<title>Measurement of trace gases by diode laser cavity ringdown spectroscopy</title>

Optical spectroscopic methods based on direct absorption offer a quantitative measurement of the absorbance, which is the product of the concentration, the molar absorption coefficient of the transition being observed and the length of the absorption path. An absorption sensitivity adequate for trace detection may be achieved by increasing the path length. One solution is offered by cavity ringdown spectroscopy (CRDS), attractive for its simplicity. We recently demonstrated that an external cavity diode laser (ECDL) can be conveniently employed for CRDS instead of a pulsed laser, contrary to previous applications. Here we extend this result to distributed feed-back (DFB) diode lasers. Paying special attention to the coupling of the laser source to the cavity, we developed an extremely simplified CRDS scheme with a sensitivity of about 10^-8/cm/(root)Hz. We then built detectors for methane and HF, working close to the optical wavelengths 1.65 and 1.31 micrometer, respectively With an optical assembly of about 50 cm length and a response time of about 1 s, these devices accurately measure atmospheric methane concentrations in the range 0.5 to 200 ppmv, and HF concentrations from 0.1 to 50 ppmv.