The main components of optical cavity ring-down spectroscopy are a laser, a ring-down cavity composed of a pair of high mirrors, and an optical detector. The optical signal emitted by the laser is collimated and injected into the ring-down cavity. The optical signal is reflected multiple times between a pair of high mirrors in the ring-down cavity. When the optical signal encounters the highly reflective mirror at the back end, a small amount of the optical signal leaves the ring-down cavity in a transmissive manner. The detector captures the transmitted light signal in real time.

When the light intensity received by the detector reaches the threshold, the laser stops emitting the light signal, and the light signal in the ring-down cavity begins to ring down, and the light intensity (1) is exponentially attenuated. The light intensity attenuation is mainly caused by the loss of the ring-down cavity itself and the selective absorption of the gas to be measured in the ring-down cavity. Therefore, the measured empty time T (no selective absorption of the gas to be measured) and ring-down time r can be obtained by measuring the concentration Concentration of the gas to be measured = K(l/t-l/).

Cavity Ring-Down Spectroscopy (CRDS) is a direct analytical method for the study of atomic and molecular content in gases. The method provides an absolute absorption coefficient (K) that can be self-calibrated. Compared with the regular trace gas analysis methods (mass spectrometry, chromatography, spectroscopy, capacitance method, electrochemical method, etc.), the optical cavity ring-down spectroscopy has the advantages of high sensitivity, fast response, high accuracy and strong stability, which makes it widely used in the fields of semiconductor, environmental protection, medical treatment and so on.