Optical cavity ring-down spectroscopy technology is an emerging method of spectrum detection. This technology is especially suitable for substance weak absorption with its characteristics of long absorption optical path and unaffected by the light intensity fluctuation. It is applied to the accurate measurement of high reflectivity of lens (accuracy up to 10-4 ～10-6) and trace gas detection with features of simple device and easy quantitative measurement.

In CNI Optical Cavity Ring-down Spectroscopy Detection System, as follow figure shows, pulsed laser as a laser source, optical resonant cavity composed of high reflectivity lens M1 and M2, the laser pulse is reflected back and forth between the two mirrors to from oscillation when pulsed laser along with the optical axis injected into the cavity. Laser pulse ring-down process and time is detected and recorded by the photodetector behind M2. Then the cavity gas concentration can be calculated In the condition of known cavity mirror reflectivity.