Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS)

FS-CRDS differs from traditional single mode, cw-CRDS by actively stabilizing the optical cavity length through the use of a co-resonant reference beam. This length stabilization in turn stabilizes the cavity's free spectral range (FSR). During a spectral scan, the laser frequency is stepped between successive TEM$_{00}$ modes, resulting in an extremely linear and stable spectral frequency axis. This stability leads to a short term noise equivalent absorption coefficient of 2.5$\times$10$^{-10}$ cm$^{-1}$ Hz$^{-1/2}$. Long-term averaging is also possible, due to the stable frequency axis, allowing for a detection limit of 1.8$\times$10$^{-11}$ cm$^{-1}$, corresponding to a line intensity of 2.5$\times$10$^{-31}$ cm/molec. Using FS-CRDS we have quantitatively measured nine ultraweak electric quadrupole transitions in the O$_{2}$ $\textit{A}$-band having intensities of 3$\times$10$^{-30}$ to 2$\times$10$^{-29}$ cm/molec. Eight of these transitions had not previously been observed.