Cavity-locked ring-down spectroscopy

We have performed cavity ring-down spectroscopy by locking a high-finesse resonator to the probe laser. We have obtained combination overtone spectra of water vapor in the ambient environment with a baseline noise of ${\text{5×10}}^{\text{−9}}$ ${\mathrm{cm}}^{\text{−1}}$ for decay constants ($R$ =99.93% reflectors) of 1 $\mu$ s. This cavity-locked approach ensures single transverse mode excitation, reduces shot-to-shot fluctuations in the decay constant to ${\text{4×10}}^{\text{−3}}$ , and eliminates oscillations in spectral backgrounds. This approach also allows ring-down decay acquisition rates limited only by the ring-down and buildup constants of the resonator, and holds the promise of offering truly shot-noise-limited cavity ring-down spectroscopy measurements.