Frequency-stabilized cavity ring-down spectroscopy

Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) is an ultrasensitive variant of traditional cw-CRDS whereby the optical cavity length is actively stabilized to an external frequency reference. This in turn stabilizes the cavity's free spectral range which provides an extremely stable, linear, and accurate frequency axis for our spectra. FS-CRDS is particularly well-suited to molecular lineshape studies. We have recently applied FS-CRDS to O$_{2}$ and CO$_{2}$ transitions in the near-infrared which are of great importance in remote sensing. Spectral signal-to-noise ratios as high as 28,000:1 have been achieved, allowing for observation of subtle lineshape effects, such as speed-dependence and collisional narrowing, and measurement of spectral parameters to the 0.1{\%}-level. In the O$_{2} A$-band we have also observed ultraweak electric quadrupole transitions as well as hyperfine structure (for the $^{17}$O-containing isotopologues).