Rapidly swept continuous-wave cavity-ringdown spectroscopy

► Continuous-wave cavity-ringdown is able to measure weak optical absorption spectra. ► Rapidly swept cavity length or laser frequency initiates optical buildup and decay. ► Cavity-excitation time needs to be much shorter than cavity-ringdown decay time. ► Highly sensitive noise-equivalent absorption of ∼5 × 10−10 cm−1 Hz−1/2 is realised. ► High-resolution spectra, spanning a wide wavelength range, can be rapidly recorded.