the modulation required for ring down. These TMRRs have two sets of TEM/sub 00/ modes (one for each polarization), offset from one another. The p-polarization resonator has a relatively low finesse, and its TEM/sub 00/ mode can be servo-locked to the laser frequency. An acousto-optic modulator (AOM) shifts the frequency of the light exciting the corresponding high-finesse s-polarized TEM/sub 00/ mode used for ring down.