Frequency-matched cavity ring-down spectroscopy

We developed a simple form of continuous-wave cavity ring-down spectroscopy (CW-CRDS) utilizing a fixed-length cavity with mode matching achieved through laser frequency modulation. We present spectra of overtone combination bands of water in the visible near 659 nm and demonstrate the absolute calibration of the spectrometer via comparison with a standard humidity sensor. Frequency-matched CRDS achieves high sensitivities (2×10−9 cm−1) in moderate integration times (3 s). The method is compared to other forms of CW-CRDS and the effects of the fixed-cavity geometry as compared to piezo-scanned cavity methods are considered.