Cavity ring-down optical spectrometer for absorption measurements using pulsed laser sources

We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b1Σg−X3Σg transition in gaseous molecular oxygen. Absorption signals of less than 1 part in 106 can be detected.