Extinction measurement with open-path cavity ring-down technique of variable cavity length

Open-path cavity ring down (OPCRD) technique with variable cavity length was developed to measure optical extinction including scattering and absorption of air in laboratory environment at 635 nm wavelength. By moving the rear cavity mirror of the ring-down cavity to change cavity length, ring-down time with different cavity lengths was experimentally obtained and the dependence of total cavity loss on cavity length was determined. The extinction coefficient of air was determined by the slope of linear dependence of total cavity loss on cavity length. The extinction coefficients of air with different particle concentrations at 635 nm wavelength were measured to be from 10.46 to 84.19 Mm^-1 (ppm/m) in a normal laboratory environment. This variable-cavity-length OPCRD technique can be used for absolute extinction measurement and real-time environmental monitoring without closed-path sample cells and background measurements.